(CNN) -- 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% 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 8,000 feet 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.
More than 50% 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 in France 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)
March 28, 1982 - El Chichon in Mexico erupts. (VEI 5)
March 27, 1986 - Augustine Volcano in Alaska sends up an ash cloud, disrupting air traffic. (VEI 4)
April 2, 1991 - The eruption of Pinatubo Volcano in the Philippines results in the loss of 350 lives, mostly from collapsed buildings. (VEI 6)
August 8, 1991 - Cerro Hudson in Chile erupts. (VEI 5)
March 20, 2010 - The volcano Eyjafjallajokull erupts in Iceland for the first time since an eruption in the early 1820s. (VEI 4)
April 14-21, 2010 - A stronger eruption of Eyjafjallajokull expels an ash cloud 30,000 feet into the air, shutting down airports in northern and western Europe for a week. The International Air Transport Association (IATA) estimates that the work disruption due to volcanic ash cost the airline industry $1.7 billion in revenue. (VEI 4)
February 1, 2014 - Mount Sinabung in Indonesia, which has been spewing gas since September, erupts, killing at least 14 people.
September 27, 2014 - Mount Ontake in Japan erupts suddenly, engulfing hikers in an ash cloud. The known death toll is 55.