A rift valley is a geographic feature caused when one arm of a triple junction between tectonic plates "fails" (i.e., stops spreading) 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 one in the world is the Great Rift Valley in East Africa, 6,000 km (3,700 mi) in length, extending from Syria in the north to Mozambique in the far south. There are rift valleys on other planets, too: Valles Marineris, on Mars, at 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 crevice in the solar system.
All of the world's largest freshwater lakes are in rift valleys, including Lake Baikal in Russia (the world's largest lake by volume, holding 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 world's largest subglacial lake, Lake Vostok, may lie in such a valley.
Some rift valleys are still geologically active, but most are dormant. If the valley continues spreading indefinitely, a continent may cleave along it, ocean water rushes in, and a new mid-ocean ridge may be formed. This was the cause of the breakup of the supercontinent Pangaea about 200 million years ago. A rift valley 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 can be found.
Aside from the Great Rift Valley in Africa, there is one right in the middle of North America, called the Midcontinent Rift System, which is more than a billion years old. During the Cretaceous period, 145 to 65 million years ago, this rift system was flooded, resulting in the Western Interior Seaway. This is why various marine fossils can be found in middle America today.