Volcano Glossary: Volcanic Terms Explained A–Z

A

ʻAʻā

Basaltic lava with a rough, jagged, clinkery surface. It forms when lava cools and degasses as it flows; the Hawaiian name is now used worldwide. Compare pāhoehoe.

Active volcano

A volcano that has erupted in the Holocene (roughly the last 11,700 years) or shows current unrest. By this definition about 1,350 volcanoes on land are potentially active.

Andesite

An intermediate volcanic rock (about 52–63% silica), typical of subduction-zone stratovolcanoes such as those of the Andes, after which it is named.

Ash (volcanic)

Fragments of rock and volcanic glass smaller than 2 mm. Unlike wood ash it is hard, abrasive and does not dissolve in water.

B

Basalt

The most common volcanic rock on Earth: dark, low in silica (about 45–52%) and erupted as runny lava. Ocean floors, Iceland and Hawaii are built mostly of basalt.

Bradyseism

Slow rising and sinking of the ground caused by magma or hydrothermal fluids below. The term is used especially for the Campi Flegrei caldera near Naples.

C

Caldera

A large basin formed when the ground collapses into an emptied magma chamber during or after a big eruption. Examples: Yellowstone, Santorini, Campi Flegrei, Tambora.

Cinder cone (scoria cone)

A small, steep volcano built from loose fragments (cinders/scoria) thrown from a single vent, often in one eruption. Parícutin (Mexico, 1943–52) is the classic example.

Conduit

The pipe-like pathway through which magma rises from the magma chamber to the vent.

Crater

A bowl-shaped depression at the top or flank of a volcano where material is erupted. Much smaller than a caldera.

D

Dacite

A silica-rich volcanic rock (about 63–68% silica) between andesite and rhyolite. Mount St. Helens and Pinatubo erupted dacite.

Dike

A sheet of magma that cuts across rock layers, usually vertical. Most magma reaches the surface through dikes; fissure eruptions in Iceland are fed by them.

Dormant volcano

A volcano that is not erupting now but is expected to erupt again. There is no sharp line between “dormant” and “active”; Vesuvius has been quiet since 1944 but is considered highly dangerous.

E

Effusive eruption

An eruption in which lava flows out relatively gently rather than being blasted apart, as at Kīlauea or in Iceland.

Eruption

Any release of magma, gas or older rock fragments from a volcano at the surface — from a quiet lava flow to a huge explosive blast.

Eruption column

The rising plume of hot gas and ash above an explosive eruption. Plinian columns can reach 20–50 km; Hunga Tonga’s 2022 plume reached about 57 km.

Explosive eruption

An eruption in which gas-rich, usually sticky magma is fragmented into ash and pumice and blasted out. Measured with the VEI.

Extinct volcano

A volcano that is not expected to erupt again because it has lost its magma supply, such as the volcanoes of the Siebengebirge near Bonn.

F

Fissure eruption

An eruption from a long crack rather than a single vent, producing curtains of lava fountains. Typical of Iceland (Holuhraun 2014–15, Sundhnúkur 2023–25).

Fumarole

A vent that emits volcanic gas and steam, often with yellow sulfur deposits around it. A sulfur-rich fumarole is called a solfatara.

G

Geyser

A hot spring that periodically erupts a column of hot water and steam, heated by magma at depth (e.g. Strokkur in Iceland, Old Faithful in Yellowstone).

H

Hotspot

A place where unusually hot mantle rises (a mantle plume) and melts through a plate far from plate boundaries, as under Hawaii and Yellowstone.

I

Ignimbrite

Rock formed by the deposit of a large pyroclastic flow, often welded by its own heat. Supereruptions leave ignimbrite sheets hundreds of km² in size.

L

Lahar

A fast-moving volcanic mudflow of ash, rock and water. Lahars killed about 23,000 people at Armero (Colombia) in 1985. The word is Indonesian (Javanese).

Lapilli

Volcanic fragments 2–64 mm in size (Latin for “little stones”).

Lava

Molten rock that has reached the surface; also the rock formed when it solidifies. Typical temperatures are about 700–1,200 °C.

Lava dome

A steep mound of sticky lava piled up over a vent. Collapsing domes can produce deadly pyroclastic flows (Merapi, Soufrière Hills).

Lava lake

A pool of molten lava that persists in a crater, as at Erta Ale, Nyiragongo, Erebus and (at times) Kīlauea.

Lava tube

A tunnel formed when the surface of a lava flow solidifies while molten lava keeps flowing beneath; after draining it leaves a cave.

Limnic eruption

A sudden release of carbon dioxide dissolved in the deep water of a lake. At Lake Nyos (Cameroon) in 1986 the gas cloud suffocated more than 1,700 people; Lake Monoun killed 37 in 1984. Because volcanic CO₂ keeps seeping in, the same lake can build up gas again — Lake Nyos has been artificially degassed with pipes since 2001.

