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Graben is a German word geologists use for ground that has dropped between two roughly parallel faults. (Its opposite, ground uplifted between two faults, is called a horst, another German word.) Grabens commonly appear in areas that have been pulled apart by tensional stresses. Horsts would appear where the land has been compressed. (Another compression landform is more common: the wrinkle ridge.)

In some cases, if tensional stresses change direction, a new set of grabens may form at an angle to the first ones. Close examination of images helps scientists determine which graben set is older. They look for which faults cut across which grabens, trying to determine a sequence. Several graben sets may occur in areas where tectonic stresses have changed direction more than once.

Roughly parallel faults scar the southern flank of Alba Mons, a large volcano. The faulting probably happened as the volcano collapsed into the empty magma chamber beneath the surface. Geologists call the raised parts horsts and the dropped-down sections grabens. The small pits at the top of the image include both collapse pits and impact craters. The collapse pits lie along the faults. (NASA/JPL-Caltech/Arizona State University)
Alba Patera Graben
(NASA/JPL-Caltech/Arizona State University)
Where fractures and grabens cut across each other, scientists can determine the relative timing of the events by examining which features intersect which others. Here several fractures that trend from upper left to lower right cut across fractures trending from upper right to lower left. Which is younger? (NASA/JPL-Caltech/Arizona State University)
Crosscutting Graben
(NASA/JPL-Caltech/Arizona State University)