notes-7 - Plate Tectonics G1) - New ideas on continental...

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Plate Tectonics G1) - New ideas on continental drift The wealth of new data from the oceans began to significantly influence geological thinking in the 1960s. In 1960 Harold Hess, a widely respected geologist from Princeton University, advanced a theory that had many of the elements that we now accept as Plate Tectonics . He maintained some uncertainty about his proposal however, and in order to deflect criticism from main-stream geologists he labelled it geopoetry . In fact, until 1962, Hess didn't even put his ideas in writing - except internally to the Navy, which funded his research - but presented them mostly in lectures and seminars. Hess proposed that new seafloor was generated from mantle material at the ocean ridges [see pages 313-314], and that old seafloor was dragged down at the ocean trenches and re-incorporated into the mantle. He suggested that the process was driven by mantle convection currents, rising at the ridges and descending at the trenches. He also suggested that the light continental crust did not descend with oceanic crust into trenches, but that land masses collided and were thrust up to form mountains. Hess's theory formed the basis for our ideas on seafloor spreading and continental drift - but it did not deal with the concept that the crust is made up of specific plates . Although the Hess model was not roundly criticized, it was not immediately accepted, (especially in the US), because it was not well supported by hard evidence. Collection of magnetic data from the oceans continued in the early 1960s, but still nobody understood the origin of the zebra-like patterns. Most assumed that they were related to variations in the composition of the rocks - such as variations in the amount of magnetite - which is a common explanation for magnetic variations in rocks of the continental crust. The first real understanding of the significance of the striped anomalies was the interpretation of a Cambridge graduate student named Fred Vine. Vine was examining magnetic data from the Indian Ocean and, like others before, he noted the symmetry of the magnetic patterns with respect to the oceanic ridge. At the same time, other researchers - led by groups in California and New Zealand - were studying the phenomenon of reversals in the earth's magnetic field. They were trying to determine when such reversals had taken place over the past several million years by analyzing the magnetic characteristics of hundreds of samples from basaltic flows. Although the phenomenon is still not well understood, it is evident that the magnetic field of the earth becomes weakened periodically and then virtually non-existent, and then becomes re-established. It is also evident that the re-established field can have the opposite polarity of the pre-existing field 1 . During periods of reversed polarity a compass would point south instead of north.
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This note was uploaded on 01/13/2012 for the course GLG 111 taught by Professor Staff during the Fall '11 term at Virginia Intermont.

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notes-7 - Plate Tectonics G1) - New ideas on continental...

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