The widely-held plate tectonics theory posits that the Earth's surface, or lithosphere, is divided into a number of different sections, known as plates. Plates may contain continents, oceans or a combination. Seven major plates compose the lithosphere: the North American, South American, Eurasian, African, Indo-Australian, Antarctic and Pacific, as well as multiple smaller plates, such as the Caribbean, Nazca and Arabian. Most of the world's earthquakes and volcanoes occur along the boundaries where plates touch one another.
The plates are composed of rock, averaging about 60 miles deep. This rock is actually lighter than the thick, viscous, fluid layer underneath it, which allows the plates to float. They may drift toward, away from or alongside one another. The plates move constantly, albeit very slowly, at rates varying between half an inch and 5 inches each year.
Earth's core is very hot. Much of this heat comes from gravitational energy that remains from the planet's formation 4.6 billion years ago; the other primary source of heat is the decay of naturally-occurring radioactive elements such as potassium, uranium and thorium. Molten rock occupies the mantle, the layer underneath the plates. The liquid rock moves in convection currents in which hot matter rises, then cools and sinks, akin to a pot of boiling soup. These currents push and pull the plates floating on top of them. Sometimes the plates can stick to one another despite the force of the underlying convection currents. Stress builds until one plate suddenly lurches all at once, resulting in the violent shaking of an earthquake.
The convection currents hypothesis is largely acknowledged as the main reason for tectonic plate shifts. However, other theories exist. For instance, perhaps gravity causes plates to slide, as it pulls hardest on the thinnest sections of plates. Or, if one plate slides underneath another, it starts to melt, and perhaps this action pulls the rest of the plate along with it.