Rift valleys form when tectonic forces deep underground exert a pulling force on the terrain. In areas where this occurs, the land splits into a steep-walled valley with a flat floor. Rift valleys can be very narrow, especially early in their formation.
Unlike river valleys, rift valleys are not primarily shaped by the erosion of flowing water. Instead, the land of a rift valley is literally torn apart by the conflicting forces acting on the local bedrock. Rift valleys can be very deep, and both such valleys known to exist in continental areas on Earth, the East African Rift and the Siberian Baikal Rift, have large, deep lakes in them.
As the walls of a rift valley are pulled apart, the floor of the valley tends to sink. If this subsidence happens faster than erosion from the walls can fill it in with sediment, the floor of the valley continues to fall relative to the surrounding terrain. This process is at work in both the Baikal rift and the East African rift. In each case, a crustal plate has begun to tear itself away from the main continental plate. Over tens of millions of years, these plates separate entirely and drift away from their parent landmass.