In deserts and in savannah grasslands important daily changes in temperature occur. This leads to exfoliation or onion peeling. This occurs in rocks of uniform structure e.g. Granite. This changes rocks into rounded boulders. In exfoliation the outer section of the rock is heated up much faster than the inner part so that where weaknesses occur there is a difference in expansion therefore loosening the outside part of the rock and as time goes on and this takes place often it crumbles off leaving rounded boulders.
Frost action occurs when ice collects within cracks in rocks during the day.
As night comes the water freezes and in so doing expands weakening the rock therefore causing fragments to detach and fall. At the bottom of cliffs scree piles can be found, made up of these rock fragments. Block disintegration Plant and animal action. The roots of plants especially trees can force joints and cracks apart in rocks. Some animals by borrowing also help to break up rocks.