![]() The East African Rift is part of the “Great Rift Valley” system discussed below. ![]() The East African Rift, the Baikal Rift Valley, the West Antarctic Rift, and the Rio Grande Rift are Earth’s major active continental rift valleys. Very few active rift valleys are found on continental lithosphere. Geologic activity beneath the underwater rift valley creates these vents, which spew superheated water and vent fluids into the ocean. Like many underwater rift valleys, the East Pacific Rise is dotted with hydrothermal vents. In the Pacific Ocean, the East Pacific Rise has created rift valleys where the Pacific plate is separating from the North American plate, Cocos plate, Nazca plate, and Antarctic plate. Over millions of years, the Mid-Atlantic Ridge has formed rift valleys as wide as 15 kilometers (nine miles). In the northern Mid-Atlantic Ridge, the North American plate and the Eurasian plate are splitting apart at a rate of about 2.5 centimeters (one inch) per year. As tectonic plates move away from one another at mid-ocean ridges, molten rock from the mantle may well up and harden as it contacts the frigid sea, forming new oceanic crust at the bottom of the rift valley. Many of Earth’s deepest rift valleys are found underwater, dividing long mountain ranges called mid-ocean ridges. The San Andreas is a transform fault that marks the roughly northward movement of the Pacific plate and the roughly southern movement of the North American plate. The Salton Trough, which stretches through the states of California (United States) and Baja California (Mexico), is a rift valley created in part by the San Andreas Fault. Rift valleys can also form at transform faults, where tectonic plates are grinding past each other. Two arms of a triple junction on the supercontinent Pangaea “opened” the ocean, while the aulacogen formed the rift valley known as the Benue Trough through what is now southern Nigeria. The Atlantic Ocean, for instance, is a result of a triple junction that started in what is now the Gulf of Guinea on the west coast of Africa. The third, “failed rift” or aulacogen, may become a rift valley. Two arms of the triple junction can split to form an entire ocean. ![]() Many rift valleys are part of “ triple junctions,” a type of divergent boundary where three tectonic plates meet at about 120° angles. Tectonic plates are constantly in motion-shifting against each other in fault zones, falling beneath one another in a process called subduction, crashing against one another at convergent plate boundaries, and tearing apart from each other at divergent plate boundaries. Tectonic plates are huge, rocky slabs of Earth's lithosphere-its crust and upper mantle. Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion. Rift valleys are found on land and at the bottom of the ocean, where they are created by the process of seafloor spreading. A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift.
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