Plate Tectonics The rocky mountains seem high and solidly fixed, but their height is a tiny fraction of the Earth's radius, and they sit atop a likewise thin solid crust on the Earth. Evidence has mounted that sections or "plates" of this solid crust move with time and have made dramatic movements during the time that life has existed on the Earth. Below is a set of sketches of continent outlines modeled after the USGS site, giving a view of the postulated movements of the continental plates. A major work called The Dyamic Earth is available through the U.S. Geological Survey : http://pubs.usgs.gov/gip/dynamic/dynamic.html . The names Pangaea for the supercontinent and Laurasia and Gondwanaland for the stage after breakup date back to the time of Wegener. Some History

Index Geophysics Concepts References: Ward and Brownlee Ch 9

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Developing the Picture of Plate Tectonics The idea of plate tectonics was resisted strongly when it was first suggested by Frank B. Taylor in 1910. But it had a persistent advocate in Alfred Wegener who argued in favor of it from 1912 until his death in 1930. Wegener pointed to the similarity of separated coastlines and the presence of similar fossils in coastal areas that are presently widely separated. It was not until the 1960s that plate tectonics began to be widely accepted. Geologist Arthur Holmes proposed the existence of convection cells in the upper mantle with hot material rising, carrying the thin crustal skin with it parallel to the Earth's surface and then sinking as it cooled. The model of plate movement was reinforced by the study of magnetic alignment of rock and the study of the dynamics of the ocean floor.

Index Geophysics Concepts References: Ward and Brownlee Ch 9

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