San Diego Geology
Geological Diversity
The rocks, mountains, faults, and fossils found in San Diego County make it one of the most interesting and diverse geological hot spots in California. From the mysterious cobbles that are found in many San Diegan's yards, the large plutons of tonalite and gabbro (not granite) in the backcountry, the colorful sedimentary layers along the La Jolla coastline, the treasured history of mining tourmaline and gold, ancient andesite volcanoes, to the fossils of Eocene monsters, San Diego County's geology nearly matches the region's biodiversity.
The region is also home to some famous geologists who have contributed significant amounts of research and inspiration to the field. The San Diego Association of Geologists is the organization many call home.
Time to pack up the rock hammer, knapsack, and preferred beverage, and explore San Diego's geological wonders.
The amazing Ramona Pluton Complex around Lake Sutherland.
Each color represents a different rock deposit.
From V. Todd et al. 2014.
Moving Cobbles
About 50 million years ago, during the Eocene Epoch, the great Ballena River tumbled down 13,000 feet high volcanic mountains and flowed across the ancient landscape of what is now Sonora, Mexico, 300 miles southeast of modern San Diego. Many of the round cobbles that formed and eventually deposited in the broad delta were made from hard, reddish rhyolite lava that had formed the mountains, along with quartzite, a blonde metamorphic rock. Then, about 20 million years ago, the movement came.
The Farallon Plate hit the North American Plate, ending the subduction zone that had created the volcanic mountain range during the Jurassic Period more than 150 million years ago. As it did so, the Pacific Plate came in from behind and pushed its way inland a bit, grabbing a slice of the North American Plate and moving it to the northwest, creating the San Andreas fault in the process. This caused, and continues to cause, a wild array of geologic activities (see graphic below from an excellent book by Keith Heyer Meldahl, 2015).
A chunk of continent that became the Western Transverse Range (WTRB in the graphic below) was pulled away, twisting and ripping it off its foundation, exposing ancient, oceanic rock which is now betweem Catalina Island and Los Angeles. The great delta of the Ballena River was ripped from its source and moved northwestward as well, scattering its cobbles (clasts) to the mesas of San Diego and the shoreline cliffs of the northern Channel Islands (especially San Miguel and Santa Rosa Islands). The photo at the top of this page shows some of these cobbles from a source near Ramona, California, stranded on top of a mountain made of tonalite, a granitic rock that was once the basement foundation of another ancient mountain chain. River cobbles on top of mountain high above the surrounding landscape?
When first described in 1892 by H.W. Fairbanks, no one had a clue where these cobbles came from. Now we know, thanks to the theory of plate tectonics, the idea that revolutionized geology in the late 1960s. In 1970s, Pat Abbott from San Diego State University did the conclusive work that finally solved the San Diego cobble puzzle. He discusses the fascinating story in his book, The Rise and Fall of San Diego (1999).
Geological Resources For San Diego County
Documents
1. Geological field trips in San Diego County, from MiraCosta College.
2. Geological map of coastal San Diego County (see #3 for explanatory document and key).
3. Pamphlet for SD County geological map.
4. Geological map for inland San Diego/Imperial Counties (see #5 for explanatory document and key).
5. Pamphlet for inland SD/I County geological map.
6. Geological map for northern San Diego County (2007). See #7 for explanatory document and key.
7. Pamphlet for northern SD County geological map (2007).
Videos
1. The ancient Calveras volcanic plug, northern San Diego County.
2. Large pegmatite crystal mine, Riverside County.
3. The building of mountains in Mission Trails Regional Park, San Diego, with Dr. Pat Abbott.
More to Come