USGS/Cascades Volcano Observatory, Vancouver, Washington
Ash and Tephra and Pumice and Scoria, etc.
- Volcanic Ash
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.
[medium size] ...
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.
[medium size] ...
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
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.
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.
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.
shapes or textures such as vesicularity that indicate they were liquid or
plastic when erupted. Blocks generally are more angular and were solid
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.
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.
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. ...
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,
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
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. ...
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.
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,
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.
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