What you are looking at is an impressive circular feature, which vaguely resembles an impact
crater.
This impressive crater in the ground measures a mile wide, 5000 feet in length, and is 400
feet deep.
However, this feature, called "hole in the ground" is not related to an impact event.
Rather, it is completely volcanic in origin.
This hole is one of several similar explosion craters in central Oregon which formed when
the area was covered with an ancient lake.
Although this vast lake is no longer present, nearby lava flows hint at the possibility
of future activity.
These features are classified as part of the Fort Rock Volcanic Field, which could produce
future explosive eruptions.
This video will discuss these fascinating volcanic features, how they formed, and mention
what future hazards are posed in the region.
The Fort Rock Volcanic Field can be found in Central Oregon, where it is 25 miles east
of Gilchrist.
The namesake of this field, Fort Rock, forms an impressive ring like structure in the center
of the volcanic complex.
So, how did Fort Rock form?
75,000 years ago, much of central and southern Oregon were covered with large lakes related
to the ongoing ice age.
One of these lakes covered almost the entirety of the valley I have shown on screen.
This lake had an area of over 900 square miles, and covered the landscape with an average
of 125 feet of water.
During this period, a volume of magma slowing rose towards the surface underneath the lake,
as this section of Oregon was highly geologically active.
The heat from the magma caused some of the water in the lake to flash to steam.
This pressure continued to build until suddenly a large section of the lake exploded in what
is termed a phreatic eruption.
As pyroclastic flows traveled over the lake into the surrounding landscape, a tall ash
plume was created.
Large boulders ejected from this eruption eventually fell back to the ground, forming
a feature referred to as a tuff ring.
Fort Rock represents this 2700 foot wide tuff ring, and has since been eroded by wind and
water.
By 20,000 years ago, the lake was still present, but had receded from its maximum extent.
To the west of the lake edge, a volume of magma once again rose to the surface.
However, due to the abundance of groundwater in the region, this water was heated and flashed
to steam.
Eventually, the pressure was too much, and a massive explosion occurred.
This explosion started several hundred feet beneath the surface, and pushed upwards, forming
a large crater.
Pyroclastic flows once again raced into the surrounding landscape, and areas within a
few miles were coated with a layer of ash.
After the dust cleared, what remained was a 7800 foot wide explosion crater which had
a depth of 400 feet.
Since this explosion crater interacted with groundwater and not water on the surface,
the feature formed was defined as a "maar" called the "Big Hole".
The most recent and famous feature in this volcanic complex formed 15,000 years years
ago through a similar process, only 5.5 miles to the east of the Big Hole.
As basaltic magma rose toward the surface once again, an explosion occurred, creating
a small crater.
Within the span of several minutes, much of the ash ejected by this explosion fell back
into the crater it originated from, once again causing pressure to build.
Eventually, this pressure was too much, and an even larger explosion occurred.
This explosion was so large and energetic that it threw boulders as large as 26 feet
wide more than 12,000 feet away.
As the dust cleared, what remained was a mile wide maar, called hole in the ground.
Since then, no additional explosive eruptions have occurred.
The existing lake has all but drained away, leaving a grassy valley in its place.
As such, it is impossible for future tuff rings like fort rock to form.
However, it is still possible for maar explosion craters to form, as groundwater is still present
in the region.
Although a cinder cone forming eruption involving the ejection of lava onto the surface is now
far more likely, explosive eruptions can still occur.
On screen, I have outlined the areas most at risk for another explosive eruption, which
includes the town of Fort Rock.
If a future eruption was to occur, it would likely be preceded by a swarm of shallow earthquakes
in the region.
If these were witnessed areas within an 8 mile radius would need to be evacuated.
Overall, the odds of a future eruption are quite low at about 1 in 25,000 each year.
I hope that you enjoyed this video!
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