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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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Photograph By Charles H. Anderson Jr.
Hawai`i Volcanoes National Park
Thurston (Nahuku) lava tube near summit caldera of Kilauea Volcano in Hawai`i Volcanoes National Park
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Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava channels and pahoehoe flows. A broad lava-flow field often consists of a main lava tube and a series of smaller tubes that supply lava to the front of one or more separate flows. When the supply of lava stops at the end of an eruption or lava is diverted elsewhere, lava in the tube system drains downslope and leaves partially empty conduits beneath the ground. Such drained tubes commonly exhibit "high-lava" marks on their walls, generally flat floors, and many lava stalactites that hang from the roof. Lava can also erode downward, deepening the tube and leaving empty space above the flowing lava.
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Photograph by J.D. Griggs on 25 February 1987
Limu formed as lava entered the sea on Kilauea Volcano, Hawai`i.
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Limu
Limu, or Limu o Pele (Hawaiian for "seaweed of Pele"), consists of thin flakes of basaltic glass that sometimes form when pahoehoe lava pours into the ocean. As waves wash atop exposed streams of lava, some water may become trapped and boil, resulting in delicate steam-filled bubbles of lava. Abrupt chilling and continued expansion of the delicate bubble walls form thin plates and shattered pieces of brownish-green to nearly-clear glass.
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Photograph by L. Keszthelyi
on 23 February 1996
Tephra generated by steam explosions built this small littoral cone along the southern shoreline of Kilauea Volcano, Hawai`i. Note pahoehoe flow nearing the ocean below the cone.
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Littoral cone
A cone of lava fragments built on the surface of a lava flow pouring into a body of water, usually the sea, is called a littoral cone ("littoral" refers to a shoreline). Lava entering the ocean heats and boils seawater, often generating steam explosions that hurl tephra onto the shore, including spatter, bombs, blocks, ash,, lapilli, and, rarely,limu. As the various tephra accumulates on the shoreline, a well-developed cone may be created.
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Photograph by C. Nye on 9 May 1994
Alaska Division of Geological and Geophysical Surveys
Photograph by R. Russell on 6 April 1977
Alaska Department of Fish and Game
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Maar
A maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table. Maars often fill with water to form a lake.
Top photo: Aerial view toward N of Ukinrek Maars, Alaska; Lake Becharof at top of photo. Water partially fills the eastern maar and completely covers a lava dome that was erupted in the 100-m deep crater during a 10-day eruption in 1977. Maar is about 300 m in diameter.
Bottom photo: Eruption column generated by phreatic and magmatic explosions rises from the larger east maar.
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Sketch by B. Myers
Sketch of a magma reservoir beneath a volcano and a conduit leading up to a lava dome at the surface. Arrow indicates direction of magma movement from a deeper source.
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Magma
Magma 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.
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Mineral
A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal morphology and physical properties such as density and hardness. Minerals are the fundamental units from which most rocks are made.
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Photograph by S.R. Brantley in September 1983
Mud volcano in the Norris Geyser Basin, Yellowstone National Park, Wyoming. The mud volcano is about 40 cm tall.
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Mud volcano
A mud volcano is a small volcano-shaped cone of mud and clay, usually less than 1-2 m tall. These small mud volcanoes are built by a mixture of hot water and fine sediment (mud and clay) that either (1) pours gently from a vent in the ground like a fluid lava flow; or (2) is ejected into the air like a lava fountain by escaping volcanic gas and boiling water. The fine mud and clay typically originates from solid rock--volcanic gases and heat escaping from magma deep below turn groundwater into a hot acidic mixture that chemically changes the rock into mud- and clay-sized fragments.
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Obsidian
Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Compared with window glass, obsidian is rich in iron and magnesium; tiny (<.005 mm) crystals of iron oxide within the glass cause its dark color.
Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow. Glass, unlike crystals, has no regular structure and therefore fractures in smooth conchoidal (curved) shapes. The intersections of these fractures can form edges sharper than the finest steel blades. For this reason, obsidian was used by many native cultures to make arrowheads and blades.
