Physical Weathering

Physical weathering is a type of weathering whose major impact is the reduction of larger rocks and minerals into smaller sized particles. This reduction exposes a larger surface area of the rocks/minerals to more weathering (both physical and chemical).

Physical weathering occurs when some activities in the environment exerts mechanical force on rocks, leading to their eventual breakdown with time. These activities make up the various processes of physical weathering such as thermal expansion and contraction, action of living organisms, etc. These processes can exert a significant pressure on whatever materials they act on, especially rocks.

Physical weathering is achieved mainly through mechanical force.

Some of the processes involved in physical weathering are:

a)   Thermal Expansion and Contraction:

This process of physical weathering is more commonly observed in the arid and humid tropical regions of the world, where the daily temperature variations can be significant.

In this process, rocks are heated by their exposure to the sun and thus they expand. Usually the surface of the rocks is the first to expand and its expansion is greater than that of the rock’s interior. As temperature reduces with the going down of the sun, the rocks will lose heat and thereby contract as a result. Just as when heated, the surface of the rocks will contract to a greater degree than the interior.

The continuous repetition of the heating and cooling cycle as well as the expansion and contraction differential will result in the rocks chipping and cracking. Therefore, a chipped rock is an evidence of physical weathering through the process of thermal expansion and contraction.

b)   Freezing and Thawing:

This process of weathering is similar to that of thermal expansion and contraction in that they both involve temperature variation as the major factor. However, freezing and thawing is limited to the temperate regions of the world.

In this process of physical weathering, liquid water enters into rocks thus wetting them. As the temperature reduces, the water in the rocks will freeze and the frozen water will exert pressure on the rocks due to its increased volume, thus widening the crevices they occupy. When temperature increases, the water will melt and move out of the crevices, leaving behind larger crevices.

These crevices will then hold more water on the next precipitation, ultimately leading to a greater widening on the next freeze. Over time, this will cause the rocks to split into smaller pieces. Frozen water can exert a pressure of 146 kg cm^-3, giving it the potential to split rocks.

c)   Grinding Action (Collision) of Particles:

The disintegration of rocks due to the collision impact of travelling particles is yet another process of physical weathering.

Water, Wind and gravity can cause some small pieces of rocks, soil particles, and other materials to move from one place to another. In the process of transportation, these particles will collide both with each other and with other stagnant rocks, leading to further disintegration.

d)   Action of Living Organisms:

The living activities of certain plants and animals do contribute to the physical disintegration of rocks and minerals.

Burrowing animals sometimes do cut into rocks, creating a cavity in the process of burrowing. Similarly, the growing roots of certain higher plants do split rocks as they penetrate the rocks.

This can be observed by noting that when certain trees such as the almond tree are planted near a wall or building, such walls will begin to crack as the almond grows. Over time, the cracks will deepen and widen and the wall will eventually fall as the roots make their way to the other side. A similar thing would happen if it were a rock instead of a wall. This is how living organisms contribute to physical weathering.

Related Articles: 
Weathering 
Chemical weathering