Within Alaska there are more than 80 active volcanoes.
One of these volcanoes in the Aleutian volcanic arc is covered by a vast 1 kilometer or 3,280
foot thick sequence of ice sheets.
This volcano has erupted 8 times since 1790, producing moderate to large explosive eruptions.
The last such eruption occurred from 1991 to 1992, sending 50 million cubic meters of
lava down Westdahl's eastern flank.
Since then, the volcano has displayed persistent signs of uplift meaning that magma is building
at shallow depths underneath the volcano.
Because of this uplift, an eruption was considered to be near imminent in the early 2000s, with
models suggesting that this volcano would erupt no later than 2010.
Of course, that year came and went without an eruption.
It is now 12 years later, and while uplift continues this volcano has still not erupted.
Luckily, a new scientific paper by Lilian Lucas and others seems to have discovered
why this towering volcano which is truly overdue for an eruption has not already erupted as
predicted.
The ice covered Westdahl volcano is located on the very end of the Alaskan peninsula,
where it is 50 miles northeast of the city of Akutan.
This vast volcano stands at 1,563 meters or 5,128 feet tall, and with its overall gentle
slopes might initially look like a shield volcano.
However, it is actually a stratovolcano.
Much of its steep summit area was lost in a major caldera forming eruption around 60,000
years ago.
This major eruption deposited large volumes of ignimbrite, forming a 9 kilometer wide
caldera.
Since this caldera formation, thick glaciers filled much of the collapsed area, and 2 of
its 3 large volcanic cones grew within the glaciated area.
These largely glaciated vents frequently produce lengthy lava flows which can be found along
every slope of this volcanic complex.
Since historical record keeping began the Westdahl volcano has experienced periods between
distinct eruptions which last between 1 and 137 years, with an average time between eruptions
being 28 years.
Although a number of factors can cause a volcanic eruption, what is most relevant at this volcano
is pressure.
At a depth of 4.5 miles or 7.2 kilometers beneath Westdahl's summit is a large body
of magma.
Ever since it last erupted in 1992 this deep magma chamber has been slowly receiving a
steading influx of new material.
This new material has over time increased the pressure within the chamber.
Once the pressure in this magma chamber is greater than that exerted on the rock overlying
the molten material, equilibrium is lost, and dike propagation begins.
Once this occurs, it is only a matter of time before a new volcanic eruption occurs.
Taking into account the depth and shape of the magma chamber, and density of rock overlying
it gave a figure which suggested an eruption would occur within 18 years or by 2010.
However, this figure failed to take into account something significant at this volcano; it
is covered by a very thick ice sheet.
Although ice is less dense than the average volcanic rock, a several kilometer thick sheet
of it increases the amount of pressure a magma chamber must have before dike propagation
can occur.
Taking this into account, the 1 kilometer thick ice sheet at Westdahl adds another 7
years between an average eruption.
This model then simulated conditions for magma chambers given varying thicknesses of glaciers
above it.
It concluded that the thicker an ice sheet is, the longer a volcano goes between distinct
eruptions.
For example, if a prolate magma chamber at a depth of 2 kilometers without an ice sheet
normally erupted every 100 years it would instead erupt once every 186 years with a
3 kilometer thick ice sheet above it.
The deeper the magma chamber is, the less of an effect an ice sheet has.
If the same magma chamber was instead located at 9 kilometers depth then the volcano would
erupt an average of once every 112 years.
These findings have implications for specific volcanic complexes in Iceland such as Grimsvotn
and the future of far northern and southern latitude volcanoes with thick ice sheets on
them.
If these ice sheets continue to decrease in thickness due to climate change, then eruptions
at relevant volcanoes will become more frequent although slightly less explosive.
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