| Number 7 | Fall, 1993 |
EUG session:Structure and dynamics of the Earth's inner and outer
core
Geodynamics Workshop In the Czech Republic
High Pressure Iron Under Heated Debate
AIRAPT Conference: Earth's core at the crossroads
British SEDI activities
Canadian SEDI activities
French SEDI activities
Japanese SEDI activities
Russian SEDI activities
US SEDI activities
AGU SEI Committee activities
IAGA Working Group I-1
IAGA Working Group V-8
The Fourth SEDI Symposium
Conference on Mathematical Geophysics
1995 IUGG General Assembly
The Fifth SEDI Symposium
This is the seventh annual issue of the newsletter of SEDI, an IUGG Union Committee to Study the Earth's Deep Interior. Requests for additional copies of this issue, or for copies of the earlier issues, should be addressed to David Loper, Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida 32306-3017, U.S.A, faxed to (904) 644-6467 or emailed to loper@gfdi.fsu.edu. Also news items for the next issue or notifications of change of address should be sent to the same address.
G. Hulot, D. Jault & J.-L. Le Mouël (IPGP, Paris) described computations of frozen-flux geostrophic velocity fields at the core's surface back to 1840. When these are used with the authors' theory to predict changes in the angular momentum of the core and the associated changes in the length-of-day over the last 150 years, they find good agreement, at least for the last century, confirming previous independent calculations. They believe that the older (pre-1900) data cannot resolve the changes adequately.
D. Gubbins (Leeds) presented a new method for inverting magnetic data directly for core motions, without calculation of an intervening magnetic-field model. Although the frozen-flux, steady solution is the zeroth-order approximation, it is also possible to add time dependency to the motions and also magnetic diffusion, although there is some trade-off between the two. The implications of the fact that the core-motions forward problem is inexact were presented by A. Jackson (Oxford U). The maximum-likelihood solution is typically not the one found by least squares, though how large the deviation is remains to be seen.
Palaeomagnetic input to the session was in the form of studies of both volcanic and sedimentary records of poles during reversals, and a study of palaeointensities. M. Prevot & P. Camps (Montpellier) described a compilation of 715 intermediate VGPs from the Cretaceous to present. Their map of these intermediate poles shows none of the longitudinal grouping seen in sediments; indeed the distribution was highly uniform. Conversely, A. Mazaud (Gif-sur-Yvette) computed fields at the CMB which would be responsible for the observed sedimentary records of the Upper Olduvai reversal (1.66 Ma); he finds flux patches on the well-known longitudes of America and its antipode. Detailed records of palaeointensity from sedimentary cores (J-P. Valet & L. Meynadier, IPGP, Paris) showed evidence of drops in intensity prior to reversals, short events and excursions. The duration of stable polarity also seems to be related to the intensity of the field.
Several papers discussed properties of the CMB and D'' zone. S. Franck & G. Kowalle (Potsdam) considered the Stefan problem of a growing interface and concluded that bumps on the CMB would be stable only for harmonic degrees <7, thus generating small-amplitude and large-scale undulations. Observations of the seismic phases Pdiff (A. Souriau, Toulouse) and PdP (M. Weber & F. Krueger, Erlangen, and J. Neuberg & T. Pointer, Leeds ) can be used to examine the CMB and D'' region. Souriau found P-wave velocity anomalies up to 2.5%; these do not appear to correlate with measured attenuations (though these are difficult to obtain), thus arguing against a thermal origin for the heteroge neity. Weber used data from the Graefenberg array to examine topside reflections from D'', finding very intermittent visibility on a scale of 10-200 km for Kurile events recorded in Europe. He confirms a 3% P-wave velocity contrast at approximately 300 km above the CMB. Krueger and co-workers presented a new method of double-beam forming by using array techniques on both source and receiver side. Pointer presented observational evidence for topside reflections in the New Zealand region using Tonga events and array data and a deconvolution method that takes the phase information of the seismic wave into account. Kirchhoff synthetics were used by J. Neuberg to explain the apparent conundrum of intermittency of reflectors on a 100 km scale despite the typica l width of the Fresnel zone for a seismic P-wave being up to 6 times this figure. The reflection coefficient is highly dependent on the angle of incidence and the impedance contrast so that in some cases only a small part of the Fresnel zone contributes t o the reflection.
B. Wood (Bristol) presented a most intriguing analysis of the feasibility of the presence of carbon as a possible light element in the core. Calculations and experiments show that no more than 4% of carbon could be present in the core, but even with concentrations as low as 0.5% the first solid to crystallize would be iron carbide (Fe3C). This also occurs in the system Fe-C-S. A plausible equation of state leads to an inner core density of 12.85 g/cc, which is well in accord with observations.
Six papers concerned themselves with convection and magnetic field generation within the Earth's core. Various new results emerged, involving the importance of the Earth's inner core, and new examples of dynamo action in kinematic and intermediate dynamo models.
P. Cardin (Ecole Normale Superieure, Paris) & P. Olsen (Johns Hopkins) have investigated convection both with and without magnetic fields. Their model uses the geostrophic approximation to reduce the 3-D fluid equations to 2-D. With no magnetic field they obtain the well-known result that the convection columns form a necklace around the inner core, and point out that for an Ekman number appropriate to the Earth, there would be 1400 columns! Within their approximation, they find that introducing a toroidal magnetic field reduces the number of columns always to 4, 5 or 6 as the regime tends towards magnetostrophic, regardless of the Ekman number.
D. Jault (IPGP, Paris) discussed new integrations of Braginsky's Model-Z dynamo model, reporting steady solutions for Braginsky's rather special choice for the distribution of the alpha-effect, namely a concentration into the equatorial belt near the surface of the core. Other choices for the alpha-effect, such as the more simple and oft-used distribution proportional to the cosine of latitude, do not produce steady solutions.
The importance of the inner core was described in two presentations by R. Hollerbach and C. Jones (both at Exeter). It was pointed out that in the inviscid limit, two flow regimes exist in the core which may not necessarily be continuous across the cylinder tangent to the inner core. It seems likely that the Lorentz force would evolve so as to cancel the shear layers which would otherwise develop. In a similar vein, when a finitely conducting inner core is introduced into the dynamo problem, it appears that toroidal field is expelled from the tangent cylinder in alpha-omega models, in order that the Lorentz torques can balance the vanishingly small viscous torques. This greatly stabilizes the magnetic field in general, diminishing the chaotic effects of the region outside the tangent cylinder. F. H. Busse and coworkers (Bayreuth) report that time-dependent flows appear to be more efficient in generating magnetic fields than do stationary flows, according to their 3-dimensional models. The kinematic dynamo problem was considered by G. Sarson & D. Gubbins (Leeds), who reported working dynamos for some previously-considered flow fields. Conducting boundaries appear to allow dynamo action to occur more easily. By studying the adjoint kinematic dynamo problem, they were able to find apparently the first working dynamo with quadrupolar symmetry.
