Geological Society Bicentenary_logo_rev_135.jpg

Working Party Matters


Membership | Terms of Reference | Task Allocation | Intranet | Geohazard Communication | Links |

 

Landslides & Slope Instability


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Subsidence & Collapse Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Seismic Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Flood Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Tsunami Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Volcanic Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Gas Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

 

Fault Reactivation Hazard


Engineering Geologists | Planners & Developers | Finance & Insurance | Member of the Public

Volcanic Geohazard: Definitions and Glossary

Index | Diagnostic Characteristics | Geographic Occurrence | Investigation & Mitigation | Key Contacts & Expert Advice |  Photo Gallery  | Essential References & Further Reading | Definitions & Glossary |

·         Aa: Pronounced "ah-ah" an Hawaiian term. This is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.

 

·         Active volcano: (a) A volcano that is currently erupting, or has erupted during recorded history. (b) 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 (there is no distinction between "active" and "dormant" in this sense).

 

·         Aerosol: Fine liquid or solid particles suspended in the atmosphere. Aerosols resulting from volcanic eruptions are tiny droplets of sulfuric acid, sulfur dioxide that has picked up oxygen and water.

 

·         Airfall: (a) Ash falling from an eruption column or ashcloud. (b) Volcanic ash that has fallen through the air from an eruption cloud. (c) A deposit so formed is usually well sorted and layered.

 

·         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: 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 kilometers 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.

 

·         Ash: (a) Fragments less than 2 millimeters (about 1/8 inch) in diameter of lava or rock blasted into the air by volcanic explosions. (b) Fragments of lava or rock smaller than 2 millimeters in size that are blasted into the air by volcanic explosions. (c) Fine pyroclastic material in fragments less than 4.0 millimeters in diameter. "Ash" in this sense is quite distinct from the ash produced by common combustion because the rocks do not catch fire and burn during a volcanic event.

 

·         Ashfall: (see airfall).

 

·         Ash flow: (a) 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. (b) A pyroclastic flow consisting predominantly of ash-sized (less than 4 millimetres in diameter) particles. Also called a glowing avalanche if it is of very high temperature.

 

·         Asthenosphere: Soft layer of the mantle, beneath the lithosphere.

 

·         Atmospheric shock wave: Strong compressive atmospheric wave driven by volcanic ejecta.

 

·         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.

 

·         Ballistic fragment: An explosively ejected rock fragment that follows a ballistic trajectory.

 

·         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 kilometers 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.

 

·         Base surge: Turbulent, low-density cloud of rock debris and water and (or) steam that moves over the ground surface at high speed. Base surges are generated by explosions.

 

·         Blocks: (a) Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. ... Fragments larger than about 2.5 inches are called blocks if they were ejected in a solid state and volcanic bombs if ejected in semi-solid, or plastic, condition. (b) Fragments of lava or rock larger than 64 millimetres in size that are blasted into the air by volcanic explosions.

 

·         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.

 

·         Bread curst bomb: see bomb

 

·         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

 

·         Central volcano: A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.

 

·         Cinders: Cinders are lava fragments about 1 centimetre (about 1/2 inch) in diameter.

 

·         Cinder cone: (a) A steep-sided volcano formed by the explosive eruption of cinders that form around a vent. (b) A small conical-shaped volcano formed by the accumulation of ejected cinders and other volcanic debris that falls back to Earth close to the vent area.

 

·         Composite volcano: (also called strato volcano)  A steep-sided volcano composed of many layers of volcanic rocks, usually of high-viscosity lava and fragmented debris such as lahar and pyroclastic deposits. Composite volcanoes erupt episodically over tens to hundreds of thousand of years and can display a wide range of eruption styles.

 

·         Conduit: A subterranean passage through which magma reaches the surface during volcanic activity.

 

·         Continental drift: Theory formed by the German scientist Alfred Wegener. According to the theory continents move in consequence of the movement of plates which move in consequence of convection currents.

 

·         Continental volcanoes: In the typical "continental" environment, volcanoes are located in unstable, mountainous belts that have thick roots of granite or granite like rock. Magmas, generated near the base of the mountain root, rise slowly or intermittently along fractures in the crust. During passage through the granite layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of non basaltic rocks.

