Geography of California
A Brief Geologic History of California
(summarized after Debora Harden, Geology of California)
Before 600 my - earliest Precambrian crystalline basement rocks are found in western Utah cutting southwest across the southernmost part of Nevada and then across southern California (Mojave Desert area). The west coast of North America was along this line about 1 billion years ago. In other words, what is now the land of California was under the Pacific Ocean, except for southernmost California.
By 600 my, rifting of the North American Plate, with sinking of the western continental margin, resulted in the creation of a passive continental margin. Sediments accumulated over 350 million years, building up thousands of meters thickness of mostly carbonate sedimentary rocks (limestone). The continental margin went from southern California across southern Nevada then northward along eastern Nevada and western Utah, up through what is now eastern Oregon and Washington. Thus the California portion of the coast faced northwest. Later metamorphosed into quartzite and phyllite, today these rocks can be seen in the Basin & Range provinces (east of Modoc Plateau) and the Mojave Desert. They are also preserved in roof pendants in the eastern High Sierra.
Then came a period (in the Devonian and Mississippian periods, around 400-320 my) of plate collision and mountain building. Evidence of this uplift and rocks made up of sediment eroded from the resulting mountains are seen in western Nevada, but also in the InyoMtns. and Death Valley area. While this uplift was occurring, other oceanic rocks were forming off the continental margin, mostly from volcanic island arcs and subduction zone metamorphic rocks. These rocks were later accreted (plastered) against the western continental margin of the North American plate, forming parts of the northern Sierra Nevada and the eastern KlamathMtns.
About 200 million years ago, in early Mesozoic time, Pangaea began to break up, creating the North American plate as we see it today. Between 100 and 50 million years ago the North American plate moved westward, breaking away from what is today the European continent. Approximately where today we see the Sierra Nevada, a subduction zone formed off the west coast of the North American plate. This subduction zone resulted in the collision and accretion of belts of oceanic rock that gradually built the continental margin westward. The youngest of these belts of accreted rock in the Sierra Nevada is about 160 million years old, middle to late Jurrasic in age.
During the Mesozoic subduction, magma rose up from beneath the descending plates, causing the formation of chains of andesitic volcanoes at the surface and plutons of granitic magma beneath them. Plutonic rocks from this period are found in the KlamathMountains, Sierra Nevada, Basin and Range, Mojave Desert and PeninsularRanges. During this time the Farallon plate was subducted and consumed beneath the North American Plate.
By 100 million years ago the subduction zone had shifted westward to the approximate position of today's Coast Ranges. Portions of the Farallon plate, along with oceanic sediments of volcanic and terrestrial detritus origin, were subducted briefly and then rose to be accreted against the North American plate, forming what we call today the Franciscan Formation.
The San Andreas transform fault system began about 28 million years ago with the collision of the Pacific plate and the North American plate. This collision caused the subduction zone along the coast to cease and the two plates began to slide past each other. The Pacific plate moves northwest relative to the American plate, a system referred to as a right lateral fault. The transform fault spread gradually northward from about the latitude of Los Angeles, carrying with it pieces of rock that crossed the plate boundaries. The Pinnacles in central California are an example of such transported materials. They are part of a volcanic complex about 23 million years old that has been carried 195 miles beyond where it was initially erupted. These rocks match a volcanic formation called the Neenach that is part of the Transverse Ranges 195 miles southeast of the Pinnacles. Other examples of features carried north along the fault include Point Reyes and Bodega Head, on the coast west of the SSU campus.
The following materials are exerpts from the California Coastal Commission's California Coastal Resource Guide, which can be ordered from University of California Press by calling 1-800-822-6657.
California's coastal mountains trace a sinuous 800-mile course from the northwest corner of Del Norte County south to the Mexican border. Except for a break in the chain at the Golden Gate, they form a continuous series of ranges and valleys, separating the coast from the Great Central Valley and the deserts of the interior. This mountainous barrier has a dramatic effect on California's climate: storms originating over the Pacific Ocean bring rain to the western slopes, while the eastern slopes remain relatively dry. Many of California's industries flourish in the climatic conditions created by the coastal mountains--the evergreen trees that support the north coast timber industry thrive on the increased rainfall and frequent fog of the region; coastal fog cools hot inland valleys just east of the coast mountains where wine grapes are cultivated; and fruit and nut trees and cool weather vegetables are grown in coastal areas from San Mateo County to San Diego.
The geologic history of California's coastal mountains begins several hundred million years ago when, according to current geologic theory, movement of the earth's crust set in motion the processes that created the coastal ranges. The geologic theory of plate tectonics describes the system of loosely interlocking plates, floating upon an underling mantle of less solid material, that cover the earth's surface. The North American Plate supports the continent of North America, and the Pacific Plate lies beneath the Pacific Ocean. About 250 million years ago these two plates, which had been gradually moving towards each other, collided; the sea floor crust of the Pacific Plate slipped beneath the continent, heating and melting as it reached the earth's interior. Between 150 and 140 million years ago this molten rock, or magma, began to push upward, forming the Klamath and Peninsular ranges.
About 30 million years ago the relative movements of the North American Plate and the Pacific Plate changed from a head-on contact to a lateral slipping against each other. This zone of slippage, extending nearly the length of the state, is called the San Andreas Fault. Along this zone, folding of the sea floor along the margin of the North American Plate resulted in the creation of the Coast and Transverse ranges, which are composed of the crushed, crumpled, and folded sea floor sediments.
