Deep Earth DIALOG

Number 9 Fall, 1995

This is the ninth annual issue of the newsletter of SEDI, an IUGG Union Committee to Study the Earth's Deep Interior. Requests for additional copies of this issue, or for copies of the earlier issues, should be addressed to David Loper, Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida 32306-3017, U.S.A, faxed to (904) 644-8972 or emailed to loper@gfdi.fsu.edu. Items for the next issue or notifications of change of address should be sent to the same address.
 
 


 

IUGG General Assembly Boulder Colorado July 2 - 14, 1995

The XXI General Assembly of the International Union of Geodesy and Geophysics was held from July 2 through 14, 1995 on the campus of the University of Colorado. SEDI was active in crafting the scientific program of the Assembly and was listed as co-sponsor of a total of eight joint symposia and workshops. The following contains reports of seven of these, plus four other symposia of interest to SEDI.
 

Earth Rotation: an Interdisciplinary Approach

A one and one-half day symposium (G JS3) on Earth's rotation, sponsored jointly by IAG, IAGA, IASPEI and SEDI, was held during the afternoon of July 7 and during both the morning and afternoon of July 10. The symposium was organized by Martine Feissel ( Observatoire de Paris), with assistance from Peter Brosche, David Cartwright, Raymond Hide, Roberto Sabadini, and John Wahr.

Earth's rotation can be affected by a wide variety of processes above, below, and at the surface of the earth. Thus, the topic is of interest to scientists in many different fields, a fact reflected in the diverse nature of the presentations in this sym posium. There were numerous talks and posters related to the effects of the atmosphere, the oceans, and changes in the distribution of fresh water (including snow and ice), at both short and long periods. There were presentations describing the many way s of detecting changes in rotation, from the use of modern space-based techniques to obtain the variability over the last few decades, to the use of historical data over the last few millenia, to the use of geologic data for still longer time periods. Th is report, however, will discuss only those presentations which were particularly relevant to Earth's deep interior, the focus of SEDI.
 
 

Nutations and Other Short-Period Fluctuations

Earth's luni-solar nutations are the periodic tipping of the earth in response to gravitational forcing from the sun and moon. Because this forcing is so well modeled [though the ever-increasing observational accuracy of VLBI demands continual improveme nts in those models; one such model was discussed by F. Roosbeek (Royal Obs. of Belgium)], observations of nutation amplitudes and phases provide information about how Earth responds to a known force, and so can be used to constrain certain properties of Earth's interior. Earth's core and mantle respond in different ways to the luni-solar force, and so there is differential rotation between the core and mantle. As a consequence, nutation observations have been useful in constraining the ellipticity of t he core-mantle boundary (cmb), and in putting bounds on core-mantle dissipative effects. This differential core-mantle motion leads also to a resonance in diurnal earth tides, and so earth-tide observations have been similarly used to constrain the bound ary ellipticity and dissipation.

 The present status of these constraints, both from a theoretical and observational point of view, was discussed in several presentations (eg. those by D. Crossley, P. Defraigne, V. Dehant, P. M. Mathews, and J. Schastok). The nutation and tidal cmb elli pticity constraints are consistent with one another, implying an ellipticity that is about 1/2 km larger than the hydrostatic value. V. Dehant (Royal Obs. of Belgium) noted that an ellipticity of that magnitude is not at all inconsistent with estimates o f the effects of mantle convection on the boundary shape. In fact, she reported that by using seismically and geodynamically consistent values for the ellipticity of constant-density surfaces throughout the mantle, it is possible to even further improve the agreement between observed and predicted nutation amplitudes. P. M. Mathews (Harvard Smithsonian Center for Astrophysics) took the interpretation of the nutation observations still further, to search for evidence of differential rotation between the inner and outer cores. By fitting parameters to the nutation amplitudes at different frequencies, he was able to numerically constrain an inner core coupling parameter that is proportional to the inner core ellipticity divided by the density jump across the inner core/outer core boundary. His proposed interpretations include an inner core boundary ellipticity that is 0.6 times the hydrostatic value, or a density jump across the inner core boundary that is about 50% larger than the PREM density jump.

 The dissipation constraints, however, are a different story. The earth tide dissipation estimates are many times larger than those derived from nutation observations. It is not clear why there is this discrepancy. It would be interesting if it could s omehow be interpreted in terms of Earth's internal structure or dynamics. It is quite likely, however, that it is due instead to errors in oceanic corrections. D. Crossley (McGill U.) argued that ocean load contributions to earth-tide observations are u ncertain enough that existing body tide dissipation estimates are probably not inconsistent with the nutation results. Improved ocean-tide estimates from satellite altimetry are likely to improve this situation in the near future. In addition, J. Schast ok (U. T&uumlbingen) concluded that past estimates of the ocean's effects on nutations could conceivably have overlooked non-negligible contributions from higher-order ocean-tide harmonics. If such contributions turn out to be important, there would pres umably be implications for the nutation results.

 The nutation cmb constraints arise because of a normal mode of Earth (called the free core nutation) that involves a relative rotation between the core and mantle. In fact, the core should possess an infinite number of gravitational/inertial modes, whic h could well involve some rotational motion of the mantle as well as non-negligible gravitational signals at Earth's surface. These modes are difficult to model [both M. Rochester (Memorial U. of Newfoundland) and S. Webb (U. Witwatersrand) described on- going efforts to calculate these motions, along with nutations, for a realistic earth], and have not yet been unambiguously observed. Expected periods are roughly 12 hours and longer. D. Crossley described the spatial characteristics of these modes, and how they depend on the stability of the fluid core. Excitation calculations predict amplitudes of somewhat less than 1 nanogal in surface gravity at long wavelengths: an amplitude regime that is, in principle, observable with existing gravimeters.
 
 

Long-Period Fluctuations

Earth's rotation rate and the position of the rotation axis both exhibit decade-scale fluctuations and secular trends. The decade fluctuations in rotation rate are believed to be due to the exchange of angular momentum between the core and mantle. G. H ulot (IPG Paris) reviewed the situation, noting that recent estimates of topographic torques due to pressure acting against the core-mantle boundary are far larger than those needed to explain the observed perturbations in rotation rate. Electro-magnetic and gravitational torques could also be important, though they are harder to estimate. Hulot noted that even if one assumes that the entire effect is due to topographic torques and is willing to adopt a model for fluid pressure at the top of the core [t he origin of this pressure is likely to be compositionally driven flow in the core, though E. Martinez (Cuidad U., Buenos Aires) suggested that Lorentz forces due to external magnetic storms could play a role in exciting core flow], it is not possible to put a meaningful upper bound on the amplitude of cmb topography. The reason is that for any realistic pressure model there is considerable cancellation in the torques acting on different parts of the cmb. A small perturbation in the pattern of the topog raphy could change the torque estimates drastically. It is thus not necessary to propose a reduction in topographic amplitude to achieve a reduction in the rotation-rate fluctuations. This is perhaps unfortunate, because any constraint on cmb topography can feed back through geodynamic models and seismic tomography results, into estimates of such things as the mantle viscosity profile, the scaling ratio between seismic velocity and density, and the depth of slab penetration. L. Nicolaysen (U. Witwaters rand), for example, argued that subduction processes in the transition zone might have a significant effect on cmb topography.

 G. Hulot went on to note that it is much harder to invoke the transfer of angular momentum between core and mantle as an explanation for the observed decade fluctuations in pole position. In fact it is not clear that any of the torques described above ar e effective enough. At the moment, the source of the decade fluctuations in pole position is not understood.

 Secular changes in pole position over the last century and in the long-wavelength, satellite-derived geoid over the last couple of decades were discussed in several presentations (eg. those by R. Eanes, D. Han, X. Jiang, L. Vermeersen, and J. Wahr). It is likely that there are significant contributions to the secular variability from post-glacial rebound, so that the observations have the potential of constraining Earth's viscosity profile. To this end, X. Jiang (U.Toronto) concluded that the magnitude of the secular change in pole position favors only a small viscosity contrast across the 670-km boundary: roughly a factor of two viscosity increase when going from the upper mantle to the lower mantle. However, the secular drift of the pole and the rel ative-sea-level data set that served as a starting point for Jiang's viscosity profile, have little sensitivity to the viscosity below about 1400 km depth. Thus, the viscosity of the lowermost mantle remains largely unconstrained by the rotation analysis .

 The secular change in the geoid does have considerable sensitivity to the viscosity in the very deepest mantle, and so it can help resolve this problem. R. Eanes (U. Texas) described the use of satellite ranging data to infer secular changes in the geoi d. The secular variation of the second-order zonal harmonic, J2, is well determined by using both LAGEOS and STARLETTE ranging data in a simultaneous solution. Odd-order and higher even-order zonal coefficients are more difficult to obtain.

 However, there are possible ambiguities in the interpretation of both the geoid and the rotation data. For example, D. Han (U. Colorado) showed that the post-glacial-rebound estimates are sensitive to assumptions about lithospheric thickness, particular ly in the case of polar drift, and noted that results for the secular change in the geoid are notably dependent on details of the assumed melting times of the Late Pleistocene ice sheets. L. Vermeersen (U. Bologna) emphasized the potentially large effect s of compressibility on the polar drift. Furthermore, J. Wahr (U. Colorado) noted that secular effects on the geoid of present-day melting of Greenland and Antarctica could well be as large as the effects of post-glacial rebound. The effects on the secu lar polar drift are apt to be relatively less important, though they could still be significant.

 There are also secular effects due to convective flow in the mantle. This is particularly an issue for polar drift, where there is geological evidence of polar wander speeds on the order of 0.3-0.5 degrees per Myr relative to the hot-spot reference fram e: roughly 1/3 to 1/2 the observed present-day polar drift. Polar wander over geologic time scales is presumably due to changes in Earth's inertia tensor associated with subducting slabs, rising plumes, and other features of convective flow. This proble m was considered by R. Sabadini (U. Milan) and by M. Richards (U. California Berkeley), who approached the problem in slightly different ways, but who both concluded that the observed geologic rate requires a lower-mantle viscosity that is at least an ord er-of-magnitude greater than the upper-mantle viscosity. R. Sabadini reached his conclusions by choosing a series of randomly-varying subduction patterns, calculating the resulting perturbations of internal mantle boundaries and of Earth's surface - calc ulations that depend on the mantle's viscosity profile, and using the results to estimate the total change in the inertia tensor. He found that the pole position varied much too rapidly unless there was at least a factor of 10 increase in viscosity betwe en the upper and lower mantles. M. Richards reached a similar conclusion using results from a numerical model of mantle convection. The model was run for different viscosity profiles to get a temporally-evolving convective flow. He found that the in ertia tensor perturbations produced by the flow change much too quickly to explain the observed polar wander, unless the lower mantle viscosity is at least 30 times greater than the upper mantle viscosity.

 R. Sabadini found supporting evidence for these conclusions by considering Earth's non-hydrostatic dynamic topography. He found, using the randomly-varying subduction patterns described above, that a near-uniform viscosity profile tends to cause a degre e-2 topography pattern that is zonally symmetric. But if the lower-mantle viscosity is much larger than the upper-mantle viscosity, then the degree-2 pattern tends to have large m = 2 terms, and to be in closer agreement with what is observed.

 There is evidence that polar wander exists on Mars and Venus as well, and this was discussed by G. Spada (U. Bologna). He noted that long-term polar wander on Earth is best excited by mass anomalies well below the surface. Long-wavelength loads on the outer surface are isostatically compensated at the base of the lithosphere. Since the lithosphere is thin, the effects on the inertia tensor are small. For Mars, on the other hand, the lithosphere is likely to be much thicker than Earth's. As a result, the mass that compensates a surface load is relatively deep inside the planet, and the net effect on the inertia tensor can be significant. Changes in surface loads on Mars can thus have a notable effect on polar wander. For Venus, there is an observed offset between the figure and rotation axes of about 1/2 degrees. Spada's results supported earlier suggestions that Venus might be undergoing a large amplitude, free wobble. Because Venus' rotation rate and precession constant are small, the free wobble period is very long - roughly 10E5 years. Since a free wobble would result in an offset between the two axes, it may be that the observed offset is a very slow periodic motion.

 The change in Earth's rotation rate over the last 600+ Myr was studied by G. Williams (U. Adelaide) using time-series analysis of marginal-marine deposits of geologic origin. He concluded that the average rate of lunar retreat over this long times span is only about half that observed at present, confirming the generally accepted view that the present rate of tidal dissipation in the oceans is anomalously high. Presumably the present-day ocean, with boundaries and depths which change over geologic time , is unusually effective in dissipating tidal energy. Williams also was able to use his estimates of the number of solar days per lunar month and the number of solar days per year, to infer that Earth's moment of inertia has changed by less than 2% over t he last 600 Myr. This argues that neither Earth's radius nor Earth's radial distribution of density has changed significantly over this time span.

 Contributed by J. Wahr (U. Colorado).
 

Geomagnetic Secular Variation and Core Flow Near the Core-Mantle Boundary

This Symposium (GJS1.05/5.13), jointly sponsored by IAG, IAGA, IASPEI and SEDI, was convened 6 July 1995. Almost all of the 14 oral and 4 poster papers presented addressed SEDI-magnetic themes such as geomagnetic jerks, 65-70 year variations, techniques of core surface field and flow estimation, and core geodynamo theory and simulation. In a time when simple magnetic Earth models, such as an insulator mantling a frozen-flux core in surficially steady, broad-scale flow can account for most of the recent geomagnetic secular variations (SV), the needs for (i) new global geomagnetic data, (ii) continued scrutiny of existing data for special geomagnetic events, and (iii) more sensitive tests of more realistic core hypotheses, are clear. If such test requir e reliable uncertainty estimates, then a thorough understanding of non-core geomagnetic fields, notably external and crustal fields, is essential.

 A. Jackson (Leeds U.) described the influence of crustal fields on core magnetic field and flow models. Crustal fields not only limit spatial resolution of the core field, but contribute to the broad-scale field mainly generated by electric current in the core. From the standpoint of core field and flow modeling, crustal fields are a source of error with covariance estimable from a statistical model of crustal magnetization. Jackson improved such models by including a correlated structure function: a correlation length of about 50 km works well when the model is compared with geomagnetic observations. Although noise, non-core geomagnetic signals, and the approximate nature of frozen-flux core or geostrophic vorticity constraints make improved resolu tion of core field and flow estimates difficult, the need for more observatory and satellite geomagnetic data remains clear.

 C. Voorhies (NASA Goddard Space Flight Center) emphasized the importance of estimating both broad-scale core surface field and flow when testing core flow hypotheses. Non-core signals in the data, and/or regularization of field models used in the estima tion limit the sensitivity of such tests. Co-estimation of the reference core field helps remove such limitations. For steady flows, it greatly reduced the significance of residuals to the sequence of geomagnetic main-field models fitted. Still lesser residuals resulted when steady fluid acceleration was estimated along with the reference field and flow. A preferred estimate gave an rms fluid speed of 7.5 km/yr and an rms acceleration 2 x 10-13 m/s*s at the core surface, indicating but weak net forces upon the fluid core.

 M. Celaya (U. Colorado) reported results in aliasing and noise in core flow inversions. The accuracy of the estimated flow was found to depend upon the fall-off of the surface kinetic-energy density spectrum with spherical harmonic degree; an inverse sq uare or faster fall-off was needed to avoid severe errors in the broad-scale flow estimate. Moreover, temporal aliasing can prevent a steady model of a time-varying flow from recovering even the time-averaged structure of the true flow. Although not eff ective in eliminating the deleterious effects of aliasing, damping can help eliminate effects of noise in SV models. Reliable covariances for the latter are not easily obtained. In a time when strong regularization conditions are routinely placed on a v ariety of seemingly overparameterized geophysical inverse problems, it was refreshing to see the hazards of underparameterization presented so clearly.

 K. A. Whaler (and R. G. Davis, U. Edinburgh) addressed the 1969 geomagnetic impulse and spin-up of the earth's liquid core. Allowing only two key coefficients of an otherwise steady flow to vary in time not only improved the fit to the SV through the 19 69 jerk, but gave good agreement with length-of-day variation data. Flows steady in a drifting frame also fit the data more closely. With the spin-up time put at 5 years, the eddy viscosity for the core was found to be 6.7 m*m/s - but twice that inferre d from decay of the nearly diurnal free wobble.

 Y. Honkura (and M. Matsushima, Tokyo Inst. Technology) addressed the computation of core motion using geomagnetic data. As should be well-known, Matsushima's method uses a much fuller core MHD model than frozen-flux methods, albeit with some assumptions about the toroidal field and poloidal flow within the core. Surface flows obtained by the frozen-flux method and by Matsushima's method were similar; however, recover of input flows generated by a dynamo simulation was at best fair for both methods. Th e frozen-flux method tended to misinterpret wave-like drifts in the field, while the truncation of Matsushima's method led to difficulty with the non-linear couplings. The dynamo simulation itself was, however, considered far from realistic.

 D. Jault (IPG Paris) discussed the bearing of secular variation analyses on geodynamo modeling (and vice-versa). Geodynamo models often feature a near balance between magnetic diffusion and advection, Lorentz and Coriolis effects of similar magnitude, a nd important small-scale structure. In contrast, SV analyses yielding core flow models often neglect diffusion, omit Lorentz effects, and presume broad-scale flow. Yet geodynamo models often adopt flow speeds from core flow models, while core flow models often adopt hypotheses from geodynamo models. However, in one model of slow variations, the main dynamo motions are in the null-space of frozen-flux flow inversions and induce observable SV indirectly. Further considerations led to the conclusions that Taylor's condition is almost satisfied and torsional oscillations exist in the core.

 S. V. Starchenko (U. Newcastle upon Tyne) presented an analytic representation of outer core MHD flow in an imposed magnetic field. This complement to the kinematic dynamo approach is designed to handle the very small, very realistic Ekman numbers so di fficult to attain in purely numerical computer simulations. It also bears on recovering appropriate boundary conditions in the inviscid limit. Flow solutions show suppression of the cylindrical Stewartson layer, but persistence of the Ekman-Hartman boun dary layers, at very small Ekman number.

