Thomas H. Jordan was awarded the Inge Lehmann Medal at the 2005 AGU Fall Meeting Honors Ceremony, which was held on 7 December in San Francisco, Calif. The medal honors outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.  

Tom Jordan entered geophysics when the plate tectonic revolution was already in full swing. The basic tenets had been worked out, but there remained two basic questions: How do continents fit into this essentially oceanic theory, and what is the style of mantle convection that accompanies the motions of the plates?

Answers to these questions had already been proposed by others. In the case of continents, it was argued that continental crust was all that distinguished continental and oceanic lithosphere. The subcontinental mantle and oceanic mantle were thought to be the same, so that the thickness of either type of lithosphere could simply be explained by conductive cooling. This proposal was simple and elegant, requiring only a modest extension of plate tectonic theory to include the continents. There were also early proposals regarding the pattern of mantle convection that necessarily accompanied surface plate motion. The prevailing wisdom at the time was that plate-related convection was superficial, restricted to the Earth’s upper mantle.

It was in such an environment that Tom made fundamental advances toward our understanding of continents and of mantle convection. Based primarily on travel-time measurements of vertically propagating shear waves, combined with observations of gravity and the composition of mantle xenoliths from old continents, he argued that the mantles beneath old continental crust and old oceanic crust were not the same, with old continents being underlain by thick roots of intrinsically lighter material.

At first glance, this proposal appeared to complicate plate tectonics. How could plates move if the continents possessed such thick roots? But he was right. The tectosphere hypothesis that describes the structure and survival of compositionally distinct, basalt-depleted continental lithospheric mantle is now the cornerstone of continental thought and has been confirmed many times over. Earth science is still struggling with the problem of how such ancient continents formed and evolved, but any such hypothesis must account for the formation of continental tectosphere.

Regarding the issue of mantle convection, Tom devised a powerful observational test for convective style, namely, tracing the fate of subducted slabs by way of the expected signature of the cold slabs on seismic velocity. If slabs could be identified in the lower mantle, then this would argue for the participation of the lower mantle in mantle convection. Again a simple test, again a controversial, unconventional result, again a fight, and again a win. Tomographic images produced decades later have confirmed the penetration of slabs into the lower mantle, consistent with whole-mantle convection.

Tom Jordan has made other advances in geophysics that are too numerous to mention. But these two contributions serve to illustrate several of Tom’s qualities that make him a scientist of the first rank: He has a taste for fundamental problems, a profound respect for seismic observations, a facility for theory that permits a remarkably powerful analysis of data, and the love of the fight that enables him to confidently take on conventional wisdom. It is truly a winning combination.

On a personal note, I have interacted with Tom as his student during those wild, controversial years, as a colleague, and as a friend. It is my pleasure to introduce the 2005 winner of the Inge Lehmann Medal, Tom Jordan.

—PAUL G. SILVER, Carnegie Institution of Washington, D.C.

I traveled to my first San Francisco AGU meeting in 1968 while still an undergraduate at the California Institute of Technology, hitchhiking up the hippie highway to see what geophysics was all about. It seems like yesterday, but I realize it was 37 meetings ago. At that time, the Earth’s interior was still represented as a layered sphere, and we had almost no information about the strange structures within its opaque depths. Pioneers like Inge Lehmann who probed this terra incognito were my heroes, so it is a special honor to receive a medal that commemorates her remarkable achievements. Our world view changed radically as we began to observe how plate tectonics and other geosystems actually worked. I remember many of the seminal discoveries by the specific AGU sessions in which they were originally presented. The exploration of three-dimensional mantle structure with the reconnaissance tools of predigital seismology made it clear that not all plates are thin and much of the return flow is not shallow, as we had been taught. Instead, the continental cratons have chemically distinct, advectively thickened keels (which I called tectosphere), stabilized for billions of years within a system of deep mantle convection. I have had a great deal of fun watching the vague outlines of this image come into vivid, multicolor focus through seismic tomography. People like me get awards like this one because we were lucky to be guided by our mentors, helped by our colleagues, and supported by our families and friends. Charles Archambeau, Gerry Wasserburg, and my father (on his way to Vietnam) kept me pointed toward science when many other directions were possible. Don Anderson, my thesis advisor, taught me to go after the big problems; later, as a friendly adversary in the unending battles about the deep structure of continents, the depth of lithospheric slab penetration, and the nature of mantle convection, he surprised me with his flexible, unconventional interpretations of the observations. From Freeman Gilbert, I learned the value of extending the computational tools for model-based inference and, from Bill Menard, the importance of style and romance in scientific research on land and at sea. I have had the privilege of studying and teaching at some great universities—Caltech, Princeton University, University of California, San Diego, Massachusetts Institute of Technology, and now the University of Southern California. At each of these institutions, my colleagues made doing science fun and productive, and the strong bonds of scientific collaboration cemented many enduring friendships. Since our first days in graduate school, Bernard Minster and I have explored many aspects of science together, not to mention life in general. My daughter, Alexandra, has continually reminded me why seeking an understanding of our planetary home is so worthwhile, and my wife, Margaret, has added new dimensions to the journey. A host of brilliant and wonderful students, Paul Silver notable among them, taught me as much as I ever taught them; they were comrades in mental expeditions to the unknown. In particular, Stu Sipkin, Ken Creager, and Art Lerner-Lam each played a special role in the research specified in Paul’s generous citation. As Paul noted, the early times were rather wild, and we lived them to the max. Nearly four decades in the business have furnished my memory with some pretty tall tales, and though my pace has slowed a bit, the adventures still continue. So, to the young scientists in this audience who have traveled to AGU to see what geophysics is all about, I want to emphasize how satisfying a scientific career can be. You should imagine the day when you will be up here accepting some AGU award. You do not have to rush it—the moment will come soon enough—but in the meantime, I hope you will be able to enjoy, as I have done, the exuberance of scientific discovery. —THOMAS H. JORDAN, University of Southern California, Los Angeles