Chapter 9: Volcanoes and Other Igneous Activity

Intraplate volcanism

Igneous activity that occurs within a tectonic plate away from plate boundaries.

Viscosity

Measure of a fluid's resistance to flow

Volatiles

Gaseous components of magma dissolved in melt. These are readily vaporize at surface pressures.

Eruption Column

Buoyant plumes of hot, ash-laden gases that can extend thousands of meters into the atmosphere.

Aa flow

A type of lava flow that has a jagged, blocky surface.

Pahoehoe flow

A lava flow with a smooth-to-ropey surface.

Lava tube

A tunnel in hardened lava that acts as a horizontal conduit for lava flowing from a volcanic vent. It allows fluid lavas to advance great distances.

Pillow lava

Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.

Pyroclastic materials

The volcanic rock ejected during an eruption, including ash, bombs, and blocks.

Scoria

Hardened lava that has retained the vesicles produced by escaping gases

Pumice

A light-colored, extremely porous igneous rock that forms during explosive volcanic eruptions.

Conduit

A pipelike opening through which magma moves toward Earth's surface. It terminates at a surface opening called a vent.

Vent

The surface opening of a conduit or pipe

Volcanic Cone

A cone-shaped structure built by successive eruptions of lava and/or pyroclastic materials.

Crater

The depression at the summit of a volcano, or that which is produced by a meteorite impact.

Parasitic cone

A volcanic cone that forms on the flank of a larger volcano.

Fumarole

A vent in a volcanic area from which fumes or gases escape.

Shield volcanoes

A broad, gently sloping volcano built from fluid basaltic lavas.

Cinder cone volcanoes (scoria cone)

A rather small volcano built primarily of pyroclastics ejected from a single vent.

Composite volcanoes (Stratovolcano)

A volcano composed of both lava flows and pyroclastic material.

Pyroclastic flow (nuee ardente)

A highly heated mixture, largely of ash and pumice fragments, traveling down the

Aerosol

Tiny solid and liquid particles suspended in the atmosphere.

Caldera

A large depression typically caused by collapse or ejection of the summit area of a volcano.

Fissure

A crack in rock along which there is a distinct separation

Fissure eruption

An eruption in which lava is extruded from narrow fractures or cracks in the crust.

Basalt plateau

The broad and extensive accumulation of lava from a succession of flows emanating from fissure eruptions.

Flood basalt

Flows of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters.

Volcanic neck (plug)

An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano. Also known as a plug.

Pipe

A vertical conduit through which magmatic materials have passed.

Intrusion

A mass of igneous rock that, while molten, was forced into or between other rocks.

Pluton

A structure that results from the emplacement and crystallization of magma beneath the surface of Earth.

Tabular

An igneous pluton that is tabular in shape

Massive

An igneous pluton that is not tabular in shape

Discordant

A term used to describe plutons that cut across existing rock structures, such as bedding planes.

Concordant

A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock.

Dike

A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.

Sill

A tabular igneous body that was intruded parallel to the layering of preexisting rock.

Columnar jointing

A pattern of cracks that form during cooling of molten rock to generate columns that are generally six sided.

Batholith

A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion.

Stock

A pluton similar to but smaller than a batholith.

Laccolith

A form of pluton that has a convex upper roof, with a flat (or approximately flat) floor and could be said to resemble a dome

Lopolith

A saucer- or lens-shaped body of intrusive igneous rock, formed by the penetration of magma between the beds or layers of existing rock and subsequent subsidence beneath the intrusion

Partial Melting

The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

Geothermal gradient

The gradual increase in temperature with depth in the crust. The average is 30°C per kilometer in the upper crust.

Decompression melting

Melting that occurs as rock ascends due to a drop in confining pressure

Phacolith

Lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline.