A slab is a portion of a tectonic plate that is being pushed under another slab. This process is called subduction, and the area where it is occurring is called the subduction zone. Plates are huge pieces of the Earth’s crust and uppermost mantle (lithosphere) and are 50 to 250 miles thick. They drift over the Earth at a pace up to 100 mm a year. Sometimes they push together or one rises on top of the other.
Slabs are responsible for about half of the movement of tectonic plates. As slabs move down into the mantle, which is a layer of molten rock, their gravity provides the energy for the movement of the plates. This happens when one plate is sliding under another one.
There is a difference in density and therefore the buoyancy between the slab and the crust of the plate. This causes slab suction force and also affects the motions of the plate. As the slab contributes its lithospheric material into the mantle, it affects the flow of the currents in the mantle. So there are two different forces: the slab pull from the upper mantle slabs and the slab suction from the lower mantle slabs. Slabs move the plates by both of these forces.
Slabs can contribute to volcanism, which is any activity related to the magma rising from the mantle and pushing through the crust. This activity usually causes volcanoes. If the viscosity of the magma is very thick, it will cause an explosive eruption. If it is thin, it will just flow onto the surrounding area of a volcano. Slabs contribute to this with flux melting. This happens when a volatile material such as water comes into contact with very hot solid rock. The water or other material decreases the melting point of the hot rock. The slab is the source of these volatile materials.
The movement of plates can cause earthquakes, which happen where plates drift apart, where they scrape against one another, or where one is thrusting against another so that one winds up above the other. A huge amount of friction occurs when one plate scrapes along the edge of another. This usually causes earthquakes without volcanic activity.