Mechanical or Physical weathering generally relies on thermal, frost and pressure actions but sometimes also includes some kind of chemical change.
Thermal: Thermal action is mostly seen in deserts, where temperatures reach extremes during day and night. The repeated fluctuation of temperature shocks the outer layers of rocks, resulting in them breaking or peeling off.
Frost: Mainly occurring in regions with moisture and extremely low temperatures, Frost action depends on the formation of ice and its expansion. Moisture and water can seep into small pores or cracks in rocks and expand upon freezing, weakening the rock’s structure. This in time can result in the rock breaking into smaller pieces.
Pressure: Pressure is a potent weathering agent and works in two main ways. Some rocks, like igneous rocks, are formed deep under the earth’s crust, which puts them under extreme pressure. However, when the top layers are eroded, the pressure is released, causing these rocks to break on the surface.
Moreover, when water, usually in seas and oceans, hits the rocks on the shore with pressure, it traps air inside the pores and cracks. This compressed air then bursts out with a lot of force when the water retreats, causing the rock to break.
Carbonation: Acid rain, which is formed when rain water mixes with gases in the air, like CO2 and SO2, can lead to the dissolution of rocks in time, causing them to break into smaller parts.
Oxidation: This process involves the change of the iron composition of rocks after coming into contact with oxygen and water. The effect, which is commonly known as ‘rusting’, weakens the rocks and causes them to crumble.
Biological: Different plants and microorganisms also release chemicals which can weaken rock composition. Examples of Biological weathering include the action of moss and lichens.