Weathering processes can be subdivided into the following:

Weathering processes can be subdivided into the following:

1. Physical weathering or Mechanical weathering.

2. Chemical weathering.

3. Biological or Organic weathering.

(1) Physical weathering processes:

When rock is broken and disintegrated without any chemical change, the process is called physical weathering or mechanical weathering. It is performed by the following processes:

Crystallization is a form of mechanical weathering. It is operative especially in arid climates. Due to dry weather, moisture is drawn to the surface of rocks.

Exposed as it is, the water evaporates and crystals form from dissolved minerals. In due course of time, the crystals grow large exerting a force great enough to spread apart individual mineral grains, and thus the rock begins to break up.

Hydration is another process which involves water, but there is no chemical change whatsoever. In this process water is absorbed by a mineral.

Because of addition of water the result is swelling and stress within the rock, which mechanically forces the grains apart as the constituents expand. Hydration causes granular disintegration.

Frost action takes place in the joints and fractures in rock, since these are areas of weakness that encourage mechanical weathering by making fragmentation easier.

This process of mechanical weathering is caused by alternating cycles of freezing and thawing of water in pores, fissures and cracks on the surface of rock. Frost action is synonymous with freeze-thaw action.

On freezing water expands nine percent of its volume and exerts a huge pressure of about 150 kg to per square centimeter. The rocks are then ruptured and their fragments are wedged apart.

This process is called as block separation. When thaw sets in the rocks, they become loose. Great blocks of rocks are thus produced which can be seen on the tops of high mountains.

Remember that this process of physical weathering is operative in middle and high latitudes and at high altitudes, where it is the most effective single agency of rock disintegration.

In favourable climatic conditions due to repea­ted freezing and thawing, small cracks be­come larger and blocks of rock are sepa­rated. Sometimes these blocks are dislodged from their original position.

This weathering action is called frost-wedging. Wedging is very common in high mountains beyond the timberline.

The bare rocks of such mountains are broken into very small fragments by frost wedging, which fall and accumulate in sloping cone-shaped piles called talus cones at the base of mountains. Such debris is angular in shape.

Pressure release jointing:

In case the overlying layers of rocks are eroded and transported away, the release of this weight-caused pressure may allow the exposed rock to expand and form new joints (called pressure-release jointing).

Thus, the work of frost-action starts with the result that layer after layer of rock peels off in curved slabs or plates. As these slabs are subjected to weathering, they slip off in process called sheeting.

This is the exfoliation process. It may be pointed out that coarse-grained crystalline granites rocks are particularly prone to this type of weathering.

The exfoliation process forms arch-shaped and dome- shaped features on the exposed landscape forming an exfoliation domes which represent the largest single weathering features on earth.

(2) Chemical Weathering

Chemical weathering is the decompo­sition of rocks which results from attack by the chemically active constituents of the soil and atmosphere.

The more complete the fragmentation of the rocks, the more intense are the chemical processes because of the greater specific surface exposed.

There are two types of chemical changes taking place in rock: firstly, the destruction of certain existing minerals, and secondly, the formation of secondary products. The chemical destruction of minerals depends on five main types of reaction given below:

(i) Solution, (ii) hydration, (iii) hydrolysis, (iv) carbonation, and (v) oxidation/reduction.

It is to be borne in mind that these reactions hardly act alone but occur in various combinations with each other. They all increase with temperature and, therefore, are more effective in low latitudes.

Solution is a less important form of chemical weathering. It does not involve chemical change. Rock is an aggregate of minerals.

There are some rock minerals which are readily dissolved in neutral or acidic water such as sodium chloride (table salt) and calcium sulphate.

Mineral salts that easily dissolve in water are called evaporates, since they are precipitated when water is saturated with them as during the evaporation process. Some minerals that are produced as a result of previous chemical weathering are also soluble.

There are certain minerals like calcium, magnesium, potassium and sodium that are most vulnerable to solution. However, some minerals are insoluble or slightly soluble in pure water. But these minerals are easily dissolved by acidic solution.

For example, rain water while passing through the atmosphere absorbs some carbon dioxide, one of its constituents.

Thus, it forms a very weak solution of carbonic acid which can dissolves a variety of minerals, particularly calcite or calcium carbonate, the main ingredient of limestone.

When acted upon by carbonic acid, calcium carbonate forms calcium bicarbonate which readily dissolves in water. Remember that solution of limestone is the outcome of both, carbonation as well as solution.

In humid tropical regions removal of calcium bicarbonate weakens rock by enlarging the pore openings. Karst topography presents a fine example of chemical weathering by the combined action of carbonation and solution.

Hydration:

Hydration is an important process in chemical weathering. As discussed earlier, hydration involves the addition of water to a mineral causing it to expand which creates stress within the rock mechanically forcing grains apart as the constituents expand.

