Earth’s mantle overturns itself about once every 200 Million years (Myrs). Prima facie evidence of this overturn is the motion of Earth’s tectonic plates, about thirteen major ones today. This number waxes and wanes as we go back in time. In fact, plate motion reconstructions based upon the magnetic isochron record of the ocean floor reveal that even during the relatively recent 100 Myrs of Earth history some plates have been lost almost entirely to subduction while new plates have appeared in their place.

Supporting evidence also comes from seismic tomograms of Earth’s interior, i.e. from “cat scans” of our...

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