Several contributions were concerned with the use of supergravimetry in studies of the core. K. Aldridge (York, Canada) & J. Hinderer (Strasbourg) reported on comparisons between short-timescale VLBI data and supergravimetric data; the latter should show signals attributable to polar motion which is measurable by VLBI. A tentative identification was made of a 14.6 hour normal mode of the atmosphere based on the VLBI data. D. Smylie (York, Canada) presented evidence of detection of the translational normal modes of the inner core, the "Slichter" triplet. The peaks seen in the product of the power spectra from four supergravimeters obey the family of splitting laws predicted by the subseismic approximation. This view was challenged by O. Jensen, D. Crossley (McGill) & J. Hinderer; they consider that the signal-to-noise ratio is too low to allow detection of resonances, and suggested that the use of phase information can often annihilate spurious peaks found in product power spectra. N. Florsch (Univ. Pierre & Marie Curie) tutored on the dangers of biasing in amplitude estimation when the signal-to-noise ratio is low.
A final hopeful note came from V. Golovkov (IZMIRAN, Moscow), who announced the exciting possibility of a new geomagnetic satellite. The joint venture between Russia and India, provisionally entitled "Unimag", would hopefully fly in late 1996 and would be the first dedicated geomagnetic satellite to fly since MAGSAT in 1980; similar accuracy to MAGSAT would be the goal.
Contributed by A. Jackson (Oxford) and J. Neuberg (Leeds).
The presentations were divided into the following groups: (1) seismic tomography and equation of state, (2) mantle rheology and inference of mantle viscosity structure, (3) mantle convection and (4) rotation of the earth. Lectures were 30 minutes each. In the evenings there were poster sessions, which began with a series of five-minute presentations.
Adam Dziewonski (Harvard) led off with a discussion of the upper-mantle tomography, where he compared his results with those of Zhang and Tanimoto (Caltech). This led to an interesting debate later between Drs. Dziewonski and Tanimoto concerning the depth extent of the slow anomalies under oceanic ridges. Dziewonski also pointed out the differences in the morphology of the megaplume structures in the lower mantle under Africa and Hawaii. An interesting observation was also underscored by Dziewonski on the changes in the power-spectrum of the the seismic tomography at around 1700 km depth. The connection between this observation and mantle avalanches was a theme repeated later in the discussions in the mantle convection session.
Ann Chopelas (Mainz) described her spectroscopic measurements based on fluorescence to yield the shear and compressional sound velocities of minerals, such as MgO, aluminum oxide and garnet up to 600 kbar. The important geophysical quantity, the ratio of the logarithmic derivatives of the density and sound velocities, was used to infer the magnitudes of the lower-mantle hot and cold thermal anomalies.
Jay Pulliam (U Washington) spoke about the confidence regions for mantle heterogeneities for both P and S waves. His inversion results were based on using the CRAY-C90 supercomputer, where matrices of the size of 5000 x 5000 were solved by the direct method.
Toshiro Tanimoto discussed the inversion of the global-scale crustal structure. Employing fundamental modes out to periods of 40 sec. and ray-tracing techniques, he found thick crust under continents and thin crust under oceans. The transition of the slow continental crust to faster lithosphere was found to occur between 50 and 80 km depth for all of the shield regions in the world. Barbara Romanowicz (UC Berkeley) then discussed the 3-D variations of seismic attenuation in the upper-mantle, based on surface waves. She found a strong correlation between the seismic and Q anomalies at depths between 200 and 400 km. The relationship of this correlation to the degree-two spherical harmonic was pointed out. Alex Forte (Harvard) then discussed the influences of lateral variations of the viscosity on mantle flow and geoid observables. These calculations were based on a variational principle. His conclusions were that there is strong insensitivity of geoid and surface topography to lateral variations of mantle viscosity. Scott King (Purdue) presented a novel technique based on a genetic algorithm for nonlinearly inverting the viscosity profiles of mantle viscosity based on geoid fit considerations. He found a multiplicity of solutions with unusual characteristics in the transition zone. One had a low viscosity zone, the other had a high viscosity layer, which Shun Karato (Minnesota) found this to be very interesting, as it might indicate the presence of some amounts of garnet in the transition zone.
Reini Boehler (Mainz) presented his results on the melting of perovskite up to 650 kbar. Extrapolation yields high melting temperatures, in excess of 7000 to 8000 K, at core-mantle-boundary conditions. This result generated considerable discussion concerning the sudden change of the homologous temperature from 0.7 in the upper-mantle to nearly 0.35 in the deep mantle. Craig Bina (Northwestern) discussed the differences in the topography between the 400 and 670 km discontinuities. Recent seismic findings of the magnitudes of the phase boundary undulations are opposite to what is expected from equilibrium phase changes.
During the poster sessions of the first evening Rolf Daessler (Potsdam) presented results of calculations of the thermal-kinetic equations associated with non-equilbrium phase-changes in subducting slabs. Karato discussed the inner-core anisotropy due to magnetic field-induced preferred orientation of iron. S. Franck (Potsdam) presented results on the preferential wavelengths arising out of chemical reactions at the core-mantle boundary. W. R. Peltier (Toronto) discussed the possibilities of an extremely low viscosity zone lying above the 670 km discontinuity, which can satisfy the geoid data. Dziewonski's colorful poster at the bar showed a series of beautiful seismic-tomography figures where the rapid change in the tomographic pattern at 1700 km was again pointed out. The effects of seismic attenuation on tomographic interpretations were pointed out by Karato's poster.
On Thursday Shun Karato started off with a lecture on mantle rheology from an experimental viewpoint. He emphasized the usage of systematics in obtaining some ideas about the hardness of garnet in the transition zone and the structural ( second-order) phase transition of perovskite in the top part of the lower mantle. The rheology of the subducting slab was also delineated. The idea that there are two hard regions inside the slab was presented. M. Kido (U Tokyo) followed with a discussion of the influences of mantle flow on dynamic sea-floor topography. Better predictions were found when the density anomalies from slabs were added to the density anomalies inferred from seismic tomographic models. W. R. Peltier discussed the pulse of the earth from the periodic oscillations found from the numerical simulations of flush events in a two-dimensional axisymmetic model. The hypothesis that the Nusselt number would decrease with increasing Rayleigh number, due to increased layering, was introduced. Ctirad Matyska (Charles U) presented results based on numerical simulations of convection with a strongly temperature-dependent thermal conductivity to show the development of megaplumes. The idea of a super thermal-attractor was introduced. The mechanism of enhanced conductivity was proposed of being capable of producing the megaplumes observed in seismic tomography of the lower mantle.