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Photograph by J.D. Griggs
on 16 July 1990
Toes of a pahoehoe flow advance across a road in Kalapana on the east rift zone of Kilauea Volcano, Hawai`i.
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Pahoehoe
Pahoehoe is a Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust. The surface texture of pahoehoe flows varies widely, displaying all kinds of bizarre shapes often referred to as lava sculpture (see pahoehoe terms below).
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Photograph by D.W. Peterson on 27 March 1984
Hundreds of strands of Pele's hair intertwined on the surface of a pahoehoe flow at Kilauea Volcano, Hawai`i. The glass strands were erupted from Mauna Ulu, a shield that formed on the east rift of Kilauea between 1969 and 1974.
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Pele's hair
Thin strands of volcanic glass drawn out from molten lava have long been called Pele's hair, named for Pele, the Hawaiian goddess of volcanoes. A single strand, with a diameter of less than 0.5 mm, may be as long as 2 m. The strands are formed by the stretching or blowing-out of molten basaltic glass from lava, usually from lava fountains, lava cascades, and vigorous lava flows (for example, as pahoehoe lava plunges over a small cliff and at the front of an `a`a flow). Pele's hair is often carried high into the air during fountaining, and wind can blow the glass threads several tens of kilometers from a vent.
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Photograph by J.D. Griggs in November 1984
Assorted shapes of Pele's tears collected a few kilometers downwind from Mauna Ulu from along the Hilina Pali Road on Kilauea Volcano, Hawai`i. U.S. dime for scale in lower right.
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Pele's tears
Small bits of molten lava in fountains can cool quickly and solidify into glass particles shaped like spheres or tear drops called Pele's tears, named after Pele, the Hawaiian goddess of volcanoes. They are jet black in color and are often found on one end of a strand of Pele's hair.
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Photograph by D.A. Swanson
on 4 April 1980
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Phreatic eruption
Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits). The intense heat of such material (as high as 1,170° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and bombs.
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Phreatic eruption at the summit of Mount St. Helens, Washington. Hundreds of these steam-driven explosive eruptions occurred as magma steadily rose into the cone and boiled groundwater. These phreatic eruptions preceded the volcano's plinian eruption on 18 May 1980.
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Photograph by Richard D. Grigg
Diver examines elongate pillowed flow lobe off the coast of Kilauea volcano, Hawai`i.
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Pillow lava
When basalts erupt underwater, they commonly form pillow lavas, which are mounds of elongate lava "pillows" formed by repeated oozing and quenching of the hot basalt. First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. This sequence continues until a thick sequence may be deposited. When geologists find pillow basalts in ancient rock sequences, they may conclude that the area was once under water.
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Photograph by J.D. Griggs on 5 December 1990
Aerial view of Hi`iaka pit crater on the east rift zone of Kilauea Volcano, Hawai`i. The floor of the crater is covered by solidified lava that poured into the crater from the lower right (note black lava flow at crater rim).
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Pit crater
Pit craters are circular-shaped craters formed by the sinking or collapse of the ground. Fissures may erupt from the walls or base of a pit crater, but pit craters are not constructional features built by eruptions of lava or tephra. Pit craters may also partially fill with lava to form a lava lake. They are common along rift zones of shield volcanoes; for example, Mauna Loa and Kilauea volcanoes in Hawai`i. No one has observed the formation of a large pit crater, but they are thought to form as a consequence of the removal of support by withdrawal of underlying magma.
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Photograph by R. McGimsey
on 18 August 1992
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Plinian eruption
Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall. Many thousands of people evacuated areas around the volcano, but about 2,000 were killed, including Pliny the Older.
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A plinian-type explosive eruption from the Crater Peak vent (hidden beneath clouds) on Mount Spurr, Alaska, sent an eruption column to a height of about 18 km above sea level
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Photograph by W.E. Scott on 27 June 1991
Dacitic pumice fragments erupted by Mount Pinatubo, Philippines, during an enormous eruption on 15 June 1991.
