Definitions of Volcano Terms

Active volcano: A volcano that is erupting at the present, or has erupted recently.

Aerosol: Small particles in the atmosphere; those from volcanoes are sulfuric.

Airfall or ashfall: The ash that falls from the eruption cloud.

Andesite: Volcanic rock; medium composition, color and force of eruption.

Volcanic ash: Fragments less than 2 millimeters (about 1/8 inch) in diameter of lava or rock blasted into the air by volcanic explosions.

Ash flow: A flow of ash.

Ballistic fragment: A piece of rock that follows a ballistic course after being erupted.

Basalt: Darkest, thinnest and least explosive of lava/rock.

Caldera: A large depression formed by a volcano; usually looks circular from above.

Conduit (volcanic): An underground passageway that magma goes through to reach the surface.

Crater: A depression that contains a volcano’s vent.

Dacite: Lighest, thickest, most explosive type of volcanic lava/rock.

Dome: Steep mountain that forms with the eruption of thick lava.

Dormant volcano: A volcano that is not erupting but is likely to do so in the future.

Ejecta: Anything thrown out by a volcano during an eruption.

Extinct volcano: A volcano that most likely won’t erupt ever again.

Fumarole: A volcanic vent that releases volcanic gases.

Fumarolic activity: A release of volcanic gases; temperatures of gases may change.

Granite: Type of igneous rock, or rock formed by hardening of lava.

Hawaiian eruption: Also called fissure eruption. Lava erupts from rifts or vents, usually basaltic lava; it flows slowly into streams or lakes of lava.

Hot Spot: Place in middle of a tectonic plate where magma comes to the surface and sometimes erupts.

Hydrothermal: Water heated by magma.

Hydrothermal alteration: The changing of rocks or minerals when they encounter water heated by magma.

Igneous rocks: Rocks created from melted rock that has hardened after cooling.

Lava : Magma that erupts onto the earth’s surface.

Lava Flow: Occurs when magma erupts from a volcanic vent nonexplosively and flows across a surface.

Lava lake: An area of pooled lava that has a cooled outer crust.

Mafic volcano: A type of that usually erupts for relatively short times (weeks to centuries) but some can become almost as large as composite volcanoes through periodic eruptions.

Magma: Melted rock that forms inside the earth; contains gases and minerals.

Mantle: An area inside of the earth in between the crust and core; approximately 3,000 km thick.

Pumice: Light igneous rock formed by the expansion of gas in an eruption.

Pyroclastic: Broken rock that is formed in a volcanic vent or eruption.  

Pyroclastic flow: Fast and hot flow of particles and gases formed during eruptions.

Rhyolite: Volcanic lava/rock that is thicker and lighter in color than most of the others.

Scoria: Lava that cooled during descent from an eruption, trapping gases inside of it.

Silica: Molecule of silicon and oxygen (SiO2) that makes up most of all volcanic rock/lava and determines the viscosity of the lava.

Silicic: Volcanic rock/lava/magma that contains lots of silica.

Strombolian eruption: An eruption in which lava explodes from the crater of a volcano to form arcs in the sky and flow down the slopes in fast-moving streams.

Subduction Zone: The place where two tectonic plates come together, one going over the other. Most land volcanoes occur at or near these boundaries.

Tectonic: Pertaining to the forces involved in the deformation of the Earth's crust, or the structures or features produced by such deformation.

Tephra: Anything solid that is erupted from a volcano explosively

Vent: The opening at the Earth's surface

Viscosity: The liquidity of a substance; how readily it flows.

Volcano: An opening in the crust of the earth that magma comes through; the landform that is formed by eruptions.

Volcanic cone or edifice: The highest parts of a volcano.

Vulcanian eruption: An eruption in which gas loaded with ash rises high above the volcano during an eruption.