Illustrated Glossary of Volcanic Terms

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'A'a:A Hawaiian term for basaltic lava flows that are typically rough and jagged with a clinkery surface.
Agglutinate:Welded, ejected material characterized by glassy material binding the pyroclasts (ejected particles), or "sintered" glassy pyroclasts.
Ash:Fine, 4.0 mm (.16 in) to 0.25mm (.01 in) in diameter, unconsolidated pyroclastic (ejected particles) material.
Basalt:A dark colored, hard, volcanic rock containing 45% to 54% silica rich minerals (SiO2) which results in low viscosity (resistance to flow). Basalt is erupted at temperatures between 2012 °F and 2282 °F. Common minerals in basalt include olivine, pyroxene, and plagioclase.
Blister:A blister is formed by the swelling of the crust that occurs as a result of the expansion of gas or vapor beneath a flow; typically about 1 meter (3.3 ft) in diameter and hollow.
Block:A solid pyroclastic fragment greater than 64 mm (2.5 in) and up to several meters in diameter, that was ejected from a volcano during an explosive eruption.
Bomb:Pyroclastic fragments greater than 64mm (2.5 in) in diameter, that were molten or plastic at the time of ejection. The shape of a bomb is determined by the viscosity of the magma, velocity and length of flight, the rate at which the lava cooled, the rate of expansion of gases, and the type of deformation that occurred upon impact.
Breadcrust bomb:Breadcrust bombs are characterized by a crust that cooled during flight and as gases within it continued to expand the crust cracked much like bread rising in an oven.
Cinder:Uncemented, glassy, vesicular (holes created by escaping gas bubbles), pyroclastic material. Cinder can be thought of as "volcanic froth".
Cinder cone:A steep, conical hill that is formed by the accumulation of cinders, spatter, and other pyroclastic material.
Cow-pie bombs:Cow-pie bombs, also known as cow-dung and pancake bombs, form from very fluid lava that is still plastic when it lands causing it to flatten upon impact; some still have a liquid core upon impact.
Crater:A circular depression in a volcano that formed from a gradual accumulation of pyroclastic material around the vent, an explosive eruption, or collapse.
Dike:A dike is tabular, intrusive, igneous rock that cuts across the layering of adjacent rock, intrudes into an existing fracture, or creates a new fracture by forcing its way through pre-existing rock.
Fault:A fracture in the Earth's crust where there has been displacement of one side relative to the other.
Fissure/vent:An elongate fracture or crack at the surface from which molten rock and volcanic gases escape onto surface. Eruptions from fissures typically dwindle to a central vent after a period of hours or days. An opening or crack in the Earth's crust from which material is ejected or flows.
Hornito:A rootless spatter cone (fed by lava from within an underlying lava tube) that has a steep sided, inverted cone shape and is formed from an accumulation of pyroclastic materials.
Inflation structure:A inflation structure occurs along a crack where swelling of underlying lava causes one side to become uplifted relative to the other, whether due to degassing or influx of more lava.
Kipuka:A mound of older land, usually covered by vegetation, that is surrounded by a younger lava flow.
Lava:Lava is magma that has erupted onto the Earth's surface and or molten rock on the surface; also used to refer to magma after it has solidified.
Lava curb:Lava curbs form when blobs of lava floating in a river of lava accumulate on the edges of the flow and begin to build out. If the curbs build out far enough on either side to connect to each other and create a crust, they create a new lava tube roof.
Lava flow:A lava flow can be described as an outpouring of molten rock onto the Earth's surface forming a river or sheet.
Lava fountains:A vertical eruption of lava from a vent or along a fissure. Lava fountains can reach a height of 2000 ft.
Lava toe:Small, bulbous extensions of lava that form at the front of pahoehoe flows by breaking through crusts on the flow front.
Lava tube:Lava tubes form when the surface flowing lava congeals forming a crust. The lava underneath the solidified crust continues to flow, now insulated from the cooling air. When the lava eruption ceases, and if the tube drains, a large tubular cave may be left behind.
