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DESCRIPTION:
Ash and Tephra and Pumice and Scoria, etc.



Volcanic Ash

Image, click to enlarge
MSH80_volcanic_ash_with_helicopter_08-22-80.jpg
For weeks volcanic ash covered the landscape around the volcano and for several hundred miles downwind to the east. Noticeable ash fell in eleven states. The total volume of ash (before its compaction by rainfall) was approximately 0.26 cubic mile (1.01 cubic kilometers), or, enough ash to cover a football field to a depth of 150 miles (240 kilometers). In this photograph, a helicopter stirs up ash while trying to land in the devastated area.
USGS Photograph taken on August 22, 1980, by Lyn Topinka.
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Image, click to enlarge
MSH80_bear_tracks_in_ash_and_pumice_october_1980.jpg
Animals also made a comeback soon after the May 18, 1980 eruption. Many smaller animals, such as gophers, mice, frogs, fish, and insects were hibernating below ground or under water on May 18, 1980, and they survived the blast. Larger animals such as bear (whose tracks are shown here), elk, deer, and coyotes have been moving back into the area as their food supplies increase. A mountain goat has even been spotted high on the flanks of the volcano.
USGS Photograph taken in October 1980, by Lyn Topinka.
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From: Kenedi, et.al., 2000, Volcanic Ash Fall -- A "Hard Rain" of Abrasive Particles: USGS Fact Sheet 027-00
Volcanic ash consists of tiny jagged particles of rock and natural glass blasted into the air by a volcano. Ash can threaten the health of people and livestock, pose a hazard to flying jet aircraft, damage electronics and machinery, and interrupt power generation and telecommunications. Wind can carry ash thousands of miles, affecting far greater areas and many more people than other volcano hazards. Even after a series of ash-producing eruptions has ended, wind and human activity can stir up fallen ash for months or years, presenting a long-term health and economic hazard.

Terminology

From: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: USGS Special Interest Publication.
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. In a major explosive eruption, most of the pyroclastic debris would consist of lapilli and ash. Volcanic bombs undergo widely varying degrees of aerodynamic shaping, depending on their fluidity, during the flight through the atmosphere. Based on their shapes after they hit the ground, bombs are variously described, in graphic terms, as "spindle or fusiform", "ribbon", "bread-crust", or "cow-dung".

Another category of ejecta far more common than volcanic bombs is scoria or cinder, which refers to lapilli- or bomb-size irregular fragments of frothy lava. If the cinder contains abundant vesicles (gas-bubble cavities), it is called pumice, which can be light enough to float on water if the vesicles are closed to rapid filling by water. In Hawaii, these fragments share a common mode or origin: all result from sudden chilling of frothy lava from which gases were escaping during fountaining. During the exceptionally high fountaining episodes of some eruptions ... an extremely vesicular, feathery ligh pumice, called reticulite or thread-lace scoria, can form and be carried many miles downwind from the high lava fountains. Even though reticulite is the least dense kind of tephra, it does not float on water, because its vesicles are open and interconnected. Consequently, when it falls on water, it becomes easily waterlogged and sinks.

If the scoria or pumice clots are sufficiently soft to flatten or splash as they strike the ground, they are called spatter. The still-molten character of spatter fragments can cause them to stick together to form welded spatter or agglutinate. Drops of lava ejected in very fluid condition and solidified in flight can form air-streamlined spherical, dumbell, and irregular shapes. Drop-shaped lapilli are called Pele's tears, after the Hawaiian Goddess of Volcanoes. In streaming through the air, Pele's tears usually have trailing behind them a thin thread of liquid lava, which is quickly chilled to form a filament of golden brown glass, called Pele's hair. Pele's hair can form thick mats downwind from high lava fountains near a vent; it also can be blown many miles from the vent.

From: Mullineaux, 1996, Pre-1980 Tephra-Fall Deposits Erupted From Mount St. Helens, Washington: USGS Professional Paper 1563
The term "tephra" refers to particles that were erupted into the air and then fell back to the ground or to deposits of those particles. The term was introduced by Thorarinsson (1944, 1954) to describe volcanic ash and coarser detritus that were projected through the air ... "Tephra" includes the materials and deposits resulting from events described as tephra fall, air fall, and pyroclastic fall but not deposits resulting from flowage events. ...

Tephra is chiefly by clast size, shape, vesicularity, and composition. Particles whose intermediate axes measure 2 mm or less are described as ash. Fine and coarse ash particles are smaller and larger than 1/16 mm across, respectively. Lapilli have intermediate axes from 2 to 64 mm in length, and blocks and bombs are more than 64 mm wide. Bombs have shapes or textures such as vesicularity that indicate they were liquid or plastic when erupted. Blocks generally are more angular and were solid when erupted.

Tephra clasts from Mount St. Helens are composed mostly of vesicular glass; pale clasts are called pumice and darker clasts scoria. Pumice and scoria clasts record eruption of new magma; that is, they represent rock material that was molten when erupted and that expanded into a froth before solidifying. The terms "pumice" and "scoria" are used both for highly vesicular particles and for some particles that are only moderately vesicular and have specific gravities greater than 1.

From: Clynne, et.al., 1998, How Old is "Cinder Cone"? -- Solving a Mystery in Lassen Volcanic National Park, California: USGS Fact Sheet 173-98
Scoria forms 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.

