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Small explosion produces light ash fall at Soufriere Hills volcano, Montserrat Photographs by M. Mangan, U.S. Geological Survey, 1997. A small explosive event from the partial collapse of the volcano's growing lava dome generated a small eruption column (top left) that was blown over the Montserrat Volcano Observatory. The approaching eruption cloud (top center) sharply reduced visibility, and ash fell on the observatory 10 minutes after the explosion (remaining images). Sand-sized ash stopped falling at this location about 30 minutes after it began, but the sky was filled with ash for at least an hour longer. Click on images for larger-sized image.

When ash begins to fall during daylight hours, the sky will turn increasingly hazy and "dusty" and sometimes a pale yellow color. The falling ash may become so dense that daylight turns to murky gray or even an "intense blackness" such that "it is impossible to see your hand when held up close to the eye." Loud thunder and lightning and the strong smell of sulfur often occurs during an ash fall. Furthermore, rain may accompany the ash and turn the tiny particles into a slurry of slippery mud. Most people also describe an intense quietness, except for thunder that may accompany the ash fall, giving a "deadness" to the normal sounds of life.

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Copyrighted photo courtesy of Douglas Miller

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 eruption cloud was blown by 90-km per hour (60 mph) winds from Mount St. Helens, 230 km (145 mi) to the west. The volcano had begun to erupt explosively less than 3 hours earlier. An advancing eruption cloud often looks like an enormous dark thunderstorm cloud or menacing weather clouds. This cloud of ash caught many communities downwind unprepared for the rain of gritty ash that followed even though it was clearly visible. Only when the ash began falling did most people realize that the dark puffy cloud was not a weather cloud.

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 Notification of advancing eruption cloud often possible

When scientists monitor a volcano using several methods, especially during times of increased activity, they typically work with many organizations, businesses, emergency-management professionals, and the media in order to disseminate current information about the likelihood of eruptive activity and the status of an ongoing eruption, including the development of an eruption column.

If scientists know that a volcano is erupting but they or others cannot see it (for example, at night or during poor weather conditions), it is not always possible to determine right away whether an eruption column with the potential for generating ash fall downwind has actually formed. Often, pilots of private and commercial aircraft are the first to verify the existence of an eruption column and a corresponding eruption cloud. Sometimes, verification of an eruption cloud capable of generating ash fall comes from small communities or individuals located within a few tens of kilometers of the volcano.

Meteorological organizations typically issue weather and wind forecasts that can serve to help determine which areas could be affected by an ash fall from a specific volcano. News organizations may use this information to report on the areas likely to get ash if an explosive eruption at a specific volcano were to occur in the next 12 to 24 hours. For notification of eruption clouds:

• learn how volcano and meteorological information is disseminated in your area or region; and
• when a volcano in your area is active and ash-producing eruptions are possible, make preparations for yourself, family, and business for an ash fall (see suggestions for home, business, and community).

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Mount St. Helens, Washington

Dick Zais, City Manager; Yakima, Washington (110 km [70 mi] east of Mount St. Helens)

"By noon, the City was engulfed in darkness and communications by home telephone were impossible. It was like an eclipse of the sun that lingered, a blinding blizzard and an electrical storm all in one. Light-sensitive street lights came on automatically, traffic stopped, and a strange quiet fell on our community; and everywhere a talcum-like sandy gray powder kept accumulating. Cars, trucks, buses, and trains, all stopped, and planes were re-routed away from the ash cloud."

"From noon until 6:00 a.m. the following morning, the City was in total darkness. Three types of ash fell alternatively on the City: dark gray sand, medium gray sand, and a light gray cement-like dust. All three grades were gritty and light, difficult to sweep or shovel, especially when dry. To make matters worse, shifting winds blew the ash everywhere, severely impairing visibility and driving in our area. It was exceedingly harmful and abrasive to mechanical and electrical equipment, especially motors of vehicles, aircraft and electronic systems. Unlike snow, however, this "precipitation" was not going to melt!"

"The ash fall was especially crippling—Yakima received 5-8 cm (2-3 in) of this material in the first 24 hours following the explosion. We estimated that several million tons of ash was deposited on the entire region."