In the afternoon the focus of many of talks was devoted to the topic of flush instabilities produced by cold material in the transition zone. This subject matter has been receiving much attention in the last two years. Volker Steinbach (Cologne) led off with a discussion of the role played by triple point in the phase diagram on enhancing flow through negative Clapeyron slopes. He also discussed the difference in the flush events brought about by considering secular cooling of the core by mantle circulation. This was followed by Paul Tackley (Caltech) who focussed the role played by the two major phase transitions in enhancing the strength of the flush event. He also showed the large difference in the perturbed moment of inertia between whole-mantle convection models and those with phase transitions. He found that layered convection produced perturbations in the moment of inertia which were one to two orders in magnitude greater than those produced by whole-mantle convection. This difference can be used to understand better the constraints of polar wander on the style of mantle convection. Satoru Honda (Hiroshima) presented 3-D cartesian numerical simulations of thermal convection with the two major phase transitions included. The effects of high Rayleigh number on enhancing the propensity of the mantle to be layered were described for surface Rayleigh number going to 4 x 10E8. In this afternoon session videos were shown for all of the talks on convection with phase-transitions. The effects of compositional boundary on mantle convection were discussed by Uli Hansen (Cologne). He pointed out the novel effects of depth-dependent properties on producing focussed plumes, which would introduce another regime of convection to the purely layered case and the whole-mantle convection mode.
During the Thursday night poster session Paul Tackley presented results on 3-D large-aspect-ratio convection with temperature-dependent Newtonian and non-Newtonian rheologies, where large aspect-ratio cells were found for effective Rayleigh numbers of around 5 x 10E5. Lada Hanyk (Charles U) presented a mathematical formulation for treating the postglacial-rebound problem from an initial-value (time-domain) standpoint. There were some discussions later between Dick Peltier and him concerning the relative merits between the time- and frequency domain approaches. Adrian Lenardic's (UC Los Angeles) poster dealt with the effects of plates and zones of weakening to produce long cold tongues at the base of the convecting layer. The development of instabilities with such a cold tongue atop the hot bottom boundary layer was emphasized to be quite different from the conventional heated boundary-layer model. Robert Bolshoi (Juelich, Germany) and Yuri Podlachikov (Amsterdam) presented models for detailed finite-element modelling of faults in the lithosphere, where fine fault-like structures can be generated self-consistently according to given criterion. Yuri Podlachikov and Sierd Cloetighn (Urije Univ, Amsterdam) presented results on sea-level variations from compressional stresses and the influences of small-scale faulting on producing intermediate time scales, less than 10E6 years, fluctuations in the sea-level. Fritz Busse (Bayreuth) presented results on the thermal-blanketing effect for a spherical-shell model. Hana Kyvalova and O. Cadek (Charles U) displayed their results of correlation between former subducting sites against five different tomographic models in the model. The results below 1500 km depth appear to be important in the connection to the potential flush events in the mantle. J. Nedoma gave an lengthy presentation of equations to be used in geodynamical modelling.
On early Friday morning at 2 o'clock Slava Solomatov (Caltech) at long last arrived at the workshop after being detained at the Czech border. Later that morning Fritz Busse led off with a general discussion of various mechanism mean-flow in various geophysical fluid dynamical situations. He called attention to the potential role played by temperature-dependent rheology in generating a mean-flow in high enough Rayleigh number convection in the mantle. Dave Stevenson (Caltech) spoke on various aspects of the interaction between mantle convection and earth's rotation. He called attention to the fact that true polar wander can take place very easily and explained why the degree two harmonic so dominant. Its relationship to the flush events was also emphasized. Jerry Moser (Charles U) discussed the role played by depth-dependent properties, such as thermal expansivity and viscosity, in producing relatively stationary large upwellings in spherical-shell convection. He also presented results on the low-dimensionality of the phase-space portraits exhibited by the moment of inertia in high Rayleigh number convection. Slava Solomatov gave a talk on the three different regimes in strongly temperature-dependent convection. As the rheological strength increases, plate-like behavior was predicted by this model based on asymptotic analysis of the mean-field solution. He and Dave Stevenson then presented later in the bar a poster on the asymptotic analysis of the thermal-kinetic equations governing non-equilibrium phase changes in slabs. There were some debates in the evening on the applicability of Rubie's kinetic data for this theoretical model.
B. Steinberger and R. O'Connell (Harvard) presented a model showing that it is possible to have a coherent motion of Pacific hotspots which are excited by flow in the mantle due to density anomalies deduced from tomography. They also predicted that in consequence of this flow in the lower mantle there would be a bias of about 2 cm/yr on absolute plate velocities. Shigeo Yoshida (U Tokyo) showed that the CMB topography can be inferred from the changes of the length of the day and geomagnetic variations from data since 1820. He also estimated the toroidal field strength to be around 150 Gauss. Bert Vermessen discussed the role played by recent tectonic uplifts on observed vertical motions and its impact on postglacial rebound. His usage of linear viscoelastic theory for treating this problem with intermediate time scales was questioned by Peltier.
On Saturday morning the workshop ended with a two hour panel session, whose members included S. Solomatov, J. Pulliam, R. Boehler, T. Tanimoto, J. Moser, S. Karato, and Yu. Podladchikov. Much was discussed on the laboratory measurements of phase changes and rheological changes during phase transitions. Another heated discussion centered on the parameterization of seismic tomographic models and the treatment of the phase boundaries in tomographic investigations. The importance of both seismic attenuation and seismic anisotropy, especially its relationship to mantle rheology , was the next topic of intense discussion. The topic then shifted back to lower-mantle rheology on the role played by orthorhombic form of perovskite. Finally the topic shifted to the issues of extremely low homologous temperatures in the lower mantle, as implied by recent measurements by Zerr and Boehler and whether or not the form of lower-mantle convection in the deep-mantle may in fact be penetrative in character.
In all, this workshop has generated many new friendships between people
from different fields, who otherwise would not have met. It has also awakened
the geodynamicists to the problems of seismologists and mineral physicists
and vice versa. The goal of having generated a viable interdisciplinary
exchange appeared to have been achieved and its success will be measured
in the future by new joint projects initiated by this gathering in southern
Bohemia.
