Volcanoes are formed by the deposition of the magma that flows or is ejected, normally from one or several circular vents, as molten or solid material.

Zoë Smith

Volcanoes are formed by the deposition of the magma that flows or is ejected, normally from one or several circular vents, as molten or solid material. Molten magma is known as lava when it reaches the Earth's surface; the solid material—classified as dust, ash, cinders, and bombs depending on size and shape—is called tephra. Volcanoes which form round circular vents are known as central volcanoes; the basin-like mouth of the vent is known as the crater. Most volcanoes tend to be conical in shape; some, however, are much larger structures with very gentle slopes. Often covering many square kilometres, they are known as shield volcanoes.

Some volcanoes are much more active than others. A few may be said to be in a state of permanent eruption, at least during the geological present. Stromboli, in the Lipari Islands, has been constantly active since ancient times; Izalco, in El Salvador, has been active since it first erupted in 1770. Other constantly active volcanoes are found in a belt, called the Ring of Fire,that encircles the Pacific Ocean.

Many other volcanoes, such as Vesuvius, in Italy, continue in a state of moderate activity and then become quiescent, or dormant, for periods ranging from months to centuries. The eruption that succeeds prolonged dormancy is usually violent. This was the case with the 1980 eruption, after 123 years of quiescence, of Mount St Helens in Washington State, United States. The massive eruption of Mount Pinatubo in the Philippines during June 1991 came after more than 600 years of dormancy.

For a long period after it has ceased to erupt either lava or tephra, a volcano continues to emit ...