Neenach Volcano is an extinct

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

in the Coastal Ranges of California. After formation, the volcanic field was split by a fault in the

San Andreas Fault Zone The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). ...

, and over the last 23 million years, the two halves of the volcano have moved about 195 miles (314 kilometers) 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 the scientific community. Ten rock types with nearly identical field, petrographic, and chemical characteristics are present in each formation in essentially the same

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...

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 A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subductio ...

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 (314 kilometers) apart and eroded down into the minor rock formations visible today.

Pinnacles

The remnants of the western portion of the volcano lie in

Pinnacles National Park Pinnacles National Park is an American national park protecting a mountainous area located east of the Salinas Valley in Central California, about east of Soledad, California, Soledad and southeast of San Jose, California, San Jose. The park's ...

(). The most prominent outcrops in the park (High Peaks, Balconies, Machete Ridge and other prominent but unnamed megaliths) consist primarily of eroded

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

flows or air falls, including volcanic

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

lava flows Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

and volcanic

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

. Remnant andesite lava flows or dikes are also present at a few locations in the park, including the Tourist Trap climbing area and along the West Fork Chalone Creek. The park encompasses approximately 42 square miles and the Neenach Volcanics represent about 20 square miles of the total.

Neenach

The remnants of the eastern portion of the volcano, called the Neenach Formation, lie about 195 miles (314 kilometers) south of the western portion near

Neenach, California Neenach ( ) is an agricultural settlement in northwestern Los Angeles County, California, with a population of about 800.The U.S. Census does not break out a separate figure for Neenach. The county registrar said in 1991 that the voting distric ...

. (). The formation covers about 10 square miles and is notable for the distinct and strongly linear boundary along its southerly side, where the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...

split the original volcano. This entire southern boundary is accessible by Pine Canyon Road (Los Angeles County Route N2) between Sandberg and Three Points. Surface rock exposures in the formation appear to consist primarily of

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

flows, which have weathered into nondescript low hills with few outcrops, unlike the dramatic megaliths in the northern portion. Satellite imagery reveals several areas of exposed rhyolitic tuff and lapilli tuff, a light green pyroclastic rock that also occurs in Pinnacles National Park and was used to construct the visitor center and other structures there. Although the entire formation appears to be on private property, a segment of the

Pacific Crest Trail The Pacific Crest Trail (PCT), officially designated as the Pacific Crest National Scenic Trail, is a long-distance hiking and equestrian trail closely aligned with the highest portion of the Cascade and Sierra Nevada mountain ranges, which lie ...

transects the Neenach from southwest to northeast, with trailheads at the intersection of Lancaster Road (County Road 138) and 269th Street West and along Pine Canyon Road about 0.9 mile east of Horse Trail Campground, which is also on the PCT.

References

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External links

Pinnacles National Monument

Volcanoes of California Extinct volcanoes of the United States Pinnacles National Park Miocene volcanoes