Cool, Sink, and Thicken

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Science  27 May 2011:
Vol. 332, Issue 6033, pp. 1011-1013
DOI: 10.1126/science.332.6033.1011-d

The oceanic lithosphere, which includes the crust and some of the brittle underlying mantle, forms at mid-ocean ridges. As this material is pushed further away from a ridge, it slowly cools over millions of years and sinks, creating deep ocean basins with measurable depths. The evolution of lithosphere thickness with age or distance from the ridge, however, is more difficult to quantify, because seismic methods often fail to resolve its lower limits (over 100 km) across an oceanic plate. Using a method based on waveform modeling of seismic wave precursors, Rychert and Shearer mapped the seismic discontinuity that defines the bottom of the lithosphere across the Pacific Ocean. Like depth, lithosphere thickness increases with distance from the ridge, in the direction that the convecting mantle below pulls it away. Moreover, the boundary layer is defined by a sharp velocity drop, signifying that the depth of the discontinuity is at least in part controlled by thermal variations between the lithosphere and the underlying viscous asthenosphere. Water or melting may also contribute to the sharpness of the discontinuity.

J. Geophys. Res. 116, 10.1029/2010JB008070 (2011).

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