Geology Chapter 5 Vocabulary

Aa lava flow

a type of basaltic lava flow with a very rough surface.

Andesite

an igneous volcanic rock with an intermediate composition.

Ash

a type of pyroclastic material that is made of pulverized rock and is very small (less than 2 mm).

Ashfall

ash that falls to the ground as the result of an ash cloud (refer to figure 5.23 page 107 for a picture of an ash cloud); ashfallsresult in a layer of ash covering the landscape; ash from ash clouds can travel hundreds of miles with prevailing winds and fall faraway from the volcano.

Ashflow (pyroclastic flow or nuee ardente)

a denser-than-air, fast moving flow of hot gases and ash.

Basalt

an igneous volcanic rock with a mafic composition.

Bimodal volcanism

eruptions of both mafic and felsic materials from a volcanic area, without any significant intermediatematerials; usually associated with continental rifting or continental hot spots.

Caldera

a large cauldron-like depression that forms by collapse after a large eruption.

Cinder cone

a small volcano made of loose pyroclastic fragments (cinders, bombs and blocks), typically cylindrical and basaltic.

Composite volcano (stratovolcano)

a large volcano made of many layers (strata) of lava flows and pyroclastic materials.

Coulee

a short thick flow usually made of rhyolite and issuing from a dome.

Crater

a roughly circular depression at the top of a volcano.

Fumarole

a type of vent from which volcanic gases escape into the atmosphere.

Felsic magma (rhyolitic magma)

magma with a high proportion of silica and a low proportion of iron/magnesium; felsic magmaforms at low temperature and is very viscous.

Fissure eruption

an eruption from a fissure or fracture rather than from a central vent; typically occurs in basaltic magma.

Intermediate magma (andesitic magma)

magma with as intermediate proportion of silica and an intermediate proportion ofiron/magnesium; intermediate magma forms at intermediate temperature and is rather viscous

Lahar

a type of debris flow that is related to a volcano and is made of a mixture of water, ash, and other debris.

Maar

a low-relief crater formed by a shallow steam explosion.

Mafic magma (basaltic magma)

magma with a low proportion of silica and a high proportion of iron/magnesium; basaltic magmaforms at high temperature and is very fluid.

Pahoehoe lava flow

a type of basaltic lava flow with a smooth ropy surface.

Phreatic eruption (steam explosion)

a steam-driven explosion that occurs when ground water is heated.

Pillow lava

elongate rounded structures found in basaltic lava flows that erupt under water or flow into water.

Pyroclastic flow

see ashflow.

Pyroclastics (tephra)

solid or semi molten material that is ejected from a volcano into the air; includes ash, cinders, blocks andbombs

Rhyolite

an igneous volcanic rock with a felsic composition.

Shield volcano

a broad volcano with low angle slopes, usually made of successive basaltic lava flows.

Steam explosion

see phreatic eruption.

Stratovolcano

see composite volcano.

Tephra

see pyroclastics.

VEI

volcanic explosivity index; a relative measure of the explosiveness of an eruption.

Vent

an opening at the earth's surface through which magma erupts or gases are emitted.

Viscosity

resistance to flow.

Volcanic dome

a very steep-sided mass of viscous magma extruded from a vent, which flowing; usually rhyolitic or andesitic incomposition.

Volcano

structure built of lava flows and/or pyroclastics erupted from a central vent