VVolcanic Ash Fall—A "Hard Rain" of Abrasive Particles

U.S. Department of the InteriorU.S. Geological Survey

USGS Fact Sheet 027-002000

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

olcanic ash consists of tinyjagged particles of rock and

natural glass blasted into the airby a volcano. Ash can threatenthe health of people and live-stock, pose a hazard to flying jetaircraft, damage electronicsand machinery, and interruptpower generation and telecom-munications. Wind can carryash thousands of miles, affect-ing far greater areas and manymore people than other volcanohazards. Even after a series ofash-producing eruptions hasended, wind and human activitycan stir up fallen ash for monthsor years, presenting a long-termhealth and economic hazard.

On the morning of May 18, 1980, manypeople in eastern Washington noticed dark,threatening clouds approaching from the west.Most thought the clouds were part of one ofthe enormous thunderstorms common in latespring. However, what they did not know wasthat at 8:32 a.m. Mount St. Helens had eruptedexplosively, blasting an enormous column ofvolcanic ash and gas more than 60,000 feetinto the air.

As the clouds drifted overhead, a rain of ashbegan to fall, plunging much of the region intodarkness that lasted all day. Homes, farms,

and roads were quickly covered by as much as4 inches of gritty ash. The smallest ash par-ticles penetrated machinery and all but themost tightly sealed structures. By the end ofthe day, more than 500 million tons of ash hadfallen onto parts of Washington, Idaho, andMontana. The ash prevented travel throughoutmuch of eastern Washington because of poorvisibility, slippery roads, and ash-damagedvehicles, stranding more than 10,000 peopleand isolating many small communities.

More than $1 billion in property and eco-nomic losses was caused by Mount St. Helens’1980 eruption—much of it by ash. Future

tial economic and health impacts of airborneand falling ash.

What is Volcanic Ash?Small jagged pieces of rocks, minerals, and

volcanic glass the size of sand and silt (less than1/12 inch or 2 millimeters in diameter) eruptedby a volcano are called volcanic ash. Very smallash particles can be less than 1/25,000th of aninch (0.001 millimeter) across.

Though called “ash,” volcanic ash is not theproduct of combustion, like the soft fluffy mate-rial created by burning wood, leaves, or paper.Volcanic ash is hard, does not dissolve inwater, is extremely abrasive and mildly corro-sive, and conducts electricity when wet.

Volcanic ash is formed during explosivevolcanic eruptions. Explosive eruptions occurwhen gases dissolved in molten rock (magma)expand and escape violently into the air, andalso when water is heated by magma andabruptly flashes into steam. The force of theescaping gas violently shatters solid rocks.Expanding gas also shreds magma and blastsit into the air, where it solidifies into frag-ments of volcanic rock and glass.

Once in the air, hot ash and gas rise quicklyto form a towering eruption column, oftenmore than 30,000 feet high. Larger rock frag-

This surreal-looking photo shows an enormous cloud of volcanic ash approaching the small town of Ephrata,Washington, on the morning of May 18, 1980. The ominous cloud was from Mount St. Helens, 145 miles to thewest. The volcano had begun to erupt explosively less than 3 hours earlier, catching many communities downwindunprepared for the destructive rain of gritty ash that followed. (Copyrighted photo courtesy of Douglas Miller.)

Volcanic ash, like this 1980 ash from Mount St. Helens,Washington, is made up of tiny jagged particles of rockand glass (see inset; magnified 200 times). Long andsevere exposure to such ash particles, without protec-tive breathing equipment, can be dangerous and harm-ful. For people with heart and lung ailments, inhalingash can cause serious respiratory problems.

large explosive eruptions in theUnited States are certain toproduce widespread ash fallsand are likely to cause evengreater losses. Since 1980,rapid population and economic

growth in the Western United States, wide-spread use of computers and electronics, andthe dramatic increase in jet-airline traffic,especially over the North Pacific, have mademore people and property in the Nation vul-nerable to the effects of volcanic ash. Know-ing the characteristics of volcanic ash andbeing prepared when a volcano shows signs ofrestlessness can significantly reduce the poten-

0

0 400 KILOMETERS

400 MILES

CANADAUNITED STATES

LongValleyCaldera

Crater Lake(Mt Mazama)

YellowstoneCaldera Complex

Mt St Helens

Glacier Peak

PACIFIC OCEAN

GLACIERPEAK ASH

erupted 14,500 years ago

MAZAMA ASHerupted 7,600 years ago

MT ST HELENS A

SH

erupted 3,800 years ago

YELLOWSTONE ASHerupted 665,000 years ago

BISHOP ASH erupted 760,000 years ago

from Long Valley

MT ST HELENS ASH 1980

Areas shown for 20th-century eruptions are ash fall thicker than about 0.2 inch (0.5 cm); areas for ancient eruptions extrapolated from known ash deposits, most thicker than 0.2 inch.

Novarupta

Mt SpurrAnchorage

PACIFIC OCEAN

Gulf of

Alaska

NOVARUPTAASH 1912

ALASKA

Area of map

(Inset map of Novarupta ash fall is at same scale.)

