Plate Tectonics

The theory of plate tectonics says that the Earth's outer layer is divided into a dozen or more brittle, rocky plates. These plates are always in motion because they are floating on the Earth's flowing upper mantle. When the plates move, the continents and ocean floor above them move as well.

Scientific evidence suggests that all seven of the continents on Earth today used to be connected in a single land mass called Pangaea. Lithospheric plate motion caused Pangaea to break up and the individual continents to change shape and move away from their placements within Pangaea. The map below shows the approximate locations of the continents on Earth's surface 250 million years ago.

Evidence for the existence of Pangaea includes the following facts:

• Fossils of similar life forms have been found on different continents.

• The coastlines of some of today's continents have matching shapes.

• Some of the different continents that exist today have similar types of rocks.

The scientific theory that explains the movements of the continents is called plate tectonics. The word "plate" is used to describe large, brittle blocks of the Earth's surface which appear to move upon the Earth's mantle as large pieces. The word "tectonics" comes from the Greek word that means "to build."

At the places where two plates meet, constructive (building) or destructive (tearing down) processes may take place. Some of those processes are discussed below.

Mountain Formation

Plates move very slowly, at a rate of one or two centimeters per year. Over hundreds of millions of years, plates sometimes collide. When continents meet head-on, the crust tends to buckle and be pushed upward or sideways. This process, sometimes called "mountain building," is how some mountain ranges are formed.

Some mountain ranges were formed so long ago that they have become eroded down to low hills. Other mountain ranges, such as the Himalayas and the Alps, are still slowly growing during the present day.

Volcano Formation

In some places, the crust on the ocean floor sinks back into the Earth's mantle. As the crust sinks, it melts, and hot melted rock rises up, squeezing through widening cracks. Magma escapes to the surface and creates volcanoes.

A volcano is an opening in the Earth's crust from which lava, steam, and/or ashes erupt or flow. When a volcano erupts, the lava flows down and hardens to form new land. This new land may take the form of a volcanic mountain, a plateau, an island, or an archipelago. An archipelago is a chain of islands. Volcanoes are usually found in the ocean or along the coast.

Volcanic eruptions can cause rapid destruction of habitats and changes to a landscape. They can also benefit the surrounding area. Volcanic ash and dust are rich with minerals. These minerals seep into the soil, making it more fertile and allowing new vegetation to grow back quickly. Another benefit is that many precious metals and gemstones can be found in cooled lava.

Sea-Floor Spreading

Plate tectonics is a relatively new scientific concept, combining the earlier theories of continental drift and sea-floor spreading. Sea-floor spreading is the movement of the Earth's crust away from the mid-ocean ridges.

During the process of sea-floor spreading, hot rock rises up from the mantle and spreads out on the surface to form the ocean floor. As the ocean floor spreads, it pushes the plates around, which in turn move the continents to new locations.

The map below shows the locations of the Earth's mid-ocean ridges, which are the sites of sea-floor spreading.

Earthquakes

Sometimes, plates do not hit head on, but rub past each other instead. Since they do not have smooth edges, the rubbing is jerky and uneven. Pressure builds up and is then suddenly released. The result is an earthquake.

An earthquake is the sudden moving and shaking of a part of the Earth's crust. Earthquakes occur along fault lines, which are cracks in the Earth's crust where lithospheric plates move past one another due to tectonic forces. There are different types of faults, and rocks may move along each of these in a different way. Examples of some different types of faults are shown in the pictures below.