Hydration causes granular disintegration and further susceptibility of the rock to chemical weathering especially by oxidation and carbonation. The process of hydration changes feldspar minerals into kaolinite clays. This process is called kaolinization.

Hydrolysis:

The chemical combination of minerals with water is called hydrolysis. Unlike hydration in which water is only absorbed, the process of hydrolysis involves the combination of water with rock minerals to form an insoluble precipitate like clay minerals and insoluble components except in the special case of carbonation where only soluble products are created.

Remember that in this process active participation of water in chemical reactions produces entirely different minerals. By-pro4ucts of this chemical weathering of granite are clay and silica.

As clay is formed from the minerals in granite, quartz particles are left behind and they are eventually converted into sand. It may be mentioned that silicate minerals are subject to great changes by the process of hydrolysis.

In many cases hydrolysis results in expansion that can lead to exfoliation. In humid tropical areas hydrolysis can occur a few meters below the surface, where it weakens and decays rocks at great depth.

Carbonation:

Carbonation represents a type of chemical weathering of rock by rainwater which in combination with dissolved carbon dioxide forms a weak carbonic acid. This changes any rock minerals containing lime, soda, potash or other basic oxides into bicarbonates.

Therefore carbonation is a very important process in the reduction of limestone terrain by dissolving minerals and carrying them away as calcium bicarbonate. This process of chemical weathering is more common in the karst topography.

Oxidation:

Oxidation is a variety of chemical weathering in which there is the chemical union of oxygen with certain rock minerals. Oxygen dissolved in water reacts with some rock minerals, especially iron, to form oxides and hydroxides.

One of the most common forms of the oxides is iron rust, derived from the chemical union of iron and oxygen. Rust is composed of two iron oxides; the minerals hematite and limonite.

This manifests itself on the rock surface as brownish or yellowish stains. Such rocks ultimately disintegrate. The results of oxidation are most readily shown when the rocks affected by carbonation contain iron, which is a very common; element.

The weathered surface of many rocks shows a yellow or brown crust; the ferrous state in which iron commonly occurs, changes into the oxidized ferric state, and this crust eventually crumbles.

Spheroidal Weathering:

This type of chemical weathering occurs below the surface. In this weathering jointed rock masses are slowly rounded by the gradual removal of their concentric outer layers. Thus, there remains a spherical boulder.

The rock shells are made loose by the process of hydrolysis. The fragmented materials are finally removed by underground water. Spheroidal weathering at depth produces boulders of decomposition which appear on ground surface following exhumation.

(3) Biological or Organic Weathering:

Besides mechanical and chemical weathering agents, there are plants and animals of various categories which are also important agencies of weathering. This is called biological or organic weathering.

Roots of trees and other plants grown on the hills penetrate into the crevices of rocks and make them wider. By this action of vegetation, water as well as air enters into the cracks in rocks; the total effect of this is the disintegration of the rocks.

However, in addition to the destructive work, the plants and trees provide protective covering to the rocks and regolith.

The roots of the plants and, trees bind the fragments of the rocks together so that they are made safer from the onslaught of different agencies of weathering.

The natural vegetation produces different kinds of acids such as humic acids, bacterial acids and other organic acids which bring about the decomposition and decay of rocks. Bacte­rial acids working in different ways help in the decomposition and decay of rocks.

Burrowing animals like rabbits, ground squirrels, the prairie dogs, the woodchucks, the earthworm, and the ants provide efficient aids to the weathering.

These animals bring soil particles to the surface and hence to a zone of greater exposure, and by doing so soil is rendered more porous, hence, more open to percolating water. Earthworms also play an important role in organic weathering.

They divide the soil into smaller parts by passing it through their intestinal tract. Earthworms and ants are by far the most important burrowing animals and are considered as the significant agencies in the production and combination of soil. Thus, they help in the final stages of rock disintegration.

Civilized man of today has come to be one of the most effective organic agents in anthropogenic weathering. His various activities like quarrying, ploughing, tunneling into the mountains or earth, and deforestation aid in the processes of weathering.

However, from the stand point of the development of landforms, the influence of man as an agent of weathering may be considered rather unimportant. At present, man has come to realize the graveness of soil erosion which washes away the most fertile layers of soil every year, and every possible measure is being taken by man to prevent it.

Despite man’s best efforts, erosion continues to be a menace. Through unwise methods of ploughing the agricultural land on steep and gentle slopes in humid regions with torrential rainfall, several millions of tons of soil is removed by running water.

According to Bennet, out of 9,250,000 acres of agricultural land surveyed in the Brazos River Watershed, Texas in the United States of America, 66 per cent is suffering from sheet erosion, the most destructive form of soil erosion.

So is the case with mining operations, where man has brought millions of tons of rocks to the surface of the earth. In similar other ways man has been an aid to weathering by preparing the land for erosion.

It must be borne in mind that in weathering the physical, chemical and biological agents actively co-operate with one another.