 

·         Cohesive lahar:  A type of volcanic debris flow that contains more than 3-5 percent clay in the deposit matrix. Such lahars are thought to form from volcanic debris avalanches. Cohesive lahars are very mobile and may flow long distance (several tens of kilometers or more).

 

·         Crater: A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.

 

·         Crust: The thin outermost layer of the Earth, forming both the continents and the ocean floors.

 

·         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.

 

·         Debris avalanche: Rapidly moving, dry flows of disaggregated rock debris, sand, and silt. Volcanic debris avalanches commonly form by some type of structural collapse of the volcano, usually the steep front of the cooled lava dome, or other parts of the upper edifice. A large portion of the volcano may become unstable, break away from the volcanic massif, and become an avalanche. A debris avalanche may be triggered by an eruption or earthquake. Debris avalanches move at velocities ranging from a few tens of meters per second to more than 100 meters per second and behave like complex granular flows or slide flows. Commonly they are quite voluminous (greater than 10 cubic kilometers) and may run out considerable distances (up to 85 kilometers) from their source. The resulting debris-avalanche deposit usually exhibits hummocky surface morphology.

 

·         Debris flow: A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow).

 

·         Density current: A gravity-induced flow of one current through, over, or under another fluid media, owing to density differences. Factors affecting density differences include temperature, salinity, and concentration of suspended particles.

 

·         Deposit: Earth material that has accumulated by some natural process. For example, a flowing mixture of water and rock debris is called a debris flow, but when the flow ceases to move, a layer of fine and coarse rock is left which is called a debris-flow deposit.

 

·         Diatreme: A general term for a volcanic vent or pipe drilled through enclosing rocks (usually flat-lying sedimentary rocks) by the explosive energy of gas-charged magmas. The diamond-bearing kimberlite pipes of South Africa are diatremes.

 

·         Dike: A tabular igneous intrusion that cuts across the host bedrock.

 

·         Diorite: A coarse, uniformly grained rock composed of feldspar and less than 50% amphibole or pyroxene. A quartz diorite has the composition of a diorite plus quartz and biotite, whereas a granodiorite has the composition of a diorite plus quartz and two feldspars. An intrusive igneous rock.

 

·         Direct blast: A hot, low-density mixture of rock debris, ash, and gases that moves at high speed along the ground surface. Directed blasts are generated by explosions.

 

·         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: (a) An active volcano that is in repose (quiescence) but is expected to erupt in the future. (b) A volcano that is not presently erupting but that is considered likely to erupt in the future

 

·         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.

 

·         Earthquake: The abrupt shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth.

 

·         Ejecta:  Material that is thrown out by a volcano, including pyroclastic material (tephra) and lava bombs.

 

·         Eruption cloud:  Cloud of gas, ash, and other fragments that forms during an explosive volcanic eruption and travels long distances with the prevailing winds.

 

·         Eruption column: The vertical portion of the eruption cloud that rises above a volcanic vent.

 

·         Eruption plume: A cloud of volcanic ash emitted from a volcanic vent or volcano.

 

·         Extinct volcano: (a) A volcano that is not expected to erupt again. (b) A volcano that is not presently erupting and is not likely to do so for a very long time in the future

 

·         Fallout: A general term for debris that falls to the earth from an eruption cloud.

 

·         Fault: A discontinuity in the earth's surface that can release underlying magma and permit it to rise to the surface.

 

·         Felsic: An igneous rock having abundant light-colored minerals.

 

·         Fissure:  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: An opening at the Earth's surface from which water vapour and other gases are emitted, often at high temperature.

 

·         Gabbro: A coarse-grained rock composed of greenish-white feldspar (mostly plagioclase) and pyroxene. Gabbro is usually very dark in colour. It is the intrusive equivalent of basalt. An intrusive igneous rock.

 

·         Glacier outburst flood: A sudden release of melt water from a glacier or glacier-dammed lake sometimes resulting in a catastrophic flood, formed by melting of a channel or by subglacial volcanic activity.

 

·         GPS: Global Positioning System, a surveying technique that uses signals from a series of artificial satellites to determine position on the Earth's surface.