The coastal mountains constitute four geomorphic provinces or geologic regions within California. The northern-most is the KlamathMountains province, which lies near the coast in northwestern Del Norte County and extends north into Oregon. The northwest-trending Coast Ranges, the largest of the state's geomorphic provinces, rises abruptly from the shore in northern HumboldtCounty and extend 400 miles south to the Santa Ynez River in Santa BarbaraCounty. The TransverseRanges lie along an east- west axis, from the Santa Barbara coast to the Mojave Desert, creating a natural barrier between Central and Southern California. The massive PeninsularRanges complete the coastal mountain system, extending south from the Los AngelesBasin to the tip of the BajaPeninsula.
In Northern California, the KlamathMountains are composed of metamorphic and granitic rock--formed as a result of extreme changes in temperature, pressure, and chemical composition that occurred when molten material from below the earth's crust was pushed to the surface. South of the KlamathMountains, the Coast Ranges lie close to the continent's edge, from HumboldtCounty to San FranciscoBay, forming a series of low mountains paralleling the coast. south of the bay, which separates the Coast Ranges into northern and southern ranges, are the Diablo, Gabilan, Santa Cruz, and Santa Lucia mountains, the highest of which reach to 4,000 feet. The sea floor sediment--sandstones and shales--that make up the Coast Rages were crumpled so completely that it is difficult to discern individual layers of sedimentation. Visible in the sea cliffs along the Northern California coast are massive and steeply dipping rock layers, called the Franciscan Formation; a spectacular example of this geology can be seen along the cliffs at Devil's Slide in San MateoCounty.
The TransverseRanges include the Santa MonicaMountains, which extend offshore to form the Northern Channel Islands of Santa Cruz, Santa Rosa, and San Miguel off the coast of Santa Barbara County. Three hundred miles east, the TransverseRanges terminate abruptly in the San Gabriel and San Bernadino mountains, dropping off into the Mojave and Colorado deserts. Severely folded, twisted , and uplifted, these mountains exhibit extreme differences in geologic age. Sedimentary rocks are most common on the western slopes such as the Santa Ynez and Santa Monica mountains, which hug the coast from Santa Barbara County south to Los Angeles County; in the rugged eastern mountains, granitic and metamorphic rock dominate. To the south, the Peninsular Ranges-- steep, narrow , and northwest trending--include in the southeast the rugged San Jacinto, Santa Rosa, Agua Tibia, and Laguna mountains that plunge into the Coachella and Imperial valleys. To the west the rolling slopes of the Santa AnaMountains gradually descend onto broad marine terraces that front the ocean; these mountains submerge westward, forming the Southern Channel Islands of Santa Barbara, San Nicolas, and San Clemente. The highest of the coastal mountains rages, the Transverse and PeninsularRanges both contain peaks of over 10,000 feet.
Dramatic changes in elevation and a variety of climatic zones contribute e to a diversity of plant life in California's coastal mountains. Conifers--redwood and Douglas-fir--cloak the windward slops of the KlamathMountains and the northern Coast Ranges. Heavy winter rainfall, summer fog, and moderate temperatures have produced redwood groves where 2,000 year-old trees tower more than 300 feet above the forest floor. South of San Francisco Bay, the slopes of the Santa CruzMountains are covered with stands of redwood while the drier regions of the southern Coast Ranges are vegetated with oaks, pines, and chaparral. As precipitation decreases southward , in the central and southern Coast Ranges, stands of hardwoods including tanbark oak, coast live oak, big- leaf maple, and madrone begin to outnumber conifers. On steeper slopes and exposed ridges where thin soils lose moisture rapidly, drought-resistant chaparral species such as chamise, manzanita, sage, and scrub oak take hold.
In the semi-arid Transverse and Peninsular ranges, chaparral is abundant on windward and southwest-facing slopes, whereas mixed- coniferous forest--Ponderosa pine, sugar pine and whit fir--grows in isolated stands in protected areas. Hardy, drought-resistant digger and knobcone pines grow on the dry, rocky slopes.
Geography of California Article Questions
1. Where was the west coast of North America about 1 billion years ago?
2. Where was the land that makes up most of what is now California?
3. When was a major period of plate collision and mountain building?
4. When did Pangaea begin to break up?
5. What formed millions of years ago approximately where we see the Sierra Nevada today?
6. During Mesozoic Subduction, what 2 things formed?
7. When did the San Andreas Fault system begin?
8. Explain how the Pinnacles in central California are an example of material that has been transported by the San Andreas Fault system.
California’s CoastalMountains
9. How do the coastal mountains affect California’s climate?
10. List 3 ways that industry has benefitted from coastal mountains.
1.
2.
3.
11. What are the Coast and Transverse mountain ranges composed of?
12. Where are each of these coastal mountain ranges?
1. KlamathMountains:
2. Coast Ranges:
3. TransverseRanges:
4. Peninsular Ranges:
13. How were the KlamathMountains formed?
14. What types of rocks are most common along the western slopes of the Santa Monica and SantaInezMountains?
15. Why are redwoods found on the windward side of these northern coastal ranges?