An elongated valley formed by the depression of a block of a planet's crust
between two faults or groups of approximately
parallel faults (see also graben). As the
faults move apart, the central area between them slips down. The friction
produced by this slippage can cause earthquakes
and also melt rock which is then released on the surface from volcanoes.
Large and well-known rift valleys include the Great
Rift Valley in East Africa, the Rheingraben, and Valles
Marineris on Mars.
AND PLANETARY SCIENCE