Contributed by David Yuen (U Minnesota).
Boehler's work and that in several other laboratories may have in fact generated as many new problems as have been solved. These issues were aired both at the spring meeting of the American Geophysical Union in Baltimore Maryland (May 24- 28, 1993) and at 'Iron Workers' symposium at the AIRAPT (International Association for the Advancement of High Pressure Science and Technology) conference in Colorado Springs June 28-July 2, 1993. The Topical Group on Shock Compression of Condensed Matter of the American Physical Society was a co-sponsor of that conference, which is summarized in the following report. The experimental spread of melting temperatures remains larger than acceptable in order to place meaningful constraints on the core. Boehler's measurements suggest that iron has quite modest melting temperatures. At 243 Gpa his melting point is only 4550 K. In contrast, separate approaches based on the analysis of shock-wave data give melting temperatures at 243 Gpa of 6800 K (3) and 5600 K (2). Still higher temperatures have been suggested (4). Although external uncertainty bounds of (1) and (2) almost touch, not all results are mutually compatible.
The consequences of such extremes in melting behavior are substantial since the thermal state of the core is an issue in discussions of energy sources for the geodynamo and as a boundary condition for heat transport into the mantle. A hot core can power the dynamo through secular cooling and can potentially add greatly to the heat budget of the mantle. Heat flux through the core-mantle boundary must necessarily produce a conductive boundary layer that can cause anomalous seismic properties (the long standing interpretation of the seismic D'' zone at the base of the mantle includes this component, although recent work highlights complexities within D''). Sufficiently high core temperatures lead to implausibly large heat flow into the mantle, whereas low core temperatures would suggest little or no mantle heat flux originates in the core and that the dynamo might derive energy primarily from buoyancy driven compositional fluxes.
Evidence is growing for an additional high-pressure solid phase (previously a subject of speculation - in the form of both rejected manuscripts and published reports (5)). Boehler and (independently) Saxena et al. (6) detected anomalies which map as a reasonable phase boundary at high pressure between epsilon-iron (hexagonal close-packed structure) and a phase of unknown structure. These results give support to the previously speculative idea (based on simplistic models of very complex physics) that a body-centered structure (bcc) could exist at high temperature, the so-called 'alpha' phase.
As reported at the recent meetings, theorists have now extended the complexity of their calculations. Deviations between experiment and calculation, which are typically larger for iron than for other transition metals, have been reduced in the new work reported by Cohen and Stixrude. Their equation of state and transition pressures are in reasonable agreement with experiments. However, they found that the bcc is not stable at high pressure and temperature.
The presence of a new phase requires an effort to map its stability range and to determine its structure. If this new phase is the equilibrium structure under inner-core conditions, its physical properties might contribute to the observed seismic anisotropy of the inner core (7). An additional phase also leads to the possibility of an additional triple point between two solid phases and the liquid, and may affect estimates of the latent heat of solidification. Whether this occurs under terrestrial core conditions remains uncertain.
The connection between the shock-wave solid-solid transition observe at 200 Gpa and the phase transitions observed in the diamond cell remains uncertain. I am inclined to believe that the shock-wave transition at 200 Gpa and the solid-solid transitions found by Boehler and Saxena and co-workers are the same. Alternative interpretations require additional solid phases and unusual phase behavior. The difference in temperature between static and dynamic experiments then must reflect errors in temperature determination for one or all experiments and/or differences in the chosen criterion for melting. A number of technical issues in data analysis and interpretation do contribute to experimental uncertainties because critical assumptions are necessary to take the 'raw' data and convert it to the pressure-temperature plane. In both the static and shock-interface experiments, radiance measurements as a function of wavelength are converted to temperatures using the Planck function and all workers assume that emissivity is independent of wavelength. Boehler argues that the systematic error introduced by such an assumption is small (several hundred degrees). This presumption is weakened by a complete ignorance of the emissivity behavior or iron under the relevant conditions and ad hoc (but physically acceptable) models can be constructed which lead to uncertainties in excess of 1000 K. Furthermore, extremely large thermal gradients along the optic path in the laser heated diamond anvil experiments (>1000 K/m) must be compared with the 0.5 to 0.8 m wavelength light emitted by the sample and detected in these experiments. Since temperatures vary by more than 500 K within one optical wavelength, details of thermal emission, optical skin depth and the properties of interfaces subject to large gradients in thermodynamic state could potentially lead to systematic errors of unknown magnitude. Such complications have not previously been explored in either experiment or theory. The shock-wave interface experiments (which also rely on Planck function fits) include an additional uncertainty associated with thermal conduction at the interface. Large corrections (>1000 K) of the experimental data are made on the basis of assumed values for thermal conductivity. Until the relevant measurements are made, these results remain substantially uncalibrated. Lastly, whether the shock-induced phase transitions occur at the equilibrium pressure remains an open question.
Although questions remain, progress over the last few years is substantial with some convergence of interpretations. Static and shock-wave experiments agree that a new high-temperature solid phase of iron exists. An effort must begin to determine the properties of the new phase. Most importantly, a new Greek letter should be assigned. It is inappropriate to call the mystery phase 'alpha' since no experiment demonstrates that it has the same structure as the ambient pressure alpha phase. Furthermore, even with the same structure, this high-pressure phase deserves a unique designation. Quantitative differences in temperatures at phase boundaries must spawn a new round of experiments in what remains a "hot" field.
1. R. Boehler, Nature, 1993.
2. J. M. Brown and R. G. McQueen, J. Geophys. Res., 91, 7485-7494,
(1986)
3. Q. Williams, R. Jeanloz, J. Bass, B. Svendsen, and T. J. Ahrens,
Science, 236, 181-182 (1987); C. S. Yoo, N. C. Holmes, M. Ross, D. J. Webb,
and C. Pike, Phys. Rev. Lett. in press 1993.
4. T. J. Ahrens, H. Tan, and J. D. Bass, High Pressure Res., 2, 145-157
(1990)
5. M. Ross, D. A. Young, and R. Grover, J. Geophys. Res. 95, 21713-21715
(1990); M. Matsui, in "Central Core of the Earth" (in Japanese), vol 2,
79-82 (1992)
6. S. K. Saxena G. Shen, and P. Lazor, Science, 260, 1312-1313 (1993)
7. R. Jeanloz and H. R. Wenk, Geophys. Res. Lett., 15, 72-75, (1988)
Contributed by Michael Brown (U Washington).
