In the morning of a day in April, 1974, residents of Sitka, Alaska woke up to find an unusual
sight.
Much to everyone's surprise, a strong plume of smoke was rising from the nearby stratovolcano
known as Mount Edgecumbe.
To further investigate, a coast guard helicopter was called in to see what portion of the volcano
was erupting.
As they flew above the summit crater, they began to see the words spray painted into
the ground "APRIL FOOL".
What had just occurred was one of the most ingenious April fools pranks, as 70 old tires
had been flown in overnight by helicopter and then set on fire by a local prankster.
It was quite a believable prank, as Mount Edgecumbe is an active volcano, with several
geologically recent eruptions.
Thus, the next time smoke rises from its summit, it will truly mean than an eruption is taking
place.
This video will discuss this Alaskan volcano, its geologic history, and mention what its
future hazards are.
The Mount Edgecumbe volcano is located in far southeastern Alaska, to the southwest
of Juneau.
The aforementioned city of Sitka is located 15 miles to the east.
Before I begin, I need to note that Mount Edgecumbe is part of a larger volcanic complex
which is the topic of this video.
The vents in this volcanic complex cover an area of 100 square miles, and encompass the
southern half of Kruzof Island.
These vents are centered on a chain of activity which is perpendicular to the plate boundary
to the west of the island.
Volcanic activity began approximately 600,000 years ago, when a series of fissures opened
and erupted large quantities of basalt lava, much like what is ongoing in Iceland.
Lavas produced from these fissures had a very low viscosity and high speed, and as a result
were able to travel long distances before cooling.
Over time, these eruptions caused the island to expand in all directions, thus building
an edifice which was nearly symmetrical.
As the many years progressed, intermittent eruptive activity continued, building spatter
cones and then cinder cones.
Eventually, the composition of lava changed, causing eruptions to become more explosive.
As the lava transitioned from basalt to basaltic andesite, and then to andesite, the cinder
cone forming eruptions stopped.
Instead, beginning 87,000 years ago, lava began concentrating in a single central volcanic
cone called Mount Edgecumbe.
Over many thousands of years, vulcanian style eruptions which produced short distance pyroclastic
flows and magnificent bombs of lava continued to build this tall volcano.
28,000 years ago, a new eruptive center called Crater Ridge began erupting large viscous
lava domes of an unusual type of lava called rhyolite to the northeast of Mount Edgecumbe.
Rhyolite eruptions are incredibly rare, and typically only seen as the most destructive
volcanoes and supervolcanoes.
By 16,000 years ago, a complex of numerous overlapping lava domes had been constructed,
which looked much like Lassen Peak in California today, but with a lighter color.
Between 16,000 and 13,000 years ago, the supply of magma to the underlying fissure system
unexpectedly increased, leading to explosive eruptions along various points of the island.
These formed peaks such as Shell Mountain to the northeast and several lava domes to
the southwest.
If you look closely at these lava domes, you can see the trend of the fissure which supplied
their molten rock.
Then, 13,180 years ago, a large body of magma rose towards the surface, causing an immense
amount of pressure to build beneath Crater Ridge.
Suddenly, this pressure was released which formed a continuous plume of ash that rose
high into the atmosphere.
Pyroclastic flows soon raced across the landscape, traveling more than 6 miles and depositing
large volumes of ash, thus expanding the island.
Then, due to the large volume of material erupted, a 7100 by 5700 foot section of ground
collapsed downwards like a sinkhole.
This formed a massive caldera which was 790 feet deep.
What had just occurred was an eruption 7 times larger than the 1980 eruption of Mount Saint
Helens.
After a period of inactivity, Mount Edgecumbe erupted once again 9,000 years ago, which
resulted in the creation of its modern summit crater.
Two more eruptions in 3810 BC and 2210 BC originated from the summit of Mount Edgecumbe
and spread ash as far away as what is now the town of Sitka.
Since then, no additional volcanic activity has occurred.
If this volcano was to erupt again, it would produce a variety of hazards.
These include lava bombs, pyroclastic flows, and a thick ash cloud which could disrupt
regional air traffic.
Luckily, the only effects the nearby town of Sitka could experience are a quantity of
falling ash.
Overall, the odds of this volcanic complex erupting each year are quite low at about
1 in 2,000 each year.
As a result of these hazards, the US Geological Survey designated Edgecumbe as a moderate
threat volcano.
I hope that you enjoyed this video!
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