 

·         Granite: (a) Igneous rocks are formed from melted rock that has cooled and solidified. When rocks are buried deep within the Earth, they melt because of the high pressure and temperature; the molten rock (called magma) can then flow upward or even be erupted from a volcano onto the Earth's surface. When magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly, usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results. A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock. (b) A coarse-grained, light-coloured rock composed of quartz and two feldspars (plagioclase and orthoclase), with lesser amounts of mica or amphibole. An intrusive igneous rock.

 

·         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.

 

·         Hawaiian eruption: "Hawaiian" eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950, or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows

 

·         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.

 

·         Hot Spot: An area in the middle of a lithospheric plate where magma rises from the mantle and erupts at the Earth's surface. Volcanoes sometimes occur above a hot spot.

 

·         Hummock: Rounded or conical mounds within a volcanic landslide or debris avalanche deposit. Hummocks contain a wide range of rock debris, reflecting the variation of deposits that previously formed the flanks of the volcano. Some hummocks contain huge intact blocks tens to hundreds of meters in diameter. Some of the original layering of lava flows and other deposits can be seen in these large hummocks, but most of the large rock fragments are thoroughly shattered. In other hummocks the rock debris is thoroughly mixed as if the material had been in a blender and thoroughly mixed together.

 

·         Hydrothermal: Pertains to hot water or the action of heated water, often considered heated by magma or in association with magma.

 

·         Hydrothermal alteration: Alteration of rocks or minerals by the reaction of hot water (and other fluids) with pre-existing rocks. The hot water is generally heated groundwater and dissolved minerals.

 

·         Hypocenter: The point within the Earth that is the center of an earthquake. The initial point of rupture during an earthquake.

 

·         Igneous: Solidified from magma; also applied to processes related to the formation of igneous rocks.

 

·         Igneous rock: Igneous rocks are formed from melted rock that has cooled and solidified. When rocks are buried deep within the Earth, they melt because of the high pressure and temperature; the molten rock (called magma) can then flow upward or even be erupted from a volcano onto the Earth's surface. When magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly, usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results. A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock.

 

·         Ignimbrite: The rock formed by the widespread deposition and consolidation of ash flows and Nueés Ardentes. The term was originally applied only to densely welded deposits but now includes non-welded deposits.

 

·         Island arc volcanoes: In a typical "island-arc" environment, volcanoes lie along the crest of an arcuate, crustal ridge bounded on its convex side by a deep oceanic trench. The granite or granite like layer of the continental crust extends beneath the ridge to the vicinity of the trench. Basaltic magmas, generated in the mantle beneath the ridge, rise along fractures through the granitic layer. These magmas commonly will be modified or changed in composition during passage through the granitic layer and erupt on the surface to form volcanoes built largely of non basaltic rocks

 

·         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.

 

·         Jokulhlaup: An Icelandic term that refers without distinction to both water floods and lahars that are generated when a volcano erupts under a glacier.

 

·         Kkipuka: 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: An Indonesian term for a debris flow containing angular clasts of volcanic material. For the purposes of this report, a lahar is any type of sediment-water mixture originating on or from the volcano. Most lahars move rapidly down the slopes of a volcano as channelized flows and deliver large amounts of sediment to the rivers and streams that drain the volcano. The flow velocity of some lahars may be as high as 20 to 40 meters per second and sediment concentrations of greater than 750,000 parts per million are not uncommon. Large volume lahars can travel great distances if they have appreciable clay content (greater than 3 to 5 percent), remain confined to a stream channel, and do not significantly gain sediment while losing water. Thus, they may affect areas many tens to hundreds of kilometers downstream from a volcano.

 

·         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.

 

·         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 lavar (to wash).

 

·         Lava delta: Lava entering the sea often builds a wide fan-shaped area of new land called a lava delta. Such new land is usually built on sloping layers of loose lava fragments and flows. On steep submarine slopes, these layers of debris are unstable and often lead to the sudden collapse of lava deltas into the sea.

 

·         Lava dome A steep-sided mass of viscous and commonly blocky lava extruded from a vent; typically has a rounded top and roughly circular outline.

 

·         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 lake: Another common lava product is the ponded flow or lava lake. The surface of lava that is ponded is smooth, broken only by polygonal cooling cracks, formed in much the same way as shrinkage cracks in mud that has been dried by the sun. ... The formation of the lava lake's solid crust by cooling can be compared to the formation of a sheet of ice on top of a body of water during a winter freeze.

