Neenach Volcano from Wikipedia

Rock formations at Pinnacles NP

Neenach Volcano is an extinct Miocene volcano in the Coastal Ranges of California. After formation, the volcanic field was split by a fault in the San Andreas Fault Zone, and over the last 23 million years, the two halves of the volcano have moved about 195 miles apart.

The correlation of the two portions, now called the Pinnacles and Neenach volcanic formations, is significant; this correlation has become a “classic example” of plate tectonics for geologists. Ten rock types with nearly identical field, petrographic, and chemical characteristics are present in each formation in essentially the same stratigraphic order. This provides ample evidence for the correlation and interpreted movement along the San Andreas Fault.

The volcano formed 23 million years ago during tectonic subduction of the oceanic Juan de Fuca Plate beneath the continental North American Plate, a process often associated with volcanic eruptions near the plate boundaries. Subsequent complex geophysical mechanisms caused a segment of the Juan de Fuca Plate to fuse to the eastern edge of the adjoining Pacific Plate, ending subduction and initiating horizontal shear movement between the plates, with the Pacific Plate moving northwesterly and the North American Plate moving in the opposite southeast direction. This transform movement began approximately 20 million years ago and the resultant shear lineation is the San Andreas Fault Zone. The Neenach Volcano was situated directly over the primary San Andreas fault, causing it to be split in two once the transform fault movement began. The average relative motion along the fault is about 1.5 cm per year, equivalent to six inches per decade. Over the course of this movement, the two halves of the volcano drifted about 195 miles apart and eroded down into the minor rock formations visible today.

The area where the Neenach volcanics are found covers 40 km², west of the Mojave Desert, but the only visible remains are a few outcroppings on hills, comprised of red-colored andesite. Being on private land, they can only be viewed from some distance.

Neenach is one of those small towns no one has ever heard off. From Palmdale, there is a choice of two routes to get there. One is the straight west from Lancaster across the featureless Antelope Valley plain. A few Joshua trees provide the only scenery. The second route follows the slower but more pleasant Elizabeth Lake Road west out of Palmdale, through a straight narrow valley, just behind one of the ridges. The road runs between nice-looking houses and goes past Hughes Lake. The valley narrows to just a few tens of meter in places while you begin to wonder about this peculiar valley. Turns out that you’re following the San Andreas Fault thru this valley. The road takes you up into the hills, past wonderful trees. In these hills near Three Points & CA-138 are the eroded remnants of the Neenach Volcano. As you drop back down onto Antelope Valley you’ll cross the California Aqueduct .

California Aqueduct carries Sierra Nevada water from the Sacramento-San Joaquin Delta up north!

Okay, geology quiz… Which side of that Miocene volcano is Pinnacles NP (about 195 miles north of Palmdale)? Hint: Check out our scenic drive at: CA 25 Road Trip1

Answer: Pinnacles NP is on the Pacific Plate west side of north-south trending CA 25 or SAF. So the Neenach volcanic formation in Palmdale is on the North American Plate or east side of the San Andreas Fault.