Guide to Geologic Hazards in Alaska - Glossary

  1. Geohazard Types
  2. Glossary
Active fault
A fault on which slip has occurred recently and is likely to occur in the future. [In terms of active faulting, "recently" is often defined as within the past 10,000, 13,000, or 1.6 million years before present depending on the source cited.] (Neuendorf, K.K.E. and others, 2005)
Ash cloud
A cloud of volcanic gases, with ash and other pyroclastic fragments (tephra), that forms by volcanic explosion; Syn: explosion cloud; ash cloud; dust cloud [volc]; volcanic cloud; volcanic plume. (Neuendorf, K.K.E. and others, 2005)
A principal hazard associated with explosive eruptions is the generation of clouds of pulverized rock debris, pumice, volcanic ash, and gas which are blasted high into the atmosphere and then drift away from the volcano with the wind. Ash-rich clouds produced during large eruptions can reach heights of 10 to 20 kilometers or more above the volcano. All aircraft should avoid flying near ash clouds, as contact with ash can damage aircraft windows and instruments, clog air vents, and shut down jet engines. (RI 2004-3)
Ash fall
Airborne ash that falls from an eruption cloud, and the resulting deposit. (Neuendorf, K.K.E. and others, 2005)
Even small amounts of fine ash fallout may cause respiratory problems in some humans and animals. Re-suspension of ash by winds may prolong the unpleasant effects of ash fallout long after an eruption. Heavy ash fallout may interfere with power generation and electrical equipment, damage air filters and gasoline engines, interrupt radio and cell phone transmissions, and greatly reduce visibility. The weight of very thick ash falls may collapse roofs, especially when mixed with rain or snow. (RI 2004-3)
Aufeis
A syn. of icing
Avalanche
A large mass of snow, ice, soil, or rock or mixtures of these materials, falling, sliding, or flowing very rapidly under the force of gravity. Velocities may sometimes exceed 500 km/hr. (Neuendorf, K.K.E. and others, 2005)
Ballistics
Explosive eruptions can blast pebble- to boulder-sized fragments of rock, ice, or pumice into the air, where they travel on arcuate, ballistic trajectories away from the vent. These projectiles, called ballistics, fall at high speed, and can injure or kill people, and crush or damage equipment and buildings. Most ballistics will fall within a few kilometers of the vent. (RI 2004-3)
Calving displacement wave
A significant volume of water displaced by glacial calving. Calving displacement waves pose a geologic hazard concern because of their potential for breaching a moraine or ice dam.
Coastal and river hazards
Alaska is surrounded by approximately 44,000 miles of coastline (including bays and fjords) and is covered by about 86,000 square miles of water. Alaska's dynamic riverine and coastal environments are vulnerable to geologic hazards because they can change rapidly in response to natural forces and human activities. Coastal and river hazards can result from storm and tidal surges, tsunamis, ice jams, ice impacts and unusually fast thaws in high snow years. Resulting impacts include flooding, erosion and damage to infrastructure.
Coastal erosion
Erosion which occurs along beaches and shorelines.
Debris avalanche
Rapidly moving masses of rock debris produced by large-scale landslides and rockfalls that travel many kilometers from their source, burying everything in their path. They can be triggered by eruptions, volcanic earthquakes, or regional earthquakes, or they may occur without an obvious cause. (RI 2004-3)
Debris flow
A moving mass of rock fragments, soil, and mud, more than half of the particles being larger than sand size. Slow debris flows may move less than 1 m per year; rapid ones reach 160 km per hour. Debris flows are very fluid and contain a high content of water. (Neuendorf, K.K.E. and others, 2005)
Directed blast
A directed blast is a laterally directed explosion of the volcano caused by rapid release of internal pressure. Most directed blasts are caused by a slope failure of newly erupted lava domes or sector collapse of the summit edifice into a debris avalanche, resulting in rapid depressurization of a shallow magma body. Directed blasts can destroy structures throughout a large radial zone extending 10 to 25 kilometers away from the source of the blast. (RI 2004-3)
Dissolved gas
Both abiotic and biologically facilitated reactions of contaminants with components of groundwater often produce species that exist in groundwater as dissolved gases. These gases are released into the air when the groundwater depressurizes under surface conditions. Examples: radon, carbon dioxide, methane, ethene, ethane, hydrogen, and acetylene.
