California sits upon two major tectonic plates, the North American Plate and the Pacific Plate. Currently, the North American Plate is moving in a westward direction, while the Pacific Plate moves steadily northward. Should this trend continue, Southern California will eventually find itself part of the Alaskan land-mass (Southern California used to be part of Baja California). The frequently violent interactions of these two plates throughout California's natural history are responsible for most of California's rugged geologic features.
Prior to the Jurassic Period, much of present day California lay submerged under the sea. One Hundred Fifty (150) million years ago, the North American Plate buckled against the Pacific Plate, causing immense compression and folding that led to the creation of a tall proto-Sierra mountain range. This range was slowly worn down by erosion, filling the future Central Valley, then ocean bottom, with sediments.
The modern Sierra has its origins 60 million years ago, as magma seeped up from the subduction zone between the two plates, creating a massive pool of granite, which slowly cooled, forming a batholith. More recently, from 12 million years to the present, the batholith has been rising, pushing upward from faults which run across the edge of the Sierra. This violent upward movement was dramatically demonstrated in 1872, when a massive earthquake near Lone Pine in Inyo County caused thrusts of upwards of 20 feet. The Klamath mountains were formed by the same process, roughly at the same time as the Sierra Nevada.
Thirty million years ago, an ocean ridge on the Pacific Plate became jammed in the subduction zone, effectively clogging it. The resultant folding and compression formed the Coastal and Transverse ranges. More dramatically, the Pacific Plate changed directions, moving northward perpendicular to the North American Plate. Here is the origin of the infamous San Andreas Fault, a strike-slip fault that begins along the north coast of California, passes almost adjacent to San Francisco, runs to the east of Los Angeles, and branches into Mexico and the Pacific Ocean. The fault has been the source of much terror for the population centers that unwittingly sprang up along it. As the two plates slide past each other, along the fault boundary, they frequently catch on each other. Tension builds up, and potential energy is stored until something gives, and the two plates jolt past one another, releasing massive amounts of energy in the form of an earthquake. Earthquakes have claimed the lives of over 1000 Californians during the past two centuries, including over 700 killed by the 1906 San Francisco Earthquake alone. The San Andreas is not the only fault in California, rather the State, particularly Southern California and the edges of the Sierra Nevada, is strewn with minor faults, all of which can cause earthquakes.
Another minor plate, the Juan de Fuca plate which runs along the Northern California coast and Oregon, is responsible for the formation of the Cascade Range. During the Quartenary Period, 10 million years ago, The Juan de Fuca plate collided with the North American plate, and was subducted under. Magma from the melting plate bubbled up, causing a series of mountain building eruptions. The Cascades remain volcanic to this day, most notably Mount Lassen, which exploded violently in 1916 and continued erupting until 1921. Mount Shasta is also an active volcano, although it has not erupted in recent memory.
Thus California, balanced on a plate boundary on the eastern edge of the "Ring of Fire," has had a violent geologic past, and promises to have an equally violent future. Slowly, it will be torn asunder by powerful processes, and reshaped by the same forces that initially molded it.