The surge of interest in physics of iron stems from advances in technologies in both diamond-anvil-cell (DAC) and shock-wave studies as well as from old and new controversies, attesting to a lack of consensus among both experimentalists and theoreticians. The advances have produced new data on iron in the pressure-temperature regime of interest for phase diagrams and for temperatures of the core/mantle and inner-core/outer-core boundaries. Particularly tantalizing in these respects is the iron phase-diagram inferred from DAC studies. Besides a possible new phase, beta, reported by Saxena (Uppsala), with a gamma-beta-epsilon triple point at about 30 GPa and 1190 K, interpretation of recent DAC results in concert with previous shock-wave results yields a possible sixth phase, theta, with an epsilon-theta-melt triple point at about 190 GPa and 4000 K reported by Boehler (Mainz). Intriguing possibilities such as these, coupled with the importance of the equation of state of iron in consideration of Earth's heat budget and the origin of its magnetic field, have also evoked considerable interest among solid-state theoreticians who argue on the basis of molecular dynamics and other first-principle methods whether the non-magnetic body-centered cubic phase of iron is stable at high pressures and temperatures associated with Earth's core. (3 out of 4 say no!) Still other theoreticians use dislocation models to calculate the melting point of iron at the inner core/outer core boundary and obtain a remarkably believable number despite the simple nature of their theory.
While the amount of data on the equation of state of iron and the number of papers on the subject, both theoretical and experimental, have increased considerably in the past three years, no consistent picture of iron has yet emerged. In fact, the number of controversies has grown, as documented by Anderson's (UC Los Angeles) paper at the AIRAPT meeting. Several possible reasons for lack of consistent experimental data are discussed in the contribution of Duba (Lawrence Livermore Nat'l. Lab.) at the AIRAPT meeting and Brown's (U Washington) comments summarizing the Spring AGU meeting. (See the previous article above.) Brown, in particular, pleads that experimentalists take a harder look at their reported uncertainties in pressure and temperature and begin a search for the physical principles overlooked on the melting measurements. Duba argues that the definition of the melt transition itself may be part of the problem.
While the major thrust of the AIRAPT meeting was on the physics of pure iron, there were notable contributions on iron alloys. Funamori and Yagi (Tsukuba) pointed out how easily hydrogen could be alloyed with pure solid iron and how significant the melting point depression would be in that case. Secco (U Western Ontario) reported on the viscosity of iron-sulfur liquids at high P, and Suzuki et al. reported that the phase diagram of rhenium is a good analog for the phase diagram of iron. The knowledge of iron's physical properties, besides Tm, was increased by several contributions.
This meeting brought out clearly our ignorance/knowledge and provided an impetus for further advances in experiment and theory. Only then will a consistent picture of the phase diagram of iron emerge. Particularly needed are synchrotron x-ray studies at high pressure and temperature in the DAC determine structure, a necessity if we are to sort the structure and the phases and to determine whether melt exists for a given set of conditions. These critical experiments, involving J. Akella (Lawrence Livermore Nat'l. Lab.), D. Mao (Carnegie Institution), and S. Saxena, are presently underway.
Geomagneticians are interested in knowing the temperature and composition of the inner core. Research on the physics of iron is correspondingly focused on Tm of pure iron at 330 GPa and the crystallographic structure of pure iron at high P and high T.
There were four new reports on Tm at 330 GPa at the Colorado Springs meeting. Boehler, who dismissed the shock wave results from the three shock wave laboratories, extrapolated his DAC values of Tm, finding Tm = 4800 K at 330 GPa. Yoo et al. (Lawrence Livermore Nat'l. Lab.) dismissing the DAC results of Boehler and the shock wave results of Brown and McQueen, found Tm = 6800 K at 330 GPa from their shock measurements. Anderson, seeking a compromise, found, by combining Boehler's DAC results with the Brown and McQueen shock wave results, that Tm = 6000 - 6200 K at P = 330 GPa, but this solution ignored the shock-wave results from Caltech and Livermore. Poirier (Inst. Physique du Globe, Paris) and Shankland (Los Alamos Nat'l. Lab.), using a solid state theory, found Tm = 5600 - 6160 K, depending on the phase (bcc, fcc, hcp). Poirier and Shankland also calculated a freezing point depression of 500 - 1000 K. These new estimates of Tm at 330 GPa are to be compared with the result of Williams et al. (1987), where Tm = 7700 ± 500 K.
There was little agreement on the structure of pure iron at inner-core conditions. The epsilon phase can be justified for the inner core only if the experiments of both Brown and McQueen and Boehler are ignored. The phase diagram presented by Williams, Knittle, and Jeanloz in 1991 would then be appropriate. On the other hand, if Boehler's experiments and those of Brown and McQueen are allowed, hcp iron cannot be the inner core phase. That is why a new phase, called theta by Boehler, is required. But what is the structure of theta? The theoreticians say this phase cannot be bcc. Could it then be fcc? Fcc seems to be allowed by both experiment and theory. This possibility harkens back to the theory of fcc at core conditions by Bukowinski (1977).
The amount of impurities needed in the iron of the inner core to match PREM density depends on the density of the iron phase dominating the inner core. An hcp inner core requires a large amount of impurities, as documented by Jephcoat and Olsen (1989). We do not yet know the density of fcc at inner core conditions, but the neutron scattering results of Stassis indicate that the bulk modulus of fcc is low (130 GPa at ambient), while r0 is high (8.00 gm/cc). This would make the density of fcc high at inner core conditions. If so, an fcc inner core phase would also require a large amount of impurities. Thus it is important to know the equation of state parameters of fcc. This problem is currently under investigation by the UCLA Mineral Physics Laboratory. Both Saxena and Boehler have suggested that the newly found beta phase (about 70 GPa and 2000 K) might be contiguous with the theta phase above 200 GPa; that is, theta and beta are one and the same.
It is clear that there are a variety of current options concerning Tm, structure, and composition available for geomagneticians to choose from. Perhaps the situation will be clarified within the next three years when the Third Ironworkers Convention will be held.
Contributed by Orson Anderson (UC Los Angeles).
A problem which has been investigated is the time dependence of alpha-omega models of the geodynamo. The observation that some patches of the earth's magnetic field drift west has traditionally been used to justify the use of alpha-omega models of the geodynamo. Unfortunately these models have been found to oscillate on a fast time scale which is inconsistent with the observations of geomagnetic reversals. A suggestion to solve this discrepancy has been put forwards by Barenghi, who has taken into account the alpha effect in the toroidal field equation, thus producing an alpha squared omega model. Steady and oscillatory Taylor states solutions have been found. The slow time dependence of the oscillations compares well with the average time scale of reversals at values of the Reynolds number of the omega effect suggested by the westward drift.
