A rift is a region where the lithosphere extends as two parts of the Earth's crust pull apart. Often a rift will form in an area of the crust that is already weakened by earlier geological activity.[1] Extensional faults form parallel to the axis of the rift.[2] An extensional fault may be seen as a crack in the crust that extends down at an angle to the vertical. As the two sides pull apart, the hanging wall ("hanging over" the sloping fault) will move downward relative to the footwall.[3] The crust thins and sinks, forming a rift basin.