Volcano TermsWith pictures

There are three main types of volcano.

Shield volcano

Volcanoes with broad, gentle slopes and built by the eruption of fluid basalt lava are called shield volcanoes. Basalt lava tends to build enormous, low-angle cones because it flows across the ground easily and can form lava tubes that enable lava to flow tens of kilometers from an erupting vent with very little cooling. The largest volcanoes on Earth are shield volcanoes. The name comes from a perceived resemblance to the shape of a warrior's shield.

Photograph by D. Little (date unknown). View of the NNW flank of Mauna Loa Volcano from the south side of Mauna Kea Volcano, Hawai`i; both are shield volcanoes.

Mount Etna, Italy

This cinder cone on the flank of Mount Etna is surrounded by a younger basaltic lava flow.

Cinder cones usually erupt lava flows, either through a breach on one side of the crater or from a vent located on a flank. Lava rarely issues from the top (except as a fountain) because the loose, non cemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent.

Composite or StratovolcanoSteep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows, are called stratovolcanoes. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions. A stratovolcano typically consists of many separate vents, some of which may have erupted cinder cones and domes on the volcano's flanks. A synonym is composite cone.

Photo: Mount Mageik volcano viewed from the Valley of Ten Thousand Smokes, Katmai National Park and Preserve, Alaska. Mageik's broad summit consists of at least four separate structures built above different vents.

Photograph by R. McGimsey on 15 July 1990

Composite Volcanoes ("Stratovolcanoes")

Composite volcanoes are built by multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma, the most common but not the only magma type, tends to form composite cones. Although andesitic composite cones are built mostly of fragmental debris, some of the magma intrudes fractures within the cones to form dike or sills. In this way, multiple intrusive events build a structural framework of dikes and sills that knits together the voluminous accumulation of volcanic rubble. Such a structure can stand higher than cones composed only of fragmental material. Composite cones can grow to such heights that their slopes become unstable and susceptible to collapse from the pull of gravity.

Famous examples of composite cones are Mayon Volcano, Philippines, Mount Fuji in Japan, and Mount Rainier, Washington, U.S.A. Some composite volcanoes attain two to three thousand meters in height above their bases. Most composite volcanoes occur in chains and are separated by several tens of kilometers. There are numerous composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".

St. Augustine volcano, Alaska. Composite cone. Photograph by Harry Glicken.

Volcanic ash

Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm (0.1 inch) in diameter, which is slightly larger than the size of a pinhead. Volcanic ash is not the same as the soft fluffy ash that results from burning wood, leaves, or paper. It is hard, does not dissolve in water, and can be extremely small--ash particles less than 0.025 mm (1/1,000th of an inch) in diameter are common. Ash is extremely abrasive, similar to finely crushed window glass, mildly corrosive, and electrically conductive, especially when wet.

Photograph by D.E. Wieprecht

BasaltBasalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move >20 km from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250° C.

AndesiteAndesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxeneand orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100° C.

Close view of andesite lava flow BrokeoffVolcano, California More about volcanic and plutonic rocks.

Arching fountain

Instead of shooting lava vertically from a vent, an arching lava fountain sends lava upward and outward. Arching fountains form when the shape or geometry of an erupting vent forces the lava outward in a continuous stream of airborne lava.

Photograph by J.D. Griggs on 25 February 1983. Arching lava fountain 10-15 m tall on the east rift zone of Kilauea Volcano, Hawai`i.

Block

A volcanic block is a solid rock fragment greater than 64 mm in diameter that was ejected from a volcano during an explosive eruption. Blocks commonly consist of solidified pieces of old lava flows that were part of a volcano's cone. This block was ejected into the air by an explosion caused by the collapse of an active lava delta at Kilauea Volcano, Hawai`i. New land built by lava often slides into the ocean, which enables seawater to mix with lava and hot rocks. Such violent mixing may trigger steam explosions that can blast hot rocks 10 to 50 cm in diameter more than 50 m inland.

Photograph by C. Heliker on January 26, 1988

Bomb

Volcanic bombs are lava fragments that were ejected while viscous (partially molten) and larger than 64 mm in diameter. Many acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include bread-crust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and “cow-dung”; bombs.

Photograph by J.P. Lockwood on July 10, 1982. These basaltic lava bombs were erupted by Mauna Kea Volcano, Hawai`i.

Caldera

A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. Calderas are different from craters, which are smaller, circular depressions created primarily by explosive excavation of rock during eruptions

Photograph by M. Williams, National Park Service, 1977.

Cinder coneA cinder cone is a steep, conical hill of volcanic fragments that accumulate around and downwind from a vent. The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles “frozen” into place as magma exploded into the air and then cooled quickly. Cinder cones range in size from tens to hundreds of meters tall.

