PINNACLES NATIONAL PARK, CA  

 

FIELD TRIP STOP - Volcanics moved by the San Andreas Fault

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LOCATION: Pinnacles National Park has two entrances, one on the east end of the Park and one on the west.  Only hiking trails connect both sides of the Park. The east entrance can be accessed from Hwy 25 and then west on Hwy 146.  The west entrance can be accessed from Hwy 101 and then east on Hwy 146. The driving distance between entrances is over 50 miles.

 

A WORD OF CAUTION: Pinnacles is unlike most National Parks. The west entrance is largely a single lane road and can be treacherous, especially if a large RV (not recommended by the NPS) is approaching you from the opposite direction.  In addition, the roads within the Park (and areas to park your vehicle, especially near trailheads) are limited. Also, roads do not wind through rock outcroppings as is common in many National Parks. Closer views of geological outcrops must be accessed by use of well-maintained hiking trails.  Another cautionary note is that hiking is discouraged in the summer months due to extreme heat and lack of available water.

 

GEOLOGIC FEATURES: San Andreas Fault; Rhyolite; Breccia; Pyroclastics; Tertiary; Miocene; Farallon Plate; Pacific Plate; Pinnacles Fault; Chalone Fault; Talus

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DESCRIPTION: Pinnacles N.P. is largely composed of 22-23 myo (Tertiary: Miocene) volcanic rock.  Magma was generated after the Farallon Plate was subducted under the North American Plate.

 

When the Farallon Plate was completely overridden by the North American Plate, magma from the newly subducted Spreading Center (with the Pacific Plate to the west and subducted Farallon Plate to the east) rose to the surface forming volcanoes of the Pinnacles Volcanic Field.  The Pacific Plate then ground against the western edge of the North American Plate forming a Transform Fault (San Andreas Fault).  The San Andreas Fault split the volcanic chain and moved 2/3 of the Pinnacles Volcanic Field about 195 miles north of its origin, located near Lancaster, CA.  We know this because of the similar unique lithology between the Pinnacles Volcanics to the north and the Neenach Volcano deposits on the east side of the fault nearly 200 miles to the south near Lancaster.

 

Volcanics were deposited above the granitic basement of the North American Plate and are composed of Rhyolite lava overlain by Pyroclastic Breccia. The layers of breccias are thought to have formed as the result of material slumping off the sides of the volcano near the vents.

 

The present day San Andreas Fault lies just east of the park.  As such, Pinnacles N.P. provides an exceptional example of tectonic plate movement along the San Andreas Fault. Two major faults occur within the park, the Pinnacles Fault near the western boundary and the Chalone Fault near the eastern boundary. The latter fault is thought to have occupied the position of the San Andreas Fault cutting through the Neenach Volcano before the San Andreas Fault shifted its present position to the east of the park.

 

Weathering and erosion, mainly of the Pyroclastics that overly the Rhyolitic flows, shaped the volcanics into the pinnacles and other rock structures observed today. Talus slopes from the eroded Pinnacles create caves popular among visitors.

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STUDENT QUESTIONS:

(1) Assuming that the Pinnacles Volcanic Rock has been displaced about 195 miles northward in 23 m.y., calculate the average rate of movement along the San Andreas Fault. 

(2) The San Andreas Fault is called a Right-Lateral Strike Slip Fault due to its relative movement on either side of the fault. Describe what is meant by this type of fault.

(3) CHALLENGE: How is the formation of Pinnacles N.P. similar in origin to that of Morrow Rock (found within this website) and why are the rocks at Pinnacles younger than those of Morro Rock (23 vs 27 myo)?

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SELECTED REFERENCES:

--Ball, Jessica. 2023. The story of the faulted field: How Pinnacles ended up nearly 200 miles north of its birthplace. USGS, California Volcano Observatory. https://www.usgs.gov/observatories/calvo/news/story-faulted-field-how-pinnacles-ended-almost-200-miles-north-its

--Ludington, Steve and Karen Gray. 1987. Petrochemical Studies of the Pinnacles Wilderness Contiguous Wilderness Study Area, Monterey and San Benito Counties, California. United States Department of the Interior, Geological Survey, Open-File Report 87-135, 11 pp.  https://pubs.usgs.gov/of/1987/0135/report.pdf

--NPS. Pinnacles National Park – Faults. Accessed on Aug. 6, 2023 from https://www.nps.gov/pinn/learn/nature/faults.htm

--NPS. Pinnacles National Park – Geology. Accessed on Aug. 6, 2023 from

https://www.nps.gov/pinn/learn/nature/geology.htm

--NPS. Pinnacles National Park - How Pinnacles Formed. Accessed on Aug. 6, 2023 from

https://www.nps.gov/pinn/learn/nature/how-pinnacles-formed.htm

--Science Partnership. Where did Pinnacles National Park Come from? Accessed on Aug. 6, 2023 from https://teachearthscience.org/Resources/Pinnacles.pdf and https://teachearthscience.org/Resources/PinnaclesKey.pdf

--US-Parks.com. Pinnacles National Park Geology. Accessed on Aug. 6, 2023 from https://www.us-parks.com/pinnacles-national-park/geology.html

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TEXT-FIGURES:

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TEXT-FIGURE 1 - FORMATION OF MORRO ROCK AND PINNACLES NATIONAL PARK

In Figure B, Spreading Center "S" has subducted under the North American Plate and led to the formation of a Transform Fault (San Andreas).  Volcanism produced by Spreading Center "S" forms the California Coastal Vo;lcanics ('v") - Morro Rock and Pinnacles National Park.

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