In the aftermath of a powerful and several month long volcanic eruption a mere 938 years
ago in the United States, a 142 square mile swath of land was made essentially lifeless.
As, this represented the area covered by more than a foot thick of volcanic ash, with a
smaller area closer to the volcanic vent which erupted coated by as much as 20 feet thick
of ash.
What remained within the broader region was the charred remnants of a thick pine forest,
having been largely burned to the ground by the heat of advancing lava flows, which covered
3 square miles of ground and were still semi molten a few weeks after the eruption ended.
In this devastated landscape, there were even still steaming pyroclastic flow deposits,
having been produced by superheated currents 1000 degrees Fahrenheit in temperature that
traveled more than a mile and a half from the erupting vent, incinerating everything
in their path.
Several thousand native people fled the area due to this eruption, who witnessed towering
eruption plumes from far away as they reached up to 66,000 feet above sea level, blocking
out the sun.
While it might sound like what I am describing is the ancient aftermath of a lesser known
yet powerful explosive volcanic eruption of Mount Saint Helens, or perhaps another stratovolcano
which has produced plinian eruptions such as Mount Edgecumbe in Alaska, this is surprisingly
not the case.
As this eruption did not originate from a U.S. state directly bordering a section of
the Pacific Ocean, but rather, the interior of Arizona.
This eruption occurred not from a stratovolcano, but rather quite strangely, a cinder cone
north of the modern town of Flagstaff which is today called Sunset Crater.
States ranging from eastern California in the Mojave Desert, to New Mexico, Utah, Nevada,
and Arizona altogether consist of more than 1,000 cinder cones which have erupted in the
last 6 million years.
These vents all erupted once and then never erupted again, occurring in numerous different
monogenetic volcanic fields.
Cinder cones are the most predictable type of volcanic vent as they almost all produce
moderate ash emissions, lava fountains, medium length lava flows, and intermittent strombolian
explosions which throw bombs of rock in a several thousand foot radius.
Thus, for most erupting cinder cones around the planet, supposing deadly gas was not blowing
in your direction and you weren't close to recent or active lava flows, you could in
my opinion generally stand a mile away from the erupting vent and be completely safe.
These eruptions are almost always strombolian in nature like seen during the 2021 eruption
of Cumbre Vieja on the island of La Palma, although sometimes cinder cone forming eruptions
also contain weak vulcanian components to them.
Yet, they are generally considered only moderately explosive eruptions, not involving pyroclastic
flows, and generally creating an eruption column no more than 10,000 feet in height,
while only having a volcanic explosivity index of a 1 or 2.
Yet, the 1085 CE eruption of Sunset Crater was 2 orders of magnitude more explosive than
a typical cinder cone forming eruption as it had a volcanic explosivity index of a 4,
created pyroclastic flows, an up to 66,000 foot high eruption column, and ejected 0.28
cubic kilometers of tephra.
So, why was Sunset Crater the most explosive cinder cone in existence, having eruptive
features similar to the most explosive of stratovolcanoes?
You might think that this explosiveness was owed to the viscosity of the lava, as more
silica rich lavas such as dacite and rhyolite are generally created in far more explosive
eruptions than those of less silica rich lavas such as basalt and picro-basalt.
Yet, Sunset Crater only erupted a traditionally nonexplosive rock type known as basalt, but
was also far more explosive than a young cinder known as SP Crater which erupted a more viscous
type of lava known as basaltic andesite.
Instead, a completely separate factor caused Sunset Crater to be the planet's most explosive
cinder cone, even producing a sub-plinian eruption phase; as the eruption involved an
absurdly high amount of gas, specifically carbon dioxide and sulfur dioxide.
The more volatiles and gas a volcanic eruption has, the more explosive it will be.
Sunset Crater's source magma was not only unusually gas rich, but at a depth of approximately
10 miles the carbon dioxide bubbles were already coming out of solution, causing the molten
rock to shoot upwards at a high rate as these gasses expanded at shallower and shallower
depths.
Additionally, this magma melted through several limestone units which released significant
amounts of carbon dioxide.
This gas expansion caused the magma conduit to repeatedly disintegrate and expand on its
edges, creating an abundant supply of already pulverized rock for the eventual eruption.
All of this allowed for pyroclastic flows and an unusually tall eruption column to be
generated.
As a final note, I would like to thank my new patron Barrett Keller for supporting this
channel!