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Here's a look at what you need to know about volcanoes, mountains that open downward to a reservoir of molten rock.

Volcano also refers to a vent in the Earth's crust from which molten rock, ash and gas emanate.

Facts: More than 80 percent of the Earth's surface, including above and below sea level, was formed by volcanoes.

Lava is molten rock from a volcano that has reached the Earth's surface.

Molten rock below the Earth's surface is referred to as magma.

Volcanoes are generally classified into four main types: - Cinder cones - the most common volcanoes, steep conical hills with a vent and a crater at the summit, usually no more than 1,000 feet high. Examples: Sunset Crater Arizona, Lassen Peak in California, and San Quintin Volcanic Field in Baja, Mexico

- Composite volcanoes or stratovolcanoes - symmetrical, cone-shaped, and have a conduit system through which magma flows to the surface through one or more vents, can reach 8,000ft in height. Examples: Mount St. Helens, Mount Fuji, Redoubt, Pinatubo, Soufriere Hills and Mount Pelée in Martinique.

Lava domes - small masses of lava that accumulate around and over the volcano's vent, then cool to break apart flowing down the dome's side. They commonly occur inside the crater of large stratovolcanoes or composite volcanoes. Example: Augustine Volcano in Alaska

Shield volcanoes - form when fluid lava cools to form a gently sloping hill. The largest group of volcanoes on Earth is of this type. Examples: Mauna Loa and Mauna Kea in Hawaii, Eyjafjallajokull and Grimsvotn in Iceland, and Novarupta in Alaska.

There are more than 500 volcanoes that have erupted at least once in the world, and 50 of those are located in the United States.

More than 50 percent of the world's active volcanoes above sea level encircle the Pacific Ocean, forming the "Ring of Fire." The ring starts at New Zealand, goes north around the eastern coast of Asia to the Aleutian Islands, and south down the western coasts of North, Central and South America.

Volcanic Explosivity Index (VEI) Proposed in 1982, measures the relative magnitude of an explosive volcanic eruption.

Similar to the Richter Scale, which measures the magnitude of earthquakes.

On a scale of 0-to-8, each whole number represents a tenfold increase of explosivity.

Uses several factors to determine a number, including volume of erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), height of eruption column, duration in hours, and qualitative descriptive terms. (USGS)

1902 - Mont Pelée erupts, destroying the town of St. Pierre, Martinique, and killing almost 30,000 people.(VEI 4)

October 24, 1902 - Santa Maria in Guatemala erupts. (VEI 6)

1907 - Ksudach in Russia erupts. (VEI 5)

1912 - Novarupta volcano erupts in Alaska. It is the largest volcanic eruption of the 20th century in the U.S., producing 21 cubic kilometers of volcanic material.(VEI 6)

January 17, 1913 - Colima in Mexico erupts. (VEI 5)

1916 - Cerro Azul in Chile erupts. (VEI 5)

January 8, 1933 - Kharimkotan in Russia erupts. (VEI 5)

October 22, 1955 - Bezymianny in Russia erupts. (VEI 5)

February 18, 1963 - Agung in Indonesia erupts. (VEI 5)

May 18, 1980 - Mount St. Helens in Washington erupts, killing 57 people and disrupting highways and railways. (VEI 5)