Harden, Chapter 1
Plate Tectonics and California
Overview
Most Californians know that earthquakes and the San Andreas fault have something to do with movement of the Earth’s plates, but few residents realize the importance of plate tectonics in shaping California. According to the plate tectonic theory, which has been widely accepted for the past 25 years, the Earth’s outer layers consist of a number of rigid plates. These plates are moving, and their encounters with each other cause many of Earth’s dynamic events. Today, plate tectonics is responsible for some obviously unwelcome aspects of our lives – earthquakes and volcanic eruptions – as well as some of the scenery that attracts people to California – rugged mountain ranges and beautiful, rocky coastlines. The region that we now call California has been at an active plate boundary for the past 230 million years, and this has been the dominant factor in California’s geologic history.
The Layered Earth
Although the deepest holes drilled into the Earth’s interior have only penetrated 10,000 meters, scientists have long known from indirect evidence that the Earth is a layered sphere. The outer most thin skin of the Earth is the crust, a rigid layer of rocks rich in silicon and oxygen. The crust is only about 5 kilometers thick beneath the oceans and 50 or more kilometers thick beneath continental mountains like the Sierra Nevada. Beneath the crust is the mantle, which extends inward about halfway to the center of the Earth and makes up more than 80 percent of the Earth’s volume. Like the crust, the mantle is composed of rocks rich in silicon and oxygen, but its rocks also contain more iron and magnesium, making them dense than those found in the crust.
Rocks believed to originate in the mantle are found in several places in California. They were brought to the surface by different processes, including volcanic eruptions and plate collisions. For example, in the Mojave Desert, bits of mantle rock have been brought to the Earth’s surface in recently erupted lava (see Chapter 7). Indirect evidence about Earth’s deeper interior is also provided by the behavior of seismic (earthquake) waves. Beneath the mantle is the core, which has a liquid outer part and a solid inner part. Both the inner and outer core are thought to be composed mostly of iron and nickel.
Question: Where is the deepest mine in the Tri-State? Does it contain scientific instruments?
Plate Tectonics and Plate Boundaries
Interactions between plates generate much of Earth’s geologic activity. California owes its active seismicity and recent volcanic eruptions to the fact that both the Pacific Plate and the Gorda Plate meet the North American Plate in California. Most of the world’s earthquakes occur at plate boundaries; in fact, the zones of concentrated seismicity are one of the main lines of evidence used to identify plate boundaries. Along plate boundaries, the different motions of the plate create stress, and that stress is relieved during earthquakes.
Plates can interact in three ways. They may separate along divergent plate boundaries, where the crust is stretched and thinned. At divergent boundaries, molten magma rises and erupts to form new oceanic crust. Today an active divergent boundary in the Gulf of California is causing Baja California, Mexico to break away from the Mexican mainland as new oceanic crust forms in the Gulf. This process is one example of sea-floor spreading, the process that gave birth to the Atlantic Ocean as the Americas separated from Europe and Africa. Today satellite systems and laser-instrumented surveying devices allow scientists to measure the opening of the Gulf of California from space. Based on the results of repeated surveys with global positioning satellite (GPS) systems, it appears that the area is spreading at the rate of about 5 c. per year.
Tectonic plates collide at convergent boundaries. At these zones, crust is either piled up into mountain ranges by buckling, faulting, and folding along the plate boundary, or shoved into the mantle during subduction. Subduction occurs if one or both of the converging plates is an oceanic plate. At the point of convergence, one plate – always the oceanic crust if the other plate is a continent – is overridden and driven beneath the other plate along a subduction zone. Along most subduction zones, a deep ocean trench marks the point where the plates converge. As the plate descends into the mantle, earthquakes are generated. Subduction zones are the only tectonic setting where earthquakes originate at great depths, and they also give rise to the largest magnitude earthquakes. See Chapters 13 and 14.
