Slabs Do Not Go Gently

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Science  29 May 2009:
Vol. 324, Issue 5931, pp. 1152-1153
DOI: 10.1126/science.1174664

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In geology textbooks, the fate of the oceanic crust seems straightforward. The ocean floor is created by upwelling of lighter magma at spreading ridges. The magma cools as it moves away from the ridge, forming a stiff layer or “plate” called the oceanic lithosphere. Having increased in density, it then descends back into the mantle in trench regions. Precise seismic tomography studies have revealed that many descending slabs have a more complex evolution and have developed tears, detached from the surface plate, or even broken up into fragments. On page 1173 of this issue, Obayashi et al. (1) not only show clear tomographic evidence for the development of a vertical tear under southwest Japan, but have also found evidence for ongoing plate rupturing. The authors correlated the images directly with measurements of stress revealed by active seismic sources.