 

·         Lava fountain: A rhythmic vertical fountain like eruption of lava.

 

·         Lava tube: During long-lived eruptions, lava flows tend to become "channelled" into a few main streams. Overflows of lava from these streams solidify quickly and plaster on to the channel walls, building natural levees or ramparts that allow the level of the lava to be raised. Lava streams that flow steadily in a confined channel for many hours to days may develop a solid crust or roof and thus change gradually into streams within lava tubes. Because the walls and roofs of such tubes are good thermal insulators, lava flowing through them can remain hot and fluid much longer than surface flows. Tube-fed lava can be transported for great distances from the eruption sites.

 

·         Lithic: (a) Of or pertaining to stone. (b) (volcanic) Pertains to pyroclastic deposits that contain abundant fragments of previously-formed rocks and/or dense fragments.

 

·         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.

 

·         Lithospheric plates: (known also as Tectonic Plates). A series of rigid slabs (16 major ones at present) that make up the Earth's outer shell. These plates float on top of a softer, more plastic layer in the Earth's mantle.

 

·         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, reticulite. As the various tephra accumulates on the shoreline, a well-developed cone may be created.

 

·         Long-period event: Discrete events with very regular, low-frequency (1-5 Hz) waveforms that resonate for many cycles.

 

·         Maar: (a) Also called "tuff cones", maars are shallow, flat-floored craters formed above diatremes as a result of a violent expansion of magmatic gas or steam. Maars range in size from 200 to 6,500 feet across and from 30 to 650 feet deep, and most are commonly filled with water to form natural lakes. (b) 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

 

·         Mafic: An igneous composed chiefly of one or more dark-colored minerals.

 

·         Mafic volcanoes: Mafic volcanoes typically erupt for brief time intervals (weeks to perhaps centuries), but some can grow almost as large as composite volcanoes. Subsequent eruptions in the region typically issue from new vents and, over tens to hundreds of thousands of years, build broad fields of many volcanoes

 

·         Magma: Molten rock containing liquids, crystals, and dissolved gases that forms within the upper part of the Earth's mantle and crust. When erupted onto the Earth's surface, it is called lava

 

·         Magma chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.

 

·         Magma polarity: Direction of magnetic poles (either normal or reversed) preserved in igneous rocks after they cool through their Curie temperatures.

 

·         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: A zone in the Earth's interior between the crust and the core that is 2,900 kilometers (1,740 miles) thick. (The lithosphere is composed of the topmost 65-70 kilometers (39-42 miles) of mantle and the crust)

 

·         Monogenetic: A volcano built by a single eruption.

 

·         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.

 

·         Mudflow: (a) The flowing mixture of water and debris (intermediate between a volcanic avalanche and a water flood) that forms on the slopes of a volcano. Sometimes called a debris flow or lahar, a term from Indonesia where volcanic mudflows are a major hazard. (b) 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

 

·         Moncohesive lahar: A type of volcanic debris flow that contains less than 3-5 percent clay in the deposit matrix. Such lahars form when melt water produced by the interaction of pyroclastic flows and snow or ice picks up locally available sediment on the flanks of a volcano, or in stream channels developed on the volcano. Noncohesive lahars usually evolve downstream into watery sediment-laden flows called hyperconcentrated flows, or floods.

 

·         Monogenetic volcano: Monogenetic volcanoes typically erupt for only brief time intervals -- weeks to perhaps centuries -- and generally display a narrower range (as compared to composite volcanoes) in eruptive behaviour. Most monogenetic volcanoes are basaltic in composition.

 

·         Nueés 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.

 

·         Obsidian: Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in colour. Compared with window glass, obsidian is rich in iron and magnesium; tiny (<.005 mm) crystals of iron oxide within the glass cause its dark colour. 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.

 

·         Oceanic volcanoes: In a typical "oceanic" environment, volcanoes are aligned along the crest of a broad ridge that marks an active fracture system in the oceanic crust. Basaltic magmas, generated in the upper mantle beneath the ridge, rise along fractures through the basaltic layer. Because the granitic crustal layer is absent, the magmas are not appreciably modified or changed in composition and they erupt on the surface to form basaltic volcanoes.

