The landscape of southern California represents one of the most geologically diverse places on Earth. The varied terrain of prominent mountains, wide basins, rolling hills, deserts, and coastal plains results from a complex geologic history spanning nearly 2 billion years. Constructive geologic processes like plate tectonics have built tall mountain ranges and generated volcanic activity. In contrast, destructive geologic processes like mass wasting and erosion from rivers, wind, and waves have acted to wear down and shape the landscape.

Very Brief Geology Lesson

The process of plate tectonics drives constructive geologic processes. The Theory of Plate Tectonics describes how Earth’s outermost layer of rock, the lithosphere, is broken into individual slabs called plates. Ocean crust, continental crust, or a combination of both makes up the upper portion of a plate. The plates are slowly moving and interacting with one another along plate boundaries, resulting in constructive geologic processes, such as volcanism and mountain building. The compressional stresses crumple the crust, like pushing a rug into a wall, causing the folding and faulting of the crust, and building up mountains. Subduction occurs when one, or both, of the colliding plates, is capped by ocean crust. During this process, the oceanic plate sinks into the mantle, destroying the ocean crust while at the same time producing magma that rises to Earth’s surface to construct volcanoes, ultimately making new rock and landforms.

Illustration of a convergent plate boundary.

Convergent plate boundary between a oceanic plate and continental plate. - Creative Commons (CC).

Constructive geologic processes can also happen where tensional stresses pull plates apart along divergent plate boundaries. On the ocean floor, divergent boundaries generate magma that rises to the ocean floor, creating volcanoes and solidifying into new rock, making new ocean crust. When tensional stresses stretch the continental crust, it fractures through continental rifting. Blocks of crust slide down along fractures (faults) making valleys, while blocks of crust on the opposite side of faults move upward to produce mountains.

Illustration of continental rifting.

Divergent plate boundary. - CC.

Plates can also slide horizontally past each other along a transform plate boundary, causing shearing stresses that fracture the crust, creating earthquake faults, like the San Andreas fault, in California.

Illustration of a transform plate boundary.

Transform plate boundary. –CC

Destructive geologic processes oppose the constructive geologic processes; Earth’s destructive geologic processes, weathering, mass wasting, and erosion act to weaken rock and wear-down landforms. For example, the erosive action of the Colorado River removing rock at a rate of about 1.5 inches per 100 years over the past four million years has resulted in carving out a canyon over a mile deep and up to 15 miles wide, the Grand Canyon. The “look” or topography of the landscape in any given place is a consequence of constructive vs. destructive geologic processes.

A Similarly Brief Geologic History of Southern California

Southern California is home to the oldest rocks in California, about 1.8 billion years old (Norris, R., Webb, R. 1990). These are in the San Gabriel and San Bernardino mountains and the Mojave desert. While the origin of these rocks isn’t certain due to their highly altered state, they probably represent part of the ancient core of the North American continent or are chunks of crust plastered onto the continent’s side by plate tectonic activity. From about a billion years to around 200 million years ago, the western margin of North America was very much like the geologically passive margin along today’s east coast. The continental shelf (shallow sea) stretched far out to the west from a shoreline that today would be situated in the Midwest. From the continental highlands to the east, sediment was eroded and deposited onto the continental margin, where it accumulated as stratigraphic layers tens of thousands of feet thick. The best display of this long-term cycle of erosion and deposition are strata (layers of sedimentary rock) making up the walls of the Grand Canyon. In southern California, these rocks have altered into metamorphic rocks.

Starting around 200 million years ago, the long slumbering western half of North America geologically woke up when the ocean crust along the western margin of the continent began to subduct beneath the continent. This event made a new plate boundary between the subducting, oceanic Farallon Plate and the overriding, continental North American Plate.
Subduction continued for about 150 million years, generating magma that rose to Earth’s surface to build up a line of volcanoes running south to north through California, just like the modern-day Cascade Range that starts in northern California and extends northward through Oregon and Washington. The magma that solidified within the crust is today exposed as granite and granite-like rocks that make up the core of California’s major mountain ranges: the Sierra Nevada Mountains, the Transverse Ranges, and the Peninsular Ranges.

Subduction in southern California ended about 23 million years ago when the plate boundary between the ocean and continental crust began to change from a convergent to a transform plate boundary (Sylvester and Gans, 2016), now known as the San Andreas fault. As the type of plate boundary changed, so did the geologic processes shaping the crust. Magma stopped being generated, so volcanoes went extinct. The crust was twisted and fractured. Blocks of crust rotated and uplifted as shear stresses deformed the crust, resulting in the squeezing up of blocks to make mountains and hills, while in other places, the crust has dropped down to form basins (broad valleys).

Starting 16 million years ago, western North America was subjected to continental rifting as tensional stresses significantly stretched the crust, making normal faults along which blocks of crust slid to form deep basins bracketed by tall mountains. This region of stretched crust extends from the eastern front of the Sierra Nevada Mountains in California, to western Utah, and includes mountains over 13,000 feet tall and deep basins, including the lowest spot in the western hemisphere, Death Valley in southern California. Concurrently, the northward movement of the Pacific Plate peeled a block of crust away from the North American plate, eventually causing it to rotate about 110o clockwise (Sylvester and Gans, 2016). Today, this “captured” block of crust is known as the Western Transverse (crosswise) Ranges. It includes the Santa Ynez in the Santa Barbara region and the Santa Monica Mountains west of Hollywood. See this excellent animation by Tanya Atwater for a video summary of this extraordinary event. Tensional stresses related to the rotation caused subsidence of crust as well, creating what is now the Los Angeles Basin.

In the early stages of the rotation event, lava rose along faults to fill gaps caused by the extension; some of this lava is today exposed in the greater Los Angeles area as volcanic rocks. As the rotation of the Transverse Ranges continued, they were also moving northward with the Pacific Plate, leading to more stretching and subsidence. Around 5 million years ago, the plate boundary, the San Andreas fault, jumped eastward. With this new orientation, movement along the fault resulted in localized compressional stresses, causing the uplift of the modern-day Transverse Ranges, including the San Gabriel and San Bernardino mountains. Sediment eroded from the growing mountains filled the Los Angeles Basin, which was deep under seawater then. In just the past 1 million years, the basin has filled to above sea level; this geologically mighty feat required a massive volume of sediment, as estimates put the bottom of the pile at 30,000 feet deep in some places.

Geologic Time Scale

The Geologic Time Scale the widely accepted system used to arrange the significant geologic and biologic events of Earth. Terms from the Geologic Time Scale are used throughout this guidebook.

Geologic time scale.