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, one stands out, the Makushin Volcano.
Its location on the chart at first seems misplaced, as the Okmok volcano which ejected 50 cubic
kilometers in one of the planet's largest recent explosive eruptions, leading in part
to the downfall of the Roman Republic is only listed as a "high threat volcano".
In contrast, its placement on this list makes perfect sense when you realize that no one
lives within direct reach of Okmok, but 4,724 residents live on top of young deposits from
Makushin at the town of Unalaska.
If this town were to have been present either 8000 or 8800 years ago, it would have been
completely annihilated by long reaching pyroclastic flows which travelled 15 miles or 25 kilometers
distant from the volcano.
Although the recent eruptions from Makushin have been relatively small, there is a threat
that such a major eruption could repeat in the future, hence its "very high threat" designation.
The Makushin volcano is located in the central Aleutian Island chain, where it is emplaced
on the northern section of Unalaska Island.
It is an unusually broad volcano, measuring 10 miles or 16 kilometers across.
This volcano began forming 930,000 years ago, when a volume of basaltic andesite composition
magma erupted onto the surface.
As it erupted, it produced a series of powerful explosions, and due in part to numerous pyroclastic
flows the existing island was expanded by several miles to the northwest via a plateau
of newly erupted ash.
Over the next several hundred thousand years, the erupted lava became more silica rich,
as it transitioned from basaltic andesite to andesite.
Over time, a large gray colored stratovolcano was constructed, which produced numerous vulcanian
explosive eruptions.
As the central volcanic peak was built to a height of more than 5,000 feet, glaciers
expanded across its edifice.
During interglacial periods when these receded, volcanic activity became possible at satellite
vents.
These secondary vents produced large lava dome forming eruptions, but also viscous andesite
lava flows.
One such lava flow is known as "lava ramp" which covered 50 square kilometers of land
in viscous lava.
One satellite vent soon became the center of activity, erupting near continuously for
several thousand years, building the 3,000 foot tall Pakushin cone with a 760 meter wide
summit crater formed from powerful explosive eruptions.
To the northwest, intrusions of magma interacted with the area's abundant groundwater, causing
it to flash to steam and explode.
These explosions carved out large craters along a northwest facing trend, some of which
are more than 2,000 feet wide.
After the most recent glacial period ended, a group of powerful explosive eruptions occurred.
One such eruption in 6790 BC began with the creation of a large bulge on the north slope
of the volcano.
This bulge eventually burst, resulting in a plinian eruption which created a 37 kilometer
or 121,000 foot high eruption column.
As the north flank of Makushin partially collapsed, it traveled northwards at incredible speeds,
the landslide only stopping 5 miles into the ocean.
Highly energetic pyroclastic flows associated with this eruption reached the modern town
of Unalaska, depositing a foot thick of superheated rock over what is now the town's main port.
Then, due to the large amount of erupted material, which totaled 7.5 cubic kilometers, a 2.5
kilometer wide section of the volcano collapsed downwards, forming a massive caldera.
A similar magnitude eruption occurred in 6050 BC, deepening the overall caldera, and depositing
10 feet of superheated tuff on the western edge of Broad Bay.
The vast majority of eruptions which have occurred since, with the exception of another
large eruption in 3650 BC, have been comparatively small, occurring as a series of small volcanic
explosions at Makushin's summit crater.
During the 20th century, Makushin erupted 10 times, producing a series of phreatomagmatic
and phreatic explosions, which widened its summit crater.
Although the next eruption from this volcano is also likely to be small, there is always
a possibility that another unusually energetic pyroclastic flow could occur from an additional
caldera forming eruption.
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