skan0001

A Global View

About 240 million years ago, all the land on the Earth’s surface was joined together in one vast supercontinent that extended from pole to pole. But by 18,000 years ago, the Earth had been transformed; the continents as we know them today had been created and had moved into their present positions. But the Earth’s surface has not stopped moving, and the process continues today.

Why does this movement occur? Deep within the Earth, pressure and heat cause rocks of the mantle to become partly molten, but near the surface a thin layer of solid rock forms the crust. As currents of heat rise and fali within the mantle, the crust breaks up into large pieces, called plates, which move about very slowly on Earth’s surface.

The movement of the plates creates powerful forces that continually create and destroy land features and reshape the Earth's surface. Most of the action takes place at the piąte edges, or boundaries, where plates move against or over each other, or dive back below the surface. As the plates move, mountains are uplifted, the sea floor cracks and spreads, volcanoes erupt, and new land is formed.

form when

molten rock—called magma— rises to the Earth's surface. Some volcanoes occur as one piąte pushes beneath another piąte. Other volcanoes result when a piąte passes over a "hot spot," an area of magma rising from the mantle.

Subduction occurs when an oceanie piąte dives under a Continental piąte and often results in volcanoes and mountains, as well as disastrous earthquakes. When a subduction earthquake occurred near Sumatra in December 2004, the resulting energy was converted into powerful tsunami waves that killed many thousands of people.

mammm

Faulting happens when two plates rub against each other, creating cracks along the piąte edges. The San Andreas Fault in California, where the Pacific and North American plates meet, ranks as one of the most destructive earthguake

Distance from the Earth ’s surface to its center: 6,370 km (3,963 miles)

90 Review 2