Geology

What Drives Plate Motions? Convection (hot rocks rise and cold rocks sink) is the ultimate driver of plate tectonics A. Forces that Drive Plate Motion The mantle consists of almost entirely solid rock, but is hot and weak enough to act like a viscous, fluid-like convective flow. The simplest type of convection is like heating a pot of water. The base, which has been heated up, becomes less dense and rises in thin sheets/blobs that spread out to the surface. As the surface cools, it densifies and the cooler water sinks back to the bottom, where it reheats, etc. This is like mantle convection. Slab pull happens when cold, dense oceanic lithosphere sinks through the less dense underlying warm asthenosphere (sink down like a rock pulled into mantle by gravity).

A. Oceanic-Continental Convergence When a plate capped with continental crust converges with a slab of oceanic lithosphere, the buoyant continental block remains ?floating?; denser oceanic crust sinks into the mantle. When oceanic slab goes down about 100km, melting is triggered in the asthenosphere above it. This is because the ?wet? oceanic rock in a high-pressure place melts at a much lower temperature than ?dry? rock of the same material does. Sediments and oceanic crusts have lots of H2O, which is carried down much depth by a subducting plate. As the plate moves down, heat + pressure drive water from voids in the rock. At a depth of 100km, the wedge of rock is hot enough so that H2O from slab, when exposed, causes some melting.