This cinder cone (Pu`u ka Pele) was erupted low on the southeast flank of Mauna Kea Volcano. The cone is 95 m in height, and the diameter of the crater at the top is 400 m. Hualalai Volcano in background.

Photograph by J.P. Lockwood on 1 December 1975

Volcanic dome

Volcanic domes are rounded, steep-sided mounds built by very viscous magma, usually either dacite or rhyolite. Such magmas are typically too viscous (resistant to flow) to move far from the vent before cooling and crystallizing. Domes may consist of one or more individual lava flows. Volcanic domes are also referred to as lava domes.

Volcanic dome atop Novarupta vent, Valley of Ten Thousand Smokes, Katmai National Park and Preserve, Alaska. The dome was erupted from the same vent that expelled about 15 km3 of magma in an enormous explosive eruption in 1912.

Photograph by T.P. Miller in June 1979

Debris avalanche

Debris avalanches are moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope. As the moving debris rushes down a volcano and into river valleys, it incorporates water, snow, trees, bridges, buildings, and anything else in the way. Debris avalanches may travel several kilometers before coming to rest, or they may transform into more water-rich lahars, which travel many tens of kilometers downstream.

A debris avalanche rushes down the side of a volcano to the valley floor. Many such debris avalanches transform into lahars and travel tens of kilometers from the volcano. Note horseshoe shaped crater on volcano's side, which is the scar created by the avalanche. Sketch and animation by B. Myers

Lahars Triggered by Melting Snow and IceMount St. Helens, Washington

Dark pathways created by lahars streak the sides of Mount St. Helens during its catastrophic eruption on May 18, 1980. The lahars were triggered by the sudden melting of snow and ice from hot volcanic rocks ejected by the initial explosive activity and subsequent pyroclastic flows.

Many of the largest and most destructive historical lahars accompanied eruptions from volcanoes mantled by a substantial cover of snow and ice. Pyroclastic flows are the most common volcanic events that generate lahars--even relatively small pyroclastic flows can quickly melt large quantities of snow and ice. The hot flowing rock debris erodes and mixes with snow and ice to form water and trigger snow avalanches on steep slopes. Lava flows moving slowly across snow usually do not melt snow and ice rapidly enough to form large lahars but the eruption of lava beneath a glacier can result in substantial ponding of water, which may lead to enormous outpourings of water.

Major, J.J, and Newhall, G.C., 1989, Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods, Bulletin of Volcanology, v. 52, p. 1-27.

Lahar

Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form in a variety of ways, chiefly by the rapid melting of snow and ice by pyroclastic flows, intense rainfall on loose volcanic rock deposits, breakout of a lake dammed by volcanic deposits, and as a consequence of debris avalanches.

A small lahar triggered by rainfall rushes down the Nima II River near the town of El Palmar in Guatemala in the image shown here. The lahar developed on the slopes of Santiaguito volcano.

Photograph by J.N. Marso on 14 August 1989

Effusive eruption

An eruption dominated by the outpouring of lava onto the ground is often referred to as an effusive eruption (as opposed to the violent fragmentation of magma by explosive eruptions). Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge, slope of the ground over which the lava travels, and duration of eruption.

For example, basalt lava may become `a`a or pahohoe, and flow in deep narrow channels or in thin wide sheets. Andesite lava typically forms thick stubby flows, and dacitelava often forms steep-sided mounds called lava domes.

Basalt lava erupts from Pu`u `O`o spatter and cinder cone at Kilauea Volcano, Hawai`i. Lava spilling from the cone has formed a series of `a`a lava channels and flows.

Photograph by J.D. Griggs on 31 January 1984

Eruption cloudA cloud of tephra and gases that forms downwind of an erupting volcano is called an eruption cloud. The vertical pillar of tephra and gases rising directly above a vent is an eruption column.

Eruption cloud is often used interchangeably with plume or ash cloud.

Eruption clouds are often dark colored--brown to gray--but they can also be white, very similar to weather clouds. Eruption clouds may drift for thousands of kilometers downwind and often become increasingly spread out over a larger area with increasing distance from an erupting vent (note fan-shaped eruption cloud in photographs at left). Large eruption clouds can encircle the Earth within days.

Photographs taken by Space Shuttle astronauts about 24 hours after the start of the eruption of Rabaul Caldera. The eruption column rose to at least 18 km above sea level where the volcanic ash and gas were blown west to form a fan-shaped eruption cloud.

Volcanic dikeDikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks. They form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Hundreds of dikes can invade the cone and inner core of a volcano, sometimes preferentially along zones of structural weakness.

