The Earth's crust is broken into moving plates of "lithosphere". There are seven very large plates, each consisting of both oceanic and continental portions, and a dozen or more small plates.

Each plate is about 80 kilometers (50 miles) thick and can be pictured as having a shallow part that deforms by elastic bending or by brittle breaking, and a deeper part that yields plastically, beneath which is a viscous layer on which the entire plate slides. The plates tend to be internally rigid, and they interact mostly at their edges.

All plates are moving relative to all others.

There are grounds for suggesting that the African plate may now be approximately fixed relative to the deep mantle, but if so it is the only such plate.

Velocities of relative motion between adjacent plates range from less than 1 centimeter (a small fraction of an inch) to about 13 centimeters (5 inches) per year.

Although these velocities are slow by human standards, they are extremely rapid by geologic ones: a motion of 5 centimeters (2 inches) per year, for example, adds up to 50 kilometers (30 miles) in only 1 million years, and some plate motions have been continuous for 100 million years.

Part 2: How the Plates Move
Part 3: The Changing Face of the Earth
Part 4: Volcanoes

From: Hamilton, 1976, Plate Tectonics and Man: Reprint from: USGS Annual Report, Fiscal Year 1976