Volcanoes and Other Igneous Activity

Magma

Molten rock beneath Earth’s surface that contains dissolved gases and crystals.

Lava

Magma that reaches Earth’s surface and flows out of a volcano or fissure.

Effusive eruption

A volcanic eruption that steadily emits lava onto the ground rather than explosively.

Viscosity

A measure of a fluid’s resistance to flow; in magma, higher viscosity means slower flow.

Eruption column

A towering cloud of gas, ash, and tephra rising above a volcanic vent during an explosive eruption.

Aa flow

A rough, jagged lava flow that forms when lava cools quickly and breaks into sharp fragments.

Pahoehoe flow

A smooth, ropy lava flow that forms when lava cools slowly and remains fluid.

Lava tube

A tunnel formed by flowing lava beneath the hardened surface of a lava flow.

Pillow lava

Rounded, pillow-shaped masses of lava formed by underwater eruptions.

Volatile

Gaseous components (like water vapor or carbon dioxide) that can escape from magma during eruptions.

Pyroclastic material

Fragments of rock, ash, and pumice ejected during a volcanic eruption.

Tephra

All solid material ejected during an eruption, regardless of size.

Scoria

A dark, vesicular volcanic rock formed from gas-rich basaltic magma.

Pumice

A light, porous volcanic rock formed from gas-rich felsic lava that cools rapidly.

Cinder cone

A small, steep-sided volcano built from pyroclastic fragments such as cinders and ash.

Composite volcano

A large, cone-shaped volcano made of alternating layers of lava flows and pyroclastic deposits.

Pyroclastic flow

A fast-moving, deadly current of hot gas, ash, and volcanic debris that flows down volcano slopes.

Nuée ardente

A type of pyroclastic flow consisting of glowing, incandescent gas and ash; French for “glowing cloud.”

Lahar

A volcanic mudflow or debris flow composed of water, ash, and rock fragments.

Tsunami

A large ocean wave usually caused by underwater earthquakes or volcanic eruptions.

Fissure eruption

A volcanic eruption occurring along cracks or fractures in the crust, producing lava flows.

Basalt plateau

A broad, elevated region formed by repeated eruptions of basaltic lava.

Flood basalt

Extensive outpourings of basaltic lava that cover large areas and form plateaus.

Volcanic neck

The solidified conduit of an ancient volcano exposed after surrounding rock erodes away.

Plug

Solidified lava that fills a volcanic vent or conduit, sometimes leading to explosive eruptions if pressurized.

Fissure

A long, narrow crack in Earth’s surface through which lava or gas escapes.

Conduit

A pipe-like channel through which magma travels to reach the surface.

Vent

The opening at Earth’s surface through which volcanic materials are emitted.

Volcanic cone

The accumulation of volcanic materials (lava, ash, cinders) around a vent forming a cone shape.

Crater

A bowl-shaped depression at the summit of a volcano formed by eruptions or collapse.

Caldera

A large, basin-like depression formed when a volcano collapses after a major eruption.

Parasitic cone

A smaller cone that forms on the flank of a larger volcano from a secondary vent.

Fumarole

A vent emitting volcanic gases such as steam and sulfur dioxide.

Shield volcano

A broad, gently sloping volcano formed by low-viscosity basaltic lava flows.

Seamount

An underwater mountain formed by volcanic activity that does not reach the ocean’s surface.

Host rock (country rock)

The preexisting rock into which magma intrudes.

Intrusion

Magma that cools and solidifies beneath the Earth’s surface forming igneous rock.

Pluton

A body of intrusive igneous rock formed from cooled magma underground.

Tabular

Having a flat, sheet-like shape, as in dikes or sills.

Massive

Large, irregularly shaped intrusive bodies without a definite form.

Discordant

An igneous intrusion that cuts across existing rock layers.

Concordant

An intrusion that forms parallel to existing rock layers.

Dike

A tabular, discordant intrusion that cuts across rock layers.

Sill

A tabular, concordant intrusion that forms parallel to rock layers.

Columnar jointing

A pattern of polygonal columns formed as lava or magma cools and contracts.

Batholith

A massive, intrusive igneous rock body covering more than 100 square kilometers.

Stock

A smaller version of a batholith, typically less than 100 square kilometers in area.

Laccolith

A mushroom-shaped intrusion that domes overlying rock layers.

Partial melting

The process by which only some minerals in a rock melt, producing magma with a different composition.

Geothermal gradient

The rate of temperature increase with depth in Earth’s crust.

Decompression melting

Melting of mantle rock caused by a drop in pressure without a change in temperature.

Ring of Fire

The seismically active zone of volcanoes and earthquakes encircling the Pacific Ocean.

Volcanic island arc

A chain of volcanic islands formed above a subducting oceanic plate.

Island arc

Another term for a volcanic island arc, formed where oceanic plates converge.

Continental volcanic arc

A line of volcanoes formed on a continent above a subduction zone.

Intraplate volcanism

Volcanic activity occurring within a tectonic plate, away from plate boundaries.

Mantle plume

A column of hot mantle material rising toward the surface, often forming hotspots.

Hot spot

A stationary area of volcanic activity fed by a mantle plume beneath a tectonic plate.

Superplume

An exceptionally large mantle plume that influences plate movements and large-scale volcanism.