OREGON STATE UNIVERSITY

Definitions

Volcano Term Definitions

a
A'a

Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.

Accidental

Pyroclastic rocks that are formed from fragments of non-volcanic rocks or from volcanic rocks not related to the erupting volcano.

Active Volcano

A volcano that is erupting. Also, a volcano that is not presently erupting, but that has erupted within historical time and is considered likely to do so in the future.

Agglutinate

A pyroclastic deposit consisting of an accumulation of originally plastic ejecta and formed by the coherence of the fragments upon solidification.

Alkalic

Rocks which contain above average amounts of sodium and/or potassium for the group of rocks for which it belongs. For example, the basalts of the capping stage of Hawaiian volcanoes are alkalic. They contain more sodium and/or potassium than the shield-building basalts that make the bulk of the volcano.

Andesite

Volcanic rock (or lava) characteristically medium dark in color and containing 54 to 62 percent silica and moderate amounts of iron and magnesium.

Aquifer

A body of rock that contains significant quantities of water that can be tapped by wells or springs.

Ash

Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted. By far the most common variety is vitric ash (glassy particles formed by gas bubbles bursting through liquid magma).

Ash Flow

A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes or even along a level surface.

Ashfall

Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.

Asthenosphere

The shell within the earth, some tens of kilometers below the surface and of undefined thickness, which is a shell of weakness where plastic movements take place to permit pressure adjustments.

Avalanche

A large mass of material or mixtures of material falling or sliding rapidly under the force of gravity. Avalanches often are classified by their content, such as snow, ice, soil, or rock avalanches. A mixture of these materials is a debris avalanche.

b
Basalt

Volcanic rock (or lava) that characteristically is dark in color, contains 45% to 54% silica, and generally is rich in iron and magnesium.

Basement

The undifferentiated rocks that underlie the rocks of interest in an area.

Bench

The unstable, newly-formed front of a lava delta.

Blister

A swelling of the crust of a lava flow formed by the puffing-up of gas or vapor beneath the flow. Blisters are about 1 meter in diameter and hollow.

Block

Angular chunk of solid rock ejected during an eruption.

Bomb

Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.

c
Caldera

The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.

Capping Stage

Refers to a stage in the evolution of a typical Hawaiian volcano during which alkalic, basalt, and related rocks build a steeply, sloping cap on the main shield of the volcano.

Central Volcano

A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.

Cinder Cone

A volcanic cone built entirely of loose fragmented material (pyroclastics.)

Cirque

A steep-walled horseshoe-shaped recess high on a mountain that is formed by glacial erosion.

Cleavage

The breaking of a mineral along crystallographic planes, that reflects a crystal structure.

Composite Volcano

A steep volcanic cone built by both lava flows and pyroclastic eruptions.

Compound Volcano

A volcano that consists of a complex of two or more vents, or a volcano that has an associated volcanic dome, either in its crater or on its flanks. Examples are Vesuvius and Mont Pelee.

Conduit

A passage followed by magma in a volcano.

Crater

A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.

Curtain of Fire

A row of coalescing lava fountains along a fissure; a typical feature of a Hawaiian-type eruption

d
Dacite

Volcanic rock (or lava) that characteristically is light in color and contains 62% to 69% silica and moderate a mounts of sodium and potassium.

Debris Flow

A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow).

Devonian

A period of time in the Paleozoic Era that covered the time span between 400 and 345 million years.

Diatreme

A breccia filled volcanic pipe that was formed by a gaseous explosion.

Dome

A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent (often circular in plane view) and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.

Dome

Mass of lava, created by many individual flows, that has built a dome-shaped pile of lava.

Dormant

The term is used to describe a volcano which is presently inactive but which may erupt again. Most of the major Cascade volcanoes are believed to be dormant rather than extinct.

Drainage Basin

The area of land drained by a river system.

e
Echelon

Set of geologic features that are in an overlapping or a staggered arrangement (e.g., faults). Each is relatively short, but collectively they form a linear zone in which the strike of the individual features is oblique to that of the zone as a whole.

Ejecta

Material that is thrown out by a volcano, including pyroclastic material (tephra) and lava bombs.

Eruption

The process by which solid, liquid, and gaseous materials are ejected into the earth's atmosphere and onto the earth's surface by volcanic activity. Eruptions range from the quiet overflow of liquid rock to the tremendously violent expulsion of pyroclastics.

Eruption Cloud

The column of gases, ash, and larger rock fragments rising from a crater or other vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the stratosphere, where high winds will carry it long distances.

Extinct

A volcano that is not presently erupting and is not likely to do so for a very long time in the future.

f
Fault

A crack or fracture in the earth's surface. Movement along the fault can cause earthquakes or--in the process of mountain-building--can release underlying magma and permit it to rise to the surface.

