Density and Convection

As we learn about density, remember that there are some factors, like temperature, that can affect the density of matter.
Convection currents in the asthenosphere occur as the semi-solid rock nearer the core heats up.  The increase in thermal (heat) energy causes more motion in the molecules.  As the molecules spread out the density of the semi-solid rock becomes less and the material rises.  As the material reaches the bottom of the lithosphere, it cools, becomes more dense and sinks. 
As long as we maintain the temperature differences between the inner parts of the Earth and the crust, this cycle will continue.
The following link shows convection currents in water as one side of the tank is heated while the other side of the tank is cooled.  Click HERE for video link

Convection Current Demonstration