Selected igneous and related terms

Aa: Aa (pronounced "ah-ah"), a Hawaiian term, is lava that has a rough, jagged, spiny, and generally clinkery surface.

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.

Airfall: Ash falling from an eruption column or ash cloud. Also called: ashfall.

Andesite: A medium-colored dark gray volcanic rock containing 53-63 percent silica with a moderate viscosity when in a molten state. Intermediate in color, composition, and eruptive character between basalt and dacite.

Ash: Fragments of lava or rock smaller than 2 millimeters in size that are blasted into the air by volcanic explosions.

Ash cloud: Cloud of ash formed by volcanic explosions or derived from a pyroclastic flow. The fine material that is generated by a pyroclastic flow and rises above it.

Ashfall: See: Airfall.

Ash-flow: A pyroclastic flow consisting predominantly of ash-sized particles. Also called a glowing avalanche if it is of very high temperature.

Basalt: Dark-colored, low-silica (less than 53 percent SiO2), low viscosity volcanic rock that is relatively fluid when molten; eruptions of basalt are generally non explosive and tend to produce relatively long thin lava flows like those common in Hawaii.

Batholith: A mass with no known bottom and an outcrop area of greater than 40 square miles (100 km2). It is typically discordant, but may be partly concordant.

Base surge: Turbulent, low-density cloud of rock debris and water and (or) steam that moves over the ground surface at high speed. Base surges are generated by explosions.

Block: Fragments of lava or rock larger than 64 mm in size that are blasted into the air by volcanic explosions.

Blowdown: Trees felled by a volcanic blast.

Caldera: A large volcanic depression, commonly circular or elliptical when seen from above.

Cenozoic: An era of geologic time from the beginning of the Tertiary period, spanning the time between 66 million years ago to the present. The Cenozoic contains the Tertiary (66-2) and the Quaternary (2-present) periods.

Cinder cone: A small conical-shaped volcano formed by the accumulation of ejected cinders and other volcanic debris that falls back to Earth close to the vent area.

Composite volcano: A steep-sided volcano composed of many layers of volcanic rocks, usually of high-viscosity lava and fragmented debris such as lahar and pyroclastic deposits. Also called: Stratovolcano.

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

Dacite: Typically light-colored, fairly silica-rich (63 to 68 percent SiO2) volcanic rock with a high viscosity when in a molten state; eruptions are commonly explosive (e.g., Mount St. Helens' eruption of May 18, 1980) and may produce voluminous tephra, pyroclastic flows, and lava domes.

Debris avalanche: The very rapid and usually sudden sliding and flowage of an unsorted mixture of soil and weathered (altered) rock that moves away from a volcano at high speed.

Debris flow: See: Lahar.

Deposit: Earth material that has accumulated by some natural process. For example, a flowing mixture of water and rock debris is called a debris flow, but when the flow ceases to move, a layer of fine and coarse rock is left which is called a debris-flow deposit.

Diatreme: A volcanic pipe consisting largely of breccia.

Dike: A tabular mass discordant with either bedding or foliation of surrounding country rocks.

Directed blast: A hot, low-density mixture of rock debris, ash, and gases that moves at high speed along the ground surface. Directed blasts are generated by explosions.

Dome: A steep-sided mound that forms when viscous lava piles up near a volcanic vent. Domes are formed by andesite, dacite, and rhyolite lavas. Also called: Lava dome.

Dormant volcano: A volcano that is not presently erupting but that is considered likely to erupt in the future.

Earthquake: The abrupt shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth.

Ejecta: Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes, lava bombs.

Eocene: An epoch of the Tertiary period, spanning the time between 58 and 37 million years ago.

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

Fumarole: An vent or opening in the ground from which hot water vapor (steam) and (or) volcanic gases are emitted.

Fumarolic activity: Volcanic gas emissions, that may be accompanied by a change in the temperature of the gases or fluids emitted.

