The Yukon area of Canada is well known for its scenic mountains, but few people are aware
of its volcanoes.
For example, the town of Fort Selkirk is built adjacent to a cinder cone which erupted about
8,000 years ago.
However, if you go back further in Yukon's geologic history, you will find that it once
contained a supervolcano.
In a series of eruptions, this supervolcano known as Bennett Lake covered the landscape
in several hundred meters of ash.
Although long extinct, its eruptive products continue to bring economic opportunities to
this region of Canada, as they are rich in both gold and silver.
In some cases, the two metals even combined with one another, forming a unique mineral
known as "electrum".
The Bennett Lake Volcano can be found stretching across the border between Yukon and British
Columbia, where it is 80 kilometers south-southwest of the city of Whitehorse.
Although it is no longer visible, this site used to contain a massive depression in the
ground known to as a caldera.
Its outline is shown on screen, measuring 30 kilometers long and 19 kilometers wide.
Approximately 58 million years ago, the Pacific Northwest had a familiar appearance but contained
a network of even taller peaks than seen today.
Offshore of Yukon, a major tectonic plate collision was ongoing as the Kula plate subducted
underneath the North American Plate.
The subducted crust then partially melted and migrated upwards, forming a chain of high
silica volcanic centers.
Beginning 57.3 million years ago, a large intrusion of molten rock erupted onto the
surface at the modern site of Bennett Lake.
After a powerful volcanic explosion created a several hundred meter wide crater in the
ground, viscous dacite magma emerged, which slowly built a lava dome.
Over the next several thousand years, more eruptions followed, building a chain of lava
domes in a semi circular shape.
This shape was the symptom of an unusually large underlying magma chamber, and a warning
sign of events to come.
Eventually, the lava dome forming eruptions became unusually explosive, as the underlying
magma chamber now contained a high amount of volatile material.
A subsequent pause in activity allowed for pressure to slowly build in the underlying
magma chamber, which culminated in a catastrophic eruption.
As this large eruption finally began, a series of vents from an underlying dike simultaneously
erupted, producing a plume of gray ash which shot 46 kilometers or 150,000 feet into the
atmosphere.
However, some of this material was too heavy to rise into the atmosphere, and as a result
part of the eruption column collapsed downwards, generating unusually energetic pyroclastic
flows.
These pyroclastic flows were up to a kilometer in height, and raced across the surrounding
landscape, travelling up mountains and seemingly defying local topography.
These flows burned all vegetation in their path, reaching more than 120 kilometers distant.
Then, due to the large amount of erupted material, a 30 kilometer wide section of ground collapsed
downwards, forming a massive caldera.
However, the eruption did not end there.
Over the ash coated landscape, dacite and rhyolite lava domes erupted in a relatively
nonexplosive manner.
Then, after a several year pause in activity, a second phase of the plinian eruption resumed.
This lengthy eruption lasted potentially as long as 100 years, containing 3 smaller VEI
7 eruptions each separated by effusive lava dome forming activity.
By the time the caldera collapsed a third time, the ground had been coated in up to
700 meters thick of ash.
After a series of less viscous andesite eruptions built a small stratovolcano, eruptive activity
took a lengthy pause.
Over a several hundred thousand year long timespan, water carved out distinctive valleys,
and parts of the caldera rim collapsed downwards, forming large debris fans.
Despite the lack of volcanic activity, the magma chamber was slowly refilling, causing
the ground overlying it to be uplifted and forming a resurgent dome.
It is during this period that warm fluids rich in gold and silver migrated to the surface,
depositing large quantities of material there is altered rock.
Eventually, alkali rich elements built up sufficiently in the magma chamber that a second
massive eruption occurred.
This eruption once again coated the ground in a several hundred meter thick layer of
ash, and caused a volcanic winter around the globe.
The caldera formed by this eruption was much smaller, as it was completely confined within
the prior caldera, measuring 14 kilometers long and 10 kilometers wide.
After this last major eruption, the ground was once again uplifted, eventually forming
a large northeast trending crack.
Despite the tens of millions of years of erosion which has occurred since, the remnants of
this crack and the underlying dike which caused it remain as a large valley which is outlined
on screen.
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