Another result which is particularly exciting has come out of the study from Chris Jones and Rainer Hollerbach (Exeter) is the possibility that the central solid core can have a big effect on the dynamo process. The dynamical effect of the inner core extends over the region inside the 'tangent cylinder', the cylinder parallel to the rotation axis which just encloses the inner core. Dynamo action is concentrated in the region outside the tangent cylinder while the quiescent region inside the tangent cylinder provides the stability needed to stop the dipole component of the field reversing with every fluctuation.
David Fearn and Mike Proctor have obtained nonlinear solutions for convection in a rapidly rotating sphere, incorporating at least part of Taylor's constraint. This links with a simplified study of Susan Ewen, Andrew Soward's research student, based on Busse's annulus model. David and Mike's work in the spherical geometry is a necessary preliminary to the solution of the full dynamo problem. Its successful accomplishment means that the group is on the way to fulfilling the main aims of the project.
The group was well represented at the Dynamo Theory programme at the Isaac Newton Institute of Mathematical Sciences, Cambridge, during the last six months of 1992, also at the EUG meeting in Strasbourg and IAGA Buenos Aires, 1993. Many of the results reported above will appear in a Special Issue of Geophysical and Astrophysical Fluid Dynamics (Guest Editor, Mike Proctor, Cambridge) devoted to papers on the geodynamo and related topics given at the Buenos Aires meeting. There have been a number of useful collaborations which include Rainer Hollerbach's visit to Prague to work with Ivan Cupal and Ron James' three month visit to Newcastle from Sydney, Australia.
Contributed by Andrew Soward (Newcastle upon Tyne).
The idea of the GGP was initiated by a group of Canadian Geoscientists running superconducting gravimeters (SGs) with the aim of providing a world-wide network of high-quality gravity measurements for a variety of geophysical problems. Initially, the GGP contained two linked components, the first being an International Observing Period, a 6-year continuous measurement campaign by existing stations with attention paid to high quality instrument siting and data acquisition systems of uniform standard. The second component was the effort in Canada to establish a further two SG stations and a data exchange centre at the University of Western Ontario, linked to the stations worldwide via INTERNET.
The international aspect of the GGP, i.e. the Observing Period (GGPOP), is now the subject of serious planning. As a first step, in Beijing this summer the Permanent Commission on Earth Tides passed the following resolution: "Considering the need to stack data from a global network of superconducting gravimeters (SGs) for several seismological and geodynamical problems, we recommend that the GGP convene a special meeting of the SG groups to establish the terms of reference of the proposed 6-year Observing Period". This resolution has provided the impetus to plan a special meeting of all interested GGP members in Luxembourg, 14 - 15 March, 1994, in the context of the Journee Luxembourgeoises de Geodynamique of the Council of Europe - European Network on Geodynamics, with the support of the International Centre of Earth Tides The data centre is still some time way from being established, but it is hoped that the two new instruments will be installed within the next year or two.
Contributed by David Crossley (McGill U).
The program "Terre Profonde" started with a summer school in the style of a Penrose Conference in Les Houches. Seventy scientists attended, with a strong group of graduate students and a large and active group of young scientists. The first day concentrated on the formation and composition of the earth's core, with lectures on the nature and amount of light elements (silicon is revived) and on the timing of core formation. Short presentations were made on the new insights provided by very high pressure and temperature experiments in the diamond-anvil cell and in the larger multi-anvil equipment, and on what is known through seismology, on the inner core, particularly its anisotropy.
The second day was devoted to core dynamics with a review on convection in the outer core and on seismological constraints on stratification and the Brunt-Väisälä frequency in the core. This was followed by short presentations, on the information provided by superconducting gravimeters, on the influence of rotation on core dynamics, and on core-mantle coupling in relation with rotation. Topics of active exchange included core viscosity structure, core flattening and nutations.
On the third day, there was a lecture on dynamo models and a review of the systematics of reversals, followed by a short presentation on magnetohydrodynamics of stars and planets and continued by a discussion on the reality and meaning of confined reversal paths and brief overviews on long term changes in reversal frequency, paleosecular variation, and paleointensity changes in various period ranges.
Attention then moved to discontinuities and boundary layers, with a lecture on phase transitions, particular emphasis being given to the D'' layer and to the 450-650 km transition zone. The constraints provided by seismic tomography on mantle structure and dynamics were explained, and the influence of mantle deformation on core rotation was described. The part played by solid state physics and thermodynamics was illustrated by lectures on the ab initio simulation of material properties and on equations of state.
The fifth topic was deep mantle heterogeneities and instabilities; heterogeneity scales in the deep mantle; plume-related problems and recent advances in supercomputing applied to plume generation; the possible role of cold subducted slabs in accounting for plate motions and deeper mantle heterogeneities, and finally new views on the role of plumes in the tectonics of Venus.
In parallel with the summer school, a call for research projects had been issued by the Institut des Sciences de l'Univers. Four cooperative research programs, under the responsibility of a coordinator for each project, were funded for two years: (1) Core seismology, gravity and dynamics, (2) Magnetism and dynamo models, (3) Physical and chemical properties of the deep mantle, and (4) Deep mantle convection.
The program "Terre Profonde" is expected to last four years. It clearly shows the recognition by the funding agencies that the label SEDI covers an active community of French geophysicists strongly involved in deep Earth research.
The French SEDI National Committee had been so far organized along more or less informal lines. It is now in the process of formalizing a bit more it structure to increase its efficiency in dealing with the various agencies.
An annual progress report of the project, Central Core of the Earth, vol. 3, was issued in March 1993, containing 45 papers, some in English and the other in Japanese with English abstracts. Copies of this report are available from Y. Honkura, Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 152 Tokyo.
The next symposium, "The Earth's Central Core," will be held in Tokyo during February 1 to 2, 1994. A monograph for the three-year project of The Earth's Central Core is scheduled to be published in 1994.
In cooperation with National Polar Institute, the National Astronomical Observatory installed a super-conducting gravimeter at Syowa Base in the Antarctica, and began its operation in March, 1993.
Contributed by T. Yukutake (Kyushu).
Two papers dealing with the mantle transition zone have appeared in the past year. The first, by Petersen, et al (N. Petersen, J. Gossler, R. Kind, K. Stammler, L. Vinnik, Precursors to SS and structure of transition zone of the north-western Pacific. Geophs. Res. Lett., 20, 281-284, 1993), reports on the structure of the phase transition zone underneath the Japan and Kuril-Kamchatka subduction zones. This study does not confirm previously reported large-scale topography on the mantle discontinuities in this region. There are indications of a strong reflector in the study region at a depth of 900 km. A second paper by Petersen, et al (N. Petersen, L. Vinnik, G. Kosarev, R. Kind, S. Oreshin, K. Stammler, Sharpness of the mantle discontinuities. Geophs. Res. Lett., 20, 859-862, 1993) presents estimates of the sharpness of the major mantle discontinuities based on observations of broad-band converted phases. Contrary to the well known laboratory estimates of the sharpness of the olivine-spinel phase transformation (around 20 km) the seismic data suggest that the discontinuity at 400-km depth is much sharper. The data for the 650-km transition is modelled by a linear gradient zone 20-30 km thick. This is in apparent contradiction with some reflection data which indicate that this transition is sharp. Most likely, the transition is composed of both a sharp discontinuity and a gradient zone.
A paper by V.N. Zharkov (The role of Jupiter in the formation of planets, Geophysical Monograph 74, IUGG 14, 7-17, 1993) describes a new cosmogonical hypothesis (for details see the abstract of the paper). In this new conception, Jupiter, which was the first of the planets to form, played a crucial role in the formation of the terrestrial planets and especially the giant planets.
A series of papers on physics of Mars interior appeared in Solar System Research, 27, Numbers 2 and 4, 1993, only two of which are described here. In the first (A. Yu. Babeyko, S. V. Sobolev and V. N. Zharkov and T. V. Gudkova, On mineralogical and velocity structure of the martian crust) models for different isotherms were calculated by the method of thermodynamic modelling. In these models the thick martian crust is divided into an upper crust (about 0-50 km), crustal transition zone (about 50-100 km) and lower crust (about 100-150 km). The distribution of seismic velocities in the transition zone and lower crust is very complicated. In second paper (V. N. Zharkov and T. V. Gudkova, On the dissipative factor of Martian interiors) the mean value of the shear dissipative factor Q(r) of the Martian interior has been estimated using the data of secular acceleration of the mean motion of Phobos. For this purpose the weight functions for tides of second and third orders are calculated. Convolution of these function with 1/Q(r) determines the delay angle of tides raised by Phobos and accordingly the secular acceleration of the satellite. The low values of Q - about 60 for the model of Mars with a liquid core and about 40 for a model with a solid core for tidal period 2 x 10000 s - point out the frequency dependence of shear Q for Martian interiors for long periods. Using a power law to estimate the quality factor in the seismic band of periods T about 1-200 s it was found that shear Q is about 110-120 and 80-85 for models with liquid and solid cores at T about 200 s and is about 250-270 and about 170-190 at T = 1 s for the same models. It is concluded that a model of Mars with a liquid core is more realistic than one with a solid core. The influence of the third-order tides on the secular acceleration of Phobos is evaluated to be smaller than 10%.
Contributed by L. Vinnik and V. N. Zharkov (Inst. Physics of the Earth, Moscow).
The status of the CSEDI initiative within NSF was reviewed by Mike Mayhew. He noted that there are four main elements necessary to make this initiative successful: a focus on 'the great problems of the Earth', formulation of well-designed hypotheses, design and execution of critical tests of the hypotheses and dissemination of the results of the tests. The initiative is progressing well within NSF with approximately $500,000 budgeted for this year and the prospect of substantial increases in future years. Also, there might be some support stemming from the high-performance computing initiative. To date, two CSEDI research projects have been funded, along with four workshops. An additional 7 workshops are in various stages of planning.
Reports of the four workshops which have been held so far were given by their convenors: that on Mantle Convection by Louise Kellogg (UC Davis), that on Geodynamics and Plate Tectonics by Mike Gurnis (U Michigan), that on Mineral Physics by Bob Liebermann (SUNY Stony Brook) and that on the Geodynamo by Paul Roberts (UC Los Angeles). Also, Chick Keller gave an overview of the LANL Mantle convection Workshops, which may serve as a paradigm for CSEDI workshops. Raymond Jeanloz (UC Berkeley) gave a brief report of the status of the CSEDI project on the core-mantle boundary and Orson Anderson (UC Los Angeles) reported on the AIRAPT Workshop on high-pressure phases of iron (see above report).
The main scientific program of the symposium consisted of six themes, each focusing on a problem of particular current interest. Theme 1, which dealt with the evidence for and against compositional stratification of the mantle from mineral physics and chemistry, was presented by Russ Hemley (Carnegie Inst), Tom Duffy (Carnegie Inst), Craig Bina (Northwestern U) and Lars Stixrude (Georgia Tech). The seismic discontinuities occurring at 410 and 660 km depth are likely to be phase transitions, but the question of a compositional difference between the upper and lower mantle has not yet been resolved. Theme 2, which dealt with seismic anisotropy and mantle deformation, was presented by Karen Fischer (UC San Diego), Shun-Ichiro Karato (U Minnesota) and Neil Ribe (Yale U). The most interesting idea to come out of this theme is that the observed seismic anisotropy of the inner core may be due to the toroidal magnetic field, raising the possibility of having a direct measure of this field, previously believed to be unobservable. Theme 3, on chemical cycling in the mantle, was presented by Don Anderson (Caltech), Terry Plank (Cornell U), Bill White (Cornell U) and Susan Schwartz (UC Santa Cruz). There is strong chemical evidence from isotopic ratios that plumes contain several % of recycled sediments. Theme 4, dealing with the effect of phase transitions on mantle convection, was presented by David Stevenson (Caltech), Paul Tackley (Caltech), Guy Masters (UC San Diego), Larry Solheim (Carnegie Inst) David Yuen (U Minnesota), Main Liu (U Missouri Columbia), Mike Gurnis and Scott King (Purdue U). It is becoming clear that phase transitions strongly affect the vigor and style of mantle convection, but the precise nature and vigor of the effect is still uncertain. Theme 5, on paleomagnetic constraints on the geodynamo, was presented by Cathy Constable (UC San Diego), Brad Clements (Florida Intl U) and Ken Hoffman (Cal Poly State U). The relation between the currently observed magnetic-field flux lobes, preferred reversal paths and cluster points of virtual geomagnetic poles remains an intriguing puzzle. Theme 6, on the temperature of the core, was presented by Quentin Williams (UC Santa Cruz), Orson Anderson, Lars Stixrude, Murli Manghnani (U Hawaii) and Tom Shankland (Los Alamos Natl Lab). While there has been some progress resolving the phase diagram of pure iron at high pressure and temperature, there is a considerable disagreement about the temperature of the liquid-solid phase transition at high pressure and considerable uncertainty about the amount (if any) of the melting-point depression due to alloying.
