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Rift valleys are formed by tectonic plate movement and are larger than canyons. The Great Rift Valley in East Africa is the largest in the world. Many of the world’s largest freshwater lakes are located in rift valleys. Some rift valleys are still active and can cause continents to split apart, as was the case with the Atlantic Ocean. The Midcontinent Rift System in North America is over a billion years old and was inundated during the Cretaceous Period, giving rise to the Western Interior Seaway.
A rift valley is a geographic feature caused when an arm of a triple junction between tectonic plates “fails” (that is, stops expanding) and leaves a remnant canyon-like structure. Unlike canyons, which are formed by river erosion, rift valleys, formed through plate tectonics, are much larger. The largest in the world is the 6,000 km (3,700 mi) long Great Rift Valley in East Africa, extending from Syria in the north to Mozambique in the extreme south. There are rift valleys on other planets as well: Valles Marineris, on Mars, 4,000 km (2,5000 mi) long, 200 km (125 mi) wide and up to 7 km (4.3 mi) deep, is the largest known fissure in the solar system.
All of the world’s largest freshwater lakes are located in rift valleys, including Lake Baikal in Russia (the world’s largest lake by volume, containing 20% of the world’s fresh water), Lake Tanganyika in Africa (second largest lake), Lake Superior in the United States (third largest), and a number of others. The largest subglacial lake in the world, Lake Vostok, could be located in such a valley.
Some rift valleys are still geologically active, but most are dormant. If the valley continues to expand indefinitely, a continent may open along it, ocean water will pour in, and a new mid-ocean ridge may form. This was the cause of the breakup of the supercontinent Pangea about 200 million years ago. A rift between Africa, Europe and the Americas began to spread, creating a new ocean, the Atlantic Ocean. Today this rift remains in the form of the geologically active Mid-Atlantic Ridge, where numerous hydrothermal vents are found.
Aside from the Great Rift Valley in Africa, there’s one right in the middle of North America called the Midcontinent Rift System that’s over a billion years old. During the Cretaceous Period, 145 to 65 million years ago, this rift system was inundated, giving rise to the Western Interior Seaway. This is why various marine fossils can be found in Central America today.
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