At the Bottom of the Oceanic Plate

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Science  23 Mar 2012:
Vol. 335, Issue 6075, pp. 1448-1449
DOI: 10.1126/science.1219658

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According to the theory of plate tectonics, huge plates below the Pacific Ocean are moving laterally at a speed of ∼10 cm/year above a weak layer called the asthenosphere. Considering that the horizontal scale of a plate can be as large as 10,000 km (the thickness is ∼100 km), how such a thin plate maintains its rigidity, while sometimes causing mega-earthquakes at the edge, is rather puzzling. Understanding what is happening at the bottom of the oceanic plate remains an open question even after some 40 years of success of plate tectonics. In the past few years, seismologists have reported sharp shallow seismic boundaries beneath oceanic basins that may define the bottom of the plate (13). On page 1480 of this issue, Schmerr (4) reports observations indicating the laterally varying nature of the bottom boundary of the oceanic plate.