 T. Shankland (Los Alamos National Lab. and J. P. Poirier, IPG Paris) reviewed mantle electrical conductivity as a background for geomagnetic variations, including the best estimate for a spherically averaged mantle conductivity as a function of radius, lateral heterogeneity, physical interpretations of conductivity variations, and the conductivity of D''. Analyses of surface data are converging on a mid-mantle conductivity of several S/m; these agree well with laboratory measurements on magesiowüstite- silicate perovskites in a diamond-anvil cell. Modest extrapolation of laboratory results indicates a conductivity of about 10 S/m just above D''. Lateral variations are likely not thermal in origin outside of subducted slabs; chemical heterogeneities ar e considered relatively less important as depth increases, notably if the deep mantle convects. A very high conductivity D'' remains possible, as perforce do appreciable electromagnetic core-mantle coupling and significant toroidal magnetic fields at the top of the core.

 Several papers addressed geomagnetic jerk events. Separation of internal, external, and externally induced internal variations can identify the origin of such events. V. A. Shapiro (et al., Inst. Geophysics, Ekaterinburg, Russia) found correlati ons between several jerks events and maxima of the 21-22 yr solar magnetic cycle, with the timing of the jerks apparently coinciding with reversals of the solar field. M. Alexandrescu (et al., IPG Paris) described a study of geomagnetic-jerk detec tion and the geodynamo using wavelet analysis; seven events identified between 1900 and 1980 are believed to originate in the core. H. Nevanlinna (Finnish Meteorological Inst.) found evidence of another geomagnetic jerk in 1870; moreover, detrended decli nation time series from 1844-1909 were found to correlate well with length-of-day time series. V. P. Golovkov (et al., Inst. Terrrestrial Magnetism, Troitsk Russia) identified nine events since 1896, fitted a flow field to a field model of the 191 5 event, and found strong similarities between the 1915 event and the geomagnetic jerk of 1970.

 Two papers identified multi-decade secular variation signals, such as a 60-70 year variation sometimes attributed to torsional oscillations of Earth's core. J. C. Gianibelli and E. A. Suarez (Argentina) analyzed Antarctic and Subantarctic geomagnetic ob servatory annual means for long period variations; in addition to 10E-13 year solar and 18.6 year lunar cycles, the found signs of a 68 year wave. U. Raval (and K. Veeraswamy, National Geophysical Res. Inst., Hyderabad) reported results of work on the 65 -year geomagnetic impulse and EM induction in the earth. Their layered-mantle model now includes lateral heterogeneity at the CMB. The 65-year impulse indeed appears to be the consequence of mantle filtering of a periodic geomagnetic signal.

 Earth's toroidal magnetic field, which is perhaps as difficult to measure as it is essential to dynamo theory, was not overlooked. H. Utada (and K. Hinata, U. Tokyo) stressed the importance of electrical potential measurement using the transoceanic cabl e network. An otherwise unmeasured toroidal magnetic field within the earth can cause steady electric potential difference across such cables; however, the AC electric field is easier to detect (if not separate from external signals). A larger cable net work is needed to separate poloidal from toroidal electric potential components.

 Contributed by C. Voorhies (NASA Goddard Space Flight Center) and M. Celaya (U. Colorado).
 

Reversals: Observation and Theory (GA1.06)

Y. Gallet and V. Courtillot (IPG Paris) noted that when plotted in order of occurrence, a change in the successive length of polarity intervals occurs at about 25 Ma. There appears to be a quasi-periodicity in reversal duration in the interval preceding this change. B. S. Singer (U. Geneva) and M. S. Pringle (Scottish U. Research and Reactor Centre) presented 40Ar/39Ar dates for volcanic records of the Matuyama-Brunhes reversal and demonstrated that the dating resolution is approaching the duration of the polarity transition. The reversal was completed before 768 ± 3 kyr. C. V. Voorhies (NASA Goddard) summarized results indicating that analysis of the spatial power spectrum of the field and recent core flow estimates indicate an equipartitioning betw een poloidal dipole and non-dipole fields.

 The debate over the interpretation of recurring geomagnetic field behavior during reversals continued at this session. C. Barton and P. McFadden (Australian Geological Survey) showed how the increasing effect of inclination shallowing with decreasing fi eld strength may provide an explanation for the observation of VGP paths which tend to fall ±90 degrees away from the sampling site longitude. L. O. Nicolaysen (U. Witwatersrand), S. K. Runcorn (Imperial College, London), K. Hoffman (California Poly. Sta te U.) and R. X. Zhu (Chinese Acad. Sci.) , each presented papers which examined how variations in properties of the lower mantle may account for preferred VGP paths or recurring flux patches during polarity transitions. K. L. Verosub, I. Aleinov and E. G. Puckett (U. California Davis) presented results of using a method of modelling flux patches on the surface of the outer core and examining the effects of varying flux patch polarity and intensity with fields observed at Earth's surface. A. Mazaud (CNR S Gif sur Yvette), J. Love and D. Gubbins (U. Leeds), presented results from the first attempt to invert paleomagnetic records of the Matuyama- Brunhes polarity transition for the radial component of the field at the core- mantle boundary. P. Ultr�-Guera rd and J. Achache (IPG Paris) suggested that rapid field directional changes reported from the Steen's Mountain polarity transition could possibly be the result of magnetic storms which occcurred while the main geomagnetic field was weak.

The afternoon session was devoted to the presentation of new results from polarity transition data from both deep sea sediments as well as from lavas and examination of some of the difficulties associated with interpreting these data as records of geomag netic field behavior.

 Contributed by B. Clement (Florida International U.).
 

Core Dynamics and the Dynamo (GA1.04)

First Session: Experiments and Observational Constraints.

K. Aldridge (York U.) reported findings from an experiment designed to detect the instabilities arising in a fluid sphere which is given a small ellipsoidal deformation. The Poincar 3 modes which are excited by this instability were discussed by G. A. Henderson (York U.), who has studied numerically the characteristic surfaces that can arise in this problem. P. Cardin (École Normale Supérieure, Paris) discussed an MHD experi ment in which a rotating disc drives liquid gallium in a strong magnetic field; the aim is to test dynamo efficiency by measuring the rate of Joule heating. S. Labrosse (IPG Paris) presented some new calculation on the thermal heating of the outer and in ner core, and J. Lister (U. Cambridge) presented a new analysis of the problem of the relative strength of thermal to compositional convection in the core.

B. Buffett (U. British Columbia) considered the question of how the anisotropy of convection in the outer core produced by rotation might alter the shape of the inner core. S.K. Runcorn (Imperial College, London) argued that the history of the lunar mag netic field suggested that its intensity was determined primarily by its energy source rather than its rotation, and hence that the absence of a dynamo in Mars and Venus was similarly due to lack of adequate convection in their cores.
 
 

Second Session: Core Dynamics and Core-Mantle Coupling.

A. Szeto and S. Xu (York U.) discussed the problems of gravitational pressure and electromagnetic coupling in the core. I. Eltayeb (Sultan Qaboos U., Oman) presented a rising plume model of compositional convection in which the stability of the plume is considered. P. Coffey (U. Newcastle upon Tyne) considered the effect of bumps on magnetoconvection, showing how the rolls induced by the bumps interact with the geostrophic flow. The ana logous problem without magnetic fields was discussed by A. Soward (U. Newcastle upon Tyne); since the geostrophic contours are no longer circular, direct driving of the geostrophic flow by the buoyancy forces becomes possible; in this way even small bum ps may give rise to significant geostrophic flows.
 
 

Third Session: Magnetoconvection and Dynamo Models.

K. Zhang (Exeter U.) presented the new results on magnetoconvection in a sphere. He showed that for typical field configurations magnetic instabilities occur when the Elsasser number exceeds ~ 10; the anomalous behaviour of magnetoconvection at low q was also discussed. W. Kuang (Harvard U.) presented the initial results of a new three-dimensional dynamo code developed at Harvard. D. Jault (IPG Paris) analysed how Taylor states can be ach ieved in dynamo models and described how retaining the azimuthal inertial term can help reach Taylor states. He also discussed the particular difficulties of obtaining a Taylor state when there is an inner core. D. Ivers (U. Sydney) continued the theme o f convergence to Taylor states, describing a method in which artificial viscosities are introduced in order to get a scheme which converge to the Taylor states.

 M. Matsushima (Tokyo Inst. Technology) described calculations for finding the helicity of core motions. C. Phillips (U. Sydney) described results from anisotropic alpha squared models, showing that oscillatory solutions can sometimes be found. T. Nakaj ima (U. California Los Angeles) described the 'mapping' method which can be used for computing intermediate dynamo models, and presented some new results on Model-Z dynamos.

 B. Farrell (Harvard U.) pointed out that because the dynamo equations were non-normal (non-self-adjoint), field amplification is possible for a finite period even if all exponentially growing modes are stable. With even small stochastic forcing, a susta ined dynamo could be achieved without requiring model instability. M. Kono (U. Tokyo) drew attention to the fact that we only have information about a small number of modes over long periods of time, so that low order models might serve to explain the li mited information available.

 C. Jones (Exeter U.) presented some models of convectively driven dynamos using the mean field or 2 1/ 2 dimensional approximation in which the radial and polar direction are fully resolved but the azimuthal direction is severely truncated. Well-behaved magnetic fields generated at value of q = k /h ~ 10 can be compared with geomagnetic field. G. Glatzmaier (Los Alamos National Lab.) described fully three dimensional convection dynamo simulations; the role of the inner core in providing some stability t o the field was emphasized. The simulations give occasional reversals of the field, and a fairly detailed picture of the reversal process was seen. J. Wicht (U. Bayreuth) described recent developments in the bifurcation approach to dynamo models, findin g both oscillatory and chaotic dynamos, mainly in the regions q > 10.

 Contributed by C. Jones (Exeter U.).
 

Structure, Evolution and Dynamics of the Core-Mantle Boundary Region

On July 7, over 100 people gathered to participate in this symposium (S JS5), convened by Raymond Jeanloz (U. California Berkeley), Bruce Buffett (U. British Columbia) and Kathy Whaler (U. Edinburgh) and cosponsored by IASPEI, IAGA, IAG and SEDI. Resear ch in seismology, geodynamics, geomagnetism, mineral physics and planetary geophysics was summarized in 21 oral and 2 poster presentations.

 The morning session began with a planetary overview by D. J. Stevenson (Caltech), who pointed out that all of the terrestrial planets are expected to have liquid cores; even if the cores do not all convect, heat conducted out of the core will drive conve ction in the mantle of each of these planets. This should give rise to a dynamic thermal boundary layer at the base of each mantle, with implications for core-mantle boundary (CMB) topography, plume generation and various mechanisms of core-mantle coupli ng, a topic of great current interest for the Earth.

 A series of seismological papers documenting the Earth's CMB began with A. Dziewonski's (Harvard U.) tomographic inversion of a new global dataset consisting of differential and absolute travel times for SS - S, ScS - S, S - SKS and SKKS - SKS phases obt ained by cross-correlation of waveforms. He found much larger anomalies (by approximately a factor 2) and improved spatial resolution relative to previous inversions.

 To compare with the global inversions, M. Wysession (Washington U.) showed how local datasets, for example S (diffracted) waveforms from an array on the Tibetan plateau, can be used to resolve vertical structure through D''. He also looked at PKP phases , and used a genetic algorithm to model data from the Missouri-Massachusetts array, a line of 20 broadband stations currently deployed between 2 permanent observatories. Though tentative, as the data are just now becoming available, the results appear pr omising.

 In another study of local anomalies at the base of the mantle, E. Garnero (U. California Santa Cruz) and coworkers used core-diffracted P-waves, and SPdKS, SKS - SPdKS and diffracted S phases to resolve a thin low-velocity zone (LVZ) adjacent to the CMB. The LVZ is up to 40 km thick, assuming a 10% velocity reduction, with the velocity still deviating from PREM well above this zone. Of three possible explanations for the LVZ, phase transitions, a chemically-distinct region and partial melting, the last was considered the most likely even though 25% melting is needed to fit the data.

B. Romanowicz (U. California Berkeley) stood in at short notice for L. Vinnik (Inst. Phys. Earth, Moscow) and other colleagues to present a study of shear-wave splitting in records from earthquakes in the Tonga-Fiji region. They found that Sdiff to a s tation in Massachusetts is delayed by ~10 seconds relative to standard Earth models. Also, SVdiff is not coupled with SHdiff, implying that the medium is transversely isotropic with a vertical axis of symmetry. Synthetic seismograms calculated by the re flectivity method give a 3 second difference in arrival time between SVdiff and SHdiff. They were able to fit the amplitude ratio and frequency trend of the data with a 200-300 km thick zone in which there is a 1% velocity difference between SV and SH. A possible explanation for the anisotropy is lattice-preferred orientation in the convective boundary layer at the base of the mantle.

 R. Cohen (Carnegie Inst. Washington) showed from first-principles quantum-mechanical calculations that the chemical behaviour of transition metals in oxides at high pressures is very different from that at low pressure. For example, FeO and NiO become increasingly different, distorting differently at high pressure. FeO, which should be a metal at ambient conditions, is an insulator at low pressure due to subtle details of the electronic band structure. However, it has a metallic phase at high pressur e, and there is reason to expect a miscibility gap between magnesiowustite and iron oxide at deep-mantle conditions.

 To complement the theoretical findings, H. K. Mao (Carnegie Inst. Washington) summarized experimental results on Fe, FeO, FeS and FeH, showing that all of these systems take on a NiAs-type crystal structure at high pressures. Mossbauer spectroscopy fur ther demonstrates that FeO loses its magnetic moment at pressures above ~100 GPa, as briefly described by R. Jeanloz and coworkers. Thus, both experiment and theory are in agreement in documenting the inter-related changes in structure and bonding charac ter (including magnetization) as FeO is taken to the conditions of the lowermost mantle and core.

 L. Kellogg (U. California Davis) discussed results of her numerical modelling of dynamics of the CMB. She models D'' as a double-diffusive convecting chemical boundary layer adjacent to the CMB, driven by heating from below and a chemical flux across t he CMB. The system is characterized by the Rayleigh, Ra, buoyancy, B, and Lewis, Le, numbers. She used Ra=10E7 with heating from below. Le, the ratio of thermal to chemical diffusivity, was set to 1000. We have no idea what the Le is likely to be; val ues over 1000 are difficult numerically, and D. A. Yuen has suggested that there is little change once Le exceeds 100. Louise presented results with B = 0 (pure thermal convection), 1 (consistent with a 2-6% density change) and 10. A layer a few hundred km thick develops, and dense material is entrained into plumes. The influx of material from the core is shut off by development of the layer, so the system remains stable and the core-mantle temperature contrast grows over time as the layer develops. I ncreasing B leads to a thinner, less heterogeneous layer. The same general structure of the layer was observed when the calculation was repeated with temperature-dependent viscosity.

 J. Wahr (U. Colorado) concluded the morning session with geodetic studies of CMB shape. He began by examining the period of the free core nutation, which depends on the dynamical ellipticity of the core and mantle deformation. The observed value impli es there is 500 m more Y(2,0) CMB topography than for hydrostatic equilibrium. The damping time for the nutation is of order 14 years, which Buffett has explained by electromagnetic core-mantle coupling. The CMB is not an equipotential surface, and othe r Y(l,m) components of non-hydrostatic topography can be added, although Y(2,0) dominates. Using a root-mean-square topography of 3.5 km, with Y(2,0) fixed, yields a period of 0.2 masec, which is less than required to fit the observations. The geodetic data suggest a 10% error in CMB topography deduced seismically, and lateral structure in the core has little effect on these results.

 The afternoon presentations began with several papers that emphasized the role of seismic-wave scattering near the CMB, as well as the long-wavelength structure of the lowermost mantle. A. Dziewonski and W. Su (Harvard U.) have estimated the lateral va riations of scattering intensity within D'' by searching for systematic variations in precursors to PKIKP in the ISC data for the years 1964-1990. The ratio of precursors to "normal" arrivals revealed systematic variations which are well correlated with positive anomalies in global S-velocity models near the CMB. In particular, the high velocities that circle the Pacific appear to be associated with large scattering intensity. The possible connection between long- and short-wavelength heterogeneity mig ht be explained by remanent slabs at the base of the mantle.

 D. Helmberger (Caltech) and coworkers presented the results of high resolution waveform modeling of the CMB region, noting that the bifurcation of SKS into SKPdS and SPdKS in some data sets is not well modeled using PREM. These records can be explained by uniformly reducing the P-velocity by about 5% in the lowermost 50 to 100 km of the mantle. More general synthetic waveforms were calculated in which the P-velocity differs at the entry and exit points of the core. These theoretical results often requ ired a 10% reduction in P-wave velocity to explain the observed waveforms if the anomalous velocity is confined to one of the two CMB crossing points. The strong SPdKS arrival is provided by a negative velocity gradient at the base of the mantle (20 to 5 0 km). Such models predict negative precursors to PcP, which are evident in some short-period data stacks, notably between Fiji and California.

 An experiment to image CMB scatterers using global network recordings of short-period precursors to PKPdf was presented by M. Hedlin and colleagues (U. California San Diego). Their objective was to use migration techniques to back-project precursor en ergy from the free surface to the CMB. Resolution studies were carried out by ray-tracing to hypothetical point scatterers near the CMB in order to identify those areas which have sufficient resolution to image small-scale CMB scatterers. The goal is to discriminate between core entry and exit point structure, and to distinguish between scatterers at the CMB and those within D''. A preliminary analysis of PKPdf precursors in the short-period GDSN data and the broadband data from the IRIS network reveal s seismograms both with and without observable precursors. Subsequent efforts will be directed at mapping the scattering intensity over the CMB.