As subduction drives the dense, water-rich rocks of the oceanic plate deeper within the Earth’s interior, water is driven out of the subduction rocks. This dewatering causes partial melting in the mantle above the subducted rocks, creating subduction-zone volcanoes. As we will see in Chapter 5, these volcanoes are quite different from those found at divergent boundaries. Today along the northern margin of California, the Gorda Plate is being subducted beneath the North American plate. As a result, northwestern California faces the threat of a great subduction-zone earthquake, and areas farther east are at risk of major volcanic eruptions from Mt. Shasta and Lassen Peak.
During convergence and subduction, pieces of the subducted plate may also be scraped off along the plate margin, where they become attached to the upper plate. These pieces are referred to as accreted terranes. Throughout California, examples of ancient collisions are found in the form of oceanic fragments attached to the North American Plate (See Chapters 8, 9, and 12). In many accreted terranes, fragments of mantle have been scraped up along with the oceanic crust, providing geologist with another opportunity to study rocks formed at great depths (See Chapter 9).
Plate boundaries where two plates are moving past each other without diverging or converging are known as transform plate boundaries. Along transform boundaries, earthquakes record the plate motions as they do along the other boundary types. However, volcanic eruptions are generally absent. The most famous transform plate boundary in the world is California’s San Andreas fault system, which separates the Pacific and North American plates. The faults of the San Andreas system show that the Pacific Plate is moving to the northwest relative to the North American Plate. The relative motion, or the displacement, on these fault is in a right-lateral direction, as illustrated in Fig. 1-7.
The actual boundary between the Pacific and North American plates is best thought of as a wide zone rather than a line. The effects of plate motion, such as active transform faults and recently uplifted mountain ranges, are seen across the width of California and even farther east. The San Andreas fault is conventionally thought of as the plate boundary, but only about half to two-thirds of the plate motion takes place within the San Andreas fault zone. In the San Francisco Bay area and south of the Transverse Ranges, plate motion takes place on other faults of the San Andreas system east and west of the San Andreas itself. Additional lateral motion takes place along the eastern edge of the Sierra Nevada, in the central Mojave Desert, and even east of California. The complexities of California geology are revealed when one realizes that even the question “Where is the exact boundary between the Pacific and North American plates?” has no precise answer.
Coastlines and Plate Boundaries
As shown on the map of the earth’s tectonic plates, not all plate boundaries are at the borders between continents and oceans. Places where the edges of a continent does coincide with a plate boundary are termed active continental margin, whereas continental margins far from active plate boundaries are known as passive continental margins. (west coast of US – active, east coast of US – passive) During its early history, in Paleozoic time, the area we now call California was a passive margin. See Chapter 7. It then became a subduction-zone active margin during the Mesozoic and early Cenozoic era. See Chapter 8 and 12. During the past 25 million years, a transform plate boundary has been present along most of California, although subduction continues along northern California.
Causes of Plate Motions
Within the Earth’s upper mantle is a layer known as the asthensosphere, a weak, semiplastic layer that underlies the moving plates or lithosphere. This layer is important in understanding plate tectonics, because the slow flow of the asthenosphere is the “conveyor belt” that moves the plates. The boundary at the base of the plates, where relatively cool lithosphere meets hotter, more plastic asthenosphere, is about 100 to 250 km beneath the Earth’s surface. Scientists believe that they have located the top of the asthenosphere using seismic techniques, but they are less certain about the mechanisms that cause plates to move.
Until recently, geologist believed that convection currents in the asthenosphere caused the rigid plates above it to move. These convection currents cause hot mantle rock to rise, cool, and then sink again, creating circulation within the uppermantle. More recent ideas about plate tectonics focus on density and gravity differences within the lithosphere as the driving forces. After molten material rises to the surface and forms new oceanic crust at divergent boundaries, the plate cools and becomes denser. As a result, the cooler part of the plate sinks back into the asthenosphere at convergent boundaries, pulling the rest of the plate with it. Geologists hypothesize that this cooling and sinking of a plate could take as much as 60 to 70 million years.
