10 Earthquakes and Tsunamis

Earthquakes and Tsunamis

Shaking earth. During an earthquake, the ground suddenly trembles. Earthquakes can happen on land or under the sea. Some earthquakes shake the ground violently. An earthquake has made the big cracks in these roads in Haiti.

Moving rock. The Earth is made of rock. Inside the Earth, there is very hot rock that moves around very, very slowly. The Earth's surface is made up of big pieces of hard rock, called plates. The plates move around on top of the hot rock. As they move, some plates push against each other. Others pull apart.

When plates push against each other for millions of years, they form mountains. Here are the edges of two plates in Iceland. The plates are moving away from each other, in opposite directions. The gap made in the Earth's surface is called a rift.

Faults. Most earthquakes happen where two plates scrape against each other. In some places there are cracks near the edges of the plates, called faults. This is the San Andreas fault in California, U.S.A. Over 10,000 earthquakes happen along its length, every year.

Sections of rock in a fault shift and push against each other. Sometimes, a big section of rock suddenly slips, making the surrounding rock shake. The shaking spreads up to the surface, causing an earthquake. As the rock in a fault settles after a big earthquake it can cause smaller quakes, called aftershocks.