 

·         Pahoehoe: (a) Pahoehoe (pronounced "pah-hoy-hoy" - a Hawaiian term), is lava that in solidified form is characterized by a smooth, billowy, or ropy surface. (b) 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.

 

·         Pegmatite: An igneous rock with very large (usually > one inch), well-formed crystals. A granitic pegmatite has the mineralogy of granite and abnormally large grains, whereas a gabbroic pegmatite has the mineralogy of a gabbro and very large grains. An intrusive igneous rock.

 

·         Peléan eruption: In a "Peléan" or "Nuee Ardent" (glowing cloud) eruption, such as occurred on the Mayan Volcano in the Philippines in 1968, a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move down-slope at velocities as great as 100 miles per hour. Such eruptive activity can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mount Pelee on Martinique, West Indies

 

·         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 is less than that of sodium and potassium oxides combined.

 

·         Phenocryst: A conspicuous, usually large, crystal embedded in porphyritic igneous rock.

 

·         Phreatic eruption: (a) An explosion of steam, water, mud, and other material. May result from heating of groundwater by magma, and may generate base surges. (b) A type of volcanic explosion that occurs when water comes in contact with hot rocks or ash near a volcanic vent, causing steam explosions. (c) 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. (d) The eruption of Taal Volcano in the Philippine Islands in 1965 typifies "Phreatic" (or steam-blast) behaviour. Here, a great column of steam, dust, ash, and cinders is blasted to a height of several thousand feet. This type of violent eruption is believed to occur when a large quantity of ground or surface water comes in contact with hot rock or magma in a volcanic vent and is instantly and explosively flashed to steam.

 

·         Phreatomagmatic: An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.

 

·         Pillow lava: Fluid lava erupted or flowing under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions. This process is repeated countless times, and the resulting protrusions stack one upon another as the lava flow advances underwater. The term pillow comes from the observation that these stacked protrusions are sack- or pillow-shaped in cross section. Typically ranging from less than a foot to several feet in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water. Much pillow lava is erupted under relatively high pressure created by the weight of the overlying water; there is little or no explosive interaction between hot lava and cold water. The bulk of the submarine part of a Hawaiian volcano is composed of pillow lavas

 

·         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 Hawaii. 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.

 

·         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: Pertaining to igneous rock bodies that form at great depth.

 

·         Polygenetic: Originating in various ways or from various sources.

 

·         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: A dense, hot, chaotic avalanche of rock fragments, gas, and ash that travels rapidly away from an explosive eruption column, down the flanks of the volcano (synonymous with 'ash flow'). Pyroclastic flows move at speeds ranging from 10 to several hundred meters per second and are typically at temperatures between 300 and 800 oC. Pyroclastic flows form either by collapse of the eruption column, or by failure of the front of a cooling lava dome. Once these flows are initiated, they may travel distances of several kilometers or more and easily override topographic obstacles in the flow path. A person could not outrun an advancing pyroclastic flow.

 

·         Pyroclastic surge: (a) Similar to a pyroclastic flow but of much lower density (higher gas to rock ratio). (b) A low-density, turbulent flow of fine-grained volcanic rock debris and hot gas. Pyroclastic surges differ from pyroclastic flows in that they are less dense and tend to travel as a low, ground-hugging, but highly mobile cloud that an surmount topographic barriers. Surges often affect areas beyond the limits of pyroclastic flows.

 

·         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.

 

·         Rhyodacite: An extrusive rock intermediate in composition between dacite and rhyolite.

 

·         Rhyolite: (a) Volcanic rock (or lava) that characteristically is light in colour, contains 69 percent silica or more, and is rich in potassium and sodium. (b) Rhyolite is a light-coloured 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.

 

·         Ring of Fire: The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.

 

·         Satellite vent: A secondary vent of flank vent at a volcanic centre.

 

·         Seamount: A seamount is an underwater mountain that rises at least 1000 meters above the sea floor. Some seamounts rise above the water's surface. Most seamounts are volcanic in orgin; only a few are non-volcanic (caused by uplifting).

