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.
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:
- Distributions of fossils, ancient glacial evidence, and rock types among the continents
- Paleomagnetic evidence or the intensity and direction of residual magnetization in ancient rocks
- Ridges and trenches on the sea floor
- Age of rocks on the sea floor
- Plate movements, originally inferred from geological evidence. These movements have recently been confirmed by Global Positioning Satellite (GPS) measurements.
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.
- The lithosphere consists of seven large and several small rigid plates, 50 to 100 km thick.
- These plates float on the asthenosphere, which extends to about 300 km in depth.
- Plates slide under, into, by, and away from each other.
- Oceans and continents can be on the same plate and move together.