Why do convection currents occur within the earth?
Magma in the lower mantle is heated by the core and rises toward the crust. It then cools and sinks back down toward the core.
Convection currents occur when a reservoir of fluid is heated at the bottom, and allowed to cool at the top.. Heat causes the fluid to expand, decreasing its density. If there is cooler material on top, it will be more compact and therefore, will sink to the bottom. The heated material will rise to the top.
Inside of the Earth, the mantle is heated by the core. When it rises to the crust, it cools off and begins to sink. This cycle runs continuously, and is responsible for tectonic plate activity. The circular motion of the convection cells carries the plates on top of them.