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Pumice
Pumice is a light, porous volcanic rock that forms during explosive eruptions. It resembles a sponge because it consists of a network of gas bubbles frozen amidst fragile volcanic glass and minerals. All types of magma (basalt, andesite, dacite, and rhyolite) will form pumice.
Pumice is similar to the liquid foam generated when a bottle of pressurized soda is opened--the opening depressurizes the soda and enables dissolved carbon dioxide gas to escape or erupt through the opening. During an explosive eruption, volcanic gases dissolved in the liquid portion of magma also expand rapidly to create a foam or froth; in the case of pumice, the liquid part of the froth quickly solidifies to glass around the glass bubbles.
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Photograph by J.D. Griggs
Reticulite erupted from Kilauea Volcano, Hawai`i.
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Reticulite
Reticulite is basaltic pumice in which nearly all cell walls of gas bubbles have burst, leaving a honeycomb-like structure. Even though it is less dense than pumice, reticulite does not float in water because of the open network of bubbles. The delicate glass threads between the bubbles are so fragile that reticulite was first called "thread-lace scoria" by the great American mineralogist, James Dana. It has also been called limu.
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Flow banding in rhyolite lava from Mono-Inyo Craters volcanic chain, California (black bands composed of obsidian)
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Rhyolite
Rhyolite is a light-colored rock with silica (SiO2) content greater than about 68 weight percent. Sodium and potassium oxides both can reach about 5 weight percent. Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix. Rhyolite is erupted at temperatures of 700 to 850° C.
More about volcanic and plutonic rocks
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Photograph by J.P. Lockwood in July 1975
Aerial view of the northeast rift zone of Mauna Loa Volcano, Hawai`i. Dark lava flows spread away from the rift zone.
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Rift zone
A rift zone is an elongate system of crustal fractures associated with an area that has undergone extension (ground has spread apart). On the great shield volcanoes in Hawai`i, a rift zone consists of many different features associated with the rise and eruption of magma from narrow dikes, including eruptive fissures, cinder and spatter cones, spatter ramparts, pit craters, lava flows, ground cracks, and normal faults.
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Rock
Rocks are naturally occurring mixtures of minerals, mineral matter, or organic materials. Three main types occur: sedimentary rocks, formed by weathering and mechanical sorting on the Earth's surface; metamorphic rocks, which are rocks that have been transformed by the effects of high temperature and pressure; and igneous rocks, derived from magma (for example, volcanic rocks ).
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Photograph by J. Lowenstern in September 1990
Scoria exposed in the cross section of a cinder or scoria cone.
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Scoria
Scoria is a vesicular (bubbly) glassy lava rock of basaltic to andesitic composition ejected from a vent during explosive eruption. The bubbly nature of scoria is due to the escape of volcanic gases during eruption. Scoria is typically dark gray to black in color, mostly due to its high iron content. The surface of some scoria may have a blue iridescent color; oxidation may lead to a deep reddish-brown color.
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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.
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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.
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Spatter cone
Photograph by T.N Mattox on 3 March 1992
Spatter and cinder cone
Photograph by C. Heliker on 2 June 1986
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Spatter and cinder cone
Long-lived basaltic lava fountains that erupt spatter, scoria or cinder, and other tephra from a central vent typically build steep-sided cones called spatter-and-cinder cones. The greatest bulk of these cones consists of spatter, but during fountaining a lava flow usually pours down one side of the cone. Eruptions that build spatter and cinder cones are much longer in duration and much more varied in intensity than those that eject only spatter to build spatter cones and ramparts.
Top photo: A small short-lived lava fountain built this elongate cone of spatter around a vent located on the flank of the much larger Pu`u `O`o spatter and cinder cone. This spatter cone is 4-5 m tall.
