Geophysics

Shaken While Stirred

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Science  11 Feb 2011:
Vol. 331, Issue 6018, pp. 650
DOI: 10.1126/science.331.6018.650-b

When oceanic plates are driven deep beneath continents at subduction zones, massive slabs of the lithosphere slowly get reincorporated by convection processes into the mantle (the large reservoir from which the seafloor originated long ago). The top and bottom boundaries of sinking slabs are roughly marked by two parallel planes of earthquake activity at depths of ∼50 to 200 km below Earth's surface. The upper seismic zone likely results from transformations of minerals in the wet crust as they are heated and squeezed; this process releases water and causes seismic activity. Based on low seismic wave anomalies in tomographic images of subduction zones in Japan, Reynard et al. conclude that the lower seismic zone is caused not by dehydration reactions but by deformation of anhydrous minerals as the slab accumulates stress and bends. Because no appreciable quantities of water are needed to explain these observations, recycling of water into the mantle from oceanic lithosphere may be limited to just the first few kilometers of descent.

Geophys. Res. Lett. 37, L24309 (2010).

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