|Number 8||Fall, 1994|
This is the eighth 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 Flu id Dynamics Institute, Florida State University,
Tallahassee, Florida 32306-3017, U.S.A, faxed to (904) 644-8972 or emailed
to firstname.lastname@example.org. Items for the next issue or notifications of change
of address should be sent to the same address.
The symposium consisted of eight half-day sessions, each consisting of a mix of invited review talks, brief oral presentations of posters, protracted coffee breaks allowing viewing of posters and finally a group discussion of the talks and posters. The c ontent of the sessions are summarized in the following seven reports (two closely related sessions being summarized in one report). The invited review talks are to be compiled into a volume, edited by David Crossley and Andrew Soward (Univ. Newcastle upon Tyne) and published as part of the Gordon and Breach Series on "The Fluid Mechanics of Astrophysics and Geophysics". Many of the contributed presentations are to be published as a special issue of Physics of the Earth and Planetary Interiors, edited by H enri-Claude Nataf (École Normale Supérieure) and David Loper (Florida State Univ.).
Not least among the attractions of the conference was the location of Whistler itself, which is a ski/summer resort some 80 miles north of Vancouver. Delegates were kept suitably conference bound for the first few days due to inclement mountain weather, but after that the sun came out for the hikes and Barbecue on Wednesday.
One of the highlights of the week was the SEDI Banquet featuring after-dinner reminiscences by SEDI gurus. Jack Jacobs (Univ. Aberystwyth) focused on the early days of SEDI (Trieste 1986 seems such a long time in the past) and Jean-Louis Le Mouël (Instit ut de Physique du Globe, Paris) gave a state-of-SEDI address that gently chided us for not being imaginative enough when it comes to selling and exploiting our science. In fact he held up as an example a recent science-fiction paperback and shared with us this most quotable quote: "What is clear is that nobody has the slightest idea what's going on in the core... Lack of knowledge won't stop the flood, of course... Flood ?... Of theoretical papers" (Paul Preuss, Core, Avon Books, 1993). Finally, and perha ps most significantly, Kurt Lambeck (Australian National Univ.), as potential incoming SEDI Chairman, highlighted some of the important science waiting to be done in the area of global geophysics that will no doubt form the essence of future SEDI symposia .
In the second invited talk, C. Yoder (Jet Propulsion Lab.) described a set of applications of modern space-geodetic techniques to problems in Earth dynamics. Yoder emphasized a number of outstanding issues, and also described attempts to use space-geodet ic data in global-change applications. In the discussion period following the talk the issue of the utility of measurements of secular variations in the long wavelength geoid in constraining deep-mantle viscosity was raised. J. Mitrovica (Univ. Toronto) s uggested that recent work (by Trupin, Mitrovica, etc.) indicated that ongoing ice/ocean mass variations could excite secular variations in the geoid which are comparable to those associated with glacial isostatic adjustment, and thus the inference of mant le viscosity based on the Jl-dot harmonics was problematic. The discussion session touched on a number of other issues raised by Yoder's talk. In particular, the connection between decadal scale length-of-day changes and core flow was addressed. J. Bloxha m (Harvard Univ.) argued that calculating torques at the CMB due to outer core flows, under the assumption of geostrophic flow, was not appropriate: indeed, theoretical considerations indicate that the geostrophic assumption leads to zero torque.
The poster presentations associated with Session 1 had a number of contributions from the SG community. These commonly considered error budgets at the growing network of stations and a variety of geophysical applications. Two posters associated with geop hysical applications of VLBI data were included. J. B. Merriam (Univ. Saskatchewan) presented work (with colleagues B. Chao and Y. Tamura) which considered estimates of mantle anelasticity in the zonal tidal band obtained from VLBI UT time series. J. Mitr ovica (Univ. Toronto) presented work (with colleagues J. Davis, P. M. Mathews, and I. I. Shapiro) dealing with their determination of the tidal h Love number parameters in the diurnal band using over a decade of VLBI group delay data.
Contributed by J. Mitrovica, Univ. Toronto.
Aldridge summarized the history of laboratory investigations on rotating fluids, in particular those related to core dynamics, beginning with Taylor's 1921 experiments showing the pervasive two dimensionality that rotation imposes on a fluid. More recent experimental work on the instability generated by an elliptic boundary, and attempts to include compositional convection and the Lorentz force were also described. Posters by J. Vanyo et al. and P. Wilde and J. Vanyo (Univ. California, Santa Barbara) des cribed recent experimental work on core motions induced by spin-up and precession.
Whaler described the recent evolution of ideas about the degree of layering in the core, and the degree of magnetic flux diffusion that is possible. Some of the problems associated with investigations of compositional convection were discussed, notably t he uncertainty as to what the lighter component in the core is, and what its miscibility with iron at core conditions might be. Poster papers W. Kuang, J. Bloxham (Harvard Univ.) and D. Fearn (Univ. Glasgow) and by J. Lister (Univ. Cambridge) and B. Buffe tt (Univ. British Columbia) (see below) also addressed this problem. Recent work on the influence of lateral variations in the mantle on core dynamics and thermodynamics suggests that core and mantle may be linked in more subtle ways than simply mechanica l coupling at the CMB.
Of the papers on core oscillations, two dealt with the translational oscillations of the inner core. D. Crossley, O. Jensen (McGill Univ.), M. G. Rochester and W.-J. Wu (Memorial Univ.) discussed the likelihood of this motion being observed at all and co ncluded that the largest of earthquakes might produce a nanogal signal in gravity, which would be observable by superconducting gravimetry, if the motion was not quickly damped. Most estimates of the damping (magnetic, viscous) favor high Q's, but are unc ertain. In a related paper, B. Buffett and D. Goertz (Univ. British Columbia) argue that magnetic damping would produce a Q ~ 2000. A new mechanism, in which a small part of inner-core boundary is subject to phase changes during the motion, may dissipate enough heat thermodynamically to damp the motion very quickly.
M. I. Bergman (Harvard Univ.) D. Fearn, P. Shannon (Harvard Univ.) and J. Bloxham explained how magnetic drag could limit the generation of convective chimneys near the inner-core boundary, and described an experimental set-up to test this idea. D. Brito , P. Cardin and H.-C. Nataf (École Normale Supérieure) described another experimental apparatus to measure the effects of magnetic fields on flow in a rotating conducting liquid. P. Cardin and P. Olson (Johns Hopkins Univ.) , and P. Olson and G. Glatzmaie r (Los Alamos National Lab.) numerically modeled the influence of the magnetic field on styles of convection in the outer core and concluded that convection patterns were sensitive to the relative orientation of buoyancy, rotation, and magnetic field. S. Ewen and A. Soward (Univ. Newcastle upon Tyne) showed that simple wave trains consisting of magneto-convective pulses are generally unstable, and conclude that other geostrophic nonlinearities not considered may also be important. M. St. Pierre (Harvard U niv.) showed numerical results that suggested that buoyant parcels rising through the core would be quickly broken up, bringing into question the notion of plumes and parcels as convection styles. Since the length scales resulting from this instability ma y be too small to handle in a global simulation of convection, St. Pierre and P. Roberts (Univ. California, Los Angeles) proposed a parametric scheme in which the magnetic field is represented as a mean and fluctuating part.
U. Hansen (Utrecht Univ.) and D. Yuen (Univ. Minnesota) showed that thermal dissipation in compositional convection can be large, and W. Kuang et al. in a study of thermal and compositional convection, found that when thermal and compositional buoyancy f luxes are comparable, the convection pattern is similar to purely thermal convection. They also showed that a stably stratified layer could form at the CMB if the convection was largely compositional, but that the presence of a thermal mode could prevent such a layer. J. Lister and B. Buffett also tackled the question of how much of the core might be stably stratified, and the consequences for core evolution if stability were achieved at various times, relative to inner core nucleation and the onset of co mpositional convection.
Contributed by J. Merriam, Univ. Saskatchewan.
The situation facing the development of MHD geodynamo theory, including the aspects described above, was reviewed by D. Fearn (Univ. Glasgow). He emphasized the importance of the small parameters, e.g. the Ekman number (~ 10-15), and the Roberts number ( q = k/h ~10-6), which can nevertheless not be neglected in the numerical integration of the dynamo problem. His talk also brought out the very considerable commitment being currently made in Britain to the solution of the geodynamo problem.
It is usually believed that, in the case of the Earth at least, the a-effect is created by asymmetric waves of planetary scale. The study of these slow "MAC-waves" is an ongoing preoccupation of a number of theoreticians, one of whom, S. London (Univ. Ho uston-Downtown), made a presentation at the present meeting. London used multiple-scale analysis and concentrated on a spherical thin-shell model of the fluid core. Slow, finite amplitude, planetary waves were the subject of an investigation by K. Hutches on (Univ. Glasgow) and D. Fearn. Their weapon was the computer and their model of the core was a cylindrical annulus. One of the first effects of nonlinearity is the creation by the waves of an axisymmetric part to the Lorentz force. Since geostrophic flo w is so easily excited in a rotating system, this component of the Lorentz force produces a disproportionately large geostrophic response in the flow. In a related but linear computational study, C. Jones, A. Longbottom (Univ. Exeter) and R. Hollerbach (L os Alamos National Lab.) included buoyancy forces and determined the 'preferred mode' of convection, i.e. the mode that is excited at the smallest value of the Rayleigh number (a non-dimensional parameter measuring the buoyancy force). They found that the azimuthal wave number of the preferred mode is of order 2 when the Elsasser number (the non-dimensional parameter measuring the Lorentz force in terms of the Coriolis force) is about 2. The results of their model in fact suggested that the Elsasser numbe r based on the toroidal field strength in the core lies between 5 and 10. They found, in this parameter range, encouraging general agreement between the field and flow produced by their model and the field and flow inferred by extrapolating the observed g eomagnetic field downwards to the CMB. They concluded that the nonaxisymmetric part of the geomagnetic field and the velocities on the core surface inferred from studies of the geomagnetic secular variation strongly support the hypothesis that large scale convection, of much the same type as theory is currently predicting, is occurring in the core.
