of the other models and theories of how the Earth works are constructed atop the reigning paradigm of geology, plate tectonics, which has only been widely accepted for less than three decades. Plate tectonics has grown from an intuitive suspicion (when maps produced by explorers showed the uncanny complementary shape of the continental shores on either side of the Atlantic, as if the continents had been unzipped), to an early-20th-century theory called continental drift, to the major set of ideas that unify today's earth sciences.

Plate tectonics is no causal force, but merely the surface manifestation of the dynamics of this "great heat engine, the Earth," said presenter David Stevenson. These surface manifestations concern civilization in an immediate way, however: earthquakes, volcanoes, and unpredicted disasters continue to cause tragic loss of life and property.

Stevenson, chairman of the Division of Geological and Planetary Sciences at the California Institute of Technology, described the scientific unifying power of plate tectonics and how it has transformed geology into a brace of strongly interdisciplinary approaches: "Geologists look at morphology, at the surface of the Earth, to try to understand past movements. Petrologists examine the rocks found at the surface of the Earth, trying to understand the conditions under which that material formed and where it came from. Volcanologists try to understand volcanos. Geochemists," he continued, anticipating the discussion of recent diamond-anvil cell experiments, "look at traces of elements transported upward by the forces within the Earth, trying to discern from them the history of the planet, its dynamic properties, and the circulation within. Seismologists look at the travel times of sound and shear waves for variations in signature that provide clues to both lateral and radial structure within the Earth," one of the primary sources of geological information on the deep planetary interior. ''Geodesists look at the motions of the Earth as a planetary body, constructing comparisons with distant reference frames, for example, by radioastrometric techniques, and the list goes on and on,'' Stevenson added. Most of these specialists feel the crucial tug of one another's insights and theories, since the Earth as a coherent system of features and phenomena may be best explained with reference to a variety of probes and explorations.

The Frontiers of Science symposium was host to a solid contingent of such earth scientists, many of whom have been major contributors to the lively debate surrounding the newer theories arising from plate tectonics. Stevenson's presentation, "How the Earth Works: Techniques for Understanding the Dynamics and Structure of Planets," served as a broad overview of the field. Emphasizing the lively premise that "ignorance is more interesting than knowledge," he re-



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