Geology

Crust on the Move

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Science  27 Jun 2008:
Vol. 320, Issue 5884, pp. 1697
DOI: 10.1126/science.320.5884.1697b

Earth's plates move at speeds on the order of about 10 cm/year, so that over 100 million years, about 1000 km of ocean crust are consumed back into the mantle along every subduction zone. The mantle is about 2500 km thick, so some swirling, mixing, bending, and storage of these relic slabs are required over several billion years of plate tectonics. Some mantle volcanic rocks carry signatures of melting of old consumed ocean crust. Two seismic studies help clarify the fate of recently subducted crust through the upper half of the mantle. Li et al. provide an updated P-wave tomographic model of Earth's mantle, which reveals large-scale (>100 km) density variations. The geometry of subducted crust, which tends to be colder and denser than ambient mantle, varies: Slabs subducted eastward beneath North and South America appear to be visible to a depth of about 1200 km. The results confirm that some crust that subducted south of Japan is ponded at about 650 km in the mantle, a region of a major phase transition, but that subducted crust to the north and south has proceeded deeper. Separately, Courtier and Revenaugh show S-wave reflections from depths of >1000 km beneath North America and the South Pacific that probably map relic slabs there. — BH

Geochem. Geophys. Geosyst. 9, 10.1029/2007GC001806 (2008); J. Geophys. Res. 113, 10.1029/2007JB005261 (2008).

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