Initial Publication Date: December 12, 2013

A Field Trip to See a Cross Section of the Crust and Samples of the Mantle

Students on field trip at Joshua Tree National Park

Dexter Perkins
Dept. of Geology and Geological Engineering
The University of North Dakota
Grand Forks ND 58202

Intended Audience: Undergraduate geology majors or members of a geology club.

Location:

This field experience begins in Sonoma County, a few hours north of San Francisco. It moves south along the coast and then cuts inland. The end of the trip is in the Mojave Desert, in southeastern California. Return to San Francisco takes 5 or 6 hours.

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From the Field Guide:

Arrive in San Francisco, pick up cars, and proceed north to the first stop at the Ring Mountain Open Space Preserve in Tibuon. Here you will see various lithologies associated with ophiolites. Continue on to camp in one of the parks near Bodega Bay—Doran Beach is especially nice.
Start with a visit to the classic blueschist locality at Ward Cr., near Cazadero. Then, in the afternoon, visit the eclogite/blueschist on the beach just north of Jenner. Return to campground for the night.
Drive south 5 hours to a campground near Pinnacles National Monument. The Pinnacles Campground on the east side of the monument is nice but expensive. (Alternatively, rustic camping is free in the Clear Creek Rec. Area near the New Idria serpentinite—approach from the south). Be sure to drive Hwy 1 south of Bodega Bay all the way south, rather than going inland. Stop at Pt. Reyes along the way, if time permits. Farther south, stop at the Anderson-Coyote Reservoir to seek xenoliths and/or visit Hollister to see the San Andreas Fault trace in town.
Visit the serpentinite body at New Idria. Collect gemmy jadeite from veins. Late in the day, drive a bit farther south to the spectacular San Andreas Fault outcrops near Priest Valley. Return to Pinnacles for the night.
Visit the classic blueschist localities at Pacheco Pass and at Panoche Pass. In the evening, a two hour hike allows you to do a spectacular loop through Pinnacles National Monument. Return to Pinnacles for night.
Take the back roads to Carmel, then stop to see geology at Pt. Lobos. Continue down the Big Sur coast, visit granulite outcrops along the coast highway and elephant seals at San Simeon. Then camp in one of the many state parks.
Continue south with a brief stop at Morro Rock. Pt. San Luis has spectacular pillow basalts and a great seafood lunch on the dock. Then continue east, perhaps stopping to see San Andreas Fault on the Carizo Plane. Near Palmdale there are some nice exposures of the San Andreas Fault in roadcut. Arfrive in Joshua Tree N.P. late in the day. Group campsites are nice but reserve ahead of time.
Drive the Geology loop in J.T., taking time to hike in to Malapais Hill to see some nice volcanics and collect (small xenoliths). In the afternoon, hike into the Wonderland of Rocks—check with rangers for directions to see spectacular weathered granite and other desert features. Spend night in J.T. again.
Drive north from the park, stopping for a while at Deadman Lake and Dish Hill to see the classic xenolith sites. These sites have mostly been picked over. Drive farther north into the Mojave Preserve and excellent camping at Hole in the Wall.
Visit Kelso Dunes, then drive north of Kelso Junction toward Baker. A right turn (east) on any of several dirt tracks (the one toward the Aiken Mine is best) provides access to many cinder cones where unlimited quantities of xenoliths may be found. They range from granulite to ultramafic. Return to Hole in the Wall for the night.
7 hour drive to Yosemite Valley
Tour Yosemite Valley
Return to San Francisco and home

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Summary:

The fieldtrip guide provided here is a summary of several trips to California that the UND Geology club has taken during the past few years. They have all been combined to provide a comprehensive trip that includes much exciting geology. The stops involved provide a glimpse of most of the major rock types found in the Earth's crust and mantle, and also some good views of the San Andreas Fault. At the start, the trip focuses on the Franciscan Terrane with visits to famous blueschist, eclogite and serpentinite occurrences. After that, the itinerary calls for driving down the California coast where granulites are seen, as well as pillow basalts and other supracrustals. Then, the path moves inland to look at granites exposed in the Mojave Desert region, and also to visit volcanic areas where crustal and mantle xenoliths are found.

Context:

This trip is appropriate for anyone with an interest in geology. A certain amount of background knowledge makes it much more meaningful. So, it is most recommended for geology majors who have completed a standard geology curriculum through a mineralogy and petrology course. UND students that go on this trip are typically juniors and seniors, although there are sometimes a few sophomores.

Goals:

The overall goal of this trip is to show students many of the things that they have heard about in their geology classes, but have not had a chance to see in the field. California has it all, and the fantastic scenery on the coast (and inland) is enough to keep everyone excited and motivated.

Design:

The trip was originally designed by making a list of some of the most highly recommended field stops in California, and then putting them in context. It was later modified as UND students and professors began to do research on crustal and mantle xenoliths. Subsequently, it was modified again to include only those places that are generally accessible during the early spring, with the possible exception of Yosemite National Park (which can be iffy in the spring).

Notes and Tips:

It is highly recommended that anyone going on this trip contact some of the geologists that have been referenced before going. Although directions are given, some of the stops and outcrops are not easily found. In other cases, private property concerns can be a problem. Finally, it is important to remember that there are lots of people in California. If you plan to go to some of these places during peak season, be prepared for traffic and delays and be sure to make campground reservations ahead of time.

Assessment and Evaluation:

Normally, not much follow up evaluation is done. The assumption is that everyone who goes on the trips will benefit from the experience.

Materials and Handouts:

Deformed rocks associated with the San Andreas Fault near Priest Valley, CA

All the material normally given to students is in the field trip guidebook (Acrobat (PDF) 2.7MB Apr3 06), except for a few appendices that have been published elsewhere. References are given so that material may be acquired.

References to Supplement Field Guide:

For key information about the xenolith localities:
Wilshire et al. (1985) Mafic and Ultramafic Xenoliths from Volcanic Rocks of the Western United States. U.S.G.S. prof. Paper 1443.
For a comprehensive summary of the geology near Cazadero (Ward Cr.):
Erickson, R.C. (1995) The Geology of the Franciscan Complex in the Ward Creek-Cazadero Area, Sonoma county, California. California Geology, November-December 1995.
Seminal paper on blueschists in the Diablo Range:
Ernst, W.G. (1965) Mineral Parageneses in Franciscan Metamorphic Rocks, Panoche Pass, California. GSA Bull. 76, 879-914.
More on blueschists in the Diablo Range: NCGS Field Trip to the Franciscan Metasedimentary Section at Pacheco Pass. Reported by Richard Cardwell. Photos by Mark Detterman and Keil Albert.
For a map and geochemical/petrolgical information on the Ring Mountain outcops: Saha, Basu, Wakabayashi, and Wortman (2005) Geochemical evidence for a subducted infant arc in Franciscan High-grade-metamorphic tectonic blocks. GSA Bulletin, v 117, 1318-1335..
To navigate the New Idria serpentinite area:
Coleman, R.G. (1986) Field Trip Guide Book to New Idria Area, California. 14th General Meeting of the International Mineralogical Association, July 1986.
For more details about seeing the San Andreas Fault in Hollister: http://sepwww.stanford.edu/oldsep/joe/fault_images/Hollister.html
Finally, these three provide more information on some of the stops in this guide, and also contain many additional stops you may care to consider:
- Alt and Hyndman (2000) Roadside Geology of Northern and Central California. Mountain Press.
- Alt and Hyndman (1975) Roadside Geology of Northern California. Mountain Press
- Sharp and Glazner (1993) Geology Underfoot in Southern California. Mountain Press.

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