Geography Study Material for Exam: Weathering and Its Major Types

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Highlights

  •  Weathering is the process of natural decay of rocks.

  • The beak down of rocks into smaller fragments by physical processes such as frost wedging and expansion and contraction of rocks in arid and semi-arid conditions is known as physical weathering.

  • We have considered mechanical and chemical weathering as separate, individual processes, but in nature, they are inseparable because many types of weathering processes are usually involved in the weathering of any outcrop.

weathering

The process by which surface and sub-surface rock disintegrate, or dissolve or are otherwise broken down is known as weathering. Rock at or near Earth's surface are exposed to physical, chemical and organic weathering processes. In other words, weathering is the weakening and disintegration of rocks in-situ or at their places.  Weathering is the process of natural decay of rocks. Weathering does not transport the weathered material; it simply generates them for transport by the agents of water, wind, glaciers,  and sea-waves.

The effect of weathering can be seen on fauna, flora and human society. (i) Soil is the product of weathering, on which agriculture depends and  which is basis of most terrestrial life. (ii) Weathering has great significance for ecology and environment, (iii) weathering disintegrate solid rock and produces loose debris and agents of erosion remove the debris and exposes fresh rock, which is then weathered, and the cycle continues. (iv) without weathering the Earth would be devoid of plant and animal life.

The major types of weathering are 1. Mechanical or Physical, (2) Chemical, and (3)organic. A brief description of the various types of weathering has been given in the following.

1. Physical or Mechanical Weathering: The beak down of rocks into smaller fragments by physical processes such as frost wedging and expansion and contraction of rocks in arid and semi-arid conditions is known as physical weathering. In physical weathering, there is the disintegration of rocks without chemical alteration. Mechanical weathering is strictly a physical process, involving no change in the rock's chemical composition. No chemical elements are added to, or subtracted from, the rock. The rock is simply broken down into small fragments by various physical stresses. The different processes of physical weathering are: (i) Ice Wedging (frost action), (ii) Sheeting, (iii) Exfoliation. (iv) Block Disintegration (v) Granular Weathering (v) Hydration.

(i). Ice Wedging: Water expands as much as 9% of its volume as it freezes. This expansion creates a powerful mechanical force called frost action. Freezing actions are important in humid micro-thermal climates and in subarctic polar regimes.Repeated cycles of water freezing and thawing break down rock segments apart. The work of ice begins in small openings, gradually expanding until rocks are leaved (split).

Ice wedging occurs under the following conditions: (i) when there is an adequate supply of moisture; (ii) where preexisting fractures, cracks, or other voids occur within the rock, into which water can enter; and (iii) where temperatures frequently rise and fall across the freezing point. Temperature fluctuation above and below the freezing point is especially important  because pressure is applied with each freeze. In areas where freezing and thawing occur many times a year, ice wedging is far more effective than in exceptionally cold areas. Ice wedging thus occurs most frequently above the timberline. It is especially active on the steep slopes above valley glaciers, where melt-water produced during warm summer days seeps into creeps into cracks and joints and freezes during the night.

(ii). Sheeting: A set of joints formed essentially parallel to the surface. It allows layers of rock to spall off as the weight of overlying rock is removed by erosion. It is especially well developed in granitic rock.

(iii). Exfoliation (Onion Weathering):  A dome-shaped feature of weathering, produced by the response of granite to the over burden removal process, which relieves pressure from the rock. Layers of rock sluff off in slabs or shells in a sheeting process.

(iv) Block Disintegration: Block disintegration occur because variations in temperature. Crystalline rocks like granite are more affected by temperature changes as particles are closely associated with each other and these particles expand and contract with increase and decrease of temperature respectively. The sedimentary rocks are less affected by temperature variations. Black disintegration is significantly found in the deserts. 

(v) Hydration (Crystallization): This is a process involving water, but little chemical change is known as hydration. It is a physical weathering process in which water is absorbed by a mineral. This addition of water initiates swelling and stress within the rock, mechanically forcing grains apart as the constituents expand. Hydration can lead granular disintegration and further susceptibility by oxidation and carbonation.

Hydration or crystallization is quite significant in the arid climates. As the water evaporates, crystals from the dissolved minerals appear on the surface. As the process continues over time and the crystals grow and enlarge, they exert a force great enough to spread apart individual mineral grains and begin breaking up the rock. Such crystallization, or salt-crystal growth, is a form of physical weathering. Crystallization is quite significant in the eastern  parts of Punjab, south-western parts of Punjab and Haryana and the south-western parts of Uttar-Pradesh.