Dissolved radon
Radon gas suspended in solution within water from underground sources (ground water).Likely to be problematic in areas containing bedrock enriched in uranium, or thorium (uraniferous granites, marine black shales, karstic-carbonates, and sedimentary and metamorphic rocks and deposits derived from the preceding lithologies).
Earthquake
A sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated strain. Syn: shock, quake, seism, macroseism, temblor (Neuendorf, K.K.E. and others, 2005)
Earthquake related hazards
Alaska is one of the most seismically active areas of the world. Our state is bordered by the circum-Pacific seismic belt (commonly known as the Pacific Ring of Fire) which is one of the earth's most active seismic features. Scientists recognize that Alaska has more and bigger earthquakes than any other region of the United States. It is impossible to predict the time and location of the next big earthquake in Alaska, however, the active geology of Alaska guarantees that major earthquakes will occur in the future.
Earthquake related slope failure
Gradual or rapid downslope movement of soil or rock under gravitational stress as a direct consequence of an earthquake event.
Erosion
The general process or the group of processes whereby the materials of the Earth's crust are loosened, dissolved, or worn away, and simultaneously moved from one place to another, by natural agencies, which include weathering, solution, corrosion, and transportation, but usually exclude mass wasting; specif. the mechanical destruction of the land and the removal of material (such as soil) by running water (including rainfall), waves and currents, moving ice, or wind. (Neuendorf, K.K.E. and others, 2005)
Eruption cloud
A cloud of volcanic gases, with ash and other pyroclastic fragments (tephra), that forms by volcanic explosion; Syn: explosion cloud; ash cloud; dust cloud [volc]; volcanic cloud; volcanic plume. (Neuendorf, K.K.E. and others, 2005)
Fault displacement
The offset of rock and Quaternary deposits including soil caused by movement along a fault.
Faulting
(a) The process of fracturing, frictional slip, and displacement accumulation that produces a fault. (b) The displacement of the crust on a fault or fault array. (Neuendorf, K.K.E. and others, 2005)
Flood
A rising body of water (as in a stream, lake, or sea, or behind a dam) that over tops its natural or artificial confines and that covers land not normally under water; esp. any relatively high stream flow that overflows its banks in any reach of the stream, or that is measured by gage height or discharge quantity. Resulting impacts include flooding, erosion and damage to infrastructure. (Neuendorf, K.K.E. and others, 2005)
Flood icing
A syn. of icing
Floodplain icing
Icing along land adjacent to a river channel. Syn: flood icing; aufeis.
Frost cracking
The contraction cracking of frozen ground, and ice on lakes and rivers, at very low temperatures; the formation of frost cracks. (Neuendorf, K.K.E. and others, 2005)
Frost creep
Soil creep resulting from frost action. (Neuendorf, K.K.E. and others, 2005)
Frost heaving
The uneven lifting or upward movement, and general distortion, of surface soils, rocks, vegetation, and structures such as pavements, due to subsurface freezing of water and growth of ice masses (esp. ice lenses); any upheaval of ground caused by freezing. Syn: frost heave. (Neuendorf, K.K.E. and others, 2005)
Frost jacking
Upward movement of improperly anchored surface structures as a result of frost heaving.
