sphere buttons

Plate Tectonics

[Click for larger image]
Earth's Major Plates
One of the most important recent scientific discoveries, involves the development of the theory of Plate Tectonics. According to this theory, the Earth's outer layer, or crust, consists of a series of plates made up of lithospheric material which floats on the denser asthenosphere of the upper mantle within the Earth's interior. Spreading centers at the bottom of the world's oceans bring up liquid rock from the asthenosphere as the plates move apart. When the liquid rock, or magma, comes in contact with the cold ocean water, it cools rapidly to form brand-new lithospheric crust. Gradually, the freshly-made rock settles into a hard, dense material called basalt. As new magma rises into the spreading center, it repeats this process, and pushes the freshly-made rock to the side. Two famous spreading centers in the ocean are the Mid-Atlantic Ridge, and the East-Pacific Rise. Spreading centers can also exist on land. The East African Rift Valley represents a boundary where a large portion of Africa is breaking off of that continent. One day, perhaps millions of years from now, water from the Indian Ocean will rush into the Rift Valley in a catastrophic flood, and East Africa will instantly become a large island. Over time, Earth's plates collide with each other, and one plate can be forced underneath another. These suture zones are like large recycling centers; lithospheric material, as it is subducted under the other plate, melts in the extreme heat of the Earth's interior to become magma once again. This hot, molten, less-dense rock, however, likes to rise toward the surface, and can be discharged onto the Earth's surface through a volcanic eruption or help to build huge mountain ranges. The most extreme example is the Peruvian Trench along the western edge of South America. Here, the Nazca Plate is being forced beneath the South American Plate. The Andes Mountains, which runs parallel to the trench, is a direct result of this dramatic, ongoing collision.
[Click for larger image]
A 3-D shaded image showing the location of the San Andreas Fault Zone
Another type of plate-to-plate interface is called a transform boundary. In this case, one plate is grinding past one another in opposite directions. The most well-known example of this in the world, is the San Andreas Fault Zone in California. Earthquakes in this region occur as one plate violently jerks past the other over short distances in a matter of seconds.
[Click for full image]
Map showing the extent of the "Ring of Fire".
Plate boundaries are commonly expressed as landforms on the seafloor or continental margins. For example, the Marianas Trench, which is the deepest location on the seafloor is located where the Pacific Plate is being subducted under the Phillipine Plate. The Himalayas formed where the Indian and Eurasian Plates collided. This collision between two large plates of lighter lithosphere has resulted in this mammoth mountain range, home of the tallest mountain in the world, Mount Everest, which towers over 5 miles above sea-level. "The Ring of Fire" describes the margins of the Pacific Plate, which is ringed by volcanoes and experiences frequent catastrophic earthquakes, from New Zealand to Indonesia to Japan and Russia's Kamchatka, through Alaska's southern fringe along the entire western coast of the Americas. Plate Tectonics could be considered one of the most important theories within our lifetimes, in that it explains the distribution of all major mountain ranges, why earthquakes and volcanoes occur, where they do, and how continents drift long distances over millions of years.

Contact Science Advisor
©2001 Museums Teaching Planet Earth