Scientists say the surface of the Earth is cracked like a huge eggshell. They call these pieces ectonic plates.As many as 20 such plates cover the Earth. They sometimes hit each other, and sometimes move away from each other. Because some continents are above two plates, the continents move when the plates do.
The movement of tectonic plates can cause earthquakes and volcanoes. Understanding this movement can help predict where earthquakes will take place. Research shows that about 90 percent of all earthquakes happen along a few lines in several places around the Earth. These lines follow underwater mountains, where hot liquid rock flows up from deep inside the Earth. Sometimes, the melted rock comes out with a great burst of pressure. This forces apart pieces of the Earth surface in a violent earthquake.
Some earthquakes take place at the edges of continents. Pressure increases as two plates move against each other. When this happens, one plate moves past the other, suddenly causing the Earth surface to split open.
One example of this pressure is found on the west coast of the United States. Part of California is on what is known as the Pacific plate. The other part of the state is on what is known as the North American plate.
Scientists say the Pacific plate is moving toward the northwest, while the North American plate is moving toward the southeast. These two huge plates come together at what is called a fault line. This line between the plates in California is called the San Andreas Fault. It is along or near this fault line that most of California earthquakes take place, as the two tectonic plates move in different directions.
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