This week marks the 100th anniversary of the largest volcanic eruption of the 20th Century, which occurred across the Shelikof Strait from Kodiak Island on the Alaska Peninsula. The Katmai-Novarupta explosion was about twice as large as the 1991 Pinatubo eruption in the Philippines and 30 times larger than the Mount St. Helen eruption of 1980. It lasted for 60 hours and buried Kodiak in a fine volcanic ash that can still be found just under the vegetation and topsoil around the island. As a wrap up to our week-long series on the anniversary, we take a look at what made the eruption so large and what would happen if it erupted again today.
In the past 100 years there have been scores of volcanic eruptions in Alaska – the northern edge of the Pacific Ring of Fire – but none were as massive as Katmai in 1912. In fact every eruption in the human history of Alaska combined wouldn’t be as large as Katmai. The actual eruption came from Novarupta – which is not a mountain, but simply a vent in the earth that drained magma from under Mount Katmai, about six miles away, and shot it 20 miles into the sky.
Katmai-Novarupta was special for that and another reason, according to Steve McNutt, the coordinating scientist with the Alaska Volcano Observatory. He said Katmai-Novarupta formed just like all the others in the Aleutian Arc, but is notable for being larger:
"As far as the processes, they were just bigger. The ash column went up on the order of 30 kilometers or so, which is higher than most others. The volume of ash was enormous and completely dwarfed Redoubt and Augustine. Redoubt and Augustine are little blips compared to this boom."
The process he describes is called subduction, where the North Pacific seafloor dives under Alaska, melts, and then bubbles to the surface as volcanoes.
"With subduction you bring down sediments and water, so you end up with a higher water content in the magma. It’s usually about 4 percent or so. The water will stay in the solution at depth, at high pressure. As you get shallower and shallower the gases start coming out of the solution and they froth just like when you open a can of soda or a champagne bottle. That’s ultimately what drives the explosion that we see, is the expansion of the steam and all the gases."
Before 1912, there was no volcano where Novarupta sits. As mentioned before, it stole magma from under nearby Katmai. McNutt says that is unique behavior, which adds to the difficulty in predicting large eruptions.
"It wouldn’t necessarily be obvious at first. That’s one of the things we don’t know that much about. Because we don’t have a good documented case when there was good instruments around, we don’t have a clear idea of exactly what happened in those final days or how you tell the difference from a small one to a big one. It seems like one thing that happens is a small one starts and it can keep going and lead to a larger one but we don’t know what that transition is and what the clues are that it is about to happen. We think that we would see substantial earthquakes and gas and deformation of the heat activity prior to such a large eruption."
When Redoubt and Augustine erupted in the last few years, airlines cancelled hundreds of flights in, to, and from Alaska and diverted hundreds more around the ash cloud. McNutt says a Katmai-size eruption today may disrupt air travel in half the globe:
"If you look at the footprint of the ash, it would shut down a lot of the air travel to Alaska for probably several weeks. It would probably shut down a lot of North America and possibly the northern hemisphere because there would be so much ash in the air and it takes on the order of a week to ten days to circle around the globe. Last summer there was an eruption of a volcano in Chile and the ash circled the globe and caused flights to be cancelled in Australia and New Zealand and that was ten days or so after the eruption occurred. With an enormous amount of ash in the air, staying in the air, being redistributed, it would have a major disruptive affect on air travel."
Climate could also be affected. When Mount Pinatubo erupted, it caused a reduction in sunlight reaching the Earth by roughly 10 percent and global temperatures dropped by one degree Fahrenheit. And remember – Katmai was twice as large Pinatubo.