Gelifluction
The progressive lateral flow of earth material under periglacial conditions; solifluction in a region underlain by frozen ground. It is commonly used as a modifying or combining term with bench, lobe, sheet, slope, and stream to indicate periglacial origin by soil flow. (Neuendorf, K.K.E. and others, 2005)
Glacial calving
The breaking away of a mass of ice from a near-vertical ice face. Occurs along the terminus of glaciers which exit into large bodies of water. (Neuendorf, K.K.E. and others, 2005)
Glacier flood
A general term describing a flood which results from glacial melting.
Glacier hazards
Outside the two ice sheets, glaciers in Alaska (and shared icefields with Canada) represent about 13% of the mountain glaciers and ice caps area on Earth. Glaciers represent an important fresh water reservoir and glacier-related tourism is an important part of the economy in Alaska. Glacier-related processes, however, can become hazards when they threaten public safety and infrastructure. A variety of glacier hazards exist in Alaska (e.g. jökulhlaups, calving, glacier surge) and many glacier-related processes lead to other (indirect) hazards (e.g. flooding and debris flows) that impact people and infrastructure in distant areas. (Echelmeyer, K.A. and others, 2002)
Glacier outburst flood
A sudden, often annual, release of meltwater from a glacier or glacier-dammed lake, sometimes resulting in a catastrophic flood. Glacial outburst floods are triggered by melting of a drainage channel, buoyant lifting of ice by water, or subglacial volcanic activity. Glacier-dammed lakes may be subglacial (below the glacier), marginal (along the sides of the glacier), or proglacial (at the glacier terminus). Proglacial and ice-marginal dams develop behind moraines or in ice-dammed valleys.
Glacier surge
[surging glacier] A glacier that alternates periodically between brief periods (usually one to four years) of rapid flow, called surges, and longer periods (usually 10 to 100 years) of near stagnation. During a surge, a large volume of ice from an ice-reservoir area is displaced downstream at speeds up to several meters per hour into an ice-receiving area, and the affected portion of the glacier is chaotically crevassed. In the interval between surges, the ice reservoir is slowly replenished by accumulation and normal ice flow, and the ice in the receiving area is greatly reduced by ablation. (Neuendorf, K.K.E. and others, 2005)
Ground ice
All ice, of whatever origin or age, found below the surface of the ground, esp. a lens, sheet, wedge, seam, or irregular mass of clear nonglacial ice enclosed in perennially or seasonally frozen ground, often at considerable depth. (Neuendorf, K.K.E. and others, 2005)
Ground motion
A general term for all seismic motion, including ground acceleration, velocity, displacement, and strain. (Neuendorf, K.K.E. and others, 2005)
Groundwater contamination
Presence of a biological, chemical, or elemental agent in groundwater in such a concentration that it renders water unfit for a particular use. Contaminants can be from human derived and/or natural sources.
Groundwater emergence
Flooding which occurs in low-lying areas when the water table rises above the land surface. Syn: groundwater flooding
Groundwater related hazards
Surface water moves downward through empty spaces or cracks in the soil, sand, or rocks. The water then fills the empty spaces and cracks above that layer. The top of the water in the soil, sand, or rocks is called the water table and the water that fills the empty spaces and cracks is called groundwater. Groundwater supplies a major portion of Alaska's drinking and industrial water. Changes in groundwater flow and supply and/or groundwater contamination can result in significant threats to human health and infrastructure.