One of the interesting aspects of the meeting was the revival in interest in precession as a source of core motion. The creation of an a-effect by precessionally driven flows was the subject of a paper by C Barenghi (Univ. Newcastle upon Tyne), R. Holler bach and R. Kerswell (Univ. Newcastle upon Tyne). They found that the intense shear layers created by a slight 'tilt over' of the Earth's rotation axis can, on average, produce a steady a-effect within the tangent cylinder, i.e. the cylinder circumscribin g the inner core and parallel to the rotation axis.
The existence of an a-effect allows the axisymmetric (zonal and meridional) components of field and motion to be studied in isolation from their asymmetric parts. In the case of kinematic theory, there are two extreme models, the a2 and aw models which differ only in the way that they create zonal field; both rely on the a-effect to produce the meridional field from the zonal field. Between these two extremes lie a range of a2w models, which create zonal from meridional field by both the a- and w-effect s. Characteristically, a2 dynamos are steady and aw models reverse polarity on the time scale, of free decay, say 103 to 104 yr. in the case of the core. Thus, even though many estimates suggest that the w-effect is much morepotent than the a-effect in cr eating zonal field, the simple aw model is unacceptable as a model of the geodynamo. There are however two main ways in which a more sophisticated aw geodynamo might be plausible. First, the oscillation is quenched by quite modest meridional flow of a typ e that might well be expected to arise in the core; second, it is also removed if the creation of zonal field by the alpha-effect supplements significantly its creation by the w-effect, which C. Barenghi argued might be the case during the long periods in which the geomagnetic field maintained one polarity. He also pointed out that the added alpha2-effect need not eliminate the oscillation; it might merely lengthen its period so much that it becomes comparable with the time scale of geomagnetic field reve rsals.
K. Hinata and Y. Hamano (Univ. Tokyo) studied an alpha2omega model with the aim of understanding how the geodynamo mechanism changes over geological time through the thinning of the fluid outer core, brought about by the growth of the solid inner core by freezing during the general cooling of the Earth. They found, in agreement with Barenghi, that, for fixed geometry, the dynamo changes from steady to oscillatory mode as the ratio of the omega-effect to the alpha-effect is increased. More interestingly, if that ratio is fixed and the depth of the fluid core is changed, the dynamo becomes increasingly of oscillatory type. In addition to the geophysical implication of that finding, they conjectured that it may explain why the dynamo in the Sun's thin conve ctive shell is oscillatory while that in Earth's thicker shell is nearly steady.
Mean field kinematic geodynamos are of course much easier subjects for study than either fully 3D kinematic models or fully MHD dynamos. They are nevertheless not trivial to solve, and the possibility of a technique that would simplify the task is of con siderable interest. Such a technique, known as the Maximally Efficient Generation Approximation (or MEGA), has been proposed. M. Kono and S. Starchenko (Univ. Tokyo) reported studies aimed at assessing the accuracy of the approximation by computing alpha- omega models using MEGA and comparing the results with those obtained from the full, unapproximated equations. The agreement between the two was, in all cases, reasonably good. For models in which the alpha- and omega-effects are concentrated in a region of the core, MEGA produces very good estimates of the critical magnetic Reynolds numbers as well as the general shapes of the poloidal and toroidal magnetic fields.
A new technique of a different type is the so-called "mapping method" of T. Nakajima (Univ. Sydney) and P. Roberts. In this approach the computational grid is chosen to simplify the integration of dynamical equations dominated by the Coriolis force, with out sacrificing the simplicity of spherical coordinates when satisfying the electrodynamic conditions at the CMB. At the present meeting they confined themselves to the asymmetric induction equation, and recovered results obtained earlier by Dudley and Ja mes for the G. O. Roberts and Gubbins dynamos as well as for three simple models proposed by themselves.
The full MHD dynamo is so forbidding that a simpler two-dimensional problem has often been studied instead. In such "intermediate models" (IMs), so-called because they are intermediate in difficulty between the kinematic models and the fully 3D MHD model s, an a-effect is invoked to prevent field collapse, but otherwise the models are fully nonlinear. They provide a suitable test bed for the elucidation of the axisymmetric force balance, which has given rise to some controversy. Because the inertial and v iscous forces are so small compared with the Coriolis and magnetic forces, it is tempting to omit them. This is the so-called "magnetostrophic approximation". The resulting equations either have no solution or an infinite number, differing only by the geo strophic flow. (This is a zonal motion independent of the coordinate parallel to the angular velocity of the Earth.) The condition for the existence of a solution is known as "Taylor's constraint", after an influential paper by J. B. Taylor in 1963, and t he resulting solutions are called "Taylor states". An alternative way of satisfying the axisymmetric dynamical balance is by including the coupling of core to mantle, the simplest way being through a weak viscous coupling. This gives rise to a different t ype of IM that was christened "model-Z" by its creator, S. I. Braginsky. Studies of models-Z of aw type were the subject of papers by A. Anufriev (Bulgarian Acad. Sci.), I. Cupal and P. Hejda (Czech Acad. Sci.), by S. Braginsky (Univ. California, Los Ange les) and P. Roberts and by D. Jault (Institut de Physique du Globe, Paris). Braginsky and Roberts aimed at a fairly complete survey of the particular model-Z investigated in 1978 by Braginsky and later by Braginsky and Roberts. They also considered its fi eld structure for large dynamo numbers. (The dynamo number is the product of the Reynolds numbers for the a- and w- effects individually.) The numerical method of Anufriev et al. allowed them to study similar models at larger dynamo numbers. They reported the existence of an unexpected bifurcation at which a new 'weak Taylor state' appeared. The integration of models such as these has in the past proved to be surprisingly difficult. In something of a breakthrough, Jault exhibited both the cause and the cu re. He found that by restoring the inertial forces to the geostrophic flow, i.e. by allowing torsional oscillations, the integrations are numerically much more stable, and can be followed to parameter ranges hitherto found to be impossible to reach. In pa rticular, he exhibited convincing IM solutions of Taylor type. This has done much to restore faith in the Taylor state as a model of the geodynamo, but strictly speaking the question of which balance (if either) obtains in the core remains open. Jault's d iscovery appears to be related to the study by D. Ivers (Univ. Sydney), which indicated that torsional oscillations should be retained even when the magnetostrophic approximation is applied to the ageostrophic part of the motion.
A model that is less challenging than fully 3D MHD models but is more difficult than the IM is the "advanced intermediate model" (AIM), in which just one Fourier mode (in longitude) of the field and motion are added to their axisymmetric components. It is unnecessary in AIM to invoke an a-effect; an axisymmetric emf arises naturally from the intera ction of those Fourier modes. Taking one particular AIM, driven by thermal convection, C. Jones, A. Longbottom (Univ. Exeter) and R. Hollerbach (Los Alamos National Lab.) succeeded in demonstrating dynamo action that in some parameter ranges produced unst eady magnetic fields exhibiting intriguing similarities with the geomagnetic field. For example, the poloidal field produced by their model is of the right order of magnitude and it undergoes periodic reversals, on a time scale long compared with a typica l fluctuation time of the field. From such 2 1/2 D models, it is in principle a short step to fully 3D-models of the geodynamo, but that step requires gigantic computer resources, in order to resolve properly fields and flows that contain many Fourier mod es in longitude.
Despite heavy use of such resources, G. Glatzmaier and P. Roberts had, at the time of the meeting, integrated their 3D convective model for only the order of 2/3 the free decay time, but their field showed every indication of permanence, and displayed fe atures that suggested that some of the preconceptions that have developed in the subject over the past 4 decades will need re-evaluation. For example, in their strong field geodynamo, the magnetic energy is nevertheless on average much the same as the kin etic energy (relative to the rotating frame); the strength of the axisymmetric parts of the toroidal and poloidal fields are comparable; fluid velocities as large as a few cm/sec and fields as large as 80 G arise within the core; it is not the region near the outer core equator but the interior of the tangent cylinder, North and South of the inner core, that is most significant for the regeneration process, etc. The finite electrical conductivity of the mantle and inner core were included in their model, both of which were free to rotate under the electromagnetic and viscous couples to which they are subjected by the fluid core, the net angular momentum of the whole system being conserved. The resulting variations in the length of day were an order of mag nitude too large but varied on roughly the right time scale. The pattern and amplitude of the radial magnetic field at the CMB resembles that of the Earth, but its secular variation is somewhat greater. The solution shows considerable variability in time but, in agreement with R. Hollerbach and C. Jones, they found that, because the inner core has a small electrical time constant (~ 0.01 free decay time), it tends to remove short time scale phenomena that would otherwise be created in the fluid core. Holl erbach and Jones argued that this influences the reversal characteristics of the geomagnetic field. Glatzmaier and Roberts showed a movie of their solution which auto-reversed occasionally when field generation North and South of the inner core happened t o become sufficiently disparate.
There can be little doubt that the fluid core is in a turbulent state and that magnetic fields and fluid motions are present that cover a wide spectrum of different length and time scales. These will not be properly numerically resolved in the foreseeabl e future. S Braginsky and P. Roberts have developed a set of equations that they argue govern the turbulently averaged fields and which model the fluid and solid core as a binary alloy of iron and some light constituent, such as S, Si, or O. (They present ed arguments that tended to favor S or Si.) They succeeded in finding a new and powerful simplification of the anelastic equations governing the fluid motion. In contrast to other recent work, they inferred that thermal convection is probably more potent as a source of convection than is compositional buoyancy created by the release of light component of core alloy at the ICB during the growth of the inner core during the general cooling of the Earth. Regarding the geodynamo as a machine doing useful work , they developed new expressions for its efficiency that incorporated the secular effects of cooling. A cognate investigation was reported by B. Buffett (Univ. British Columbia) and J. Lister (Cambridge Univ.), who presented a model for thermal evolution of the core and estimated the relative importance of the two driving mechanisms. They found that thermally driven dynamos are viable, even in the absence of compositional convection. When both mechanisms operate, the energy supply to the dynamo can increa se significantly. They argued that thermal convection must have been the primary source in the early Earth when the inner core was small or absent, but that compositional convection is the primary source of energy for the geodynamo today. Like Braginsky a nd Roberts, they inferred that the Nusselt number of core convection is probably small even though core convection is strongly supercritical.
