The United States contains 162 active volcanoes.
Of these, 85 are located in Alaska.
Although many of the volcanoes close to cities such as Anchorage are well known, other more
remote volcanoes are not.
This video will discuss an interesting volcano which as of May 13th of 2021, was just been
placed on a raised alert status.
The volcano in question is called Great Sitkin, and last erupted in 2019.
This video will discuss this volcano, and mention what its future hazards are.
The Great Sitkin volcano can be found in the western part of the Aleutian island chain.
The closest settlement to this volcano, Adak, lies 26 miles to the west southwest.
As we initially look at this volcano, you will note a raised rim to the east of the
modern summit.
This raised rim represents an older part of the modern volcano which remained after a
western section collapsed to the northwest in a massive landslide.
Despite appearances, the Great Sitkin volcano is not responsible for the creation of the
majority of Great Sitkin island.
Before this volcano existed, a large shield volcano stood a mile to the west of the modern
edifice.
This unusual shield volcano erupted both basalt and andesite, which cover slightly more than
80% of the island.
This age of this older volcano is not known, but likely was 2 to 3 million years old.
The modern Great Sitkin stratovolcano emerged out of this vast shield volcano sometime in
the last 100,000 years.
When this new volcano first erupted, it consisted of numerous explosions which ejected bombs
of lava in a several mile radius.
The reason for its explosiveness was two-fold.
Compared to the prior shield volcano it was more siliceous.
Additionally, when the lava interacted with groundwater, it converted it to steam, thus
generating a series of explosions.
Over the span of tens of thousands of years, the Great Sitkin stratovolcano was built to
a height of 6200 feet.
However, its height was beginning to cause structural instability on the volcano's west
side.
Why?
It is easiest to explain its structural instability through a cross section diagram.
As you can see, the modern volcanic edifice is built on top of a much older unit.
However, its center has over time shifted further to the northwest, thus causing its
mass to push the volcano in that direction.
After a series of smaller landslides, another eruption occurred.
This eruption deposited a large quantity of ash on top of the existing edifice.
Eventually, the weight of the newly deposited ash caused the volcano to catastrophically
collapse to the northwest.
As the volcano lost several thousand feet in height, a massive tsunami was generated.
In the aftermath of this eruption, a large several mile long scar remained.
Eventually, a new volcanic cone grew out of the collapse scar through the formation of
overlapping ash and domes of lava.
This occurred sometime in the last 10,000 years.
Since then, the volcano has produced numerous low intensity explosive eruptions which were
periodically interspersed by large volume plinian eruptions.
One of the 8 documented plinian eruptions was large enough that it created a small caldera,
measuring about 4000 feet across.
The most recent of these large volume eruptions happened sometime during the 1700s.
However, not all of the eruptions from this volcano have occurred from its summit.
Several eruptions occurred from the flank of Great Sitkin, including one eruption 9,000
years ago, which traveled down Sitkin Creek on the east of the island.
Additionally, several lava domes exist in a northwest trending pattern from its summit.
The most recent lava dome building eruption occurred in 1974, which built an andesite
lava dome in the summit crater.
Since then, Sitkin has produced three small eruptions, the last of which occurred in June
of 2019.
In Summary, the Great Sitkin volcano is considered a high threat volcano by the US geological
survey as its numerous mildly explosive eruptions are interspersed by high volume explosive
eruptions about once every 1,200 years.
Luckily, no one lives on the island which hosts this volcano.
The main hazard Great Sitkin poses in the future is to air traffic.
Every time an explosive eruption occurs, it will disrupt air travel in the region since
even small volumes of ash can cause airplane engines to fail.
Additionally, due to prevailing winds, if a large eruption was to occur, both Adak and
Atka could be coated with up to an inch of ash.
I hope that you enjoyed this video!
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