The state of Alaska contains 86 active volcanoes.
These volcanoes are all collectively rated by the US Geological Survey as ranging between
"very low threat" and "very high threat" designations.
These designations generally seem self explanatory, as for example Mount Redoubt is rated as a
very high threat volcano based on its proximity to Anchorage.
Yet, of the volcanoes in the highest threat category, the Akutan volcano initially seems
misplaced.
Although it is frequently active, none of its recent eruptions have been particularly
destructive.
However, a mere 1600 years ago the volcano had a far larger eruption.
This eruption not only deposited up to 6 feet thick of ash, but also ejected lava bombs
all the way to the modern town of Akutan.
If this town were to have existed during that eruption, it would have largely been destroyed.
The Akutan volcano is located in southwest Alaska in the Aleutian Island chain where
it is centered on the island of Akutan.
The town of Akutan is located approximately 8 miles or 12.5 kilometers to the east of
its summit.
The most prominent feature visible at Akutan is its nearly 2 mile wide caldera.
Within this caldera is a deep crater lake, a young cinder cone, and a series of black
andesite lava flows.
This caldera formed during the volcano's largest known eruption.
However, if we look to the southwest we can see the rim of another caldera which also
formed in the last 15,000 years.
Before the modern Akutan volcano began to form, an ancestral volcano was active and
created the vast majority of the modern island between 3.3 and 1.4 million years ago.
The modern Akutan volcano began forming 580,000 years ago when a volume of highly viscous
dacite magma erupted onto the surface.
This magma caused a series of explosions to occur as it interacted with shallow groundwater
and surface ice.
After the explosive phase ended, a 700 foot or 213 meter wide lava dome was constructed.
Over the subsequent tens of thousands of years, additional similar eruptions occurred on the
northern third of the island which at the time represented the main eruptive center.
Volcanic activity eventually shifted to the center of the island, which over time built
a towering gray volcanic cone out of layers of basaltic andesite lava and ash.
Although this main peak continued to largely represent the center of volcanic activity,
occasional satellite vents became the dominant feature for extended periods of time.
One such center was active around 250,000 years ago, where low viscosity lava flows
built the Flat Top eruptive center.
Its eruptions produced several rivers of basaltic lava which flowed towards the ancient coastline.
Another center developed 150,000 years ago, which over time created a 2,000 foot tall
satellite cone called Cascade Bright out of basaltic andesite lavas.
Eruptive activity later shifted back to the center of the island, which over time built
a twin peaked summit.
However, the western of the two volcanic cones soon became highly unstable, as fumaroles
altered its edifice to shades of yellow and red.
Then, 11,700 years ago, a plinian eruption occurred.
As a large volume of volcanic rock was ejected high into the atmosphere, pyroclastic flows
raced to the west of the cone.
Then, due to the large volume of erupted material, this volcanic cone collapsed downwards, forming
a western facing caldera.
The northernmost cone soon became the center of eruptive activity, which also experienced
a caldera forming eruption.
This caldera forming eruption occurred in 340 AD, sending bombs of lava into the town
of Akutan and covering the entirety of the island with a thick layer of ash.
In total, 1 cubic kilometer of volcanic rock was ejected and the eruption had a volcanic
explosivity index rating of a 5.
In the years following this eruption, a new cinder cone was built in the center of the
caldera.
In 1852, a new satellite cinder cone erupted lava to the northwest of the main cone, forming
Lava Point and building a lava delta.
Since then, Akutan has erupted 36 times, producing small scale explosive eruptions in its caldera
while also occasionally sending viscous lava flows to the north through a breach in its
crater wall.
This volcano erupted almost every year from 1972 to 1992, but has not erupted since.
A major volcanic eruption nearly occurred in 1996, when a large crack formed between
Akutan's summit and Lava Bright.
This was caused by the intrusion of a dike of magma, which will likely be the site of
the next eruption from this volcano.
Today, numerous hot springs and geysers can be found on the eastern flank of Akutan.
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