(CNN) Here's a look at volcanoes, mountains that open downward to a reservoir of molten rock.

Facts:

Volcanoes are generally classified into four main types:

- Cinder cones: These are the simplest type of volcanoes.

Examples: These are the simplest type of volcanoes. They are hills, often steep and formed by an accumulation of congealed lava around a vent. When a cinder cone erupts, the ground shakes as magma rises. Then, a powerful blast throws lava, ash, and gas into the air.Examples: Paricutin in Mexico, Sunset Crater in Arizona.

- Lava domes: These domes are small masses of lava that accumulate around and over the volcano's vent. Lava domes commonly are found inside the craters or on the flanks of large composite volcanoes.

Example: These domes are small masses of lava that accumulate around and over the volcano's vent. Lava domes commonly are found inside the craters or on the flanks of large composite volcanoes.Example: Mont Pelée.

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Volcanic Explosivity Index (VEI):

Major eruptions: Selected VEI 4 and above (1902-current):

Note: Dates listed cover the day of peak explosive activity.

May 6, 1902 - Soufrière St. Vincent on the island of Saint Vincent erupts, resulting in about 1,680 deaths. (VEI 4)

May 8, 1902 - Pelée on Martinique erupts, killing about 28,000 people. (VEI 4)

October 24, 1902 - Santa Maria in Guatemala erupts, resulting in about 2,500 deaths. (VEI 6)

June 6, 1912 - Novarupta volcano erupts in Alaska, killing two people. Novarupta volcano erupts in Alaska, killing two people. It is the largest volcanic eruption of the 20th century. (VEI 6)

January 20, 1913 - Colima in Mexico erupts. The number of deaths is unknown. (VEI 5)

May 19, 1919 - Kelut in Indonesia erupts, resulting in 5,110 deaths. (VEI 4)

March 17, 1963 - Agung in Indonesia erupts, killing more than 1,000 people. (VEI 5)

March 29, 1982 - El Chichón in Mexico erupts, resulting in 1,879 deaths. (VEI 5)

June 15, 1991 - Pinatubo in the Philippines erupts, killing about 350 people. (VEI 6)