Glacier outburst flood: A sudden release of melt water from a glacier or glacier-dammed lake sometimes resulting in a catastrophic flood, formed by melting of a channel or by subglacial volcanic activity. Also know as a jkulhlaup, an Icelandic term.

Graben: An elongate crustal block that is relatively depressed (down dropped) between two fault systems.

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.

Holocene: An epoch of the Quaternary period, spanning from 8,000 years ago until the present.

Horst: an elongate crustal block that is relatively upthrown between two normal faults.

Hydrothermal: Pertains to hot water or the action of heated water, often considered heated by magma or in association with magma.

Hydrothermal alteration: Alteration of rocks or minerals by the reaction of hot water (and other fluids) with pre-existing rocks. The hot water is generally heated groundwater and dissolved minerals.

Laccolith: A concordant, floored, mushroom-shaped.

Lahar: A flowing mixture of water-saturated rock debris that forms on the slopes of a volcano, and moves downslope under the force of gravity, sometimes referred to as debris flow or mudflow. The term comes from Indonesia.

Lapilli: Fragments of lava or rock between 2 and 64 mm in size that are blasted into the air by volcanic explosions.

Lateral blast: An explosive event in which energy is directed horizontally instead of vertically as in an eruption column.

Lava: Molten rock that erupts from a vent of fissure, see Magma.

Lava dome: See: Dome.

Lava flow: Stream of molten rock that erupts relatively non explosively from a volcano and moves slowly downslope. Also, a solidified tongue-like or sheet-like body formed by outpouring lava.

Lithospheric Plates: A series of rigid slabs (16 major ones at present) that make up the Earth's outer shell. These plates float on top of a softer, more plastic layer in the Earth's mantle. Also called: Tectonic Plates.

Lopolith: A large, concordant basin-shaped mass. The constituent rocks are generally layered.

Maar: A low-relief explosion crater the walls of which consist largely or entirely of loose fragments of country rock and possibly some ejecta.

Mafic: Term used to describe volcanic rock or magma composed chiefly of dark-colored, iron- and magnesium-rich minerals.

Magma: Molten rock that contains dissolved gas and crystals, formed deep within the Earth. When magma reaches the surface, it is called lava.

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.

Mesozoic: The era of geologic time between the Paleozoic and the Cenozoic, spanning the time between 250 and 66 million years ago. The Mesozoic is dividing into the Triassic (250-205), Jurassic (205-135), and Cretaceous (135-66) periods.

Miocene: An epoch of the Tertiary period, spanning the time between 24 and 5 million years ago.

Mudflow: See: Lahar.

Neck: The mass that plugs the conduit of a former volcanic vent. The term is most often applied to erosional remnants of such masses. Also called a pipe or plug.

Nueé Ardente See: Pyroclastic Flow.

Oligocene: An epoch of the Tertiary period, spanning the time between 37 and 24 million years ago.

Pahoehoe: Pahoehoe (pronounced "pah-hoy-hoy"), a Hawaiian term, is lava that in solidified form is characterized by a smooth, billowy, or ropy surface.

Paleocene: An epoch of the Tertiary period, spanning the time between 66 and 58 million years ago.

Paleozoic: An era of geologic time, from the beginning of the Precambrian to the beginning of the Mesozoic, spanning the time between 570 and 250 million years ago. The Paleozoic era contains the Cambrian (570-500), Ordovician (500-425), Silurian (425-400), Devonian (400-365), Mississippian (365-310), Pennsylvanian (310-290), and Permian (290-270) periods.

Phreatic eruption: A type of volcanic explosion that occurs when water comes in contact with hot rocks or ash near a volcanic vent, causing steam explosions, or explosions of steam, water, mud, and other material. May result from heating of groundwater by magma, and may generate base surges.

Phreatomagmatic eruption: An explosion composed of magmatic gases and steam derived from groundwater or surface water, combined with lava and other debris.

