Describe tectonic plates and explain how they "float" on the Earth's surface (what do they float on and what causes their movement). (site 1)

Tectonic plates is a theory that the lithosphere is separated into plates that move over the asthenosphere, the molten upper portion of the mantle. Oceanic and continental plates come together, spread apart, and interact at boundaries all over the planet.
Tectonic plates float on top of the molten rock and moving around the planet. The heat from radioactive processes within the planet's interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.