Ice avalanche
A sudden fall, down a steep slope, of ice broken from an ice sheet or glacier (most commonly from a hanging glacier). (Neuendorf, K.K.E. and others, 2005)
Ice dam
A river obstruction formed of floating blocks of ice that may cause ponding and widespread flooding during spring and early summer. Syn: ice barrier. (Neuendorf, K.K.E. and others, 2005)
Ice jam
(a ) An accumulation of broken river ice lodged in a narrow or obstructed part of a channel; it frequently produces local floods during a spring breakup. Cf:ice gorge. (b) An accumulation of large fragments of lake ice or sea ice thawed loose from the shore during early spring and subsequently piled up or blown against the shore by the wind, often exerting great pressures. (Neuendorf, K.K.E. and others, 2005)
Ice push
The lateral pressure exerted by the expansion of shoreward-moving ice, especially of lake ice. Syn: ice shove; ice thrust (Neuendorf, K.K.E. and others, 2005)
Iceberg
A piece of floating or grounded (stranded) glacier ice of any shape, detached (calved) from the terminus of a glacier into a body of water. (Neuendorf, K.K.E. and others, 2005)
Icing
A surface ice mass formed during the winter in a permafrost area by successive freezing of sheets of water that may seep from the ground, or from a spring or river. (Neuendorf, K.K.E. and others, 2005)
Inorganic contamination
Ground and/or surface water contamination resulting from substances lacking an elemental combination of carbon, hydrogen and oxygen. In the context of geologic hazards, examples could include: salt, iron, calcium salts, metals and other mineral materials.
Jokulhlaup
An Icelandic term for glacier outburst flood (Neuendorf, K.K.E. and others, 2005)
Lahar
A mudflow composed chiefly of volcaniclastic materials on the flank of a volcano. The debris carried in the flow includes pyroclasts, blocks from primary lava flows, and epiclastic material. Syn: mudflow (Neuendorf, K.K.E. and others, 2005)
Hot volcanic debris erupted onto the flanks of the volcano can melt snow, forming floods or fast-moving slurries of water, mud, rocks, and sand. Lahars may also occur when thick deposits of proximal ash are rapidly eroded by heavy rainfall following eruptions. (RI 2004-3)
Lahar runout flow
Hyperconcentrated streamflows that form by downstream transformation of lahars through loss of sediment and dilution by streamflow (P 1447-B)
Landslide
A general term covering a wide variety of mass-movement landforms and processes involving the down slope transport, under gravitational influence, of soil and rock material en masse. Usually the displaced material moves over a relatively confined zone or surface of shear. The wide range of sites and structures and material properties affecting resistance to shear result in a great range of landslide morphology, rates, patterns of movement, and scale. Landsliding is usually preceded, accompanied, and followed by perceptible creep along the surface of sliding and/or within the slide mass. Terminology designating landslide types generally refers to the landform as well as the process responsible for it, e.g. rockfall, translational slide, block glide, avalanche, mudflow, liquefaction slide, and slump. (Neuendorf, K.K.E. and others, 2005)
Lateral spread
(a) Lateral movements in a fractured mass of rock or soil, which result from liquefaction or plastic flow of subjacent materials. (b) Lateral movement of intact blocks of soil or rock material which slide on a gently inclined planar surface, such as a bedding plane or a liquefied sand layer. (Neuendorf, K.K.E. and others, 2005)
Lava flow
A lateral, surficial outpouring of molten lava from a vent or a fissure; also, the solidified body of rock that is so formed. (Neuendorf, K.K.E. and others, 2005)
Liquefaction
In saturated, cohesionless soil, the transformation from a solid to a liquid state as a result of increased pore pressure and reduced effective stress [in response to severe ground shaking resulting from an earthquake]. (Neuendorf, K.K.E. and others, 2005)
Loose snow avalanche
Avalanches which occur in freshly fallen snow that has a lower density. They are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usual (Tremper, Bruce, 2008)
Mudflow
A general term for a mass-movement landform and a process characterized by a flowing mass of predominantly fine-grained earth material possessing a high degree of fluidity during movement. The degree of fluidity is revealed by the observed rate of movement or by the distribution and morphology of the resulting deposit. Mudflows are intermediate members of a gradational series of processes characterized by varying proportions of water, clay, and rock debris. The water content of mudflows may range up to 60%. The degree of water bonding, determined by the clay content and mineralogy, critically affects the viscosity of the matrix and the velocity and morphology of the flow. With increasing fluidity, mudflows grade into loaded and clear streams; with a decrease in fluidity, they grade into earthflows. Also spelled: mud flow. (Neuendorf, K.K.E. and others, 2005)
Naled
A Russian term for icing; aufeis. (Neuendorf, K.K.E. and others, 2005)
Organic contamination
Contamination resulting from substances containing any member of a large class of chemical compounds whose molecules contain carbon. Typically caused by surface spills and underground storage tank leakage.