M

Maar

A broad, shallow crater formed by a phreatomagmatic explosion when magma meets groundwater; often filled with a lake. The Eifel maars in Germany are classic examples.

Magma

Molten or partly molten rock below the surface, usually containing crystals and dissolved gas. Once erupted it is called lava.

Magma chamber

A body of magma stored in the crust, usually several kilometres deep. Modern research shows most are mushy zones of crystals with pockets of melt rather than big liquid caves.

Mantle plume

A column of hot rock rising slowly from deep in the mantle that feeds a hotspot.

Mud volcano

A cone or pool where pressurized mud, water and gas (often methane) reach the surface. Most are not related to magma; Lusi in Java (since 2006) is the best-known example.

O

Obsidian

Natural volcanic glass formed when silica-rich lava cools too quickly for crystals to grow. It breaks with razor-sharp edges and was used for tools.

P

Pāhoehoe

Basaltic lava with a smooth, billowy or ropy surface. Compare ʻaʻā.

Phreatic eruption

A steam-driven explosion caused when groundwater is suddenly heated; no fresh magma is erupted. Often almost without warning (Ontake 2014, Whakaari 2019).

Phreatomagmatic eruption

An explosive eruption caused by magma meeting water, producing fine, wet ash (e.g. Surtsey 1963, Eyjafjallajökull 2010 under the ice).

Plinian eruption

A sustained, highly explosive eruption with a towering column, named after Pliny the Younger, who described Vesuvius in AD 79.

Pumice

Light, frothy volcanic rock full of gas bubbles; it often floats on water. Formed from gas-rich, silica-rich magma.

Pyroclastic flow (pyroclastic density current)

A ground-hugging avalanche of hot gas, ash and rock, often faster than 100 km/h and hotter than 200–700 °C. The deadliest volcanic hazard.

Pyroclastic (tephra)

Any fragments thrown out by an explosive eruption: ash (<2 mm), lapilli (2–64 mm) and blocks or bombs (>64 mm).

R

Rhyolite

The most silica-rich volcanic rock (about 68–77% silica). Its sticky magma drives the largest explosive eruptions, including Yellowstone’s.

Ring of Fire

The roughly 40,000 km horseshoe of subduction zones around the Pacific Ocean that hosts about 75% of the world’s active volcanoes.

S

Sand boil (sand volcano)

A small cone of sand formed when water under pressure pushes sand up to the surface, typically during earthquake liquefaction or behind river dikes during floods. Despite the name it has nothing to do with magma.

Shield volcano

A broad volcano with gentle slopes built from many fluid basaltic lava flows, such as Mauna Loa and Kīlauea.

Silica

Silicon dioxide (SiO₂), the main ingredient of magma. More silica makes magma more viscous and eruptions more explosive.

Stratovolcano (composite volcano)

A steep cone built from alternating layers of lava and pyroclastic material, e.g. Fuji, Vesuvius, Mayon. The most dangerous volcano type.

Strombolian eruption

Mildly explosive, rhythmic bursts of glowing lava fragments caused by large gas bubbles bursting at the surface, as at Stromboli.

Subduction zone

A plate boundary where one plate sinks beneath another. Water released from the sinking plate melts the mantle above and feeds explosive volcanoes.

Supervolcano

Informal term for a volcano that has produced a VEI 8 eruption (more than 1,000 km³ of material), such as Yellowstone or Toba.

T

Tuff

Rock made of compacted volcanic ash. Tuff from the German Eifel was used for many Romanesque churches in the Netherlands.

V

VEI (Volcanic Explosivity Index)

A 0–8 scale for the size of explosive eruptions, based mainly on erupted volume; from VEI 2 upward each step is about ten times larger.

Vent

The opening at the surface through which volcanic material erupts.

Viscosity

A fluid’s resistance to flow. High-viscosity (sticky) magma traps gas and erupts explosively; low-viscosity magma flows easily.

Vog

Volcanic smog: a haze of sulfur dioxide and sulfate particles, well known downwind of Kīlauea.

Volcanic bomb

A blob of lava larger than 64 mm thrown from a vent that partly solidifies in flight, often getting a streamlined or ‘bread-crust’ shape.

Volcanism (volcanic activity)

All processes by which magma and its gases rise through the crust and reach the surface, including eruptions, hot springs and fumaroles.

Volcano

An opening in a planet’s crust where magma, gas and rock fragments reach the surface — and the landform built up around it. Named after Vulcano, the Italian island linked to the Roman fire god Vulcan.

Volcanogenic

Formed by volcanic processes — e.g. volcanogenic sediments, or volcanogenic massive sulfide (VMS) ore deposits formed by hot springs on the sea floor.

Volcanology

The scientific study of volcanoes, magma and eruptions, including monitoring and hazard assessment. A scientist in this field is a volcanologist.

Vulcanian eruption

Short, violent explosions that clear a plugged vent and throw out ash and blocks, named after Vulcano (eruption of 1888–90). Sakurajima is a modern example.