Within the Aleutian Peninsula are some of the most active volcanoes within the United
States.
One of these volcanoes is covered by a thick sequence of glaciers, which largely hides
its most prominent feature.
You see, the raised rim seen here on the Veniaminof volcano represents the edge of a massive 11
by 8 kilometer wide caldera, which formed in one of the planet's largest eruptions in
the last 10,000 years.
This massive eruption occurred 3750 years ago, and sent superheated pyroclastic flows
which scorched the land in a 60 kilometer or 37 mile radius.
Since this catastrophic eruption, sequences of lava flows have filled in its central caldera,
the most prominent feature of which is a cinder cone that now stands at over 300 meters or
1,000 feet in height.
This cinder cone has been the site of numerous historical eruptions, 15 of which have occurred
since 1983.
As a result, Veniaminof is one of the most likely volcanoes within Alaska to cause flight
disruptions due to its frequent emissions of fine ash particles.
Although its recent eruptions are not very large, its caldera forming eruption occurred
during a period of time when large swaths of the peninsula were rendered uninhabitable.
The ice covered Veniaminof volcano is centered 285 miles west-southwest of the city of Kodiak,
and is southwest of two other volcanoes with similarly large calderas.
Veniaminof is one of the most voluminous volcanoes in all of Alaska, and this is reflected by
its age.
It began forming around 260,000 years ago, when a mixture of low silica basalt and basaltic
andesite lavas erupted onto the surface.
These eruptions were less explosive than their modern counterparts, and began constructing
a wide volcanic shield with a steeper central volcanic cone.
While the vast majority of eruptions occurred at this central cone which continued to grow
taller and taller, it was not the only location where eruptive activity occurred.
On occasion, significant quantities of lava would erupt on the volcano's flanks, forming
cinder cones and covering several square miles in a fresh layer of rock.
After more than 100,000 years of eruptive activity, the nature of Veniaminof's volcanism
changed.
Instead of producing basalt and basaltic andesite lavas, it began producing almost exclusively
andesite lavas as of 150,000 years ago.
Sometime around this major change, the existing volcanic cone suffered a catastrophic collapse,
causing several cubic miles of rock to slide to the southeast.
This cone was subsequently rebuilt over the next 30,000 years, until it was once again
at approximately the same prior height.
The next major eruptive change came between 80,000 and 46,000 years ago, when a series
of more than a dozen vents erupted along a 30 mile stretch of land which went almost
all the way to the Bering Sea.
This created a group of explosion craters, cinder cones, and lava flows.
33,000 years ago, a powerful eruption occurred on the northwest section of the volcano, resulting
in the creation of a small caldera.
This caldera was subsequently largely filled in by lava domes, only to be deepened during
this volcano's most powerful volcanic eruption.
In 1750 BC, a large volume of dacite composition magma exploded upwards from the volcano's
summit, producing a towering plume of ash which rose to a height of more than 40 kilometers.
Part of this eruption column then collapsed downwards, resulting in pyroclastic flows
that burned the landscape in a 50 to 70 kilometer radius.
Then, due to the large amount of erupted material, an 11 by 8 kilometer wide section of ground
collapsed downwards, forming a massive caldera.
In total, this eruption ejected 50 cubic kilometers of tephra, meaning the eruption had a volcanic
explosivity index of a 6.
Interestingly, this powerful eruption occurred around the same time as two other caldera
forming eruptions from immediately adjacent volcanoes.
Between 1900 BC and 1645 BC, large swaths of the peninsula were rendered uninhabitable
due to thick deposits of ash via VEI 6 eruptions from Veniaminof, Black Peak, and Aniakchak.
The good news is that another caldera forming eruption of Veniaminof is unlikely for some
time, at least while it continues to produce frequent smaller scale eruptions confined
to its caldera rim.
Although several nearby towns could be placed at risk due to lahars or glacial floodsgenerated
by an unusual flank eruption, this scenario is quite unlikely.
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