The whole basis of the mental pictures which many scientists conjure up in their brains when attempting to visualize the MHD of the core was questioned by P. Lorrain and D. Crossley (McGill Univ.), who argued that the frozen-flux picture of MHD is fatall y flawed. Their argument is based on the premise that, when the conductivity s becomes large, the current density J = sigma(E + V x B) becomes large, and the state E-> -V x B is not realized.
Ultimately, the theory of the Earth's magnetism will have to withstand the test of comparison with observations. The data can be roughly be divided into two parts: the ancient and modern. By 'ancient' we mean that obtained from paleomagnetic studies; by 'modern' we mean that derived from satellites, land-based observatories, and archeomagnetism. The ancient data was reviewed by K. Hoffman (California Poly. State Univ.), who also gave interpretations in the light of modern geodynamo theory. Although recen t paleomagnetic data favor a preferred orientation of the geomagnetic axis during polarity reversals, Hoffman argued that this might be an artifact partly caused by the time-averaging intrinsic to sedimentary data. He showed, using information extracted f rom volcanic rocks only, that the virtual geomagnetic poles show a tendency to remain during reversals in some areas for long periods (e.g. NW Australia) and do not show any preferred longitude band. Hoffman concluded that some agency must exist that cont rols CMB boundary conductivity. He also emphasized that the existence of a slower evolution of the magnetic field in the form of the change in average reversal rate is well confirmed. This rate is currently about 5/million years, but was essentially zero in the Cretaceous (about 100 Myr ago) and in the Permian (about 250 Myr ago). This also suggests that slowly changing conditions in the lower mantle are exerting significant effects on the geodynamo mechanism.
C. Johnson and C. Constable (Univ. California, San Diego) addressed the question of the character of the long-term field as observed in the paleomagnetic record: does it have any long-term biases not obvious in the governing equations? Although the data are not sufficient to make definitive statements, they find the evidence is strongest for the "far-sided" effect, in which the virtual geomagnetic pole occurs opposite to the geographic pole. Turning now to the modern data, V. Golovkov (Izmiran, Troitsk) presented the results of recent study that utilized four centuries of data from Veinberg's catalog. The perplexing stability of the geomagnetic field in North America was confirmed, e.g. the zero declination isoline did not move during this time, to the a ccuracy of the data and analysis. He contrasted this large but unchanging anomaly with that under Siberia.
To make reliable contact between the surface and near-surface observations of the field and the geodynamo, it is necessary to extrapolate those observations down to the core surface. A. Goodacre (Ottawa) showed, with some simplifying assumptions, that th e core fluid motion can be shown to resemble the present-day plate motions on the surface of the Earth. He thus concluded that the secondary motions in the core reflect or is influenced by the convection in the lower mantle. The general philosophy of extr apolating the surface field to the CMB was analyzed by W. Webers (GeoForschungCentrum, Potsdam), who paid particular attention to the regularization of the extrapolated field.
A. Jackson (Oxford Univ.) used such extrapolations, and an assumed tangentially geostrophic force balance, to infer core surface motions. He found evidence for East-West oscillations below the western Pacific and large changes in zonal flow energy at the end of the last century, which he suggested were associated with the large change in the length of the day at that time. Most encouraging was the degree to which his model fitted the observatory data. The entire field of extrapolation and core surface mo tion was reviewed by C. Voorhies (NASA Goddard), who paid particular attention to the successes and shortcomings of the steady-flow approximation and the steady-acceleration approximation, in which it is supposed that, superimposed on a steady surface flo w, there is also a constantly accelerating part. He described results obtained by using the Definitive Geomagnetic Reference Field. He reported that steadily accelerating flows gave a good account of the field, and provided estimates of the rms flow speed (7.50 km/yr.) and of the rms acceleration (0.183 km/y2). He described what has been done to include both mantle conductivity and an assumed steady diffusion of magnetic flux in the core fluid at the CMB.
These studies rest largely on the properties of the electromagnetic equations, i.e. the kinematic approach to geomagnetic data analysis; except for the tangentially geostrophic approximation, dynamic aspects are ignored. A more ambitious attack was made by M. Matsushima (Tokyo Inst. Tech.) who, like the above authors, tried to determine the core surface motions, but who used the full MHD equations to obtain a model consistent with the present geomagnetic field and its secular variation. Only the shape of the poloidal velocity field was assumed in this work. By this approach, he could obtain core surface motions that resemble those described by Jackson and Voorhies. He could also estimate the rms field strength in the core. That of the toroidal field was about 20 Gauss, so that his model is of alpha2omega-type. J. Love and D. Gubbins (Univ. Leeds) argued that the geodynamo would minimize the ohmic dissipation implied by the induction equation. They fitted such a model to the observations in order to learn something about both the motions in the core and the geodynamo.
An attempt to bridge the gap between the ancient and the modern was made by G. Hulot, et al (Institut de Physique du Globe, Paris) who proposed a stochastic model of the geomagnetic field. This model assumes an axial dipole that changes only slowly with time, superimposed on which is a field varying rapidly, i.e. the secular-variation time scale. They used recent and archeomagnetic data to define the statistics of their model and then used those statistics to produce numerical simulations of the field ov er a period of 105 yr., which they found were not unlike the temporal behavior of sedimentary data. The numerical simulations also showed occasional "excursions" reminiscent of the excursions seen in sedimentary data. This suggests that, though infrequent , excursions are a pervasive characteristic of the geomagnetic field.
Contributed by Masaru Kono (Univ. Tokyo) and Paul Roberts (Univ.
California, Los Angeles)
The poster session covered many related topics: Earth evolution, MORB-source depletion in the upper mantle; pooling of subducted crustal material at the CMB; layering in the mantle and the source of hotspots; plume interactions and spatial variations in convection experiments; laboratory and numerical models of subducted slabs and subduction zones; tilting of continental cratons due to subduction; chemically distinct regions in the mantle; effects of cooling at the CMB; effects of curvature on three-dime nsional convection models; and the effect of continents in controlling Wilson cycle flow reversals in the mantle convective circulation.
Contributed by Gary Jarvis, York Univ.
A follow-up presentation by J. Bloxham and W. Kuang (Harvard Univ.) emphasized the practical difficulties that arise in the direct calculation of topographic coupling using estimates of the fluid flow at the core surface. It was noted that the topographi c torque vanishes for a core in geostrophic balance and thus the ageostrophic part of the flow is responsible for topographic coupling. Based on the time scale of angular momentum transfer, it was argued that the magnetic field is essential to this transf er of angular momentum. Using a numerical model they found that the topographic couple is very sensitive to the magnetic field within the core. Optimism was expressed for the prospect of placing constraints on the magnetic field using observations of the decadal variations in the length of day.
A somewhat different point of view was expressed by S. Yoshida and Y. Hamano (Univ. Tokyo) who calculated the geomagnetic field variations that might arise from changes in the length of day. They assumed that the effects of core-mantle boundary undulatio ns allow length-of-day variations to cause geomagnetic variations and attempted to explain the decadal variation of the sectoral component of the magnetic field. They inferred both the boundary topography and toroidal magnetic field required to reproduce the observed variations in the sectoral components. The estimated boundary topography was well correlated with the pattern of seismic heterogeneity in the lower mantle and the toroidal field strength was approximately 150 G. The length-of-day variations w ere attributed to climate change, although questions were raised about whether the atmosphere possessed sufficient angular momentum to cause the decadal variations.
Possible connections between climate, length of day and the magnetic field were revisited in a poster presentation by H. Greiner-Mai and H. Jochmann (GeoForschungZentrum, Potsdam). Similarities in the spectra of global temperature and magnetic field sugg ested a possible influence of the magnetic field on long-term climate changes. Changes in the length of day at periods of roughly 70 years and longer were attributed to core-mantle coupling, while changes over shorter periods were thought to be caused by atmospheric excitations. Identifying the nature of the physical interactions between the magnetic field and climate was emphasized as a key area of future research.
Another study relating Earth rotation and magnetic field variations was presented by Y. Yokoyama (Univ. Industrial Technology). Two main decadal variations with periods of 30 and 60 years are observed in both rotation and field, which implies core-mantle coupling on these time scales. While the 60-year rotational variation is due mainly to an axial torque, the 30-year variation is due predominantly to an equatorial torque. The corresponding 30-year magnetic variation exhibits a 180-degree phase change wh ich is not evident in the Earth rotation variations. It was indicated that these features provide useful constraints on the possible core-mantle coupling mechanisms.
In a related study, H. Greiner-Mai examined the possibility of non-axial rotations of the outer core based on geomagnetic variations. The interpretations were based on similarities between the spectra of the magnetic field variations and polar motion. It was argued on the basis of the spectra that coupling responsible for the polar motion could not result from magnetic torques, presumably because the non-axial rotation of the outer core was too small. A more effective coupling mechanism was required to a ccount for the observations.
Improvements in the analysis of Earth rotation observations were also discussed in this session. The poster by R. Gross (Jet Propulsion Lab.) examined the diverse measurement techniques used to determine Earth rotation and how these measurements could be combined to generate a time series that spans the longest possible interval. In this study, all available independent observations of the Earth's rotation were combined in a self-consistent manner that took into account the diverse nature of the raw obse rvations. The decadal variations were isolated from the combined Earth rotation series, and the torques that must act on the solid Earth to generate these observed decade-scale variations were computed.
Several presentations dealt with theoretical or experimental studies of fluid flow in the vicinity of the CMB. B. Buffett (Univ. British Columbia) presented a calculation of the magnetic field induced by the flow of the core past a bumpy mantle with fini te conductivity. The induced magnetic field was shown to be as large as 1 - 2 G for typical seismic estimates of CMB topography. The wavelength of the induced field was determined by the wavelength of the boundary topography and it was suggested that the induction due to long-wavelength topography might contribute the apparently static features observed in the magnetic field.
R. Davis (Leeds Univ.) and K. Whaler (Univ. Edinburgh) presented some preliminary experimental investigations of the spin-up of a neutrally buoyant fluid. Particular attention was given to the role of boundary topography. The experiments showed that mid- and high-latitude topography had the largest effects, producing Taylor columns for westward flow and damped Rossby waves for eastward flow. Strong time-dependence was also observed for westward flow with the detachment of patches of cyclonic vorticity an d cyclonic-anticyclonic vortex pair interaction over the topography. Possible connects between the experimental results and the core flow inferred from secular variation were noted. Improved estimates of the core flow were then obtained from the secular-v ariation coefficients by making a simple extension of the steady-flow approximation. They permitted the steady velocity field to drift with respect to a frame of reference fixed to the mantle and obtained improved fits to the secular variation coefficient s. In particular they found that the largest improvements were produced over a 20 year interval centered on the 1970 magnetic jerk.
