Sea-floor spreading provided proof of Wegener's theory of continental drift, but because both oceanic and continental crust appear to move, a new theory was developed to explain both continental drift and sea-floor spreading. This is the theory of plate tectonics, which states that Earth's lithosphere is divided into tectonic plates that move around on top of the asthenosphere.

What causes the plates to move? Scientists are not sure but have possible answers, which all involve the plates being affected by heat and gravity. The diagram below shows the three possible driving forces of plate tectonics. Click on each driving force for a description.