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

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.

Limu O Pele (Pele Seaweed)
Delicate, translucent sheets of spatter filled with tiny glass bubbles.

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.

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.

Magmatic
Pertaining to magma.

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 great...

Mantle
The zone of the earth below the crust and above the core.

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.

Myth
A fictional story to explain the origin of some person, place, or thing.

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. Obsidian

Oceanic Crust
The earth's crust where it underlies oceans.

Pahoehoe
A Hawaiian term for lava with a smooth, billowy, or ropy surface. Click here to view a photo of pahoehoe.

Pali
Hawaiian word for steep hills or cliffs.

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 (Explosion)
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.

Pit Crater
A crater formed by sinking in of the surface, not primarily a vent for lava.

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 Eruption
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.

Plug Dome
The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.

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
Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.

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.

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

Relief
The vertical difference between the summit of a mountain and the adjacent valley or plain.

Renewed Volcanism State
Refers to a state in the evolution of a typical Hawaiian volcano during which --after a long period of quiescence--lava and tephra erupt intermittently. Erosion and reef building continue.

Repose
The interval of time between volcanic eruptions.

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.

Ridge, Oceanic
A major submarine mountain range.

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.

Rift Zone
A zone of volcanic features associated with underlying dikes. The location of the rift is marked by cracks, faults, and vents.

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

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.

Seafloor Spreading
The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as plates are separating.

Seamount
A submarine volcano.

Seismograph
An instrument that records seismic waves; that is, vibrations of the earth.

Seismologist
Scientists who study earthquake waves and what they tell us about the inside of the Earth.

Seismometer
An instrument that measures motion of the ground caused by earthquake waves.

Shear Waves
Earthquake waves that move up and down as the wave itself moves. For example, to the left.

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.

Shoshonite
A trachyandesite composed of olivine and augite phenocrysts in a groundmass of labradorite with alkali feldspar rims, olivine, augite, a small amount of leucite, and some dark-colored glass. Its name is derived from the Shoshone River, Wyoming and given by Iddings in 1895.

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.

Solfatara
A type of fumarole, the gases of which are characteristically sulfurous.

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

Spatter Rampart
A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent.

Specific Gravity
The density of a mineral divided by the density of water.

Spines
Horn-like projections formed upon a lava dome.

Stalactite
A cone shaped deposit of minerals hanging from the roof of a cavern.

Stratigraphic
The study of rock strata, especially of their distribution, deposition, and age.

Stratovolcano
A volcano composed of both lava flows and pyroclastic material.

Streak
The color of a mineral in the powdered form.

Strike-Slip Fault
A nearly vertical fault with side-slipping displacement.

Strombolian Eruption
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.

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.

Tephrochronology
The collection, preparation, petrographic description, and approximate dating of tephra.

Tilt
The angle between the slope of a part of a volcano and some reference. The reference may be the slope of the volcano at some previous time.

Trachyandesite
An extrusive rock intermediate in composition between trachyte and andesite.

Trachybasalt
An extrusive rock intermediate in composition between trachyte and basalt.

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.

Tremor
Low amplitude, continuous earthquake activity often associated with magma movement.

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

Tuff
Rock formed of pyroclastic material.

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

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.

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 ...

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

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

Volcanic Arc
A generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic rocks formed there.

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

Volcanic Cone
A mound of loose material that was ejected ballistically.

Volcanic Neck
A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone.

Volcano
A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure (usually conical) that is produced by the ejected material.