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How volcanoes work, explained by a volcanologist

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volcano summary

Volcanoes and geothermal energy

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Written by Barbara B. Decker, Robert W. Decker•All

Fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Nov 12, 2024 • Article History

Key People:: Sir William Hamilton; Arthur L. Day; Walter Herman Bucher

Related Topics:: hot spring; mud volcano; fumarole; volcanic neck; hot-spot volcano

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News •

Highest Alert for Volcano Unleashing Columns of Hot Clouds • Nov. 9, 2024, 3:52 AM ET (Newsweek)

Volcano erupts in eastern Indonesia, killing at least 10 people • Nov. 4, 2024, 11:15 PM ET (CNN)

Indonesia: Volcano eruption kills several • Nov. 4, 2024, 9:09 AM ET (Deutsche Welle)

Volcano erupts in eastern Indonesia, killing 6 • Nov. 3, 2024, 3:17 PM ET (South China Morning Post)

Living on a Volcano’s Edge, Italians Practice for Disaster • Oct. 27, 2024, 4:15 AM ET (New York Times)

Geothermal energy is plentiful, but geothermal power is not. Temperatures increase below Earth’s surface at a rate of about 30 °C per km in the first 10 km (roughly 90 °F per mile in the first 6 miles) below the surface. This internal heat of Earth is an immense store of energy. In the upper 10 km of rock beneath the conterminous United States, it amounts to 3.3 × 10²⁵ joules, or about 6,000 times the energy contained in the world’s oil reserves. The problem in utilizing geothermal energy is extracting it.

geothermal energy resources in the United States

The natural escape of Earth’s heat through its surface averages only 0.06 watt per square metre (0.006 watt per square foot). To make geothermal power practical, some special situation must exist to concentrate Earth’s heat energy in a small area. Underground reservoirs of steam or hot water that can be funneled into a drill hole provide this special situation. Some geothermal steam wells can produce 25 megawatts of thermal power, an amount equal to the normal heat flux of more than 400 square km (150 square miles) of land surface. The key to this concentration is the transfer of heat from deeper levels to the near surface by the ascending magma associated with volcanism. Magma at temperatures close to 1,200 °C (2,200 °F) moves upward to depths of only a few kilometres, where it transfers heat by conduction to groundwater. The groundwater then circulates by convection and forms large underground reservoirs of hot water and steam. Some of this thermal water may escape to the surface as hot springs or geysers.

Holes drilled into a subsurface geothermal system allow rapid transfer of hot water or steam to the surface. At the Geysers, a geothermal field north of San Francisco, superheated steam is directly tapped from porous underground reservoirs. In most other geothermal fields, the hot water is at or below its subsurface boiling temperature—about 300 °C (570 °F) at a depth of 1 km (0.6 mile). The hot water and steam produced from geothermal wells are used as the energy source to drive turbine generators in electric power plants. Hot water from lower-temperature geothermal reservoirs can be used for space heating and other applications. This form of geothermal power is utilized extensively in Iceland.

Some geothermal systems act as natural distilleries in the subsurface, dissolving trace amounts of gold, silver, and other rare elements from their host rocks. These elements may then be deposited at places where changes in temperature, pressure, or composition favour precipitation. Many hydrothermal ore deposits have been formed by once active—and in a few cases still active—geothermal systems. Gold is one more legacy of volcanism.

Robert W. Decker Barbara B. Decker

List of the world’s major volcanoes

The table displays a list of the world’s major volcanoes by region.

