Carnegie Institution, Washington, D.C.
Revealing the mysteries of the earth's deep interior: Plates, plumes, and the birth of modern seismology
The long-held view of a stable layered earth was turned on its head by the remarkable discovery of sea floor spreading in the 1960s. Cherished ideas of a static earth and immobile continents died almost overnight, and by 1970 scientists had shown that the outer rind of the earth consisted of a mosaic of rigid plates in continual motion relative to one other. The theory of plate tectonics postulated an earth in constant upheaval, with plumes rising from great depth and lithospheric plates plunging deep into the mantle. Plate tectonics was a massive revolution in earth sciences and an unparalleled success at shedding light on a host of previously unexplained observations. At the same time, however, it raised a whole new set of fundamental questions about forces arising from deep within the earth that shape the surface of the planet. Ironically, the oceans which played the dominate role in shaping the theories of sea-floor spreading and plate tectonics, were discovered to be everywhere less than about 200 my old. It is in the complex mosaic of continental structures that lie hidden the clues to how plate tectonics and the earth itself have evolved over a 4.5 billion year history.
The plate tectonics revolution confronted (and inspired) seismologists with two monumental challenges: (1) how to map subtle structural manifestations deep in the mobile earth that are the indicators of the forces that drive the great plate tectonic events at the earth's surface, and (2) how to image the deep structure of the continents in three-dimensions and on geologic scales precise enough to explain how continents formed and evolved over geologic time. These challenges were met through a national coalition of universities and research institutions that drove the development of a new generation of powerful seismograph systems. The most revolutionary of these were portable seismic instruments of unprecedented versatility, capable of continuous broadband digital recording for years in even the most remote reaches of the globe. Large numbers of these advanced systems, deployed as arrays in key regions, act in essence as a downward pointing "inverted telescope". Seismologists using tomographic techniques (similar to those used in medical CAT scans) are able to analyze the "telescope's" seismic wave observations of local and distant earthquakes to obtain clear images of the earth's interior. These images indicate not only the dynamical workings deep within the earth that drive plate tectonics, but they allow us for the first time to map the deep continental structures that reveal how the planet has been shaped over its history. This talk will describe how seismic images are obtained and the vital clues they yield for solving mysteries as diverse as the causes of earthquakes and mountain building to the origin of diamonds formed deep in the ancient roots of continents.
About
Dr. James
Education:
- B.S., Stanford University, 1962
- M.S., Stanford University, 1963
- Ph.D., Stanford University, 1967
(All degrees in geophysics)
Honors:
- Distinguished IRIS/SSA Lecturer, 2003-2004
- Fellow, American Geophysical Union
- Fellow, Royal Astronomical Society (2000)
- National Science Foundation Graduate Fellow (1963-1966)
Positions held:
- Carnegie post-doctoral fellow, 1966 -1968
- Carnegie staff member, 1968-present
Brief summary of recent
research and activities:
I have devoted most of my professional career to studying the structure, evolution,
and formation of continental lithosphere. Among my major contributions
I include: (1) a plate tectonic synthesis for the evolution of the Andean
orogenic belt and the influence of subduction zone processes on mountain building,
and (2) the chemical nature and seismic structure of the crust and deep mantle
roots beneath ancient continental nuclei, particularly in South America and
southern Africa. My Andean studies were concentrated primarily
in Peru, Bolivia, Chile, and Colombia. For the past decade my work has
involved portable broadband array seismology imagery to investigate how continents
that formed in the early earth differ geophysically and geochemically from continental
masses that formed in later geologic time. I have undertaken portable
array field experiments in southern Africa, Brazil, Venezuela, Galapagos, Azores,
as well as in the U.S. and Canada. I was one of the early organizers
of PASSCAL (Program for Array Seismic Studies of the Continental Lithosphere),
the portable array program within IRIS, and I currently serve as Chairman of
the PASSCAL Committee.
Societies:
- American Geophysical Union
- Society of Exploration Geophysicists
- Royal Astronomical Society
- Seismological Society of America
Some recent publications:
James, D.E., F.R. Boyd, D. Schutt, D.R. Bell, and R.W. Carlson, Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa, G-cubed, in press, 2003.
James, D.E., F. Niu, and J. Rokosky, Crustal Structure of the Kaapvaal Craton and its Significance for Early Crustal Evolution, Lithos, in press, 2003.
James, D.E., Imaging crust and upper mantle beneath southern Africa: The southern Africa broadband seismic experiment, The Leading Edge, 22, 228-249, 2003.
James, D.E. and M.J. Fouch, Formation and evolution of Archaean cratons: Insights from southern Africa, in The Early Earth: Physical, Chemical and Biological Development ( C.M.R. Fowler, C.J. Ebinger, and C.J. Hawkesworth, eds. ), Geological Society, London, Special Publications, 199 , 1-26, 2002.
James, D.E., M.J. Fouch, J.C. VanDecar, S. van der Lee, and Kaapvaal Seismic Group,Tectospheric structure beneath southern Africa , Geophys. Res. Lett. 28 , 2485-2488, 2001.
James, D. E., and I. S. Sacks, Cenozoic formation of the central Andes: A geophysical perspective, Geology and Ore Deposits of the Central Andes, B.F. Skinner, ed., Special Publication No. 7, Society of Economic Geologists , 1-25, 1999.
VanDecar, J.C., D.E. James, and M. Assumpção, Seismic evidence for a fossil mantle plume beneath South America and implications for plate driving forces, Nature, 378 , 25-31, 1995.
Books:
James, D. E., ed., Encyclopedia of Solid Earth Geophysics , Van Nostrand Reinhold Co., Inc., New York, 1328 pp., 1989.
