What is the primary force that causes the sea floor to spread and continents to drift

Thermal convection is the primary force that causes the seafloor to spread and continent's to drift.

From seismic wave velocities we know that the asthenosphere behaves in ductile manner, that is even though it is solid it can flow under stress and behave like a liquid. If this is the case, then it can also convect.

Convection is a mode of heat transfer wherein the heat moves with the material. Convection is caused when material that occurs at a deeper level is heated to the point where it expands and becomes less dense than the material above it.

When this occurs, the hot less dense material rises. In a confined space, rising hot material will eventually cool and become denser than its surroundings. This cool dense material must then sink. This gives rise to convection cells, with hot rising currents and cool descending currents.

If the asthenosphere is in fact moving as a result of convection, then convection could be the mechanism responsible for plate tectonics.

Hot rising currents would occur beneath oceanic ridges. Magma intruding into the ridge would push lithosphere apart at the ridge. As the new lithosphere cools, it will slide off the topographic high that results from the upwelling of the mantle and will eventually become cold and dense. This dense lithosphere will tend to pull the rest of the lithosphere downward. A combination of dragging the lithosphere along the top of the convection cell, ridge push, sliding, and slab pull all appear to be contributing factors to the cause of plate tectonics.

Learn more about continental drift, sea floor spreading and plate tectonics from links below:

www.tulane.edu

www.divediscover.whoi.edu

www.nationalgeographic.org

sciencing.com

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