Mantle convection is the slow creeping motion of Earth's rocky
mantle in response to perpetual gravitationally unstable variations in its density. Material near the surface of Earth, particularly oceanic
lithosphere, cools down by
conduction of heat into the oceans and atmosphere, then thermally contracts to become dense, and then sinks under its own weight at convergent
plate boundaries. This
subducted material sinks to some depth in the Earth's interior where it is prohibited, by inherent density stratification, from sinking further. This stoppage creates a
thermal boundary layer where sunken material soaks up heat via thermal conduction from below, and may become buoyant again to form upwelling
mantle plumes.
Styles of convection
There is a current debate within the geophysics community as to whether convection is likely to be 'layered' or 'whole'. Geochemical studies of mid ocean ridge basalts (MORB) and tectonically emplaced upper mantle material have found it to be depleted in 'incompatible' elements such as U and Th; whereas material thought to be derived from the lower mantle i.e. ocean island basalts (OIB) is found to be enriched in incompatible elements. This suggests that the upper and lower mantle are not well mixed so that convection cells are layered with the boundary being between the upper and lower mantle at about 670 km depth. However, geophysicists have employed seismic tomography and have detected features reminiscent of slabs at subduction zones penetrating through the 670 km and down to at least 1800 km depth, this supports the whole mantle convection hypothesis.
See also