2. Chemical Weathering: Chemical reactions that act on rocks exposed to water and the atmosphere so as to change their unstable mineral components to more stable forms. oxidation, carbonation, solution and hydrolysis are the most common reactions. Chemical weathering is synonymous with decomposition.
During chemical weathering, rocks are decomposed, the internal structure of the minerals is destroyed, and new minerals are created.. Thus, there is a significant change in the chemical composition and the physical appearance of the rock. Water is of prime importance in chemical weathering, because it takes part directly in the chemical reactions and it acts as a medium to transport elements of atmosphere to the minerals of the rock. It is also important because it removes the products of weathering to expose new, fresh rock, which in turn can be weathered. The rate and degree of chemical weathering, therefore, are greatly influenced by the amount of precipitation.

(a) Dissolution:  The simplest form of chemical weathering occurs when a mineral dissolves into solution. For example, when sodium chloride (common table salt) dissolves in water. Water is called the universal solvent because it is capable of dissolving at least 575 of the natural elements and many of their components. It readily dissolves carbon dioxide, thereby yielding precipitation containing carbonic acid. This acid is strong enough to react with many minerals, especially limestone, in a process called 'carbonation' Carbonation simply means reactions whereby carbon combines with minerals. Such chemical weathering transforms minerals containing calcium, magnesium, potassium, and sodium into carbonates.

When rainwater attacks formations of limestone, the constituent minerals are dissolved and wash away with the rain water. The entire landscape composed of limestone are dominated by the chemical weathering process of carbonation. These are regions karst topography.

b) Oxidation: A chemical weathering process whereby oxygen oxidizes (combine with) certain metallic elements to form oxides, most familiar as "rusting" of iron in a rock or soil that produces a reddish-brown stain of iron oxide (Fe2 O3).

(c) Carbonation:  A process of chemical weathering by a weak carbonic acid (water and carbon dioxide) that reacts with many minerals, especially limestone, containing calcium, magnesium, potassium, and sodium transforming them into carbonates.

(d)  Spheroidal Weathering:   It is a chemical process that occurs when water penetrates joints and fractures in rock and dissolves the rocks cementing minerals. The resulting rounded edges and corners resemble exfoliation but do not result from pressure release jointing. This type of weathering and rock decay in turn opens spaces for frost action. For example,  a boulder can be attacked from all sides, shedding spherical shells of decayed rock like the layers of an onion. As the rock breaks down, more and more surface area is exposed for further weathering. 

(e) Hydrolysis:  The chemical union of water and mineral is known as hydrolysis. The process involves not merely absorption of water, as in the sponge, but a specific chemical change on which a new mineral is produced. A good example of hydrolysis is the chemical weathering of feldspar. Feldspar is an abundant mineral in a great many igneous, metamorphic, and sedimentary rocks. 

We have considered mechanical and chemical weathering as separate, individual processes, but in nature, they are inseparable because many types of weathering processes are usually involved in the weathering of any outcrop. The mechanical fracturing of a rock increases the surface area, where chemical actions take place and permits deeper penetration for chemical decomposition. Chemical decay, in turn, facilitates mechanical disintegration. One process may dominate in a given area, depending on the climate and rock composition, but mechanical and chemical weathering processes generally attack the rock at the same time.

3. Organic Weathering: Plants and animals including man largely control the breakdown of rocks. In all types of climates and biomes, biotic communities play some role in one way or the other. The vegetation protects the rocks by binding them through their roots but different but different types of acids produced by them facilitate biochemical weathering. The burrowing animals, worms, and other organisms help in the gradual breakdown of rocks. Burrowing animals include gophers, foxes, rabbits, jackals, rats, and termites, etc. which dug out burrows in the rocks. Recently, man has become the most powerful weathering agent because of the development of modern technologies.

Products of Weathering: The major products of weathering are: (i) a blanket of loose, decayed rock debris, known as regolith, and (2) rock bodies modified into spherical shapes. The results of weathering can be seen from the driest deserts and the frozen wastelands to the warm, humid tropics. The most obvious product of weathering is a blanket of loose, decayed rock debris known as regolith, which forms a discontinuous cover over the solid, unaltered bedrock it. In addition, there is a universal tendency for weathering processes to form rounded, or spherical, surfaces on a decaying rock body.

Regolith and Soil: The term regolith comes from the Greek work rego, meaning 'blanket' (blanket rock). It is a layer of soft, disaggregated rock material formed in place by the decomposition and disintegration of the bedrock that lies beneath it. Within the regolith, the individual grains or small groups of mineral particles are easily separated, one from the other. The thickness of the regolith ranges from a few centimeters to hundreds of meters, depending on the climate, type of rock, and length of time that weathering processes have been operating. The transition of bedrock to regolith can be seen in road cuts and stream valleys.
Gravel, sand, silt and mud deposited by streams, wind and glaciers are sometimes referred to as transported regolith, in order to distinguish them from the residual regolith produced by weathering. Many types of transported regolith or superficial deposits have been identified. The uppermost layer of the regolith is the soil. It is composed chiefly of small particles of rock and minerals plus varying amounts of decomposed organic matter. The soil is widely distributed and so economically important that it has acquired a variety of definitions. A classification of soils will be discussed in the subsequent article.

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