My Science Box

Here's a great web resource:

http://wise.berkeley.edu 

Summary
What drives the motion of the Earth’s tectonic plates? Partly, it is convection, the process by which heat energy is transferred by currents in a liquid or gas. Convection currents within the mantle carry tectonic plates along with the slowly moving mantle like giant rafts carried along by a current in a river. To help students understand this idea, soapy water in a pie pan is heated from below and convections currents can be observed forming and moving in the soapy water. Several prelude demonstrations help students recognize that hot things rise and cold things sink.