rift valley

Geol.

1. graben.

2. a subsea chasm extending along the crest of a mid-ocean ridge, locus of the magma upwellings that accompany seafloor spreading.

[1890-95]

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Elongated trough formed by the subsidence of a segment of the Earth's crust between dip-slip, or normal, faults.

Rift valleys are usually narrow and long and have a relatively flat floor. The sides drop away steeply in steps and terraces. Rift valleys are found on the continents and along the crests of oceanic ridges. They occur where two plates that make up the Earth's surface are separating (see plate tectonics). Submarine rift valleys are usually centres of seafloor spreading, where magma wells up from the mantle. The most extensive continental rift valleys are those of the East African Rift System; other notable examples include Russia's Baikal Rift Valley and Germany's Rhine Rift Valley.

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(Africa),also called  Eastern Rift Valley, or Great Rift System, 

      major branch of the East African Rift System (q.v.).

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