Major volcanoes of the world
region		elevation*		first recorded eruption	comments
region		feet	metres	first recorded eruption	comments
*Elevation figures may differ from other sources.
Source: Lee Siebert and Tom Simkin, Volcanoes of the World: An Illustrated Catalog of Holocene Volcanoes and Their Eruptions (2002– ), Smithsonian Institution, Global Volcanism Program Digital Information Series, GVP-3, http://www.volcano.si.edu/world.
Mediterranean	Etna, Sicily, Italy	10,991	3,350	1500 BC	The tallest active volcano in Europe, Etna has had recorded eruptions for millennia, including a huge blast in 1669 that left 20,000 people dead.
	Vesuvius, Campania, Italy	4,203	1,281	217 BC	This storied volcano, which in AD 79 destroyed the city of Pompeii, is still active and is surrounded by several million people living in the area of Naples.
	Stromboli, Eolie Islands, Italy	3,038	926	350 BC	Nearly continuous small eruptions of incandescent lava have given this volcano, located on an island north of Sicily, the nickname "lighthouse of the Mediterranean."
	Vulcano, Eolie Islands, Italy	1,640	500	360 BC	Vulcano, the ancient archetype of the term volcano, has not erupted since the late 19th century.
	Thera (Santoríni), Cyclades, Greece	1,204	367	197 BC	The scenic island of Thera is actually the remains of a volcano that exploded about 1500 BC in one of the largest eruptions of historic times.
Atlantic Ocean	Teide, Tenerife, Canary Islands	12,188	3,715	1396	The highest peak in the Atlantic Ocean, Teide was observed in eruption by Christopher Columbus and still emits hot gas from vents on its slopes.
	Fogo, Cape Verde Islands	9,281	2,829	1500	The highest peak in the Cape Verde Islands, Fogo rises from a caldera created by an ancient volcanic collapse.
	Beerenberg, Jan Mayen, Norway	7,470	2,277	1732	Located on an island in the Arctic Circle, Beerenberg is the northernmost active volcano on Earth.
	Tristan da Cunha, South Atlantic	6,758	2,060	1961	The only recorded eruption of this remote island-volcano occurred in 1961, forcing the evacuation of its small population for two years.
	Askja, Iceland	4,974	1,516	1875	Askja's three large calderas are dotted by volcanically active fissures and cones.
	Hekla, Iceland	4,892	1,491	1104	One of Iceland's most active volcanoes, it has an elongated profile caused by frequent eruptions of lava from a long fissure running parallel to the Mid-Atlantic Ridge.
	Deception Island, South Shetland Islands	1,890	576	1800	This ring-shaped island-volcano off the Antarctic Peninsula has a natural inner harbour formed from a huge caldera.
	Surtsey, Iceland	554	169	1963	This volcanic island emerged from the Atlantic Ocean in a fiery eruption in 1963.
North America	Citlaltépetl, Veracruz-Puebla, Mexico	18,619	5,675	1545	This volcano has been dormant since 1846; its snowcapped cone is the third highest peak in North America.
	Popocatépetl, México-Puebla, Mexico	17,802	5,426	1345	Popocatépetl (Aztec for "smoking mountain") still spreads ash on occasion over the surrounding region.
	Rainier, Washington, U.S.	14,409	4,392	1825	This dormant volcano is one of the premier mountain-climbing destinations in the United States.
	Colima, Colima, Mexico	12,631	3,850	1519	This active volcano frequently ejects ash plumes and lava bombs.
	Hood, Oregon, U.S.	11,240	3,426	1859	The highest peak in Oregon and visible from Portland, this dormant volcano is the focal point of Mount Hood National Forest.
	Lassen, California, U.S.	10,456	3,187	1914	A series of eruptions from 1914 to 1917 marked the southernmost volcanic activity recorded in the Cascade Range.
	Redoubt, Alaska, U.S.	10,197	3,108	1902	Lava domes on this volcano occasionally collapse, melting glaciers and sending mudflows streaming down nearby river valleys.
