II. Physical Weathering Process

Physical weathering is the mechanical disintegration of rock-forming minerals by different geomorphic agents. The main factors responsible for it are (i) temperature change, (ii) the crystallization of water or other crystal growth, (iii) pressure-release mechanism, (iv) mechanical action of plants and animals. These factors act slow but can cause great damage to the rocks because of continued stress or fatigue developed in the rock.

♤ Expansion by unloading – pressure release (unloading) mechanism causes disintegration of rock. It is because of continued erosion by various geomorphic agents. Fractures develop roughly parallel to the surface. This process has been termed exfoliation. Exfoliated sheets may measure thousands of meters.

♤ Temperature change and expansion – thermal expansion of rock is the cause of rock cracking and disintegration. If you travel to arid-tropics, it is possible that you may hear sounds like rifle shots which are actually cracking of the rock as they contract. The theory is that rocks are poor conductors of heat. Due to strong diurnal heating, the outer layers of the rock warm up considerably, but do not transmit heat to the inner layers. During night when temperature falls, same layer gets contracted. This should lead to the setting up of stresses in the rock, causing fracturing parallel to the surface

♤ Salt weathering – a number of salts such as Sodium Chloride, Calcium sulphate may enter rocks in dissolved form. On drying and crystallization they expand and set up a disruptive effect. Expansion of these salts depends on temperature and their own thermal properties. Force exerted by crystallization is sometimes more than the tensile strength off rocks, thus causes splitting. Areas with alternating wetting and drying conditions favour salt weathering.

♤ Frost action and crystal growth – frost action is one of the most important weathering processes in cold climates. When water fills the pores, cracks and crevices in rocks and then freezes, it expands and exerts a bursting pressure. The rocks are fractured, cracked. In this process, rate of freezing is important. Freezing also penetrates to a greater depth when the ground is bare rather than forest covered.

These processes – chemical and mechanical – are not stand alone activities. Different processes acted upon same rock and produced net resultant weathered material together. For instance, both chemical and mechanical weathering processes further weaken the joints, the layers

thereby peeling off in sheets. It is probably best to conclude that chemical weathering and pressure release ally with temperature changes to produce rock disintegration. It is likely that hydration process may also be involved when crystallization takes place. Another instance is of hydration where hydration itself is a mechanical effect, but it occurs intimately with hydrolysis in such a manner that it is difficult to draw any hard and fast line here between mechanical and chemical weathering. Actions of plants, human and animal affect both chemical and mechanical weathering.