Volcano Finder - World Volcanoes
There are four types of eruption processes that produce volcanic ash:
(1) decompression of rising up magma gas bubble growth, and breakup of the foamy magma in the volcanic vent (magmatic)
(2) explosive mixing of magma with ground or upper surface water (hydrovolcanic)
(3) fragmentation of rock during rapid expansion growth of steam and/or hot water (phreatic)
(4) abrasion during rapid collision of ash grains.
Variations in eruption method and the characteristics traits of volcanic ashes created during explosive eruptions vary on many factors, including magmatic temperature gas, content viscosity(thickness), and crystal content of the magma before eruption, and the ratio of magma to ground or surface water and physical properties of the rock encircling the vent. Volcanic ash is comprised of rock and mineral pieces and glass shards. Glass shard forms and sizes depend upon size and shape of gas bubbles current within the magma immediately before eruption and the processes responsible for fragmentation of that magma. Shards span from slightly curved, thin glass plates, which were smashed from large, thin enclosed bubble walls, to hollow needles broken from pumiceous melts holding gas bubbles stretched into thin tubes by magma movement within the volcanic vent. Pumice fragments make up the grainier grained portions of the glass fraction. Particle sizes range from meters for large blocks ejected near the volcanic vent to nanometers for fine powdery ash and aerosol droplets inside well dispersed eruption plumes. Most of the discharge from volcanic eruptions falls out within a few tens of kilometers of the source. Of major anxiety to aviation are widespread eruption plumes which are scattered downwind at elevations of 8 to 20 kilometers and can drift down-range for thousands of kilometers.
Pyroclastic fallout (ash fall or tephra fall)
Volcanic ash (material less than 2 millimeters in width) or tephra (material greater than 2 millimeters in width) is created when magma is finely fragmented by vesiculation (the formation of vesicles) or when previously solidified rocks are smashed by the explosion of ground water into steam. Ash falls are deposited below an eruption column and for many kilometers downwind; the ash fall becomes much finer grained with ever increasing distance from the volcano. Strong eruption plumes can carry the finest lightest ash into the stratosphere, where strong winds spread it over many thousands of kilometers. Even a very small ash fall poses a serious nuisance to agricultural crops, exposed people, delicate machinery, electronics, and computers. When thick or wet, the ash can become as heavy as cement, and it can cause some roofs to collapse. Wind-borne ash is a serious abrasive danger to aircraft engines and instruments and great care is taken to track and avoid high moving ash fallout.
Lava flows are streams of super hot molten rock that either feed out quietly from a vent or are fed by active lava fountains. Fluid basalt flows can creep and move at velocities from 15 to a fast 50 kilometers per hour on steep slopes, and travel up to many kilometers from their orginal source. Viscous andesite lava flows move only a few kilometers per hour and rarely extend more than 8+ kilometers from their home vent. Lava flows destroy everything in their path, but most move slowly enough that people can escape.
Lava Domes are lava (dacite or rhyolite) that is too sticky and tacky to flow very far from its vent forms steep sided mounds called lava domes.
An eruption column is described as the vertical or sub-vertical part of the emissions originating from an explosive volcanic vent. Eruption columns vary from very low, small-sized bursts to huge convective structures that quickly transport ash, volcanic gases, and suspended particle air into the stratosphere. They have a lower gas-thrust part (the jet of material exiting the vent) that commonly represents less than 10% of the total height. Above this is the umbrella region, a region of momentum-driven rise with considerable lateral spreading movement. Characteristics of eruption columns differ according to the style of explosive eruption.
Volcanic gases most common gases found with active volcanoes are water vapor, hydrogen, carbon dioxide, sulfur dioxide, hydrogen sulfide, helium, and deadly carbon monoxide and hydrochloric acid. Lesser amounts of hydrofluoric acid, nitrogen, argon, and other compounds are commonly associated with live volcanoes as well. Volcanic gases rarely reach or drift to populated areas in high lethal concentrations, although sulfur dioxide can react with the atmosphere downwind and fall as corrosive acid rain to cause metal corrosion and a host of other area effects. People with bad respiratory or heart diseases are especially susceptible to volcanic gases (fume) effecting their breathing. Carbon dioxide is heavier than air and likes to seek out and settle in depressions, where it can occur in deadly invisible lethal concentrations and cause suffocation. On occasion, toxic concentrations of fluorine gas from hydrofluoric acid have been adsorbed onto ash and ingested by unsuspecting livestock or leached into area domestic water supplies.
Pyroclastic flows and pyroclastic surges
These are large mixtures of hot rock fragments and gases that can fall away from their source vents at super hurricane velocity. Pyroclastic flows are very dense and most are confined to steep enclosed valleys around a volcano; the largest flows can travel tens and even hundreds of kilometers beyond a volcano. Pyroclastic surges are fierce, low-density variants of pyroclastic flows. Some unusually rapid hot pyroclastic flows or surges start from laterally directed explosions from a vent. Because of their high speed and very high temperature, pyroclastic flows and surges kill or destroy almost all that is in their path by depth, knock down power, and fire(heat).
