Earth’s crust rides on a sea of molten rock called magma, which bubbles to the surface wherever two plates meet. Earthquakes work like pressure valves for this magma (known as lava when it reaches the surface). In ‘’effusive’’ volcanoes (such as the famous volcanoes of Hawaii, U.S.A.), the lava flows at a steady rate, often forming new mountains and islands. Volcanoes with a lot of gases dissolved in their magma and a high content of a chemical known as silica have ‘’explosive’’ eruptions in which they literally blow their tops. Potentially much deadlier than effusive eruptions, explosive volcanoes vaporize the landscape in every direction with hot gas and carpet the terrain with choking ash. The explosive eruption of Mount Vesuvius in A.D.79 destroyed the ancient Roman town of Pompeii, burying it under 13 to 20 feet (4 to 6 m) of hot ash. The geyser-riddled Yellowstone National Park in Wyoming, U.S.A., sits above a ‘’supervolcano’’ that last blew its top 640,000 years ago.
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