CA Geology


The Precambrian era spans from the formation of the Earth, 4.6 billion years ago, to the Cambrian Explosion 541 million years ago. Geologists know little about this time because the majority of these rocks have been heavily metamorphosed and subject to extensive heat and pressure, altering their state. California was completely underwater as the coastline was still east of it. The Golden State’s oldest rocks are over a billion years old and lie within the eastern deserts and eastern Transverse Ranges. The west coast of the continent was to the east of most of the rest of the state. Geologists infer this from the distribution of these Precambrian rocks.

Paleozoic Era:

The Antler Orogeny in Nevada occurred about 400 million years ago. An orogeny is when part of the earth’s crust is compressed, causing a mountain range to form. In this case, an island arc rose off shore. Island arc collisions caused by this orogeny continued until 215 million years ago. However, although this theory is widely accepted, there remains alternate hypotheses. Here is one from the USGS. The Sonoma Orogeny starting 215 million years ago formed another island arc whose rocks are now contained in the Sierra Nevada and parts of the Klamath Mountains.
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The plate to the left describes the Antler and Sonoma Orogeny events.

Mesozoic Era:

During the Mesozoic, California split further apart from Europe and Pangaea. 210 million years ago marked the Nevadan orogeny, or the uplift of the ancestral Nevadan mountains. This orogeny was caused by the formation of the large section of granitic rocks that are now comprising of most of the Sierra Nevada. By 150 mya (million years ago) the coastline was around the edge of the present day Central Valley and was composed of the Nevadan mountains caused by this uplifting. Intrusive granitic rocks continued to form near the Sierras until about 100 mya.



Cenozoic Era:

By the Eocene epoch, large rivers and tributaries flowed through the Western United States. Additionally, the Nevadan mountains that were uplifted in the Mesozoic were gradually reduced to small hills. The San Andreas fault was formed by the contact between the Pacific and North American plates around 30 mya. The below diagram represents the San Andreas Fault.

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Around 20 mya, volcanic activity emerged in the Sierra Nevada and Mojave regions. Deep sea basins collected along the present day coast between San Francisco and Orange County in 10 mya. For example, the Monterey Formation is consists of one of these basins. Fossil sites containing Monterey Formation sediments include Gaviota, El Capitan, and Jalama Beach. The rocks in these basins are primarily composed of marine organisms instead of non-aquatic sediments. Around 5 mya, orogenies accelerating, uplifting present day mountain ranges including the Sierras, Transverse Ranges, and Coastal Ranges. Subduction in Northern California formed the volcanoes in Cascades.

Works Cited:

“California Geologic History,” (Colorado University) Powerpoint

“A Brief Geologic History of California,” (Sonoma University, July 23, 2003)