The Earth is not a perfect sphere. It is flattened at the poles and bulges at the equator. By tracking the orbit of satellites, it is possible to determine accurately the shape of the Earth. The shape of the Earth, called the geoid, is represented in the global geoid height map at right. Red areas are higher while blue areas are lower.
Concentrations of earthquakes outline several large segments of the lithosphere called plates. The lithospheric plates "float" on the asthenosphere and move about the Earth's surface. Some plates carry whole continents with them. The theory that describes these plates and their movement is called plate tectonics.
At the mid-ocean ridges, new rock is produced by volcanism and the plates move away from each other. Where two plates approach each other, one is thrust downward into the mantle where it is heated and melted.
On this map of the Earth, each red triangle represents the location of an active volcano. Volcanoes are concentrated along plate boundaries. Oceanic ridges are found where plates spread apart (diverge). Most surface volcanoes are located near converging plate boundaries (subduction zones), where two plates collide and one plate is driven beneath the other. Exceptions include volcanic Islands in the mid-Pacific Ocean, which are formed as the plate moves over hotspots in the Earth's mantle.
Two hundred million years ago all the Earth's continents formed a single land mass called Pangea.
The continents began to 'drift' apart about 150 million years ago. Today, the 'drifting' continues. For example, every year North America moves 2-3 centimeters (about 1 inch) farther from Europe.