The world’s most famous supervolcano is the Yellowstone Caldera which is located in
Wyoming.
Yet, it is not the only supervolcano on the planet, as these incredibly large volcanoes
have affected almost every country if you go back far enough in Earth's geologic history.
One of these ancient supervolcanoes can be found in the United Kingdom, where it has
been largely hidden by hundreds of millions of years of erosion.
Although it is extinct, its eruptions were responsible for the creation of unusual geologic
formations which can be found near the town of Glen Coe.
The supervolcano I am referring to is also called Glen Coe, which produced an eruption
300 times larger than the 1980 eruption of Mount Saint Helens.
Although the rocks of this ancient supervolcano have been highly deformed and stretched over
time, its approximate caldera outline is shown on screen.
It measures 13.8 kilometers long and 8 kilometers wide, and was likely once a bit larger.
This caldera represents a giant collapse scar which originated from one of the planet's
most explosive eruptions during the Silurian period of geologic history.
Approximately 420 million years ago, what is now Scotland was part a large landmass
which connected what is the New England area of the United States, Newfoundland, Greenland,
and the far northern edge of Canada.
This area was a prominent volcanic island arc formed by the collision of two tectonic
plates to the east of the island chain.
It was largely surrounded by a vast ocean which contained numerous unusual creatures
such as the 2 meter long eurypterid, or the beetle like arthropods known as trilobites.
On land, this arc was populated by numerous tall stratovolcanoes which had a generally
gray color, erupted andesite, and contained large summit craters which emitted volcanic
gasses.
At least one of these ancient stratovolcanoes was located adjacent to the town of Glen Coe,
where it was situated in a geologic feature known as a graben.
Grabens are an elongated block of the Earth's crust which lie between two faults and are
displaced downwards compared to the blocks on either side.
It can be interpreted that these faults acted as a path of least resistance, allowing for
the growth of a volcano.
Over time, numerous vulcanian eruptions coated the area in hundreds of meters thick of both
andesite lavas and ash.
Then, the volcano seemingly stopped erupting, as underneath its edifice an unusually large
magma chamber began to build in volume.
This magma chamber began leaching material from the surrounding crust, causing its composition
to change from andesite to rhyolite.
For tens of thousands of years following this change, the rhyolite erupted onto the surface
but instead of forming a stratovolcano created a series of large explosion craters from which
lava domes erupted.
These lava domes experienced frequent partial collapses, resulting in pyroclastic flows.
However, despite the volume of erupted material, it was insufficient to relieve the amount
of pressure building in the underlying magma chamber.
Finally, a far larger batch of magma intruded into the crust from the magma chamber, causing
a massive eruption to begin on the surface.
As several vents along the graben faults simultaneously erupted, a plume of ash shot 49 kilometers
into the atmosphere.
Kilometer high pyroclastic flows subsequently raced across the landscape, travelling more
than 75 kilometers distant and even entering the adjacent ocean.
Over the span of 48 hours, the area looked like it was in continuous nighttime as the
sheer quantity of ejected ash blocked all sunlight.
This ash alongside pyroclastic flow deposits formed a unique rock type known as ignimbrite.
Ash continued piling up in the area, reaching more than 200 meters thick in some locations.
As the underlying magma chamber was now partially empty, the ground overlying it collapsed downwards
to fill the empty space, resulting in the creation of a large caldera.
In total, 300 cubic kilometers of volcanic rock was ejected, meaning the eruption would
have been rated on the volcanic explosivity index as a 7.
What remained post eruption was a 1500 meter deep caldera in the ground and a landscape
completely void of vegetation and covered in superheated ash.
Nothing in a 50 kilometer radius survived the eruption.
In the many millions of years following this caldera's formation, volcanic activity moved
elsewhere in the United Kingdom, eventually transitioning towards Ben Nevis.
During the most recent ice age, glaciers carved away at the rock units of Glen Coe, exposing
its numerous volcanic layers to the world.
Today, what remains is a fascinating example of a megaeruption during a different time
in the planet's geologic history.
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