The thought of powerful explosions occurring, lava being thrown into the air, and pyroclastic
flows racing across a mountainous region of Colorado due to a volcanic eruption might
sound like an absurd instance of fiction.
Yet, despite common perception, a volcanic eruption is more likely than you might think
within the state, as Colorado contains a single active volcano.
To prove this point, I present an ominous depression in the ground which is centered
northeast of the town of Dotsero and a mile north of I-70.
This depression is in fact a volcanic crater referred to as a "maar", which formed in a
powerful explosive eruption that carved out a complex crater in the ground.
It took a truly immense amount of energy to create this 2460 foot long, 2260 foot wide,
and 250 foot deep crater.
And, this crater used to be significantly deeper, originally going 1300 feet into the
ground.
Yet, it has been mostly filled in since its formation, which occurred only a little over
4,000 years ago in 2150 BCE.
However, Dotsero Crater is not the only feature that remains from this climactic eruption.
There is also a 10,340 foot long crack that runs through the crater, creating a subsided
fissure up to 20 feet deep and 100 feet wide.
This crack marks the maximum dimensions of the original intruding body of magma in the
subsurface, which had a north-northeast orientation.
As this magma intruded into the subsurface 4150 years ago, it pushed overlying rock not
only upwards but also outwards, pushing apart 310 million year old sedimentary rock.
After thousands of earthquakes, with at least 1 likely reaching the threshold of a magnitude
5 event, lava finally erupted.
Initially, small amounts of molten rock were weakly emitted from the entire nearly 2 mile
long fissure.
However, within an hour of the eruption's onset, parts of the fissure had become clogged
with lava, causing underlying molten rock to divert elsewhere to only a few select points.
Whereas before the entire fissure was erupting, now only 7 small 100 foot wide sections were
ejecting molten rock in lava fountains, while also producing a distinguishable roar.
Strombolian eruptive activity had commensed, and the construction of multiple cinder cones
was now ongoing.
While some of these cinder cones would soonafter cease their activity, having not grown more
than 50 feet above the ground, two vents remained active.
As the days progressed, a blocky flow of a'a' lava advanced downhill in a narrow valley
at a rate of less than 100 feet an hour, eventually reaching an alluvial plain, and spreading
out in the shape of a fan.
The emplaced hard rock may have even temporarily diverted the Eagle River further to the south,
creating violent explosions as lava fell into its mass.
Eventually, the majority of lava output ceased, by which time approximately one third of a
square mile had been coated in a layer of lava.
As a gray coated cinder cone continued to grow, now reaching a height of 250 feet above
the hillside, strong ash emissions began in the place where lava was previously the main
output.
Yet, something was occurring only a few hundred feet north of this cone.
As, underground, there was a fault line that was heavily saturated with groundwater.
Magma at depth was continuing to advance closer to the surface in some locations, and this
heated overlying rocks.
Eventually, the heat became so great that any groundwater present flashed to steam.
The expansion of water to steam caused it to expand more than 1,000 times its original
volume, placing incredible strain on the overlying rock.
Eventually, the overlying rock gave way in a tremendous explosion.
As large chunks of Pennsylvanian age sandstone were thrown several thousand feet into the
surrounding landscape, a towering 4 mile high eruption column was generated.
Part of this column then collapsed back to the ground, and raced to the southwest at
a speed of 85 miles per hour, traveling an entire mile before it ceased.
Continued explosions subsequently occurred within the newly formed Dotsero Crater, forming
a deeper and deeper edifice.
By the time the eruption had ended, the area at the very edge of the crater rim had been
buried by 65 feet thick of ash and volcanic debris.
In hindsight, it might initially appear that the Dotsero eruption was a simple one off
event, meaning it will never erupt again.
Yet, on a hillside 7 miles to the west there is another young looking vent; a gray cinder
cone.
Known as Willow Peak, while this feature has not been dated, I estimate its age to be 80,000
years old.
Using this small sample size, Dotsero might simply be the second vent in a small scale
volcanic field which seemingly erupts about once every 40,000 to 60,000 years.
This means that any region within 8 miles of one of these two vents could be the site
of a future volcanic eruption, although likely tens of thousands of years from now.
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