A volcano is a geological landform (usually a mountain) where magma (rock of the earth`s interior made molten or liquid by high pressure and temperature) erupts through the surface of the planet. Although there are numerous volcanoes (some very active) on the solar system`s rocky planets and moons, on Earth at least, this phenomenon tends to occur near the boundaries of the continental plates. However, important exceptions exist in hotspot volcanoes.
The word volcano comes from the Roman god of fire, Vulcan. Vulcan was said to have had a forge (a place to melt and shape iron) on Vulcano, an active volcano on the Lipari Islands in Italy.
Magma is molten rock within the Earth`s crust. When magma erupts through the earth`s surface it is called lava. Lava can be thick and slow-moving or thin and fast-moving. Rock also comes from volcanoes in other forms, including ash (finely powdered rock that looks like dark smoke coming from the volcano), cinders (bits of fragmented lava), and pumice (light-weight rock that is full of air bubbles and is formed in explosive volcanic eruptions - this type of rock can float on water).
Volcanic eruptions can cause great damage and the loss of life and property.
The largest volcano on Earth is Hawaii`s Mauna Loa. Mauna Loa is about 6 miles (10 km) tall from the sea floor to its summit (it rises about 4 km above sea level). It also has the greatest volume of any volcano, 10,200 cubic miles (42,500 cubic kilometers). The most active volcano in the continental USA is Mt. St. Helens (located in western Washington state).
The largest volcano in our Solar System is perhaps Olympus Mons on the planet Mars. This enormous volcano is 17 miles (27 km) tall and over 320 miles (520 km) across.