Periglacial slope failure
Gradual or rapid downslope movement of soil or rock under gravitational stress. (Neuendorf, K.K.E. and others, 2005)
Permafrost and periglacial related hazards
Permafrost isn't a geologic hazard until it starts to melt. Unfortunately, permafrost is very sensitive to disturbance by humans and nature. As a result, it is important to consider whether or not permafrost is present when developing any kind of infrastructure. Researchers suggest that permafrost is getting warmer due to rising temperatures at northern latitudes.
Pyroclastic density current
A gravity-controlled, laterally moving mixture of pyroclasts and gas. It is a more general term that includes pyroclastic flow and pyroclastic surge (Neuendorf, K.K.E. and others, 2005)
Pyroclastic flows (incandescent flows of ash, gas, and coarse volcanic debris) and surges (hurricane-force blasts of turbulent hot gas and ash clouds) can race down slopes at speeds as great as 100 meters per second, travel 10 to 30 kilometers from the vent, and overtop hills and other topographic obstacles. (RI 2004-3)
Pyroclastic fall
Aerial dispersal of rock material formed by a volcanic explosion or expulsion from a volcanic vent.
Pyroclastic flow
A density current of pyroclastic material, usually very hot and composed of a mixture of gases and particles. A syn. of ash flow used in a more general sense in that an ash flow is composed of ash-sized pyroclasts. (Neuendorf, K.K.E. and others, 2005)
Pyroclastic surge
Low-density, dilute, turbulent pyroclastic flow. Types of pyroclastic surges include base surges, ash-cloud surges, and ground-surges. (Neuendorf, K.K.E. and others, 2005)
Radon gas
A radioactive, gaseous isotope of radon; it is a member of the thorium series and a daughter of radium-224. (Neuendorf, K.K.E. and others, 2005)
Radon hazards
Radon is a radioactive gas that is known to occur in Interior Alaska, especially in the hills around Fairbanks. Radon gas is only a problem when it is trapped in spaces where people live and work. Radon mitigation is most effective during initial home construction but be retrofitted in many circumstances.
Retrogressive thaw
A slope failure or slump resulting from thawing of ice-rich permafrost. (Neuendorf, K.K.E. and others, 2005)
River erosion
Erosion which occurs along rivers and streams, includes both bank erosion and stream bottom erosion.
River icing
Icing in stream courses and rivers. Syn: flood icing; aufeis.
Rock avalanche
The very rapid downslope flowage of rock fragments, during which the fragments may become further broken or pulverized. Rock avalanches typically result from large rockfalls and rockslides, and their patterns of displacement have led to the term rock-fragment flow (Varnes, 1958). Characteristic features include chaotic distribution of large blocks, flow morphology and internal structure, relative thinness in comparison to large areal extent, high porosity, angularity of fragments, and lobate form. Cf: debris flow. (Neuendorf, K.K.E. and others, 2005)
Rockfall
The relatively free falling or precipitous movement of a newly detached segment of bedrock (usually massive, homogeneous, or jointed) of any size from a cliff or other very steep slope; it is the fastest form of mass movement and is most frequent in mountains during spring when there is repeated freezing and thawing of water in cracks in the rock. Syn: sturzstrom (Neuendorf, K.K.E. and others, 2005)
Saltwater encroachment
Displacement of fresh water or groundwater by the advance of salt water due to its greater density, usually in coastal and estuarine areas but also by movement of brine from beneath a playa lake toward wells discharging fresh water. Encroachment occurs when the total head of the salt water exceeds that of adjacent fresh water. Syn: sea-water intrusion, salt-water intrusion, sea-water encroachment. (Neuendorf, K.K.E. and others, 2005)
Saltwater intrusion
Synonym of saltwater encroachment. (Neuendorf, K.K.E. and others, 2005)
Sea ice
Any form of ice originating from seawater. Sea ice poses a geologic hazard when it destabilizes shorelines via erosion of tidal flats and bluff bases. (Neuendorf, K.K.E. and others, 2005)
Sea level rise
