On October 11th of 2022, the alert level was raised at the Grímsvötn volcano in Iceland,
increasing from green to yellow.
This change was made due to the fact that a section of the volcano's ice cap began to
rapidly subside on October 5th, and as of the writing of this video has sunk by 4.3
meters or 14.1 feet.
This subsidence indicates that a glacial flood is about to occur or is already underway which
is known as a jokulhlaup.
While the exact relationship between these jokulhlaups and volcanic eruptions is still
debated, it is widely accepted that on several occasions a jokulhlaup preceded or coincided
with a volcanic eruption.
This occurred in 1922, 1938, 1954, and most recently in 2004.
Although jokulhlaup's occur around the planet for varying reasons, when they originate from
the Grimsvotn volcano, which is largely buried underneath a thick ice sheet, they almost
certainly are volcanic in origin.
Since Grimsvotn has a presumably large volume magma chamber as indicated by its ability
to produce highly voluminous effusive and explosive eruptions, it also has a large geothermal
system consisting of abundant fumaroles which are buried beneath the thick ice sheet.
These fumaroles regularly release volcanic gasses at above the boiling point of water,
which melts sections of the overlying glaciers, creating a thick subglacial lake which can
have on the order of several cubic kilometers of water.
The rate of melting responds to the movement of magma at depth meaning that it can increase
or decrease.
All of this water eventually floods at a path of least resistance, creating a jokulhlaup
that floods distant drainages of the volcano.
You might hear all of this and think that the relationship between jokulhlaups and volcanic
eruptions couldn't be more self-explanatory, as it is true that fresh lava erupted onto
the surface or beneath a thick ice sheet can generate a glacial flood.
However, this doesn't describe the full picture as shown by Grimsvotn producing a jokulhlaup
in 2004 five days before the eruption began.
The reason this was believed to have occurred relates to the pressure exerted by the mass
of overlying water and glacial ice on the volcano's magma chamber.
While meltwater is piling up beneath the thick ice sheet, the notable mass of this material
makes it incredibly difficult for a volcanic eruption to occur.
For an eruption to occur, the tensile stress on rock overlying the magma chamber needs
to become positive.
This can occur quite quickly with the removal of a sufficient amount of overlying water
and ice, causing dike propagation to begin and often leading to a volcanic eruption.
Thus, although a volcanic eruption can trigger a jokulhlaup a jokulhlaup can also trigger
a volcanic eruption.
However, most jokulhlaups from the Grimsvotn volcano have not led to volcanic eruptions,
although statistics seems to suggest that each event carries with it a very low double
digit percentage chance of causing a volcanic eruption or occurring alongside one.
For example, since Grimsvotn's last eruption in 2011, it has produced 10 separate jokulhlaups,
not including the one which is ongoing.
And, Grimsvotn often produces volcanic eruptions which are completely unrelated to jokulhlaups.
With this being said, this large caldera complex volcano has been the most historically active
in Iceland, at least since 1000 CE.
During that time it has produced 78 volcanic eruptions, although there is a potential undercounting
of these events as in the last 100 years 18 eruptions have occurred.
Grimsvotn's recent eruptions have been universally explosive to a degree, so if it was to erupt
it would likely affect regional air traffic.
However, Grimsvotn's 2011 eruption which was rated as a VEI 4 was unusually large, representing
the volcano's largest in 138 years.
Most of Grimsvotn's eruptions are two orders of magnitude smaller, rating in as a VEI 2.
And, although unlikely, the possibility always exists that this volcano's next eruption could
occur via a lengthy series of fissures away from overlying glaciers.
Thanks for watching!
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