California is host to a number of large active volcanoes such as Mammoth Mountain and Mount
Shasta.
However, towards the northern part of the state, there are a large group of lava fields
which few are aware of.
These lava fields stretch between Lassen Peak to the south and Medicine Lake to the north,
and contain 6 different volcanoes.
Although none of these volcanoes have erupted in the last 10,000 years, they have each produced
eruptions recent enough to create black lava flows visible from satellite.
Of these volcanoes, the most recent to produce an eruption was Brushy Butte, which is a complex
of overlapping shield volcanoes.
Since this volcano has been active for more than 1 million years, it will eventually erupt
again.
The Brushy Butte volcanic complex is located in the northeast corner of California.
Specifically, this volcano is 68 miles northeast of the city of Redding.
Just like Lassen Peak to the south, the small Brushy Butte volcanic complex is considered
to be part of the Cascade Mountain range.
The reason why this volcano exists is due to the collision of the Juan de Fuca plate
underneath the North American plate offshore of California.
The melted crust from this collision then migrated upwards, before erupting on the surface
and forming a chain of volcanoes.
The Brushy Butte volcanic complex began forming 1.34 million years ago, when magma erupted
onto the surface via a small regional fault, thus forming a chain of small spatter cones.
Over the span of several years, 5 square miles were covered with a thick layer of runny lava,
sometimes reaching up to 150 feet thick.
The next eruption from this volcano occurred 300,000 years later 5 miles to the north,
and created a thin layer of lava in a week long eruption.
Beginning approximately 600,000 years ago, eruptions at this volcanic complex became
far more frequent.
One notable eruption occurred 570,000 years ago, which began as a large volume of magma
pushed upwards towards the crust, where there was a small lake on the surface.
Eventually, the heat from this magma caused much of the lake water to flash to steam,
and subsequently explode in a massive eruption.
Due to the nature of this explosion, the vast majority of ash fell back to the ground close
to the crater formed by this eruption.
By the time the eruption had ended, a 350 foot tall and 3100 foot wide volcanic cone
remained which was called a tuff ring.
Over the next 400,000 years, intermittent effusive eruptions deposited lava across the
field, but none were large enough to form the outline typically seen at a shield volcano.
Then, 190,000 years ago, this would change as lava began erupting in a new central location.
If you zoom in on the center of the modern lava field, you can see a number of small
craters with increased vegetation on the bottom.
For example, the crater shown on screen is 1000 feet wide and 50 feet deep.
This crater, like the others in the region, used to be the site of a large lava pool which
would overflow during a decade long eruption.
After the eruption ended, the lava drained away, and the overlying ground collapsed like
a sinkhole.
To the east of this crater you can see a number of nearby lava tubes.
During several decade long eruptions, the gentle slope of the volcano was not enough
to carry the erupted lava more than a mile or two.
As a result, much of the lava went underground, and carved its way through the rock at a much
steeper angle, causing it to eventually erupt at a more distant location.
After the eruption stopped, the lava drained away, forming a system of caves.
In total, 29 distinct eruptions occurred in a span of 190,000 years.
Over time, these eruptions created an 11 mile wide and 550 foot tall shield volcano called
Brushy Butte.
The most recent of these occurred 18,000 years ago out of the raised spatter cone shown on
screen.
Due to the long geologic history of this volcano, it will erupt again.
Over its more than 1 million year long history, an eruption has occurred an average of once
every 37,000 years.
The next eruption from this volcano will likely originate from the same location as its last
eruption, and deposit a new layer of fluid lava over a wide area.
Lavas from such an eruption could travel up to 6 miles away, eventually overrunning nearby
towns.
With this being said, if this volcano was to erupt again, its eruption would most likely
be completely safe to view.
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