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Rift and subduction zones

Plates pull apart from each other along rift zones , such as the Mid-Atlantic ridge, driven by upwelling currents in the mantle ( [link] ). A few rift zones are found on land. The best known is the central African rift—an area where the African continent is slowly breaking apart. Most rift zones, however, are in the oceans. Molten rock rises from below to fill the space between the receding plates; this rock is basaltic lava, the kind of igneous rock that forms most of the ocean basins.

Rift zone and subduction zone.

Illustration of Rift and Subduction Zones. The upper panel shows a rift zone beneath an ocean. At left is a vertical scale of 100 km, from the ocean surface down to the top of the mantle’s partially melted zone, which is labeled at the bottom of the diagram. At top center the mid-ocean rift zone is shown, with arrows pointing left and right indicating the direction of plate motion. Directly below the rift zone magma rises up to fill the spaces and cracks between the separating plates, creating mountains and volcanoes. At far right, the thickness of the crust is indicated, consisting of the basalt from the volcanoes and sediment from their erosion. The thickness of the lithosphere is also shown, from the ocean surface down to the top of the mantle’s partially melted zone. Finally, at the left and right portions of the illustration the older rocks are labeled, with arrows pointing away from the rift zone. The further from the rift, the older the rocks. The lower panel shows a subduction zone beneath an ocean. At left is a vertical scale of 100 km, from the ocean surface down to the top of the mantle’s partially melted zone, which is labeled at the bottom of the diagram. At top center the oceanic trench is labeled. To the right of the trench ocean crust and sediments are indicated, with arrows pointing left showing the motion of the crust toward the trench. At the trench, the ocean crust is forced beneath the continental crust, which is labeled on the left of the diagram. The ocean crust moves down toward the partially melted zone. As it does so, the melting ocean crust becomes hot enough to rise up to the surface (to the left of the trench in this diagram) and create the volcanoes of an island chain. At far right the thickness of the lithosphere is shown, from the ocean surface down to the top of the mantle’s partially melted zone.
Rift and subduction zones are the regions (mostly beneath the oceans) where new crust is formed and old crust is destroyed as part of the cycle of plate tectonics.

From a knowledge of how the seafloor is spreading, we can calculate the average age of the oceanic crust. About 60,000 kilometers of active rifts have been identified, with average separation rates of about 4 centimeters per year. The new area added to Earth each year is about 2 square kilometers, enough to renew the entire oceanic crust in a little more than 100 million years. This is a very short interval in geological time—less than 3% of the age of Earth. The present ocean basins thus turn out to be among the youngest features on our planet.

As new crust is added to Earth, the old crust must go somewhere. When two plates come together, one plate is often forced beneath another in what is called a subduction    zone ( [link] ). In general, the thick continental masses cannot be subducted, but the thinner oceanic plates can be rather readily thrust down into the upper mantle. Often a subduction zone is marked by an ocean trench; a fine example of this type of feature is the deep Japan trench along the coast of Asia. The subducted plate is forced down into regions of high pressure and temperature, eventually melting several hundred kilometers below the surface. Its material is recycled into a downward-flowing convection current, ultimately balancing the flow of material that rises along rift zones. The amount of crust destroyed at subduction zones is approximately equal to the amount formed at rift zones.

All along the subduction zone, earthquakes and volcanoes mark the death throes of the plate. Some of the most destructive earthquakes in history have taken place along subduction zones, including the 1923 Yokohama earthquake and fire that killed 100,000 people, the 2004 Sumatra earthquake and tsunami that killed more than 200,000 people, and the 2011 Tohoku earthquake that resulted in the meltdown of three nuclear power reactors in Japan.

Fault zones and mountain building

Along much of their length, the crustal plates slide parallel to each other. These plate boundaries are marked by cracks or faults . Along active fault zones, the motion of one plate with respect to the other is several centimeters per year, about the same as the spreading rates along rifts.

One of the most famous faults is the San Andreas Fault in California, which lies at the boundary between the Pacific plate and the North American plate ( [link] ). This fault runs from the Gulf of California to the Pacific Ocean northwest of San Francisco. The Pacific plate, to the west, is moving northward, carrying Los Angeles, San Diego, and parts of the southern California coast with it. In several million years, Los Angeles may be an island off the coast of San Francisco.

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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