Long-term increases in mean sea level. The expression is popularly applied to anticipated ecstatic sea level changes due to the greenhouse effect and associated global warming. (Neuendorf, K.K.E. and others, 2005)
Seasonal melt flood
Flooding due to seasonal melting of ice and snow
Seiche
A free or standing-wave oscillation of the surface of water in an enclosed or semi-enclosed basin (as a lake, bay, or harbor) that varies in a period, depending on the physical dimensions of the basin, from a few minutes to several hours, and in height from several centimeters to a few meters; that is initiated chiefly by local changes in atmospheric pressure, aided by winds, tidal currents, and small earthquakes; and that continues, pendulum fashion, for a time after cessation of the originating force (Neuendorf, K.K.E. and others, 2005)
Slab avalanche
Slab avalanches occur when a more cohesive or harder layer of snow sets on top of a less cohesive or softer and weaker layer of snow. Slab avalanches often involve large volumes of fast moving snow. (Tremper, Bruce, 2008)
Slope instability hazards
Geologic hazards are often interrelated. This is especially true for unstable slopes. Slopes may become unstable for a variety of reasons such as the character of a geologic formation or the occurrence of earthquakes, rain, fire, melting permafrost, or rapid deglaciation. In addition, rapid down slope movement of large quantities of material into or under water can generate tsunamis. Surficial and engineering geologic maps and geologic hazard reports often contain information about landslides in an area.
Slump
A landslide characterized by a shearing and rotary movement of a generally independent mass of rock or earth along a curved slip surface (concave upward) and about an axis parallel to the slope from which it descends, and by backward tilting of the mass with respect to that slope so that the slump surface often exhibits a reversed slope facing uphill. (Neuendorf, K.K.E. and others, 2005)
Slush avalanche
Flow of a partially or totally saturated snow cover, common during rapid spring breakup in arctic and subarctic alpine regions. Slush avalanches can cascade from slopes as gentle as 3 or 4 degrees--practically level ground. Considerable earth and rock material may also be carried downward. Syn: Slush fall.
Snow avalanche
An avalanche consisting of relatively pure snow, although considerable earth and rock material may also be carried downward. Syn: snowslide. (Neuendorf, K.K.E. and others, 2005)
Soil creep
The gradual, steady downhill movement of soil and loose rock material on a slope that may be very gentle but is usually steep. (Neuendorf, K.K.E. and others, 2005)
Solifluction
The slow viscous downslope flow of waterlogged soil and other unsorted and saturated surficial material, normally at 0.5-5.0 cm/yr; esp. the flow occurring at high elevations in regions underlain by frozen ground (not necessarily permafrost) that acts as a downward barrier to water percolation, initiated by frost action and augmented by meltwater resulting from alternate freezing and thawing of snow and ground ice. (Neuendorf, K.K.E. and others, 2005)
Storm surge
An abnormal, sudden rise of sea level along an open coast during a storm, caused primarily by strong winds offshore, or less frequently by a drop in atmospheric pressure, resulting in water piled up against the coast. It is most severe when accompanied by high tide. Syn: surge [waves], hurricane surge, storm wave. (Neuendorf, K.K.E. and others, 2005)
Strudel scour
Drainage of large volumes of fresh water through sea ice at holes and cracks (strudel) causes scour depressions in the sea floor below. In Alaska these scour depressions have been observed to be more than 4 m deep and as much as 20 across. (Reimnitz, Erk and others, 1974)
Subsidence
The sudden sinking or gradual downward settling of the Earth's surface with little or no horizontal motion. The movement is not restricted in rate, magnitude, or area involved. Subsidence may be caused by natural geologic processes, such as solution (karst phenomena), thawing, compaction, slow crustal warping, or withdrawal of fluid lava from beneath a solid crust; or by man's activity, such as subsurface mining or the pumping of oil or groundwater. (Neuendorf, K.K.E. and others, 2005)
Earthquakes can cause rapid subsidence due to land level changes.