Felsic

An igneous rock having abundant light-colored minerals.

Fire fountain

A rhythmic vertical fountainlike eruption of lava.

Fissures

Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.

Flank Eruption

An eruption from the side of a volcano (in contrast to a summit eruption.)

Fluvial

Produced by the action of of flowing water.

Fracture

The manner of breaking due to intense folding or faulting.

Fumarole

A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles.

g
Geothermal Energy

Energy derived from the internal heat of the earth.

Geothermal Power

Power generated by using the heat energy of the earth.

Graben

An elongate crustal block that is relatively depressed (downdropped) between two fault systems.

Guyot

A type of seamount that has a platform top. Named for a nineteenth-century Swiss-American geologist.

h
Hardness

The resistance of a mineral to scratching.

Harmonic Tremor

A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.

Heat transfer

Movement of heat from one place to another.

Heterolithologic

Material is made up of a heterogeneous mix of different rock types. Instead of being composed on one rock type, it is composed of fragments of many different rocks.

Holocene

The time period from 10,000 years ago to the present. Also, the rocks and deposits of that age.

Horizontal Blast

An explosive eruption in which the resultant cloud of hot ash and other material moves laterally rather than upward.

Horst

A block of the earth's crust, generally long compared to its width, that has been uplifted along faults relative to the rocks on either side.

Hot Spot

A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs and may not be associated with ocean ridges.

Hot-spot Volcanoes

Volcanoes related to a persistent heat source in the mantle.

Hyaloclastite

A deposit formed by the flowing or intrusion of lava or magma into water, ice, or water-saturated sediment and its consequent granulation or shattering into small angular fragments.

Hypabyssal

A shallow intrusion of magma or the resulting solidified rock.

i
Ignimbrite

The rock formed by the widespread deposition and consolidation of ash flows and Nuees Ardentes. The term was originally applied only to densely welded deposits but now includes non-welded deposits.

Intermediate

A descriptive term applied to igneous rocks that are transitional between basic and acidic with silica (SiO2) between 54% and 65%.

Intrusion

The process of emplacement of magma in pre-existing rock. Also, the term refers to igneous rock mass so formed within the surrounding rock.

j
Juvenile

Pyroclastic material derived directly from magma reaching the surface.

k
Kipuka

An area surrounded by a lava flow.

l
Laccolith

A body of igneous rocks with a flat bottom and domed top. It is parallel to the layers above and below it.

Lahar

A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow.

Lapilli

Literally "little stones." Round to angular rock fragments, measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state.

Lava

Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.

Lava Flow

An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet-like body formed by outpouring lava.

Lava Lake

A lake of molten lava, usually basaltic, contained in a vent, crater, or broad depression of a shield volcano.

Lava Tube

A tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away.

Lithic

Of or pertaining to stone.

Lithosphere

The rigid crust and uppermost mantle of the earth. Thickness is on the order of 60 miles (100 km). Stronger than the underlying asthenosphere.

Luster

The reflection of light from the surface of a mineral.

m
Maar

A volcanic crater that is produced by an explosion in an area of low relief, is generally more or less circular, and often contains a lake, pond, or marsh.

Mafic

An igneous composed chiefly of one or more dark-colored minerals.

Magma

Molten rock beneath the surface of the earth.

Magma Chamber

The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.

Magnitude

A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs.) The number scale for magnitudes is logarithmic rather than arithmetic. Therefore, deflections on a seismograph for a magnitude 5 earthquake, for example, are 10 times greater than those for a magnitude 4 earthquake, 100 times greater than for a magnitude 3 earthquake, and so on.

Matrix

The solid matter in which a fossil or crystal is embedded. Also, a binding substance (e.g., cement in concrete).

Miocene

An epoch in Earth's history from about 24 to 5 million years ago. Also refers to the rocks that formed in that epoch.

Moho

Also called the Mohorovicic discontinuity. The surface or discontinuity that separates the crust from the mantle. The Moho is at a depth of 5-10 km beneath the ocean floor and about 35 km below the continents (but down to 60 km below mountains). Named for Andrija Mohorovicic, a Croatian seismologist.

Monogenetic

A volcano built by a single eruption.

Mudflow

A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.

n
Nuees Ardentes

A French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force and moves hurricane speed down the mountainside.

o
Obsidian

A black or dark-colored volcanic glass, usually composed of rhyolite.

p
Pahoehoe

A Hawaiian term for lava with a smooth, billowy, or ropy surface.

Pele Hair

A natural spun glass formed by blowing-out during quiet fountaining of fluid lava, cascading lava falls, or turbulent flows, sometimes in association with pele tears. A single strand, with a diameter of less than half a millimeter, may be as long as two meters.

