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Volcanic eruptions. Like earthquakes, volcanic eruptions usually occur near the boundaries of the earth's plates, either on land or under the sea. During a volcanic eruption, molten rock, or lava, flows out of the crater resulting in the formation of a volcanic cone. These mountains are called volcanoes. An active volcano is one that always shows signs of volcanic action. Volcanic gas, steam and hot gases may release into the air. This type of activity might signal a forthcoming eruption. At the time of a full-scale eruption ash, lava and volcanic "bombs" might also be thrown out from the crater. Paricutin, a volcano in western Mexico, erupted suddenly in 1943 and stopped erupting just as suddenly in 1952. The first sign was a crack in the ground of a cornfield, followed by rising hot gas and steam. Thirty minutes later, explosions began. Clouds of gas and ashes shot as high as 6 km into the air. For the rest of its active life, the explosions continued. The ashes that fell to the ground formed a cone-shaped mountain that now rises 410 m above the level of the cornfield. Because Paricutin has shown no further signs of volcanic actions since 1952, it is said to be dormant, or sleeping. At some future time, it may once again wake up to send more ashes to fall on the earth. Mount St Helens, located in southwestern Washington, had been dormant for over 100 years when it erupted on May 18, 1980. The eruption blew off 390 m of the mountain's top, sent smoke and ash as high as 18 km, and took 61 lives. It was the first volcanic eruption to take place in the United States since 1917. Volcanic eruptions have killed nearly 200,000 people in the last 600 years. Despite their destructiveness, volcanic eruptions provide useful materials. Volcanic rock is widely used for road building. Volcanic ash fertilizes land, and some chemicals can be made from volcanic sulfur. What is seafloor spreading? What does the theory of plate tectonics state? According to the theory of plate tectonics, on what do the plates move? How does this theory relate to scientific knowledge about earthquakes and volcanic eruptions? Why is the Pacific rim called the «Ring of Fire»? How do seismologists measure the intensity of earthquakes?
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