Surface fault rupture
Dislocation of the surface of the earth related to motion along the fault at depth. Ground cracking related to surface fault rupture can occur along the fault and away from the fault.
Thawing permafrost
Thawing of any soil, subsoil, or other surficial deposit, or even bedrock, occurring in the arctic, subarctic, and alpine regions at variable depth beneath the Earth's surface in which a temperature below freezing has existed continuously for a long time (from two years to tens of thousands of years) [Permafrost]. This definition is based exclusively on temperature, and disregards the texture, degree of compaction, water content, and lithologic character of the material. The thickness of permafrost ranges from over 1000 m in the north to 30 cm in the south; it underlies about one fifth of the world's land area. Syn: pergelisol, perennially frozen ground. (Neuendorf, K.K.E. and others, 2005)
Thermokarst
Karst-like topographic features produced in a permafrost region by the local melting of ground ice and the subsequent settling of the ground (Neuendorf, K.K.E. and others, 2005)
Tsunami
Tsunamis are sea waves produced by any large-scale, short-duration disturbance of the ocean floor. These may be caused by shallow submarine earthquakes, but also by submarine slumps, subsidence, or volcanic eruptions. Tsunamis generated thousands of miles away can travel relatively quickly and be very destructive. Tsunami warning systems can provide ample warning for these far-field tsunamis if properly implemented. Locally generated tsunamis can reach the shore within seconds of initial wave propagation. In such cases, ground shaking is the only warning people will have.
Uplift
Any force that tends to raise an engineered structure and its foundation relative to its surroundings. It may stem from pressure of subjacent ground, surface water, or expansive soil under the base of the structure; or from lateral forces such as wind or earthquake shaking that tend to cause overturning. (Neuendorf, K.K.E. and others, 2005)
Regional or local uplift of the land surface related to movement on faults, this can occur rapidly during an earthquake or slowly during strain accumulation between earthquakes.
Volcanic ash
Fine pyroclastic material (under 2.0 mm diameter; under 0.063 mm diameter for fine ash). The term usually refers to the unconsolidated material but is sometimes also used for its consolidated counterpart, tuff. (Neuendorf, K.K.E. and others, 2005)
Volcanic gas
Volatile matter, released during a volcanic eruption, that was previously dissolved in the magma. (Neuendorf, K.K.E. and others, 2005)
Volcanic vents often emit steam and gases, including hydrogen sulfide and carbon dioxide, in concentrations that are potentially harmful to humans. Lava flows and pyroclastic flows also emit large amounts of steam and gas. Dangerous invisible, odorless volcanic gases can collect in low-lying areas near fumaroles, making it inadvisable to descend into craters, caves, or fissures occupied by fumaroles. (RI 2004-3)
Volcanic landslide
The steep cliffs that form the caldera walls, bedrock cliffs, volcanic cones, and steep lava flow fronts are prone to rockfalls and small landslides. The frequency and size of such events is likely to increase during eruptions and earthquakes. (RI 2004-3)
Volcano hazards
Alaska contains over 130 volcanoes and volcanic fields which have been active within the last two million years. Of these volcanoes, about 90 have been active within the last 10,000 years (and might be expected to erupt again), and more than 50 have been active within historical time (since about 1760, for Alaska). Volcanoes produce a wide variety of natural hazards. Some, such as ash clouds and pyroclastic flows, occur as eruption processes. While others, such as water contamination and lahars (mudflows) can occur even when a volcano is quiescent.