A theoretical study by K. Moffatt (Univ. Cambridge), D. Loper, and H. Shimizu (Florida State Univ.) investigated the horizontal velocity and poloidal magnetic field produced at the top of the core by a small buoyant parcel in the uppermost core in the li mit of small Rossby number and small magnetic Reynolds number. Viscous effects were neglected, leaving one dynamic parameter, the Elsasser number (denoted as 1/N2), measuring the strength of the Lorentz force relative to the Coriolis force. In the limit o f strong Coriolis force, a localized buoyancy distribution produces a disturbance of velocity and magnetic field extending a distance 1/N2 in the direction of the angular velocity vector (a foreshortened Taylor column), while the limit of strong Lorentz f orce, a localized buoyancy distribution produces a disturbance of velocity and magnetic field extending a distance 1/N in the direction of the applied magnetic field. The linearized problem of a Gaussian buoyant parcel near the CMB was analyzed using a do uble Fourier transform in the limit of strong Coriolis force. The horizontal velocity distribution at the CMB is found to be dominantly geostrophic in this limit (i.e., with no horizontal divergence to dominant order), and the poloidal field produced by t he upwelling parcel is small, of order 1/N2 times the magnetic Reynolds number.
C. Denis (Univ. Liége) examined the question of boundary conditions at the CMB for tidal deformations and seismic normal mode spectra. It was shown that the choice of free-slip or no-slip boundary conditions at a fluid-solid interface can cause large dif ferences in the calculated results. The calculations indicated that the choice of boundary conditions at the CMB significantly altered certain normal modes and the load (Love) numbers. The choice of boundary conditions at the inner core boundary was found to be less important.
In a related study, C. Denis and Y. Rogister (Univ. Liége) considered the static deformation of the Earth and the liquid-core paradox. The paradox was first noted by Longman who showed that static solutions require either a solenoidal displacement field in the core or a neutral stratification. Physical and mathematical arguments were presented to identify the source of the paradox and to reveal the shortcomings of the concept of static Love numbers in planets with fluid cores.
V. Zharov and N. Chujkova (Sternberg State Astronomical Inst.) examined the possible consequence of convective flow on the position of the inner core. Flow along the rotation axis in the fluid core might lift the inner core, leading to a change in the co re's angular momentum and a variation in the Earth's gravity field. The angular momentum changes would be most easily detected in polar motion observations. Quantitative predictions were made for two possible situations.
A global dynamic view was presented by A. Vogel (Free Univ. Berlin). An attempt was made to construct a pattern of mantle-wide convection which relates the dynamics of the core-mantle boundary with lithospheric structures. The idea of convection tectonic s was introduced as a conceptual framework to explain the entirety and complexity of global geotectonic processes.
C. Denis and P. Varga (Geodetic and Geophysical Research Inst., Sopron) presented some new information which indicated that the deceleration of the Earth in the Proterozoic was significantly smaller than it was in the Paleozoic. These new data solve the old problem of the Moon being within the Roche limit in too near a geological past, but raise the question of why the rate of deceleration should be so small over much of Earth's history. Changes in the ellipticity of the Earth, which accompany changes in the Earth's rotation, were calculated as a function of geological time. The slow decrease in the surface flattening builds up significant membrane stress in the layer situated above the isotherm 600 C. It was suggested that this stress might be responsib le for the original breakup of the surface layer into lithospheric plates.
The long-standing issue of Chandler-wobble excitation was examined by M. Furuya, Y. Hamano (Univ. Tokyo) and I. Naito (National Astronomical Observatory, Mizusawa). Although the atmospheric role is generally favored, its contribution to the Chandler is s till estimated to be insufficient and other sources, such as seasonal redistribution of water mass, are invoked to make up the shortfall. In this study, however, the authors demonstrate that the Chandler wobble is resonantly excited by atmospheric winds. The non-seasonal wobble was determined using the atmospheric angular momentum variation and the results were compared with the observed wobble. The wind-induced wobble yielded the correct amplitude when the Chandler period was adjusted to 430.3 days and t he predicted phase agrees well with the observed phase. Atmospheric mass redistribution, which has been believed to be the primary candidate for atmospheric excitation, acted to cancel out the overshoot of the wind effect.
The session was concluded with the invited oral presentation of R. Boehler (Max-Planck-Institut für Chemie, Mainz) on core-mantle melting and chemical interactions. A review of the melting results on iron showed that large systematic deviations exist between static and shock experiments. Shock temperature measurements are still subject to large experimental uncertainties and there is evidence of overshoot from shock measurements at lower pressures. Static measurements from three groups show excellent agreement and extrapolation of his own measurements to 2 Mbars yields a temperature at the inner core - outer core boundary of slightly below 5000 K. Various amounts of oxygen do not seem to lower the melting point of pure iron at pressures of the outer core. These measurements result in a temperature jump across the core-mantle boundary of at least 1300 K. This may not effect the viscosity in the lower mantle to the extent previously assumed because the melting measurements on (Mg,Fe)SiO3 perovskite yie ld extremely high melting temperatures for the lower mantle, in excess of 7000 K. New measurements on MgO and (Mg,Fe)O show a surprising crossing of their melting curves with that of perovskite near 500 kbar, lowering the upper bounds for the solidus at t he core-mantle boundary to about 5000 K. It was shown that MgO and MgSiO3 perovskite react entirely differently with molten iron at the P-T conditions of the core-mantle boundary. Dry MgSiO3 seems to be inert to molten iron whereas MgO qualitatively shows strong reactions. This could lead to a dense, iron-rich magnesiowustite phase that could gravitationally segregate to the bottom of the lower mantle and may provide an alternative explanation of the change in physical properties in the D''region.
Contributed by B. Buffett, Univ. British Columbia.
D. G. Isaak, O. L. Anderson, K. Masuda and D. Guo (Univ. California, Los Angeles) considered the uniqueness of the assumption that iron in the inner core is in the hcp phase and examined the hypothesis that the inner core could be in the fcc phase. Altho ugh the theoretical models developed by Cohen and Stixrude give virtually the same densities at high P (and low T) for both phases, is does not necessarily follow that these two phases would have the same density at high P and high T. The thermal pressure arising from vibrational energy has to be considered in order to get the correct temperature effect. Experimental work on the bulk modulus of fcc iron at low P and high T (1400 K), along with estimates of the thermal expansivity, allow an estimate of the thermal equation of state of iron in the fcc phase. From this work, it is found that fcc iron at inner-core conditions might have the same density as hcp iron, or it could be a few tenths of a gram less dense because of uncertainties in the extrapolation s. Further work is needed, but the possibility remains that the amount of light impurities in the core is less than that calculated by Jephcoat and Olson.
X. Song and D. Helmberger (California Inst. Tech.) gave an update
on the reported anisotropy of seismic waves in the inner core. They confirmed
earlier reports of an axisymmetric anisotropy of about 3%, the fast direction
being parallel to the Earth's ro tation axis. They explored the depth dependence
of the anisotropy and found a 1% velocity increase at the top 300 km and
a 3% increase below, indicating a layered structure at the equator of the
O. Anderson, K. Masuda and D. Guo reported on the calculation
of thermal expansivity, a, at high P and T for silicate perovskite, using
their new thermodynamic theory. They found that the isotherms of a(P) converge
at high P (about) 40 GPa), making a at high P independent of temperature.
This convergence makes the value of a at pressures throughout the lower
mantle independent of the model parameters: about 2 x 10-5 K-1 at the top
of the lower mantle and about 1 x 10-5 K-1 at the bottom. Thus one can
spe ak of a consensus on a at lower-mantle conditions from one thermodynamic
model to another. For other physical properties, such as, for example,
the isentropic density distribution, there is no consensus of values from
one model to another. The value of th e Grčneisen parameter, g, is found
to be critical to the calculations, and future research on physical properties
will center on finding g for silicate perovskite.
S. Karato (Univ. Minnesota) discussed the physical properties that, from his viewpoint, most affect mantle dynamics: density and rheology. He pointed out that while rheology is important, many dynamic studies consider only density contrasts - a problem a rising from the paucity of rheological data.
Recent rheological measurements (up to the equivalent of 300 km depth) have made it possible to calculate viscosity due to diffusion creep, especially in olivine. Studies of the degree of softening (arising from changes in grain size) associated with pha se changes have been very revealing. These experimental studies have shown that crystal structure may be more important than density in controlling rheological properties. A significant grain-size reduction leads to a significant rheological weakening. Th is has led to some very interesting hypotheses about the dynamics of the subducting oceanic lithosphere. (The reporter would add two other physical properties that are important to mantle dynamics: thermal expansivity - quite apart from density - and phas e changes.)
W. R. Peltier and X. Xiang (Univ. Toronto) discussed the nature
of three variables that constrain the viscosity of the lower half of the
lower mantle. Two are associated with the non-tidal acceleration of planetary
rotation, and one is associated with th e ongoing secular drift of the
pole of rotation towards Greenland. They discussed the extent to which
the relatively high viscosity in the lowermost part of the lower mantle
required by the good data is allowed by the non-tidal acceleration of rotation
an d polar wander measurements (best explained by post-glacial rebound).
P. Wu (Univ. Calgary) discussed post-glacial rebound in terms
of non-linear rheology. Laboratory results indicate that the upper mantle
may deform under a power-law creep rheology, while linear rheology is sufficient
to explain the observations of reboun d. An ongoing investigation is exploring
the possibility that the rheology if the whole mantle may be seen as obeying
a linear law, even though the creep law for individual grains may be non-linear.
Contributed by O. Anderson, Univ. California, Los Angeles.
