The rate of spreading along the Mid-Atlantic Ridge averages about 2. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers.
Seafloor spreading over the past to million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge.
Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. The map also shows Reykjavik, the capital of Iceland, the Thingvellir area, and the locations of some of Iceland's active volcanoes red triangles , including Krafla.
The consequences of plate movement are easy to see around Krafla Volcano, in the northeastern part of Iceland.
Here, existing ground cracks have widened and new ones appear every few months. From to , numerous episodes of rifting surface cracking took place along the Krafla fissure zone. Some of these rifting events were accompanied by volcanic activity; the ground would gradually rise m before abruptly dropping, signalling an impending eruption. Between and , the displacements caused by rifting totalled about 7 m.
When the continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes.
The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone. East Africa may be the site of the Earth's next major ocean. Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about million years ago. Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa the Horn of Africa a large island.
The size of the Earth has not changed significantly during the past million years, and very likely not since shortly after its formation 4. The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised.
Such destruction recycling of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks is subducted under another.
The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates.
If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. Trenches are the deepest parts of the ocean floor and are created by subduction. Places where these breaks occur are called faults.
A well-known example of a transform plate boundary is the San Andreas Fault in California. Virgin Islands. Home Ocean Exploration Facts What features form at plate tectonic boundaries? What features form at plate tectonic boundaries? The San Andreas Fault is the transform plate boundary where a thin sliver of western California, as part of the Pacific Plate, slides north-northwestward past the rest of North America. In the Caribbean Sea, the U.
Virgin Islands lie along a transform plate boundary where the small Caribbean Plate moves eastward past the oceanic part of the North American Plate. Lillie, New York, W. Norton and Company, pp. This feature includes the famous San Andreas Fault, responsible not only for destructive earthquakes, but also for the spectacular scenery of the San Francisco Bay area and other coastal regions of California.
The San Andreas Fault is responsible for most of the movement in western California, causing a sliver of the state to slide past the rest of the continent. In Mexico, a combinatiion of divergent and transform plate boundary motion is opening the Gulf of California, causing the Baja Peninsula to separate from the rest of Mexico. Letters in ovals are abbreviations for NPS sites listed above. Marshak, , W.
Cabrillo National Monument south of San Diego also lies within the broad zone of deformation between the two plates. The transform plate boundary between the Pacific and North American Plates in western California formed fairly recently. About million years ago, a large tectonic plate called the Farallon Plate started to subduct beneath the western edge of North America.
This resulted in a line of volcanoes stretching all the way from what is now Alaska to Central America. Beginning about 30 million years ago, so much of the Farallon Plate was consumed by subduction that the Pacific and North American plates were in contact, forming the San Andreas transform plate boundary in western California.
Over time, the San Andreas transform plate boundary has grown longer as the Farallon Plate split into two separate plates—the Juan de Fuca Plate on the north, and the Cocos Plate on the south. Remnants of the ancient volcanic mountain chain remain. In central and southern California, for example, the volcanoes have largely eroded away and massive areas of granite from the cooled magma chambers form portions of the Sierra Nevada Mountains, including Yosemite National Park.
Forty million years ago, a large tectonic plate, known as the Farallon Plate, was between the Pacific and North American plates. A transform plate boundary developed where the Pacific Plate was in contact with the North American Plate and the volcanism ceased in central California.
Farther east, the continent began to rift apart in the Basin and Range Province. The Sierra Nevada are the eroded remnants of the volcanic arc developed when the Farallon Plate subducted beneath the continent.
The San Andreas Fault is just one of several faults that accommodate the transform motion between the Pacific and North American plates.
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. In contrast to convergent and divergent boundaries, crust is cracked and broken at transform margins, but is not created or destroyed. Virgin Islands. Home Ocean Exploration Facts What are the different types of plate tectonic boundaries?
What are the different types of plate tectonic boundaries? There are three kinds of plate tectonic boundaries: divergent, convergent, and transform plate boundaries.
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