ANCHORAGE - Scientists continue to monitor two volcanoes that the Alaska Volcano Observatory says could send dangerous ash into the air at any time.
Mount Spurr, 80 miles west of Anchorage across Cook Inlet, shook itself from a 12-year sleep in early July and has been in Code Yellow status ever since, with daily small earthquakes.
Code Yellow indicates an eruption is possible and could occur with no warning, according to the Alaska Volcano Observatory.
Mount Veniaminof, about 500 miles southwest of Anchorage on the Alaska Peninsula, changed from Code Green, or "dormant," to Code Yellow about Jan. 1. On Jan. 10, the observatory upgraded its activity to Code Orange, indicating the volcano is "in eruption."
Ash plumes from Veniaminof can be seen on sunny days and have been photographed from planes. Even when clouds obscure the summit, seismic records indicate the eruption is continuing, said John Power, a geophysicist at the observatory.
"We have some magma at Veniaminof," he said.
Reports have come from Perryville, 22 miles south-southeast of the volcano's summit, that its plumes have been flashing orange at night, Power said.
The volcano is experiencing what scientists call a "Strombolian eruption," a low-level, continuous eruption accompanied by minor ash plumes. The category is named, Power said, for an Italian volcano that appears to have been erupting for about 2,500 years.
Both volcanoes are near enough to major airways that scientists are monitoring them every day, Power said. Volcanic ash, if blown high enough, poses a serious threat to aircraft.
The Veniaminof plume is apparently rising no higher than 12,000 feet, not enough to interfere with trans-Pacific air routes. Scientists and others are worried about that possibility, Power said. The ash can pose risks to smaller planes at lower altitudes. Pilots on the Alaska Peninsula are "taking steps to steer away," he said.
Spurr is in its seventh month of elevated earthquake activity, according to the observatory. The quakes are too small to feel.
About 15 per day occur about four miles below the mountain's summit, according to the observatory's Web site. The average has been as high as 20.
The mountain has not shown signs of an imminent eruption.
Since July, volcanologists have been analyzing data collected by instruments on and off the mountain. They now feel, Power said, that another group of small earthquakes has been occurring beneath Spurr's summit since 2003, but at the base of the earth's crust, 12 to 25 miles down.
Power said the two groups of earthquakes probable are linked to magma moving into cracks in the crust.
"There's some increased magmatic activity, and that's what's causing the shallower seismicity and the melting of the ice cap," he said.
In mid-July and early August, observers flying above Spurr noticed small flows of mud and rock and a recently formed "ice cauldron" in the summit ice cap, according to the Web site.
That sink hole, a collapse experts said has been caused by increased heat from below the summit, was about 165 feet in diameter and about half that in depth and contained a pond of icy meltwater. It has since grown.
Mount Spurr erupted three times in 1992, spewing noxious ash over Anchorage and other cities.
Ash that fell on Anchorage in August 1992 was only a few millimeters thick, according to observatory scientists. However, people were compelled to wear face masks, cover computers and change auto filters as they stirred up clouds of gritty ash wherever they walked and drove.
Before 1992, Spurr last blew its top in 1953. Both eruption sequences occurred below Crater Peak, a separate volcanic vent on Spurr, about 2.5 miles south of the summit.
The last time the summit vent is known to have erupted is about 5,000 years ago, according to the observatory.