Regional Earth Science Topics for California about Regional Earth Science Topics for California - California is a perfect place to study the various branches of earth science in an integrated way. The geology, meteorology, plate tectonics, mineralogy, and hydrology of the Golden State all contribute to the quality of life of California.
Tectonic Activity and Earthquakes
Consider ﬁrst the impact of plate tectonics on California. The state is marked by the great San Andreas fault, a transform plate boundary that extends nearly the length of the state from north east to southwest. California residents know that the San Andreas fault and its many branches are the major source of the state’s infamous earthquakes. The San Andreas fault is located where the Paciﬁc and North American plates meet. As these plates shift relative to one another, they stick and slip. To mitigate the hazards posed by the geological features of particular locations, the United States Geologic Survey publishes geologic hazard maps. Geologic hazard maps indicate what the geologic risks of various regions are, from radon exposure to ﬂooding. The California Geologic Survey publishes seismic hazard zone maps which target seismic hazards speciﬁcally, including earthquakes, landslides, and liquefaction. City planners and builders use the maps to identify regions where risk-reduction measures need to be employed prior to development. However, geologic hazard maps are useful to all Californians. If you live in California, knowing where to get geologic hazard maps and how to interpret them is one step you can take towards being earthquake prepared. Figure 1 is a geologic hazard map for eastern San Jose. On this map, areaswith a liquefaction hazard are shown in green. These are places where liquefaction has occurred in the past or where there is an elevated risk of it in the future. Areas colored blue are places where earthquake-induced landslides have occurred in the past or where they pose a signiﬁcant risk in the future. Keep in mind that such a map cannot be interpreted as indicating all the areas susceptible to the denoted risks. Also, all areas designated as at risk don’t necessarily share the same level of risk. Though useful, geologic hazard maps are not perfect risk indicators because geologic hazard prediction isn’t an exact science at this time.
The tectonic forces that produce California’s destructive earth quakes have an upside—they also have created California’s beautiful mountainous terrain. California’s coastal mountains extend 800 miles from the northwest corner of Del Norte County to the U.S.–Mexican border. This mountain chain breaks in the California’s mountains have also been shaped by Ice Age glaciers and modiﬁed by erosion from wind and rain. California’s coastal mountains form a barrier, trapping moisture in the air that blows in from the Paciﬁc Ocean with the prevailing westerly winds. As discussed in Chapter 24, moist air is forced up the windward side of the mountains, cools as it rises, becomes saturated, and produces precipitation. Air blowing down the leeward side of the mountains is depleted of much of its water content, which keeps the eastern slopes of California’s mountains relatively dry. This wet-dry climate gives rise to many of California’s agricultural resources. The evergreen trees of the north coast grow in the rainy climate there. The extensive forests of evergreens provide habitats for a number of native species and are the basis of the north coast timber industry. Likewise, fruit and nut trees and cool weather vegetables grow in the cool coastal climates found from San Mateo to San Diego. The Sierra Nevada mountains range in height from about 9000 feet (2700 m) to over 14,505 feet (4421 m) at Mt. Whitney. Thus, the Sierra Nevada trap moisture in the air that has blown over the coastal mountains. As a result, locations such as Death Valley which lie on the east side of the Sierra are some of the driest in the state. vicinity of San Francisco, but otherwise forms a nearly continu ous series of smaller ranges and valleys (Figure.2). The other prominent mountain range inCalifornia is the Sierra Nevada (Spanish for “snowy range”). The Sierra Nevada stretches 400 miles from Freydonyer Pass in the north to Tahachapi Pass in the south. It is bounded on the west by the Central Valley and on the east by the Great Basin. How, exactly, were California’s majestic mountains built?
The coastal mountains began to form about 250 million years ago when the Paciﬁc Plate and North American Plate collided. Although these plates form a transform boundary today, the plates were actually converging 250 million years ago. When they collided, the Paciﬁc Plate subducted beneath the North American Plate, creating an ocean trench in the subduction zone. As oceanic crust sank deeper into the mantle, it formed molten rock (or magma). Eventually, magma erupted as lava and accumulated, forming a volcanic island arc as described in Chapter 22. Over time, the volcanic arc gave rise to the Sierra Nevada. The coastal range formed in the vicinity of the trench. As the Paciﬁc Plate subducted at the trench, magma generated by the subduction process gradually rose through the mantle and crust deforming overlying rock strata. The deformed strata plus accumulating sediment from the land, as well as rock scraped off the subducting plates, eventually aggregated to form an accretionary wedge. This wedge of accreted rock and deformed strata gradually rose to great heights as the coastal mountains. Thus, the coastal and Sierra Nevada ranges both substantially formed about 150 million years ago as a result of processes associated with the convergence of the Paciﬁc and North American Plates.