Bottom photo: Lava fountain from the main vent of Pu`u `O`o adds new tephra to its towering spatter and cinder cone. In 1986, the cone was about 255 m tall. The summit was built higher than the main vent (about 86 m higher) as tephra from dozens of tall fountains between 1983 and 1986 was blown by the persistent trade winds toward the southeast.
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Photograph by C. Heliker on 3 April 1992
Lava pours from the base of spatter cones built atop a fissure on the southwest flank of Pu`u `O`o (note slope in lower left), Kilauea Volcano, Hawai`i.
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Spatter cone
Spatter cones are low, steep-sided hills or mounds of welded lava fragments that form along a linear fissure or around a central vent. As lava fragments erupt into the air, they often do not have time to cool completely before hitting the ground. Consequently, the fragments "spatter" as they land and bond to the underlying lava fragments, which are still hot and sometimes oozing down the side of the cone.
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Photograph by J.D. Griggs on 26 March 1984
In less than 24 hours, these low lava fountains built a spatter rampart along the sides of the eruptive fissure on Mauna Loa Volcano, Hawai`i. Note person for scale in lower left.
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Spatter rampart
Lava fountains that erupt from an elongate fissure will build broad embankments of spatter, called spatter ramparts, along both sides of the fissure. The spatter commonly sticks together, or agglutinates, when it lands and is buried by later spatter. In contrast to these low linear fortifications, spatter cones are more circular and cone shaped--the only real distinction between the two structures is their shape.
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Photograph by Charles H. Anderson Jr.
Photograph by T.N. Mattox on 24 August 1993
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Spatter
Very fluid fragments of molten lava ejected from a vent that flatten and congeal on the ground are called spatter. Typically, spatter will build walls of solidified lava around a single vent to form a circular-shaped spatter cone or along both sides of a fissure to build a spatter rampart.
Top photo: Clumps of molten lava (spatter) hurled above the rim of a spatter cone have already started to cool and develop a thin black skin on their surface. Width of the image is about 3 m.
Bottom photo: Close view of cooled, solidified spatter fragments hurled from an active littoral cone on the south shoreline of Kilauea Volcano. The impact of the molten spatter hitting the ground flattened the fragments into roughly circular disks.
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Photograph by Charles h. Anderson Jr.
Mount St. Helens is a Stratovolcano typically consists of many separate vents.
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Stratovolcano
Steep, 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.
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Photograph by B. Chouet in December 1969
Photograph by B. Chouet in 1992
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Strombolian eruption
Strombolian eruptions are characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly. The lava fragments generally consist of partially molten volcanic bombs that become rounded as they fly through the air.
Top photo: Close view of Stromboli Volcano erupting incandescent molten lava framgents.
Bottom photo: View of Stromboli Volcano on the island of Stromboli in the Mediterranean.
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Photograph by K. Shickman on 23 January 1990
A tree mold glows several hours after pahoehoe lava surrounded a tree and burned its trunk until the tree fell onto the lava flow; the trunk is visible in upper right. The glow is from the hot lava below.
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Tree mold
Fluid basaltic lava may preserve the shapes of trees and other objects by solidifying around them. Tree molds are formed when lava surrounds a tree, chills against it, and then drains away. The standing structure left behind is often called a lava tree.
Tree trunks engulfed and incinerated by lava leave cylindrical hollows, or tree molds, where lava solidified against them; tree molds often preserve the original surface texture of the tree. Tree molds are found within standing lava trees and on the surfaces of lava flows. They are common in pahoehoe flows and occasionally found in `a`a flows.
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Photograph by D. Wieprecht
Tephra erupted by Mount St. Helens on 18 May 1980 ranging in size from ash (left 2 piles) to lapilli (right 2 piles).
Photograph by E. Wolfe on 24 June 1991
Volcanic ash falls to ground and creates darkness, Mount Pinatubo in the Philippines.
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Tephra
Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Tephra includes large dense blocks and bombs, and small light rock debris such as scoria, pumice, reticulite, and ash.
