Vol. 5, No. 12 December 1995 INSIDE • New Members, Fellows, Student Associates, p. 247 GSA TODAY South-Central Section Meeting, p. 250 • A Publication of the Geological Society of America • Northeastern Section Meeting, p. 253 Seismic Images of the A B Core-Mantle Boundary Michael E. Wysession, Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130 C D ABSTRACT INTRODUCTION Seismology presents several ways While most geologists, including of providing images of the geologic specialists in the field of seismology, structures that exist in the lowermost study rocks at Earth’s surface, more mantle just above the core-mantle attention also is being paid to the boundary (CMB). An understanding planet’s other major boundary, that of the possibly complex geophysical between the core and mantle. With a E F processes occurring at this major density jump of 4.3 kg/m3 between discontinuity requires the combined the silicate lower mantle and the liquid efforts of many fields, but it is the iron outer core, as well as a tempera- role of seismology to geographically ture increase of possibly 1500 °C map out this largely uncharted terri- between the lower mantle adiabat and tory. Seismic phases that reflect, outer core, the core-mantle boundary diffract, and refract across the CMB (CMB) may well be Earth’s most signifi- can all be used to provide different cant and dramatic discontinuity. Our Figure 1. Images from a motion picture showing the propagation of seismic shear energy information in different ways. increasing knowledge of this highly through the mantle (Wysession and Shore, 1994). The images correctly show the locations of Profiles of core-diffracted and core- variable and heterogeneous region has the seismic shear wave fronts at (A) 320, (B) 540, (C) 800, (D) 1040, (E) 1360, and (F) 1800 s reflected waves are especially power- come though the combined efforts of after the occurrence of a 600-km-deep earthquake at the lower left of the images. Red is out of the page; blue into the page. Amplitudes are normalized and raised to a power of 0.8 to ful when used as differential geoscientists in a wide array of fields, enhance smaller features. Images were made by interpolating between a grid of 72,846 syn- traveltimes in relation to direct and an important part of this effort has thetic seismograms calculated by the superposition of all torsional normal modes (28,585) with phases. The resulting seismic maps been the use of seismology to map out periods greater than 12 s. show long-wavelength lateral hetero- the structures that exist there. Because geneity in the lowermost few hun- of the limitations of imaging a surface dred kilometers of the mantle (a nearly 3000 km beneath us through a region called D”) with a magnitude heterogeneous mantle, our images lack mantle. Variations in the wobble of The examination of the CMB using of at least 6%, which is comparable clear resolution. In a sense we are like Earth’s axis of rotation and in the the seismic waves from large earth- only to Earth’s upper few hundred the seafaring explorers of 500 years ago length of days are also a result of quakes has a long history. R. Oldham kilometers. The maps of lateral seis- who had mapped out the outlines of mass variations and provide constraints first identified the core in 1906, and mic variations show significant con- the world’s continents but still knew on the topography of the CMB. I. Lehman discovered the inner core tinent-sized features that are most little of what lay within them. In this Effects of CMB topography and the in 1936. By the 1940s, scientists like likely a result of the convective article I discuss a few attempts to get thermal variations of the lowermost K. Bullen had not only determined dynamics occurring at the base of clearer maps of the “continents” at the mantle create observable variations in reasonable radial models of Earth’s seis- the mantle. The geophysics of the CMB, speculate about what these maps the geomagnetic field by affecting core mic velocities, but had even noted the CMB and lowermost mantle proba- may mean, and describe some of the flow. Geodynamic modeling, both unusual behavior of the then-named bly has many analogies with that of directions that may be taken to develop experimental and numerical, is provid- D” layer at the bottom of the mantle. Earth’s lithosphere and crust, and a sharper image. ing realistic time histories of the pat- As late as the 1980s most seismologists variations in the structure of D” may The red-and-blue seismic maps terns of convection that might occur in observed a decrease in D” velocities