Weathering – Chemical + Physical + Biological

by Devender

0 1532

Weathering:

Denudation is the process of wearing away of the earth’s surface which is carried out in four phases - Weathering, Erosion, Transportation & Deposition.

The warm wet climate promotes chemical weathering
The dry climate provides good conditions for physical weathering

Chemical Weathering

It is extremely slow and gradual decomposition of rocks due to exposure to air & water. Regolith is weathered material from the rock or mineral remains of decomposed rocks.

For Example - When Granite is exposed to weather, it is found to be rough-surfaced because it is mainly made up of Quartz, Feldspar & Mica. Feldspar is more quickly weathered than Quartz, therefore, it is worn away. Eventually, leaving loosened quartz crystals.

When soil cover is present on the rock, the chemical weathering of the rock gets enhanced
The soil absorbs rainwater and keeps rock in contact with moisture
The rainwater absorbs organic acids from the soil
Thus, rainwater becomes a stronger weathering agent than pure water

Types of Chemical weathering:

1 Solution:

Many minerals get dissolved with water and especially with rainwater that contains enough carbon dioxide to make it a weak acid. For example - In the case of limestone, rainwater dissolves calcium carbonate from which rock is chiefly formed. Therefore, joints & cracks in the rock are quickly widened, warning it out easily.

Rocks are more resistant to harbor moisture if they have fewer joints and cracks
All rocks are exposed to the solution up to some extent
The extent up to which it is exposed depends on the mineral composition of the rock, its structure, density & climatic conditions

2 Oxidation:

It is the weathering that occurs due to the reaction of oxygen in presence of air & water with minerals present in the rock.

For example - Most rocks contain a certain amount of iron. When iron comes in contact with air, it is changed into iron oxide & finally into rust. It crumbles easily, loosening the overall structure of the rock.

3 Decomposition by Organic Acids:

The soil around the rocks has bacteria that thrive on decaying plant or animal material. These bacteria produce acids which when dissolved in water, speed up the weathering of rocks.

Microorganisms & plants like mosses or lichens can live on bare rock damp surface
They absorb chemical elements from the rocks as food & produce organic acids
Therefore, they become the agent of both Chemical & Mechanical weathering.

Physical Weathering

It is the disintegration by mechanical process and also known as Mechanical Weathering. There are two types of physical weathering:

By insolation
By Frost

1 By Insolation:

Block Disintegration

It mainly takes place in dry desert areas which are hot at day and cold by night. It leads to expansion & contraction of rock that sets up stresses in the rock which leads to its disintegration.

Granular Disintegration

There are different minerals present in rocks which lead to different rates of expansion & contraction of rock bringing fragmentation of rock. For example - Granite

Exfoliation

Stress occurs naturally near the surface & where there are sharp angles in the rock. Thus, rectangular blocks gradually become round by splitting away of sharp corners. It leads to peeling off of the outer layer of rock.

Exfoliation also occurs due to repeated wetting and drying of the rock. When the rock is wet, its outer surface expands and when it becomes dry, it shrinks. It finally leads to the peeling of the outer layer of the rock.

2 By Frost:

It mainly occurs at high altitudes and cold climates. During the daytime, cracks & joints inside rock fill with water while at night, that water gets frozen. Hence, leads to an increase in the volume of water in the rock.