As tephra falls to the ground with increasing distance from a volcano, the average size of the individual rock particles becomes smaller and thickness of the resulting deposit becomes thinner. Small tephra stays aloft in the eruption cloud for longer periods of time, which allows wind to blow tiny particles farther from an erupting volcano.
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Photograph by J.D. Griggs on 23 September 1984
Tumulus about 30 m in diameter on the Hilina Pali road on the south flank of Kilauea Volcano, Hawai`i.
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Tumulus
The surfaces of pahoehoe flows on flat or gentle slopes often exhibit elliptical, domed structures called tumuli. A tumulus is created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward. Since the solid crust is brittle, it usually breaks to accommodate the "inflating" core of the flow. Such fractures generally extend along the length of a tumulus, and are frequently accompanied by smaller irregular cracks down the sides. Lava commonly squeezes out through these fractures, and sometimes drains from the tumulus to leave a hollow shell.
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Photograph by D.E. Wieprecht
Volcanic ash collected in Randle, Washington, located about 40 km NNE of Mount St. Helens. The north edge of the eruption cloud of May 18, 1980, passed over Randle and deposited between 1 and 2 cm of ash on the community. At the same distance along the axis of the eruption cloud, however, about 7 cm of ash and larger-sized tepra fell to the ground.
SEM image provided by A.M. Sarna-Wojcicki
Close view of a single ash particle from the eruption of Mount St. Helens; image is from a scanning electron microscope (SEM). The tiny voids or "holes" are called vesicles and were created by expanding gas bubbles during the eruption of magma.
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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.
Volcanic ash is created during explosive eruptions by the shattering of solid rocks and violent separation of magma (molten rock) into tiny pieces. Explosive eruptions are generated when ground water is heated by magma and abruptly converted to steam and also when magma reaches the surface so that volcanic gases dissolved in the molten rock expand and escape (explode) into the air extremely rapidly. After being blasted into the air by expanding steam and other volcanic gases, the hot ash and gas rise quickly to form a towering eruption column directly above the volcano.
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Photograph by K.A. McGee on 19 September 1995
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 tonnes of irritating sulfur dioxide gas (SO2) gas each day.
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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.
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Photograph by C. Heliker on 21 February 1997
Small lava fountain erupts from a new vent on the flank of Pu`u `O`o spatter and cinder cone on the east rift zone of Kilauea Volcano, Hawai`i.
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Vent
Vents are openings in the Earth's crust from which molten rock and volcanic gases escape onto the ground or into the atmosphere. Vents may consist of a single circular-shaped structure, a large elongate fissure and fracture, or a tiny ground crack. The release of volcanic gases and the eruption of molten rock will result in an assortment of constructional features ranging from enormous shield volcanoes and calderas to fumaroles and small rootless hornitos.
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Photograph by E. Klett on 27 January 1994;
U.S. Fish and Wildlife Service
Snow-covered Kanaga Volcano in Alaska erupts a small column of tephra, gas, and steam. Kanaga is a stratovolcano. View is toward the west.
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Volcano
A volcano is a vent at the Earth's surface through which magma (molten rock) and associated gases erupt, and also the cone built by effusive and explosive eruptions.
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Photograph by J.W. Ewert on 5 October 1998
Eruption column caused by a vulcanian-type explosive eruption rises above Tavurvur Volcano in Rabaul Caldera, Papua New Guinea.
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Vulcanian eruption
A vulcanian eruption is a type of explosive eruption that ejects new lava fragments that do not take on a rounded shape during their flight through the air. This may be because the lava is too viscous or already solidified. These moderate-sized explosive eruptions commonly eject a large proportion of volcanic ash and also breadcrust bombs and blocks. Andesitic and dacitic magmas are most often associated with vulcanian eruptions, because their high viscosity (resistance to flow) makes it difficult for the dissolved volcanic gases to escape except under extreme pressure, which leads to explosive behavior.
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