Each year there are millions of earthquakes. Most are mild tremors that cause no damage. They usually go unnoticed. But a few earthquakes can be devastating, like the one that struck San Francisco in 1989 and destroyed hundreds of homes. Earthquakes happen under the ocean, too. If the quakes are strong enough, the shocks create enormous waves. Such powerful waves roll across the ocean at high speeds and may cause more damage when they hit land than the earthquake itself. What causes Earth to quiver and shake? This basic introduction explains why Earth's surface sometimes moves. It shows young readers how scientists believe the earth is made, from its hot iron core to the enormous floating plates that form the cool outer crust where we live. David L. Harrison and Cheryl Nathan examine the nature and causes of earthquakes in vivid, easy-to-understand language and bold illustrations.