 The next two presentations examined how the heat flux at the CMB can influence the structure and dynamics of the core. J. Lister (U. Cambridge) and B. Buffett examined the conditions under which a stably stratified layer can develop at the top of the co re. They showed that the presence of a thermally stratified layer is controlled by the heat flux taken up by the mantle across the CMB, and its ratio (Nu) to the conductive heat flux along the adiabat in the core. If Nu < 1, as some recent estimates sug gest, a stratified layer develops as a consequence of the competition between the negative thermal buoyancy flux from the CMB and the positive compositional buoyancy flux from the inner-core boundary. Using simple fluid-mechanical and thermodynamic model s, they showed that a 100-km stratified layer would develop if Nu = 0.9. However, modest lateral variations in the heat flux at the CMB leads to large fluid velocities, which appear to be inconsistent with geomagnetic secular variations. This observatio n argues against a stratified layer, and for a heat flux at the CMB which is greater than that conducted along the adiabat (e.g., Nu > 1).

 G. Glatzmaier (Los Alamos National Lab.) and P. Olson (Johns Hopkins U.) used three-dimensional magneto-convection simulations to investigate how a heterogeneous heat flux at the CMB influences convection and magnetic field generation in the core. Cold regions in the mantle enhance the outward flux of heat from the core, promoting thermal convection. The downwelling motions are typically narrow with relative large velocities, while the return flow is broad and weak. In contrast, warm regions within t he mantle can cause a localized heat flux into the core, thus producing a stratified region. The character of the magnetic field generation is strongly influenced by the geometry of the core, with the field generation being most intense inside a cylinder which is parallel to the rotation axis and tangent to the solid inner core. Outside this cylinder, the field generation is less active, but more of the field diffuses into the mantle, contributing to the observable part of the field.

 R. Coe and J. Glen (U. California Santa Cruz) gave a summary of our present understanding of geomagnetic reversals based on paleomagnetic records. Except for the chronology of reversals over the last 150 myr, many of the features of magnetic reversals are poorly constrained. The long-standing figure of 5000 years for the average duration of a reversal is vulnerable to systematic errors. The question of whether reversal pole paths of the past 15 myr are confined to certain geographical regions is sti ll unresolved. Even more controversial is the hypothesis that reversals may be punctuated by extraordinarily rapid field changes, of order 1,000 times faster than ordinary secular variations. Despite the uncertainty, there is growing consensus that syst ematic features may be present in the global geometry of the field during a reversal. To overcome the difficulties posed by the brevity of reversals, they emphasized the need for high-resolution records from different kinds of rocks, a more global distri bution of recording sites, and some means of establishing common time lines for the transition field records of a given reversal observed from different sites. New records from lava flows and sediments will help to resolve some of these questions, but n ew methods of data analysis must also be explored.

 The next two presentations dealt with experimental determinations of iron and silicate melting temperatures and the constraints they impose on the thermal regime near the CMB. R. Boehler (Max-Planck Inst.) summarized the results of both static diamond-c ell experiments and shock temperature measurements on iron. Using these results he argued that the core temperature at the CMB is approximately 4000 K. Experimental data on iron alloys suggest that the melting-temperature depression due to the light ele ments oxygen and sulphur is probably negligible at high pressures. This would yield a temperature drop of approximately 1500 K if the geotherm of the lower mantle is near adiabatic. Recent melting experiments on the two major mantle components (Mg,Fe)SiO 3-perovskite and (Mg,Fe)O-magnesiowüstite place an upper bound of 5000 K for the solidus temperature of the lower mantle.

 T. Ahrens and K. Gallagher (Caltech) reported on shock-temperature measurements on iron and on olivine in the high pressure phase, the latter indicating a melting temperature of approximately 4200 K at the CMB. Consequently, they argued that the CMB tem perature cannot exceed 4200 K. Experimental data on iron were re-analyzed using new data for the high-pressure thermal diffusivity of the window materials that were used in the experiments. Improved corrections to the Hugoniot temperature lower the infe rred melting temperatures of iron by as much as 700 K. The revised melting temperature at the inner-core boundary is near 6000 K. If the addition of light alloying elements depresses the melting temperature to ~ 5500 K, then the temperature at the CMB i s approximately 4500 K.

 B. Buffett described some consequences of a heterogeneous D'' on core-mantle coupling. The presence of lateral variations in electrical conductivity, arising from either chemical heterogeneity in D'' or topography on the CMB, was shown to contribute sig nificantly to the magnetic field at the top of the core. Motion of the fluid core past the mantle heterogeneity can further amplify the induced field. Solutions for the full hydromagnetic disturbance in the fluid core indicate that this induced field ca n be as large as 2 Gauss for plausible levels of mantle heterogeneity and boundary topography. Such large perturbations represent a significant fraction of the total field at the CMB, and may account for those features which appear stationary. These mag netic perturbations also affect core-mantle coupling. Estimates of the magnetic shear stress on the mantle can increase by a factor of two or more due to the additional field, while the effect of fluid pressure acting on the boundary topography is only w eakly dependent on magnetic forces near the CMB. Both topographic and electromagnetic torques are found to be viable mechanisms for explaining the length-of-day variations.

 Further efforts to model the seismic structure of the core-mantle transition zone were described by A. Rodgers (U. California Santa Cruz) and R. Engdahl (USGS, Denver). They described an extensive set of traveltimes for P, PcP, ScS, and ScP which were a ssembled from ISC and EDR catalogs for the period January 1, 1964 to June 30, 1994. Events were relocated using residuals of both up-going (pP, pwP, and sP) and down-going (P and PKPdf) rays calculated relative to the ak135 model [Kennett, Engdahl, and B uland, 1995]. Phases were re-identified relative to the updated event locations, and estimates of CMB structure were then obtained taking account of current mantle models. Long-wavelength P-wave velocity heterogeneity in D'' is fairly well constrained b y global travel times up to degrees 4 or 5, but smaller scale structure is not reliably resolved with the current data set.

 A thermodynamic model for the chemical evolution of the CMB was presented by E. Majewski (Polish Acad. Sci.). The elements of his model included the high-pressure diffusion of FeO from the mantle into the fluid core; the melting of FeS-troilite at the b ase of the mantle; and the diffusion of liquid iron from the core into D''. The CMB was treated as a phase boundary and a surface of solution, while the fluid core was treated as a solution layer. It was assumed that the dissolved FeO and the FeS-troili te melt are transported by thermal convection from the CMB to the inner-core boundary. There, the FeO precipitates and the FeS crystallizes. Diffusion-limited rates of mass transfer of FeO and FeS-troilite are then found to be 2200 kg/m*m/year and 400 kg /m*m/year, respectively.

 M. Osmaston (Surrey, UK) presented an alternative explanation of the geomagnetic field, and its reversal, based on thermoelectric currents at the CMB. He argued that small differences in temperature across the CMB, possibly associated with sites of core upwelling and downwelling, may set up electric current loops within the lower mantle and outermost core. Under these circumstances, the morphology of the resulting field would be determined by the pattern of thermal convection in the core. The polarity and strength of the overall field would depend on the spacing of sites of anomalous electric potential on the CMB. Magnetic reversal could then be explained by changes in the electric potential, which would alter the current loops. When points of oppos itely-signed potential differences are nearly equally spaced, small changes could produce frequent reversals. Such a model demands a new view of westward drift and of the core motions inferred from secular variation.

 Contributed by B. Buffett, R. Jeanloz and K. Whaler.
 

Modelling the Earth's Interior

In the next few years, it is likely that an effort will be made to generate new reference Earth models and such models may not be solely seismological. This session (S JS8), cosponsored by IAG, IASPEI, IAVCEI and SEDI, was designed to get a cross secti on of people working on different aspects of the structure of the Earth's interior together to think about how their work fits together and contributes to the concept of a reference model. The talks therefore cover the range of mineral physics, geodynami cs, geomagnetism, and seismology.

 The session began with R.C. Liebermann, B. Li (both at SUNY Stony Brook) and G. D. Gwanmesia (Deleware State U.) discussing the impact of new multi-anvil Brillouin scattering experiments on our understanding of the transition zone. These experiments ca n now be conducted up to about 12 GPa (though still at low temperature). Experiments on poly-crystalline magnesium silicates show simple linear pressure dependence of both compressional and shear velocity for the high-pressure polymorphs and that the pol ycrystalline samples behave quite isotropically (note, for example, that single crystal b phase is extremely anisotropic). They conclude that the 410 km discontinuity can be modelled with a mantle which has 45-65 percent of olivine but that the b phase h as little velocity signature. They also pointed out the possibility that majorite garnet in the transition zone might be capable of explaining the high velocity gradients in this region.

 P. Shearer (U. California San Diego) followed with a discussion of his recent efforts to stack SS precursors to isolate the effect of the 520 km discontinuity. The latest stacks are hand-edited and restricted to bounce points in oceanic regions to reduc e waveform distortion. These stacks show strong 410 and 660 km discontinuity, a weak 520 km discontinuity (which is demonstrably not a sidelobe artifact of the other jumps) and evidence for an enhanced velocity gradient below the 660 km discontinuity. T he stacks give an impedance contrast of 2.2-3.6% at 520 in good agreement with the work of Revenaugh and Jordan. It is interesting to note that the 520 is not seen in P'P' precursor studies and is not seen in refraction type studies implying that the imp edance contrast is due mostly to a density change rather than a velocity change. This result also seems to be in agreement with the emerging mineral physics data.

 The next talk, by L. Stixrude (Georgia Inst. Tech.), also dealt with transition-zone discontinuities but was concerned with their apparent sharpness to seismic waves. He presented simple analytical results for the equilibrium shape of a phase transition assuming ideal solution theory. The sigmoidal shape means that seismology may see a sharp boundary at an increased apparent depth. In fact, a wide transition might actually look sharper than a narrow transition. The boundary properties would also vary geographically because of the temperature dependence of the shape of the phase transformation. He also pointed out that the presence of a third non-transforming phase in a ternary system can actually narrow the transformation (coexistence) zone. All o f these factors lead to the conclusion that the observed seismological sharpness is not in conflict with current mineral-physics interpretations of the major discontinuities.

 D. Weidner and Y. Wang (SUNY Stony Brook) concentrated on buoyancy effects caused by possible chemical stratification and by phase transformations. They pointed out that experimental high-pressure values for the thermal expansion of perovskite are actua lly in quite good agreement and can admit a lower mantle of pyrolitic composition. This would remove any chemical buoyancy effects between the lower and upper mantles. They also questioned the apparently convection-inhibiting effect of the spinel-perovs kite transformation at 660 km. A complete discussion must include the garnet mineralogy as well as the olivine mineralogy and the garnet-perovskite transition has a generally positive clapeyron slope with a density jump twice as large as the spinel-perov skite transformation. It is possible that the buoyancy effect near 660 km due to transformations in the complete pyrolite assemblage may actually help convection rather than hinder it.

 In the next talk, P. Defraigne (Royal Obs. Belgium; talk given by V. Dehant) considered the effects of one-cell versus two-cell convection on fitting the geoid, dynamic topography, and plate velocities using mantle tomography. They found that the influe nce of the lithosphere is very important and they get a better match of geoid and plate velocities with one-cell convection but with a large viscosity jump. They also considered the effects of phase transitions and chemical boundaries. A chemical bounda ry apparently leads to too much topography on the mantle discontinuity but a phase transition gives about 1/3 of the effect of the chemical boundary. They also found that model SH12WM13 gives the right sign for the degree 2 topography predicted for the c ore-mantle boundary by geodetic studies.

 The next talk returned to seismology where G. Ekstr&oumlm and A. Dziewonski (Harvard U.) discussed a new global mantle model of shear velocity, S20U7L5, which is expanded to degree 20 in spherical harmonics. The enhanced resolution of this model over ea rlier ones is made possible by the addition of high-resolution Love and Rayleigh wave phase-velocity maps in the 50-140 sec period range to the inversion. In the model, the separation of cratonic areas in Eurasia at 50 km depth is visible. Ridges are st ill clear at 150 km depth but disappear by 200 km (except under the Indian Ocean). At 200 km, cratonic roots are still visible with the fastest anomaly being under W. Africa. A point comparison of the Canadian Shield with the body-wave model SNA is very good (except for artifacts associated with the 220 km discontinuity in PREM). A similar point comparison with the "tectonic North America" model TNA is also quite good except that TNA has significantly lower velocities in the transition zone.

 J. VanDecar, D. James (both at Carnegie Inst. Washington), and M. Assumpcao (U. Sao Paolo) presented interesting evidence for a fossil plume head under the Parana Basin in the next talk. This study took advantage of a small array deployed in the area wh ich recorded a good azimuthal distribution of events allowing both P and S tomography to be done beneath the array. Independent inversions of P and S wave relative travel times (made using cross correlation techniques) both show a low velocity, plume-lik e structure reaching to at least 600 km depth. The authors interpret this as a fossil plume head (the plume is now at Tristan da Cunha) which would require that significant coupling of lithospheric plate motions to mantle flow is possible.

 The afternoon session began with a change of field to geomagnetism. There is considerable interest in the question of whether core motions are affected by mantle properties. A. Jackson (Leeds U.) discussed the constraints that can be used to determine core motions more reliably. In particular, he noted a new constraint that appears in the magnetostrphic limit. Kelvin's theorem applies to patches of the core bounded by null-flux curves (curves where the radial component of the magnetic field is zero). These null-flux curves are material curves and move geostrophically. A consequence of this result is that the area of patch projected onto the equatorial plane is invariant with time. It remains to be seen if secular-variation models can be produced w hich satisfy this constraint.

 J. X. Mitrovica (U. Toronto), J. L. Davis (Harvard-Smithsonian Center for Astrophysics) and J. M. Johansson (Chalmers U. Tech., Sweden) next discussed constraints on the viscosity profile in the mantle from glacial loading data. The modelling of relativ e-sea-level (RSL) variations in the past has led to a wide variety of viscosity models. The authors argue that this is due to the sensitivity of the data to details of the ice-load history and to the previous reliance on simple forward-modelling interpre tation techniques. To minimize sensitivity to ice-load details, data should only be taken from the far field or from right in the middle of the previously glaciated area. Formal inversion shows that models can be found which reconcile the data. In part icular, the RSL data are relatively insensitive to the viscosity in the lowermost mantle so a high viscosity (needed to model the geoid data) could be accomodated here.

 D.V. Helmberger (California Inst. Tech.), E.J. Garnero (U. California Santa Cruz) and X.D. Song (Columbia U.) summarized their recent work on velocity structure in the deep Earth. The velocity structure in the lowermost mantle has been constrained usin g SmKS phases. They find that S-SKS and SKKS-SKS times generally agree with the predictions of tomographic maps revealing an anomalously low velocity region in the middle of the Pacific. The SPdKS (SKS with P diffractions along the CMB) phases which sam ple this region are also strongly delayed. This effect can be modelled by introducing a 10% drop in P velocity in a thin zone above the CMB, 10 to 40 km thick. The physical explanation for such a zone remains a matter for conjecture.

 S.P. Grand (U. Texas) showed results from seismic data inversion. His tomography shows amplitudes of heterogeneities larger in the upper 300 km of the mantle and in the lowest 200 km, but does not reveal structure in the transition zone. In the lower m antle, both long-wavelength anomalies (mostly associated with high velocities) and narrow features (mostly associated with slow velocity zones) appear. There is no massive pile-up of the slab near 660 km depth; they enter slowly in the mantle. At the CM B, there are long-wavelength anomalies as well as narrow ones. Active upwellings exist in the lower mantle. These results support convection occurring across the whole mantle.

 Y. Ricard (Ecole Normale Supérieure, Lyon) combined information from present and past slabs and hotspots, from the age of the crust, and from the chemistry of the mantle and crust to deduce the temperature and pressure corrections as well as location s of phase transitions. He could get from this a model of lateral heterogeneities inside the Earth's mantle including shallowest layers (water, ice, sediments, crust) and the thickness of the lithosphere and could compare with the existing tomography mod els. This comparison showed a 70% correlation. The amplitudes are either similar or larger than some of them; in particular he got degree-two anomalies of about half the tomography results deduced from eigen-mode data. This shows that on the one hand, tomography models are due to the ingredients used above, and on the other hand, the tomography inversions are smoothing the mantle heterogeneities which are probably much shallower and short wavelength.

 H. P. Bunge (Los Alamos National Lab.) , M. A. Richards (U. California Berkeley) and J. R. Baumgardner (Los Alamos) have performed simulations of mantle convection with different viscosity profiles and different Rayleigh numbers varying with time. They concluded that the viscosity stratification has a profound effect on the plan form of the 3-D convection. The resultant flow field is remarkably similar to Earth's subduction-dominated convection (with downwellings of the cold upper boundary layer becomi ng sheetlike and elongated). The likely effects of an endothermic phase change at 670 km depth are relatively mind by comparison. Their calculations suggest also that the dynamics of mantle flow is strongly influenced by relatively modest radial variati ons in mantle viscosity.

 Y. S. Zhang and T. Lay (U. California Santa Cruz) used Love and Rayleigh waves together with a map of oceanic age in order to parameterize the oceanic lithosphere thickness. In the residual maps, they found that the hotspots features clearly appeared. They also concluded that there are differences in the velocity gradients between young and old areas. A continuous thickening of the oceanic lithosphere was found beyond 100 Ma. The thermal state, however, was different under different oceans which coul d not be explained by a simple thermal boundary layer model.

 J. H. Woodhouse (Oxford U.) and J. Trampert (EOPGS, Strasbourg) used Love and Rayleigh surface waves in order to infer crustal and upper mantle structure. They obtained the general properties of these models; in particular they saw that the main contin ental roots extend to 150 km depth; they could also relate the aging of the lithosphere with the variations of the seismic velocities. Their resulting crustal thickness distribution shows good agreement with that expected on the basis of a simple regiona lization the principal part of which is the known large difference between the continental and oceanic crustal structures. They also concluded from their simulations that some features can be explained by partial melting.

 Y. Le Stunff, C. W. Wicks and B. Romanowicz (U. California Berkeley) performed a comparison between short-period and broad-band data. This comparison allowed them to point out some defocusing problems. Still, they were able to see reflectors in the dee p mantle at 785 km and 1180 km depth.

