The Science of Sustainability

Our Newest National Park: Pinnacles National Monument

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View from the Pinnacles, on the Pacific plate, to the North America plate across the San Andreas fault. Photo courtesy Mike Prasad of Flickr under Creative Commons license

One of the last things the 112th Congress did in 2012 was to enact legislation that turns Pinnacles National Monument into the nation's 59th national park. It will replace Yosemite as the nearest national park to the Bay Area, so we should familiarize ourselves with its wealth of natural attractions — including the geological.

Pinnacles has a lot to offer besides rocks and tectonics. It's a crucial northern outpost of the California condor and a stronghold of good habitat for all kinds of animals and plants. The park is reported, among other things, to be the most species-rich area in the world for wild bees. The park's plant checklist is 20 pages long, covering five major habitats. The views from its namesake peaks are phenomenal, and its night skies are dark as can be.

The pinnacles of Pinnacles are spires of volcanic rocks: rhyolite, dacite, andesite. These are remnants of a small volcanic complex that dates from early Miocene time, about 23 million years ago. That is quite a bit older than the volcanic centers of the Bay Area proper, which date from the late Miocene, but its tectonic origin is the same: the rearrangement of the Pacific and North America plates that created the San Andreas fault system.

The original volcano was probably not right on the new fault but east of it, and it was located near Lancaster in Southern California. We know this because soon after the volcano finished its business, the fault system took hold of it and wrenched it apart. Pinnacles is the western portion. The eastern portion is still down there, where geologists know it as the Neenach Volcanics. That's where I picked up this specimen of rhyolite, just like what you'll see at Pinnacles.

Rhyolite from 3 Points Road. Photo by Andrew Alden

The Pinnacles/Neenach connection, or piercing point, documents about 315 kilometers of movement across the San Andreas fault. It's one of the fault's best-known piercing points.

At some point during its northward journey, the Pinnacles volcanics sank into a basin created by a spur fault. Otherwise it might have been entirely eroded away by now. Instead it's an island of high lava, sitting between Salinian granite on the west and unrelated sedimentary rocks across the fault on the east. This is very similar to the geologic setting of Fremont Peak, up the fault a little ways. Here's the park boundary superimposed on the state geologic map.

Rocks in the park: pink, Mesozoic granite; Ti/Tv/Tvp, Pinnacles volcanics; Mc, Miocene conglomerate.

The erosion of the Pinnacles has created some unusual features, most notably a number of deep crevices that have become covered by large fallen boulders (talus). The cover of boulders, in fact, has rendered the crevices into caves. If you've visited limestone caves and lava tubes, you'll want to add these rare talus caves to your collection. I know I will.

Learn more about Pinnacles geology at the Pinnacles National Monument site. Rock climbers will want to bookmark the Friends of Pinnacles, who work to keep the park at its best.

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Category: Blog, Geology

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Andrew Alden

About the Author ()

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for About.com since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as "Best of the East Bay" from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • SJG

    What is the benefit of turning into a park? I can already see a con: there will be an entrance fee and camping will also cost $. Hmm I think I liked it as a National Monument before…

  • SJG

    What is the benefit of turning into a park? I can already see a con: there will be an entrance fee and camping will also cost $. Hmm I think I liked it as a National Monument before…

  • Jennifer Moline

    There already has been a charge to enter Pinnacles and to camp there. Making it a national park offers Pinnacles more funding and protections.