They also saw water streaming down the glacier, indicating heat from magma is reaching higher elevations of the mountain.
"The glacier is sort of falling apart in the upper part," research geologist Kristi Wallace said.
The signs of heat add to concerns that an eruption is near, which could send an ash cloud about 100 miles northeast toward Anchorage, the state's largest city, or onto communities on the Kenai Peninsula, which is even closer to the mountain on the west side of Cook Inlet. It would be the first eruption since 1990.
Particulate released during an eruption has jagged edges and can injure skin, eyes and breathing passages, especially in young children, the elderly and people with respiratory problems.
It can also foul engines. An eruption in December 1989 sent out an ash cloud 150 miles that flamed out the jet engines of a KLM flight carrying 231 passengers on its way to Anchorage. The jet dropped more than two miles before pilots were able to restart the engines and land safely.
A week ago, the observatory detected a sharp increase in earthquake activity below the volcano and upgraded its alert level to orange, the stage just before full eruption. The warning that an eruption was imminent caused a rush on dust masks and car air filters in Anchorage.
Alaska's volcanoes typically start with an explosion that can shoot ash 50,000 feet high and into the jet stream, but there are warning signs because magma causes small earthquakes as it moves.
Geologist Jennifer Adleman said the observatory has been recording quakes up to magnitude 2.1 but not at the frequency that preceded the last two eruptions in 1989 and 1990.
"We're looking for an increase of seismicity to match the precursor activity," Wallace said. "We haven't seen that yet."
On the Net:
Alaska Volcano Observatory:
Ash hazards: http://volcanoes.usgs.gov/hazards/