Within the United States there are three supervolcanoes which are considered to be active.
The most famous of these is the Yellowstone volcano in Wyoming.
However, there is also the Long Valley volcano within California, and the Valles caldera
in New Mexico.
Yet, if you go back to between 15 and 35 million years ago, supervolcanoes were surprisingly
abundant in Colorado, New Mexico, Nevada, Utah, and Arizona.
One of the ancient supervolcanoes in Arizona can be visited via the Chiricahua National
Monument where rhyolitic ignimbrite hundreds of feet thick was deposited in a single eruption.
One way to tell that you live in an area which was once effected by a voluminous supervolcano
related eruption is a layer of volcanic rock that is a mixture of tuff and ignimbrite which
is hundreds to thousands of feet thick.
If you live in or around Kingman Arizona, this image of a thick volcanic deposit will
likely look familiar to you, especially due to a prominent banded layer of material visible
in several roadcuts.
Known as the Peach Springs Tuff, it is an eruption deposit more than 25 feet thick in
Kingman which originated in a catastrophic supervolcanic eruption 18.8 million years
ago.
This supervolcano was not located within Kingman but rather a location 25 miles to the west-southwest,
just west-northwest of the city of Oatman.
Known as the Silver Creek Caldera, this long extinct volcano produced a massive eruption
18.8 million years ago which devastated an area larger than the entire U.S. state of
New Jersey, rendering it uninhabitable for some time.
To understand just how catastrophic this eruption was, I have overlayed the area significantly
effected, albeit sourced from a scientific paper referenced on screen, which experienced
voluminous and long distance pyroclastic flows.
For context, pyroclastic flows are a fast moving mixture of superheated volcanic gas,
ash, and fragments of lava which race across the landscape surrounding a volcano at speeds
of sometimes more than 100 miles per hour, and generally travel downhill.
For comparison, the 1980 eruption of Mount Saint Helens was catastrophically large, ejecting
1.3 cubic kilometers of volcanic rock.
Yet, its pyroclastic flows only traveled a mere 5 miles from its summit while its lateral
blast travelled up to 19 miles distant.
In the case of the Silver Creek Caldera, depending on the exact direction, pyroclastic flows
travelled between 27 and 154 miles distant, making it to where the modern cities of Searchlight,
Lake Havasu City, Parker, Needles, and just to the very edge of the city of Barstow.
And, this area was covered in an average thickness of 40 meters or 131 feet of volcanic rock,
with the thickest layers of material being around 220 meters or 722 feet thick.
However, not much is currently known about the geologic history of its source caldera.
However, it is quite likely that while a large volume magma chamber was building in the crust
and using the fault line as a path of least resistance, small eruptions likely occurred
on the surface.
These eruptions likely produced explosion craters known as maars from within which viscous
flows of lava would erupt, forming rhyolite lava domes.
After tens and perhaps hundreds of thousands of years of only minor volcanic eruptions,
something triggered a massive release of pressure from the volcano.
Suddenly, a large volume of magma raced towards the surface, being propelled by a high concentration
of gas, which generated several magnitude 6 earthquakes in rapid succession.
Then, several vents along a chain of activity erupted simultaneously, blasting a plume of
ash quickly to a height of 164,000 feet or 50 kilometers into the atmosphere, potentially
higher.
Part of the eruption column then collapsed downwards, creating up to mile high pyroclastic
flows which seemingly defied topography and even raced up mountains.
These pyroclastic flows quickly sped across the landscape for a lengthy period of time,
which per a scientific paper referenced on screen, moved at an average speed of between
11 and 44 miles per hour.
Also per the paper, this was supplied by an eruption rate of around 50 million cubic meters
of material per second, which at that rate would have lasted for at least 2.5 to 10 hours.
Then, due to the massive amount of material which was ejected, the magma chamber was severely
drained.
This caused the rock overlying it to collapse downwards, forming a massive caldera.
In total, at least 1,300 cubic kilometers of volcanic rock was ejected meaning the eruption
had a volcanic explosivity index of an 8.
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