Convection is the movement of heat by moving mass at the same time. Often it is caused by a change in density of the mass. Hotter materials tend to have less density than cooler materials.

The hot materials rise while cooler materials begin to descend.

Within the AESTHENOSPHERE pressure and temperature produces weak rock structures that provide a base for convection. Hot mass can rise while cooler masses can sink. Inside the lower areas of the earth heat moves by convection.

This is the process that will be studied in greater detail when we look at PLATE TECTONICS. It supplies the driving force for "continental drift", "sea floor spreading" and the creation of "new rock".