A Book and Map Highlight Mount Diablo’s Geology

In connection with the NCGS’s 75th anniversary, a new book-length scientific report on the geology of Mount Diablo and a companion geologic map and guide were published in 2021. The book updates decades of geologic insight into the origin and history of this iconic mountain and its place in the regional tectonic setting on the North America plate boundary. The map is the first of its kind for Mount Diablo and introduces visitors to Mount Diablo State Park’s unique geologic history.

The Book

The new volume, Regional Geology of Mount Diablo, California: Its Tectonic Evolution on the North America Plate Boundary, was published by the Geological Society of America as Memoir No. 217. Edited by NCGS members Raymond Sullivan, Doris Sloan, Jeffrey R. Unruh and David P. Schwartz, it contains 17 research papers by 33 authors, plus an insert presenting a detailed and updated geologic map of the Mount Diablo region. Purchase it for $42 on the GSA website.

Regional Geology of Mount Diablo was assembled under the leadership of Ray Sullivan, Professor Emeritus of Geology at San Francisco State University. Its publication commemorates the 75th anniversary of the Northern California Geological Society. Save Mount Diablo and members of the NCGS provided much of the funding for the book.

This collection includes the most recent interpretations of the mountain’s structure and explains its recent uplift. Mount Diablo might look like a volcano, but its peaks are composed mainly of ancient oceanic rocks and its flanks of younger sedimentary rocks: in brief, “while the rocks are old, the mountain itself is young.” Chapters include a study of the landscape, details of the rocks, geologic history of the area, mining and petroleum discoveries, and the age and origin of volcanic ash beds in the younger sedimentary rocks on the flanks. The main sources of these ash beds are from Nevada, Yellowstone, Cascade Range and Sonoma. A short phase of volcanic activity some 7½ million years ago on the east side of Mount Diablo formed a series of domes along Marsh Creek Road.

According to Prof. Sullivan, “the new work contains the most complete studies on the geology of Mount Diablo. They point to the origin of Mount Diablo as a relatively recent event in the long and complex geological history of the California Coast Ranges.”

View the table of contents at https://rock.geosociety.org/store/TOC/MWR217.pdf

The Map

The new Geologic Guide to Mount Diablo State Park, a folded two-sided map published by the Mount Diablo Interpretive Association (MDIA), is available at www.mdia.org or at the Mount Diablo State Park Summit and Mitchell Canyon Visitor Centers.

The simplified geological guide, edited by NCGS member Greg Bartow, consists of an updated geologic map with roads and trails on one side, and a description of the geology on the other side. The geologic map, by Russell Graymer and Victoria Langenheim of the U.S. Geological Survey, is the same one presented in GSA Memoir 217. The guide also includes a map showing the epicenters of 7,200 earthquakes that have occurred in the vicinity of Mount Diablo over the last 36 years. These earthquakes illustrate the active nature of the fault systems that continue to form Mount Diablo.

Stephen Smith, MDIA President, said, “This geologic guide will provide a new resource for visitors to appreciate the unique nature of the park.”

Mount Diablo State Park draws more than 600,000 visitors each year to explore the park and take in the spectacular views. MDIA is a nonprofit, all-volunteer organization that assists the California Department of Parks and Recreation in maintaining and interpreting the park. Through education, sponsored activities, and publications, MDIA fosters appreciation and the enlightened use of Mount Diablo State Park.

Copies of the geologic map can be purchased online at mdia.org for $7.00 (shipping included), or for $6.00 at the Visitor Centers at Mount Diablo State Park Summit and Mitchell Canyon.