In geology , a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley , which may be filled by water forming a rift lake . The axis of the rift area may contain volcanic rocks , and active volcanism is a part of many, but not all active rift systems.
Major rifts occur along the central axis of most mid- ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates.
Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an aulacogen.