 M. H. Ritzwoller and J. M. Wahr (U. Colorado) used normal mode data in order to obtain tomography models (estimation of structure coefficients for mantle spheroidal modes at all degrees up to 8) and boundary structures like at 670 km depth. They used a priori kernels relating the volumetric structure and the boundary topographies on four mantle boundaries: the free surface, the CMB, and the 400 km and 670 km depth boundaries which can be modeled with a phase transition. The kernels are the transfer fun ctions induced by internal loading by tomography lateral heterogeneities in the mantle. They depend on the nature of each boundary (chemical or phase), the scaling between density and the estimated seismic velocities, the radial viscosity profile, the de nsity jump across each boundary, whether an a priori slab model has been employed and its density, and the Clapeyron slope for the phase boundaries. They simultaneously inverted for the normal-mode structural coefficients and the Earth's long-wavelength geoid to estimate volumetric structure, boundary topography and the radial dependence of the scaling between density and seismic velocity. They obtained reliable topography at 660 km depth of about 10 km.

 G. Pari and W. R. Peltier (U. Toronto) have used tomography models in order to compute the heat-flux field considering an endothermic phase transition at 660 km depth. They then constrain their computations by the observed geoid as well as the observed heat flux data. In their model they have advective heat fluxes underneath ocean, and conductive underneath continent. Their modeling allowed them also to constrain the viscosity profile inside the mantle. Their conclusions are the following: 1. The ge oid and free-air gravity are well described by tomography based internal-loading theories for which it is assumed that the flow is whole-mantle in style. Layering of the flow destroys the fit to the gravity field. 2. The surface pattern of heat-flux ano malies is poorly described by whole-mantle internal loading theories. However good descriptions can be achieved for models in which it is assumed that the flow is rigidly layered at 660 km depth. 3. A possible resolution of this problem might be found i n models which will provide a more realistic treatment of the dynamic role of the endothermic phase transformation at 660 km depth.

 Contributed by G. Masters (U. California San Diego) and V. Dehant (Royal Obs. Belgium).
 

Mantle Dynamics and the Geological Record.

The goal of Earth sciences is ultimately to explain all surface observations with a physical process whose origin and cause may be found either at the surface itself or deeper within the Earth. In the last decade with the advent of seismic tomography th e dynamics of the mantle and the structure of the deep interior have become more and more connected to the features we observe at the surface. A first step was to use the picture of density heterogenity obtained from seismic tomography to calculate the l ong-wavelength geoid and present-day plate motions, providing a connection between mantle dynamics and surface geophysical and geological observables.

 In more recent years considerable advances have been made in understanding this connection as geodynamicist started using the wealth of information contained in the geological record to construct dynamical models of the mantle, or the effect of dynamical processes on the geological record {Richards and Engebretson, 1992; Ricard et al., 1993}, such as continental tilting {Mitrovica, et al., 1989} and continental flooding {Gurnis, 1988}. Session S JS7: Mantle Dynamics and the Geological Reco rd, sponsored by IASPEI and SEDI, reflected this renovated and important effort in geodynamical studies.

 The session was characterized by talks on a variety of subjects, starting with models of mantle flow to show the subduction control on the tilting of the Russian platform during the Devonian and the Permian. The models by R. Pysklywec, J. X. Mitrovica, A. Rutty (U. Toronto) and C. Beaumont (Dalhousie U.) , showed that west-dipping subduction under the Euramerican plate can explain the eastward tilting of the Russian platform as observed in the geological record. Their models are consistent with a varie ty of geological data. The plate tectonic history of the last 200 My, was used to construct a model of mantle heterogeneity with which to calculate flow models that could accurately predict plate motions over the Cenozoic [Lithgow-Bertelloni and Richards ] and examine the cause of plate rearrangements.

 P. Puster, B. H. Hager, and T. Jordan (Massachusetts Inst. Tech.) performed numerical mantle convection calculations with plates, whose geometries evolve with time. The results were characterized by a measure of the size of the plate and its velocity an d were compared to similar measures for the plate tectonic record of the last 120 Ma (plate area and velocity). They found their results to be most consistent with the geological record for a lower mantle 30 times more viscous than the upper mantle. Cal culations for a mantle intermittently layered by the effects of a strongly endothermic phase transition lead to results inconsistent with the Earth's plate history.

 J. Baumgardner (Los Alamos National Lab.) presented the results of 3-D spherical finite-element mantle-convection code with plates and a resolution of 50 km throughout the mantle. The code included the effects of phase transitions at 410 and 660 km depth , and lateral and radial viscosity variations. Therefore, this simulate the effects of spreading and subduction. The results of calculations showing plate evolution throughout the Phanerozoic were in agreement with many aspects of the geological record.

The neo-tectonic uplift of many continental areas (plateaus and mountain ranges) which began in the Oligocene, was explained by E. V. Artyushkov (Inst. Physics of the Earth, Moscow) and A. W. Hoffman (Mainz) by the upwelling of asthenospheric material fr om the deep mantle, subsequently spreading under the continental lithosphere. Finally, results for plate-tectonic evolution from a 3-D mantle convection code with lateral and radial varying viscosities were shown by X. Sun and L. Han (Peking U.).

 Contributed by C. Lithgow-Bertelloni (Dept. of Terrestrial Magnetism, Carnegie Inst. of Washington).
 

Modelling Earth's Structure and Dynamics

Symposium (S JS4), organised jointly by IAG, IAGA and IASPEI consisted of a day-long oral session and a related poster session scheduled on the Thursday of the first week. Convenors were S. Constable (U. California San Diego), J.-P. Poirier (Inst. Physi que du Globe, Paris), A. Duba (Lawrence Livermore National Lab.) and I. Jackson (Australian National U.). There was a significant overlap in scope with another joint symposium S JS8 'Modelling the Earth's Interior' held on Thursday of the second week, wh ich might be avoided by more careful planning for future meetings. Perhaps because so many participants attended only for one week, however, both symposia were actually quite well subscribed. Attendance at the oral sessions averaged 30 - 50, and discuss ion was generally lively.

 The scope was intentionally broad and attracted contributions ranging from the crust to the inner core. Some of the highlights were as follows. S. Crampin (U. Edinburgh) explained how the deformation of heavily fractured and fluid-saturated crustal roc ks leads to seismologically observable alignment of fluid-filled microcracks, and H. J. Müller (GeoForshungsZentrum, Potsdam) presented the results of high P-T ultrasonic measurements of elastic wave speeds in crustal rocks. Turning to the upper mantle, S. V. Sobolev (U. Karlsruhe) derived a model for the temperature variation beneath Western Europe, consistent with both seismic shear wave tomography and heat flow, by explicitly incorporating the effects of viscoelastic dispersion which increases the tem perature sensitivity of VS. In studies relevant to the transition zone, T. Inoue (Ehime U.), H. Yurimoto (Tokyo Inst. Tech.) and Y. Kudoh (Tohoku U.) reported the synthesis of specimens of the b-phase of Mg2SiO4 containing up to 3 wt% H2O with implicatio ns for water storage within the transition zone, while D. L. Farber, Q. Williams (U. California Santa Cruz) and F. J. Ryerson (Lawrence Livermore National Lab.) demonstrated that experimentally observed fast diffusion of divalent cations in the b- and g- polymorphs of Mg2SiO4 may explain the relatively high electrical conductivity of the transition zone. Ita et al. (Purdue U., Caltech) described the development of more realistic mantle-convection models. Anal-yses of the elasticity and density of the lower mantle based on recent improvements in our knowledge of the P-V-T equation-of-state of MgSiO3 perovskite, presented by O. L. Anderson (U. California Los Angeles) and K. Masuda (Geological Survey of Japan) and I. Jackson and S. Rigden (Australia n National U.) indicate that a pure perovskite lower mantle is too incompressible to match seismological observations. Admixture of magnesiowüstite in proportions appropriate for the pyrolite composition seems to provide a better match. W. Su, A. M. Dzi ewonski and J. Tromp (Harvard U.) presented an analysis based on PKIKP travel times and anomalously split normal modes which reinforces previous suggestions of anisotropy extending throughout the inner core, with a symmetry axis inclined about 10¼ from th e rotational axis.

 Contributed by I. Jackson (Australian National U.).
 

Physical and Chemical Evolution of the Earth

The attendance to this symposium (S JS2), convened by T. J. Ahrens (Caltech) and S. Franck (Potsdam) and cosponsored by IASPEI and IAVCEI, was excellent and cross-disciplinary, from geochemists and mineral physicists to seismologists and geomagneticians.

At the beginning, planetesimals impacted the Earth. Impacts do more than make craters, they may also strip out the atmosphere. H. J. Melosh (U. Arizona) showed that, if the release isentrope of the shock falls into the liquid-vapor domain, the expandin g vapor plume may carry the atmosphere with it, provided the projectile is fast enough and large enough. Numerical simulations by G. Chen & T. J. Ahrens (Caltech) of an impact shock-wave on a planet with a proto-atmosphere indicate that an impact which i nduced a free-surface velocity of the solid-earth of 2 km/s blows off 0.1% of the atmosphere, whereas melting, vaporization and complete blow-off of the atmosphere occur when the solid-earth free-surface velocity reaches 8 km/s.

 Whether or not a deep magma ocean can be differentiated, leading to a differentiated lower mantle is a matter of current debate. Y. Abe (U. Tokyo) investigated the role of thermal blanketing by the atmosphere: with no atmosphere, the deep magma ocean co ols rapidly and the lower mantle is weakly differentiated; if the cooling time increases due to thermal blanketing, differentiation can occur. A low degree of partial melting, with high solid viscosity and low fluid viscosity favors differentiation.

D. J. Stevenson (Caltech) suggests that core formation can be compared in many ways with basalt formation at ridges: in both cases, initial small-scale equilibration between melt and solid is followed by disequilibration during transport and by later sta ge processes and metasomatism. Combining physical models with mineral physics information, one can draw inferences on core composition. Stevenson addressed the issue of silicon in the core. Discounting the possibility that silicon is already present as a reduced phase, he calculates the solubility product of SiO2 and finds that, although its silicon content is small, the core may be saturated in silicon. Underplating of silicate out of the core onto the mantle might be an energy source for driving the geodynamo.

 K. Kuramoto (Tokyo Inst. Technology) calculated the equilibrium partitioning between volatiles and molten silicate and iron during the melting events leading to core formation. In the case of volatile-poor planetesimals, as is probably the case for the Earth, carbon is mostly partitioned into the core, while hydrogen goes as water into the silicates.

Q. Williams & E. Knittle (U. California Santa Cruz) used experimental observations of chemical reactions between high-pressure silicates and molten iron, in diamond anvil cells, to investigate the constraints on the nature of chemical elements in the cor e. Static determinations of the equation of state of e-FeSi have been performed (K0 = 209 ± 6 GPa; K'0 = 3.5 ± 0.4). The value of K'0 is too low to account for the variation of seismic velocities with depth, leading to the conclusion that silicon is not a major component in the core and that core and mantle were not equilibrated.

 The process of core-mantle separation is examined again by M. J. Drake (U. Arizona), in the light of recent experimental determinations at high temperature and pressure (up to 70 kbar) of the partition coefficients of siderophile elements between metal a nd silicate. The partition coeficients of Ni, Co and P into iron decrease as pressure increases. The conclusion is that the core has probably never been in equilibrium with the mantle. In two companion contributions, V. Rama Murthy & S. Karato (U. Minn esota) also address the problem of metal-silicate equilibrium during core formation. Only small droplets of iron, less than 1 cm in diameter, could equilibrate, but the larger volume fraction of the core presumably comes from blobs larger than 100 m and is out of equilibrium. The apparent excess of siderophile elements in the mantle is linked to the assumption that metal has equilibrated with silicates at low pressure and temperature; if a large fraction of the core has not equilibrated, its composition is mostly preterrestrial, implying that the main light element candidate is sulphur, rather than oxygen or silicon.

Talks on the second day covered a quite wide range of topics, often only loosely related. The sessions were diminished somewhat by the absence of a number of scheduled speakers. Dispersion and absences combined to make the second day somewhat less sati sfying than the first day, but the quality of many of the presentations was nonetheless very high.

 C. T. Herzberg (Rutgers) presented an interesting analysis seeking to relate the petrology of basalts, picrites and komatiites to the mantle conditions under which partial melting took place. By using lab data on the content of MgO, CaO and alum-ina wit h pressure and temperature, Herzberg infers that the 3.5 Ga Barbeton komatiites were formed by a melting process that segregated melts at quite high pressure (perhaps 10 GPa) and with a temperature excess of around 300K relative to current mantle. Part of this temperature excess could be due to hotter plumes (a bigger temperature difference between mean mantle and plumes than we see now) but most of it is likely due to the higher mean temperature of the mantle at that time.

 E. Ohtani (Tohoku U.) discussed the partitioning of siderophiles at high pressure and temperature and the implications for core formation. His experiments on partitioning between molten silicates and molten iron indicate that while some elements (Ni, Co) exhibit substantial pressure dependence, others (Fe, V, Mn, P) do not. He presented a model in which he sought to explain observed mantle abundances by starting with chondritic abundances and then modifying these by a combination of volatility effects a nd partitioning into the core forming melt. He was able to construct a model that worked for many but not all elements: P and Mn remain a problem.

 C. Agee (Harvard U.) discussed his work on mantle differentiation from a magma ocean. He argued that lower mantle melting occurs at temperatures well below the melting point of perovskite and may have magnesiowüstite as the liquidus phase. He proposed that it is then possible to reconcile the observed upper mantle Fe/Mg ratio with a chondritic starting material by the extraction of FeO (in magnesiowüstite).

 In what was perhaps the most intriguing talk of the day, A. Dziewonski (Harvard U.) presented comparisons of global tomography for both S and P-wave, from which he attempted to separate the lateral variations in the ratio of shear modulus to density from those which are due only to changes in the ratio of bulk modulus to density. Not surprisingly, the former are far larger than the latter, but somewhat surprisingly, the two are anticorrelated throughout much of the mantle. A substantial positive correl ation was only observed in (roughly) the transition zone, and especially at low harmonic degree. Dziewonski suggested that the negative correlation can be explained by assuming that all the variations in the deep Earth (roughly the lower mantle) are due only to temperature because the variation of shear modulus with temperature is larger than the variation of density with temperature which is in turn (perhaps) larger than the variation of bulk modulus with temperature. In this way, the ratio of shear mo dulus to density can decrease as the temperature increases (at fixed pressure), whereas the ratio of bulk modulus to temperature would increase. It is unclear whether mineral physics is compatible with this interpretation (which requires a very small var iation of bulk modulus with temperature). Alternative interpretations would call for a combined effect of compositional and thermal variations or would argue that the 'signal' (variations in bulk modulus/density) is simply too small compared with the she ar modulus effect to be extracted reliably.

 In an added paper, J. Faust (U. California Santa Cruz), working with E. Knittle, described diamond-cell experiments designed to determine the buoyancy of oceanic crust at great depth in the mantle. They obtained generally good agreement with the earlie r pioneering work of Irifune and Ringwood but concluded that there may be a somewhat greater positive buoyancy of crust than previously estimated, perhaps around 0.2 g/cc density deficit relative to neighboring normal mantle for the entire depth range fro m 660 to roughly 1600 km. If the crust were to delaminate it would straddle 660km, keeping part of it in the transition zone.

Contributed by J.-P. Poirier (IPG Paris) and D. J. Stevenson (Caltech).
 

Deja Q: Anelasticity of the Earth's Deep Interior

A great deal of interest in the past decade has focused on the development of elastic tomographic models on a number of length scales in the mantle. Similar, although much more poorly focused, seismic models of Q in the upper mantle, lithosphere and crust have emerged more recently. Such models are considerably harder to estimate accurately than their elastic counterparts, primarily due to the fact that they are typically constructed from seismic amplitudes rather than travel times. Elastic an d anelastic structures both impact amplitudes, but their effects are hard to separate with confidence. Nevertheless, substantial progress appears to have been achieved lately. Simultaneously, experimental geophysicists are just beginning to experience s ubstantial success in their attempts to measure attenuation (as a function of temperature, pressure, dislocation density, and composition) in realistic geological samples at seismic frequencies (0.001 - 1 Hz). Such measurements are extremely important in order to interpret tomographic models (moduli are frequency dependent) and the emerging 3-D Q models.

 The purpose of this session (SW11) cosponsored by IASPEI and SEDI, was to bring together seismologists and experimental geophysicists to attempt to illuminate the state of anelasticity research in each of these disciplines. Although the session was sent enced to taking place on the last day of a two-week meeting, attendance was surprisingly good.

 The session began with invited talks by two experimentalists who have made significant advances in shear modulus and shear attenuation measurements in the past several years. I. Jackson (Australian National U.) described advances in forced torsion-oscill ation measurements made on fluid saturated Carrara marble, iron, and olivine. Jackson's experiments are investigating the impact of composition, fluid saturation, temperature, and pressure on attenuation. In particular, he is approaching detailed measur ements on ultramafics at temperatures rising to about 1300C, which is of relevance to upper mantle conditions, and he is investigating impacts of porosity and fluid saturation in crustal rocks at lower temperatures and pressures. I. Getting (U. Colorado) also described his first measurements of shear modulus and attenuation of single crystal MgO at seismic frequencies using forced harmonic torsion with particular attention to estimates of the temperature derivatives of shear modulus and attenuation as a function of seismic frequency and ambient temperature. The results presented represent about a decade of research at each of these two labs in Canberra and Boulder.

 A lively but quite sobering discussion followed these talks, out of which emerged an apparent consensus that the impact on shear modulus and attenuation of dislocation density, and more generally sample deformation, requires much further study. Expected variations in dislocation density (caused by geographical variations in shear stresses) could have as large of an impact on attenuation in the upper mantle as lateral temperature variations. This is both good and bad news. The good news is that 3-D Q models when combined with 3-D elastic models and properly interpreted may place constraints on the stress state of the upper mantle. The bad news is that 'proper interpretation' is very difficult, indeed, since the way to separate temperature and dislocation density effects on attenuation remains unclear. More basic experiments designed to understand the physics of attenuation are still needed before seismologists will be able to use the emerging measurements with confidence to interpret their e lastic and anelastic tomographic models.

