U.S.   Geological Survey  Fact Sheet 027-00 Online Version 1.0

**Volcanic   Ash Fall–A "Hard Rain" of Abrasive Particles**

| 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. |
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On the morning of May 18,   1980, many people in eastern Washington noticed dark, threatening clouds approaching   from the west. Most thought the clouds were part of one of the enormous thunderstorms   common in late spring. However, what they did not know was that at 8:32 a.m.   Mount St. Helens had erupted explosively, blasting an enormous column of volcanic   ash and gas more than 60,000 feet into the air. 

|  | 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 the west. The volcano       had begun to erupt explosively less than 3 hours earlier, catching many       communities downwind unprepared for the destructive rain of gritty ash that       followed. (Copyrighted photo courtesy of Douglas Miller.) |
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As the clouds   drifted overhead, a rain of ash began to fall, plunging much of the region into   darkness that lasted all day. Homes, farms, and roads were quickly covered by   as much as 4 inches of gritty ash. The smallest ash particles penetrated machinery   and all but the most tightly sealed structures. By the end of the day, more   than 500 million tons of ash had fallen onto parts of Washington, Idaho, and   Montana. The ash prevented travel throughout much of eastern Washington because   of poor visibility, slippery roads, and ash-damaged vehicles, stranding more   than 10,000 people and isolating many small communities. More than $1 billion   in property and economic losses was caused by Mount St. Helens' 1980 eruption—much   of it by ash. Future large explosive eruptions in the United States are certain   to produce widespread ash falls and are likely to cause even greater losses.   Since 1980, rapid population and economic growth in the Western United States,   widespread use of computers and electronics, and the dramatic increase in jet-airline   traffic, especially over the North Pacific, have made more people and property   in the Nation vulnerable to the effects of volcanic ash. Knowing the characteristics   of volcanic ash and being prepared when a volcano shows signs of restlessness   can significantly reduce the potential economic and health impacts of airborne   and falling ash. 

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| Volcanic       ash, like this 1980 ash from Mount St. Helens, Washington, is made up of       tiny jagged particles of rock and glass (photo on right; magnified 200 times).       Long and severe exposure to such ash particles, without protective breathing       equipment, can be dangerous and harmful. For people with heart and lung       ailments, inhaling ash can cause serious respiratory problems. |

#### What is   Volcanic Ash? 

Small jagged   pieces of rocks, minerals, and volcanic glass the size of sand and silt (less   than 1/12 inch or 2 millimeters in diameter) erupted by a volcano are called   volcanic ash. Very small ash particles can be less than 1/25,000th of an inch   (0.001 millimeter) across. 

Though called   "ash," volcanic ash is not the product of combustion, like the soft   fluffy material created by burning wood, leaves, or paper. Volcanic ash is hard,   does not dissolve in water, is extremely abrasive and mildly corrosive, and   conducts electricity when wet. 

Volcanic ash   is formed during explosive volcanic eruptions. Explosive eruptions occur when   gases dissolved in molten rock (magma) expand and escape violently into the   air, and also when water is heated by magma and abruptly flashes into steam.   The force of the escaping gas violently shatters solid rocks. Expanding gas   also shreds magma and blasts it into the air, where it solidifies into fragments   of volcanic rock and glass. 

Once in the   air, hot ash and gas rise quickly to form a towering eruption column, often   more than 30,000 feet high. Larger rock fragments more than 2 inches across   ejected by the explosion typically fall within a few miles of the eruption site.   However, wind can quickly blow fine ash away from the volcano to form an eruption   cloud. As the cloud drifts downwind from the erupting volcano, the ash that   falls from the cloud typically becomes smaller in size and forms a thinner layer.   Ash clouds can travel thousands of miles, and some even circle the Earth. 

| **VOLCANIC         ASH FALL FROM ANCIENT AND MODERN ERUPTIONS IN THE WESTERN UNITED STATES**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
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|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | Ash from         Mount St. Helens—Yakima, Wash., May 1980. | Ash from         Spurr Volcano—Anchorage, Alaska, August 1992). Copyright Anchorage         Daily News. |
| Ash from         Mount St. Helens—Ephrata, Washington, May 1980. Copyright Douglas         Miller.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Ash fall         from the two largest eruptions in the United States in the 20th century         (yellow) is Ash from Mount St. Helens' Ephrata, Wash., May 1980 ((c) dwarfed         by ash fall from ancient eruptions. More than $1 billion in losses was         caused by Mount St. Helens'1980 eruption—much of by ash. Ash falls         from future eruptions are certain to be widespread and hazardous. To reduce         health risks, people should avoid breathing ash, wear dust masks and goggles,         not use contact lenses, and stay indoors when possible.