This dike was exposed when a new pit crater formed in about 1880 A.D. in the northeast corner of the summit caldera of Mauna Loa Volcano. The dike is about 1.5 m wide.

Photograph by J.P. Lockwood in March 1983

FaultFaults are fractures or fracture zones in the Earth's crust along which one side moves with respect to the other. A fault scarp is a cliff or steep slope that sometimes forms along the fault at the surface. There are many types of faults (for example, strike-slip, normal, reverse, and thrust faults) ranging in size from a few tens of meters to hundreds of kilometers in dimension.

Aerial view toward the NE of the Pu`uKapukapu fault scarp (maximum height about 320 m) in the Hilina fault system, south flank of Kilauea Volcano, Hawai`i. In Hawai`i, these tall cliffs are called "pali's".

Photograph by D.A. Swanson on 24 June 1971

FissureIn geology, a fissure is a fracture or crack in rock along which there is a distinct separation; fissures are often filled with mineral-bearing materials. On volcanoes, a fissure is an elongate fracture or crack at the surface from which lava erupts. Fissure eruptions typically dwindle to a central vent after a period of hours or days. Occasionally, lava will flow back into the ground by pouring into a crack or an open eruptive fissure, a process called drainback; sometimes lava will flow back into the same fissure from which it erupted. Eruptive fissure on southeast rim of Kilauea caldera, Hawai`i. This eruptive fissure was active briefly during an eruption in July 1974. Note prominent spatter ramparts on right, and subdued rampart on left, built by the ejection of lava along the fissure. The smooth texture of the surface on the lip of the fissure (lower right) is evidence that lava drained back into the fissure toward the end of the 1974 eruption.

Photograph by S.R. Brantley on 14 August 1998

FumaroleFumaroles are vents from which volcanic gas escapes into the atmosphere. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic flows. They may persist for decades or centuries if they are above a persistent heat source or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools.

Close view of a fumarole on Kilauea Volcano. Elemental sulfur vapor escaping from the fumarole has cooled to form yellow-colored crystals around its margins.

Photograph by R.L. Christiansen on 27 July 1973

Volcanic gas

Magma contains dissolved gases that are released into the atmosphere during eruptions. Gases are also released from magma that either remains below ground (for example, as an intrusion) or rises toward the surface. In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems. The most common gas released by magma is steam (H2O), followed by CO2 (carbon dioxide), SO2 (sulfur dioxide), (HCl) hydrogen chloride and other compounds. Sulfur dioxide and other volcanic gases rise from the Pu`u `O`o vent on Kilauea Volcano, Hawai`i. During periods of sustained eruption from Pu`u `O`o between 1986 and 2000, Kilauea emitted about 2,000 to 1,000 metric tons of irritating sulfur dioxide gas (SO2) gas each day. Photograph by K.A. McGee on 19 September 1995

Geyser

Most geysers are hot springs that episodically erupt fountains of scalding water and steam. Such eruptions occur as a consequence of groundwater being heated to its boiling temperature in a confined space (for example, a fracture or conduit). A slight decrease in pressure or an increase in temperature will cause some of the water to boil. The resulting steam forces overlying water up through the conduit and onto the ground. This loss of water further reduces pressure within the conduit system, and most of the remaining water suddenly converts to steam and erupts at the surface.

Castle Geyser erupts water and steam, Yellowstone National Park, Wyoming.

Photograph by S.R. Brantley in September 1983

HornitoA small rootless spatter cone that forms on the surface of a basaltic lava flow (usually pahoehoe) is called a hornito. A hornito develops when lava is forced up through an opening in the cooled surface of a flow and then accumulates around the opening. Typically, hornitos are steep sided and form conspicuous pinnacles or stacks. They are "rootless" because they are fed by lava from the underlying flow instead of from a deeper magma conduit.

This hornito formed on the surface of a pahoehoe flow during the Mauna Ulu eruption on the east rift zone of Kilauea Volcano, Hawai`i. Photograph by D.A. Swanson on 21 May 1970

MagmaMagma is molten or partially molten rock beneath the Earth's surface. When magma erupts onto the surface, it is called lava. Magma typically consists of (1) a liquid portion (often referred to as the melt); (2) a solid portion made of minerals that crystallized directly from the melt; (3) solid rocks incorporated into the magma from along the conduit or reservoir, called xenoliths or inclusions; and (4) dissolved gases.

This sketch shows a classic cartoon view of a magma reservoir beneath a volcano and a conduit leading up to a lava dome at the surface. The arrow indicates direction of magma movement from a deeper source.

USGS Photo Glossary of volcanic terms

Appreciation to the USGS for providing the photos and information used here to create a glossary of terms.

courtesy of USGS Earthquake Hazards Program

http://volcanoes.usgs.gov/images/pglossary/index.php