California’s mountains are less famous for their geologic history and climactic effects than for their cultural signiﬁcance. After all, it was in the Sierra Nevada range that gold was famously discovered. Up until about 200 years ago, gold was both plen tiful and easy to ﬁnd in alluvial deposits in the Sierra Nevada. (An alluvial deposit is an accumulation of gravels, rocks, clay, silt, etc., which has been transported by streams.) Because gold has a high speciﬁc gravity and is very durable, it collects in stream beds after weathering and erosion of the primary source. Thus gold could be prospected along stream beds by anyone with a simple gold pan and some patience. The California Gold Rush began when James Marshall discovered pieces of the shiny metal at Sutter’s Mill on the American River, about 50 miles from Sacramento in Coloma. By 1849, prospectors were ﬂocking to the area and by 1865, $750,000,000 in gold had been mined. The California Gold Rush was an event of worldwide sig niﬁcance. Hopeful gold prospectors came to the Sierra Nevada from all over the globe, particularly Mexico, China, Germany, France, and Turkey. Some traveled over land along the Ore gon–California trail while others journeyed by ship around the tip of South America. Either way, the trip was ultimately fruitless for most latecomers. Alluvial deposits, at ﬁrst so easy to tap, soon became depleted. Today, mining for gold continues. Hobbyists enjoy panning for gold while large-scale operations employ costly mining methods to obtain and process large bodies of ore. California’s most important gold deposits are thought to be in the Sierra Nevada, Klamath Mountains, and Mohave Desert though large deposits have been discovered in the Peninsular and Transverse Ranges and in the Northern Great Valley. Low-grade, unmined deposits are scattered throughout the state. In 2001, California ranked fourth in the United States in gold production. That year alone, California produced 449,000 troy ounces with a value of about $122 million. Besides gold, fossil fuels and rich soil are two of California’s other natural resources that relate to its geology. Cali fornia obtains about half of its crude oil supply from oilﬁelds in state. According to the U.S. Department ofEnergy, California ranked third in the nation in its production of crude oil. In recent years, California ranked second in the total amount of energy produced but 48th in the amount consumed per person. Thus, Californians can proudly claim to be some of the nation’s best energy conservationists. Oil is found off shore as well as underground in 18 of the 58 counties in California. For this, Californians can thank multitudes of marine organisms that died and anaerobically decayed millions of years ago,
Agriculture and Water Supply
The rich soil of California’s Central Valley is another one of th state’s resources derived from its geologic past. The Central Valley is about 50 miles wide and 450 miles long and lies between the coastal mountains and the Sierra Nevada. The Central Valley is a broad, fertile plain and the most productive agricultural area west of the Rocky Mountains; about 60 percent of California’s productive farmland is located in the Central Valley. During the time that the California landmasses were forming, the Central Valley existed as an ocean trench and seabed. Over time, sediments from surrounding landmasses were deposited into the sea bed forming the valley ﬂoor. Despite its rich soil, the Central Valley could not have become the “fruit bowl of the world” without water for crop irrigation. No resource is more vital to California than water, and throughout California’s history many battles have been waged to obtain access to it. Advocates for agriculture vie with others representing industry, urban centers, recreation, and environmental conservation. There is a regional character to the water rights controversy as well because some 75 percent of California’s water originates in the northern third of the state, but 80 percent of the demand comes from the southern two-thirds of the state.
However, potential fresh water shortages in California have largely been addressed by California’s dams and water transport system. California has the most sophisticated water storage and transport network in the entire world system of dams, reservoirs, pumping plants, and aqueducts transports about halfof the state’s water supply for distances up to hundreds of miles. San Francisco, for example, derives much of its public water supply over 100 miles to the east from the Hetch Hetchy Dam and Reservoir in Yosemite National Park. The hub of the California water supply system is the Sacramento–San Joaquin Delta. The delta is a region where two of California’s largest rivers meet. The rivers mingle with the San Francisco Bay to form the largest estuary in the world, an aerial photograph shows that the delta is a maze of winding channels. The delta has a total length of about 700 miles. Small town communities, bustling ship ports, marinas, industries, and historical sites are found along its shores while water sports enthusiasts and some 500 species of wildlife can be found immersed in delta waters. The delta itself receives runoff from over 40 percent of the state’s land area. This runoff is principally in the form of streams that are fed by melting snow in the high Sierra Nevada. Since two-thirds of the state’s residents receive at least a portion of their drinking water from the delta, and millions of acres of irrigated farmland rely on it, this is a critical water resource for all regions of the state northern, central, and southern. The two major water projects that divert water directly from the delta are the State Water Project and the Federal Central Valley Project. The State Water Project is a water distribution system that delivers water to agricultural and urban areas in the Bay Area, Silicon Valley, the San Joaquin Valley, the Central Coast, and Southern California. The Federal Central Valley Project serves farms, homes, and industry in the Central Valley, urban centers in the San Francisco Bay Area, and is the primary source of water for much of California’s wetlands. These projects serve multiple purposes in that they produce hydroelectric power and provide ﬂood protection, navigation, and water puriﬁcation including salinity control.
