California has a number of strange volcanoes which have erupted at some point since the
end of the last ice age.
This includes the fumaroles and mud volcanoes of the very small Salton Buttes volcano adjacent
to the Salton Sea and the Lassen Peak volcano which last erupted in 1917.
Yet, if we go back further in the state's geologic history we can find a truly remarkable
geologic oddity.
What you are looking at is the remnant of a long extinct volcano northeast of Greenfield,
California.
Known as the Neenach volcano, it contains thousands of remarkable pillars of primarily
rhyolitic lava and ash, being protected in the Pinnacles National Park.
Yet, this is only half of the volcano.
So, where is the other half?
This other half is quite aways distant and was not transported due to a large volume
landslide.
Instead, it is located 195 miles to the southeast in part of the Mojave Desert west of the city
of Lancaster.
And, if you line up these two features which are seemingly cut in half, they align quite
well.
So, how did these features end up so far apart?
The answer is the most infamous geologic feature in California; the San Andreas Fault.
As a reminder, there are four types of fault lines.
3 of them have a large vertical motion component to them while the fourth, a strike slip fault
moves primarily horizontally.
The San Andreas fault happens to be one such strike slip fault which is slightly more than
700 miles in length, stretching from the eastern section of the Salton Sea all the way to 30
miles west-southwest of the city of Ferndale at an offshore location, where it connects
with the boundary of the Juan de Fuca Plate.
The San Andreas fault represents the boundary of two tectonic plates where to the east is
the North American Plate and to the west is the Pacific Plate.
This right lateral strike-slip fault moves at a rate of approximately 2 inches a year
which is about the same rate at which fingernails grow.
Thus, although it has been at different average speeds in the many millions of years it has
existed, a total of 23 million years of movement separated the two parts of the Neenach volcano
by 195 miles, representing an average yearly speed during that time of 0.537 inches or
1.364 centimeters.
And, both sections of the fault moved at about the same rate meaning that the original location
of this volcano is now 16 miles west of where the Missouri Triangle area is in California.
When the Neenach volcano's first eruption began 23 million years ago, it started with
a bang via a series of powerful explosions due to the type of lava it erupted, rhyolite.
Since rhyolite has a very high viscosity it is difficult for underlying gas to escape
so it often builds up in pressure until a threshold is reached, generating an explosion.
These explosions created pyroclastic flows, traveling up to 5 miles distant and coating
the landscape in ash.
As the eruption continued, a rhyolite lava dome began to grow which was about half as
tall as it was wide.
This lava dome would experience additional partial collapses, creating breccia and more
pyroclastic flows.
The next eruption from this volcano occurred in a different location 7 miles to the north
as a dominant central vent had not yet developed.
Instead, the next several dozen eruptions occurred from scattered vents which each erupted
once in various locations across an area of 100 square miles in what was termed the Neenach
Volcanic Field.
Eventually, less silica rich rocks began to erupt and occasional less viscous dacite and
andesite lava flows erupted.
A dominant vent developed within the next 200,000 years, and this much like Lassen Peak
was built to a height of more than 5,000 feet above its surrounding landscape.
Due to the nature of these very steep lava domes, even when an eruption wasn't occurring
landslides were somewhat frequent.
After the volcano became extinct it was split by the San Andreas fault and sections of it
moved 195 miles apart.
As a final note, the Neenach volcano is not the only ancient volcano to have been split
by faulting.
As, in Arizona, the ancient Hamblin volcano over 13 million years split into three separate
pieces.
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