Loess:Unconsolidated, silt-sized particles with accessory clay and sand particles that are deposited primarily by the wind. Loess that has filtered down into cracks in the lava and between the cinders provides the growth medium for vegetation.
Magma:Magma is molten rock beneath the Earth's surface.
Pahoehoe:A Hawaiian term for a basaltic lava flow that has a smooth, billowy, or ropy surface.
Pit crater:Also known as a volcanic sink, is a circular-shaped depression with steep to vertical walls that formed by collapse of the ground that results from the removal of support such as from the withdrawal of the underlying magma.
Pressure plateau:A pressure plateau forms from a sill-like injection of new lava beneath the crust of an earlier flow that has not completely solidified.
Pressure ridge:Elongated uplift of the congealing crust of a lava flow believed to be caused by the pressure of the underlying, still flowing, lava.
Pumice:Pumice is a light colored, frothy volcanic rock having the composition of rhyolite. It is often buoyant enough to float on water.
Pyroclastic:Pyroclastic is a term that refers to volcanic rock material that is formed by a volcanic explosion or by ejection from a volcanic vent.
Rafted block:A volcanic fragment that was caught up in a lava flow and detached from its source, such as a piece of crater-wall carried off much like an iceberg.
Ribbon bombs:Ribbon bombs are strands of fluid lava ejected from a vent that take the shape of thin twisted "ribbons".
Shelly pahoehoe:A type of pahoehoe lava with a surface that consists of broken blisters and small open lava tubes. In the Craters of the Moon lava field surface crusts are typically about 10 cm (3.9 in) thick.
Shield volcano:A broad, gently sloping volcano that has a flattened dome shape, not unlike that of a knights shield. Shield volcanoes usually cover a large area and form from overlapping and interfingering, low viscosity, lava flows.
Slabby pahoehoe:A type of pahoehoe with a surface that consists of a jumbled arrangement of jagged plates, or slabs, of pahoehoe that were rafted, sheared, tilted, upturned, overturned and heaped on each other
Spatter:An accumulation of very fluid pyroclasts (ejected material).
Spatter cone:A spatter cone is a low, steep sided cone formed from the accumulation of spatter ejected from a vent or fissure.
Spatter rampart:A broad, elongate embankment of spatter that is built by a curtain of fire and forms along either side of a fissure.
Spindle bomb:Volcanic bombs with a twisted shape; spindle bombs form from blobs of fluid lava that often take on a smooth stoss side (front side), a rougher lee side (backside) marked by ribs and fluting caused by frictional resistance to air, and have prominent, usually twisted, projections on either side that form as ribbon bombs separate.
Spiny pahoehoe:A type of pahoehoe with a surface that consists of elongate vesicles that formed from stretching of very viscous lava, giving it a surface texture of small ridges or spines.
Squeeze up:A bulbous blob of viscous, molten lava that was forced, by pressure, up through a fracture or opening in solidified lava.
Tachylite:A black, green or brown volcanic glass that forms when basaltic magma is rapidly chilled.
Tension fractures:Tension fractures result from stresses that pull rocks apart.
Tree mold:A tree mold or lava tree forms when lava flows around a tree and chills, leaving behind a "mold" of the space occupied by the tree, or impression of the charred wood. Tree molds can also be horizontal if the tree was knocked down by the lava flow.
Tumulus:A tumulus is a dome or mound shaped structure on the crust of a lava flow caused by pressure from the difference in rates of flow beneath the crust. Unlike a volcanic blister a tumulus is a solid structure.
Vesicles:A small cavity in a volcanic rock that was formed by the expansion of escaping gas bubbles.
Volcanic rift zone:An elongate system of crustal fractures associated with underlying dike complexes.
Volcano:A vent in the Earth's surface through which magma and associated gases and ash erupt. The structure produced by ejected material.
Xenolith:An inclusion of a foreign body of rock in an igneous rock.

Last Updated: 27-Sep-1999