Tracking the Movement of Plates
Because all of the Earth’s plates are mobile, the plate boundaries provide clues only to the relative motions between plates. The actual movement directions and rates for individual plates – as they would be seen from space, for example – are measured in several ways. GPS systems are one method for precisely determining plate motions or the vertical growth of mountain ranges. GPS data are currently being used for many geologic applications, including the monitoring of plate motions For example, the Southern California Integrated GPS Network (http://www.scign.org) is a network of approximately 250 GPS receivers in the Los Angeles Basin and surrounding region. This network continuously records the tectonic movement of the Earth’s crust and can measure changes in the ground surface on a scale of millimeters. The Southern California Earthquake Center, NASA’s Joint Propulsion Laboratory, and the U.S. Geological Survey established the network following the 1994 Northridge earthquake. See Chapters 13 and 14.
Geologists can also determine the long term movement of plates at hot spots. These are concentrated areas of very high heat flow deep within the mantle that cause rocks to melt above them. Because these hot spots are located below the plates, scientists believe that the spots remain fixed while the plates move over them. Directly above the hot spots, lava rises through the plate to form volcanoes. As the plate moves, new volcanoes appear in the part of the plate newly positioned over the hot spot. After millions of years of plate motion over a fixed hot spot, a chain of volcanoes and volcanic rocks is left as a marker of the direction and the long-term rate of plate motion. In this way, the Hawaiian islands record the movement of the Pacific Plate. Similarly, the volcanic rocks of eastern Oregon, Idaho, and Wyoming record the movement of the North American plate over the Yellowstone hot spot.
The absolute direction of the movement of the Pacific Plate is northwestward, whereas that of the North American plate is southwestward. Where the two plates meet along the northwest-oriented boundary, the relative motion between them results in the right-lateral motion that characterizes the San Andreas fault system. See Chapter 14. A small amount of this motion, about 10 percent, is convergent, resulting in the formation of mountains, as discussed next.
Deformation
Movement between plates creates forces that act on the rocks located near plate boundaries. In any given area, force applied to a particular body of rock, regardless of its size, create stress of the rock. Tectonic stress causes rocks and landforms to be disrupted, creating features that geologists can use to interpret the forces that created them. By studying these patterns of deformation and analyzing their geometry, geologists can infer the modern stress conditions, as well as the ancient forces recorded in geologic features of an area. However, because plate boundaries are not simple planes between homogeneous blocks of rock, the patterns of deformation are often complex and difficult to interpret.
In general, convergent motion causes rocks to be compressed. Compression causes rocks to be crumpled and buckled by folding and faulting, all of which act to shorten the Earth’s crust. Reverse and thrust faults result from shortening, and these are the types of faults found in regions undergoing compression, such as the Transverse Ranges. See Chapter 16.
Divergent motion causes the crust to be extended. As the crust is extended or stretched, blocks of rocks can slide apart, creating depressions. In these settings, including California’s Basin and Range Province, see Chapter 7, normal faults result. Further complexity can be added to the picture. For example, a mountain range might be uplifted in response to compression, and the uplift itself might at the same time cause extension along the crest of the range.
An area affected by purely transform motion is subjected to shear stress, creating transform or strike-slip faults like the major faults of the San Andreas system. See Chapter 14. However, the forces operating along the San Andreas fault system are not created by pure transform motion. In many areas, convergent motion accompanies the transform motion along the plate boundary; geologists refer to this combination of forces as transpression. As a result, vertical movements are recorded during earthquakes, and entire mountain ranges have been buckled and uplifted in the fault zone during the past few million years. California’s Coast Ranges, Transverse Ranges, and even the Sierra Nevada are testimony to the significant recent compression along the plate boundary. During the geologically recent past, a significant component of extension accompanied transform motion along the San Andreas system, caused by transtension along the plate boundary. This past extension is record in a series of fault-related marine basins and volcanism, as reconstructed from rocks of that period.
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