	Shishaldin, Alaska, U.S.	9,373	2,857	1824	The snow-covered, beautifully symmetrical cone of this Aleutian volcano frequently spouts ash and lava.
	Parícutin, Michoacán, Mexico	9,210	2,807	1943	One of the youngest volcanoes on Earth, it was born of a continuous eruption of lava and ash that lasted from 1943 to 1952.
	St. Helens, Washington, U.S.	8,363	2,549	1831	A gigantic eruption in 1980 devastated 550 square km (210 square miles) of mountain terrain and spread ash over Washington, Idaho, and Montana.
	Pavlof, Alaska, U.S.	8,264	2,519	1790	This active volcano forms a striking landform with its nearby twin peak, Pavlof Sister.
	Veniaminof, Alaska, U.S.	8,225	2,507	1838	Two cones rise from a huge glacier-filled caldera at the top of this volcano.
	Katmai, Alaska, U.S.	6,716	2,047	1912	The summit of this volcano collapsed in 1912 following the gigantic explosion of nearby Novarupta Volcano—the largest eruption recorded in the 20th century.
	El Chichón, Chiapas, Mexico	3,773	1,150	1982	A powerful blast in 1982 destroyed most villages within 8 km (5 miles) and left behind a crater that is now filled with an acidic lake.
Central America and Caribbean	Tajumulco, Guatemala	13,845	4,220	1821(?)	The highest volcano in Central America, its twin peaks rise almost from sea level near the city of San Marcos.
	Acatenango, Guatemala	13,044	3,976	1924	This peak and its twin, Fuego, rise above the old colonial city of Antigua Guatemala.
	Fuego, Guatemala	12,346	3,763	1524	This twin of Acatenango is one of Central America's most active volcanoes, frequently erupting ash and lava.
	Irazú, Costa Rica	11,260	3,432	1723	The highest volcano in Costa Rica, it is a popular tourist spot that offers views of the country's Pacific and Atlantic coasts.
	Poás, Costa Rica	8,884	2,708	1828	Two of the three craters at the summit contain lakes that regularly attract tourists and observers.
	Pacaya, Guatemala	8,373	2,552	1565	Lava explosions are visible from Guatemala City, 30 km (18 miles) away.
	Santa Ana, El Salvador	7,812	2,381	1521	El Salvador's highest volcano.
	San Miguel, El Salvador	6,988	2,130	1510	The slopes of this active volcano are home to coffee plantations and to the important city of San Miguel.
	Izalco, El Salvador	6,398	1,950	1770	Born of a series of eruptions in 1770; its frequent glowing activity caused it to be called the "lighthouse of the Pacific."
	San Cristóbal, Nicaragua	5,725	1,745	1528	The highest volcano in Nicaragua.
	Concepción, Nicaragua	5,577	1,700	1883	This active volcano rises from the northern half of Ometepe Island in Lake Nicaragua.
	Arenal, Costa Rica	5,436	1,657	1922	Frequent explosions and effusions of lava have made this volcano a popular destination for sightseers.
	Pelée, Martinique	4,583	1,397	1792	An eruption of hot gas and ash in 1902 killed 26,000 people in the port city of Saint-Pierre.
	Momotombo, Nicaragua	4,255	1,297	1524	The cone of this volcano, rising on the northern shore of Lake Managua, is a prominent landmark.
	Soufrière, Saint Vincent and the Grenadines	4,003	1,220	1718	An eruption in 1902 killed 2,000 people and ruined half the island of Saint Vincent.
South America	Guallatiri, Chile	19,918	6,071	1825	This volcano is occasionally observed venting steam and gas.
	Cotopaxi, Ecuador	19,393	5,911	1534	Though Cotopaxi is Ecuador's best-known volcano, it has not erupted since the first half of the 20th century.
	El Misti, Peru	19,101	5,822	1542	This volcano, its symmetrical cone towering over the city of Arequipa, holds an important place in Peruvian culture.
	Lascar, Chile	18,346	5,592	1848	This active volcano frequently ejects clouds of ash and pumice.
