Peter Cawood on When Plate Tectonics Started
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The heat liberated during the formation of our planet created an ocean of magma.  As it began to cool, the Earth differentiated into a dense metallic core surrounded by a less dense rocky mantle.  At some point, we know that the surface of the Earth must have formed itself into the rigid blocks we call plates, and that these plates began to move and interact with each other as parts of the global process we call plate tectonics.  But did the plates form and did plate tectonics start soon after the Earth differentiated into the metallic core and rocky mantle over four billion years ago, or was there a long interim period before the initiation of plate tectonics?  There is an ongoing debate about this, with some finding evidence for plate tectonics as far back as the Hadean, i.e., over 4 billion years ago, and others suggesting that it was not until the Neoproterozoic, about 750 million years ago, that modern plate tectonics got going.  Peter Cawood is a professor in the School of Earth, Atmosphere, and Environment at Monash University in Melbourne.  His research aims to discover the origin of the Earth’s continents by studying the ancient interiors of continents called cratons.   It is these cratons that preserve the clues as to the conditions and processes that prevailed in the early Earth, which in turn can tell us when and how plate tectonics started.   
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