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Rabaul Caldera, Papua New Guinea (Lauer, 1995)

Following a 27-hour period of intense earthquake activity, Tavurvur and Vulcan volcanoes on opposite sides of the Rabaul caldera erupted on September 19, 1994, early in the morning. Nearby Rabaul Town (right, click for large-sized image) was covered with ash as thick as 1.5 m, and an estimated 90,000 people were displaced from the area.

"The ash was mushrooming out in thick clouds-but there was no noise. The earth had stopped moving. It was 6.15 am. We watched in awe. The clouds began to drift towards us... What a tremendous experience! But now the sky was darkening and black specks of ash were falling on us like light rain. There was also an overpowering small of sulphur."

7.30 am, after driving to a new location, about 8 km (5 mi) from Rabaul town. "Soon the clouds would reach Kulau too. Suddenly there were fears of being overcome by poisonous gas. The smell of sulphur was sickening and the air was a strange, yellowish colour. The the power went off. In a moment of panic, people, including our party which had now increased to eleven, decided to head for the safety of Kerevat, using the North Coast Road. Progress was extremely slow as the road was choked with half the population of Rabaul, all with the same idea. Eventually the vehicle traffic halted altogether. Due to unusual thermal patterns Kerevat was deluged by mud rain and the road quickly became impassable." The party returned to Kulua.

"The next day brought much of the same-but it seemed to drag on and on. The atmosphere was very heavy. There was no power. Every time we left the house we collected ash and pumice on our skin and in our hair. And the noise-the thunder and lightning was constant, with some almighty cracks. It was obvious that objects were being struck."

Five days later, describing a visit home to retrieve belongings: "There were parts of trees all over the road, flattened vegetation everywhere and everything covered in ash and mud. Our houseyard looked like a moonscape—house, water tanks, plants—all a drab grey. The trees along the bank had snapped and lost most of their foliage... inside the house was a depressing mess. The floors were covered in ash. There was even ash on the ceiling above the louvres and caked along the top of the curtains."

Nine days after the eruption: "The ash was dreadful all day yesterday. It was clearly visible in the air and I could feel and see it on my skin every time I stepped outside. I can't get used to wearing the mask and goggles. I found the smell of the masks nauseating so I've taken to wearing a folded handkerchief over my nose and mouth like a "baddie" in a Western movie."

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Darkness from additional ash falls, and ash covering and infiltrating everything not covered or protected. During a heavy ash fall and for several days after, normal community and business services are typically severely limited or completely unavailable. Transportation systems are likely to be shut down or restricted—roads may be impassable or purposefully blocked, airports temporarily closed. People will be stranded away from home. Electrical power may be intermittently unavailable when conditions favor arcing on transformer insulators or if power generators have to be shut down for clean-up operations. Light ash fall may result in some of these conditions, but the main challenge will likely be the persistent ashy conditions and effort needed to clean and remove the ash. The main thing to expect after an ash fall is a time-consuming, costly, and "dirty" effort by everyone in the affected area to remove and dispose of the ash and to keep ash from entering homes, businesses, and waste-water systems. People need to coordinate their clean-up activities so that ash is only moved once and does contaminate a previously-cleaned site. Because water will be in high demand, a rationing strategy may be necessary so that a community's fire-fighting ability is not reduced. After a heavy ash fall, surviving roofs may collapse if the ash becomes saturated before it is removed or they may have suffered structural damage. Days to months of billowing ash and coordinated clean-up operations, including need to identify disposal sites.

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References

Lauer, S.E., 1995, Pumice and ash, an account of the 1994 Rabaul volcanic eruptions: CPD Resources, Australia, 80 p.

Zais, R., 1999, City of Yakima: Presentation on the eruption of Mount St. Helens to Regional Council Civil Defense in New Plymouth, New Zealand.

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URL: http://volcanoes.usgs.gov/ash/ashfall.html Page Contact Information: GS-G-HI_Ash@usgs.gov Page Last Modified: Tuesday, 3 February 2009