Although both Alaska and Yukon contain volcanoes which have produced eruptions in recent times,
none of these are located in close proximity to the community of Old Crow in Yukon.
The closest active volcano to this community is the Fort Selkirk volcano which is 325 miles
to the south-southeast.
Yet, in and around this settlement is a notably white ash layer which is several centimeters
thick.
This ash layer was thus called the "Old Crow Tephra" and for many years its source remained
a mystery.
While volcanoes far to the southwest in Alaska such as Mount Churchill, Novarupta, Aniakchak,
Black Peak, and the Fisher caldera have produced voluminous explosive eruptions in the last
20,000 years, none of these were the source of the ash layer which was dated to 124,000
years old.
Instead, it originated from a volcanic eruption an order of magnitude larger than the aforementioned
caldera forming eruptions at each volcano, at a complex known as the Emmons Lake Caldera.
The Emmons lake caldera is located in southern Alaska on the Aleutian Peninsula where it
is 62 miles west northwest of the city of Sand Point.
Emmons Lake contains Alaska's largest known caldera to form in the last 1 million years,
measuring 18.2 kilometers long and 10.5 kilometers wide.
Its caldera rim is up to 610 meters or 2,000 feet higher than the caldera bottom, contains
a series of young eruption products, a caldera lake, and a series of post caldera stratovolcanoes.
These stratovolcanoes are located in a straight line of activity 13 miles in length, consisting
of when measured from southwest to northeast: Mount Emmons, Hague, Double Crater, Little
Pavlof, Pavlof, and the Pavlof Sister volcanoes.
The Emmons lake volcano has been erupting intermittently for more than 500,000 years,
which before its first caldera forming eruption consisted of an unusually voluminous complex
of lava domes, stratovolcanoes with glacier covered peaks, cinder cones, and flank lava
flows.
However, these products produced by explosive eruptions would not last long as Emmons Lake
would go on to produce six separate caldera forming eruptions according to the Alaska
Volcano Observatory between 400,000 and 20,000 years ago.
You can see the remnants of at least 1 of these older calderas on the southwestern section
of the complex.
By 200,000 years ago, a large caldera likely already existed but was perhaps a third of
its modern size.
Over time, the large magma chamber of Emmons Lake seemingly became more and more silica
and volatile rich, which primed the volcano for another powerful eruption.
Then, 124,000 years ago, after perhaps dozens or hundreds of feet of ground uplift, a powerful
volcanic eruption began on the surface.
As Emmons Lake explosively erupted at a rate of several hundred thousand cubic meters of
rock per second, a plume of ash rose to a height of 50 kilometers or 164,000 feet.
Some of this material was too heavy and collapsed downwards while other material sort of figuratively
boiled over the erupting vents, creating voluminous pyroclastic flows which were more than 1,000
feet tall and swept over the tops of adjacent mountains.
Travelling at a speed of more than 100 hundred miles per hour, these would go on to completely
decimate the landscape within a 30 kilometer or 18 mile radius.
However, although current evidence has not proven that these flows travelled beyond this
distance, eruptions of this scale often result in pyroclastic flows which exceed a maximum
runout distance more than 2 or 3 times this length, meaning that pyroclastic flows likely
raced across the adjacent sea, crossing sections of it and reaching adjacent islands.
Due to prevailing wind patterns at the time of the eruption, the majority of ash drifted
towards the northeast.
This ash would go on to cover almost the entirety of eastern and central Alaska and large sections
of western Yukon.
Then, due to the large amount of erupted material totalling 200 cubic kilometers of volcanic
rock meaning the eruption had a volcanic explosivity index of a 7, the magma chamber of Emmons
Lake had largely been drained.
This caused a large section of overlying rock to collapse downwards to fill the empty space,
forming a massive caldera.
In the more than 100,000 years since another caldera forming eruption has occurred and
numerous vents have filled in large sections of the caldera.
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