A volcano is a vent and associated landform where molten rock, ash, and gases from Earth’s interior reach the surface. Most volcanoes are concentrated at tectonic plate boundaries, though some occur above mantle hotspots, and eruptions range from effusive lava outpourings to highly explosive events with significant hazards to people, infrastructure, and climate.
Last updated September 7, 2025
A volcano is a vent in Earth’s crust (and the landform built around it) through which molten rock, hot fragments, and gases are erupted, with the term also applied to similar features on other planets and satellites. Encyclopaedia Britannica. Most of the world’s active volcanoes lie along the margins of Earth’s moving plates, with many encircling the Pacific in the Ring of Fire, although some form within plates above mantle plumes known as hotspots. U.S. Geological Survey; USGS; USGS.
Distribution and tectonic setting
Volcanic activity is closely linked to Plate tectonics, with subduction zones producing chains of arc volcanoes, divergent boundaries such as mid‑ocean ridges generating widespread submarine volcanism, and intraplate Hotspot systems like Hawai‘i creating linear island chains. U.S. Geological Survey; USGS. About 1,350 potentially active volcanoes are recognized on land worldwide, excluding those along spreading‑center ridges, and roughly 500 have erupted in historical time. USGS. The Smithsonian Institution’s Global Volcanism Program catalogs about 1,250 Holocene volcanoes (active within the past 12,000 years) and typically observes 40–50 continuing eruptions at any given time. Smithsonian GVP. Submarine volcanism at spreading ridges accounts for the majority of magma reaching Earth’s crust each year and forms a nearly 60,000‑km global ridge system. National Geographic.
Principal types and landforms
Volcanoes are commonly classified by morphology and eruptive products into Shield volcanoes, Stratovolcanoes (composite volcanoes), cinder cones, and lava domes. USGS. Shield volcanoes such as Mauna Loa are built by numerous fluid basaltic flows to produce broad, gently sloping edifices, sometimes among the largest mountains on Earth. USGS. Stratovolcanoes like Mount Fuji are steep‑sided edifices composed of interlayered lava, ash, and other pyroclastic deposits, typically associated with subduction zones. Britannica. Cinder cones form from accumulations of scoria around a single vent, often with summit craters, as at Parícutin in Mexico. USGS. Lava domes grow by extrusion of viscous magma, and large roof‑collapse events can produce block‑and‑ash flows. Encyclopaedia Britannica. Calderas—large depressions formed by roof collapse after major evacuations of shallow magma—cap some volcanic systems and may host post‑caldera domes and lakes. USGS.
Magma, eruptive styles, and products
Magma composition, temperature, gas content, and viscosity govern eruptive behavior, with basaltic magmas generally hotter and less viscous than andesitic or rhyolitic magmas and thus more prone to effusive activity. Tulane University. Basaltic lavas commonly form pāhoehoe (smooth, ropy) and ‘a‘ā (rough, rubbly) textures, and transitions between them result from changing shear and cooling conditions during flow. USGS; USGS. Volcanic gases are dominated by water vapor, carbon dioxide, and sulfur dioxide, with minor H2S, CO, HCl, and HF, influencing plume chemistry and hazards. USGS. Explosive eruptions generate pyroclastic materials from ash to bombs and can produce pyroclastic density currents (pyroclastic flows) that are extremely hot and fast‑moving. USGS; USGS.
Eruptive magnitude is commonly expressed with the Volcanic Explosivity Index (VEI), a logarithmic, semi‑quantitative scale (0–8) based primarily on erupted tephra volume and plume height. USGS; Journal of Geophysical Research. The VEI formalism, introduced by Newhall and Self (1982), enables comparisons among historic and prehistoric eruptions across orders of magnitude. NASA/ADS abstract; USGS.
Hazards and impacts
Volcanic hazards include lava flows, tephra fall (ashfall), ballistic projectiles, volcanic gases, debris avalanches, pyroclastic flows, and lahars (volcanic mudflows), each with distinct footprints and timescales. USGS; USGS. Lahars can initiate during eruptions by rapid snow‑ and ice‑melt or by crater‑lake breaches and can also recur for years to decades when fresh ash is remobilized by rainfall, traveling tens of kilometers along river valleys. USGS; USGS. Volcanic ash poses serious hazards to aviation, prompting a global network of Volcanic Ash Advisory Centers; in the United States, the Washington and Anchorage VAACs issue operational advisories and graphics for airborne ash. NOAA/NWS. Large explosive eruptions can inject sulfur‑bearing aerosols into the stratosphere, affecting radiative balance and temporarily cooling global climate, as documented following the 1991 Pinatubo eruption. USGS.
