Weathering is defined as the physical disintegration and chemical alteration of Earth materials by the action of water, air, and organisms. For example, the breakdown of rocks in situ, their disintegration and decomposition without distant transportation of the products. The process of creation of new relief features and wear and tear of Earth materials are constantly going on in all parts of the globe. Orogenesis builds new mountain ranges but the landscape is modified through folding, faulting, and volcanic eruptions. As a result of the waring away of Earth, the surface is lowered and leveled. This process is called denudation. Similarly, the other forces of change which are active agents of weathering cause erosion, transportation, and deposition.
Types of Weathering:
The process of weathering can be classified into the following types:
(1). Mechanical weathering
(2). Chemical Weathering
(3). Biological Weathering
Mechanical or physical weathering means the destruction of rocks through the action of frost in the mountains and changes of temperature in the deserts.
(1). Weathering Through Frost:
Frost is a major agent of mechanical weathering in the mountains. According to H.J de Blij, “In high altitude zones where extreme cooling and warming alternate, the water penetrates into the rocks joint planes freezes and thaws repeatedly during a single season, wedging apart large blocks and boulders and loosening them completely. Then depending upon the relief, these pieces of rocks may remain more or less where they are or may roll or fall down-slope.”
A surface covered by such scattered stones is called boulder field, block spread, felsenmeer, or rock sea. Thus frost is a major sculpturing agent of the pyramidal peaks or fretted ridges of a glaciated mountainous area.
(2). Weathering Through Rain:
Rain also contributes significantly either in the shape of rain wash or in the form of river erosion. After a heavy rain, the entire slope becomes unstable. Similarly, when a drought succeeds a rainy season the rocks give up absorbed moisture and crumble into small fragments. When a mass of clay dries out it shrinks and breaks up.
(3). Weathering Through Temperature:
Temperature is also a key player in the process of weathering. Changes in the temperature in dry regions bring about contraction and expansion of rocks. As a result, they break into smaller fragments, and this process is known as block disintegration. Similarly, exfoliation or spelling takes place in some rocks when a shell splits off from the interior due to a difference in temperature. Moreover, an occasional shower of rain in a dry desert can cause a sudden chilling of the rock and its breakup into fragments.
(4). Weathering through Pressure:
Sometimes disintegration of rocks takes place due to pressure release called “unloading”. When denudation removes top layers of the rock, the pressure is released and the rock expands to form new curvilinear joints As a result, curved rock shells pull away from the mass and this process is called sheeting. Domes of the Yosemite Valley (California, USA) are the best example of unloading.
Long exposure to the weathering agents causes changes in the chemical composition of a rock. As a result, some minerals such as augite, olivine, hornblende, muscovite, biotite, and orthoclase are altered. Rock salt is completely removed in solution. However, some minerals such as quartz remain unaffected and resist the change. But the agents of change succeed in loosening quarts grains. Thus a rock as hard as granite is torn apart by the weathering process.
In a humid climate, water is abundant which is essential for chemical weathering. When water containing carbon dioxide enters calcareous rocks (limestone or chalk), it acts as an acid and dissolves them in the form of calcium bicarbonate. Similarly, rainwater breaks up feldspars through a process called hydrolysis. There are five main types of chemical weathering, which are detailed in the following lines;
(1). Solution (Rainwater absorbs other minerals).
(2). Carbonation (Carbonic acid eats away rocks mass).
(3). Hydrolysis (Minerals are moistened).
(4). Oxidation (Volume of minerals expands due to water).
(5). Hydration (Volume of minerals expands due to water).
In spheroidal weathering, water affects the outer shell of rocks as basalt causing the rock to swell and expand. As a result, shells of decayed rock go on breaking away. Thus the blocks more and more rounded.
(1). Solutional Weathering:
Water is known as a universal solvent because it has the ability to dissolve certain minerals. Thus it washes out of the rocks and soil minerals by dissolving them in the shape of a solution. This action changes the composition of rocks. The process of weathering through the solution is very common in warm-wet climates.
(2). Weathering by Carbonation:
Water absorbs carbon dioxide to forms carbonic acid. When this weak acid reacts with limestone and dolomite, thus chemical weathering takes place. This process also results in the formation of caves and subterranean corridors. Similarly, the bacterial thriving in the solid covering rocks produces acids. The acidic water causes the weathering of the rocks. Some microorganisms and plants such as mosses absorb chemical elements from the rocks and produce organic acid which brings changes in the rocks.
(3). Hydrolytic weathering:
In this form of chemical weathering, the rock minerals are moistened and transformed into other mineral compounds. As a result, expansion in volume takes place which leads to the breakdown of rocks. For example, hydrolysis of feldspar yields a clay mineral (Silica) in solution and carbonate or bicarbonate of sodium, potassium, or calcium in solution. The new minerals are weaker and softer. Similarly, in granite boulders, hydrolysis combines with other processes to cause the separation of outer shells. Granite too breaks down to form friable clay mass mixed with resistant quartz crystals and mica flakes.
Oxidation is the chemical process in which oxygen combines with other elements to form oxides like iron oxide and aluminum oxide. These oxides give reddish color to the rocks and soils. Thus oxidation takes place in rocks that contain iron or aluminum elements. According to F.J Monkhouse, ”The weathered surface of many rocks reveals a yellow or brown crust; the ferrous States in which iron commonly occurs change into an oxidized ferric state, and this crust readily crumbles. In deserts, a varnish rind or film of iron or manganese oxide is formed on rocks by solution drawn to the surface through intense evaporation…..once the rind is broken the whole rock may be removed by eroding forces.”
This process involves the addition of water in the minerals such as feldspar. As a result, the volume of the minerals is increased leading to the break-up of the rock.
The third type of weathering is known as biological weathering. In biological weathering, some plants such as algae, mosses, lichens, and other vegetation retain water on the surface of the rock, and organic acids assist to decay the rock beneath. For example, vegetation increases the acid content, which disintegrates Calcareous rock. Similarly, water containing bacterial also decompose limestone and dolomite. Moreover, animal life such as rabbi, moles, worms also contributes to change the surface material. For example, the burrowing animals loosen the surface material and worms bring a large quantity of fine material to the surface of Earth.
Human beings are also a major agent of biological weathering. They contribute to the changes as detailed under:
(1). By quarrying and mining.
(2). By polluting air, water, and soil through the addition of domestic or industrial wastes.
(3). By farming and fertilizing.
(4). By exploding nuclear devices.
(5). By cutting rock-sides in order to make roads and ways.