KUCB KIAL Unalaska Community Broadcasting

Not All Eruptions Are Equal For Submarine Bogoslof Volcano

Feb 7, 2017

Bogoslof Volcano.
Credit T. Keith, U.S. Geological Survey

Bogoslof Volcano has exploded more than two dozen times since December, but not all eruptions are created equal.

On Friday, for instance, the Aleutian volcano spit ash about 20,000 feet into the air during a brief half-hour blast that dusted Bogoslof Island and not much else. It was a far cry from last Monday’s eruption.

That event lasted eight hours, spewing ash 35,000 feet high and coating Unalaska -- more than 50 miles away -- with a fine layer. 

Other explosions have produced cloudy plumes of ice and even swallowed up acres of land, reshaping Bogoslof’s geography.  

Scientists at the Alaska Volcano Observatory say the variety of eruption styles largely comes from its status as a submarine volcano.

“Everything that happens out at Bogoslof involves magma and sea water,” said AVO geologist Chris Waythomas.

That’s because most of the volcano is hidden beneath the Bering Sea. The speck of land sitting above sea level is just the summit, and the vent inside is sometimes above water and sometimes below.

When Bogoslof erupts, it pushes hot magma to the surface. If the vent is above water, the volcano can easily throw ash far and wide. Even if it’s below water, the volcano sometimes explodes with enough force that magma expels all the sea water and still produces a sizeable blast.  

But if there’s less energy behind the eruption, Waythomas said the magma meets resistance. It has to explode through the sea water, which suppresses ash or creates water vapor instead.

"When it rises, all the water vapor cools, condenses, and forms ice pellets, which we see as this white, cold cloud extending away from the volcano,” he said.

Waythomas said it’s hard to predict what you’re going to get with Bogoslof -- sooty ash or icy vapor, a far-reaching explosion or just a modest plume.

In any case, he thinks it’s safe to say Bogoslof’s not done yet.

“No indication that it’s slowing down all that much," he said. "We still think we have a way to go with this.”

That could mean weeks -- or even months -- of more eruptions. But for Unalaskans, Waythomas said last week’s ash fall should be as bad as it gets.

“It’s hard to imagine this volcano will produce bigger ash clouds than that," he said. "It wasn’t a lot of ash, but it still caused consternation and a lot of canceled flights, so be ready for that.”

In the last two weeks, PenAir has canceled at least 10 flights to and from the island because of volcanic activity.