What type of force causes exfoliation

Exfoliation is where layers of rock split off parallel to the surface of a larger rock body.

In smaller rocks, heating and cooling (especially in a desert where it is very hot in the day and cold at night) can cause the rock to expand and contract near the surface and break off in layers.

Large, thick layers of rock split off parallel to the ground surface in some cases, and these might be the result of the fact that the rock used to be under the ground, and when erosion removed the rocks on top of it, it expanded because it had less weight on it.