Ch. 6 Volcanoes and Other Igneous Activity

magma

A body of molten rock found at depth, including any dissolved gases and crystals.

lava

magma that reaches Earth’s surface

effusive eruptions

A quiescent eruption that produces mainly outpourings of fluid lava.

viscosity

a measure of a fluid’s resistance to flow

eruption columns

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

aa flows

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

pahoehoe

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

lava tubes

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

pillow lavas

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

volatiles

Gaseous components of magma dissolved in melt. Volatiles readily vaporize (form a gas) at surface pressures.

pyroclastic materials or tephra

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, glassy vesicular rock commonly having a granitic composition.

fissure

A crack in rock along which there is a distinct separation.

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 or pyroclastic materials.

crater

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

calderas

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

parasitic cones

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

fumaroles

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

shield volcanoes

A broad, gently sloping volcano built from fluid basaltic lavas

seamounts

An isolated volcanic peak that rises at least 1000 meters (3000 feet) above the deep-ocean floor

cinder cones or scoria cones section

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

composite volcanoes or stratovolcanoes

A volcano composed of both lava flows and pyroclastic material.

pyroclastic flow or nuée ardentes

A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground

lahar

A mudflow on the slope of a volcano that results when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

tsunamis

The Japanese word for a seismic sea wave.

fissure eruptions

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

basalt plateaus

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

flood basalts

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

volcanic necks or plugs

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

host rock or country rock

Preexisting crustal rocks intruded by magma. Host rock may be displaced or assimilated by magmas.

intrusions or plutons

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

tabular

Describing a feature such as an igneous pluton that has two dimensions that are much longer than the third.

massive

Refers to 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.

dikes

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

sills

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.

batholiths

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

stocks

A pluton similar to but smaller than a batholith.

laccoliths

A massive igneous body intruded between preexisting strata.

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 30C per kilometer in the upper crust.

decompression melting

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

Ring of Fire

The zone of active volcanoes surrounding the Pacific Ocean.

volcanic island arcs

A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occurring

continental volcanic arc

Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.

intraplate (within the plate) volcanism

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

mantle plume

a mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core-mantle boundary.

superplumes

A large mantle plumes. Superplumes are thought to be responsible for creating basalt plateaus.