likewise be a combined result of ther- that we produce, which represent the the lower mantle. Mineral physicists, relative to the rest of the mantle, which mal, chemical, and mineral phase velocities with which P and S waves through both high-pressure diamond made sense thermodynamically; if the variations. Interpretations of the seis- propagate through a given region, do anvil experiments and theoretical equa- CMB is a chemical boundary between mic images, requiring knowledge of not mean very much by themselves. tions of state, are delineating the kinds rock and iron, then heat must be the mineral physics of expected min- However, these two velocities are a of materials we might expect to occur conducted across it, and a thermal eralogical assemblages at these function of density, rigidity, and at these great pressures and tempera- boundary layer will likely form at the depths, suggest that the CMB plays a incompressibility, which are compli- tures. The stories emerging about the bottom of the mantle. This thermal very important role in controlling cated functions of temperature, compo- CMB are quite exciting, involving ris- boundary layer will have temperatures the dynamics of the core and lower sition, and mineralogical phase. Hope ing hot plumes, sinking cold mantle, hotter than the rest of the lower man- mantle, and therefore of the evolu- for better understanding exists because laterally swept mantle dregs, core-man- tle adiabat and will have appropriately tion of the interior of Earth. different disciplines complement each tle chemical reactions, and core-mantle slower velocities. other in providing constraints about dynamic coupling, but because they are The 1980s, however, brought two the state of the deep Earth. Long wave- compatible with evidence across many seismological findings of primary length signals in the geoid are affected independent disciplines, they are not by mass variations in the lowermost quite as speculative as they may seem. Core-Mantle continued on p. 239 Editor’s Note: Each year the David and Lucile Packard Foundation awards 20 Fellowships for Science and Engineering in a national GSA Today To Get Smaller—and Larger competition to promising young scien- tists and engineers working in fields In a step strongly urged by many readers, we’re making your 1996 GSA Today smaller in for- that receive relatively less popular mat, but thicker: down to 81⁄2 by 11 inches and up to 40 pages. The new format will make the attention than high-energy physics, magazine easier to handle and read, will make possible the addition of special supplements, and space, and medicine. 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The science article in this the Washington Report, Forum, and a list of geological meetings; classified and display advertising issue is one of several in which Packard will continue, too. Fellows in earth science report on Look for the new size in January, and watch these pages for other improvements in research in their field. your new GSA Today! IN THIS ISSUE Seismic Images of the GSA Core-Mantle Boundary ....... 237 GSA Today to Get Larger ........... 237 Officers 1996 Officers and Councilors ....... 238 and GSA Today Science Editor ........... 238 Southeastern Section Corrections . 238 Councilors New USGS Program ............... 238 GSA On the Web .................. 238 for 1996 Washington Report ................ 241 Penrose Conference Scheduled ..... 242 Eldridge M. Moores George A. Thompson David A. Stephenson David E. Dunn GeoVentures ...................... 242 President Vice-President Past President Treasurer University of California Stanford University South Pass Resources, Inc. University of Texas—Dallas Award Nominations Deadlines ...... 243 Davis, California Stanford, California Scottsdale, Arizona Richardson, Texas GSAF Update ...................... 244 SAGE Remarks ..................... 245 Councilors (1994–1996) Councilors (1995–1997) Councilors (1996–1998) International Division News ........ 246 1995 Honorary Fellows ............ 246 Keros Cartwright Maryellen Cameron Joanne Bourgeois Illinois State Geological Survey National Science Foundation University of Washington New Members, Fellows ............ 247 Champaign, Illinois Arlington, Virginia Seattle, Washington New Student Associates ............ 248 John A. Cherry James A. Helwig John E. Costa Book Nook ........................ 248 University of Waterloo Mobil Oil Corporation U.S. Geological Survey Book Reviews ...................... 248 Waterloo, Ontario, Canada Dallas, Texas Vancouver, Washington South-Central Section Meeting ....