A Sooty Reminder of Nature’s Power : Volcanoes: Second eruption since June dusts Alaska with a gritty ash that hampers breathing, fouls car and plane engines, and turns into a watery muck when it rains.

From above, it looked like a nuclear mushroom cloud, a billowing black column flashing with lightning and rising 11 miles into the afternoon sky. On the ground in Alaska’s largest city, the western horizon turned blue-black, with eerie yellow and green fringes like an approaching hurricane, and within hours the whole city was swallowed into darkness.

Then it started raining sand.

For six hours last Tuesday, Anchorage was showered in ash from Mt. Spurr, an 11,069-foot volcano 80 miles away. The eruption, the second from Spurr since June, came with little warning and dumped as much as a quarter-inch of gritty, gray soot over a huge swath of populated Alaska. The ash swirled in clouds like a black blizzard and more or less shut down the city.

For Anchorage, a city of 230,000 people who put up with everything from earthquakes to moose grazing in their vegetable gardens, the eruption was the latest reminder of the awesome natural forces at work in America’s most rugged state.

Since 1986, three volcanoes within 175 miles of here have erupted with multiple blasts that dusted the city with ash.

The volcanoes--Spurr, Redoubt Volcano and Augustine Volcano--are all part of a string of active volcanoes rising, cone-shaped, from an ice-covered mountain range on the western edge of Cook Inlet, a broad finger of saltwater that reaches up from the North Pacific.

It is the biggest concentration of active volcanoes in the world, part of the so-called “ring of fire” circling the Pacific Basin from South America to the Cascades to Asia. Nearly 50 volcanoes in Alaska have erupted since 1700.

Among them are Novarupta--which erupted in 1912, spitting out so much grit that it dusted Seattle and produced haze measured all the way to North Africa--as well as nearly anonymous Aleutian volcanoes, some of which have erupted in the last few years but went virtually unnoticed because they are so isolated.

“It’s an exceptional laboratory,” said Joe Dorava, a scientist with the Alaska Volcano Observatory, a state-federal agency that has set up a network of electronic monitors on many of the volcanoes. Scientists are working on several research projects they hope will help them predict eruptions and avoid disasters in more populated parts of the world.

No one has been killed in any of the recent Alaska volcano blasts, primarily because no one lives very close to them. Aside from some seasonal fishing and hunting camps, offshore oil platforms and a few widely scattered villages, many volcanoes lie in the part of the world where there are more bears than people. Anchorage is separated from them by the inlet.

But the volcanoes are close enough to populated Alaska--and busy airline pathways--to cause major inconveniences, and last week’s blast produced the worst ash fall in Anchorage since the last time Mt. Spurr blew in 1953.

Air traffic came to an abrupt halt Tuesday, stranding thousands of travelers in the heart of the summer tourist season. Offices closed, gas stations ran out of automobile air filters within hours, and many people wore surgical-style face masks to keep from wheezing as they hosed down driveways and flower beds.

Everything was caked in dust for the rest of the week, the sun was blocked out by a gloomy brown haze, and cars and trucks whipped the ash into choking clouds. A drizzle fell most of the day Friday, settling the dust clouds a little but turning everything on the ground to silty, sticky muck that was more difficult to clean up.

Air traffic was returning to normal by the weekend, while traces of airborne grit were reported as far as British Columbia.

Volcanic ash is extremely corrosive and acidic, resembling tiny shards of glass, and it can destroy jet engines if sucked into turbines, causing planes to stall in midair. The ash often does not show up on radar.

In December, 1989, a KLM Boeing 747 bound for Anchorage from Europe temporarily lost power in all four engines and plunged from 25,000 to 13,000 feet as it flew though a cloud thrown up from Redoubt Volcano. The pilot was able to restart the engines, but the aircraft was heavily damaged.

Last week’s eruption canceled hundreds of flights. Anchorage International Airport, with the air conditioning turned off to keep from sucking in ash, became a giant, stuffy bunkhouse, with hundreds of stranded travelers sprawled on the floor and sitting, dazed, in coffee shops and bars.

“We just finished sleeping in the woods for three weeks, so this ain’t so bad,” said Harold Lee, a fisherman from Morgantown, W.Va., as he sat with his buddy, Gaspard Criner, on sleeping bags in an airport corridor. “At least there’s a bathroom down the hall.”

In the city, police reported an increase in fights because so many people were cooped up indoors. It was a bank guard’s nightmare--half the people in town were wearing masks--but there was no large increase in crime.

City street crews swept up the dust with snow removal equipment and planned to store the ash for use on icy roads this winter. At least one enterprising gravel company owner was planning to use it in concrete. City refuse officials pleaded with residents not to fill trash cans with ash because some dumpsters became so heavy that garbage trucks could not lift them.

It has been possible to forecast some of the recent Alaska volcano blasts, much as the U.S. Weather Service predicts storms or floods, because many have been preceded by intense shaking. But Mt. Spurr went off last week almost unexpectedly, showing that volcanoes, like earthquakes, are still largely unpredictable.

The June 27 Spurr blast, on a flank of the mountain known as Crater Peak, sent an ash cloud north into Denali National Park instead of into Anchorage, and left a gaping hole in the mountain. Because a rock plug did not form over the vent, as often occurs, little seismic activity could be detected before the latest eruption. Rather than building up pressure first, rumbling and then exploding--like a pot of boiling water covered with a lid--the volcano let loose an explosion of hot ash.

The volcano continues belching a column of white smoke from its vent and scientists say they are not sure whether Mt. Spurr will blow again or go back to sleep.

Vulnerable to Volcanic Ash

Anchorage, Alaska’s largest city with 230,000 people, has been more vulnerable to volcanic ash falls in the last few years than any other American city. Three times since 1986, ash from volcano eruptions has closed airports, fouled car engines and raised dust to unhealthy levels.

* Last week, the eruption of Mt. Spurr caused the largest fall in recent times. About a quarter-inch of gritty soot fell in a six-hour period.

* Lighter falls occurred with the eruptions of the Redoubt Volcano in 1989 and Mt. Augustine in 1986.

Key Volcanos

1. Mt. Wrangell

2. Mt. Spurr

3. Redoubt Volcano

4. Iliamna Volcano

5. Augustine Volcano

6. Mt. Katmai

7. Novarupta

8. Mt. Trident