 With a single exception, the remainder of the talks were by seismologists. The exception was S. Dickman's (SUNY, Binghamton) talk on Q estimates at tidal periods (9 days, 2 weeks, 1 month). The estimation of mantle Q from tidal measurem ents is complicated by the frequency variation of dynamic contributions from the oceans, atmosphere, and the outer core. Dickman corrected 9 years of VLBI length-of-day observations using models of these effects, and inferred that constant-Q- anel astic models cannot extend from the seismic band down to these much longer periods. This may be suggestive that the mechanism of Q at these periods may be different than the seismic mechanisms.

 Seismic talks reviewed work revealing Q variations on a number of length scales. Three invited talks were related to global-scale structures. In the first, J. Durek (U. California Berkeley) reviewed his efforts to produce more accurate sphericall y averaged whole-earth Q models from radial, inner core and mantle mode data. His efforts were catalyzed by the large recent event in Northern Bolivia on June 9, 1994, and are distinguished from other recent studies in that he has attempted to con strain compressional Q in the upper mantle since the ratio of compressional to shear Q may be diagnostic of the mechanism of attenuation operating at a given depth. He discussed the difficulty of estimating compressional Q, but posi ted preliminary estimates of 1400 in the upper mantle, 350 in the asthenosphere, and very much higher values in the lower mantle. His shear Q estimates of 120 in the upper mantle and 340 in the lower mantle imply a shear/compressional Q rat io of about 15% in the asthenosphere but only about .03% in the lower mantle. From this he hypothesizes that the mechanism of Q in the asthenosphere differs from that at greater depth. In the second invited talk, B. Romanowicz (U. California Berk eley) reviewed her most recent models of lateral variations in Q in the upper mantle determined from surface wave amplitude measurements and discussed the complementarity of elastic and anelastic maps (anelasticity is more strongly temperature depe ndent). After reviewing the difficulties involved with estimating 3-D Q models, she noted that her models change character with depth in a coherent fashion. At depths above 250 km, there is a strong age and tectonic correlation of the observed lat eral variations in Q. At greater depths, however, this correlation disappears and a correlation with hotspot distribution emerges. In the final invited talk on global-scale Q, J. Bhattacharyya (U. California San Diego) presented work on in terpeting SS-S body wave differential attenuation maps. Such measurements provide a depth-integrated estimate of the lateral variations in Q which Bhattacharyya notes possess a strong tectonic correlation and are well correlated with the surface-w ave Q maps. However, his maps are not particularly well correlated with known shear-velocity variations, except at degrees 1 and 2, with degree 2 particularly well correlated with shear-velocity variations between 300 - 550 km depth. He hypothesi zed that this may be evidence for a substantial component of these large scale Q variations originating quite deep in the upper mantle. Bhattacharyya also reviewed attempts to model Q variations in D''. The last global-scale talk was by S. Tsuboi (U. Tokyo), who presented preliminary observations of singlet frequency excitations and speculated how they might be useful to estimate Q.

 The session closed with two invited talks and a contributed talk on lithospheric/crustal Q. T. Takanami (Hokkaido U.) presented a Q model of the northeastern Japan arc beneath the Tohoku volcanic zone constructed using an AR decomposition filter to measure spectral amplitude ratios at high frequencies. E. Roth of (Washington U.) presented a model of the Q structure beneath the Fiji Plateau and the Lau Backarc determined from a deployment of 11 PASSCAL stations. M. Flanagan (U. California San Diego) presented a related poster. Finally, J. Lees (Yale U.) took the audience to much smaller features discussing problems associated with making Q measurements in the uppermost crust. These measurements are important since they may constr ain crack density and fluid saturation, both of which affect earthquake mechanics significantly.

 Contributed by M. Ritzwoller (U. Colorado).
 

Thermoelastic Properties of Deep Mantle Phases

The workshop (SW2), which was convened by O. L. Anderson (U. California Los Angeles) and I. Jackson (Australian National U.) and sponsored by IASPEI and SEDI, provided a forum for the discussion of the latest developments in experimental techniques for t he measurement of thermoelastic properties, and of progress in the modelling of such properties. There was also lively discussion on just where we stand in resolving the properties of various high-pressure phases, notably magnesium silicate perovskite.
 
 

Advances in experimental techniques.

Papers by A. Yoneda, G. Chen, I. C. Getting and H. A. Spetzler (U. Colorado) and B. Li, R. C. Liebermann (SUNY Stony Brook), I. Jackson and G. D. Gwanmesia (Deleware State U.) described results from ultraso nic studies which have the potential to provide measurements of elastic moduli under conditions of simultaneous high pressure and temperature. These developments will shorten the extrapolation required to seismological conditions. The preliminary result s of Li et al, suggest that the wave speeds at transition-zone pressures may be overestimated somewhat by extrapolation of data obtained over more limited ranges of pressure, in general accord with other recent studies at very high pressures. Comp lementary information is being provided by spectroscopic measurements which constrain the Gr&uumlneisen parameter, and through the pressure dependence of entropy, the thermal expansivity (A. Chopelas, Mainz). Progress in synchrotron-based X-ray diffractio n studies under high P-T conditions was described in studies of single-crystal orthopyroxene (Y. Zhao, D. Shiferl and T. J. Shankland, Los Alamos National Lab.) and of a majorite-pyrope solid solution (Y. Wang, F. Guyot and P. Gillet, Ecole Normale Superi eure, Lyon).
 
 

Progress in modelling thermoelastic properties.

I. Inbar and R. E. Cohen (Geophys. Lab., Carnegie Inst. Washington) have modelled the thermal expansivity and temperature dependence of bulk modulus of MgO using ab initio molecular dynamics. T heir calculations are in excellent agreement with experiment. Similar methods have been used by G. D. Price, L. Vocadlo and A. Patel et al (U. College, London) to model the thermoelastic properties of MgSiO3 perovskite and to determine the correla tions in atomic motions to be included in a classical calculation of the Grüneisen parameter (F. D. Stacey, U. Queensland). Other presentations explored the sensitivity of P-V-T equations-of-state and derived thermoelastic qualities to various approximat ions to the density g(n) of lattice vibrational states (Debye, multiple Einstein, Kieffer, experimental g(n) from inelastic neutron scattering), and found that outcomes are generally insensitive to the details of g(n). (S. M. Rigden and I. Jackson, Austr alian National U.; K. Wang and R. R. Reeber, U. North Carolina; Wang et al, Stony Brook; Chopelas, Mainz). It was emphasized that all thermoelastic parameters are interconnected thermodynamically, and in particular that the parameter dT and thus ( dKT/dT)P is constrained by the value of (dKT/dP)T (O. L. Anderson and K. Masuda, Geol. Survey Japan).
 
 

alpha, gamma, (dKT/dT)P and d2KT/dPdT for MgSiO3 Perovskite.

Although a final consensus remains elusive, the impact of the latest P-V-T data for MgSiO3 perovskite from the groups in Stony Brook and Tokyo is to increase the likelihood that silic ate perovskites are 'normal' high-pressure phases with high KT, low a and low-to-normal (dKT/dT)P. The advances in data quality, and in understanding the role of non-hydrostatic stress and of the marginal metastability especially of Fe - bearing perovski tes, mean that the results of the pioneering studies of Knittle et al and Hemley et al (also used in the analysis of Stixrude et al) have been largely superseded by the data emerging during 1994/5 from the Stony Brook and Tokyo labora tories. These new studies suggest that average values of a over extended ranges of temperature are > 2.5 x 10E-5 1/K with a mild pressure dependence consistent with (dKT/dT)P ~ - 0.02 GPa 1/K. Corresponding values of g deriving from analysis of P-V-T d ata, spectroscopy and Barton-Stacey modelling are 1.2 - 1.5, with q = (dlng/dlnV)T ~ 1 (e.g. Anderson, U. California Los Angeles). A dissenting view was presented by Patel et al whose MD calculations, based on pair potentials fitted to the elastic constants, suggest substantially larger values of a and (dKT/dT)P in general accord with the data for (Mg,Fe)SiO3 perovskite. D. G. Isaak (U. California Los Angeles) addressed the sensitivity of thermoelastic properties to composition in Mg-Fe solid sol utions, concluding that iron concentration generally has a negligible effect on KT, dKT/dT, gamma and alpha in mantle minerals. From analyses of a subset of the published P-V-T data for MgSiO3 perovskite, Jackson and Rigden concluded that d2KT/dPdT is ~ 1 x 10E-4 1/K, within the range for oxides and silicates, previously defined by Isaak.

 Contributed by I. Jackson (Australian National U.)
 

Summaries of other meetings


 

Deep Earth and Planetary Volatiles

Some 90 scientists gathered at Caltech September 21 - 23, 1994 to participate in an International Conference on Deep Earth and Planetary Volatiles. The Conference was hosted by Caltech with the Lunar and Planetary Institute providing logistical and fina ncial support. Support was also provided by NSF and guidance came from both the Conference Organizing Committee and Coordinating Committee for Cooperative Studies of the Deep Earth's Interior.

 Volatile condensation, delivery and loss via impact, and other processes on all the planets were surveyed. W. Kaula (U. California Los Angeles) pointed out that although comets did indeed impact and deliver volatiles to the terrestrial planets, on Earth and Venus their impact was at such high velocity that their volatiles were all blown-off and terrestrial planetary vola-tile budgets were not augmented by this process. D. Paige (U. California Los Angeles), however, suggests that water from comets could successfully be delivered to Mercury and could be seen from Earth-based radar studies of the shadowed crater walls at the Mercurian poles. He proposes that NASA fly a mission to Mercury to collect evidence to test this hypothesis.

 H. W&aumlnke and G. Dreibus (Mainz) reviewed their previous hypothesis of a C1 carbonaceous chondrite volatile-bearing veneer accreting onto the terrestrial planets and pointed out that the Earth has water both in its interior and on its surface. In con trast, Mars' water budget appears to be in a near-surface reservoir as the SNC meteorites are from deep-seated sources and appear to be relatively dry and water-free. Wanke attributed this to the absence of plate tectonics on Mars.

 Noble gas constraints on the evolution of the solid earth and atmosphere were discussed by a number of speakers. C. Allegre (University of Paris VII) presented new isotopic data on basalt glasses from Loihi Seamount (Hawaii) which confirm the discovery of elevated 20Ne/22Ne ratios in the Hawaiian plume. There now seems to be a consensus that the entire mantle is characterized by solar neon ratios. Conference participants recognized and discussed the paradoxical observation that the Earth's atmosphere and mantle have Ne compositions which are isotopically distinct and which cannot be explained by known nuclear processes. Leading explanations for this observation are fractionation of neon from the atmosphere after the bulk of terrestrial degassing, or the production of the atmosphere from sources other than the Earth's interior, e.g., from cometary inputs. K. Zahnle (NASA Ames) described his model for mass-fractionating loss of Ne from planetesimals, and indicated that such a process occurring in the early Earth's atmosphere could fractionate neon appropriately without extreme loss of neon and without measurably fractionating other noble gas isotopic ratios. G. Wasserburg and D. Porcelli (Caltech) discussed the implications of a steady state mass tra nsport model (originally used for helium isotopes) for Ne and Xe in the Earth. Their self-consistent model suggests that the atmosphere carries only a small fraction of rare gases derived from the Earth's interior, the balance being delivered from presen tly unknown extraterrestrial sources. These discussions demonstrated that the origin of the atmosphere is now seriously in doubt.

 Also poorly constrained is the flux of volatiles into the mantle at subduction zones (planetary ingassing). The model of Wasserburg and Porcelli, and considerations of terrestrial xenon described helium and neon measurements of be slab-derived melts whi ch suggest that neither atmospheric He nor Ar are going down. The case for subducted Xe remains open.

 In contrast, there is little doubt that water and carbon dioxide are being subducted, but their fate is obscure. G. Nolet (Princeton U.) showed results of high resolution mantle tomography which reveal the presence of distinct low-seismic-velocity zones at ~100 and ~300 km depth extending outward from several subducting slabs. He interpreted these as a consequence of dehydration of various hydrous mineral phases and loss of volatiles into the mantle wedge.

 The geophysical consequences of water in planetary silicate mantles were the subject of several papers. King et al. (Purdue U.) pointed out how the uneven presence of volatiles in subducting slabs on the earth affects the time scale that material s in subduction regions pile-up above the spinel-to-perovskite-plus-periclase (670 km) endothermic transition zone and induce changes in the rate of Tackley-type avalanches in the mantle. They also point out that concentrations of volatiles in the mantle could give rise to regions of buoyant plumes.

 LPI's W. Kiefer pointed out that volatiles do not effect mantle convection as much as is widely assumed and S. Franck and C. Bounama (Potsdam) conducted parameterized convection modeling of the terrestrial planets and also concluded that little water rem ains in the interior of Mars and Venus.

 B. Wood (Bristol) summarized the capacity of major mantle phases to contain water and CO2. New data (J. Smythe, Colorado) showed that the b-phase of Mg2SiO4, wadsleyite could contain 600 ppm of OH- compared to olivine's 200 ppm. This observation and M eade's recent discovery of the surprisingly large capacity of MgSiO3 (perovskite) for OH- as well as the stability of the water-bearing phase Dcc ((Mg, Fe) SiH2O4), which according to X. Li and R. Jeanloz (Berkeley) can coexist with MgSiO3 (perovskite) i n the lower mantle. The range of stability of Mg(OH)2 allow huge mantle budgets of water as pointed out by T. Duffy (Geophysical Lab., Carnegie Inst. Washington). Further support for this possibility came from new thermochemical data for hydrous mineral s from A. Navrotsky's laboratory (Bose et al., Princeton U.) and studies of the proposed lower mantle hydrous "Phase E" by T. Kawamoto et al. (Tempe). Also, the discovery by Jeanloz's group that magnesite-siderite is stable with respect t o the perovskite phases in the lower mantle allows plausible concentrations of carbon (2000 ppm) in the lower mantle.

 The proceedings will be edited by K. Farley and published by the American Institute of Physics. Contributed by T. J. Ahrens and K. Farley.
 

Developments in High-Pressure, High-Temperature Research and the Nature of the Earth's Deep Interior

This Royal Society Discussion Meeting, organized by A. P. Jephcoat, R. J. Angel and R. K. O'Nions and held in London January 25-26, 1995, brought together workers in fields of experimental high-P-T research (mineral physics) to discuss current issues and controversies with geochemists, seismologists and geodynamicists. The goal was to not only force a reappraisal of the role and current state of experimental studies, but to place them in the context of overlapping research on the Earth's interior in a short, two-day exchange with emphasis on open discussion. The meeting attracted unusual media interest, reflecting the public interest in the deep Earth.

 S. K. Saxena (Uppsala) chaired the first session where T. J. Ahrens (California Inst. Tech.) began with a presentation of the latest implications on shock temperature measurements for core temperatures and thermodynamic calculations on the ehcp-liquid ph ase line of iron. He confirmed that preliminary measurements of the window thermal diffusivity under shock conditions appeared to be lower than originally assumed, leading to revised temperatures in closer agreement with the diamond-anvil cell (DAC) melt ing data for Fe of Boehler and Saxena. This result was also supported by the available data on the bulk modulus (K0T) of e iron used in the thermodynamic model, although sensitivity to choice of K0 appeared high.

 R. Boehler (Mainz) reviewed developments in laser-heated DAC technology, emphasizing the difficult problems which have already been overcome. He stressed the advantages of a larger spot obtained from higher-power lasers and direct sampling of the emitte d light vs inversion techniques. He suggested complete convergence of shock and his DAC results if the revised shock temperatures remain ~1000 K too high due to overshoot of equilibrium conditions. Melting temperatures of iron at core pressures are not strongly dependent on O or S alloying component and further, as yet unknown, phases of iron at core conditions were again postulated.

 The criteria used to diagnose melting of samples in the DAC were discussed by R. Jeanloz and A. Kavner (UC Berkeley). They stressed the problems inherent in most methods including observation of fluid motion within the DAC, quenched texture, reflectivity or the power-temperature correlation. Tradeoffs exist between reliability of determination of onset of melting and precision of measurement. Of the methods employed, bracketing the appearance of fluid is robust and there is a need to refine these crit eria as used by different groups. Imaging techniques will in future provide reliable methods for the determination of the temperature distribution in the DAC.

A. G. Duba (Lawrence Livermore National Lab.), presided over the second session dedicated to a variety of high-P, -T experimental work. A. P. Jephcoat and S. P. Besedin (Oxford U.) reported on melting experiments in the DAC showing a video of the melti ng of iron in an argon pressure medium, and highlighted the difficulty in discriminating between the melting of iron and melting of the pressure medium based on a reflectivity technique. They pointed out that the argon melting curve could intersect the Boehler-Saxena iron melting curve as low as 40 GPa. This generated much discussion on how well the melting curve of argon is known.

 J-P. Poirier (Institute de Physique du Globe, Paris) considered the effects of the partial pressure of oxygen and non-stoichiometry in magnesiowüstite-perovskite assemblages synthesized in the DAC, concluding that the effect of oxygen partial pressure is similar to the effect of applied pressure in that both result in structural compression that decreases the activation energy of hopping. Extrapolations of measured conductivity values to the CMB are therefore apparently not much affected by the redox st ate.

 H-k. Mao (Geophysical Lab., Carnegie Institution of Washington) summarized DAC techniques, in particular new pioneering Brillouin scattering results, and made the philosophical experimental point that progress in high-P, -T research is evolving continuou sly and depends largely on developing the techniques to characterize the sample effectively under extreme conditions with a careful reduction in errors through improved measurement and sampling techniques.

 T. Yagi and N. Funamori (U. Tokyo) reviewed their extensive work on the experimental determination of the equation of state (EOS) of perovskite obtained by in situ (cubic-anvil) diffraction techniques to near 30 GPa and 2000 K and the implications for lo wer mantle composition based on a thermal EOS. Mantle temperature estimates constrain total iron to between 10% and 15%. A best fit was obtained for a molar admixture of perovskite to magnesiowüstite between 2:1 and 3:1 respectively for Fe/(Fe+Mg) = 11%, though they stressed that precise compositional constraints remain strongly dependent on the experimental uncertainties in the EOS determination for specific compositions, despite the improved methods.