Besides good soil and a steady water supply, California crops need something else to thrive—a favorable climate. Many people come to California expecting a warm and uniform climate. They are surprised to ﬁnd out that California actually has varying regional climates, from the cool, moist, and foggy North Coast to the hot, arid, southern California deserts. At one measuring station in California, annual precipitation has exceeded 161 inches; others have gone for more than a year with no measurable rain. Much of the variability of California’s climate relates to its topography. As previously mentioned, the coastal mountains and Sierra Nevada greatly affect precipitation patterns by creating areas of rain shadow. Also, proximity to the ocean has a large effect on California climate. As discussed in Chapter 25, any area near a large body of water tends to have a moderate climate due to the high speciﬁc heat capacity of water. So the coastal regions of California generally have maritime climates with warm winters and cool summers, small daily and seasonal temperature variation, and high humidity. As distance from the ocean increases, so do maritime inﬂuences. Eastern parts of California experience a continental climate warmer summers, colder winters, greater daily and seasonal temperature ranges, and generally lower humidities.
A third important factor in California’s climate is the effect of atmospheric and ocean circulation patterns. There is a semipermanent high-pressure air mass above the Northern Paciﬁc Ocean. In the summer, the pressure center moves toward the north, and blocks storm systems moving toward the coast. As a result, California receives little rainfall from this source during the summer. In the winter, the high-pressure zone moves southward, permitting storm systems to move across California. The Paciﬁc high-pressure zone also affects ocean circulation. Remember that air ﬂows along a pressure gradient from high to low pressure. This means that air ﬂows southward out of the North Paciﬁc high, which sets up ocean currents moving southward along the coast. Since this current is slightly offshore, there is a zone of upwelling and cold water temperatures adjacent to the coast as water from deeper layers is drawn into circulation.
El Niño and Weather Patterns
Ocean and atmospheric circulation patterns in California are affected by El Niño. You may have heard of El Niño in the news, but not really know much about it. El Niño is a cyclical weather disturbance that affects many parts of the globe, but impacts California particularly strongly. Under normal conditions, trade winds blow from areas of high to low pressure along the equator, dragging the warm equatorial surface waters along with them. As warm surface waters move westward, deeper, colder waters to the east rise upward to occupy the space left vacant by the warm surface water as previously mentioned. The upwelling cold waters, rich in nutrients, attract a variety of sea life. Upwelling of these cold waters has been especially important to the ﬁshing industry along the west coast of South America where people earn their living catching anchovies that come to feed in the nutrient-rich waters. Fishing is not good during October when the trade winds slacken, reversing the normal westward ﬂow of warm tropical surface waters. As the warm surface waters drift eastward, upwelling diminishes and so does the ﬁshing industry. People along the South American coast refer to this occurrence as El Niño because it appears to begin each year around the traditional December celebration of Christmas (in Spanish el niño is the Christ child). Under normal conditions, the trade winds pick up again in early spring, the surface waters are again blown westward across the ocean, and everything returns to normal. There are some years, however, in which the trade winds fail to strengthen, and the warm surface waters remain off the coast of South America for a year or longer. During these abnormal conditions, upwelling of cold water ceases, and South American ﬁshing industries fail. Although a small El Niño occurs each year, it is this extended El Niño that is referred to as the El Niño condition.
The El Niño condition inﬂuences climate on both sides of the tropical Paciﬁc Ocean. Under normal conditions, upwelling cold water on the eastern side of the Paciﬁc coincides with dry and cool air, high pressures, and clear skies. On the western side of the Paciﬁc, surface waters warm the surrounding air. As the warm and moist air rises, low pressures and storms develop on the warm western side of the Paciﬁc. During an extended El Niño condition, the pattern is reversed. Warm water, rising warm and moist air, low pressures, and storms are found on the eastern side of the Paciﬁc rather than the western side. This exchange of pressure systems and weather patterns between east and west upsets the climate on the west coast of the United States California in particular. El Niño varies in intensity, but it has been known to deliver great storms to northern and southern California along with ﬂoods and mudslides. Temperature-sensitive crops and animal species are impacted. For example, sea lion populations on San Miguel Island have declined in El Niño years because ocean temperatures are too warm to sustain the ﬁsh sea lions eat. Yet, El Niño is cyclical. Recovery comes after the storms have passed. El Niño, earthquakes, mountain building, gold, rich soil, and varied topography are but a few of the physical factors that shape life in California. To understand this place, or any other, it is important to know the underlying science!
Conceptual Integrated Science