Volcano shakes itself from 12-year slumber, surprising scientists:
Last time Mount Spurr erupted, Anchorage Bowl became ash tray

Anchorage Daily News

(Published: July 28, 2004)

Mount Spurr, the volcano on Anchorage's doorstep, is kicking up once again, the first time since it erupted 12 years ago, scientists said this week.

Tiny earthquakes by the hundreds have been rumbling beneath the mountain across Cook Inlet from the city, according to the Alaska Volcano Observatory in Anchorage.

The observatory on Monday raised its official level of concern from Code Green, or "No eruption anticipated," to Yellow, meaning "An eruption is possible in the next few weeks and may occur with little or no additional warning."

Scientists hastened to say the earthquake swarm does not necessarily presage an eruption of Spurr, which blew its top three times in 1992 and, in the August '92 explosion, spread a thin, obnoxious layer of ash over Anchorage.

"The most likely scenario," geophysicist John Power said, "is that the earthquakes will die off." That's what commonly occurs.

But it's also true, Power said, that when volcanoes blow, their eruptions most often follow just such a swarm of quakes.

The current activity is different from the swarm that preceded the 1992 eruptions, according to the observatory. The earthquakes then occurred deep below Crater Peak, one of two main vents. Crater Peak, the site of Spurr's previous eruption in 1953, is about 21/2 miles south of the 11,070-foot summit, the main vent.

The last time the summit vent exploded -- that scientists know about -- was 5,000 years ago, Power said.

"It is unique," he said of the current swarm. "I didn't expect this out of Spurr at this point in time."

The mountain's recent activity began slowly in February and intensified on July 4. An average of 20 quakes are now occurring every day, a rate higher than at any time since 1992.

"We feel it's gone on long enough and intensely enough for us to put the color code to yellow," said Power, who works for the U.S. Geological Survey, one of three federal and state partners in the volcano observatory.

Tuesday afternoon, in the Observatory's Volcano Crisis Room on the campus of Alaska Pacific University, Power pointed to a small zig-zag ink splotch on a seismograph drum that showed otherwise straight lines. He said it showed an earthquake had occurred below Spurr at 1:30 p.m.

A computer screen showed other quakes. The temblors occur as deep as four miles below the surface, according to the observatory.

Eleven sensors at locations on Mount Spurr pick up the smallest earth shudders, which a person standing on the mountain would not feel, Power said. The largest recent quake, at 8 p.m. July 12, measured only 1.4 in magnitude.

The quakes are the only confirmed volcanic activity at Spurr so far, indicative of the movement deep below the mountain of magma, or molten rock and gases, that is beginning to seek an outlet, according to Power.

If the magma should rise farther up long vents toward the surface, observers might detect heating, ground swelling and fumaroles at the top, he said.

Scientists received a pilot's report on July 11 of a sulfur smell and steam coming from the volcano, but the sighting could not be confirmed, partly because of cloud cover, Power said.

The observatory has notified the Federal Aviation Administration, the National Weather Service, the Alaska Division of Emergency Services and other agencies about the change in the color code, Power said.

Spurr, 80 miles west of Anchorage in the Tordrillo Mountains, is one of more than 40 active Alaska volcanoes along an arc of mountains and islands from the Tordrillos south and west to the far Aleutians.

Three of the volcanoes closest to Anchorage recently blew their stacks within a six-year span, causing major problems. Besides Spurr, Augustine Volcano in lower Cook Inlet went off in March 1986, and Redoubt Volcano across the Inlet from Kenai erupted over several weeks in 1989 and 1990.

Shortly after Redoubt's Dec. 15, 1989, eruption, a KLM jetliner flew into the plume as if it were a cloud bank, at about 28,000 feet over Talkeetna. All four engines seized up and shut off after sucking in the abrasive ash.

The jetliner, carrying more than 230 people, fell silently from the sky for more than two miles. At 13,300 feet, the crew managed to restart the engines and land safely at Anchorage in a plane that had sustained $80 million in damage.

The highest danger from volcanoes is to airplanes, Power said. Since 1980, at least 14 other aircraft flying along North Pacific air routes have been damaged because crews unwittingly flew through volcanic plumes, according to the Geological Survey.

Spurr began stirring in August 1991 with a flurry of earthquakes beneath Crater Peak. The peak erupted on June 27, 1992; winds carried the plume north, to the west of Fairbanks, away from population centers.

But it blew again on Aug. 18, an explosive eruption that belched ash more than eight miles up. Winds aloft directed the cloud eastward, directly over Anchorage and farther on to Valdez and Cordova.

A layer of sand-like particles fell softly until it had blanketed all of Anchorage and spread a pall of gray gloom. The airport closed for 20 hours, construction work and outdoor activities halted, air filters and mouth-and-nose masks flew off the shelves, and many residents shut themselves indoors the next day. Quite a few of them got drunk and cranky, police said at the time.

Because vehicles kicked up clouds of ash on the roads and windblown particles obscured the sun, it was easy to think the layer of ash was an inch thick or more.

But it was "much less than that," Power said. Only three millimeters fell.

"A very little bit of ash, for people who've lived in Anchorage for any amount of time, is a nasty thing," he said.

Daily News reporter Peter Porco can be reached at pporco@adn.com or 257-4582.