The link with large scale dynamics is also strengthened, with more joint inversions [A. Forte (Institut de Physique du Globe, Paris) and R. Woodward (US Geological Survey, Albuquerque); P. Shearer & J. P. Morgan (Univ. California, San Diego)]. With the p rovocative title "Global seismic tomography - what use is it anyway?", G. Masters (Univ. California, San Diego) offered a very fair and complete review of the important findings and limitations of global tomography. By showing results of synthetic tomogra phic inversions performed on numerical convection models (computed by Tackley et al.), he gave the start of a new era, in which seismologists and dynamicists understand and accept each other's strengths and weaknesses, an approach also advocated by H.-C. Nataf, Y. Ricard (École Normale Supérieure), J.-P. Montagner and P. Lognonné (Institut de Physique du Globe, Paris). Contributions in large-scale dynamics included investigation of convection in presence of heterogeneities in boundaries [K. Kurita and M. Kuri (Univ. Tsukuba)] and plume trajectories in a convecting mantle [R. O'Connell and B. Steinberger (Harvard Univ.)].
Much of the discussion was around the effect of the endothermic phase transition at 660 km. It is clearly recognized as one of the major ingredients of mantle convection. The key question is: how much layering does it produce in the real Earth? And the m easure of layering can very well be different for various phenomena: vertical flow at short- and long-wavelength, temperature drop across the thermal boundary layer, dynamic topography, etc. From the modeling perspective, there are two big question marks: the treatment of the kinetics of the transition, and the influence of subducting plates on the overall behavior. From the observational point of view, several contributions indicated a better agreement of seismological results with a rather permeable 660 km boundary. Nevertheless, the transition zone remains one of the least resolved region of the mantle.
Efforts were also directed at considering self-consistent models, in which all the consequences of layering are introduced (thermal boundary layers, and the effect on the viscosity profile, deflection of the boundary, etc.).
Overall, a very constructive session, well in the multidisciplinary spirit of SEDI. However, the debate would have benefited from views of geochemists and regional tomographers, who were not present.
Contributed by Henri-Claude Nataf, École Normale Supérieure.
R. Kind (Potsdam) and collaborators, using converted phases and underside reflection studies, presented evidence for localized sharpness of the 410 km boundary (< 5 km) but reported that the 600 km boundary appeared more gradual (similar to 20 km). Bound aries were also seen at 220 km and 900 km in at least some areas but not at 520 km. A. Yamazaki and K. Hirahara (Kyoto) found sharp boundaries at both 660 km and 410 km near the Tonga subduction zone. V. S. Solomatov and D. J. Stevenson (California Inst. Tech.) considered ways to produce a sharp 660 km boundary, arguing that boundary sharpness was not explicable in terms of equilibrium thermodynamics but must involve kinetic effects of hysteresis in the phase transitions. C. A. Stewart (New York) consider ed the kinetic delay in transformation of olivine to be controlled by deviatoric stress. Inhibition to convection through 660 km by the negative Clapeyron slope of the phase transition at that depth is accepted as the principal cause of slab compression a nd deformation as observed by S.-X. Zhang and A. Y. Li under Izu-Bonin. C. R. Bina (Northwestern Univ.) and G. R. Hellfrich suggested that the Clapeyron slope of the 660 km transition is less than that at 10 km although undulations of the 660 km boundary appear to be greater. It may be possible to reconcile these apparently conflicting observations in terms of the kinetic effects discussed by other speakers. There is also the important difference that the Clapeyron slopes are opposites in sign; the 660 km transition inhibits convection but the 410 km transition assists it. No speakers referred specifically to the entropies of the transitions. A Clapeyron slope of any sign and magnitude is only important if the entropy and volume changes are substantial. I t is evident that we are still groping for a satisfying treatment of the effects of phase transitions on mantle convection. Neither can we yet claim to have resolved the question of a compositional variation across 660 km. R. J. Hemley and C. R. Bina (Nor thwestern Univ.) reported that Fe enrichment of the lower mantle was disallowed but Si enrichment was possible. However, there were some unresolved questions about relevant equations of state and I. N. S. Jackson (Australian National Univ.) showed that ne ither equations of state nor earth-model results were yet adequate to settle the composition question.
Two papers argued for substantial undulation of the core-mantle boundary. A. M. Forte and A. M. Dziewonski (Harvard Univ.) interpreted the lower-mantle velocity anomalies in recent tomographic models to relate directly to density variations and hence to a convective flow pattern that would cause substantial (plus/minus 6 km) boundary topography. J. Bloxham and W. Kuang (Harvard Univ.) found that undulations of several kilometers were needed to explain observed decade-scale length-of-day variations by cor e-mantle coupling. They pointed out that conventional hydrodynamic core-mantle coupling had been misunderstood and could not have the required effect, but they developed instead a theory of magneto-topographical coupling. If electromagnetic core-mantle co upling is found to be clearly inadequate then such topography will be unavoidable but is not yet universally agreed to be necessary. Direct observation remains elusive except for the excess ellipticity. P. M. Mathews and I. I. Shapiro convincingly confirm ed the 0.5 km excess bulge of the core-mantle boundary (CMB) first reported by T. Herring and others on the basis of forced nutation observations by V.L.B.I.
There is now general agreement that the layer immediately above the CMB (D'') differs from the overlying mantle and that the difference is not explicable simply as a thermal boundary layer, although the necessity for core cooling demands that D''includes a thermal boundary. The nature of D''is subject to widely differing ideas. A crucial question is the sharpness and depth variability of the top of D''. H.-C. Nataf and S. Houard (École Normale Supérieure) suggested that a phase transition is probably res ponsible. If so it should be marked by a discontinuity in properties and should be a global feature. This is given some support by the widespread observation of D''reflections, although, in common with other observers Nataf and Houard have not detected re flections everywhere. T. Shibutani (Kyoto Univ.) and collaborators gave additional support for the D''discontinuity with evidence of a 1% to 1.5% velocity jump near 289 km above the CMB under the Western Pacific. G. Poupinet (Grenoble) and A. Souriau soug ht evidence of rapid lateral variations within D''that would give inclined areas of the upper boundary and cause complex reflections of the kind analyzed by J. Neuberg and T. Ponter (Univ. Leeds). J. M. Kendall and P. M. Shearer (Univ. California, San Die go) found D''thickness ranging between 150 km and 250 km, which appears difficult to reconcile with a phase transition, and perhaps favors lateral thermal and chemical heterogeneity. Also, we should still allow that a distinct D''layer may be completely a bsent from some areas. Substantial lateral velocity variations near the base of the mantle reported by R. Valnzuela et al. (St. Louis Univ.) are consistent with very strong lateral gradients in D''structure that may obscure any radial stratification. The difficulties in interpreting the D''region are compounded by the possibility that D''is anisotropic, which was considered by V. Maupin (Oslo).
Ideas about the origin of D''are also diverse. D. J. Stevenson (California Inst. Tech.) considered several alternatives, dismissing all but the suggestion that D''contains remnants of subducted slabs. But no more than 10% of the slab material can reach t he core-mantle boundary, without requiring too high a fraction of the geothermal flux to originate in the core and then where is the other 90%? Can we expect cool slab material to survive intact to this depth? Another suggestion that has been the subject of strenuous debate in recent years is a contribution to D''by chemical reaction across the CMB. T. J. Ahrens and X. Song (California Inst. Tech.) gently rewarmed the discussion by showing that a reaction in which iron is lost by pervoskite and free SiO2 is formed is thermodynamically admissible. A. M. Dziewonski and W.-J. Su reported that tomographic anomalies in D''were correlated with those above it and questioned the distinctness of D''. The commonly assumed but not universally agree idea that D''is t he source of plume was implicit in a 3-D numerical model study by P. E. van Keken and C. W. Gable (Los Alamos National Lab.) of the interaction of a plume with a boundary marking a strong viscosity contrast (660 km). They found plume pulsation, postulated to account for the production of chains of discrete islands by hot-spots, to occur only with high viscosity contrasts.
J. Troup (Harvard Univ.) presented a new analysis, with a larger number of free-oscillation modes than hitherto, of the mode splitting by inner-core anisotropy. He matched the data to a model having varying anisotropy within the inner core and found that it was consistent also with observations that P waves are faster in the axial direction than in equatorial directions. The reality of inner-core anisotropy cannot now be doubted. Its cause is a matter of fundamental interest. The most probable explanatio n to date appears to be one presented at the Mizusawa SEDI meeting by M. Kumazawa, who suggested that growth of the inner core by solidification of outer-core material occurred more strongly at the equator and that the resulting excess ellipticity of the inner core continuously relaxed towards equilibrium, causing crystal alignment by the deformation. Plausibility of this process relies upon a persistent difference between the outer-core circulations in equatorial and polar regions. That such a difference is expected was shown by G. A. Glatzmaier (Los Alamos National Lab.) and P. L. Olson (Johns Hopkins Univ.) who reported numerical simulations of magneto-convection in the core with different behavior within and outside an axial cylinder tangent to the in ner core. It remains to show that Kumazawa's theory is consistent with Troup's model.
Attenuation in the inner core is much stronger than was supposed until recently. A. Souriau and P. Roudil (Toulouse) found Q < 200 in the outer 300 km of the inner core for waves in the frequency range 0.1 Hz to 1 Hz. There appears to be an absorption pe ak at about 1 Hz. It is unlikely that the much higher Qs at free oscillation periods can be dramatically revised downwards. Thus the strong frequency-dependence of Q in the inner core is believed to be real. That a low Q in hot iron at seismic frequencies is to be expected was the conclusion of laboratory studies by I. N. S. Jackson (Australian National Univ.). For iron of two difference carbon contents at 1200 degrees C and 300 MPa he found not only very low Q but a corresponding strong frequency depende nce of the shear modulus. At 1 s period the value was only half that at ultrasonic frequencies and at 300 s it was a further 16% less. Not many years ago the high value of Poisson's ratio for the inner core appeared paradoxical. Now we have more explanati ons than we need!
Contributed by Frank Stacey (Univ. Queensland) and Thorne Lay
(Univ. California, Santa Barbara).
