An hourslong eruption of Bogoslof volcano in the Aleutian Islands dropped a small amount of ash on nearby Unalaska overnight Monday from a massive plume that drifted over the Pacific Ocean.
Laura Kraegel, a reporter at Unalaska public radio station KUCB, said that ashfall from the eruption was relatively light Tuesday morning.
"It's less than a millimeter but there's a sulfurous smell, so it's definitely apparent," Kraegel said.
Kristi Wallace, a geologist at the Alaska Volcano Observatory, said locals were sending photos of the ashfall, which was still considered "trace" because it hadn't exceeded a millimeter.
"I would not describe it as a continuous layer, so when it falls on snow you can see the snow in between and when it falls on cars it appears as droplets," Wallace said. "I don't think what they got is anything anyone could take out a ruler and measure."
The National Weather Service's Anchorage office received reports of trace ashfall in Unalaska from the plume by Tuesday morning. Joshua Maloy, an aviation meteorologist at the office, said the eruption, which began at 8:20 p.m. Monday according to the Alaska Volcano Observatory, had apparently ended by 3 a.m. Tuesday.
"It had been in a continuous state of eruption for several hours but at the present time, as far as what we can tell from our satellite imagery, it has stopped erupting," Maloy said.
Bogoslof, on an island about 60 miles west of Unalaska, has been intermittently active since mid-December. An eruption last week sent an ash cloud over Unalaska, but no land-based ashfall was reported.
An advisory issued by the Unalaska Department of Public Safety just before 6 a.m. mentioned the ashfall, as well as a series of tips on how to cope with more ash should it arrive.
Jennifer Shockley, the department's deputy director, said ashfall in town occurred during the predawn hours and tapered off by about 8 a.m.
"We can look out across the bay a couple miles away and see that the formerly white snow is now a dusty pale gray," Shockley said. "It seems to be fairly widespread in our area."
About a dozen flights to or from Unalaska over the past 10 days had been canceled due to previous ashfall in the region, according to Shockley.
In its 1 a.m. statement on the eruption, the observatory reported that the eruption was accompanied by increased seismic activity.
"Numerous lightning strikes have been detected in the vicinity of Bogoslof from the resulting volcanic cloud," AVO staff wrote. "At (12 a.m. Tuesday), a continuous volcanic plume extends for a distance of 125 miles towards the southeast over Unalaska Island at an altitude of about 20,000 to 25,000 feet."
AVO geologist Cheryl Searcy confirmed that the eruption had apparently ended based on infrasound readings from Bogoslof. Locals were continuing work to measure the Unalaska ashfall, however.
"They were uncertain if it was continuously falling now, and we have observers putting out whiteboards to determine if it was still falling," Searcy said. "The winds are not as strong as is usually seen, and because of that the ash is moving a bit more slowly."
AVO on Monday increased the volcano's aviation color code to red and its alert level to warning, its highest indicators of further potential activity.
Although no initial reports of aircraft diverted due to the ash plume had reached the weather service by Tuesday morning, Maloy said the ash was still being tracked as the cloud gradually dissipates over the Pacific.
"We do believe that the ash will be out there for six hours up to 12 hours," Maloy said.
Wallace said Monday's overnight ash, the first from Bogoslof reported to have fallen on a community rather than water or unoccupied land, didn't measure up to the March 2016 eruption of Pavlof volcano on the Alaska Peninsula. That blast coated Nelson Lagoon in at least 4 millimeters of ash, and affected flights across Alaska as ash from the plume drifted inland.
Although Unalaska received only the "feather edge" of heavier fall from the main plume, Wallace said, that was still enough to leave residents asking questions Tuesday.
"I think some people are waking up this morning and saying, 'What happened to my town?' — that's what one woman said," Wallace said.