 The first session of the second day gave time to seismology and dynamics of the mantle and core, steered by U. Christensen (Göttingen).

 Seismological constraints on deep earth structure were presented by G. Masters (U. California San Diego). He illustrated the general convergence of the tomographic models from different groups in largescale features, although some discrepancies remain. T he most recent models show no change in character at the 660km discontinuity, supporting the idea that an endothermic phase transition is not a barrier to large-scale flow in the mantle. This observation contrasts with the suggestion (W.R. Peltier, Toron to) that inhibited flow arising from an endothermic phase transition at the 660km discontinuity would lead to a thermal boundary layer, and a concomitant dipolar viscosity variation across the interface (large increase and decrease in viscosity from the whole mantle convection case on either side of 660km). Disruption of the thermal boundary layer episodically would lead to intermittent transport of cold material into the lower mantle and may reconcile the different viscosity results which result from m odeling the geoid and post-glacial rebound data.

 P. L. Olson and G. A. Glatzmaier (Johns Hopkins and Los Alamos National Lab.) presented results of numerical calculations of magnetoconvection in the Earth's core in the presence of inhomogeneous boundary conditions, as are likely to result from a convec ting mantle. The calculations prescribe a toroidal magnetic field, and invoke variable heat-flux boundary conditions. They report that beneath warm mantle stable stratification occurs, whereas enhanced convection develops beneath cold mantle, breaking th e columnar style of convection which would otherwise occur with homogeneous boundary conditions. The net result is that it is difficult to generate magnetic fields with flux patches which are symmetric about the equator, as exhibited by the current magne tic field, through the influence of such variable heat-flow boundary conditions.

 G. R. Helffrich (U. Bristol) chaired the final session on broader chemical issues in the deep Earth.

 R. J. Angel (U. Bayreuth) and N. L. Ross (U. College London) reviewed details of crystal chemical compression mechanisms. Structures with a large number of internal degrees of freedom (for example the olivine structure) can exhibit inhomogeneous strain on compression and changes in the compression mechanism without necessarily showing a volume discontinuity. Anomalous equation-of-state behavior is likely to result if used outside the range of measurements with parameters obtained only over these anomal ous compression regions.

 R. J. Hemley and R. E. Cohen (Geophysical Lab., Carnegie Institution of Washington) reviewed progress in first-principles modeling of a variety of materials, and stressed the change in character of chemical bonding that takes place under pressure that ca n result in different chemical properties in the deep Earth. Comparing with experimental results, materials such as FeO, exhibit substantial chemical and physical property change while for other materials, such as MgO, high pressure behavior can be re latively well predicted on the basis of lower pressure measurements. Valence and chemical reactivity are also likely strongly affected by pressure in the Earth.

 J. H. Jones (NASA Johnson Space Center, Houston) discussed chondrite models of the composition of the mantle and core. The extent to which a particular sample of the upper mantle can be distinguished as primitive, chemically processed (even by small amo unts), or representative of a non-standard chondrite class generated much discussion. The principle argument was that Mg/Si and Al/Si ratios are determined by a nebular process and that a CV chondrite model is closer (though not perfect) than the CI stan dard model in explaining refractory lithophile major and trace element abundances.

 B. J. Wood (U. Bristol) contributed to the proceedings with a paper on water and carbon in the Earth's mantle. In particular, the width of the 410 km discontinuity at the top of the transition zone (as identified with the olivine-betaMg2SiO4 transition ) is strongly dependent on water content. Current seismological constraints on the width of the transition fix the olivine H2O content in the range 0-500ppm. Similar considerations apply to the 670 km transition. Carbonates, on the other hand, are refr actory and will survive subduction, probably entering the lower mantle. Indications that the transition zone is reduced would also suggest reduction of carbonates to free carbon (diamond) at these depths.

 The proceedings will be published in Phil. Trans. Roy. Soc. A early 1996 and separately in book form. Contributed by A. Jephcoat (Oxford U.) and A. Jackson (Leeds U.)
 

Global Seismology and Large-Scale Dynamics: An EGS Symposium

The symposium, held April 5, 1995, in Hamburg, consisted of 14 papers. In spite of the title, emphasis was very much on the global seimological picture and only to a lesser extent on the significance these results have for our understanding of large-sca le dynamics in the earth. Our knowledge of the structure of the earth is steadily improving so a similar symposium is planned for the next EGS General Assembly in The Hague in 1996. We would strongly encourage papers from those researchers who are willi ng to develop dynamic models and speculate on their significance. Improved seismological models will, of course, also be very welcome. They are essential to our understanding of the Earth.

 Highlights of the session in Hamburg are as follows. An invited paper by L. P. Vinnik (Inst. Phys. Earth, Moscow) et al. on a low velocity zone at 350-500 km depth beneath the Kaapvaal craton, which was observed using P-S conversions. He related this to the passage of the deep rooted African plate over hotspots. C. W. Wicks and M. A. Richards (U. Washington) reported on discontinuities in the lower mantle observed by P triplications and S to P conversions from Tonga events recorded at the Warra munga array. J. Gossler and R. Kind (GeoForschungsZentrum, Potsdam) presented observations of SS precursors from upper mantle discontinuities and correlated the thickness of the mantle transition zone with oceanic and continental lithospheric structure. H . Paulssen (U. Utrecht), in an invited paper, discussed problems of delay time tomography in strongly heterogeneous mantle structures caused by the size of the Fresnel zones and suggested waveform inversions. G. Laske (Geophysics Inst., Karlsruhe) and G. Masters (U. California San Diego) showed that the amplitude and the polarization of surface-waves are of crucial importance for resolving the structure of the upper mantle. E. Chaljub et al. (IPG, Paris) compared theoretical seismograms with obser vations of weak phases caused by global upper mantle discontinuities, focusing on, the existence of the 520 km discontinuity, but without coming to a final conclusion. H. Pedersen(U. Grenoble) reported on the computation of complete theoretical seismogr ams for two dimensional media and oblique incident waves using the Indirect Boundary Element Method.

 Contributed by R. Kind.
 

Mathematical and Computational Issues in the Geosciences

Five SEDI researchers attended a meeting on Mathematical and Computational Issues in the Geosciences, sponsored by the SIAM (Society of Industrial and Applied Mathematics) Activity Group on Geosciences, held February 8-10, 1995, in San Antonio, Texas.

The group included Scott King (Purdue U.); Marc Parmentier (Brown U.); Dan Rothman (Massachusetts Inst. Tech.); Y. F. Sun (Lamont-Doherty Earth Observatory) and David Yuen (U. Minnesota). We organized a small 'mini symposium,' on High Performance Comp uting and Solid Earth Dynamics and Structure. The mini symposium included thirty minute presentations on: High Performance Computing and Mantle Dynamics (King), Mantle Dynamics and Melt Migration (Parmentier), Lattice Gases and Multiphase Flow Through P orous Media (Rothman), and Physical Wavelet Characterization and High Resolution Inversion (Sun). In addition, at the meeting wide poster session, there was a poster by Yuen entitled, "Effects of Viscous Heating in Temperature-Dependent Viscosity Conve ction." The oral session was well attended and the poster session was very active.

 Aside from the exchange of information at the meeting itself, there are two additional results that can be linked to our attendance:

 1) We learned about a new journal, "Computational Geosciences," that is being edited by Mary Wheeler. David Yuen is now writing an article on parallel visualization to be submitted to this new journal.

 2) The talk given by King was the basis for the recent IEEE Journal of Computational Science and Engineering article, "A Numerical Journey to the Earth's Interior."

 We feel that it was fruitful to bring people from a deep earth background together with people who are in more applied areas, who were well represented at this meeting. We were among a limited few participants at this meeting who come from traditionally, non-industry related fields. We hope this meeting opened some dialogue for starting interaction between these groups.

 Contributed by S. King (Purdue U.).
 

SEDI-sponsored EUG Symposium Strasbourg, 12-13 April 1995

Symposium VIII of the European Union of Geosciences, entitled "Composition, Structure, and Dynamics of the Deep Earth" was co-sponsored by both SEDI and EGS (European Geophysical Society). It lasted two days, and attracted 79 communications. It was of v ery high quality. The abstracts have appeared in TERRA abstracts, supplement #1 to TERRA nova, vol. 7, 1995.
 

Workshop on Numerical Modeling on Mantle Convection and Lithospheric Dynamics

This workshop, one of a continuing series, took place from 16 to 19 September 1995 on the westfrisian island Vlieland in the Netherlands. Since the first workshop in 1987, it has been the spirit of the workshop to bring together scientists who are acti vely working in the field of numerical modeling to discuss in depth technical and conceptual questions. The 1995 workshop was organized by the Netherlands (U. Hansen and M.J.R. Wortel, both at Utrecht U.). Scott King (Purdue U.) and Kevin Furlong (Penns ylvania State U.) helped organize the US participation of the meeting. In all, 55 participants from nine countries (United States, Canada, Japan, Israel, Czech Republic, Hungary, France, Germany, and the Netherlands) participated in the workshop. The me eting was supported by The Royal Dutch Academy of Science and The National Science Foundation of the United States.

 In the spirit of a workshop, a limited number of participants were asked to present 30 minute overview talks on a limited number of topics. All participants were encouraged to bring one or more poster presentations. After the afternoon session there wa s a two to three hour time slot for discussion at the poster sessions. This was a very lively time and many of the new techniques and technical results were presented as posters.

 The first session opened with review talks by M. Gurnis (Caltech) and S. King highlighting studies that have attempted to incorporate plates into models of mantle convection. K. Furlong followed with a discussion of the effect on mid-ocean ridges of an increase in viscosity due to partial melt (as reported by Kohlstead's group) and the resulting effect on the heat budget of the lithosphere-asthenosphere system. R. Govers (Utrecht U.) discussed some of the problems associated with mantle boundary condit ions on lithosphere problems and the need for interaction between mantle and lithosphere models. U. Christensen (Göttingen) observed that we have a reasonably good understanding of rheology when breaking or faulting is not a factor but, our understanding of 'rheology' in the case of faulting is poor. Christensen questioned whether we can produce a continuum description of all aspects of lithospheric deformation or whether we need the modeler imposed structures like faults and weak zones in a continuum d escription. This question was a continuing theme of the workshop.

 The afternoon session was highlighted by technical aspects of modeling lithospheric and mantle dynamics. S. Balachandar (U. Illinois), P. Tackley (U. California Los Angeles), D. Yuen (U. Minnesota) and J. Schmalzl (U. Utrecht) discussed high performance computing in geodynamics. Balachandar and Tackley focussed on parallel computing, Yuen discussed communications and visualization and Schmalzl discussed advances and improvements in visualization packages that make processing and interpreting geodynamic al results easier. The summary of the session chair (A. Malevsky, U. Montreal) clearly indicated that parallel computing is still not a routine process for problems in geodynamics.

 The poster session on lithospheric dynamics concluded the first day. The poster session on lithosphere-mantle interac-tion consisted of contributions from a number of international research groups. A couple of trends, or related issues, were identifie d by the discussion panel. Two different approaches to incorporating lithospheric plates in models of mantle convection were noted: lithospheric plates imposed to mantle convection as an initial condition versus approaches to plates as a kinematic boun dary condition to mantle convection. Posters in the first category were presented by Zhong, Gurnis and Moresi ("Constraints on the time-dependence of lithosphere-mantle dynamics from vertical motions", "Incorporation of faults in models of mantle convect ion"). Posters by Olbertz, Wortel and Hansen ("Simulation of trench migration and its effects on the subduction process") and Cserepes ("Interaction of convection and plate-like surface motion") fall into the second category.

 The second theme of the workshop was the role of elasticity in evaluating topography of the free surface. In many of the mantle-convection models pressure below the free surface is used as a proxy for surface topography. The relevance of this point dep ends on the length and time scales studied. In the poster session, expressions of mantle convection in the topography and bathymetry of the earth was addressed by Zhong, Gurnis and Moresi, Schott and Schmeling ("Delamination and detachment of a lithosphe ric root", Marquart and Schmeling ("Subduction processes: 2-D versus 3-D modeling - first results"), and Karpychev ("Plate tectonics contribution to the geoid and dynamic topography").

 The last theme identified by the panel was initiation, interaction and seismicity of faults. Heimpel and Olson ("A seismodynamical model of lithospheric deformation: development of continental and oceanic fault networks"), Lyakhovsky and Ben-Zion ("The dynamics of distributed faulting") and Podlachikov ("2-D Numerical modeling of elasto-visco-plastic lithospheric deformation") show that it is possible to reproduce the Gutenberg-Richter frequency-magnitude relation using different approaches. One can no te, however, that some of these approaches are less realistic in that they do not incorporate current knowledge of rock rheology.

 The first talk on the second day certainly broadened the view of most of the participants. In this presentation, T. Spohn (M&uumlnster) described a series of dynamical problems from other planets, highlighting the fact that a range of Rayleigh numbers ( 104 - 1010) and core-to-planetary-radius ratios (0.25 to 0.75) are represented throughout the solar system. He encouraged participants broaden their studies to include a wider range of parameters that could further our understanding of global dynamics o n other planetary bodies.

 The mid-morning session focused on rheological properties of plate boundaries. K. Furlong discussed the 'history dependence' of plate boundary rheology, showing an example of the western US as the plate boundary evolved. A. Poliakov discussed a continu um description of faulting that he used in modeling salt diapers and could have application to plate boundaries. S. Zhong (Caltech) discussed the fault model he had developed with M. Gurnis. This renewed the discussion over the philosophical issue of us ing a fault or weak boundary prescribed by the modeler versus a continuum rheology where 'faults' or some mechanism of stress concentration developed without the direct input of the modeler.

 The afternoon session included talks by P. Tackley and P. Bunge (LANL/Berkeley) discussing the role of phase changes and viscosity stratification in 3-D convection models. Bunge found that many of the effects of ponding of downgoing cold material and t he eventual avalanche into the lower mantle attributed to phase changes can also be reproduced with a high viscosity lower mantle and little, or no affect from the phase change. Tackley also discussed the influence of strongly varying material properties on the planform of 3-D convection. P. van Keken (U. Michigan) discussed the implications of recent melting temperature measurements of Perovskite on a homologous temperature based estimate of lower mantle viscosity stratification and the effect of this viscosity model on convection. van Keken's viscosity profile is similar to one group of viscosity models derived from using seismic tomography to drive mantle flow and explain plate motions and the geoid. U. Christensen showed a series of calculations w ith a phase change and imposed plate velocities to study the influence of trench rollback on slab penetration and avalanches.

 The summary of the poster session from the previous day started the last morning session. The results were compiled by H. Schmeling (Frankfurt) and O. Cadek (Prague). There were as many, if not more 3-D results than 2-D results, marking the first time that 3-D convection was the dominant focus. There was also a good deal if interest in lateral variations in viscosity and this seems to be making its way into 3D models. It seems that the technical limitations have been overcome.

 The meeting concluded with presentations by S. Balachandar and M. De Jonge (U. Utrecht), both treating aspects which did not fall into the category of numerical modeling in the usual sense. Balachandar demonstrated how eigenvalue decompositions of flow or temperature fields can be used to determine structures in complex flows with a compact representation. He illustrated this method with global seismic tomography and found that there is some unusual structure between 700 and 850 km depth (a depth recen tly noticed by several seimology groups). de Jonge discussed his work on the detailed tomography of the subduction zones in Indonesia, where he employed around 3.5 million rays. He addressed the thermal structure of slabs by using forward modeling and a prodigious inversion effort, including the use of geological information as a part of the boundary conditions.

 In addition to the formal program, the rather isolated location of the Island helped to create an environment that was ideal for scientific exchange. Informal conversations and discussions continued late into the night around the bar. While very brief, most of the participants felt it was an interesting and productive workshop.

 Contributed by Scott King and Ulli Hansen .
 

National and Other Activities


 

Russian SEDI Activities

A research group was created in 1994 to work on a project named "The Earth's core as a total source of gravity and magnetic fields anomalies and geodynamic effects". The group consists of six staff members of the Sternberg Astronomical Institute, two po st-graduates and two students of the Moscow State University. The project has been supported by the Russian State Foundation on Fundamental Research. Accomplishments of the project are to be summarized in a a special issue of "Trudy Gosudarstvennogo As tronomicheskogo Instituta im. P.K. Sternberga", Tom LXV, Chast 1, Izdatelstvo Moskovskogo Universiteta, 1996. The titles of these articles are as follows:

 N.A. Chujkova. Preface.
P.A. Stroev, E.D. Koryakin, A.N. Grushinsky. The Global Distribution of Average (5 degrees to 5 degrees) Depths of Mohorovicic Discontinuity on the Earth.
N.A. Chujkova, T.G. Maximova. A Spherical Harmonic and Statistical Analysis of the Moho Surface Depths.
N.A. Chujkova, A.N. Grushinsky, T.G. Maximova. The Spherical Harmonic and Statistical Analysis of the Earth Equivalent Rock and its Isostatic Compensation.
N.A. Chujkova, S.A. Kazaryan. The Earth's Core as the Possible Source of the Gravity anomalies.
N.A. Chujkova, N.V. Alakhverdova. The Earth Core's Internal Structure as a Total Source of Gravity and Magnetic Fields Anomalies: Preliminary Results.
Yu.V. Barkin. To Inner Core Dynamics.
S.L. Pasynok. On the Polar Free Oscillation of the Earth's Inner Core.
N.A. Chujkova, K.V. Semenkov. Dependence of the Reversals Frequency of the Geomagnetic Field on the Position of Solar System in the Galaxy.
V.E. Zharov. The mass and the moment of inertia of the Earth using a new model of Moho surface.
Also, ten reports on the project will be presented at a Geodynamics seminar on November 16, 1995. For more information on this project, please contact Dr. Nadezhda A. Chujkova; email address - chujkova@sai.msu.su.
Contributed by Nadezhda A. Chujkova and Lev Vinnik.
 