Printed on recycled paper

ments more than 2 inches across ejected bythe explosion typically fall within a few milesof the eruption site. However, wind canquickly blow fine ash away from the volcanoto form an eruption cloud. As the cloud driftsdownwind from the erupting volcano, the ashthat falls from the cloud typically becomessmaller in size and forms a thinner layer. Ashclouds can travel thousands of miles, andsome even circle the Earth.

Some Effects of Volcanic AshWhen volcanic ash accumulates on buildings,

its weight can cause roofs to collapse, killing andinjuring people. A dry layer of ash 4 inches thickweighs 120 to 200 pounds per square yard, andwet ash can weigh twice as much. The load ofash that different roofs can withstand beforecollapsing varies greatly—flat roofs are morelikely to collapse than steeply pitched ones.

Because wet ash conducts electricity, it cancause short circuits and failure of electroniccomponents, especially high-voltage circuits andtransformers. Power outages are common in ash-fall areas, making backup power systems impor-tant for critical facilities, such as hospitals.

Eruption clouds and ash fall commonly inter-rupt or prevent telephone and radio communica-tions in several ways, including physical damageto equipment, frequent lightning (electricaldischarges), and either scattering or absorptionof radio signals by the heated and electricallycharged ash particles.

Volcanic ash can cause internal-combustionengines to stall by clogging air filters and also

damage the moving parts of vehicles and ma-chinery, including bearings and gears. Engines ofjet aircraft have suddenly failed after flyingthrough clouds of even thinly dispersed ash.Roads, highways, and airport runways can bemade treacherous or impassable because ash isslippery and may reduce visibility to near zero.Cars driving faster than 5 miles per hour on ash-covered roads stir up thick clouds of ash, reduc-ing visibility and causing accidents.

Ash also clogs filters used in air-ventilationsystems to the point that airflow often stopscompletely, causing equipment to overheat.Such filters may even collapse from the addedweight of ash, allowing ash to invade build-ings and damage computers and other equip-ment cooled by circulating outside air.

Agriculture can also be affected by volcanicash fall. Crop damage can range from negli-gible to severe, depending on the thickness ofash, type and maturity of plants, and timing ofsubsequent rainfall. For farm animals, espe-cially grazing livestock, ash fall can lead tohealth effects, including dehydration, starva-tion, and poisoning.

Like airborne particles from duststorms,forest fires, and air pollution, volcanic ashposes a health risk, especially to children, theelderly, and people with cardiac or respiratoryconditions, such as asthma, chronic bronchitis,and emphysema.

The best time for communities, businesses,and homeowners to make preparations for arain of volcanic ash is before an eruptionoccurs. When an explosive eruption does

COOPERATING ORGANIZATIONSFederal Aviation Administration

National Oceanic and Atmospheric Administration,National Weather Service

For more information contact:U.S. Geological Survey

David A. Johnston Cascades Volcano Observatory5400 MacArthur Blvd., Vancouver, WA 98661

Tel: (360) 993-8900, Fax: (360) 993-8980http://vulcan.wr.usgs.gov/

orAlaska Volcano Observatory

4200 University Drive, Anchorage, AK 99508Tel: (907) 786-7497, Fax: (907) 786-7425

http://www.avo.alaska.edu/or

U.S. Geological Survey Volcano Hazards Programhttp://volcanoes.usgs.gov/

See also What are Volcano Hazards? (USGS Fact Sheet002-97) and Volcanic Ash—Danger to Aircraft in the North

Pacific (USGS Fact Sheet 030-97)

Christopher A. Kenedi, Steven R. Brantley,James W. Hendley II, and Peter H. Stauffer

Graphic design bySusan Mayfield and Sara Boore

Banner design by Bobbie Myers

occur, warning of advancing ash clouds mayprecede actual ash fall by only minutes orhours. By developing community emergency-response plans that can be activated when avolcano is threatening to erupt, the harmfuland disruptive effects of ash can be greatlyreduced. So that the public can be warned ofimpending eruptions and advancing ashclouds, the U.S. Geological Survey (USGS)and cooperating organizations operate instru-ment networks that monitor more than 40active volcanoes in the United States.

VOLCANIC ASH FALL FROM SOME ANCIENT AND MODERN ERUPTIONS IN THE WESTERN UNITED STATES

Ash fall from thetwo largest erup-tions in the UnitedStates in the 20thcentury (yellow) is

Ash from MountSt. Helens—Yakima, Wash.,May 1980

Ash fromMount St. Helens—Ephrata, Wash.,May 1980 (© Douglas Miller)

Ash from Spurr Volcano—Anchorage, Alaska,August 1992(© Anchorage Daily News)

dwarfed by ash fall from ancienteruptions. More than $1 billion inlosses was caused by Mount St.Helens’ 1980 eruption—much of itby ash. Ash falls from future erup-tions are certain to be widespreadand hazardous. To reduce healthrisks, people should avoid breath-ing ash, wear dust masks andgoggles, not use contact lenses,and stay indoors when possible.