Short essay on the Process of Weathering

Introduction:

Weathering it the general term applied to the combined action of all processes causing rocks to be disintegrated physically and decomposed chemically because of exposure at or near the earth’s surface.

In particular, weathering occurs, where rocks and minerals come in contact with the atmosphere, surficial water, and organic life under conditions that are normal to the surface of the earth.

Weathering is the initial stage in the process of denudation. An essential feature of the process is that it affects rocks in situ; no transportation is involved. The products of rock-weathering tend to accumulate in a soft surface layer, called regolith. The regolith grades downward into solid, unaltered rock, known simply as bed­rock.

Weathering helps erosion, but is not a part of it. There can be weathering without erosion and erosion without weathering.

Types of weathering:

There are three main types of weathe­ring:

1. Physical weathering.

2. Chemical weathering.

3. Biological weathering.

Factors affecting weathering:

All the processes of weathering are affected by rock structure, climate, topography and vegetation.

Rock structure refers to mineralogical composition, joints bedding planes, faults, fractures, pores and its integral hardness. The degree of weathering of the source area, i.e., the area where weathering operates is controlled by the nature of the pre-existing rocks to a greater extent.

Climate is the sum-total of the meteorological elements like temperature, moisture, including both humidity and precipitation, wind, air-pressure and evaporation. Climate determines whether physical or chemical weathering will predominate and the speed with which these processes will operate.

Topography directly affects weathering by exposing rocks and indirectly through the amount of precipitation, temperature and vegetation.

Surfaces covered with vegetation are protected and bare sur­faces are weathered to a greater extent.

Agents of weathering:

The principal agents of the transforma­tion of rocks in the mantle of waste are water, oxygen, carbon- dioxide, acids, organisms, and variations of temperature. These agents affect equally good in case of both physical and chemical processes of weathering.

1. Physical weathering processes:

This process refers to the mechanical disintegration of rocks in which their mineralogical com­position is not changed. This is brought about chiefly by tempera­ture changes, e.g., thermal expansion and contraction. The followings are some of the important processes of physical weathering.

(a) Exfoliation:

In this case thin sheets of rock split off owing to differential expansion and contraction during heating and cooling over the diurnal temperature range.

Sometimes, it is the result of unloading in which case, because of the removal of the overlying rocks, the pressure on the igneous rocks beneath them is also diminished and this results in the expan­sion of igneous rocks and in the formation of large scale fractures parallel to the surface topography. Sheets between the fractures are detached from the main mass which thus suffers fragmentation.

(b) Crystal growth:

The soluble constituents of the rocks or minerals enter the rocks through fractures and joints, along with water. With the evaporation of water the solution is precipitated to form crystals or crystalline aggregates and as they grow, they exert large expansive stresses, which help in breaking up some rocks.

(c) Freezing of water:

Water, as we know, expands by about 9 05 percent in volume when it freezes. The water seeps down into the fracture and under suitable climatic condition, begins to freeze at the top of the fracture first. As freezing continues, the pressure exerted on the walls becomes more and more intense, which results in widening the existing fracture and new fractures form. This is the dominant mode of weathering, in climates where there is repeated freezing and thawing. This is also known as Frost action.

(d) Differential expansion:

Rock-forming minerals expand when heated, but contract when cooled. Where rock surfaces are exposed daily to intense heating by direct solar rays, alternating with intense cooling by long wave radiation at night, the resulting expan­sion and contraction of mineral-grains tends to break-them apart.

The intense heat of forest and bush fires is known to cause rapid flaking and scaling of exposed rock-surfaces.

2. Chemical processes of weathering:

It is also known as mineral alteration, consists of a number of chemical reactions, all these reactions change the original silicate minerals of igneous rock, the primary minerals, into new compounds, the secondary minerals, that are stable in the surface environment.

Besides, sedimentary and metamorphic rocks are also substantially affected by the chemical processes of weathering. Chemical weathering is more important than mechanical weathering in almost all the climatic regions.

The atmosphere contains a number of constituents that can react with minerals. Most important of these are water, carbon- dioxide, and oxygen. The effectiveness of these chemical constituents depends on the composition of the rocks and size of the particles that make them up.

For example, Quartz is a very stable substance, so rocks composed primarily of quartz decompose very slowly; whereas the ferromagnesian minerals are highly susceptible to chemi­cal weathering.

Three processes are notably responsible for chemical weather­ing:

(a) Oxidation.

(b) Hydration.

(c) Carbonation.

(a) Oxidation:

The presence of dissolved oxygen in water in contact with mineral surfaces leads to oxidation ; which is the chemi­cal union of oxygen atoms with atoms of other metallic elements. Oxygen has a particular affinity for iron compounds and these are among the most commonly oxidised materials.

(b) Hydration:

The chemical union of water with a mineral is called hydration. It is sometimes confused with ‘hydrolysis’, the reaction between water and a compound. The process of hydration is particularly effective on some aluminium bearing minerals, such as feldspar.

(c) Carbonation:

Carbon-dioxide is a gas and is a common constituent of the earth’s atmosphere. Rain water in course of its passage through the atmosphere, dissolves some of the carbon- dioxide present in the air. It thus turns into a weak acid called carbonic acid, H₂C0₃, and is the most common solvent acting on the crust. The effect process is well noticed in the limestone or chalk areas in the humid regions of the world.

Besides the above, another process known as “Solution” is quite significant in bringing about the chemical weathering of rocks. In this case, some of the minerals get dissolved by water and thus removed in solution, for example gypsum, halite etc.

3. Biological-weathering:

This process of weathering is mainly related to the activities of various organisms. Organisms, mainly plants and bacteria, take part in the transformation of rocks at the surface, in the following ways:

(a) Bio-physical processes.

(b) Bio-chemical processes.

(a) Bio-physical processes:

(i) Plant-roots, growing between joint blocks and along minute fractures between mineral grains, exert an expansive force tending to widen those openings and sometimes create new fractures.

(ii) Insects like earth-worm, snail etc. loosen the soil cover and create suitable conditions for the various external agencies to have their own action on the underlying rocks, which ultimately lead to rock weathering.

(b) Bio-chemical processes of weathering:

(i) Sometimes, cer­tain groups of bacteria, algae and mosses break rock-forming silicates down directly, removing from them elements like silicon, potassium, phosphorous, calcium, magnesium, that they need as nutrients. These transformations occasionally occur on a large scale and are decisive in the alteration of parent rocks and facilitate rock weathering.

(ii) After the death of animals or plants, with their subsequent decay and degeneration, chemically active substances are produced, which are capable of bringing about rock-weathering. For example, humic acid which is formed during decay and degeneration of plant life is capable of bringing about rock weathering effectively, to some extent.

Thus, in nature, the processes of weathering are being carried .out by various external agencies.