The massive depression in the ground which you are looking at is a caldera which formed
during the climatic eruption of Crater Lake approximately 7,700 years ago in Oregon.
While spectacular, this more than 2,000 foot deep caldera is not the only similar edifice
in the United States, as a surprisingly high number of large calderas exist in Alaska.
These include calderas at Mount Katmai, Okmok, Aniakchak, the Fisher Caldera, Black Peak,
Veniaminof, and a little known volcano known as Ugashik-Peulik.
As per the name, the Mount Peulik volcano represents the more recent stratovolcano in
the complex while the Ugashik volcano represents the older of the two, containing a 3 mile
wide and 2.65 mile long caldera which is 880 feet deep.
This caldera is quite scenic, often being snow covered, although it is clearly older
than may of the other mentioned calderas within Alaska as its rim shows a higher degree of
erosion.
The major volcanic eruption which formed this caldera likely occurred 40,000 years ago,
and was comparible in size to the 1991 eruption of Mount Pinatubo, registering in on the volcanic
explosivity index as a 6.
However, the Ugashik caldera and the lava domes within it are not the only interesting
aspect of this volcanic complex.
Its northern and western sectors contain more than 1,000 small hills and mounds, some of
which are more than 20 feet in height.
These mounds are classified as "hummocky terrain" and owe their existence to a catastrophic
collapse and directed blast at the volcano which was somewhat similar tothat witnessed
at Mount Saint Helens in 1980.
And, the Peulik volcano has been the site of two historical eruptions during the 1800s
while the Ugashik volcano is one of the few locations in Alaska to have geologically recently
erupted rhyolite lava.
For context, this remote volcano is located on the Aleutian Peninsula, far southwest of
Anchorage where it is 66 miles south of the city of King Salmon.
Although the exact age of when this volcano began to form due to subduction of the Pacific
Plate underneath the North American plate is unclear, we do know that it had erupted
at least once by 171,000 years ago.
During the early phase of this volcano, a group of overlapping lava domes and a medium
sized volcanic cone was slowly constructed out of both andesite and dacite lavas in variably
explosive and effusive eruptions.
Two peaks were constructed during this period.
Eventually, a large and volatile rich magma chamber grew underneath the volcano, allowing
for a powerful plinian eruption to occur which caused its large caldera to form 40,000 years
ago.
During the next 28,000 years, 4 dacite and rhyolite composition lava domes erupted within
the caldera, creating gray colored block edifices.
A fifth dome, aka the northeast most one, may have then erupted around 10,000 years
ago.
And only after all of this had occurred did the Peulik volcano begin to grow.
However, it at first primarily erupted less silica rich basalt and basaltic andesite lavas.
After frequent cone building eruptions, a powerful lateral eruption occurred, causing
several hundred million cubic meters of rock to slide to cover an area of 29 square miles.
Another smaller lateral blast and collapse likely occurred around the same time which
was directed to the north and northwest.
Several flank andesite lava flows thereafter erupted to the north-northwest and a lava
dome grew within the summit of the collapse crater.
The Alaska Volcano Observatory notes that at least 6 Holocene eruptions occurred at
this volcano, the last of which was in 1852 although more likely occurred to quickly build
this stratovolcano in only the last 11,000 years.
While a future eruption at some point from Mount Peulik in the next 10,000 years seems
highly likely, the same cannot be said of the Ugashik caldera as it appears far less
active.
The good news is that assuming Peulik is the source of future eruptions and due to the
nearest town being Pilot Point 45 miles to the west-southwest and the lack of closer
settlements, hazards from potential future pyroclastic flows and lava flows are unlikely
to destroy any stuctures and instead only damage sections of unpopulated forest most
likely witin a 5 mile radius of the volcano although further traveling hazards beyond
this specified distance are possible as shown by the debris avalanche deposits which reach
a distance of 6.8 miles or 11 kilometers to Mount Puelik's west.
However, ashfall during any future eruption could be sufficient to damage roofs at nearby
cities during a larger event, perhaps on the order of the 1977 Ukinrek Maars eruption,
which could contaminate the water supply at effected cities, although even a small ash
emission could be sufficient to heavily disrupt regional air traffic.
As a result of these hazards, the U.S. Geological Survey designated Ugashik-Peulik as a high
threat volcano.
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