	Ruiz, Colombia	17,457	5,321	1570	In 1985 a relatively mild eruption melted a glacier on this volcano, triggering mudflows that buried several villages and killed more than 25,000 people.
	Sangay, Ecuador	17,159	5,230	1628	This steep-sided, glacier-covered volcano is almost constantly erupting.
	Tungurahua, Ecuador	16,479	5,023	1640	Lava and mixtures of hot ash and gas frequently flow down the sides of this steep and active volcano.
	Guagua Pichincha, Ecuador	15,695	4,784	1566	Rising just west of the capital city of Quito, this volcano regularly has small eruptions.
	Llaima, Chile	10,253	3,125	1640	Ash columns and lava flows frequently issue from this glacier-clad volcano, which, like Villarrica, is a popular ski area.
	Villarrica, Chile	9,340	2,847	1558	Ash columns and lava flows frequently issue from this glacier-clad volcano, which, like Llaima, is a popular ski area.
	Hudson, Chile	6,250	1,905	1891	This southernmost volcano in the Chilean Andes exploded violently in 1991.
Pacific Ocean	Mauna Kea, Hawaii, U.S.	13,796	4,205	…	This dormant volcano, often snowcapped because of its great height, is home to an astronomical observatory.
	Mauna Loa, Hawaii, U.S.	13,681	4,170	1832	Every three to four years, this huge shield volcano erupts with fountains and streams of incandescent lava.
	Erebus, Ross Island	12,447	3,794	1841	The southernmost active volcano in the world, Erebus ejects lava almost constantly.
	Kilauea, Hawaii, U.S.	4,009	1,222	1790	Since 1983 Kilauea has been in almost continual eruption, producing rivers of lava that flow to the sea 50 km (30 miles) away.
	Loihi, Hawaii, U.S.	−3,199	−975	1996	Frequent effusions of lava from this submarine volcano will bring it to the surface between 10,000 and 100,000 years from now.
East Asia	Klyuchevskaya, Kamchatka, Russia	15,863	4,835	1697	The highest and most active volcano on the Kamchatka Peninsula.
	Fuji, Honshu, Japan	12,388	3,776	781	Dormant since 1707, this volcano has a graceful conical form and is a sacred symbol of Japan.
	Tolbachik, Kamchatka, Russia	12,080	3,682	1739	Tolbachik is enormous, composed of a rounded shield volcano and a conical stratovolcano.
	Shiveluch, Kamchatka, Russia	10,771	3,283	1854	Rising dramatically from a plain north of Klyuchevskaya Volcano, Shiveluch occasionally spouts plumes of ash.
	Ontake, Honshu, Japan	10,049	3,063	1979	Second only to Mount Fuji in elevation and popular esteem, Ontake is an object of worship for pilgrims who climb it each year.
	Bezymianny, Kamchatka, Russia	9,455	2,882	1955	This volcano was considered dormant until 1955–56; in the latter year a gigantic blast created a large crater that has since been filled by a growing lava dome.
	Alaid, Kuril Islands, Russia	7,674	2,339	1790	The largest volcano in the Kuril Islands, Alaid has been observed ejecting huge plumes of ash.
	Aso, Kyushu, Japan	5,223	1,592	864	The gigantic caldera of this volcano is occupied by many peaks, including Naka-dake, one of Japan's most active volcanoes.
	Unzen, Kyushu, Japan	4,921	1,500	1663	A 1792 eruption caused a debris avalanche and tsunami that killed more than 10,000 people.
Southeast Asia and Oceania	Kerinci, Sumatra, Indonesia	12,467	3,800	1838	Located in a national park in central Sumatra, Kerinci is the highest volcano in Indonesia.
	Rinjani, Lombok, Indonesia	12,224	3,726	1847	The caldera at the summit of this volcano is the site of a lake and of an active cone that frequently issues lava.
	Semeru, Java, Indonesia	12,060	3,676	1818	The highest volcano on Java, Semeru is almost continuously erupting lava or ash.
	Slamet, Java, Indonesia	11,247	3,428	1772	Slamet periodically ejects plumes of ash, often in conjunction with mild ground tremors.
	Raung, Java, Indonesia	10,932	3,332	1593	Explosive eruptions frequently originate in vents located within a broad caldera at the peak of this volcano.