From: Hoblitt, Miller, and Scott, 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest, USGS Open-File Report 87-297
Tephra consists of fragments of lava or rock blasted into the air by explosions or carried upward by a convecting column of hot gases ... These fragments fall back to earth on and downwind from their source volcano to form a tephra, pyroclastic-fall, or volcanic "ash" deposit. Large fragments fall close to the erupting vent, and progressively smaller ones are carried farther away by wind. Dust-size particles can be carried many hundreds of kilometers from the source. ... Tephra deposits consist of combinations of pumice, glass shards, dense-rock, and crystals that range in size from ash (less than 2 mm), through lapilli (2-64 mm), to blocks (greater than 64 mm). Eruptions that produce tephra range from those that eject debris only a few meters into the air, to cataclysmic explosions that throw debris to heights of several tens of kilometers. ...

From: Miller, 1989, Potential Hazards from Future Volcanic Eruptions in Calfornia: U. S. Geological Survey Bulletin 1847
During many volcanic eruptions, fragments of lava or rock are blasted into the air by explosions or carried upward by a convecting column of hot gases. These fragments fall back to earth on and downwind from their source vent to form a pyroclastic-fall or "ash" deposit. Pyroclastic-fall deposits, referred to as tephra, consist of combinations of pumice, scoria, dense-rock material, and crystals, that range in size from ash (<2mm) through lapilli (2-64mm) to blocks (>64mm). Eruptions that produce tephra range from short-lived weak ones that eject debris only a few meters into the air, to cataclysmic explosions that throw debris to heights of several tens of kilometers. Explosive eruptions that produce voluminous tephra deposits also commonly produce pyroclastic flows.

Hazards

From: W. E. Scott, R. M. Iverson, J. W. Vallance, and W. Hildreth, 1995, Volcano Hazards in the Mount Adams Region, Washington: USGS Open-File Report 95-492
Small explosions that accompanied past lava-flow eruptions at Mount Adams and other volcanoes in the region were strong enough to hurl lava blocks from vents, and probably created clouds of tephra that rose thousands of meters into the atmosphere. In depositing only a few millimeters of tephra for tens or, rarely, a few hundred kilometers downwind, such clouds offer little threat to life or structures. But tephra clouds can create tens of minutes to hours of darkness as they pass over a downwind area, even on sunny days, and reduce visibility on highways. Deposits of tephra can short-circuit electric transformers and power lines, especially if the tephra is wet, which makes it highly conductive, sticky, and heavy. Tephra injested by vehicle engines can clog filters and increase wear. Tephra clouds often generate lightning that can interfere with electrical and communication systems and start fires. Finally, and perhaps most importantly, even small, dilute tephra clouds pose a significant hazard to aircraft that fly into them. ...

From: Miller, 1989, Potential Hazards from Future Volcanic Eruptions in California: USGS Bulletin 1847, 17p.
Close to an erupting vent, the main hazards to property posed by eruptions of tephra include high temperatures, burial, and impact of falling fragments; large falling blocks can kill or injure persons who cannot find shelter. Significant property damage can result from the weight of tephra, especially if it is wet, and 20 centimeters or more of tephra may cause structures to collapse. Hot tephra falling near a volcano may set fire to forests and structures. Farther away, the chief danger to life is the effect of ash on the respiratory system. Even 5 centimeters of ash will stop the movement of most vehicles and disrupt transportation, communication, and utility systems. Machinery is especially susceptible to the abrasive and corrosive effects of ash. These effects, together with decreased visibility or darkness during an eruption, may further disrupt normal transportation, communication, and electrical services; they can also result in psychological stresses and panic among people whose lives may not be endangered.

From: Myers and Brantley, 1995, Volcano Hazards Fact Sheet: Hazardous Phenomena at Volcanoes, USGS Open-File Report 95-231
An explosive eruption blasts molten and solid rock fragments (tephra) into the air with tremendous force. The largest fragments (bombs) fall back to the ground near the vent, usually within 2 miles. The smallest rock fragments (ash) continue rising into the air, forming a huge, billowing eruption column. Volcanic ash is composed of fragments of rock, minerals, and glass that are less than 2 millimeters (0.08 inch) in diameter.

Eruption columns can be enormous in size and grow rapidly, reaching more than 12 miles above a volcano in less than 30 minutes. Once in the air, the volcanic ash and gas form an eruption cloud. Eruption clouds pose a serious hazard to aviation. During the past 15 years about 80 commercial jets have been damaged by inadvertently flying into ash, and several have nearly crashed. Large eruption clouds can travel hundreds of miles downwind from a volcano, resulting in ash fall over enormous areas. Ash from the May 18, 1980, eruption of Mount St. Helens was deposited over 22,000 square miles of the western United States. With increasing distance downwind from a volcano, the ash particles become smaller and the thickness of the resulting layer decreases. Minor ashfall can be a nuisance to people and damage crops, electronics, and machinery; heavy ashfall can collapse buildings.

From: Wright and Pierson, 1992, Living With Volcanoes, The U.S. Geological Survey's Volcano Hazards Program, USGS Circular 1973
Vigorous eruption plumes can carry the finest ash into the stratosphere, where strong winds distribute it over many thousands of kilometers,. Even a small ash fall poses a serious nuisance to people, crops, machinery, and computers. When thick or wet, it can cause roofs to collapse. Windborne ash is a serious threat to aircraft.


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02/08/05, Lyn Topinka