Plate tectonics

Plate tectonics <<tehk TAHN ihks>> explains how parts of Earth's surface were formed. It is a theory, or idea, based on known facts. Plate tectonics tells why volcanoes are found in certain places, why there are high mountains and deep canyons in the oceans, and how mountains form.

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Plate tectonics: Earth's surface

Earth has a hard outer shell. Scientists who study Earth believe that this shell is made up of about 30 pieces, or plates. Some of these plates are huge. For example, the crust under most of the Pacific Ocean is a single plate.

The plates sit on top of a layer of rock. The rock is solid, but it is so hot that it has melted, and this melted rock flows. This flow of melted rock makes the plates move. They move very slowly—only about 4 inches (10 centimeters) each year. That is about as fast as a hair grows. The continents sit on the tops of the plates. So when the plates move, the continents move, too.

How plates move

In some places, the plates move apart from each other. Those places are mostly on the ocean floor. If plates on a continent move apart, water rushes in to form rivers, lakes, and even oceans.

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Plate tectonics: Continental drift

In other places, the plates move toward each other and push against each other. When that happens, the edge of one plate sinks and slides under the other plate.

The sinking plate can make trenches, which are like huge, deep valleys. Sometimes it causes earthquakes. Often volcanoes form along the top plate. Sometimes layers of the top plate crumple like a napkin as the other plate slides under it. Then high mountains are pushed up.

Where plates move

Scientists can measure how fast plates move and where they are moving. For example, they know that the Atlantic Ocean is slowly getting wider. The Pacific Ocean is slowly shrinking.

Scientists have also traced these movements backward in time. They believe the continents were once one big continent. They call it Pangaea <<pan JEE uh>>, which means “all earth.” Pangaea was surrounded by one big ocean called Panthalassa <<PAN thuh LAS uh>>, or “all sea.”

About 200 million years ago, Pangaea broke into two huge continents, Gondwanaland and Laurasia. Gondwanaland included what are now South America, India, Australia, Africa, and Antarctica. Laurasia was made up of what are now Asia, Europe, and North America.

Both of these huge continents broke into smaller pieces. They drifted for millions of years until they reached the places where they are today. For example, Africa and South America were once next to each other. When they broke apart, Africa drifted to meet Asia and Europe. South America drifted to meet North America.

The plates are still moving. Scientists believe that millions of years from now, the continents and oceans will be different in size and shape than they are today.

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