One main part of the symposium contained investigations on the formation and evolution of the planet Earth on a global scale. D. J. Stevenson (together with V. S. Solomatov) presented both an overview on the modeling of the earliest Earth and new results on the physics of crystallisation of a terrestrial magma ocean. Stevenson pointed out that giant impacts could transport energy deep into the interior and make it relatively hotter than upper regions. Melting and vigorous convection can produce crystalli sation without substantial chemical differentiation of the deep mantle, and isotope systematics may preserve a record of this period. C. A. Steward, M. R. Rampino, and C. Robinson presented a new possibility for the triggering of a mantle plume: according to their results, a giant meteorite impact can produce a shock wave front that produces peak pressures and deviatoric stress at 400 km depth that can significantly enhance the kinetics of nucleation and growth of spinel in olivine. This transformation an d the corresponding back transformation result in a dissipation of the mechanical shock energy as heat that can influence plume location in the convectively unstable mantle. R. Honda, H. Mizurani, and T. Yamamoto investigated core formation simultaneously with Earth accretion and showed how iron drops could sink through plastic flow such that a core would be mostly completed by the end of the accretion process.
The effect of water-dependent creep rate on the volatile exchange between mantle and surface reservoirs was modeled by S. Franck and C. H. Thuermer. They found a rapid outgassing of mantle water within a time scale of less than 200 Myr. Such an event is supported by geochemical investigations of noble-gas isotopic ratios. The importance of deep fluid flow for the chemical composition and structure of the Earth's crust and mantle was shown by M. V. Rodkin. He discussed Benioff zones and processes at the A rchaeozoic-Proterozoic boundary as examples of the resulting changes in composition. P. J. Tackley, D. J. Stevenson, G. A. Glatzmaier, and G. Schubert investigated 3-dimensional spherical models of mantle convection that include the phase transitions at 4 00 km and 670 km depth. They varied the Clapeyron slope for these transitions within the current range of uncertainty and demonstrated the accumulation of cold material, huge catastrophic "avalanches", and broad cylindrical downwellings to the base of the mantle. M. F. Osmaston presented a new view of the global evolution of the Earth's interior taking into account the effect that under certain conditions silicate melts will be denser that the local solid mantle and therefore will gravitate downward. He p roposed a new 4-step scenario for the Earth history with the occurrence of subduction-like processes very early after planetary formation.
The interpretation of seismic-velocity heterogeneities not only as lateral temperature variations but also as consequences of possible significant chemical heterogeneities was given by A. M. Forte, A, M. Dziewonski, and R. J. O'Connell. They found that i t is possible to divide certain tomographic models into one part correlated with differences between continents and oceans (extending to depths exceeding 400 km) and another part of primarily thermal origin. V. A. Nikolaev estimated the world stress field using tidal triggering by taking into account the difference of earthquake numbers occurring during compressive and tensile phases of the tidal vector modulus and its components. He showed that the temporal variation of the predominant phases distributio n hints at a dominantly West-East tension during this century.
There were further contributions related mainly to the present states of certain parts of the Earth's interior, to the discussion of regional effects, and to inferences from laboratory models. Thus, from laboratory studies T. Yagi and H. Yusa showed that a new, unquenchable high-pressure form of Ca3A12Si04 is stable in the orthorhombic perovskite crystal structure above 30 GPa; with a bulk modulus of 288 GPa it is a possible lower-mantle phase for calcium. From shock wave equations of state T. J. Ahrens and T. Duffy obtained thermal expansion coefficients of perovskite and perovskite + magnesiowustite crystal structures at pressures above 100 GPa that favor iron and silica enrichment in the lower mantle. As a caution to over-interpretation of laboratory data S. Rigden and I. Jackson demonstrated that systematic errors can enter when, say, compositional models extrapolated from laboratory conditions have densities differing by less than 1%. F. Mulargia and F. Quareni showed that, at temperatures only half that of the melting temperature, the leading-order anharmonic parameters differ from the quasi-harmonic terms by as much as tens of percent in the Grč§neisen parameter and specific heat, again indicating the importance of anharmonicity in equations of st ate applied to the deep Earth.
Considering electrical conductivity, T. J. Shankland, J.-P. Poirier, and J. Peyronneau demonstrated that conductivities of perovskite and perovskite + magnesiowustite measured in diamond-anvil cells match those determined from electromagnetic induction s tudies for the uppermost mantle; the parameters used in extrapolation and interpolation leave so little room for temperature or pressure to exert much influence that any lateral conductivity variations might better be attributed to compositional rather th an thermal effects.
After extensive corrections for electrical currents induced around New Zealand H. W. Dosso, J. Chen, C. J. Bromley, F. H. Charnalaun, M. R. Ingham, and D. McKnight found anomalous electrical conductors associated with faults and local geological features related to the plate subduction boundary. H. Muller compared laboratory measurements of compressional and shear-wave velocities made at crustal temperature and pressure of rocks from the Erzgebirge in Saxony with a seismic profile in the region to produc e a velocity-depth model and to infer the probable partial melting in the deep crust of this region. E. S. Husebye and J. E. Lie described how 1730 km of reflection profiling beneath the Skagerrak Sea defined a seismic reflector indicative of a structural fabric created during the Proterozoic; reactivation of this zone of weakness during formation of the Oslo Rift could explain the presumed simple shear extension of the Skagerrak Graben. Because the usual elastic or isostatic models to explain the support mechanism of seamounts produces a lithosphere that appears to be too thick, Y. Tomoda and Y. Akiyama developed a model in which young seamounts are dynamically supported by their underlying, less dense asthenosphere.
A conference volume to be published in Physics of the Earth and Planetary Interiors is planned.
Reported by S. Franck (Univ. Potsdam) and T. J. Shankland (Los
Alamos National Lab.).
Presentations by Ritzwoller et al., and Rodgers et al. addressed the issue of resolving internal boundary topography from volumetric heterogeneities. They argued that a combination of normal-mode data and body-wave travel times is necessary to separate t he two effects, while differential body-wave travel times, such as SS-S, are particularly unable to make the difference. This problem, as well as the concern of mapping heterogeneities at the wrong depth, because of surficial effects, was mentioned in sev eral presentations.
One way to constrain shallow layers is to use shorter period (30 s) surface waves. This was the approach followed by Trampert and Woodhouse, and by Tromp and Ekstrom. Improvements on the data side also included mapping VS/VP and attenuation (Woodhouse an d Robertson).
Theoretical advances were presented by Clevede and Lognonne, who found a way to invert seismograms efficiently by using multiplet modulation functions, while Li and Romanowicz perform a non-linear waveform inversion considering cross-modal coupling. Bias es of tomographic inversion were pointed out by Trampert and Snieder, who propose ways to reduce spectral leakage.
The link between tomography and geodynamics was the topic of several talks of that session. A model constrained by the two ends was proposed by Forte et al., while Maruyama et al., and Corrieu et al., pointed out the link between past tectonics and prese nt heterogeneities. Particularly impressive is the correspondence between lower-mantle fast anomalies and the inferred position of old slabs. Dziewonski & Su showed a model with slow anomalies extending very deep into the mantle, beneath ridges.
The base of the mantle is found to be very heterogeneous, and its importance for understanding mantle dynamics was pointed out by Obayashi et al., and Wysession.
Impressive computations of convection in a 3D box with variable viscosity were presented by Tackley.
Lithosphere, asthenosphere, ridges and hotspots
Lay and coworkers reviewed recent advances in the mapping of fine structures beneath the lithosphere, and pointed out an apparent connection at depth between ridges and some hotspots, and a correlation between VS anomalies and surface volume flux. At a s maller scale, Achauer et al. compared VP and density anomalies in two different rift environments. They find that in the Kenyan rift, velocity anomalies are very large (± 10%), implying a lower density/velocity variation than usually assumed. Both talks r aised the question of relating seismic velocity to temperature. This was taken up by Jackson and Fitz Gerald, who measured the shear modulus of dunite over a large range of frequencies. The results are fundamental for relating seismic velocity and attenua tion anomalies to temperature. Anelasticity is shown to increase the sensitivity of shear modulus to temperature by at least 50%, when going from ultrasonic to seismic frequencies.
Several new models of regional tomography were presented, from the scale of Mount Erebus (Luo and Dibble) to that of Antarctica (Roult et al.), in many different tectonic environments (Taiwan: Ma and Zhao; Iceland: Kaban et al.; Central Asia: Zielhuis; A ustralia: Vahau; etc.). An iterative non-linear travel-time tomography method was tested by Kamiya & Koketsu in the Japanese region.
Several studies pertained to the dynamics of hotspot plumes, from tail to head, with particular emphasis on the time variations (Ribe and Christensen; Houseman; Bercovici; Theissing and Spohn).
Phase transitions and slab dynamics
Van der Hilst and Engdahl reviewed how recent tomographic images of slabs have greatly enriched our vision of mantle dynamics. They proposed a simple scenario, illustrated by laboratory slab experiments, in which the morphology of slabs is controlled by trench retreat and an increase of viscosity at the 660 km discontinuity. Fixed trenches produce slabs that go straight into the lower mantle, while retreating trenches causes the slab to lay down at the discontinuity. A similar unifying idea was behind th e models of Yamanaka and Seno, who give a good fit to stress distribution in slabs, provided the lower-mantle viscosity is high enough (3-7 1022 Pa s).
The key role of phase transitions in mantle dynamics was recalled by Tackley et al., who pointed out the differences, for various global observables, between the different possible dynamic regimes. Robust methods to learn more about the actual regime in the mantle from tomographic images were proposed by Puster and Jordan.
The basic physics of phase transitions was not ignored: Morris pointed out the importance of the small-scale mass-transfer that must take place during phase transitions. A better understanding and modeling of this effect is crucial for the analysis of th e kinetics of the transitions. This, in turn, is very important for both the dynamical models with phase transition, and the inferences made from the topography of the seismic discontinuities, which is just starting to be determined by seismologists (Vasc o et al.).
This report was prepared by Henri-Claude Nataf (École Normale
As the first speaker, S. Balachandar pointed out, there are three main issues for high-performance computing: efficient and accurate computations, data compression and storage, and interpretation and post-processing large data sets. The first issue is th e area most people think of when they hear the phrase, "high performance computing," but data compression and post-processing present equally important challenges. For example, storing the output from a three-dimensional computation, which has 256 nodes a long each side, with four variables at each node-be it mantle convection, ocean or atmospheric circulation, or seismic wave modeling-for 5,000 time steps requires 2.6 terabytes of storage. While computers and algorithms to consider such a grid may exist, the infrastructure in networking and disk storage makes storing the results of such a computation unfeasible within the present infrastructure.
