turned often to the theme of describing Earth processes that "we don't understand."

Marcia McNutt, a session participant from the Massachusetts Institute of Technology, has led numerous marine geophysical expeditions to the South Pacific and has contributed significant new additions to the theory of hot spots. Hot spots, upwelling plumes of hot rock and magma that emanate from the deeper layers of the Earth, may provide important clues to one of the basic features of the Earth's dynamics, mantle convection, a process modeled by another session participant, Michael Gurnis from the Ann Arbor campus of the University of Michigan. Another contributor, Jeremy Bloxham from Harvard University, has conducted an exhaustive study of the Earth's magnetic field by reexamining worldwide historical data spanning centuries and has made some strong inferences about the Earth's core. This deep region has also been explored by simulating the conditions of high pressure and temperature that prevail there, an approach described for the symposium's audience by Russell Hemley of the Carnegie Institution of Washington, where pioneering work has been accomplished in the Geophysical Laboratory through the use of diamond-anvil cells.

The session was organized by Raymond Jeanloz of the University of California, Berkeley, and Sue Kieffer from Arizona State University, whose departmental colleague Simon Peacock was also present. The presenters were also joined in the audience by a pair of prominent geologists from the University of California, Santa Cruz—Elise Knittle, a collaborator with Jeanloz, and Thorne Lay, who has provided pioneering seismological insights about the boundary between the Earth's central core and the mantle above it, a region often referred in its own right as the core-mantle boundary. Together these geologists from many allied disciplines ranged over many of the issues in modern earth science, touching on such basic questions as how the Earth came to be, what it looks like inside, how it works, what early scientists thought about it, and how such views have evolved, as well as what theories now dominate scientific thinking and how scientists develop and test them. In considering these and other questions, they provided a vivid picture of a science almost newly born, one in which the technological and conceptual breakthroughs of the latter half of the 20th century have galvanized a truly interdisciplinary movement.


The universe was created at the Big Bang, probably about 13 billion years ago, but more than 8 billion years passed before the

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