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Mantle Flow Drives the Subsidence of Oceanic Plates

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Science  02 Apr 2010:
Vol. 328, Issue 5974, pp. 83-85
DOI: 10.1126/science.1185906

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Sinking Sea Floors

The depths of ocean bottoms are constantly fluctuating at a very slow rate in response to the generation (at mid-ocean ridges) and consumption (at subduction zones) of sea-floor material. Because older sea floor is susceptible to sinking as it cools, it has been assumed that sea-floor depth varies directly with its age. However, Adam and Vidal (p. 83; see the Perspective by Tolstoy) now show that the depth of the Pacific Ocean actually varies in response to the mantle underlying the oceanic crust. This effect is clear when sea-floor depth was measured along lithospheric flow lines—which represent the movement of oceanic crust triggered by mantle convection. Because the ocean bottom does not flatten as predicted by previous models, there is no need to invoke any additional heat supply to sustain old oceanic crust in thermal models of the mantle.

Abstract

The subsidence of the sea floor is generally considered a consequence of its passive cooling and densifying since its formation at the ridge and is therefore regarded as a function of lithospheric age only. However, the lithosphere is defined as the thermal boundary layer of mantle convection, which should thus determine its structure. We examined the evolution of the lithosphere structure and depth along trajectories representative of the underlying mantle flow. We show that along these flow lines, the sea-floor depth varies as the square root of the distance from the ridge (as given by the boundary-layer equation) along the entire plate, without any flattening. Contrary to previous models, no additional heat supply is required at the base of the lithosphere.

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