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Photo glossary of volcano terms
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Glowing `a`a flow front advancing over pahoehoe on the coastal plain of Kilauea Volcano, Hawai`i.
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`A`a flow
`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified `a`a flow makes walking very difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an `a`a flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an `a`a flow.
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Close view of andesite lava flow
Brokeoff Volcano, California
Learn more about this andesite lava flow, including a photograph and description of its features.
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Andesite
Andesite 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 (clinopyroxene and 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.
More about volcanic and plutonic rocks.
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Basalt rock sample
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Basalt
Basalt 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.
More about volcanic and plutonic rocks.
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Photograph by C. Heliker
on January 26, 1988
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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.
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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.
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Photograph by J.P. Lockwood on July 10, 1982
These basaltic lava bombs were erupted by Mauna Kea Volcano, Hawai`i.
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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 breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs.
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Photograph by M. Williams, National Park Service, 1977
Aniakchak Caldera formed during an enormous explosive eruption that expelled more than 50 km3 of magma about 3,450 years ago. The caldera is 10 km in diameter and 500-1,000 m deep. Subsequent eruptions formed domes, cinder cones, and explosion pits on the caldera floor.
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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.
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Photograph by J.P. Lockwood on 1 December 1975
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.
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Cinder cone
A 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.
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Close view of dacite lava from the May 1915 eruption of Lassen Peak, California
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Dacite
Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000°C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome.
More about volcanic and plutonic rocks
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Sketch and animation by B. Myers
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.
Animation sequence (Quicktime®, 1.2 Mb)
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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.
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Photograph by J.P. Lockwood in March 1983
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.
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Dike
Dikes 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.
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Photograph by J.D. Griggs
on 31 January 1984
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.
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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 dacite lava often forms steep-sided mounds called lava domes.
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Photographs provided by NASA, STS064-116-064
September 20, 1994
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. A smaller eruption cloud (bottom photograph, lower right) was blown northward by lower-level winds.
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Eruption cloud
A 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 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.
Eruption cloud is often used interchangeably with plume or ash cloud.
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Photograph by D.A. Swanson on 24 June 1971
Aerial view toward the NE of the Pu`u Kapukapu 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".
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Fault
Faults 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.
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Photograph by S.R. Brantley on 14 August 1998
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.
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Fissure
In 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.
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Photograph by R.L. Christiansen on 27 July 1973
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.
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Fumarole
Fumaroles 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.
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Photograph by S.R. Brantley
in September 1983
Castle Geyser erupts water and steam, Yellowstone National Park, Wyoming.
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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.
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Photograph by D.A. Swanson
on 21 May 1970
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.
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Hornito
A 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.
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Photograph by J.D. Griggs on 13 January 1987
This kipuka formed during the Pu`u `O`o-Kupaianaha eruption on the east rift zone of Kilauea Volcano, Hawai`i.
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Kipuka
A Hawaiian term for an "island" of land completely surrounded by one or more younger lava flows. A kipuka forms when lava encircles a hill or a slight rise in the ground as it moves downslope or across relatively flat ground. Because they are surrounded by more recent flows, kipukas are often covered with mature vegetation.
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Photograph by J.N. Marso on 14 August 1989
A small lahar triggered by rainfall rushes down the Nima II River near the town of El Palmar in Guatemala. The lahar developed on the slopes of Santiaguito volcano.
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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.
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This lapilli was erupted by Pu`u Pua`i (gushing hill) from the summit of Kilauea Volcano in 1959 and fell to the ground about 800 m downwind. See summary of this spectacular eruption.
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Lapilli
Rock fragments between 2 and 64 mm (0.08-2.5 in) in diameter that were ejected from a volcano during an explosive eruption are called lapilli. Lapilli (singular: lapillus) means "little stones" in Italian. Lapilli may consist of many different types of tephra, including scoria, pumice, and reticulite.
Rounded tephra particles in this size range are called accretionary lapilli if they consist of tiny ash grains stuck together. Ash sometimes form such rounded particles in an eruption column or cloud, owing to moisture or electrostatic forces.
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Photograph by J.D. Griggs on 13 November 1985
Lava moves across the ground as a pahoehoe flow, Kilauea Volcano, Hawai`i
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Lava
Lava is the word for magma (molten rock) when it erupts onto the Earth's surface. Geologists also use the word to describe the solidified deposits of lava flows and fragments hurled into the air by explosive eruptions (for example, lava bombs or blocks). Lava is from the Italian word for stream, which is derived from the verb lavare--to wash.
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Photograph by Charles H. Anderson Jr.
Lava dome in the crater of Mount St. Helens Washington. The Lava Dome in the Crater of Mount St. Helens stop gowing in Oct. 1986. The dome is about 1000 feet high.
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Lava dome
Lava 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.
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Photograph by J.D. Griggs on 30 March 1984
Aerial view of a channelized `a`a flow on Mauna Loa Volcano, Hawai`i.
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Lava flow
Lava flows are masses of molten rock that pour onto the Earth's surface during an effusive eruption. Both moving lava and the resulting solidified deposit are referred to as lava flows. Because of the wide range in (1) viscosity of the different lava types (basalt, andesite, dacite, and rhyolite); (2) lava discharge during eruptions; and (3) characteristics of the erupting vent and topography over which lava travels, lava flows come in a great variety of shapes and sizes.
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Photograph by J.D. Griggs on 5 October 1983
Lava fountain of the Pu`u `O`o cinder and spatter cone on Kilauea Volcano, Hawai`i.
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Lava fountain
A jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock is called a lava fountain. Lava fountains typically range from about 10 to 100 m in height, but occasionally reach more than 500 m. Lava fountains erupt from isolated vents, along fissures, within active lava lakes, and from a lava tube when water gains access to the tube in a confined space (see type of explosive activity where lava enters the sea, from the USGS Hawaiian Volcano Observatory).
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Photograph by Charles H. Anderson Jr
in Feb. 1998
Aerial view of a lava lake atop the Kupaianaha vent on the east rift zone of Kilauea Volcano, Hawai`i. The fume rising from the end of the lava lake.
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Lava lake
Lava lakes are large volumes of molten lava, usually basaltic, contained in a vent, crater, or broad depression. Scientists use the term to describe both lava lakes that are molten and those that are partly or completely solidified. Lava lakes can form (1) from one or more vents in a crater that erupts enough lava to partially fill the crater; (2) when lava pours into a crater or broad depression and partially fills the crater; and (3) atop a new vent that erupts lava continuously for a period of several weeks or more and slowly builds a crater higher and higher above the surrounding ground.
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