The mantle rock beneath the earth's plates is extremely hot because of the radioactive iron core that bombards it with particles, as well as the gravitational pressure at that depth. The pressure is what keeps the mantle semi-solid, not allowing it to fully melt. As a result, the mantle is constantly in motion. Plate tectonics describes the effect the mantle's motion has on the topmost crust.
Convection is another reason the mantle flows. Heat is transferred to an object by a surrounding liquid if the liquid or object is moving. If the liquid moves more quickly than the object, then the liquid transfers energy to the molecules of the object.
The lower part of the mantle is closer to the hot iron core and is warmer than the higher part. Convection causes the lower, warmer portion of the mantle to rise and the upper, cooler mantle to fall in a continuous cycle. These convection cells create the motion that slowly drives the tectonic plates around the Earth.
When there are deep fractures in the crust and upper mantle at fault lines caused by earthquakes, the hot mantle rises through the weaker rock. It forms bubbles as it rises, which further breaks the rock because bubbles expand. This melted mantle is called magma and it flows or explodes to the surface as lava.