Monitoring and forecasting
Volcano monitoring integrates seismicity, deformation (GPS, tilt, InSAR), gas emission rates and chemistry (e.g., SO₂ flux), hydrologic and thermal measurements, and remote sensing to detect unrest and inform hazard mitigation. USGS. Coordinated observatory operations and international programs support crisis response and capacity building to reduce risk from future eruptions. USGS. Satellite observations can characterize plume heights, ash dispersal, and gas composition during remote or submarine events, providing critical information for aviation and emergency management. NASA Earth Observatory; NASA JPL.
Notable eruptions (selected examples)
The April 1815 eruption of Mount Tambora in Indonesia (VEI 7) is the largest observed eruption in recorded history and contributed to the “Year Without a Summer” in 1816 through stratospheric aerosol loading. Britannica; Britannica. The May 18, 1980, eruption of Mount St. Helens (Washington, USA) produced a catastrophic sector collapse, lateral blast, Plinian column to >24 km, pyroclastic flows, and widespread ashfall. USGS; USGS. The June 15, 1991, eruption of Mount Pinatubo (Philippines) was a VEI 6 event that injected ~20 million tons of SO₂ into the stratosphere, causing measurable global cooling and extensive regional lahar hazards. USGS; USGS. The January 15, 2022, submarine eruption of Hunga Tonga–Hunga Ha‘apai generated the highest volcanic plume detected in the satellite era, reaching the mesosphere and injecting unprecedented water vapor into the stratosphere. NASA Earth Observatory; NASA JPL.
Submarine and extraterrestrial volcanism
Most volcanic activity by volume is submarine, especially along mid‑ocean ridges where decompression melting produces new oceanic crust; such ridges form the planet’s longest continuous mountain system. National Geographic. The term “volcano” is also applied beyond Earth, with active volcanism documented on bodies such as Jupiter’s Io and enormous shield volcanoes on Mars, although this article focuses on terrestrial volcanism. Encyclopaedia Britannica.
Study and terminology
Volcano science, or volcanology, draws from geophysics, geochemistry, petrology, and geomorphology to reconstruct eruptive histories, evaluate hazards, and guide monitoring and response. Encyclopaedia Britannica; [The Encyclopedia of Volcanoes](book://Haraldur Sigurdsson|The Encyclopedia of Volcanoes|Academic Press|2015).
A volcano is a vent and associated landform where molten rock, ash, and gases from Earth’s interior reach the surface. Most volcanoes are concentrated at tectonic plate boundaries, though some occur above mantle hotspots, and eruptions range from effusive lava outpourings to highly explosive events with significant hazards to people, infrastructure, and climate.
Last updated September 7, 2025
A volcano is a vent in Earth’s crust (and the landform built around it) through which molten rock, hot fragments, and gases are erupted, with the term also applied to similar features on other planets and satellites. Encyclopaedia Britannica. Most of the world’s active volcanoes lie along the margins of Earth’s moving plates, with many encircling the Pacific in the Ring of Fire, although some form within plates above mantle plumes known as hotspots. U.S. Geological Survey; USGS; USGS.
Distribution and tectonic setting
Volcanic activity is closely linked to Plate tectonics, with subduction zones producing chains of arc volcanoes, divergent boundaries such as mid‑ocean ridges generating widespread submarine volcanism, and intraplate Hotspot systems like Hawai‘i creating linear island chains. U.S. Geological Survey; USGS. About 1,350 potentially active volcanoes are recognized on land worldwide, excluding those along spreading‑center ridges, and roughly 500 have erupted in historical time. USGS. The Smithsonian Institution’s Global Volcanism Program catalogs about 1,250 Holocene volcanoes (active within the past 12,000 years) and typically observes 40–50 continuing eruptions at any given time. Smithsonian GVP. Submarine volcanism at spreading ridges accounts for the majority of magma reaching Earth’s crust each year and forms a nearly 60,000‑km global ridge system. National Geographic.
Principal types and landforms
Volcanoes are commonly classified by morphology and eruptive products into Shield volcanoes, Stratovolcanoes (composite volcanoes), cinder cones, and lava domes. USGS. Shield volcanoes such as Mauna Loa are built by numerous fluid basaltic flows to produce broad, gently sloping edifices, sometimes among the largest mountains on Earth. USGS. Stratovolcanoes like Mount Fuji are steep‑sided edifices composed of interlayered lava, ash, and other pyroclastic deposits, typically associated with subduction zones. Britannica. Cinder cones form from accumulations of scoria around a single vent, often with summit craters, as at Parícutin in Mexico. USGS. Lava domes grow by extrusion of viscous magma, and large roof‑collapse events can produce block‑and‑ash flows. Encyclopaedia Britannica. Calderas—large depressions formed by roof collapse after major evacuations of shallow magma—cap some volcanic systems and may host post‑caldera domes and lakes. USGS.