A new coordinating Committee for CSEDI was chosen for the coming year. Rick O'Connell continues to serve as Chairman of CSEDI.
Contributed by D. Loper (Florida State U).
At each of the three most recent AGU meetings, SEDI activities have been the focus of union sessions: at the Fall 1992 meeting in San Francisco a two-day session "Studies of the Earth's Deep Interior: Addressing Fundamental problems" drew 55 papers; at the Spring 1993 meeting in Baltimore a single day session "Multidisciplinary Studies of the Earth's Deep Interior: Boundaries and Discontinuities" drew 20 papers; and at the Fall 1993 meeting in San Francisco a single day session "Studies of the Earth's Deep Interior: Upwellings and Downwellings" drew 31 papers. In addition to providing an interdisciplinary forum, free from the constraints of sessions organized within the AGU section structure, these sessions have helped raise the visibility of SEDI in North America.
The committee intends to argue for a union session at each AGU meeting, and welcomes suggestions from the SEDI community for possible themes. In addition, the committee would welcome suggestions for other meetings, such as Chapman Conferences, that could usefully serve the SEDI community. These should be sent to J. Bloxham, Department of Geological Sciences, Harvard Univ., Cambridge MA 02138, USA.
Contributed by Jeremy Bloxham (Harvard U).
A symposium on "Structure and Dynamics of the Earth's Inner and Outer Cores" sponsored by SEDI was held in Strasbourg on 8 April as part of the biennial Congress of the European Union of Geosciences. (See the report on page 1.) Problems of paleomagnetic reversals paths and the magnetohydrodynamics of the outer core received special attention at this symposium.
In addition to these major meetings numerous workshops have taken place at the national level which attest to the growing interest in the origin of geomagnetism and its temporal variations.
Contributed by F. H. Busse (Bayreuth)
Contributed by C. Barton (Australian Geol. Survey).
The Symposium program consists of eight half-day sessions on "Modern geodetic constraints on Earth structure", "Physical and chemical properties of the deep Earth", "Seismology and large-scale dynamics", "Core-mantle interactions and the CMB", "Mantle mixing and the cooling Earth: A symposium in memory of J. Tuzo Wilson", "Core dynamics and magnetohydrodynamics", "Earth rotation and internal structure" and "The geodynamo: theory and observational constraints". Invitations have been mailed to a select group of speakers who are deemed to be in the forefront of new developments in deep Earth geophysics. A Conference Grant has been requested from the Natural Science and Engineering Council of Canada to cover the cost of the invited speakers. It is anticipated that funds to cover the additional conference expenses will come from the registration fees.
The organizers received a total of 168 positive responses to the first circular, and are anticipating that the total number of attendees may exceed 250. The second circular is due to be mailed out near the end of the year. If you have not indicated your interest in attending and wish to receive the second circular, write to SEDI 94 Secretariat, Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7, send a fax to (514) 398-4680 or an email message to sedi94@erda.geophys.mcgill.ca.
This report was contributed by D. Crossley (McGill U).
It is hoped that such a network may soon be established for SEDI. In anticipation of that event, anyone wishing to be on the SEDI email network when it becomes established should send a blank message to loper@gfdi.fsu.edu with the subject 'SEDI subscribe'.
SEDI is six years old and is now well established, thanks to the efforts of its first Chairmen, Ned Benton and Durk Doornbos, and its active and devoted Secretary, Dave Loper. Its life is punctuated by the increasingly attended SEDI Symposia, of which it can truly be said that "it is where the action is" in the field of deep Earth studies: dynamo theory, core-mantle coupling, very high-pressure mineral physics, seismology of the core ... to name only a few of the topics which made the third Symposium in Mizusawa so successful.
1993 is a transition year, between the Mizusawa Symposium and the 1994 one organized by the Canadian SEDI group, to be held at Whistler Mountain. However, all major Earth sciences meeting held this year (Spring and Fall AGU, EUG etc.) have included one or several highly successful SEDI sessions. It seems therefore that in a few years SEDI has met most of the objectives that presided at its foundation: it is now clearly identified with an active community of deep Earth geophysicists and it is involved in many national and international projects.
We must now see to it that SEDI does not fall victim of its success, by extending its interest to shallower and shallower regions of the Earth (at what depth does the deep Earth begin?), running the risk, in so doing, of losing its identity by becoming co-extensive with solid Earth geophysics. But I have no doubt that wisdom will prevail.
Jean-Louis Le Mouël
Durk graduated from the University of Groningen in 1965 and received his Doctorate in Geophysics (cum laude) from the University of Utrecht in 1974. He served there as senior lecturer until 1980, when he moved to the NORSAR Institute in Kjeller, Norway, as senior scientist. In 1985 he was appointed Professor of Physics of the Solid Earth in the Department of Geophysics at the University of Oslo, the first in this discipline at the university.
In addition to serving as Chairman of SEDI, Durk held a number of other important scientific posts both in Norway and internationally. He was a member of the Norwegian ILP Committee from 1983 to 1988. He was also a member of the Norwegian Geophysical Society, and in 1988 was elected chairman of its section for seismology and solid Earth physics. In this capacity, he served as the Norwegian delegate to IASPEI. Durk was elected chairman of the Norwegian National Committee for Geophysics and Geodesy in 1991 and in this capacity served as the representative of Norway to IUGG. He was elected Member of the Norwegian Academy of Sciences in 1988.
Durk devoted his scientific career to the exploration of the Earth's deep interior. His research stressed the links between the Earth's mantle and outer core, which he studied using data based on the recordings of seismic waves. Much of his research dealt with the quantification of the topography of the core-mantle boundary. The hallmark of his work was a careful and definitive analysis of the seismic waveforms which carry the most information of the region being studied. In recent years he worked actively for the systemization of global collection of seismic data, particularly through the ISOP project, which would give more detailed insight into the Earth's deep interior.
Durk had many ideas for future research, some of which he planned to develop during his upcoming sabbatical year in Paris and Pasadena. This work would undoubtedly have provided new and important results concerning the physics and structure of our planet.
We are all poorer because of his untimely death. A memorial fund has been established in Durk's name to support a prize for a young scientist. Donations should be made payable to IASPEI and sent to E. R. Engdahl, IASPEI Secretary-General, U.S. Geological Survey, DFC, Box 25046, Stop 967, Denver, CO 80225, USA.
A more complete obituary, from which much of the above has been abstracted, appeared in the June 8, 1993, issue of EOS.