Volcano is a place where ash, gases, and molten rock from deep underground erupt onto the surface. The word volcano also refers to the mountain of erupted rock and ash that often accumulates at such a place.
Volcanic eruptions result from magma (molten rock below the ground). Magma usually forms 30 to 120 miles (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.
If magma accumulates at a high enough rate, the volcano erupts almost continuously. With magma that accumulates more slowly, the eruption may halt for periods while new magma replaces that which has erupted.
The violence of an eruption depends largely on the amount of gas dissolved in the magma and the magma's viscosity (resistance to flow). Magmas with little gas produce relatively calm eruptions in which lava flows quietly onto the surface. Magmas with much gas can shoot violent jets of gas and ash high into the air. Viscous (thick and sticky) magmas tend to erupt more violently than runnier, more fluid magmas. Water mixing with the erupting magma can make any eruption more explosive.
Volcanoes can create many dangers. Hot ash, gas, lava, and mud can bury or burn people and buildings near an erupting volcano. The most violent eruptions launch large clouds of ash and gas high into the atmosphere, causing problems far from the volcano itself.
Volcanoes also provide benefits. Erupted materials contain many nutrients and can break down to form fertile soils. Volcanic activity provides an important source of geothermal energy, energy from Earth's interior heat. Geothermal energy can power electric generators and heat water and buildings. Undersea volcanoes have built up over time to form islands on which millions of people live. Volcanoes also have inspired myths and legends in many cultures. The word volcano comes from Vulcan, the Roman god of fire. Scientists who study volcanoes are called volcanologists.