French SEDI Activities

France has an on-going program "Terre Profonde" that funds national research on SEDI related topics. The budget is about 1.2 MFF. It covers research expenses (but not the salaries) around several themes, such as: material properties at high pressure; man tle convection and plumes; the Earth's magnetic field; seismology and gravity. It also contributes to the animation of working groups and meetings. A small group on the 'geodynamo', animated by Philippe Cardin and Dominique Jault is particularly active. For further informatin, contact Vincent Courtillot (courtillot@ipgp.jussieu.fr).
 

US SEDI Activities: New NSF Program

The Geophysics Program of the Division of Earth Sciences of the United States' National Science Foundation has recently (on September 27, 1995) announced the start of a new "Special Emphasis" program for CSEDI (Cooperative Studies of the Earth's Deep Int erior, named after the US National initiative for SEDI). The proposal submission deadline will be December 15 of each year, with an anticipated annual budget of $1.5M. The funds for this Special Emphasis Research Opportunity come from the Earth Sciences Division Discipline Programs, and do not represent new resources to the EAR community. The text of the announcement is as follows.

"The Division of Earth Sciences (EAR) at the National Science Foundation (NSF) invites the submission of proposals for collaborative, interdisciplinary studies of the Earth's interior within the framework of the community-based initiative known as Cooperative Studies of the Earth's Deep Interior (CSEDI). Funding for this Special Emphasis Area will support basic research on the character and dynamics of the Earth's mantle and core, their influence on the evolution of the Earth as a whole, and on processes operating within the deep interior that affect or are expressed on the Earth's surface.

 "CSEDI is a community initiative that has been organized by members of the SEDI (Studies of the Earth's Deep Interior) committee of the IUGG (International Union of Geodesy and Geophysics) and the SEI (Studies of the Earth's Interior) committee of the Am erican Geophysical Union. A science plan has been developed with broad community input and support and reflects the scientific objectives of the initiative. This initiative grew out of the realization that the most important problems related to Earth's in terior need a multi-disciplinary effort that brings to bear in a coherent way creative and imaginative thinking about the state and dynamics of Earth's interior, along with the utilization of the most advanced computational, experimental, analytical, and observational techniques. Ultimately, the goal of such efforts is to determine as quantitatively as possible how the Earth's interior works, and how processes in Earth's deep interior control the structure and evolution of Earth as a whole. To do so requ ires the resolution of many first-order issues leading to an understanding of:

 - the engine in Earth's deep interior that drives plate tectonics and the physical and chemical aspects of plate motions and mantle convection, and, hence, the physical, mechanical, and chemical evolution of the lithosphere and deep mantle;

 - the generation and changes of Earth's magnetic field;

 - the character and dynamism of Earth's interior as expressed on or near Earth's surface throughout Earth history;

 - the generation of mantle plumes and their role in mantle flow and the generation of large igneous provinces;

 - the dynamical, thermal, electromagnetic, and chemical interactions that occur across the core-mantle boundary zone; and

 - the mechanical, thermal, and chemical interactions across the mantle transition zone.

 "The opportunity for rapid progress in this research activity derives largely from the timely coincidence of advances in several disciplines. Seismic imaging of the Earth's deep interior provides insights into the convective and thermal patterns in the m antle. Advances in high pressure materials research allow for direct laboratory investigations of the pressure-temperature-composition and mechanical properties of the deep interior. Isotopic measurements of crustal and mantle-derivative rocks reveal che mical signatures that indicate recycling of the deep interior. Modeling of the Earth's magnetic field has illuminated possible relations between convection in the Earth's core and structures in the lowermost mantle, potentially providing new insights int o the geodynamo. Geodetic techniques have provided new and unexpected probes of the deep interior. Advances in computational techniques allow complex simulations of flow and convection in the mantle and core. Individually these are all important advances, but the aim of this funding opportunity is to link these advances into coordinated and integrated studies that will allow significant new insights into an understanding of the processes operating in the deep interior and how they govern the evolution of the surface of the Earth.

 "The function of the National Science Foundation CSEDI special emphasis is to provide support for truly integrated, multi-disciplinary studies so that accelerated progress can be made on these fundamental problems of the Earth's deep interior. Emphasis w ill be placed on cooperative, multi-disciplinary efforts that are fully integrated and for which the value of the collaboration can be shown to exceed the contributions from individual studies. In recognition of the potential and of the impediments to in- depth collaboration among component disciplines, the project description, budget, and work schedule should emphasize the specific steps and mechanisms required to assure successful integration at all stages of the research."

 For further information on the NSF program, contact Robin Reichlin, CSEDI Director, Division of Earth Sciences, Room 785, National Science Foundation, Arlington, VA 22230, (703) 306-1556, rreichli@nsf.gov. For further information on CSEDI, contact R. J. O'Connell, Department of Earth and Planetary Science, Harvard University, 20 Oxford Street, Cambridge, Cambridge, MA 02138.
 

International Geomagnetic Reference Field 1995 Revision

The International Geomagnetic Reference Field (IGRF) is a series of mathematical models of the main geomagnetic field and its secular variation produced an maintained by Working Group of IAGA Division V: Analysis of the Global and Regional Geomagnetic F ield and its Secular Variation. Each model comprises a set of spherical harmonic coefficients up to a set truncation level. The models are defined for successive 5-year epochs. At the final epoch, presently 1995.0, a secular variation model is included for forward continuation of the field for the following 5 years.

 A 7th generation (1995) revision of the IGRF was adopted by the International Association of Geomagnetism and Aeronomy (IAGA) during the XXI General Assembly of the International Union of Geodesy and Geophysics held in Boulder, USA in July 1995. The IGR F now consists of a new set of IGRF models at 5-year epochs from 1900.0 to 1940.0, the existing DGRF models at 5-year epochs from 1945.0 to 1985.0 (IAGA, 1991; Langel, 1992), a new DGRF 1990 model that replaces IGRF 1990, and a new IGRF 1995 model that in cludes secular variation (SV) terms for forward continuation of the 1995 field to the year 2000.0. Coefficients for dates between the 5-year epochs are obtained by linear interpolation between the corresponding coefficients for neighbouring 5-year epochs .

 The main field models are truncated at N=10 (120 coefficients), which is the practical compromise adopted for producing well-determined main field models while avoiding most of the contamination from crustal fields. Main field coefficients are rounded t o the nearest nanotesla to reflect the limit of resolution of the observational data. The prospective secular variation model is truncated at N=8 (80 coefficients) with coefficients rounded to the nearest 0.1 nT/yr, again reflecting the resolution of the available data. When converting between geodetic and geocentric coordinates, use of the IAU ellipsoid (International Astronomical Union, 1966) is recommended; it has an equatorial radius of 6378.160 km andflattening 1/298.25.

 The IGRF 1995 model will be superseded when a definitive model of the main field at 1995.0 is adopted at some later date. Similarly, the IGRF models for 1900 to 1940 may be revised later. Details about the the derivation of the 1995 revision of the IGR F will appear in a special issue of the Journal of Geomagnetism and Geoelectricity in 1996. The name IGRF refers collectively to the entire series of spherical harmonic models; if a particular epoch model is intended, the reference must be specific, e.g. , IGRF 1995 or DGRF 1980.

 ASCII files of the IGRF coefficients and computer programs for synthesising field components are available from the World Data Centres listed below and also from cooperating national geomagnetic observatory agencies and geological surveys throughout the world.
 
 

World Data CenterA: Solid Earth Geophysics
National Geophysical Data Center
NOAA, Code E/GCI
325 Broadway
Boulder, CO 80303-3328, USA
Tel: +1-303-497 6521
Fax: +1-303-497 6513 Email: info@ngdc.noaa.gov

 World Data Center B2
Russian Geophysical Committee
Academy of Sciences of Russia
Molodezhnaya 3
Moscow 117 296, RUSSIA
Tel: +7-930 0546
Fax: +7-930 5509
Email: sgc@adonis.iasnet.com

 World Data Centre C1: Geomagnetism
British Geological Survey
Murchison House, West Mains Road
Edinburgh, EH9 3LA, UK
Tel: +44-131-667 1000
Fax: +44-131-668 4368
Email: drbar@wpo.nerc.ac.uk

 World Data Center A: Rockets and Satellites
NASA/Goddard Space Flight Center
Code 633
Greenbelt, MD 20771, USA
Tel: +1-301-286 6695
Fax: +1-301-286 1771
Email: request@nssdca.gsfc.nasa.gov

 The new models adopted for the 1995 revision of IGRF are based on candidate models provided by NASA's Goddard Space Flight Center (Terry Sabaka, Rich Baldwin, and Bob Langel), The Institute of Terrestrial Magnetism, Ionospheric and Radio Wave Propagation IZMIRAN (Vadim Golovkov), and jointly by the US Navy (John Quinn and Rachel Coleman) and the British Geological Survey (Susan Macmillan and David Barraclough). We thank the staff of magnetic observatories and survey organisations world-wide for providing the data on which IGRF depends. For further information about IGRF, contact C. E. Barton, Australian Geological Survey, GPO Box 378, Canberra ACT 2601, Australia. Fax: +61-6-249 9986, email: cbarton@agso.gov.au.
 

American Geophysical Union Committee on the Study of the Earth's Deep Interior

The American Geophysical Union Committee on the Study of the Earth's Deep Interior promotes AGU activities pertaining to the Earth's deep interior. In the last year, the committee sponsored a wide number of Union and jointly-sponsored special sessions a t the Fall and Spring AGU meetings. Highlights included a two-day Union session at the Fall 1994 meeting called "Earth's Structure from Crust to Core", a special session on the multidisciplinary studies of the Earth's Core-Mantle Boundary and numerous ad ditional special sessions sponsored jointly by the AGU Tectonophysics, Geodesy, Geomagnetism and Paleomagnetism, Seismology, and Volcanology sections. The AGU SEDI Committee also co-sponsored, with Cooperative Studies of the Earth's Deep Interior, an eve ning reception attended by about 100 AGU members to discuss the NSF CSEDI Initiative. Among the special sessions sponsored by the AGU SEDI committee at the Spring, 1995 AGU Meeting, was a Union session on "Mantle Structure, Dynamics, and Geochemical Rese rvoirs". The AGU SEDI committee meets annually at the Fall AGU meeting. For more information, contact Louise Kellogg, AGU-SEDI committee chair, Department of Geology, University of California, Davis, CA 95616, U.S.A. Email kellogg@ymir.ucdavis.edu
 

Business Activities


 

SEDI Business meeting July 6, 1995

The 1995 business meeting of SEDI was held in the Engineering Center at the University of Colorado the evening of July 6, 1995, with Chairman Jean-Louis LeMouel(IPG, Paris) presiding. The meeting was attended by 60 people from 14 countries.

 The meeting began with the Chairman giving a brief history of SEDI, including its relationship with the IUGG, and describing some of its past activities. The Secretary then gave a brief report on the membership of SEDI; there are presently about 580 nam es on the active mailing list. About 60% of these are on the email network; this number will be raised to about 85% following the IUGG Assembly.

As an introduction to the election of officers, the Chairman reviewed the administrative structure of SEDI. This consists of a Chairman, a Vice Chairman, a Secretary and an Advisory Committee. The latter is to represent the membership of SEDI as a whol e and to advise the Chairman as needed. The Chairman at his discretion may chose a subset of this committee to act as an executive committee. The slate of names drawn up by the nominating committee was then presented to those in attendance. Following o ne nomination from the floor, the slate of officers was approved without dissent. The officers of SEDI for 1995-99 are as follows. Chairman: Kurt Lambeck - Australia, vice Chairman: Masaru Kono - Japan, Secretary: David Loper - USA, Advisory Committee: Jeremy Bloxham - USA, Bruce Buffett - Canada, Ulrich Christensen - Germany, Ivan Cupal - Czech Republic, Ibrahim Eltayeb - Near East, Fu Rongshan - China, Yoshio Fukao - Japan, David Gubbins - UK, H. K. Gupta - India, Raymond Hide - UK, Albrecht Hofman - Germany, Gauthier Hulot - France, Ian Jackson - Australia, Jean-Louis Mouël - France, Guy Masters - USA, Henri-Claude Nataf - France, Richard Peltier - Canada, Tomas Shankland - USA, Frank Stacey - Australia, Lev Vinnik - Russia, Takesi Yukutake - Japan, Vladimir Zharkov - Russia.

 The next items of business were reports of various activities. Kurt Lambeck(Australian National U.) gave a brief summary of plans for the 1996 SEDI Symposium to be held in Brisbane, 23-27 July, 1996, and fielded questions regarding the program, coordian tion with the Western Pacific AGU meeting and travel support. Next Jerry Schubert (U. California Los Angeles) reported on plans for a Gordon Conference on the Composition, Structure and Dynamics of the Earth's Interior to be held at Plymouth State Colleg e in New Hampshire, June 30 - July 5, 1995. There was some discussion of the similarity of its program to that of the SEDI symposium and of the closeness in time of the two meetings. Brief reports of French (by V. Courtillot, IPG, Paris), Japanese (M. K ono, U. Tokyo), Canadian (D. Crossley, McGill U.) and USA (O. Anderson, U. California Los Angeles) SEDI activities were given. H.-C. Nataf (Ecole Normale Sup�rieure, Paris) reported on the European Business meeting of SEDI held at the EUG assembly in Apr il of this year, and discussed the problem of coordinating SEDI activities between the EGS and EUG.

 The final item of business was the passing of the SEDI gavel from Jean-Louis LeMouel to Kurt Lambeck.

 Secretary, David Loper, 1995/07/25.
 

European SEDI business

After the European SEDI business meeting held in Strasbourg during the 1995 EUG meeting, two actions have been taken at the European level: 1) in response to a joint query of the Presidents of EUG and EGS, a letter was sent to them expressing the view of the European SEDI community that a closer cooperation between the two Associations was needed in order to avoid conflicts between meetings. 2) contacts were taken to build up an EC network proposal on the geodynamo (numerical and experimental approaches ). The proposal should be prepared before the EC march 1996 deadline.

 The European Secretary of SEDI (Henri-Claude Nataf : nataf@geophy.ens.fr) welcomes input from European members on activities to be established at the European level.
 

SEDI mailing list and Email list

SEDI is a rather informal organization. The only requirement to join is an expression of interest; it has no dues or formal requirements. However, to remain a member of SEDI, it is necessary to renew this expression of interest every four years, in pha se with the IUGG General Assemblies. A query was sent with the 1994 newsletter and the names of those who responded were retained, while those who did not were dropped. The current list contains the names of 597 people, to whom this newsletter is being sent. There are a total of 321 names that are being dropped from the active list. If you know of a colleague who did not recieve this newsletter, it is a likely indication that the person has been dropped. To be reinstated, any person need only send a n expression of interest to the secretary (see the first paragraph of this issue).

 The SEDI email list now contains the names of 515 people - 86% of thoseon the mailing list. If you receive this newsletter but have not been receiving any SEDI email messages, you likely are not on the email list. To add your name to the SEDI email lis t, send to SEDI-mail-request@gfdi.fsu.edu the following two-line message:
 
 

subscribe

 quit
 
 

This will prompt the system to send you a message informing you that you have been added to the list and containing information how the system operates.

 If you wish to send a message to all on the SEDI email list, address it to SEDI-mail@gfdi.fsu.edu.
 

Future Meetings of Interest


 

SEDI 96: Plans and Preparations

The 1996 SEDI Symposium, 23-27 July, in Brisbane, Australia, will be incorporated into a wider Western Pacific Geophysics Meeting (WPGM) run by the American Geophysical Union (AGU). AGU will be handling all the logistics for the meeting, in standard AGU style. A first call for papers appeared in the September 19 issue of EOS, with a deadline for receipt of abstracts in Washington of March 15, 1996. Announcements about registration, hotel bookings and pre-bookings for social events and tours will also appear in EOS. For SEDI participants who are AGU members, this will be a straightforward and familiar process, but AGU membership is not a requirement. Non-members of AGU should write, expressing interest in the meeting, to Brenda Weaver, American Geoph ysics Union, 2000 Florida Ave. NW, Washington DC 20009, USA; Fax +1-202-328-0566, E-mail: bweaver@agu.org. Brenda has promised to send all relevant details to enquirers. Brenda already has the names and addresses of those who sent inquiries to Frank Sta cey.

 Two other independent groups are organising symposia as part of the WPGM. The Solid Earth Geophysics Group of the Geological Society of Australia has a several day symposium on the Australian lithosphere and an international group of geodesists has plan ned sessions on GPS techniques. Each of the independent groups will constitute on of the approximately 15 parallel sessions at the meeting. SEDI has been assigned one of the larger rooms for the full week of the meeting (Tuesday 23 to Saturday 27 July, 1996, inclusive) and poster space nearby.

 The meeting venue is the Brisbane Convention and Exhibition Centre, which began full operation in May 1995. It is an impressive building with excellent facilities, adjacent to the Southbank Parklands which were developed as a recreational area on the si te of the 1988 Brisbane Expo. The Centre forms part of a cultural complex with the Queensland Art Gallery, Museum, Concert Hall, Theatres and State Library. Most of the hotels are in the city area, a 20 minute walk across the Brisbane River, and there a re frequent trains from the South Brisbane station, immediately adjacent to the Convention Centre. The centre itself has its own snack bar and there are more than 20 food outlets, from upmarket restaurants to take-away with picnic tables, in the Parkland s area.

 July in Brisbane is mid-winter, but at 23.5o S many people find this the most comfortable time of year. This is one of the drier months and days are normally sunny, reaching 21oC on average. Evenings are cool, with an overnight minimum averaging 10oC, so that some warm clothing is needed.
 