Volcanic debris flows (mudflows or lahars)
Debris flows are a wet flowing mixture of water-saturated debris, some where between a debris avalanche and a water flood, it typically moves at speeds of several tens of miles per hour on steep volcano slopes, slowing to less than 10 mi per hour on gentle hills and slopes. Debris flows can travel tens of miles down valley and devastate distant unsuspecting communities, as with Colombia during the 1985 eruption of Nevado del Ruiz.
Volcanic landslides are gravity driven slides, often rapid, of a mass of rock, soil, vegitation (trees and plants) that can occur in all sizes, from those involving a small amount of loose debris on the surface of a volcano to massive failures of the entire summit and (or) the flanks of a volcano. Volcanic landslides may not be associated with just eruptions; very heavy rainfall or a large earthquake can trigger landslides on steep volcanic slopes. Good examples are Quake Lake in Wyoming and hillside collapses in Bluebird Canyon and Ventura, California. Landslides that have evolved into a chaotic tumbling flow are termed debris avalanches.
A Caldera is a large depression commonly formed by collapse failure of the surface ground following a massive explosive eruption of a large sub-surface body of stored magma. The huge calderas at Yellowstone and Long Valley are associated with eruption of silicic magma as pyroclastic flows as are evidenced in these areas. Many people don’t even know they are on a caldera, since they are so huge(see Long Valley Caldera Map). Kilauea caldera, by contrast, is thought to be associated with slow draining of basaltic magma from beneath Hawaii’s Kilauea's summit. The caldera now water filled by Oregon's Crater Lake was produced by an monstrous eruption that destroyed a volcano the size of Mount St. Helens and sent masses of volcanic ash as far east as Nebraska. A eruption and caldera creation of this scale would be in today’s world a “extinction level event", since that much ash would block the sun and kill crops for many years.
Key to Volcano Finder Map Volcano Types:
Caldera - A large volcanic collapse depression, commonly circular or elliptical when seen from above.
Cinder cone - A steep-sided volcano formed by the explosive eruption of cinders that form around a vent.
Complex volcano - A volcano composed of a mixture of landforms. In most cases, they occur because of changes either in eruptive habit or in location of the principal vent area.
Crater rows - An area of congealed lava produced by isolated lava fountains along a fissure (volcanic vent).
Fissure vent - Linear volcanic vents through which lava erupts, usually without any explosive activity. The vents are usually a few meters wide and may be many kilometers long.
Fumarole field - Areas where there are cracks in the ground that allow gases to reach the surface.
Hydrothermal field - An area where water heated by magma or in association with magma reaches the surface.
Lava dome - A rounded, steep-sided mount that forms when very viscous lava is extruded from a volcanic vent.
Maars - Shallow, flat-floored craters formed above diatremes (volcanic vents or pipes drilled through rocks by the explosive energy of gas-charged magmas) as a result of a violent expansion of magmatic gas or steam. Maars often fill with water to form a lake.
Pyroclastic cone - A volcanic cone composed of fragmented material ejected from a volcano.
Scoria cones - A steep-sided volcano formed by the explosive eruption of a cinder cone. Scoria is formed when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles.
Shield volcano - A volcano that resembles an inverted warrior's shield, with broad, gentle slopes, built by multiple eruptions of fluid basalt lava. Basalt lava tends to build enormous, low-angle cones because it flows across the ground easily and can form lava tubes that enable lava to flow tens of kilometers from an erupting vent with very little cooling.
Somma volcano - A large volcanic collapse depression that is partially filled by a new central cone.
Stratovolcano - A steep-sided volcano built by lava flows and tephra deposits. Tephra is solid material of all sizes explosively ejected from a volcano into the atmosphere.
Subglacial volcano - A volcanic form produced by eruptions beneath a glacier or beneath the surface of a lake within a glacier.
Submarine volcano - A volcanic form produced by eruptions in the ocean.
Tuff cone - A volcanic cone formed by the interaction of basaltic magma and water.
Tuff rings - Shallow, flat-floored craters formed by the interaction of magma and water.
Volcanic field - A collection of cinder cones and (or) lava flows.
Volcano Eruption Time Frame:
Last known eruption B.C. (Holocene)
Last known eruption in 1964 or later.
Last known eruption from 1900-1963, inclusive.
Last known eruption from 1800-1899, inclusive.
Last known eruption from 1700-1799, inclusive.
Last known eruption from 1500-1699, inclusive.
Last known eruption from A.D. 1-1499, inclusive.
Last known eruption B.C. (Holocene)
Quaternary eruption(s) with the only known Holocene activity being hydrothermal.
Undated, but probable Holocene eruption.
Uncertain Holocene eruption.
Cool Volcano Links:
Volcanic Explosivity Index
Earthquake Fault Maps
Source: National Atlas of the United States, Smithsonian Institution, ESRI
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