Pele Tears

Small, solidified drops of volcanic glass behind which trail pendants of Pele hair. They may be tear-shaped, spherical, or nearly cylindrical.

Peralkaline

Igneous rocks in which the molecular proportion of aluminum oxide is less than that of sodium and potassium oxides combined.

Phenocryst

A conspicuous, usually large, crystal embedded in porphyritic igneous rock.

Phreatic Eruption

An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.

Phreatomagmatic

An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.

Pillow lava

Interconnected, sack-like bodies of lava formed underwater.

Pipe

A vertical conduit through the Earth's crust below a volcano, through which magmatic materials have passed. Commonly filled with volcanic breccia and fragments of older rock.

Plastic

Capable of being molded into any form, which is retained.

Plate Tectonics

The theory that the earth's crust is broken into about 10 fragments (plates,) which move in relation to one another, shifting continents, forming new ocean crust, and stimulating volcanic eruptions.

Pleistocene

A epoch in Earth history from about 2-5 million years to 10,000 years ago. Also refers to the rocks and sediment deposited in that epoch.

Plinian

An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic.

Plug

Solidified lava that fills the conduit of a volcano. It is usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.

Pluton

A large igneous intrusion formed at great depth in the crust.

Polygenetic

Originating in various ways or from various sources.

Precambrian

All geologic time from the beginning of Earth history to 570 million years ago. Also refers to the rocks that formed in that epoch.

Pumice

Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.

Pyroclastic Flow

Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed.

q
Quaternary

The period of Earth's history from about 2 million years ago to the present; also, the rocks and deposits of that age

r
Rhyodacite

An extrusive rock intermediate in composition between dacite and rhyolite.

Rhyolite

Volcanic rock (or lava) that characteristically is light in color, contains 69% silica or more, and is rich in potassium and sodium.

Synonyms: rhyolites
Rift System

The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their on-land counterparts such as the East African Rift.

Ring of Fire

The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.

s
Scoria

A bomb-size (> 64 mm) pyroclast that is irregular in form and generally very vesicular. It is usually heavier, darker, and more crystalline than pumice.

Seamount

A submarine volcano

Shearing

The motion of surfaces sliding past one another.

Shield Volcano

A gently sloping volcano in the shape of a flattened dome and built almost exclusively of lava flows.

Silica

A chemical combination of silicon and oxygen

Sill

A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes.

Skylight

An opening formed by a collapse in the roof of a lava tube

Spatter Cone

A low, steep-sided cone of spatter built up on a fissure or vent. It is usually of basaltic material

Spines

Horn-like projections formed upon a lava dome.

Stratovolcano

A volcano composed of both lava flows and pyroclastic material.

Strombolian

A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater

Subduction Zone

The zone of convergence of two tectonic plates, one of which usually overrides the other.

Surge

A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column accompanying a volcanic eruption or crater formation.

t
Talus

A slope formed a the base of a steeper slope, made of fallen and disintegrated materials.

Tephra

Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.

Trachyte

A group of fine-grained, generally porphyritic, extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase.

Tsunami

A great sea wave produced by a submarine earthquake, volcanic eruption, or large landslide.

Tuff Cone

A type of volcanic cone formed by the interaction of basaltic magma and water. Smaller and steeper than a tuff ring.

Tuff Ring

A wide, low-rimmed, well-bedded accumulation of hyalo-clastic debris built around a volcanic vent located in a lake, coastal zone, marsh, or area of abundant ground water.

Tumulus

A doming or small mound on the crest of a lava flow caused by pressure due to the difference in the rate of flow between the cooler crust and the more fluid lava below

u
Ultramafic

Igneous rocks made mostly of the mafic minerals hypersthene, augite, and/or olivine.

Unconformity

A substantial break or gap in the geologic record where a rock unit is overlain by another that is not next in stratigraphic sucession, such as an interruption in continuity of a depositional sequence of sedimentary rocks or a break between eroded igneous rocks and younger sedimentary strata. It results from a change that caused deposition to cease for a considerable time, and it normally implies uplift and erosion with loss of the previous formed record.

v
Vent

The opening at the earth's surface through which volcanic materials issue forth.

Vesicle

A small air pocket or cavity formed in volcanic rock during solidification

Viscosity

A measure of resistance to flow in a liquid (water has low viscosity while honey has a higher viscosity.)

Volcanic Complex

A persistent volcanic vent area that has built a complex combination of volcanic landforms.

Vulcanian

A type of eruption consisting of the explosive ejection of incandescent fragments of new viscous lava, usually on the form of blocks.

x
Xenocrysts

A crystal that resembles a phenocryst in igneous rock, but is a foreign to the body of rock in which it occurs.