The impact of the colliding plates can cause the edges of one or both plates to buckle up into a mountain ranges or one of the plates may bend down into a deep seafloor trench. A chain of volcanoes often forms parallel to convergent plate boundaries and powerful earthquakes are common along these boundaries.
The Pacific Ring of Fire is an example of a convergent plate boundary. At convergent plate boundaries, oceanic crust is often forced down into the mantle where it begins to melt. Magma rises into and through the other plate, solidifying into granite, the rock that makes up the continents.
Volcanoes are one kind of feature that forms along convergent plate boundaries, where two tectonic plates collide and one moves beneath the other.
This photo shows an explosion near the summit of the West Mata volcano within the Pacific Ocean; the image area is about 1. Download larger version jpg, 1. Heat within the asthenosphere creates convection currents that cause tectonic plates to move several centimeters per year relative to each other. If two tectonic plates collide, they form a convergent plate boundary. The plate boundary is a broad zone of deformation with a width of about 60 miles kilometers.
Along much of the boundary, the bulk of the motion occurs along the San Andreas Fault. Other parks in the region, namely Pinnacles, Channel Islands and Joshua Tree national parks, Cabrillo National Monument and Santa Monica Mountains National Recreation Area, reveal evidence of the shearing, rotation, and uplift that occurs within the broad zone of deformation between the two plates.
Deformation along the transform plate boundary in California can be visualized by placing a deck of cards between your hands in a praying position. Imagine that your left hand is the undeformed Pacific Plate, your right hand the intact North American Plate. Notice what happens as you move your left hand away and slide your right hand toward you. The cards slip along their faces, forming a broad zone of shearing between your unaffected hands. For western California, each slipping card face would be a fault surface.
The broad zone of transform motion between the Pacific and North American plates formed numerous slivers of mountain ranges with narrow valleys in between. The valleys are commonly due to erosion along individual fault lines. Lillie, Wells Creek Publishers, 92 pp. The broad zone of shearing at a transform plate boundary includes masses of rock displaced tens to hundreds of miles, shallow earthquakes, and a landscape consisting of long ridges separated by narrow valleys.
The San Andreas Fault is just one of many active earthquake faults in a broad zone of shearing along the transform plate boundary in the San Francisco Bay Area.
These forces also create a sheared-up landscape that includes spectacularly beautiful coastlines and economically important harbors. Thousands of earthquakes over millions of years have built this landscape not only along the major fault line—the San Andreas Fault—but also on other faults within the broad zone of shearing between the Pacific and North American plates. For example, rocks found today in Point Reyes National Seashore north of San Francisco were originally part of the line of granite rocks formed beneath ancient subduction zone volcanoes.
The plate motion has plucked the rocks from their original position and moved them more than miles north-northwestward to their current position at Point Reyes. Other rocks in the San Francisco Bay Area were originally part of an accretionary wedge, similar to rocks found today in the coastal ranges of the Cascadia Subduction Zone in northern California, Oregon, and Washington.
The transform plate boundary is a broad zone forming as the Pacific Plate slides northwestward past the North American Plate. It includes many lesser faults in addition to the San Andreas Fault. Parks near the coast, including Point Reyes National Seashore, Golden Gate National Recreation Area, and Pinnacles National Park, contain volcanic and plutonic rocks that were plucked from the edge of the North American Plate and transported tens to hundreds of miles northwestward as part of the Pacific Plate.
Parks in the Sierra Nevada, including Yosemite, Kings Canyon, and Sequoia national parks, contain granite-type rocks that cooled within magma chambers beneath ancient subduction zone volcanoes. National Park Service sites along the transform plate boundary in California contain rocks formed during the earlier subduction that occurred in western North America. Like modern subduction zones, the region had an accretionary wedge Coast Range , a forearc basin Great Valley , and a volcanic arc Sierra Nevada.
Rocks have been disrupted by shearing and other forces associated with the transform plate motion and, in some instances, transported northward a long distance from where they originally formed. The magnitude 7. It caused extensive damage to the city, including fires that lasted for several days, and killed an estimated 3, people. The Earthquake Trail at Point Reyes weaves back and forth across the fault line.
Exhibits along the trail include the reconstruction of a fence that was offset 16 feet 5 meters during the earthquake. Doing some quick math, one can appreciate how dramatically plate-tectonic forces can affect the landscape, even in our lifetimes.
The average movement of the Pacific Plate past the North American Plate in California is about 2 inches 5 centimeters per year. The 16 feet about inches, or 5 meters of offset along the fence line thus carries a powerful message. Every century or so a large earthquake is necessary to release stress accumulated along large segments of the San Andreas Fault that lock rather than slip smoothly.
The movements of these plates can account for noticeable geologic events such as earthquakes, volcanic eruptions, and more subtle yet sublime events, like the building of mountains. Teach your students about plate tectonics using these classroom resources. Explore the boundaries between Earth's tectonic plates with MapMaker, National Geographic's classroom interactive mapping tool.
In , after decades of tediously collecting and mapping ocean sonar data, scientists began to see a fairly accurate picture of the seafloor emerge. The Tharp-Heezen map illustrated the geological features that characterize the seafloor and became a crucial factor in the acceptance of the theories of plate tectonics and continental drift.
Today, these theories serve as the foundation upon which we understand the geologic processes that shape the Earth. Scientists look to these landforms and geologic events as evidence of plate tectonics, helping them both understand what happened in the past as well as predict what Earth will look like in the future. The five activities in the Plate Tectonics module build a systems view of plate tectonics, engaging students in data exploration about plate boundaries and experimentation via computer-based models of plate motion.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image by Naeblys. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Also called a collision zone. Ring of Fire. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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