	Agung, Bali, Indonesia	10,308	3,142	1808	The highest and most sacred point in Bali, it is traditionally considered to be a throne of the gods and the centre of the world.
	Merapi, Java, Indonesia	9,737	2,968	1548	Located in a densely populated region, Merapi ("Mountain of Fire") is known for ejecting dangerous mixtures of hot gas and ash.
	Apo, Mindanao, Philippines	9,691	2,954	…	The highest peak in the Philippines, Apo is the site of a national park.
	Marapi, Sumatra, Indonesia	9,485	2,891	1770	Sumatra's most active volcano, Marapi has had more than 50 reported eruptions, most of them small or moderate explosions.
	Tambora, Sumbawa, Indonesia	9,350	2,850	1812	In 1815 Tambora blew 1,400 metres (4,600 feet) off its top in the largest explosive eruption ever recorded.
	Ruapehu, North Island, New Zealand	9,176	2,797	1861	Permanently snow-covered above the tree line, this volcano is in a popular ski area.
	Papandayan, Java, Indonesia	8,743	2,665	1772	The collapsed summit of this volcano contains a famous area of sulfurous vents known as Kawah Mas ("Golden Crater").
	Mayon, Luzon, Philippines	8,077	2,462	1616	The Philippines' most active volcano, Mayon frequently ejects plumes of ash and streams of lava.
	Canlaon, Negros, Philippines	7,989	2,435	1866	The highest point in the Visayan Islands, Canlaon is located in a national park that includes craters, hot springs, and a variety of wildlife.
	Ulawun, New Britain, Papua New Guinea	7,657	2,334	1700	The highest peak on the island of New Britain, Ulawun ("North Son") forms a pair with its nearby twin, Bamus ("South Son").
	Kelud, Java, Indonesia	5,679	1,731	1000	The summit crater contains a lake that caused devastating mudflows in previous eruptions but is now controlled by drainage tunnels.
	Lamington, New Guinea, Papua New Guinea	5,512	1,680	1951	Lamington was not known to be a volcano until 1951, when a powerful blast swept the area with hot gas and ash, killing 3,000 people.
	Pinatubo, Luzon, Philippines	4,875	1,486	1991	A gigantic explosion in 1991 may have produced more ash and smoke than any other volcanic eruption of the 20th century.
	Krakatoa (Krakatau), Sunda Strait, Indonesia	2,667	813	1680	In 1883 a series of tremendous explosions collapsed the island of Krakatoa; since 1927 a new cone, Anak Krakatau ("Son of Krakatoa"), has risen from the submerged caldera of the old volcano.
	Taal, Luzon, Philippines	1,312	400	1572	Taal Volcano, also called Volcano Island, rises from Taal Lake, which fills the caldera of an ancient volcano.
Africa and Indian Ocean	Cameroon, Cameroon	13,435	4,095	1650	Lava frequently flows down the flanks of this volcano, located near the Gulf of Guinea in West Africa.
	Nyiragongo, Congo (Kinshasa)	11,384	3,470	1884	Lava flowing from its crater in 1977 and 2002 killed thousands of people and damaged the city of Goma.
	Nyamulagira, Congo (Kinshasa)	10,033	3,058	1865	The most active volcano in Africa, it often emits lava and ash, though fatalities are rare.
	Ol Doinyo Lengai, Tanzania	9,482	2,890	1880	Masai for "Mountain of God," Ol Doinyo Lengai still issues lava from its summit crater.
	Fournaise, Réunion	8,632	2,631	1640	One of the world's most active volcanoes, it regularly issues flows of lava and is the site of a French volcanic observatory.
	Karthala, Comoros	7,746	2,361	1808	Lava flowing from the summit and flanks of this shield volcano frequently reaches the sea.
	Erta-Ale, Ethiopia	2,011	613	1906	Ethiopia's most active volcano; its summit caldera frequently overflows with lava.
	Barren Island, Andaman Islands, India	1,161	354	1787	Located in the Andaman Sea some 1,300 km (800 miles) east of the Indian mainland, Barren Island is the only historically active volcano on Indian territory.