The magnetic stripes frozen into the sea floor as it forms at mid-ocean ridges record the Earth’s magnetic field at the time of formation.  Reversals in the Earth’s magnetic field define the edges of these stripes, in effect time-stamping the sea floor position.
Chuck DeMets is Emeritus Professor of Geoscience at the University of Wisconsin, Madison.  He studies the magnetic anomalies in seafloor rocks to reconstruct plate motions at a temporal resolution five times better than has been done hitherto.  This has revealed unexpected speed-ups and slow-downs in plate motions that provide juicy puzzles for geodynamicists.  In the podcast he focuses on the detailed motions of the Indian plate that show, among other things, that its northward movement actually sped up for a period after the collision with Eurasia before it settled down to a steady slower speed.
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