What you are looking at is the spectacular and deep remains of the formerly 12,000 foot
tall Mount Mazama volcano in Oregon.
Whereas it used to be a very tall stratovolcano, it experienced a catastrophic eruption in
5680 BCE, where 120 cubic kilometers of volcanic rock was ejected.
This drained the volcano's underlying magma chamber, causing the ground overlying it to
collapse downwards to fill the empty space, forming a massive caldera.
While this 8 to 10 kilometer or 5 to 6 mile wide depression in the ground is quite remarkable,
it is not the only recent caldera to form within the state.
To the northeast is the Newberry volcano, which is the 2nd largest volcano by volume
in the Cascade Range.
While the vast majority of active volcanoes in the Cascade Range are stratovolcanoes meaning
they generally have steep slopes, the Newberry volcano is instead a massive shield volcano.
However, unlike the shield volcanoes in Hawaii, Newberry has a history of producing highly
explosive eruptions.
For example, its last eruption in 690 CE was 1/4th the size of the 1980 eruption of Mount
Saint Helens, and caused ash to fall as far away as in Idaho.
This eruption produced a highly unusual solid black lava flow that contained millions of
tons of obsidian.
While this eruption was quite large, it does not compare to the size of the same volcano's
caldera forming eruptions.
Newberry's caldera measures 8 kilometers long and 6 kilometers wide, containing two large
lakes, the Big Obsidian lava flow, and an assortment of other recent eruptive centers.
This caldera has formed through not one but two large explosive eruptions.
One such eruption occurred 300,000 years ago, and the most recent one occurred 75,000 years
ago.
Both of these eruptions were approximately the same size, each ejecting around 10 cubic
kilometers of tephra in VEI 6 eruptions.
While the Newberry volcano has been erupting intermittently for more than 600,000 years,
the actual shield only began forming 400,000 years ago.
Fountains of lava erupted spectacularly onto the surface, then fell back to the ground
where they moved at an unusually fast rate due to their low viscosities.
Since lava could easily travel several miles, it constructed a wide volcano with gentle
slopes instead of a narrower volcano with steeper slopes as it typical for volcanoes
with higher viscosity lavas.
By 300,000 years ago, Newberry had been built to an impressive height of around 9,000 feet
above sea level.
For comparison, this is 1,000 feet taller than its highest point today.
While almost all of the erupted lavas at that time had a composition of basalt, an underlying
rhyolitic composition magma was building underneath the volcano.
Eventually the pressure and gasses in this chamber became too great, triggering a highly
explosive eruption.
A similar process occurred 75,000 years ago, deepening the existing caldera.
Although we aren't exactly sure how those two eruptions would have looked, I will be
using some modelling based on similar eruptions and known evidence.
For example, the earlier of the two caldera forming eruptions deposited an unusually thick
layer of material on the volcano's eastern flanks.
While this could be simply due to prevailing winds at the time of the eruption, it could
also mean that the main vent originated in the eastern section of the caldera.
Here is likely what occurred during the massive eruption 75,000 years ago.
As a massive amount of magma intruded into the shallow crust, it interacted with the
area's water table.
This caused a significant quantity of water to flash to steam, eventually triggering a
powerful explosion.
As the eruption began, a 30 kilometer or 98,000 foot high plume of ash rose high into the
atmosphere.
As part of the eruption column collapsed downwards, it produced unusually energetic pyroclastic
flows which raced down Newberry's flanks.
These flows likely travelled between 15 to 20 kilometers distant, reaching the edge of
La Pine, Three Rivers, and Sunriver.
Then, due to the large amount of erupted material, the volcano partially collapsed, losing more
than 1,000 feet in height.
Some of Newberry's flanks were likely covered by more than 20 feet thick of ash.
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