The United States contained 162 active volcanoes. As of the making of this video, three of these are
erupting right now. One of these erupting volcanoes is located in Alaska and produced one of
the largest eruptions on the planet since the end of the last ice age. The volcano I am referring to
is called Semisopochnoi, which is located in the Aleutian islands. This video will discuss this
volcano, and mention what its future hazards are. The Semisopochnoi volcano is located in the far
southwestern portion of Alaska in a remote section of the Aleutian islands. Specifically, it can be
found 161 miles west of the closest city, Adak. Like the majority of the Aleutian islands,
this island, called Semisopochnoi is completely volcanic in origin. This volcano began forming
500,000 years ago, when it started as a submarine volcano at the bottom of the Bering Sea. Once
Semisopochnoi reached the surface, the lava which erupted from it spread outwards in every direction
due to its relatively fast moving lava. For the next several hundred thousand years, continued
eruptions from this massive shield volcano enlarged the island until it was 12 miles wide.
However, over this long timespan, the silica rich crust had slowly changed the lava present
in the underlying magma chamber. As indicated by the geologic record, eruptions slowly became
more explosive as the lava which erupted deposited basaltic andesite instead of basalt.
The formation of the volcanic cone called Ragged Top on the southeastern section of the
island marked the transition of the volcano from a shield volcano to a stratovolcano.
By 30,000 years ago, the eruptive activity had ceased at this cone and shifted to the northern
half of the island. Over the next 20,000 years, frequent explosive eruptions built a 3700 foot
tall feature known as Anvil Peak. Underneath this cone, magma continued to become more and
more silicous, as lava which erupted now had the composition of andesite. As a result, pressure and
volcanic gasses began to built at an extreme level underneath the island, causing high amounts of
uplift on the surface. Then, 8,000 years ago, this pressure was released in one of the largest
eruptions of the last 10,000 years. A large eruption in the center of the island escalated,
causing additional vents to open up in the surrounding area. As hundreds of millions of
tons of ash were ejected high into the atmosphere, the underlying magma chamber began to show signs
of structural instability. Eventually, the weight of the overlying rock and lava was so large that
it caused a 5 mile wide section of the crust to collapse downwards, forming a large caldera.
Pyroclastic flows from this eruption reached every section of the island, and covered
large sections with more than 50 feet of ash. In the aftermath of this caldera formation,
two eruptions occurred on the western third of the island. The first of these constructed
a small cinder cone, which partially overlapped a section of the caldera wall.
As a result, material which erupted to the east fell into the caldera, giving this cone an unusual
shape resembling 3/4 of a cone. The second eruption created a 2000 foot wide explosion crater
just to the northwest as the result of intense heat interacting with a small lake on the island.
After the construction of two additional cinder cones, eruptions shifted to a different section
of the island. Over the next several thousand years, eruptions built a new stratovolcano
related cone within the caldera rim known as Mount Cerberus. This cone has since
been the site of the vast majority of recent eruptions, including its current 2021 eruption.
However, other eruptions have also taken place to the south, constructing two smaller satellite
cones to the south of Sugarloaf Peak. One of these cones produced an eruption in 1987.
Since 1985, Semisopochnoi has erupted four times. The vast majority of eruptions at this volcanic
complex are quite small, resulting is only minor deposition of ash and moderately sized
flows of lava. Luckily, no one lives within 150 miles of this volcano. If Semisopochnoi
was to experience another large eruption, it would pose little hazard outside of generating a large
ash cloud which would cause flight disruptions over a large section of the Pacific.
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