The state of California contains the 3rd most active or potentially active volcanoes in
the United States, totalling 11 volcanoes.
These range from the little known Salton Buttes volcano in southern California which is associated
with fumaroles and mud pots, to the Long Valley supervolcano, the towering lava domes of the
Mono Inyo field, the glaciers on Mount Shasta, or even the Lassen Peak volcano which last
erupted in 1917.
Yet, these are not the only California volcanic features of a fairly recent geologic origin.
As, if you go offshore to the west of the state and into the Pacific Ocean, you will
find a series of towering seamounts, with one example being the Davidson Seamount.
This seamount is indeed an ancient but now extinct volcano which towers more than 7200
feet from the ocean floor, covering nearly 180 square miles.
Other examples include the Gumdrop, Pioneer, and Guide Seamounts, which you might notice
all are elongated in a north-northeast orientation.
But, something is seemingly missing from this cluster of volcanoes; despite having the profile
of large submarine shield volcanoes, they do not contain a summit caldera or series
of collapse craters.
Yet, there are also a series of similarly aged seamounts which are aligned in a perfectly
perpendicular direction towards the east-southeast.
The most notable of these are the cluster of 5 Taney Seamounts which stretch across
a 40 mile long trend, with the oldest vents being towards the northwest end and the youngest
vents being at the southeast end.
Altogether, within 200 miles offshore of central and southern California there are at least
25 seamounts which altogether formed in about the same geologic period.
These Seamounts which are all extinct formed almost all collectively between 26 and 10
million years ago with one seamount, the San Juan Seamount, erupting until 2.8 million
years ago.
Yet, as we can see by the modern plate boundaries, these volcanoes did not form due to the collision
of two tectonic plates, nor did they form due to the spreading apart of two tectonic
plates at what is known as a mid ocean ridge.
Rather, these seamounts could all be considered the remnants of intraplate volcanoes, much
like how Sunset Crater which is part of the San Francisco volcanic field in Arizona is
an intraplate volcano.
In the case of the Central California Seamounts, their origin can in fact be traced to the
absense of the San Andreas Fault.
The San Andreas fault in California is a strike slip fault that marks the boundary between
the North American Plate to the east and the Pacific Plate to the west.
Yet, this infamous fault line did not always exist as it only began forming 12 million
years ago and had largely expanded to its modern length around 7 million years ago.
Note how this period seemingly coincides with when the majority of seamounts ceased their
volcanic activity; this is no coincidence.
As, the seamounts which trend in a north-northeast direction represent a section of the crust
that once was a mid ocean ridge.
There, the crust spread apart to the east and west, allowing for molten rock in the
mantle to erupt onto the surface via a series of fissures, forming new oceanic crust.
However, this spreading did not always occur in an even manner at different segments, causing
what are known as transform faults to form between them with fracture zones forming parallel
with them.
The transform faults would be the site of eventual east-southeast trending seamounts
such as the Taney Seamounts.
Yet, these seamounts did not form right on top of an active plate boundary, as they instead
began erupting around 8 million years after the mid ocean ridge and transform faults vanished,
as they would go on to represent weak points in the crust.
These weak points, much like two major fault lines which border the Uinkaret volcanic field
in Arizona would go on to provide a path of least resistance for chunks of the upper mantle
which just so happened to be slightly warmer than the surrounding regions.
This molten rock intruded into the crust before erupting onto the ocean floor, forming chains
of volcanoes, some of which had calderas or pit craters, over millions of years.
Thanks for watching.
If you would like to request a specific topic, please leave a comment below.
Additionally, I would like to thank Gregory Miller for becoming a new patron on this channel's
Patreon page!