- from Volcanoes
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Why do volcanoes exist? Because the surface of our Earth is moving all the time.
The Earth’s outer layer is split into plates, like a jigsaw puzzle. The plates are each about 50 miles thick. They float like rafts on a softer layer of rock. As the plates float, they bump together or move apart. When plates run into each other, one plate goes under the other. This is called subduction. When a plate sinks low enough, the rock melts and makes new magma for volcanoes. When plates move apart, magma comes up to the surface. The area where that happens is called a rift zone. The lava that erupts builds wide plains and volcanic mountains.
Other volcanoes take shape in the middle of plates. They form over places in the mantle where the magma is superhot. It’s like someone turned up the gas burner on a stove. As a plate moves over a hotspot, the magma melts through it.
One of America’s most active volcanoes is Mount St. Helens, in Washington State. Another is Kilauea, in Hawaii.