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11: Coast Ranges

Last updated

Mar 26, 2025
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- 10.7: Detailed Figure Descriptions
- 11.1: A Complicated Land

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Introduction to the Coast Ranges

The Coast Ranges are a series of mountains and valleys trending northwest-southeast along the western edge of North America. The California Coast Ranges begin near Santa Barbara and extend northward to the Oregon border, following the coastline of California. The Coast Ranges are a major division of the physiographic provinces of California, and abut the Klamath province and Great Valley to the east, and the Transverse province to the south. Although the Coast Ranges are not immediately adjacent to the Sierra Nevada, in some places they are geologically similar, as portions of the former Sierra Nevada have been amalgamated into the Coast Ranges through tectonic violence.

We can look across California from west to east and see the Coast Ranges descending into the plains of the Central Valley, then rising again into the Sierra Nevada, as shown in (Figure $\PageIndex{1}$ ):

Almost every major event in California’s geologically brief lifespan has left indelible imprints in the rocks of the Coast Ranges. Magma that cooled as the supercontinent of Pangaea began to break up? Check. Agonized seafloor rocks being forced underneath continents, even as a daring few escaped onto the continent? Yep. Bizarrely transformed metamorphic rocks thoroughly altered at temperatures less than your kitchen’s oven? Uh huh. The San Andreas ripping apart western California and implanting magma bodies which to this day feed hot springs and geysers? You bet.

In the 1994 Boomer nostalgia film Forrest Gump, the titular character, played by Bay Area native (and former community college student) Tom Hanks unwittingly intersects with major historical events such as the Vietnam War and Watergate, along the way meeting figures such as Elvis Presley, John Lennon, JFK, LBJ, and Nixon. The Coast Ranges are like Forrest Gump. They’ve unintentionally been in the right place to witness a lot of the important geologic events in California.

In this chapter we will divide the complexity of the Coast Ranges into broad geographic categories. We will talk about two regions:

the Northern Coast Ranges and
the Southern Coast ranges

The Northern Coast Ranges are primarily composed of sedimentary rocks that were deposited during the Mesozoic and Cenozoic Eras. The Southern Coast Ranges are generally younger, and are composed of sedimentary rocks that were deposited during the Cenozoic Era. For time ranges of these time periods it may be useful to review the timeline from the chapter Geologic Time.

We will also parse the major rock units in the Coast Ranges, including:

The Coast Range Ophiolite
The Franciscan Complex
The Great Valley Sequence
Quaternary rocks, including volcanics

For fun we will examine some of the most interesting features of the Coast Ranges, including:

Geothermal fields of hot bubbling mud fed by the wake of the San Andreas fault
A coal mining region, right here in California
Metamorphic rocks so altered they could be described as Seussian

Figure $\PageIndex{1}$ : Location of the Coast Ranges Province of California. “The Coast Ranges Province of California” by Emily Wright, a derivative of the original work, is licensed under CC-BY-NC.

Learning Objectives

By the end of this chapter, you should be able to:

Describe how plate tectonics influences the geology of the Coast Ranges.
Explain our current understanding of the origins of the Coast Range Ophiolite, Great Valley Sequence, and Franciscan Complex.
Explain the origins and organization of ophiolite sequences generally.
Explain Coast Range geomorphic features associated with transpression and transtension.
Describe San Francisco Bay during the last glacial maximum.

11.1: A Complicated Land
This page discusses the Coast Ranges of California, where the North American and Pacific plates interact, creating geological complexity. First mapped by Professor Andrew Lawson in 1895, it features the significant San Andreas Fault. The region requires an understanding of plate tectonics due to its diverse rock types. Notably, the northern and southern Coast Ranges differ in geology, with the southern part having more petroleum resources and dramatic faulting.
11.2: The Coast Range Ophiolite
This page discusses the Coast Range Ophiolite (CRO), a geological formation in California characterized by pillow basalts, sheeted dikes, and gabbro from the Jurassic period. It marks the oldest rock unit in the area and highlights significant tectonic processes, specifically the obduction of oceanic lithosphere rather than subduction, which resulted in its current location far from the ocean.
11.3: The Great Valley Sequence
This page describes the geological history of California's Central Valley, revealing its origin as a forearc basin influenced by ancient volcanoes. Erosion transported volcanic debris, forming the Great Valley Sequence, which includes various sedimentary rocks. Although mostly covered by newer sediments, some formations show tectonic uplift. Additionally, the nearby Franciscan formation is related to the subduction of the Farallon Plate, highlighting the area's complex geology.
11.4: The Franciscan Complex
This page discusses the Franciscan Complex, a key geological formation in western California, known for its diverse rock types formed through extensive metamorphism and tectonic activity linked to the breakup of Pangaea. Additionally, it references several scholarly works on geology and physics, including a 1982 bulletin, a 2014 book by David L. Goodstein, a 2018 study by Mulcahy et al., and a 2015 paper by Wakabayashi, providing citations for further exploration of the subject.
11.5: Cenozoic Events
This page explores the geological history of the San Andreas Fault and its formation linked to Pangaea's breakup and subduction events. Key features include the Salinian Block's unusual granitic rocks, younger volcanic progression due to mantle activity, and the impact of fault movements on the Coast Ranges.
11.6: Quaternary Events
This page discusses the Quaternary period, highlighting the transition from the Pleistocene to the Holocene and significant human developments post-Ice Age. It mentions the transformation of San Francisco Bay and early human interactions with extinct species. Additionally, it reflects on the importance of curiosity and inquiry in life, emphasizing that the pursuit of knowledge adds excitement and depth to human experience.
11.7: Representative Coast Range Rocks
This page discusses various geological rock types found in California, including Serpentinite, Eclogite, Blueschist, radiolarian chert, and Greywacke sandstone. Serpentinite, the state rock, originates from metamorphosed mafic rocks, while Eclogite forms under extreme conditions. Blueschist arises in subduction zones from basaltic protoliths, and radiolarian chert is composed of marine plankton silica. Greywacke, the predominant rock in the Franciscan Complex, results from underwater landslides.
11.8: Chapter Summary
A summary of the Coast Ranges chapter.
11.9: Detailed Figure Descriptions
Descriptions of complex images within this chapter, as well as additional guidance for users who have difficulty seeing and interpreting images.

Thumbnail: "Coast Ranges Province" is licensed under CC BY-NC 4.0 by Allison Jones.

Introduction to the Coast Ranges

Learning Objectives

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