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A few hundred years ago, scientists believed that volcanoes were burning mountains of fire. Today we know they are really openings or vents into the hot interior of the planet. About 1500 volcanoes around the world are classified as active. Nearly 90% of these rest in the ring of fire, a band circling the Pacific Ocean. Their location is no accident. While the earth's surface look's stable, it's crust is made up of immense slabs of rock, like puzzle pieces that constantly shift. Where these tectonic plates interact, volcanoes often form. Friction from shifting plates melts the earth's crust, causing rock beneath the crust to liquefy. This molten rock, or magma, then becomes a volcano by erupting through rifts in the plates. Once magma escapes from the earth, it's called lava. But not all lava is the same. Runny lava flows quickly, and forms the gentle slopes of "shield" volcanoes. Thick, sticky lava doesn't flow as far,and tends to form the steeper slopes of "composite" volcanoes. Composite volcanoes can erupt violently, hurling ash and lavaat speeds of up to 200 miles per hour. These eruptions sometimes blow away large chunks of the volcanoes itself. Some volcanoes, like Hawaii's Kilauea, form over hot spots in the earth's crust, where magma rises in the middle of a tectonic plate. Kilauea is one of the most active volcanoes on the planet, and has erupted continuously since 1983. Because Kilauea is a shield volcano, this constant eruption is a gentle one. Lava oozing form Kilauea constantly hardens into new land. Making Hawaii the only state that's still growing. Living near a volcano can be hazardous, as the residents of Pompeii learned in the year 79A.D., when Mount Vesuvius erupted, spewing superheated ash, poisonous gas, and rocks. This deadly combination, called a pyroclastic flow, is far more dangerous than a lava flow. Vesuvius buried Pompeii, claiming the lives of 2,000 people. Though volcanoes can be deadly, they also provide benefits. Volcanoes created 80% of the earth's surface, as well as much of the air we breathe today. Volcanoes create rich soil for farming. And many countries harness their subsurface heat to create geothermal energy. Both creators and destroyers, volcanoes prove that, beneath it's calm surface, Earth remains a living planet and a restless one.
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