11.8: Chapter Summary

11.1: A Complicated Land

• The Coast Ranges are defined by tectonic complexity. They lie along the boundary between the North American and Pacific plates, shaping their dynamic geology.
• Plate tectonic theory is essential to understanding the Coast Ranges. Before the theory was developed, geologists could describe the region’s features but not explain why different rock types were juxtaposed.
• The Coast Ranges contain diverse and seemingly contradictory rock formations. Rocks such as pillow basalts, eclogites, and unaltered sedimentary rocks are found together due to tectonic accretion.
• Dating rock assemblages in the Coast Ranges is challenging. Many rocks did not originally form in California but were transported and altered by tectonic forces, making it difficult to assign a single "age" to an assemblage.
• The Coast Ranges lack simple, layered rock sequences. Unlike the Grand Canyon, where sedimentary layers are neatly stacked, Coast Range rocks are faulted, folded, and jumbled together.

11.2: The Coast Range Ophiolite

• The Coast Range Ophiolite is a relic of an ancient ocean floor. It’s a mix of mantle rock, oceanic crust, and deep-sea sediments, all jammed together and stranded on land by tectonic forces.
• Most ophiolites come from mid-ocean ridges, but the Coast Range Ophiolite is likely from a forearc setting, meaning it formed near a subduction zone, where one plate was diving under another.
• Subduction gave it a one-way ticket to California. As oceanic crust was being shoved down into the mantle, bits of it were scraped off and plastered onto the growing edge of North America, creating the Coast Range Ophiolite.
• Over millions of years, this once-deep-sea rock got fractured, metamorphosed, and chemically altered, transforming parts of it into serpentinite, California’s state rock.
• The Coast Range Ophiolite is old, but figuring out exactly how old is tricky. The best estimates place it at around 165 to 175 million years old, making it Jurassic in age.

11.3: The Great Valley Sequence

• The Great Valley Sequence consists of marine sedimentary rocks deposited in a forearc basin.
• These sediments were eroded from the Sierra Nevada volcanic arc and transported westward.
• Much of the sediment was deposited by turbidity currents, forming turbidites.
• The sequence records deep marine environments including submarine fans and channels.
• It lies structurally between the Sierra Nevada batholith and the Franciscan Complex.
• The rocks are now exposed along the western edge of the Great Valley, where they have been uplifted and folded.

11.4: The Franciscan Complex

• The Franciscan Complex formed in a subduction zone at California’s ancient continental margin.
• It includes a mix of oceanic crust, seafloor sediments, and rocks scraped from the subducting plate.
• Rocks were subjected to intense pressure and low temperatures, producing metamorphic rocks like blueschist.
• Chert, greenstone, greywacke, and serpentinite are common rock types in the Franciscan.
• Tectonic forces created a chaotic mixture of rock types, sometimes called a mélange.
• Fossils of deep-sea organisms, like radiolaria, are found in chert layers.

11.5: Cenozoic Events

• The subduction of the Farallon Plate ended during the Cenozoic as the Pacific-Farallon Ridge was consumed.
• A transform boundary developed between the Pacific and North American Plates, creating the San Andreas Fault system.
• New faults formed inland from the plate boundary, including the Hayward and Calaveras Faults.
• The Coast Ranges experienced significant uplift beginning in the Miocene and continuing into the present.
• Volcanic activity shifted inland and northward as subduction ended.
• Ongoing plate movement continues to cause earthquakes and shape the landscape of the Coast Ranges.

11.6: Quaternary Events

• The Quaternary period includes the Pleistocene, Holocene, and potentially the Anthropocene epochs.
• Ice Age climate fluctuations influenced sea level, erosion, and ecosystems in the Coast Ranges.
• The region hosted large Ice Age mammals like Columbian mammoths and saber-toothed cats. Many of these species went extinct at the end of the Pleistocene, possibly due to climate change and human activity.
• The Holocene epoch saw the development of modern climates and ecosystems.
• Some scientists propose the Anthropocene to mark significant human impact on Earth's systems.

11.7: Representative Coast Range Rocks

• The California Coast Ranges are an exceptional place to view a myriad of rock types including: Serpentinite, radiolarian chert, greywacke, blueschist, and eclogite. 
• Serpentinite is California’s state rock and forms through metamorphism of mantle rocks at subduction zones.
• Radiolarian chert forms from the accumulation of silica-based microfossils on the deep ocean floor.
• Greywacke sandstone is a poorly sorted sedimentary rock common in the Franciscan Complex.
• Greenstone is metamorphosed basalt, often altered by low-grade metamorphism in subduction settings.
• Blueschist forms under high-pressure, low-temperature conditions and contains blue minerals like glaucophane.
• Eclogite is a high-pressure metamorphic rock formed deeper in subduction zones, less common at the surface.