Alaska in its recent history has been the site of several supermassive volcanic eruptions.
One such relatively recent eruption formed an 11 mile or 18 kilometer long hole in the
ground, representing a massive caldera collapse.
Yet, despite producing one of the largest volcanic eruptions in the last 10,000 years
of the planet, very few people are aware of the volcano which produced this eruption.
The volcano I am referring to is called the Fisher Caldera, which is still quite active
and likely to erupt again in the future.
The Fisher Caldera can be found in the eastern part of the Aleutian Island chain where it
is located 65 miles east northeast of the city of Akutan.
From satellite, the truly impressive size of the Fisher Caldera complex can be observed.
Its most prominent feature is an 11 mile long and 7 mile wide caldera, which formed in a
supermassive eruption in 7372 BC.
During that eruption, a group of 8 stratovolcanoes simultaneously collapsed, creating a more
than 1000 foot deep crater in the ground.
This vast volcanic complex began forming 660,000 years ago, when a volume of basaltic lava
erupted onto the surface.
This initial eruption was unusually explosive, as although the erupting lava was low viscosity,
it had a high gas content.
Intermittent vulcanian explosions occurred as lava was cleared from the erupting crater,
producing pyroclastic flows every few hours.
As ash continued to be ejected, lava output increased, forming a stratocone.
After several thousand years of eruptions, a 1500 foot tall stratovolcano was built.
Volcanic activity during the next 300,000 years occurred largely through monogenetic
vents which erupted once then never erupted again, distributing basaltic lava over wide
swaths of the island.
Then, a volume of more viscous dacite lava erupted, building a complex of lava domes
on what is now the eastern caldera rim.
Increasing volcanic activity began around 40,000 years ago, constructing two separate
andesite stratovolcanoes.
These formed alongside a series of cinder cones which created towering lava fountains.
Volcanic activity then shifted further westwards, resulting in the near simultaneous formation
of 5 closely spaced stratovolcanoes.
Despite their proximity, these volcanoes represented distinct systems which were fed by individual
magma chambers of differing compositions.
Erupted volcanic rocks ranged from basalt to trachyte to dacite.
By 15,000 years ago, there were 8 closely spaced snow covered stratovolcanoes present.
However, at depth something unusual occurred that changed the composition and gas content
of magma.
The active volcanoes on the caldera rim would go on to produce increasingly explosive eruptions,
producing pyroclastic flows which reached more than 10 miles distant.
Then, 10,000 years ago, two large batches of magma intruded underneath the caldera,
one to the northeast and one to the southwest.
Since it had a differing composition, it reacted with overlying magmas, causing surrounding
rock to fracture and greatly weaken.
Soon, a massive amount of pressure began to build.
Then, in 7372 BC, one of the world's largest recent explosive eruptions began.
As the large volume of magma broke through the surface in two locations, it created twin
eruption columns.
Soon, these columns partially collapsed, creating unusually energetic pyroclastic flows.
These pyroclastic flows raced across the landscape in all directions, but the most energetic
ones were directed to the east and northeast.
They burned all vegetation in their path, depositing a several foot thick layer of superheated
rock as far as 40 miles away.
As the day seemingly turned to night, dozens of feet thick of ash quickly piled up which
drifted in a northeast direction.
Then, due to the large volume of erupted material, a massive section of weakened ground collapsed
downwards to fill a near empty magma chamber, forming Alaska's largest caldera.
In total, 56 cubic kilometers of volcanic rock were ejected.
In the several thousand years following this eruption, shallow remaining magma interacted
with the caldera's crater lake, which for a time was 300 feet thick.
This formed a series of explosion craters visible around the complex.
Over the next 4,000 years, a new stratovolcano called the turquoise cone was constructed,
which reached more than 1,000 feet in height.
Then, in 3170 BC, this volcano completely collapsed to the northeast, leaving behind
a smaller caldera and ejecting 14 cubic kilometers of volcanic rock.
Volcanic activity subsequently shifted to the east, building Mount Finch, which produced
explosive eruptions in 1795, 1826 to 1827, and 1830.
Although this complex has not erupted since then, it still contains actively emitting
fumaroles and hot springs.
However, unless another caldera forming eruption was to occur, the island's only settlement
called False Pass would not be in any danger apart from possibly experiencing quantities
of falling ash.
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