How can a large block of granite form layers like an onion?

When a huge block of granite is exposed to the surface, then due to the changes in the prevailing temperature condition at that place, the rock layers will experiences certain changes.

The rocks will gradually expand due to high temperature, and again the rocks undergoes cooling due to the decreasing temperature. In addition to that, the water that gets accumulated at the joints and fractures, also initiates in the freeze-thaw process. This constant cooling and heating of the rock layers and due to the stress acting on that, because of the varying temperature condition, will lead to the breaking or peeling of layers, such as the onion layers, which is commonly know as exfoliation.