General Terms 'A'a: Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments. Click here to view a photo of 'a'a. Accidental: Pyroclastic rocks that are formed from fragments of non-volcanic rocks or from volcanic rocks not related to the erupting volcano. Accretionary Lava Ball: A rounded mass, ranging in diameter from a few centimeters to several meters, [carried] on the surface of a lava flow (e.g., 'a'a) or on cinder-cone slopes [and formed] by the molding of viscous lava around a core of already solidified lava. Acid: A descriptive term applied to igneous rocks with more than 60% silica (SiO2). Active Volcano: A volcano that is erupting. Also, a volcano that is not presently erupting, but that has erupted within historical time and is considered likely to do so in the future. Agglutinate: A pyroclastic deposit consisting of an accumulation of originally plastic ejecta and formed by the coherence of the fragments upon solidification. Alkalic: Rocks which contain above average amounts of sodium and/or potassium for the group of rocks for which it belongs. For example, the basalts of the capping stage of Hawaiian volcanoes are alkalic. They contain more sodium and/or potassium than the shield-building basalts that make the bulk of the volcano. Andesite: Volcanic rock (or lava) characteristically medium dark in color and containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Ash: Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted. By far the most common variety is vitric ash (glassy particles formed by gas bubbles bursting through liquid magma). Ashfall (Airfall): Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered. Ash Flow: A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes or even along a level surface. Asthenosphere: The shell within the earth, some tens of kilometers below the surface and of undefined thickness, which is a shell of weakness where plastic movements take place to permit pressure adjustments. Avalanche: A large mass of material or mixtures of material falling or sliding rapidly under the force of gravity. Avalanches often are classified by their content, such as snow, ice, soil, or rock avalanches. A mixture of these materials is a debris avalanche. Basalt: Volcanic rock (or lava) that characteristically is dark in color, contains 45% to 54% silica, and generally is rich in iron and magnesium. Basic: A descriptive term applied to igneous rocks (basalt and gabbro) with silica (SiO2) between 44% and 52%. Bench: The unstable, newly-formed front of a lava delta. Blister: A swelling of the crust of a lava flow formed by the puffing-up of gas or vapor beneath the flow. Blisters are about 1 meter in diameter and hollow. Block: Angular chunk of solid rock ejected during an eruption. Bomb: Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact. Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades. Capping Stage: Refers to a stage in the evolution of a typical Hawaiian volcano during which alkalic, basalt, and related rocks build a steeply, sloping cap on the main shield of the volcano. Eruptions are less frequent, but more explosive. The summit caldera may be buried. Central Vent: A central vent is an opening at the Earth's surface of a volcanic conduit of cylindrical or pipe-like form. Cinder Cone: A volcanic cone built entirely of loose fragmented material (pyroclastics.) Cleavage: The breaking of a mineral along crystallographic planes, that reflects a crystal structure. Composite Volcano: A steep volcanic cone built by both lava flows and pyroclastic eruptions. Compound Volcano: A volcano that consists of a complex of two or more vents, or a volcano that has an associated volcanic dome, either in its crater or on its flanks. Examples are Vesuvius and Mont Pelee. Conduit: A passage followed by magma in a volcano. Continental Crust: Solid, outer layers of the earth, including the rocks of the continents. Usage of continental crust. Continental Drift: The theory that horizontal movement of the earth's surface causes slow, relative movements of the continents toward or away from one another. Country Rocks: The rock intruded by and surrounding an igneous intrusion. Crater: A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent. Curtain of Fire: A row of coalescing lava fountains along a fissure; a typical feature of a Hawaiian-type eruption. Dacite: Volcanic rock (or lava) that characteristically is light in color and contains 62% to 69% silica and moderate a mounts of sodium and potassium. Debris Avalanche: A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the major avalanche involved in the eruption of Mount St. Helens, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material. Most of the May 18, 1980 deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche. Debris Flow: A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow). Diatreme: A breccia filled volcanic pipe that was formed by a gaseous explosion. Dike: A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes. Dome: A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent (often circular in plane view) and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. Dormant Volcano: Literally, "sleeping." The term is used to describe a volcano which is presently inactive but which may erupt again. Most of the major Cascade volcanoes are believed to be dormant rather than extinct. Ejecta: Material that is thrown out by a volcano, including pyroclastic material (tephra) and lava bombs. Episode: An episode is a volcanic event that is distinguished by its duration or style. Eruption: The process by which solid, liquid, and gaseous materials are ejected into the earth's atmosphere and onto the earth's surface by volcanic activity. Eruptions range from the quiet overflow of liquid rock to the tremendously violent expulsion of pyroclastics. Eruption Cloud: The column of gases, ash, and larger rock fragments rising from a crater or other vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the stratosphere, where high winds will carry it long distances. Eruptive Vent: The opening through which volcanic material is emitted. Extinct Volcano: A volcano that is not presently erupting and is not likely to do so for a very long time in the future. Usage of extinct. Extrusion: The emission of magmatic material at the earth's surface. Also, the structure or form produced by the process (e.g., a lava flow, volcanic dome, or certain pyroclastic rocks). Fault: A crack or fracture in the earth's surface. Movement along the fault can cause earthquakes or--in the process of mountain-building--can release underlying magma and permit it to rise to the surface. Felsic: An igneous rock having abundant light-colored minerals. Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected. Flank Eruption: An eruption from the side of a volcano (in contrast to a summit eruption.) Fracture: The manner of breaking due to intense folding or faulting. Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles. Geothermal Energy: Energy derived from the internal heat of the earth. Geothermal Power: Power generated by using the heat energy of the earth. Graben: An elongate crustal block that is relatively depressed (downdropped) between two fault systems. Harmonic Tremor: A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault. Heat transfer: Movement of heat from one place to another. Horizontal Blast: An explosive eruption in which the resultant cloud of hot ash and other material moves laterally rather than upward. Horst: A block of the earth's crust, generally long compared to its width, that has been uplifted along faults relative to the rocks on either side. Hot Spot: A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs and may not be associated with ocean ridges. Hot-spot Volcanoes: Volcanoes related to a persistent heat source in the mantle. Hyaloclastite: A deposit formed by the flowing or intrusion of lava or magma into water, ice, or water-saturated sediment and its consequent granulation or shattering into small angular fragments. Hydrothermal Reservoir: An underground zone of porous rock containing hot water. Hypocenter: The place on a buried fault where an earthquake occurs. Usage of hypocenter. Ignimbrite: The rock formed by the widespread deposition and consolidation of ash flows and Nuees Ardentes. The term was originally applied only to densely welded deposits but now includes non-welded deposits. Intensity: A measure of the effects of an earthquake at a particular place. Intensity depends not only on the magnitude of the earthquake, but also on the distance from the epicenter and the local geology. Intermediate: A descriptive term applied to igneous rocks that are transitional between basic and acidic with silica (SiO2) between 54% and 65%. Intrusion: The process of emplacement of magma in pre-existing rock. Also, the term refers to igneous rock mass so formed within the surrounding rock. Juvenile: Pyroclastic material derived directly from magma reaching the surface. Kipuka: An area surrounded by a lava flow. Laccolith: A body of igneous rocks with a flat bottom and domed top. It is parallel to the layers above and below it. Lahar: A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow. Usage of lahar. For a larger discussion on lahars, click here. Landsat: A series of unmanned satellites orbiting at about 706 km (438 miles) above the surface of the earth. The satellites carry cameras similar to video cameras and take images or pictures showing features as small as 30 m or 80 m wide, depending on which camera is used. Usage of Landsat. Lapilli: Literally, "little stones." Round to angular rock fragments, measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state. Lava: Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock. Lava Dome: Mass of lava, created by many individual flows, that has built a dome-shaped pile of lava. Lava Flow: An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet-like body formed by outpouring lava. Lava Fountain: A rhythmic vertical fountainlike eruption of lava. Lava Lake (Pond): A lake of molten lava, usually basaltic, contained in a vent, crater, or broad depression of a shield volcano. Lava Tube: A tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away. Lithosphere: The rigid crust and uppermost mantle of the earth. Thickness is on the order of 60 miles (100 km). Stronger than the underlying asthenosphere. Maar: A volcanic crater that is produced by an explosion in an area of low relief, is generally more or less circular, and often contains a lake, pond, or marsh. Mafic: An igneous composed chiefly of one or more dark-colored minerals. Magma: Molten rock beneath the surface of the earth. Magma Chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano. Magmatic: Pertaining to magma. Magnitude: A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs.) The number scale for magnitudes is logarithmic rather than arithmetic. Therefore, deflections on a seismograph for a magnitude 5 earthquake, for example, are 10 times greater than those for a magnitude 4 earthquake, 100 times greater than for a magnitude 3 earthquake, and so on. Mantle: The zone of the earth below the crust and above the core. Monogenetic: A volcano built by a single eruption. Mudflow: A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar. Nuees Ardentes: A French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force and moves hurricane speed down the mountainside. Usage of Nuees Ardentes Obsidian: A black or dark-colored volcanic glass, usually composed of rhyolite. Oceanic Crust: The earth's crust where it underlies oceans. Pahoehoe: A Hawaiian term for lava with a smooth, billowy, or ropy surface. Click here to view a photo of pahoehoe. Pele Hair: A natural spun glass formed by blowing-out during quiet fountaining of fluid lava, cascading lava falls, or turbulent flows, sometimes in association with pele tears. A single strand, with a diameter of less than half a millimeter, may be as long as two meters. Pele Tears: Small, solidified drops of volcanic glass behind which trail pendants of Pele hair. They may be tear-shaped, spherical, or nearly cylindrical. Peralkaline: Igneous rocks in which the molecular proportion of aluminum oxide isless than that of sodium and potassium oxides combined. Phenocryst: A conspicuous, usually large, crystal embedded in porphyritic igneous rock. Phreatic Eruption (Explosion): An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved. Phreatomagmatic: An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma. Pillow lava: Interconnected, sack-like bodies of lava formed underwater. Pipe: A vertical conduit through the Earth's crust below a volcano, through which magmatic materials have passed. Commonly filled with volcanic breccia and fragments of older rock. Pit Crater: A crater formed by sinking in of the surface, not primarily a vent for lava. Plate Tectonics: The theory that the earth's crust is broken into about 10 fragments (plates,) which move in relation to one another, shifting continents, forming new ocean crust, and stimulating volcanic eruptions. Plinian Eruption: An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic. Plug: Solidified lava that fills the conduit of a volcano. It is usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away. Plug Dome: The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged. Pluton: A large igneous intrusion formed at great depth in the crust.Pumice: Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles. Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent. Pyroclastic Flow: Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed. Rhyodacite: An extrusive rock intermediate in composition between dacite and rhyolite. Rhyolite: Volcanic rock (or lava) that characteristically is light in color, contains 69% silica or more, and is rich in potassium and sodium. Ridge, Oceanic: A major submarine mountain range. Rift System: The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their on-land counterparts such as the East African Rift. Rift Zone: A zone of volcanic features associated with underlying dikes. The location of the rift is marked by cracks, faults, and vents. Ring of Fire: The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean. Scoria: A bomb-size (> 64 mm) pyroclast that is irregular in form and generally very vesicular. It is usually heavier, darker, and more crystalline than pumice. Seafloor Spreading: The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as plates are separating. Seamount: A submarine volcano. Seismograph: An instrument that records seismic waves; that is, vibrations of the earth. Seismologist: Scientists who study earthquake waves and what they tell us about the inside of the Earth.. Seismometer: An instrument that measures motion of the ground caused by earthquake waves.. Shearing: The motion of surfaces sliding past one another. Shear Waves: Earthquake waves that move up and down as the wave itself moves. For example, to the left.. Shield Volcano: A gently sloping volcano in the shape of a flattened dome and built almost exclusively of lava flows. Silica: A chemical combination of silicon and oxygen. Sill: A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes. Skylight: An opening formed by a collapse in the roof of a lava tube. Solfatara: A type of fumarole, the gases of which are characteristically sulfurous. Spatter Cone: A low, steep-sided cone of spatter built up on a fissure or vent. It is usually of basaltic material. Spatter Rampart: A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent. Spines: Horn-like projections formed upon a lava dome. Stalactite: A cone shaped deposit of minerals hanging from the roof of a cavern. Stratovolcano: A volcano composed of both lava flows and pyroclastic material. Strombolian Eruption: A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater. Subduction Zone: The zone of convergence of two tectonic plates, one of which usually overrides the other. Surge: A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column accompanying a volcanic eruption or crater formation. Talus: A slope formed a the base of a steeper slope, made of fallen and disintegrated materials. Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air. Tephrochronology: The collection, preparation, petrographic description, and approximate dating of tephra. Tilt: The angle between the slope of a part of a volcano and some reference. The reference may be the slope of the volcano at some previous time. Trachyandesite: An extrusive rock intermediate in composition between trachyte and andesite. Trachybasalt: An extrusive rock intermediate in composition between trachyte and basalt. Trachyte: A group of fine-grained, generally porphyritic, extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase. Tremor: Low amplitude, continuous earthquake activity often associated with magma movement. Tsunami: A great sea wave produced by a submarine earthquake, volcanic eruption, or large landslide. Tuff: Rock formed of pyroclastic material. Tuff Cone: A type of volcanic cone formed by the interaction of basaltic magma and water. Smaller and steeper than a tuff ring. Tuff Ring: A wide, low-rimmed, well-bedded accumulation of hyalo-clastic debris built around a volcanic vent located in a lake, coastal zone, marsh, or area of abundant ground water. Tumulus: A doming or small mound on the crest of a lava flow caused by pressure due to the difference in the rate of flow between the cooler crust and the more fluid lava below. Ultramafic: Igneous rocks made mostly of the mafic minerals hypersthene, augite, and/or olivine. Vent: The opening at the earth's surface through which volcanic materials issue forth. Usage of vent. Vesicle: A small air pocket or cavity formed in volcanic rock during solidification. Viscosity: A measure of resistance to flow in a liquid (water has low viscosity while honey has a higher viscosity.) Volcano: A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure (usually conical) that is produced by the ejected material. Volcanic Arc: A generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic rocks formed there. Volcanic Complex: A persistent volcanic vent area that has built a complex combination of volcanic landforms. Volcanic Cone: A mound of loose material that was ejected ballistically. Volcanic Neck: A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone. Vulcan: Roman god of fire and the forge after whom volcanoes are named. Vulcanian: A type of eruption consisting of the explosive ejection of incandescent fragments of new viscous lava, usually on the form of blocks. Water Table: The surface between where the pore space in rock is filled with water and where the the pore space in rock is filled with air.