 

·         SAR interferometry: Synthetic aperture radar (SAR) interferometry is a powerful remote sensing technique for measuring the distance to the Earth's surface. It requires that the surface is imaged by the radar at least twice from almost the same position in space. The property measured is the phase of the reflected radar signal. The differences in phase between the two images produce interferometric fringes that will represent both the topography of the surface and any changes in position of the surface during the period between the acquisition of the two images (Murray and others, 2000).

 

·         Scarp: A low concave cliff or series of cliffs that marks the detachment zone of a landslide or slope failure.

 

·         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 colour, mostly due to its high iron content. The surface of some scoria may have a blue iridescent colour; oxidation may lead to a deep reddish-brown colour.

 

·         Seamount: A submarine volcano.

 

·         Seismicity: Pertaining to earthquakes or earth vibration.

 

·         Seismograph: A scientific instrument that detects and records vibrations (seismic waves) produced by earthquakes.

 

·         Seismic swarm: A series of earthquakes, occurring in a limited area over a relatively short period of time.

 

·         Shield volcano: The molecule formed of silicon and oxygen (SiO2) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma's silica content, the greater its viscosity or "stickiness."

 

·         Shoshonite: A trachyandesite composed of olivine and augite phenocrysts in a groundmass of labradorite with alkali feldspar rims, olivine, augite, a small amount of leucite, and some dark-colored glass.

 

·         Silica: The molecule formed of silicon and oxygen (SiO2) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma's silica content, the greater its viscosity or "stickiness."

 

·         Silicic: Term used to describe silica-rich volcanic rock or magma.

 

·         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: 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 consist of spatter, but during fountaining 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.

 

·         Spatter rampart: A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent.

 

·         Spatter ridges: 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 consist of spatter, but during fountaining 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.

 

·         Spreading ridge: Places on the ocean floor where lithospheric plates separate and magma erupts. About 80 percent of the Earth's volcanic activity occurs on the ocean floor.

 

·         Stratovolcano: (also called a stratocone or composite cone). A steep-sided volcano, usually conical in shape, built of lava flows and fragmental deposits from explosive eruptions.

 

·         Strombolian eruption: In a "Strombolian"-type eruption observed during the 1965 activity of Irazu Volcano in Costa Rica, huge clots of molten lava burst form the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.

 

·         Strike-slip fault: A nearly vertical fault with side-slipping displacement.

 

·         Strombolian: An eruption style characterized by pulse like explosive bursts and low-level emission of ash and pyroclastic debris. Usually each burst lasts for only a few seconds, and sustained eruption columns generally do not develop.

 

·         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.

 

·         Swarm: A group of many earthquakes of similar size occurring closely clustered in space and time with no dominant main shock.

 

·         Talus: A slope formed at the base of a steeper slope, made of fallen and disintegrated materials.

 

·         Tectonic: Refers to earthquakes generated by faulting rather than by volcanic activity.

 

·         Tephra: (a)  Solid material of all sizes explosively ejected from a volcano into the atmosphere. (b) Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. Fragmental volcanic products between 0.1 to about 2.5 inches in diameter are called lapilli; material finer than 0.1 inch is called ash. Fragments larger than about 2.5 inches are called blocks if they were ejected in a solid state and volcanic bombs if ejected in semi-solid, or plastic, condition.

 

·         Tertiary: Period of geologic time from about 65 million years ago until 1.8 million years ago. It contains the Palaeocene, Eocene, Oligocene, Miocene, and Pliocene epochs.

 

·         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: Widely spaced, fast-moving ocean wave(s) most commonly initiated by sudden displacements of the sea floor during earthquakes or submarine landslides. Volcanic eruptions can also cause tsunamis if unconsolidated volcanic sediment flows rapidly or falls into the water as in a catastrophic slope failure from a steep-sided volcanic cone or edifice, or if explosive eruptions occur at or near sea level. Tsunamis are capable of inundating significant portions of the coastline, especially if the wave energy is focused by narrowing of inlets and bays.

 

·         Tuff: Volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuffs commonly are composed of much shattered volcanic rock glass--chilled magma blown into the air and then deposited. If volcanic particles fall to the ground at a very high temperature, they may fuse together, forming a welded tuff

 

·         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.

 

·         Tuya: (a) A volcano that erupted under a glacier. (b)  A tuya is a volcano that erupts initially beneath a glacier, melts through the ice, and develops an upper, subaerial part, which commonly consists of a flat-to0ped form capped by a lava flow

 

·         Ultramafic: Igneous rocks made mostly of the mafic minerals hypersthene, augite, and/or olivine.

