(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 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.
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.
Major Eruptions (1902 - current): 1902 - Mont Pelée erupts, destroying the town of St. Pierre, Martinique, and killing almost 30,000 people.
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.
1914 - 1917 - Lassen Peak in California erupts, covering over 16 square kilometers but resulting in no deaths.
May 18, 1980 - Mount St. Helens erupts, killing 57 people and disrupting highways and railways.
March 24 - April 15, 1984 - The eruption of Mauna Loa in Hawaii, the largest active volcano on Earth, threatens the city of Hilo on the island of Hawaii until the lava flow stops less than three miles from the city.
1986 - Augustine Volcano in Alaska sends up an ash cloud, disrupting air traffic. It previously erupted in 1883, resulting in a 9 meter high tsunami 80 kilometers away. No one died and damage was minimal because the tsunami hit at low tide.
1989-1990 - Debris flows from Redoubt Volcano in Alaska temporarily close the Drift River Oil Terminal and cause a 747 jet to momentarily lose power.
1991 - The eruption of Pinatubo Volcano in the Philippines results in the loss of 350 lives, mostly from collapsed buildings.
1995 - Soufriere Hills Volcano erupts on Montserrat in the West Indies, evacuating the southern half of the island and destroying the capital city of Plymouth.
March 20, 2010 - The volcano Eyjafjallajokull erupts in Iceland for the first time since an eruption in the early 1820s.
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.
May 21, 2011 - The Grimsvotn volcano, Iceland's most active volcano, erupts and grounds hundreds of international flights into and out of Iceland due to a cloud of volcanic ash.
May 25, 2011 - European air travel returns to normal.
June 4, 2011 - The Puyehue-Cordon Caulle volcano complex in southern Chile erupts. Authorities evacuate about 3,500 people from the area. Ash piles as high as 30 centimeters (12 inches) on highways through Patagonia.
October 15-17, 2011 - An ash cloud from the Puyehue volcano in Chile causes flight cancellations in Argentina, Brazil, Chile and Uruguay. The volcano has been sporadically active since late May.
May 7, 2013 - In southeastern Luzon, Philippines, the Mayon volcano erupts with ash and rock for about 73 seconds, producing a cloud that reaches 500 meters above the crater, killing five climbers and injuring eight other members of the same party.
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)
May 18, 1980 - Mount St. Helens registers a VEI 5 with an erupted volume of approximately 1 km.