National Geographic Volcano101
Mount Vesuvius, a volcano near the Bay of Naples in Italy, is hundreds of thousands of years old and has erupted more than 50 times. Its most famous eruption took place in the year 79 A.D., when the volcano buried the ancient Roman city of Pompeii under a thick carpet of volcanic ash.
Volcanic eruptions result from magma (molten rock below the ground). Magma usually forms 50 to 200 kilometers beneath Earth's surface. It rises because it is less dense than the surrounding rock. Rising magma can collect below or inside a volcano in a region called a magma chamber. As the magma accumulates, the pressure inside the chamber increases. When the pressure becomes too great, the chamber breaks open, and magma rises in the volcano. If magma reaches the surface, an eruption occurs. The hole through which the magma erupts is called a vent.
Source: World Book Online, 2014
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