 

·         Unconformity: A substantial break or gap in the geologic record where a rock unit is overlain by another that is not next in stratigraphic succession, such as an interruption in continuity of a depositional sequence of sedimentary rocks or a break between eroded igneous rocks and younger sedimentary strata. It results from a change that caused deposition to cease for a considerable time, and it normally implies uplift and erosion with loss of the previous formed record.

 

·         VEI: The Volcanic Explosivity Index (VEI) was proposed in 1982 to attempt to standardize the assignment of the relative size of an explosive eruption, using ejecta volume as well as the other criteria mentioned earlier. The VEI scale ranges from 0 to 8. A VEI of 0 denotes a non explosive eruption, regardless of volume of erupted products. Eruptions designated a VEI of 5 or higher are considered "very large" explosive events, which occur worldwide only on an average of about once every 2 decades. The May 1980 eruption of Mount St. Helens rated a VEI of 5, but just barely; its lateral blast was powerful, but its output of magma was rather small. The VEI has been determined for more than 5,000 eruptions in the last 10,000 years. None of these eruptions rates the maximum VEI of 8. For example, the eruption of Vesuvius Volcano in A.D. 79, which destroyed Pompeii and Herculaneum, only rates a VEI of 5. Since A.D. 1500, only 22 eruptions with VEI 5 or greater have occurred: one VEI 7 (the 1815 Tambora eruption), four of VEI 6 (including Krakatau in 1883), and seventeen of VEI 5 (counting Mount St. Helens in 1980 and El Chichon, Mexico, in 1982). Considered barely "very large," the eruption of Mount St. Helens in May 1980 was smaller than most other "very large" eruptions within the past 10,000 years and much smaller than the enormous caldera-forming eruptions-which would rate VEI's of 8-that took place earlier than 10,000.

 

·         Vent: An opening in the Earth's surface through which magma erupts or volcanic gases are emitted.

 

·         Vesuvian eruption: In a "Vesuvian" eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D.79, great quantities of ash-laden gas are violently discharged to form a cauliflower-shaped cloud high above the volcano.

 

·         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 (opening) in the surface of the Earth through which magma erupts; it is also the landform that is constructed by the erupted material.

 

·         Volcanic avalanche: (also called a debris avalanche) A large, chaotic mass of soil, rock, and volcanic debris moving swiftly down the slopes of a volcano. Volcanic avalanches can also occur without an eruption as a result of an earthquake; heavy rainfall; or unstable soil, rock, and volcanic debris. 

 

·         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 landslide: The downslope movement of soil, rock debris, and sometimes glacial ice, with or without water, from the flank of a volcano.

 

·         Volcanic neck: A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone.

 

·         Volcanic tremor: Continuous seismic signal with regular or irregular sine wave appearance and low frequencies (0.5-5 Hz). Harmonic tremor has a very uniform appearance, whereas spasmodic tremor is pulsating and consists of higher frequencies with a more irregular appearance.

 

·         Volcaniclastic: A volcanic rock or unconsolidated deposit composed of pre-existing fragments, particles or clasts of volcanic origin.

 

·         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.

 

·         Vulcan: Roman god of fire and the forge after whom volcanoes are named.

 

·         Volcanologits: A person who studies volcanoes and their behaviour.

 

·         Volcanology: The science of volcanoes.

 

·         Vulcan: God of fire of the ancient Romans. The volcano is named after Vulcan.

 

·         Vulcanian eruption: The eruptive activity of Paricutin Volcano in 1947 demonstrated a "Vulcanian"-type eruption, in which a dense cloud of ash-laden gas explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone.

 

·         Xenocrysts: A xenocryst (meaning "foreign crystal") is a foreign crystal that is located in an igneous rock.

 

·         Xenolith: A xenolith (meaning "foreign rock") is a foreign rock that is located in an igneous rock.

 

Index | Diagnostic Characteristics | Geographic Occurrence | Investigation & Mitigation | Key Contacts & Expert Advice |  Photo Gallery  | Essential References & Further Reading | Definitions & Glossary |


Engineering Group Working Party on Geological Hazards