2.11-B Mantle Convection: Buoyancy and Uplift: Vertical Movements of the Crust

Mantle convection is a cause of vertical crustal movement. Based on studies of Earth’s gravitational field, it became clear that up and down convective flow in the mantle also affects the elevation of Earth’s major landforms. The buoyancy of hot rising material accounts for broad up-warping in the overlying lithosphere, while downward flow causes down-warping.

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Southern Africa is one region where large-scale vertical motion is evident. Here much of the region exists as an expansive plateau having an average elevation of nearly 1,500 meters, or 5,000 feet. Geologic studies have shown that southern Africa and the surrounding seafloor have been slowly rising for the past 100 million years, even though it has not experienced a plate collision for nearly 400 million years.

Evidence from seismic tomography indicates that a large, mushroom shaped mass of hot mantle rock is centered below the southern tip of Africa. This super plume extends upward about 2900 km, 1800 miles, from the mantle core boundary and spreads out over several thousand kilometers. Researchers have concluded that the upward flow of this huge mantle plume is sufficient to elevate southern Africa.

Extensive areas of downwarping have also been discovered. For example, large nearly circular basins are found in the interiors of some continents. Studies indicate that many major episodes of crustal downwarping are not caused by the weight of accumulating sediments. Rather, they show that the formation of basins promoted the accumulation of vast quantities of sediments. Several of these downwarped structures exist in the United States, including the large basins of Michigan and Illinois.

Similar episodes of large-scale downwarping are known on other continents, including Australia. The cause of these downward movements followed by rebound may be linked to the subduction of slabs of oceanic lithosphere. One proposal suggests that when subduction ceases along a continental margin, the subducting slab detaches from the trailing lithosphere and continues its descent into the mantle. As this detached lithospheric slab sinks, it creates a downward flow in its wake that tugs on the base of the overriding continent. In some situations the crust is apparently pulled down sufficiently to allow the ocean to extend inland. As the oceanic slab sinks deeper into the mantle, the pull of the trailing wake weakens and the continent floats back into isostatic balance.

Playlist of Geology and Earth Science Videos from Snow Mountain
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