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#### Some Effects   of Volcanic Ash

When volcanic   ash accumulates on buildings, its weight can cause roofs to collapse, killing   and injuring people. A dry layer of ash 4 inches thick weighs 120 to 200 pounds   per square yard, and wet ash can weigh twice as much. The load of ash that different   roofs can withstand before collapsing varies greatly—flat roofs are more   likely to collapse than steeply pitched ones. 

Because wet   ash conducts electricity, it can cause short circuits and failure of electronic   components, especially high-voltage circuits and transformers. Power outages   are common in ash-fall areas, making backup power systems important for critical   facilities, such as hospitals. 

Eruption clouds   and ash fall commonly interrupt or prevent telephone and radio communications   in several ways, including physical damage to equipment, frequent lightning   (electrical discharges), and either scattering or absorption of radio signals   by the heated and electrically charged ash particles. 

Volcanic ash   can cause internal-combustion engines to stall by clogging air filters and also   damage the moving parts of vehicles and machinery, including bearings and gears.   Engines of jet aircraft have suddenly failed after flying through clouds of   even thinly dispersed ash. Roads, highways, and airport runways can be made   treacherous or impassable because ash is slippery 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, reducing visibility and causing accidents. 

Ash also clogs   filters used in air-ventilation systems to the point that airflow often stops   completely, causing equipment to overheat. Such filters may even collapse from   the added weight of ash, allowing ash to invade buildings and damage computers   and other equipment cooled by circulating outside air. Agriculture can also   be affected by volcanic ash fall. Crop damage can range from negligible to severe,   depending on the thickness of ash, type and maturity of plants, and timing of   subsequent rainfall. For farm animals, especially grazing livestock, ash fall   can lead to health effects, including dehydration, starvation, and poisoning.   

Like airborne   particles from duststorms, forest fires, and air pollution, volcanic ash poses   a health risk, especially to children, the elderly, and people with cardiac   or respiratory conditions, such as asthma, chronic bronchitis, and emphysema.   

The best time   for communities, businesses, and homeowners to make preparations for a rain   of volcanic ash is before an eruption occurs. When an explosive eruption does   occur, warning of advancing ash clouds may precede actual ash fall by only minutes   or hours. By developing community emergency-response plans that can be activated   when a volcano is threatening to erupt, the harmful and disruptive effects of   ash can be greatly reduced. So that the public can be warned of impending eruptions   and advancing ash clouds, the U.S. Geological Survey (USGS) and cooperating   organizations operate instrument networks that monitor more than 40 active volcanoes   in the United States.

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

*Graphics   by Susan Mayfield and Sara Boore  Banner design by Bobbie Myers*

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For more   information or copies of the paper version of this fact sheet contact:  U.S. Geological Survey  David A. Johnston Cascades Volcano Observatory  5400 MacArthur Blvd.  Vancouver, WA 98661  Telephone (360) 993-8900  Fax (360) 993-8980 

Visit the   Cascades Volcano Observatory on the web

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

Visit the   Alaska Volcano Observatory on the web

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**COOPERATING   ORGANIZATIONS**  Federal Aviation Administration  National Oceanic and Atmospheric Administration,  National Weather Service

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**Related   Fact Sheets**

What   Are Volcano Hazards? (USGS Fact Sheet 002-97)

Volcanic   Ash—A Danger to Aircraft in the North Pacific (USGS Fact Sheet 030-97)   

See   a list of other   volcano-related fact sheets published by the U.S. Geological Survey

 

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**U.S.   GEOLOGICAL SURVEY–REDUCING THE RISK FROM VOLCANO HAZARDS**
Learn more   about volcanoes and the hazards they pose at the USGS   Volcano Hazards Program website
URL of this page: https://pubs.usgs.gov/fs/fs027-00/Maintained by: Michael DigglesCreated 3/15/00Last modified: 10/13/04 (mfd)
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