Magma, eruptive styles, and products
Magma composition, temperature, gas content, and viscosity govern eruptive behavior, with basaltic magmas generally hotter and less viscous than andesitic or rhyolitic magmas and thus more prone to effusive activity. Tulane University. Basaltic lavas commonly form pāhoehoe (smooth, ropy) and ‘a‘ā (rough, rubbly) textures, and transitions between them result from changing shear and cooling conditions during flow. USGS; USGS. Volcanic gases are dominated by water vapor, carbon dioxide, and sulfur dioxide, with minor H2S, CO, HCl, and HF, influencing plume chemistry and hazards. USGS. Explosive eruptions generate pyroclastic materials from ash to bombs and can produce pyroclastic density currents (pyroclastic flows) that are extremely hot and fast‑moving. USGS; USGS.
Eruptive magnitude is commonly expressed with the Volcanic Explosivity Index (VEI), a logarithmic, semi‑quantitative scale (0–8) based primarily on erupted tephra volume and plume height. USGS; Journal of Geophysical Research. The VEI formalism, introduced by Newhall and Self (1982), enables comparisons among historic and prehistoric eruptions across orders of magnitude. NASA/ADS abstract; USGS.
Hazards and impacts
Volcanic hazards include lava flows, tephra fall (ashfall), ballistic projectiles, volcanic gases, debris avalanches, pyroclastic flows, and lahars (volcanic mudflows), each with distinct footprints and timescales. USGS; USGS. Lahars can initiate during eruptions by rapid snow‑ and ice‑melt or by crater‑lake breaches and can also recur for years to decades when fresh ash is remobilized by rainfall, traveling tens of kilometers along river valleys. USGS; USGS. Volcanic ash poses serious hazards to aviation, prompting a global network of Volcanic Ash Advisory Centers; in the United States, the Washington and Anchorage VAACs issue operational advisories and graphics for airborne ash. NOAA/NWS. Large explosive eruptions can inject sulfur‑bearing aerosols into the stratosphere, affecting radiative balance and temporarily cooling global climate, as documented following the 1991 Pinatubo eruption. USGS.
Monitoring and forecasting
Volcano monitoring integrates seismicity, deformation (GPS, tilt, InSAR), gas emission rates and chemistry (e.g., SO₂ flux), hydrologic and thermal measurements, and remote sensing to detect unrest and inform hazard mitigation. USGS. Coordinated observatory operations and international programs support crisis response and capacity building to reduce risk from future eruptions. USGS. Satellite observations can characterize plume heights, ash dispersal, and gas composition during remote or submarine events, providing critical information for aviation and emergency management. NASA Earth Observatory; NASA JPL.
Notable eruptions (selected examples)
The April 1815 eruption of Mount Tambora in Indonesia (VEI 7) is the largest observed eruption in recorded history and contributed to the “Year Without a Summer” in 1816 through stratospheric aerosol loading. Britannica; Britannica. The May 18, 1980, eruption of Mount St. Helens (Washington, USA) produced a catastrophic sector collapse, lateral blast, Plinian column to >24 km, pyroclastic flows, and widespread ashfall. USGS; USGS. The June 15, 1991, eruption of Mount Pinatubo (Philippines) was a VEI 6 event that injected ~20 million tons of SO₂ into the stratosphere, causing measurable global cooling and extensive regional lahar hazards. USGS; USGS. The January 15, 2022, submarine eruption of Hunga Tonga–Hunga Ha‘apai generated the highest volcanic plume detected in the satellite era, reaching the mesosphere and injecting unprecedented water vapor into the stratosphere. NASA Earth Observatory; NASA JPL.
Submarine and extraterrestrial volcanism
Most volcanic activity by volume is submarine, especially along mid‑ocean ridges where decompression melting produces new oceanic crust; such ridges form the planet’s longest continuous mountain system. National Geographic. The term “volcano” is also applied beyond Earth, with active volcanism documented on bodies such as Jupiter’s Io and enormous shield volcanoes on Mars, although this article focuses on terrestrial volcanism. Encyclopaedia Britannica.
Study and terminology
Volcano science, or volcanology, draws from geophysics, geochemistry, petrology, and geomorphology to reconstruct eruptive histories, evaluate hazards, and guide monitoring and response. Encyclopaedia Britannica; [The Encyclopedia of Volcanoes](book://Haraldur Sigurdsson|The Encyclopedia of Volcanoes|Academic Press|2015).
