The most volcanically active area within California is located near the town of Mammoth Lakes.
Four separate volcanoes, including the long valley supervolcano and the Mammoth Mountain
volcano are located in the vicinity.
Altogether, these volcanoes have erupted 8 times in the last 1,000 years, with the most
recent eruption occurring at Mono Lake in 1890.
These volcanoes have created many beautiful features including hot springs, lava domes,
and maar explosion craters.
One of the many beautiful features created by this long chain of volcanoes is a grouping
of volcanic columns.
These columns have a beautiful hexagonal patterning to them, and altogether are known as Devil’s
Postpile.
They took to be man made due to their geometric shape yet are completely natural in origin.
So, how did this volcanic outcropping form?
This video will discuss this geologic oddity which is protected as part of Devils Postpile
National Monument.
The Devils Postpile National Monument can be found in central eastern California near
the border with Nevada.
Specifically, it is 6 miles west of the town of Mammoth Lakes.
First, some clarification.
In this section of California there is a long, tabular body of magma underground called a
dike.
This dike has created a 25 mile or 40 kilometer long zone of activity which has created three
separate volcanoes.
These are Mammoth Mountain, Mono Inyo, and Mono Lake.
In all likelihood, these distinct volcanoes could technically be classified as a single
volcano, but that isn’t important for the topic of this video.
This volcanic chain initially began erupting dark colored basaltic lava approximately 400,000
years ago on the western edge of the Long Valley caldera.
This fresh lava contrasted with the much lighter colored rhyolite lavas of the much larger
supervolcano, allowing for the 2 volcanic systems to be easily differentiated.
Over tens of thousands of years, the line of potential eruptions expanded to both the
north and south.
Then, 80,000 years ago, a fresh volume of low silica magma rose towards the surface.
After a series of earthquakes related to the underground movement of this magma, it erupted
in a spectacular lava fountain.
As large quantities of lava gushed out of this newly formed vent, a river of lava was
created which flowed into a narrow valley.
Over the span of several weeks, a dark black lava flow piled up within the valley until
it was approximately 500 feet or 152 meters thick.
Eventually, this eruption ended.
Due to the thick mass of recently erupted lava, it took a very long time for it to cool.
The lava at top cooled first.
It consisted of numerous evenly spaced centers.
As it cooled, the lava contracted towards these centers, in an even manner.
Due to the overall crystal structure, this occurred from 6 different sides.
Soon, the lava cracked as it contracted, resulting in the creation of numerous closely spaced
hexagons.
Over time, these primarily hexagon fractures propagated downwards to the rest of the overall
lava flow.
Later, during the last glacial period, glaciers swept over the landscape, removing a large
chunk of the top section of these basalt columns.
After these retreated what remained was a rounded top where hexagons are exposed.
What you see today largely represents the bottom of the overall lava flow.
Overall, these basalt columns are quite large.
Some measure more than 3.5 feet in diameter and 60 feet long.
Due to the near vertical deposition of these columns, they look like tall posts stacked
in a pile.
As a final note, other interesting basaltic columns can be found at Devil’s Tower in
Wyoming, Giant’s Causeway in Ireland, and Panska Skala in the Czech Republic.
These beautiful features can form almost anywhere there is a thick basaltic lava flow.
I hope that you enjoyed this video.
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