 

The Scientific Program

The September 19 EOS gave only a very preliminary version of the program for the fifth SEDI symposium. Some more detail follows here. The now traditional SEDI program arrangement will be continued, with one or two invited papers in each half-day sessio n followed by brief oral presentations of the larger number of poster papers. The detailed arrangements will depend upon the papers submitted but (with room for expansion of particularly popular sessions) presently planned session topics are (with sessio n chairs):
 
 

23 July a.m. SD02.1 Seismology: structure of the lower mantle, especially D'' and the transition zone. (B. L. N. Kennett, RSES, A. N. U., Canberra 0200, Australia. FAX: +61-6-257-2737, E-mail: Brian@rses.anu.edu.au).

 23 July p.m. SD02.2 Seismology: anisotropy and attenuation in the lower mantle and core (B. L. N. Kennett).

 24 July a.m. SD04 Geodesy: mantle viscosity, core ellipticity and related problems. (K. Lambeck, RSES, A. N. U., Canberra 0200, Australia. FAX: +61-6-249-5443, E-mail: Chair.SEDI@anu.edu.au).

 24 July p.m. SD05 Mantle convection, depth of subduction and the source of plumes. (G. Houseman, Dept. of Earth Sciences, Monash U., Clayton, Victoria 3168, Australia. E-mail: greg@pegasus.earth.monash.edu.au).

 25 July a.m. SD01.1 High pressure properties of silicates, oxides and the deep mantle. (I. N. S. Jackson, RSES, A. N. U., Canberra 0200, Australia. FAX: +61-6-249-0738; E-mail: Ian.Jackson@anu.edu.au).

 25 July p.m. SD01.2 Phases and properties of iron and the core. (O. L. Anderson, IGPP, UCLA, Los Angeles 90024, USA. FAX: +1-310-206-3051 and S. Saxena, Uppsala).

 26 July a.m SD03.1 The dynamo - theoretical and numerical advances. (D. Ivers, School of Mathematics and Stats., U. Sydney, F07, Sydney, NSW 2006, Australia. FAX: +61-6-692-4534, E-mail: ivers_d@maths.su.oz.au).

 26 July p.m. SD03.2 Mantle influences on dynamo behaviour. (P. McFadden, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia. FAX: +61-6-249-9986, E-mail: pmcfadde@agso.gov.au).

 27 July Available for program extension.

 Abstracts must be submitted to AGU in its specified format and will be published in a special WPGM supplement to EOS. The preferred AGU method of submitting abstracts is now by electronic mail, using the AGU web address: http://www.agu.org which has the advantage that abstracts are immediately available to anyone on the world-wide-web. Please also send abstract copies to the SEDI program chair, Prof. K. Lambeck (address as under SD04 above) who should be contacted about any query on the overall program . Session chairs should be consulted about details of individual sessions. Contributors to the session SD01.2 on iron and the core are asked to send an abstract copy to Prof. O. L. Anderson (address above).
 
 

Social Program

There are three components to the social program: evening activities, daytime activities and pre- and post-conference touring. The development of a strong social program is based on the expectation that some participants will wish to take advantage of a visit to Queensland to see the Great Barrier Reef and as a consequence partners will be more numerous than at most scientific meetings. With this in mind Mike and Jo McElhinny made a personal 'fact-finding' tour of reef trips and facilities to recommend those that are both good and good value, as reported in EOS, December 6, 1994. We have now negotiated with a tourist operator to offer the recommended trips at substantial discounts for early bookings made through AGU. Details and booking arrangements will appear in EOS and will be sent to non-AGU participants by Brenda Weaver.

 The following evening activities are planned:
Mon, July 22, 5 pm - 8 pm Convention Centre
Registration and ice-breaker. Included in registration fee.
Tues, July 23, 5.15 pm - 7.15 pm Convention Centre
Wine and cheese tasting. Sampling some local products with an expert commentary. Pre-book with registration.
Wed, July 24 Queensland Art Gallery and Queensland Museum, both very close to the Convention Centre, stay open to 8pm on Wednesday evenings.
Thurs, July 25, 6pm Public Lecture.
This will be on a geophysical topic of wide public interest. Free to the public as well as WPGM participants.
Thurs, July 25, 7.30pm Barbecue (Australian style).
This will be outside (weather permitting) so bring a jacket or pullover. Pre-book with registration.

 Fri, July 26, 6pm Brisbane Planetarium Southern Sky Program. Limit 132 (Another night can be booked if there is a high demand). Pre-book with registration. The planetarium is situated in the new Botanical Gardens at the foot of Mount Coottha. The co st of hiring buses to take participants the short distance from city hotels exceeds the cost of taxis if more than two share a cab. Negotiations on this detail are still in progress but we will probably recommend sharing a taxi for the journey to and fro m the planetarium (~A$8 each way). The group entry fee for the planetarium is A$6, booked through the AGU registration process. For those with own transport or willing to pay for a modest taxi diversion a drive to the top of Mount Coottha will be reward ed (on a clear evening) with a magnificent view across Brisbane.

Daytime Social Activities

A lounge area with coffee facilities in the Forest Sculpture area on the Plaza level of the Convention Centre will be set up as a social meeting area. It will have a desk that will be manned each morning by volunteer tour advisers. Many of the possible activities can be arranged at short notice (with the advisers' help if needed) but those proposed for the 24th, 26th and 27th must be pre-booked. The following are suggestions, including details of the three tours that require pre-booking.
 
 

Tues, 23 July. Forest Sculpture meeting area, Plaza level of the Convention Centre. Meet Jo McElhinny and Joy Stacey for a potted orientation course on Brisbane, touring possibilities, places to eat, ferry and bus services. Then:

 Option 1: Accompany Joy on a guided walk around the Southbank Parklands and Cultural Precinct with a stop for morning coffee.

 Option 2: Accompany Jo on a Brisbane City Council bus tour of significant sights and buildings (A$12).

 12.30: Both groups meet for lunch at the Australian Perspectives Gallery, and aboriginal art gallery.

 1.30 or so. Introduction to the more interesting shops.

 Wed, 24 July. Moreton Experience. A day tour to Moreton Island with guide Alan Gennings. This is a still largely unspoilt sand island 40 km off the coast with interesting natural history and evidence of early aboriginal settlements. Hotel pickup 7 to 7 .20 am. Adult A$65. Lunch provided. Number limited to 28 and pre-booking is essential. The tour may be repeated on Friday if demand is high.

 Thurs, 25 July. Brisbane River Cruise to Lone Pine Koala Sanctuary. 10 am start from North Quay, near to northern (city) end of Victoria Bridge. Close contact with a range of local wild-life. Mirimar Cruises offer this trip daily and pre-booking is n ot necessary. Adult fare A$15 and Sanctuary admission A$12. Light lunches are available at the Sanctuary cafe.

 Fri, 26 July. Lamington National Park. Coach departs hotel 8.30am for Binna Burra Lodge in one of the best remaining examples of Queensland Coastal Rainforest. A lodge naturalist will conduct guided walks. Cost A$67 including lunch at the lodge. Pre-b ook with registration.

 Sat, 27 July. Coach trip to Glasshouse Mountains and Noosa. Depart from hotel 8.30 am, return 6 pm. The Glasshouses are a series of volcanic plugs rising from what is now an almost flat plain. A stop at a reptile park is timed to see crocodiles fed. The stop in Noosa (now Australia's premier beach resort) will suffice for a walk in the Noosa National Park, where a pocket of rainforest reaches the beach, or the Noosa River estuary and for lunch (own expense). Afternoon tea at Montville, a village hi gh in the Blackall Range, is included in the fare - A$61. Pre-book with registration.

 Contributed by F. D. Stacey (U. Queensland).
 

Third Annual CSEDI Meeting 
Santa Fe, January 1996.

The third "annual" meeting of Cooperative Studies of the Earth's Deep Interior (CSEDI), will be held on January 12-13, 1996, in Santa Fe, New Mexico. The IGPP Center at Los Alamos National Laboratory, headed by Charles Keller, is hosting this meeting and Rick O'Connell, Chair of the CSEDI committee is the principal scientific organizer. The last meeting, also in Santa Fe in October 1994, was very well received, and the organizers hope that this one will equally successful. The focus of the meeting this year will be to discuss areas of particular interest to the CSEDI program and community. An excellent turn out is expected from all of the major disciplines, and a provocative agenda is scheduled which should help to catalyze discussions of new collabor ative research undertakings. Several major CSEDI themes will be addressed with a combination of broad overview talks and short-presentation. These topics include: Core mantle boundary (R. Jeanloz), Deep mantle structure and geochemistry (Bill White), M ineral properties, seismic properties and mantle structure (Tom Ahrens), Geodynamo modelling (Gary Glatzmaier), Mantle convection and plate tectonics (Mark Richards), Earth models - dynamic and seismic (Brad Hager). There will also be brief reports on al l of the CSEDI Workshops held in 1994 and 1995 and discussions of the CSEDI program and ways that software development can be supported. This is also the time to discuss the relation between the CSEDI program and the community, and the role and activiti es of the CSEDI Coordinating Committee. Most importantly, we want to include scientific discussions of new developments and results of interest to the CSEDI community. Please contact the organizers if you desire to make a presentation.

The meeting will be held at La Fonda Hotel in Santa Fe, New Mexico, which is located in downtown Santa Fe, within walking distance of restaurants, shops and museums. The registration fee is $90.00 fee; after December 20, 1995, a $35.00 late fee will be charged. The fee will entitle the participant to two continental breakfast; coffee breaks; a registration reception on Thursday, January 11 from 6:00pm - 8:00pm; and a conference reception on Friday, January 12th at 5:30pm, immediately following the Frid ay session. Guest tickets for the receptions will be $15.00/ea. A detailed agenda will be sent out soon. To obtain further information about the scientific program, contact R. J. O'Connell, Department of Earth and Planetary Science, Harvard University, 20 Oxford Street, Cambridge, Cambridge, MA 02138; email oconnell@vortigern.harvard.edu; phone: (617) 495-2532. For information about registration, contact Shirley Roybal, IGPP, MS C305, Los Alamos National Laboratory, Los Alamos NM 87545; phone (505) 6 67-0920; fax (505 665-3107; email srr@kokopelli.lanl.gov.
 

Composition, Structure and Dynamics of the Earth's Interior

A Gordon conference on this topic will be held at Plymouth State College, Plymouth, New Hampshire, June 30-July 4, 1996. The Conference program has been finalized and all discussion leaders and principal speakers have agreed to participate. Briefly, to pics and speakers are as follows.
Mapping Earth's Interior. Discussion leader, T. Jordan; speakers, S. Grand and A. Dziewonski.
The Fate of Slabs. Discussion leader, K. Creager; speakers, R. van der Hilst, E. Takahashi and C. Litgow-Bertelloni.
Posters and videos. Discussion leader, D. Stevenson.
Patterns of Mantle Convection. Discussion leader, L. Kellogg; speakers, J. Phipps Morgan, P. J. Tackley and I. Jackson.
Geochemical Reserviors and Mantle Mixing. Discussion leader, D. Anderson; speakers, R. Carlson and U. Christensen.
Plates and Mantle Convection. Discussion leader, D. Bercovici; speakers, M. Gurnis, S-I. Karato and Y. Ricard.
Posters and videos. Discussion leader, B. Romanowicz.
Dynamics of the Core. Discussion leader, D. Loper; speakers, D. Gubbins, G. Masters and P. Olson.
Unresolved problems. Discussion leader, R. O'Connell; speakers, D. Turcotte and R. Jeanloz.

 A proposal has been submitted to NSF for additional Conference funding to help defray expenses of attendees. A final decision about the level of NSF support is pending.

 Formal announcement for the Conference will appear in the February 9, 1996, issue of Science. The announcement will include an application form which must be returned to the Gordon Conferences offices by all aspiring participants. Because of logistical requirements (housing and feeding of conferees) attendance will be limited to at most 135 individuals. While a small number of conferees have been invited (e.g., discussion leaders and principal speakers), most of the participants will be determined by a pplication. Since the Conference may well be oversubscribed, the earlier one submits an application the better. Applications are already available and can be obtained by contacting the Gordon Conferences at Conference Application, Gordon Research Confer ences, University of Rhode Island, P.O. Box 984, West Kingston, RI 02892-0984 USA (mail) or fax 401 783 7644, phone 401 783 4011/3372, email app@grcmail.grc.uri.edu. Applicants are encouraged to send short abstracts of what they would present in a poste r paper.

 Again, submit your applications as early as possible. Upon approval of your application, you will be sent registration material that must be completed and returned to the Gordon Conferences office. Your place at the conference is assured only upon compl etion of the entire process, i.e., submittal of an application, approval, and return of registration materials.

 Either of the Conference co-chairs can be contacted at the addresses below for further information.

 Gerald Schubert, Co-Chair
3806 Geology Bldg.,
BOX 951567
Dept., ESS, UCLA
Los Angeles, CA 90095-1567
tel 310 825 4577
fax 310 825 2779
or 310 206 5031
gschubert@mgnvax.ess.ucla.edu
gschuber@artemis.ess.ucla.edu

J. Michael Brown, Co-Chair
Geo Bldg., #202
BOX 351650
Atmospheric Sciences
University of Washington
SeattleWA 98195
tel 206 543 9419
fax 206 543 0489
brown@geophys.washington.edu
 

EGS Symposia

There are two sessions of SEDI interest scheduled for the European Geophysical Society XXI General Assembly to be held in The Hague, The Netherlands 6 - 10 May 1996. Session SE1.1 on "The Earth's Mantle and Core: Dynamics, Structure, and Composition" ai ms at bringing together recent results from seismology, high pressure physics and geodynamics in order to better understand the structure of, and processes at the Core-Mantle Boundary (CMB). This includes in particular questions like: Is D" primarily a c ompositional or a thermal boundary layer? What temperature gradients exist across D" and CMB and how are these maintained? Are plumes likely to form at the CMB? What is the nature of the seismic reflector at the top of D"? Besides addressing those que stions, contributions dealing with coupling mechanisms between the core and the lower mantle are especially invited. Other topics will include: Composition of mantle and core, Thermodynamics of the transition zone, Thermal and compositional convection i n the core, Generation of the Earth's magnetic field. (The last three of these topics will e coordinated with session SE1.). Conveners are H. Paulssen (Utrecht) and H.-C. Nataf (Paris). This symposium is sponsored by SEDI (Studies of the Earth's Deep I nterior). For more information, contact Hanneke Paulssen, Dept. of Geophysics, Utrecht University, P.O. Box 80021, 3508 TA Utrecht. The Netherlands; Tel: +31(30)2535089; Fax: +31(30)2535030; e-mail: paulssen@geof.ruu.nl.

 The theme of session EGS1 01 on "Circulation in the Atmosphere, the Oceans and the Earth's Interior" is large scale circulation caused by lateral and vertical gradients in density. This session will concentrate on the stability and temporal variability of the buoyancy driven circulation in each of these fields. Of particular interest are the existence of multiple equilibria and transition to complex temporal behavior in these type of flows. Contributions are invited on these issues with respect to the thermohaline ocean circulation, the large scale atmospheric circulation (e.g. Hadley circulation) and large scale mantle circulation. The session is convened by U. Hansen (Utrecht U.), H. A. Dijkstra (Utrecht) and F. T. M. Nieuwstadt (Delft). For more information contact U. Hansen, Dept. of Geophysics, Earth Science Institute, Utrecht University, PO Box 80021, Budapestlaan 4, NL- 3508 TA Utrecht, The Netherlands; email: Hansen@geof.ruu.nl; Fax (+) 30 253 5030.

 The deadline for the receipt of abstracts is 15 December 1995.
 

Rock Magnetism Conference

The biennial Rock Magnetism Conference at Santa Fe, New Mexico, is in theplanning stages. The proposed dates are June 27 to June 30, 1996. A major themeis expected to be old pitfalls and new techniques for obtaining high resolutiontimeseries of geomagn etic paleointensity from sediments and rocks. For moreinformation, contact the Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, fax - 612-625-3819.
 

Planetary and Cosmic Dynamos

An International Workshop on "Planetary and Cosmic Dynamos" will be held in the Chateau Trest (120km south of Prague) 18-23 August 1997, following the IAGA Assembly in Uppsala Sweden. The meeting will bring together dynamo specialist in geophysics and a strophysics. It is in essence a continuation of similar meetings held in 1979 at Alsovice Czechoslovakia, 1983 at Potsdam, Germany, 1988 at Liblice Chateau, Czechoslovakia and 1992 at Potsdam. For more information contact Ivan Cupal, Czech Geophysical I nstitute, Academy of Sciences, Bocni II, 14131, Prague 14131, Czech Republic, fax - 42-2-761-549; email - ic@ig.cas.cz.

Chairman's Statement

The principal SEDI activity in 1995 centered around the Boulder-IUGG meeting in July 1995 through initiation and support of a variety of symposia. Jean-Louis Le Mouël chaired his last business meeting at that time and it is appropriate to thank him for guiding SEDI through the past few years to a robust forum for scientific discussion on issues concerning the nature of the Earth's deep interior. For better or worse I was elected Chairman with Masaru Kono as Vice Chairman. More important, David Loper w as re-appointed as secretary. It is no secret that he has been the driving force behind SEDI and without his continual effort, including prodding Chairmen into periodic bouts of activity, the organisation would only be a shadow of itself. An important a spect of David's role is also a mercenary one in that administrative support for SEDI is partly funded through NSF.

 One important outcome of the Boulder meeting was the continuing support of SEDI by the IUGG. SEDI's inter-disciplinary nature is appreciated by the IUGG as a counter balance to the more discipline-structure of the Associations of the Union. Unfortunate ly the IUGG support translated only into minor financial support and SEDI remains a mechanism for organising interdisciplinary sessions on the deep earth without a significant financial basis. The IUGG Secretariat did seek to explore the possibility of e stablishing a closer link with SEDI, something not encouraged by the latter.

 The main SEDI activity for the coming year will be the biennial meeting, to be held in Brisbane, Australia, from 23-27 July. The format and scope of the meeting will follow the successful Whistler meeting with the emphasis being placed on the physics an d chemistry on the lower mantle and core. Details of the meeting will be found elsewhere in this issue of the Deep Earth Dialog. I urge you to attend!

 Kurt Lambeck, Australian National University.

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