As many speakers pointed out, moderate sized 3-D fluid dynamical calculations are now feasible on computers ranging from large workstations, or clusters of workstations, to massive parallel supercomputers like the Intel Delta or Thinking Machines CM5. H. -P. Bunge showed that a 3-D finite element code using explicit message passing and PVM communication software could be used on a cluster of workstations to solve some previously-unaddressable geodynamical problems. In addition, the use of a widely-impleme nted message passing package makes such a code easier to port across platforms.
There were two main thrusts of the fluid algorithm talks: solving the resulting sparse matrix equations, and exploring high-order methods.
For large simulations, the most widely-used numerical methods are based on spectral discretizations and the resulting matrix equations are solved using a multi-grid technique. Paul Tackley showed that these methods are feasible on a massively parallel MI MD platform using standard FORTRAN and message-passing libraries. The implementation of his code required parallel FFT and Legendre transform routines. As Tackley showed, the multi-grid solver and FFT routines performed very well as the number of processo rs and size of the problem increased; however, the Legendre transform did not perform as well as the problem size increased. Later in the session, Bengt Fornberg presented an effectively FFT based periodic pseudo-spectral approximation to avoid the proble ms presented by the Legendre transforms in spherical geometries.
For problems with large variations in material properties over a small spatial scale, such as temperature-dependent viscosity in mantle convection, spectral methods usually give way to local methods, such as finite difference or finite element techniques . In these problems, the main obstacle is the solution of the resulting sparse matrix equation. Because of the size of the resulting matrices, an iterative method is required for problems of reasonable size. As M. Parmentier pointed out, multi-grid offers the best theoretical convergence for constant material property problems; he was able to solve moderate sized 3-D problems on a workstation with a multi-grid finite-difference technique. G. Davies extended a multi-grid finite-difference technique to solv e problems with 500-3000 fold variations in viscosity by reducing the amplitude of the viscosity variations on successively coarser grids. S. King presented the overlapping Schwartz method (OSM), a relatively new method for solving matrix equations arisin g from PDE's. OSM is a domain decomposition method where preconditioning of the subdomains overlaps the domains. Overlapping reduces the residual on the edge nodes of the subdomain, accelerating the convergence of the method. King found encouraging result s on 3-D temperature-dependent problems with a factor of 1,000 variation in viscosity. S. Balachandar showed that spectral- transform methods can be used for temperature-dependent viscosity 3-D convection up to an effective Rayleigh number of 6 x 106. The momentum equation is solved iteratively using the GMRES technique, where the depth-dependent part of the momentum equation operator is treated as a preconditioner. This preconditioner guarantees accurate satisfaction of mass conservation and boundary con ditions and also accelerates convergence.
P. van Keken implementing DASPK, a code using the differential-algebraic method for time-stepping with a commercially available finite- element code SEPRAN. The resulting finite-element code is fifth-order accurate in time, which can save considerable co mputer time for strongly time-dependent flows. A. Malevsky discussed the implementation of the method of characteristics on the Connection Machine with fourth- order spatial accurate bi-cubic splines. He stressed the efficiency of higher-order finite-diff erence methods on distributed memory parallel computers. T. Larsen presented results for 3-D convection up to Rayleigh number of 108 for various order of finite-difference schemes, based on Fornberg's algorithm. She found that higher-order methods, such a s 8th order, are far superior to lower-order (second-order) methods in the high Rayleigh number regime. In her poster, S. Zhang also presented higher-order finite-difference methods as applied to 3-D spherical-shell convection, where spherical harmonics u p to degree 256 and 150 higher-order finite-difference points were used in the radial direction. The 3-D structures were visualized by a volume rendering technique with a resolution up to 300x300x300. Efficient algorithms for visualizing these large data- sets efficiently were presented in the neighboring poster by David Reuteler.
L. Trenish presented examples of visualization taken from IBM Visualization Data Explorer. Issues concerning data management and implementation on various platforms were also displayed. The problems encountered in processing and visualizing of large data -sets taken from oceanographic expeditions were presented by Liu et al., who have employed the GLORIA system for this purpose. Numerical simulations of deep ocean convection models using the CM5 were reported by Logan Kuiper in his poster. Finally, there were presentations by the research group from the Earth Resources Group at MIT of seismic wave calculations from a distributed memory parallel computer.
The session promoted an exchange of technical information across disciplines not usually found at AGU sessions. It is our hope that, in the future, we can broaden the range of participants to include areas of the Union that were not well represented in t his session. We believe that exchange of information across traditional discipline boundaries could be very beneficial in the area of High Performance Computing.
Submitted by S. D. King, Purdue Univ., and D. A. Yuen, Univ. Minnesota.
27 talks of 25 minutes each were presented in Reinhausen which ranged from dynamical problems of the Moon and Mars to the experimental investigation of the motion of solitary hot blobs rising in cold glucose syrup. Particular attention was directed towar ds the evolution in time of convection systems. Although presentations of numerical models predominated, results of several laboratory experiments were also presented. Many students presented the results of their diploma theses which are the product of a one-year project in contrast to doctoral theses which require an average period of 3 years. Since the Workshop was designed to stimulate discussing and interaction, participants were encouraged to talk on work in progress and outlines of future projects. There is no place here to list all topics discussed at the Workshop. It suffices to say that it has provided valuable experience for students entering the field of geodynamics and that it has accomplished its goal of an increased interaction among German geodynamicists very well.
Contributed by F. H. Busse, Univ. Bayreuth.
A new program, "Decoding the Earth's Whole History," is expected to start in April, 1995, with Professor M. Kumazawa (Univ. Tokyo) acting as principal investigator. The program, comprising modeling of mantle convection and core dynamics, attempts to deco de information contained in deep-sea sediments back to 4 x 109 years ago with special emphasis on the events at the boundaries between the Archean and Proterozoic and between the Permian and the Triassic.
Another program, "A geophysical network on the Pacific hemisphere" is now being prepared by a group headed by Professor Y. Fukao ( Univ. Tokyo) and is to be initiated in 1996. The program is to extend a network of geophysical observations including seism ograms, magnetometers and GPS instruments both on the sea floor and on islands over the western half of the Pacific. The ultimate goal is to clarify the structures and dynamics of the Earth's deep interior based on the data acquired by the network, includ ing the dynamics of the mantle and core and the coupling between the two.
Contributed by T. Yukutake, Kyushu Univ.
The CGU and CANSEDI have also recently hosted the successful SEDI 94 in Whistler, British Columbia. Thanks to David Crossley, Gary Jarvis, and the rest of the local organizing committee for all the work they put into this conference.
The single large collaborative project of CANSEDI is the operation of the superconducting gravimeter at Cantley, Quebec. This instrument was installed in late 1989 and operated continuously until the fall of 1993. During this period it produced some of t he highest quality data yet obtained from such an installation. A persistent problem however, had been the long term drift, which, at half a m gal /day, was ten times larger than some superconducting gravimeters had achieved. To correct this problem, the instrument was shut down in late 1993, and returned to GWR for servicing. It will be returned to service in the fall of 1994, and will participate in the Global Geodynamics Program.
It is worthwhile at this point to review the remarkable operating characteristics of the Cantley gravimeter, some of which are common to all superconductors, but some are unique to this instrument. Despite the large amplitude of the drift, it was very ne arly linear. After four years of operation we could just detect a non-linear component of drift, and not all of this could be attributed to the instrument because of a large unmodeled hydrological signal. Cantley is almost unique among operating stations in the characteristics of the seasonal surface water storage - there is very little run-off from October to March, followed by a rapid snow melt. This signal is on the order of ten microgal in gravity, and makes the largest single contribution to long per iod residuals. Don Bower, of the Geological Survey of Canada, has discussed a model for this phenomenon at the SEDI 94 meeting.
Although superconducting gravimeters are difficult to calibrate, compared with mechanical spring instruments, they maintain their calibration exceptionally well. Indeed, month-by-month measurements of the admittances of the principal tides at Cantley var y by only one part in 104, and there is no evidence for any long-term change in calibration over four years. The phase of the major tidal admittances is also remarkably stable, varying by no more than 0.01 throughout the observation period.
Non-linear tide signals of about 20 nanogal have been detected, but they are almost certainly the loading effect of non-linear tides on the east coast of North America. This means that any possible non-linear response in the instrument is at least 93 dB below the linear response.
It has been shown that the Cantley instrument faithfully tracks subtidal band signals with amplitude less than a ngal. This is an important result, because the signal level of core modes is unlikely to be much larger than a nanogal, and so detection depe nds critically on how well the instrument functions at this level. We can make this claim because atmospheric pressure variations in the subtidal band generate gravity signals of a few ngal or less, and we can accurately calculate these signals in gravity , given the pressure. For example, the S7 harmonic of a day, and its annual and semi-annual modulations, have amplitudes of about a microbar. The measured variations in gravity at these frequencies are less than a ngal, and they agree, in amplitude and in phase, with what is predicted on the basis of the pressure measurements. A similar story is told at other harmonics of a day in the subtidal band.
Contributed by Jim Merriam, Univ. Saskatchewan
A workshop on "Geomagnetic Polarity Reversals and Field Behavior From ODP Sediments" was convened by B. Clement (Florida International Univ.) 7-8 November 7-8 1994 at Florida International University, Miami, FL. The deep sea sediment cores obtained by th e Ocean Drilling Program provide an opportunity to obtain a broad geographical distribution of records of geomagnetic polarity reversal transitions. The primary goal of the workshop, sponsored by JOI/USSAC, was to organize a collaborative study of existin g ODP cores containing polarity transition records and to identify new drilling sites likely to enhance the geographical coverage of transition records. Topics and issues discussed include cataloging new and existing transition records, assessing the fide lity of the paleomagnetic record, comparison of igneous and sedimentary records, and the characteristics of sites which produce high resolution records.
An informal Workshop on "Structure of the CMB and D''region" was held at UC Berkeley on September 10-11. This multidisciplinary workshop, organized by R. Jeanloz and B. Romanowicz (Univ. California, Berkeley), focused on new seismological observations be aring on the core-mantle boundary and D'' region. The objective is to consider the implications of the new data, and to outline possible directions for future research.
