Exploring The Planets - Earth

The Shape of The Earth
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.	Earth's Geoid 70k JPEG courtesy National Geodetic Survey, US Dept of Commerce

Plate Tectonics

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.

Moving Plates
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.
114k JPEG from Earth's Dynamic System. Image courtesy Center for Earth and Planetary Studies, National Air and Space Museum.	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.

Drifting Continents
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.