The state of Alaska contains more than 80 active volcanoes, some of which in relatively
recent times have produced eruptions so large that the recent eruption of Hunga Tonga Hunga
Ha'apai looked like a mild burp.
For example, the 1912 eruption of the newly formed Novarupta volcano was 60 times larger
than the aforementioned eruption, causing the formation of a caldera at an adjacent
volcano and causing measurable ash to fall as far away as Seattle.
However, this eruption was not the largest to occur within the state in the last 10,000
years.
That title instead belongs to the Aniakchak volcano, which is characterized by a prominent
6 mile or 10 kilometer wide caldera, and is filled with recent volcanic cones.
In 1645 BC, it ejected 75 cubic kilometers of volcanic rock, which led to a volcanic
winter around the planet.
This volcano last produced a major explosive eruption in 1931, and will certainly erupt
again.
The Aniakchak volcano is located in southwest Alaska on the aptly named Alaska peninsula
where it is 220 miles south-southwest of the city of Kodiak.
Looking 65 miles to the southwest are two other similarly massive calderas, which exist
at both the Veniaminof and Black Peak volcanoes.
These closely spaced calderas indicate that the geochemistry is correct and sufficient
underlying magma exists to produce very large explosive eruptions.
We aren't exactly sure how old this volcano is, but a best guess would place Aniakchak's
oldest volcanic rocks at about 300,000 years in age.
Like other nearby volcanoes, it began forming due to offshore plate subduction several hundred
thousand years ago.
When this magma reached the surface, it erupted in an explosive manner, slowly building a
large volcanic cone.
This volcanic stratocone would go on to produce numerous eruptions from which layers of basaltic
andesite lava were interspersed with thick layers of ash.
While the vast majority of eruptions occurred at the central volcanic cone which was built
to a height of several thousand feet, occasional lava domes were built out of dacite and rhyolite
on a series of exterior ring fractures.
While these eruptions continued, the central volcanic cone reached a height of 7,000 feet
or 2,130 meters.
Meanwhile, at depth, the underlying magma was not only becoming more enriched in alkali
elements such as sodium and potassium, but also more silica rich.
This resulted in a series of rhyolite lava domes, indicating that the underlying magma
chamber had reached a critical point, as its high viscosity caused an immense amount of
pressure to build.
Then, in 6300 BC, a massive eruption began in the center of the complex.
As an existing lava dome exploded, an immense amount of ash shot high into the atmosphere.
Bombs of lava the size of cars were ejected several miles distant, and an immense amount
of ash piled up on the ground.
After several days of activity, this ash totaled more than 100 feet thick in some locations.
Strangely, this large eruption is not associated with the formation of a caldera, suggesting
that the magma chamber which fed it was very deep.
In total, 30 cubic kilometers of volcanic rock was ejected.
Over the next 4800 years, a vent at the northeast of the complex was most active, producing
a series of plinian explosive eruptions.
These eruptions slowly became more frequent and explosive until a caldera forming event
occurred.
Then, in 1645 BC, the largest volcanic eruption within Alaska during the last 10,000 years
began.
As a plume of ash shot more than 45 kilometers into the atmosphere, part of the eruption
column collapsed, producing unusually long reaching pyroclastic flows.
These flows completely burned and buried the landscape in superheated ash, destroying vegetation
at more than 50 miles or 80 kilometers from their source.
Then, due to the large amount of erupted material, the underlying magma chamber was largely emptied
which caused the ground overlying it to collapse downwards forming a 6 mile wide caldera.
A thick lake later filled the several thousand foot deep caldera in the ground, and several
lava domes erupted into it.
This lake later completely drained away to the southeast, and was followed by the creation
of several volcanic cones such as Vent Mountain and Surprise Cone which produced vulcanian
explosive eruptions.
In 1540, a volcanic eruption equal in size to the 1980 eruption of Mount Saint Helens
occurred from two of these vents.
The most recent eruption of Aniakchak occurred in 1931 when a plinian eruption occurred from
a vent on the western edge of the caldera wall, forming a grouping of 100 meter deep
and 250 meter wide explosion craters.
Today, the only settlement within direct reach of pyroclastic flows and lahars is the nearby
town of Port Heiden, which could be heavily impacted by an unusually large eruption.
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