CSEDI co-sponsored a meeting on Planetary Volatiles at California Institute of Technology last month. T. Ahrens (California Inst. Tech.) was the organize). A report of this meeting will appear in the next issue of the DIALOG.
CSEDI has endorsed a proposal, submitted by G. Schubert (Univ. California, Los Angeles), to initiate a Gordon Conference on "Composition, Structure and Dynamics of the Earth's Interior."Workshop on Geomagnetism in Studies of the Earth's Interior
An Indo-US Workshop on "Geomagnetism in studies of the Earth's
interior" was held in Pune, India 22-26 August 1994. The workshop was organized
by J. Heirtzler (NASA Goddard) and M Rajaram (Indian Inst. Geomagnetism)
and hosted by the Institute. The goal of the workshop was to promote a
better understanding of geomagnetism in the study of the Earth as that
is done in the two countries and to explore where joint projects might
be undertaken. A report of the workshop is available from either organizer.
Contributed by Louise Kellogg, Univ. California, Davis
The most interesting and lively portion of the meeting was the presentation and discussion of the two invitations to hold the next SEDI Symposium in 1996. The Australian invitation was presented by Kurt Lambeck, and that for France was presented by Phil lipe Cardin. The Australian proposal would have the symposium held in Brisbane the week of 23-27 July 1996, in parallel with the Western Pacific AGU meeting. The advantage of this arrangement is that AGU will handle most of the organizational matters. Pre- and post-symposium excursions are being organized. The French proposal would have the symposium in the historic town of Tours the week of 7-12 July 1996. The proposed title of the symposium is "From mantle to core & vice-versa." Points raised in th e discussion included the following. The meeting of the Committee on Mathematical Geophysics will probably be held in Cambridge in 1996; the SEDI symposium should be coordinated with this to avoid scheduling conflicts. Some expressed concern about the co st of flights to Australia making the meeting there too costly for students. However, the time is peak season for flights to Europe and off season for Australia. Also Australia is inexpensive for Chinese scientists. Faced with two strong and attractive in vitations, those in attendance at the business meeting finally made the best choice - both were accepted! The one substantive modification was that the French invitation was accepted for 1998 rather than 1996. In summary, the 1996 Symposium will be held i n Brisbane, Australia, the week of 23-27 July 1996 and the 1998 symposium will be held in Tours, France, with the exact time not yet determined. A summary of current plans for the SEDI 96 Symposium are found below; the first announce ment is enclosed with this issue of the DIALOG.
The final item of business was a brief announcement concerning the publication of the proceedings of the 1994 symposium. The invited review lectures are being compiled into a Doornbos Memorial Volume entitled Earth's Deep Interior, edited by David Crossl ey). This volume will be part of a series of volumes The Fluid Mechanics of Astrophysics and Geophysics, edited by Michael Ghil and Andrew Soward, published by Gordon and Breach. A number of the contributed presentations have been submitted for publicatio n in a special issue of Physics of the Earth and Planetary Interiors, guest editors Henri-Claude Nataf and David Loper.
Contributed by David Loper, Florida State Univ.
Two further prizes were awarded from presentations at the SEDI symposium in Whistler, Canada in August 1994. These went to Dr. R. Hollerbach of the Institute of Geophysics and Planetary Physics, Los Alamos National Laboratory, for his work on the import ance of the inner core for the nature of the Earth's dynamo, and to Dr. R. E. Cohen of the Carnegie Institution of Washington for work on theoretical models of the material properties of iron under the conditions prevailing in the Earth's core.
The fund is currently administered by an informal committee chaired by Brian Kennett, 2nd Vice-President of IASPEI, and a more formal structure will be set in place at the forthcoming IUGG meeting in Boulder.
Contributions for the Doornbos Memorial Fund should be sent to Dr. E. R. Engdahl, IASPEI Secretary General, U.S. Geological Survey, DFC, Box 25046, Stop 967, Denver CO 80225, USA.
Contributed by Brian Kennett, Australian National Univ.
In response you should receive a message informing you that you
have been added to the list of subscribers and explaining in more detail
how the network operates.
Also, please encourage colleagues who are not on the SEDI mailing
list to use a copy of the inquiry sheet to indicate their interest in being
placed on the mailing list.
Shortly thereafter the European Union of Geosciences will meet in Strasburg 9-13 April 1995. This meeting will include Symposium VIII "Composition, structure and dynamics of the deep Earth", co-sponsored by SEDI and EGS and consisting of part 1 "Structur e and composition of the deep Earth convened by W. Spakman (Univ. Utrecht) and F. Guyot (Institut de Physique du Globe, Paris) and Part 2 "Modelling in deep Earth geodynamics" convened by U. Hansen (Univ. Utrecht), P. Cardin (École Normale Supérieure) and R. Sabadini (Univ. Bologna). A business meeting for those interested in SEDI will take place at this occasion. Further information will be sent to SEDI members on the email network. (See article on page 19.)
A conference entitled Plume 2 will be convened by D. L. Anderson
(California Inst. Tech.), S. R. Hart (Woods Hole Oceanographic Inst.) and
A. W. Hofmann (Max Planck Inst. fčr Chemie, Mainz) at Schloss Ringberg,
Bavaria, 16-21 July 1995. This is a sequel to a conference on the same
topic held at California Institute of Technology in 1991. For more information,
contact Dr. Kerstin Lehnert, Max Planck Inst. fčr Chemie, Postfach 3060,
D-55020 Mainz, Germany; email email@example.com; phone 49-6
131-305260; fax 49-6131-371051.
In addition to these Union and inter-Association symposia, there
will be a number of sessions of interest in the programs of the individual
Associations, including IAG, IAGA, IASPEI and IAVCEI.
Brisbane is a city with a little over a million people on the east coast of Australia at latitude 27.5S. The sub-tropical latitude and coastal situation give a well moderated climate. Average July daily high and low temperatures are 21C and 12C, so that a jacket or pullover is needed morning and evening but the usual mid-day weather is sunny and warm. Average July rainfall is 65 mm, which makes it one of the drier months. The climate is ideal for eating al fresco at the many restaurants scattered through the South Bank Parklands and these offer plenty of scope for relaxed lunch breaks.
One feature of the internationalization of AGU is its formation of Regional Advisory Committees (RACs). It was AGU's RAC for Australia and New Zealand that invited AGU to hold its 1996 WPGM in Brisbane. Three of its members were appointed as a local orga nizing committee (M. McElhinny, N. S. W., Chair, F. Stacey, Brisbane; R. Walcott, Wellington). The invitation was finally accepted in January 1994, by which time we had had 12 months of thinking and planning. One idea was to emphasize special interest top ics that would provide scientific high points and this was the motive for inviting SEDI to make it a joint meeting. The SEDI Symposium will be a special focus for the meeting.
Recognizing the need for the SEDI program to be effectively independent, a SEDI Symposium program committee has been formed (K. Lambeck, Canberra, Chair; G. Houseman, Melbourne; D. Ivers, Sydney; I. Jackson, Canberra; B. Kennett, Canberra; P. McFadden, C anberra). This committee will arrange the week long symposium coordinating its plan with the AGU program committee that will be responsible for the other sessions of the WPGM (B. Fraser, Newcastle, Chair; C. Barton, Canberra, solid earth geophysics member ). F. Stacey has assumed chairmanship of a local organizing committee for matters specifically concerning SEDI, but will, of course, not be acting independently of the WPGM local organizing committee, chaired by M. McElhinny.
The meeting will run for 5 days, Tuesday to Saturday, and SEDI sessions will include all of the established SEDI interests. The present intention is that SEDI should be one of several parallel sessions, AGU-style, but the scope of the conference centre a llows the program committee the option of parallel SEDI sessions or a separate room for nuts-and-bolts workshops on more specialized topics, if this appears appropriate. There is a large area for poster papers, adjacent to refreshments, and SEDI will have its own clearly identified poster area. In anticipation of strong interest in the SEDI sessions, one of the larger rooms in the conference centre will be assigned to them.
Administration of the meeting will be handled by AGU in its normal way. Calls for papers, registration, and housing request forms will appear in EOS and abstracts will be printed in a special WPGM Supplement to EOS. Intending SEDI participants who are me mber s of AGU are asked to use the EOS forms for these purposes, identifying their section interest as SEDI. Participants who are not AGU members should contact Frank Stacey for this information (Physics Department, The University of Queensland, Brisbane 4072, Australia; FAX +61-7-365-1242). Further information will be circulated to the SEDI mailing list when the program committee has details to report.
Joy Stacey has prepared an extensive list of activities in and around Brisbane for accompanying persons (and participants skipping a session or two!). Some, such as the Art Gallery, Museum and the City Heritage Trail of architectural features are very cl ose. There are boat trips up river to a koala sanctuary and down river to islands in Moreton Bay (Brisbane coastline is protected from open ocean by large sand islands, with many smaller ones in the 30 km wide bay). There is an excellent stand of the loca l coastal rain forest at Maiala National Park on Mount Glorious, west of Brisbane, requiring a half day trip, and full day coast trips visit the open ocean beach resorts south (Gold Coast) and north (Sunshine Coast). Depending on demand, group tours can b e arranged.
Pre- and Post-conference Touring
Most of the Queensland coastline is "protected" by the Great Barrier Reef which extends 2000 km from Papua-New Guinea to the northern end of Fraser Island, 400 km north of Brisbane. Many of the reef tour boats operate out of Cairns, 1400 km north of Bris bane, which has an international airport used by QANTAS and several overseas carriers. Carriers not stopping at Cairns have arrangements with domestic airlines to add a side trip to Cairns without extra fare. Recognizing that conference participants may w ish to take advantage of this situation but that most travel agents can only go by the literature themselves receive, the local organizing committee has made a careful study of the possibilities, to give informed advice.
Mike and Jo McElhinny made an extended tour of reef trips to select those they regard as both good and good value. Their recommendations are detailed in an article that Mike has written for EOS. Non-members of AGU who may be interested should write to Fr ank Stacey (Dept. Physics, Univ. Queensland, Brisbane 4067, Queensland, Australia) for a copy.
Contributed by Frank Stacey, Univ. Queensland.
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