How is a rift valley formed?

A rift valley is a lowland region that forms where Earth’s tectonic plates move apart or rift. Faulting due to divergence creates the rift system. Continental rifts are potential places where new oceans are expected to form. Rift valleys can also form at transform faults, where tectonic plates are grinding past each other. The Salton Trough, which stretches through the states of California (United States) and Baja California (Mexico), is a rift valley created in part by the San Andreas Fault. 

Further Reading:

• Major Landforms of the Earth
• Geography Notes