Pleistocene: An epoch of the Quaternary period, spanning the time between 2 million years ago and the beginning of the Holocene at 8,000 years ago.

Pliocene: An epoch of the Tertiary period, spanning the time between 5 and 2 million years ago.

Pluton: A general term for any igneous intrusion.

Precambrian: All geologic time before the Paleozoic era. This includes about 90% of all geologic time and spans the time from the beginning of the earth to 570 million years ago.

Pumice: A light-colored, frothy volcanic rock containing abundant trapped gas bubbles formed by the explosive eruption of magma. Commonly perceived as lumps or fragments of peas size and larger but can also occur abundantly as ash-size particles. Because of its numerous gas bubbles, pumice commonly floats on water.

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

Pyroclastic flow: A hot (300-800 °C (570-1470 °F)), dry, fast-moving (10 to more than 100 meters per second (20 to more than 200 miles per hour)) and high-density mixture of ash, pumice, rock fragments, and gas formed during explosive eruptions or from the collapse of a lava dome. Moves away from a volcano at high speeds.

Pyroclastic surge: Turbulent, low-density cloud of hot rock debris and gases that moves over the ground surface at high speed. Similar to a pyroclastic flow but of much lower density (higher gas to rock ratio).

Quaternary: Period of geologic time from about 2 million years ago until the present. It contains the Pleistocene (2 million - 8,000) and Holocene (8,000 - present) epochs.

Rhyolite: Typically a light-colored crystalline or black glassy volcanic rock or magma, containing more than 68 percent silica with a very high viscosity when in a molten state.

Seismograph: A scientific instrument that detects and records vibrations (seismic waves) produced by earthquakes.

Sheet: A general term for a tabular mass such as a dike or sill.

Shield volcano: A volcano shaped like an inverted warrior's shield with long gentle slopes produced by eruptions of low-viscosity basaltic lava.

Silica: The molecule formed of silicon and oxygen (SiO2) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma's silica content, the greater its viscosity or "stickiness."

Silicic: Term used to describe silica-rich volcanic rock or magma.

Sill: A tabular mass concordant with bedding or foliation of surrounding country rock.

Spreading Ridges: Places on the ocean floor where lithospheric plates separate and magma erupts. About 80 percent of the Earth's volcanic activity occurs on the ocean floor.

Stock: A mass with the characteristics of a batholith but with an outcrop area of less than 40 square miles (100 km2). Depth of erosion may control naming a mass a stock rather than a batholith.

Stratovolcano: See Composite volcano.

Subduction Zone: The place where two lithosphere plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between the two plates.

Tectonic Plates: See: Lithospheric Plates.

Tephra: Materials of all sizes and types that are erupted from a volcano and deposited from the air.

Tertiary: Period of geologic time from about 66 million years ago until 2 million years ago. It contains the Paleocene (66-58), Eocene (58-37), Oligocene (37-24), Miocene (24-5), and Pliocene (5-2) epochs.

Tremor: See: Harmonic Tremor.

Vent: An opening in the Earth's surface through which volcanic materials (magma and gas) escape. Vents can be at a volcano's summit or on its slopes; they can be circular (craters) or linear (fissures).

Viscosity: Measure of the fluidity of a substance. Basalt is less viscous than dacite.

Volcano: A vent (opening) in the surface of the Earth through which magma erupts and also the landform that is constructed by the erupted material.

Volcanic avalanche: A large, chaotic mass of soil, rock, and volcanic debris moving swiftly down the slopes of a volcano. Volcanic avalanches can also occur without an eruption as a result of an earthquake; heavy rainfall; or unstable soil, rock, and volcanic debris. Also called: Debris Avalanche.

Volcanic cone or edifice: Used to describe the uppermost slopes and summit area of a volcano.

Volcanic landslide: The downslope movement of soil, rock debris, and sometimes glacial ice, with or without water, from the flank of a volcano. See: Debris Avalanche.