Mount Spurr stirs from its 12-year slumber

Posted: Thursday, July 29, 2004

ANCHORAGE - Mount Spurr is showing some life these days, 12 years after the volcano last erupted.

The Alaska Volcano Observatory this week raised its official level of concern to yellow, signifying an eruption is possible in the next few weeks.

A swarm of tiny earthquakes has been rumbling beneath the mountain across Cook Inlet 80 miles west of Anchorage.

The series of earthquakes does not necessarily presage an eruption of Spurr, according to scientists. The volcano was last significantly active in 1992 and, in an August explosion that year, spread a thin layer of ash over Anchorage.

"The most likely scenario," geophysicist John Power said, "is that the earthquakes will die off."

Eruptions, however, most often follow just such a pattern of quakes, said Power, who works for the U.S. Geological Survey, one of three federal and state partners in the Anchorage-based observatory.

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.

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. The temblors occur as deep as four miles below the surface.

Eleven sensors at locations on Mount Spurr pick up the smallest earth shudders, 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.

Mount Spurr is one of more than 40 active Alaska volcanoes along an arc of mountains and islands from the Tordrillo Mountains south and west to the far Aleutians.

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