
Plate Tectonics

If you look at the world map shown below, you may notice that some of the continents could fit together like pieces of a puzzle.

Picture of world map

Although human history does not record a time when these puzzle pieces were together, all the present continents were once together in a single landmass that scientists called Pangaea (Greek for "all earth"). Over millions of years, continental masses have moved, sometimes toward each other so that they've collided, and sometimes away from each other so that they've been torn apart. In fact, landmasses--our continents--and the rest of the outer "skin" of the earth are continually moving. According to scientists, these movements occur at rates of several centimeters (or inches) per year. The theory that explains how the landmasses once fit together and how the outermost layers of the earth continue to move is plate tectonics, one of the grand unifying theories of geological science. To understand how plate tectonics has affected Ohio and what Ohio was like around 450 million years ago, read about life in the Ordovician .

Main Evidence for Plate Tectonics

The shapes of the continents are one of the first pieces of evidence that led scientists to theorize that the continents were previously joined together into one giant landmass. But other evidence also contributes to the theory of plate tectonics:

Main Features of Plate Tectonics

There are three main features of plate tectonics: plates, plate boundaries, and continental drift in combination with seafloor spreading.

Active Plate Movements

The most active areas on the planet for earthquakes and volcanoes are the subduction zones around the Pacific Ocean. Here the Pacific Plate is pushing under the surrounding plates, which are often bordered by landmasses. The stresses of this movement cause dramatic local shifts in the earth's crust, common in Japan, Alaska, and California earthquakes. And, as the Pacific crust slides under the surrounding plates, it causes melting of rocks above it, forming magma that rises up through the lithosphere to erupt at volcanoes in Japan, Indonesia, and the western coast of the Americas from Alaska through the United States to Mexico and Chile.

Plate Tectonics: A Summary

The following image illustrates the forces that cause plate tectonics. Below the image, four points summarize the internal and external processes that have caused the earth to be structured as it is now--and as it will continue to change in the future.

Picture of forces that cause plate tectonics

Last Updated: September 16, 2010