In this May 24, 2018 photo from the Federal Emergency Management Agency, lava erupts from a fissure in the Leilani Estates neighborhood near Pahoa on the island of Hawaii. Three lava flows from eruptions of Kilauea volcano are now flowing into the ocean off Hawaii's Big Island. Hawaii County officials say the third flow started pouring into the sea Thursday. Lava is spewing from a fissure in a rural community that's feeding the two other flows that are reaching the waves. (Grace Simoneau/FEMA via AP)
Grace Simoneau/FEMA via AP
In this May 24, 2018 photo from the Federal Emergency Management Agency, lava erupts from a fissure in the Leilani Estates neighborhood near Pahoa on the island of Hawaii. Three lava flows from eruptions of Kilauea volcano are now flowing into the ocean off Hawaii's Big Island. Hawaii County officials say the third flow started pouring into the sea Thursday. Lava is spewing from a fissure in a rural community that's feeding the two other flows that are reaching the waves. (Grace Simoneau/FEMA via AP)
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PAHOA, HI - MAY 5: In this handout photo provided by the U.S. Geological Survey, lava errupts from a new fissure from Luana Street after the eruption of Hawaii's Kilauea volcano on May 5, 2018 in the Leilani Estates subdivision near Pahoa, Hawaii. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
Handout/Getty Images North America/Getty Images
PAHOA, HI - MAY 5: In this handout photo provided by the U.S. Geological Survey, lava errupts from a new fissure from Luana Street after the eruption of Hawaii's Kilauea volcano on May 5, 2018 in the Leilani Estates subdivision near Pahoa, Hawaii. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
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Lava shoots into the night sky from active fissures on the lower east rift of Kilauea volcano, Tuesday, May 15, 2018 near Pahoa, Hawaii. (AP Photo/Caleb Jones)
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(CNN) —  

Here’s a look at volcanoes, mountains that open downward to a reservoir of molten rock.

Facts:

More than 80% of the Earth’s surface, both above and below sea level, is of volcanic origin.

Lava is molten rock from a volcano that reaches the Earth’s surface. Liquid rock below the Earth’s surface is referred to as magma.

More than 50% of the world’s active volcanoes above sea level encircle the Pacific Ocean, forming the “Ring of Fire.” The ring stretches from New Zealand to the coast of South America.

In 2010, a volcano eruption in Iceland disrupted air travel for days across Europe. The eruption expelled an ash cloud 30,000 feet into the air. The International Air Transport Authority estimated that the airline industry lost $1.7 billion in revenue due to the disruption.

In 2018, two significant eruptions took place in populated areas. The Kilauea volcano in Hawaii erupted for three months, spewing lava hundreds of feet in the air and destroying about 700 homes. One death was reported. In Guatemala, the Fuego volcano erupted and killed at least 165 people. The two volcanoes erupted in different ways. Slow-moving lava was the destructive force in Hawaii while the Guatemalan volcano unleashed a lethal mix of ash, rock and volcanic gases.

According to the National Oceanic and Atmospheric Administration (NOAA), there have been more than 600 significant volcanic eruptions worldwide between 1800 and 2020.

Volcanoes are generally classified into four main types:
- Cinder cones: 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.

- Composite volcanoes or stratovolcanoes: These are symmetrical, cone-shaped and have a conduit system through which magma flows to the surface. They can rise 8,000 feet above their surroundings.
Examples: Mount St. Helens, Mount Fuji, Mount Shasta, Mount Cotopaxi, Mount Hood and Mount Rainier.

- 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: Mont Pelée.

- Shield volcanoes: Form when lava cools to form a gently sloping dome. The Hawaiian island chain was created by oceanic shield volcanoes.
Examples: Mauna Loa and Kilauea in Hawaii.

Deadliest Volcanic Eruptions since 1500 AD.

Volcanic Explosivity Index (VEI):

The Volcanic Explosivity Index, which measures the relative explosiveness of volcanic eruptions, was devised in 1982. On a scale of 0-8, each number represents a tenfold increase in explosive power.

The VEI uses several factors to determine a number, including volume of erupted material, height of eruption column and duration in hours.

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

Sources: NOAA Significant Volcanic Eruption Database and Smithsonian volcano database

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. 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)

January 21, 1951 - Mount Lamington in Papua New Guinea erupts and kills nearly 3,000 people. (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)

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

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

October 26, 2010 - Merapi in Indonesia erupts, resulting in 386 deaths. (VEI 4)