Within the little travelled interior of Iceland, after a several hour long drive on what could
barely pass as a "road" is a magnificent volcano.
At this volcano are a series of lava flows stretching for as far as the eye can see,
a massive caldera with a crater lake, and beautiful flat topped features known as tuyas.
All of these beautiful features can be found at the volcano known as Askja.
It produced one of the largest volcanic eruptions in the history of Iceland in 1875, and today
is showing signs of unrest as an intrusion of magma is occurring 3 kilometers underneath
its edifice.
From satellite, the most obvious feature in this volcanic complex is its large lake filled
caldera which measures 4.5 kilometers wide.
However, if you zoom out, there is an even larger caldera present.
In total, this complex contains three calderas, each of which formed in incredibly large volume
explosive eruptions.
However the group of three partially overlapping calderas are not the entirety of this volcano.
The Askja volcano also contains a network of smaller shield volcanoes, tuff rings, spatter
cones, and fissures which are all emplaced along a 100 kilometer long chain of activity.
This chain of activity is known as the northern volcanic zone, and signals a divergent plate
boundary where the Eurasian plate is spreading apart from the North American plate.
This movement creates a series of faults and fractures in the crust, from which the underlying
hotspot injects new magma into the crust, eventually form a chain of volcanoes.
The Askja volcano began forming approximately 200,000 years ago, when a volume of basaltic
lava intruded into one of these faults in the crust.
Initially, it created a chain of spatter cones in a manner similar to the 2021 eruption of
Geldingadalir.
However, this eruption was a bit more explosive due to the relatively higher gas content contained
in the magma.
Eventually, several of the spatter cones merged into a single large shield volcano, which
would later become the center of Askja's activity.
Over tens of thousands of years, a large shield volcano was constructed at this center.
However, during this timespan a volume of magma deep in the crust had been leaching
material from the surrounding crust, slowly becoming more silica rich.
Eventually, this magma transitioned to a highly explosive composition matching rhyolite.
A subsequent episode of intense rifting triggered a batch of underlying basaltic magma to intrude
into rhyolite, which caused an immense amount of pressure to build.
Then, a plinian eruption began on the surface, as a column of ash was ejected more than 35
kilometers into the atmosphere.
After several cubic kilometers of rock was ejected, a 4.3 kilometer wide section of the
volcano collapsed downwards, forming a large caldera.
Although the climate of the planet was still quite similar to its modern counterpart at
the time of this eruption, it was not long until glaciers swept across the landscape
and buried this volcano in more than a kilometer thick of ice.
For more than 80% of Askja's lifespan, it was located underneath a thick ice sheet which
caused the nature of its eruptions to change.
Subglacial eruptions are generally not able to move very far horizontally, which results
in the creation of very steep volcanic cliffs.
As erupting lava interacts with the overlying glacier, explosions occur, causing the lava
to violently fragment into many little pieces.
After the eruption ends, glacial movement and subglacial runoff moves the brecciated
material, distributing what is known as "hyaloclastite" rock.
A large section of hyaloclastite can be found to the east of its recent caldera, which measures
more than 300 meters thick in some locations.
After the glaciers retreated, crustal pressure was released, which triggered the volcano's
largest explosive eruption in 8910 BC.
This eruption created a 10 kilometer wide caldera.
In the thousands of years following this eruption, lavas refilled the dropped block within the
caldera.
Then, in 1875, another highly explosive eruption occurred, forming the modern crater lake filled
smaller caldera.
This eruption also created a maar known as Viti, which today contains a popular hot spring
where waters are always between 25 and 30 degrees Celsius.
Although the ongoing intrusion of magma is occurring underneath the caldera, if it were
to erupt it would most likely cause a small flank eruption on the edge of the caldera.
Such an eruption would only be moderately explosive, and cover several square kilometers
of land in lava.
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