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Oregon State University - Volcano index Category: Sciences > Geology and volcanology Date & country: 27/01/2011, USA Words: 205

Accessory
A mineral whose presence in a rock is not essential to the proper classification of the rock.

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

Accretionary Lava Ball
A rounded mass, ranging in diameter from a few centimeters to several meters, [carried] on the surface of a lava flow (e.g., 'a'a) or on cinder-cone slopes [and formed] by the molding of viscous lava around a core of already solidified lava.

Acid
A descriptive term applied to igneous rocks with more than 60% silica (SiO2).

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 (Airfall)
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.

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.

Basic
A descriptive term applied to igneous rocks (basalt and gabbro) with silica (SiO2) between 44% and 52%.

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.

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. Eruptions are less frequent, but more explosive. The summit caldera may be buried.

Central Vent
A central vent is an opening at the Earth's surface of a volcanic conduit of cylindrical or pipe-like form.

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.

Compression Waves
Earthquake waves that move like a slinky. As the wave moves to the left, for example, it expands and compresses in the same direction as it moves.

Conduit
A passage followed by magma in a volcano.

Continental Crust
Solid, outer layers of the earth, including the rocks of the continents.

Continental Drift
The theory that horizontal movement of the earth's surface causes slow, relative movements of the continents toward or away from one another.

Country Rocks
The rock intruded by and surrounding an igneous intrusion.

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

Craton
A part of the earth's crust that has attained stability and has been little deformed for a prolonged period.

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

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 Avalanche
A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the major avalanche involved in the eruption of Mount St. Helens, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material. Most of the May 18, 1980 deposits in the upper valley of the North Fork Toutle Riv...

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

Detachment Plane
The surface along which a landslide disconnects from its original position.

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.

Dike
A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes.

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.

Dormant Volcano
Literally, "sleeping." 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.

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.

Episode
An episode is a volcanic event that is distinguished by its duration or style.

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.

Eruptive Vent
The opening through which volcanic material is emitted.

Evacuate
Temporarily move people away from possible danger.

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

Extrusion
The emission of magmatic material at the earth's surface. Also, the structure or form produced by the process (e.g., a lava flow, volcanic dome, or certain pyroclastic rocks).

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
See also

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.

Formation
A body of rock identified by lithic characteristics and stratigraphic position and is mappable at the earth's surface or traceable in the subsurface.

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.

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.

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.

Hydrothermal Reservoir
An underground zone of porous rock containing hot water.

Hypabyssal
A shallow intrusion of magma or the resulting solidified rock.

Hypocenter
The place on a buried fault where an earthquake occurs.

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.

Intensity
A measure of the effects of an earthquake at a particular place. Intensity depends not only on the magnitude of the earthquake, but also on the distance from the epicenter and the local geology.

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.

Joint
A surface of fracture in a rock.

Juvenile
Pyroclastic material derived directly from magma reaching the surface.

Kipuka
An area surrounded by a lava flow.

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.

Landsat
A series of unmanned satellites orbiting at about 706 km (438 miles) above the surface of the earth. The satellites carry cameras similar to video cameras and take images or pictures showing features as small as 30 m or 80 m wide, depending on which camera is used.

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

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 Fountain
A rhythmic vertical fountainlike eruption of lava.

Lava Lake (Pond)
A lake of molten lava, usually basaltic, contained in a vent, crater, or broad depression of a shield volcano.

Lava Shields
A shield volcano made of basaltic lava.