WEATHERING OF ROCKS

Weathering is defined as a process of decay, disintegration and decomposition of rocks under the influence of certain physical and chemical agencies.

Disintegration: It may be defined as the process of breaking up of rocks into small pieces by the mechanical agencies of physical agents.

Decomposition: It may be defined as the process of breaking up of mineral constituents to form new components by the chemical actions of the physical agents.

Denudation: It is a general term used when the surface of the earth is worn away by the chemical as well as mechanical actions of physical agents and the lower layers are exposed.

The process of weathering depends upon the following three factors:

i) Nature of rocks

ii) Length of time

iii) Climate

Two Chief types of weathering are commonly distinguished on the basis of type of agency involved in the process and nature of the end product.

They are: i) Physical or mechanical weathering

ii) Chemical weathering

Physical weathering: It is the physical breakdown of rock masses under the attack of certain atmospheric agents. A single rock block is broken gradually into smaller irregular fragments and then into particles of still smaller dimensions. It is the most active in cold, dry and higher areas of the earth surface.

Physical Factors

Out of the two types of weathering, i.e., disintegration and decomposition, the disintegration process is accomplished in nature by a greater variety of natural agencies like wind, rivers, glaciers, dashing waves and tides, gravity (waterfalls, landslides, avalanches. meteorites), exfoliation, frost wedging, frost heaving and miscellaneous.

Wind: Wind is a relatively weak natural force because of the medium, i.e., air. In spite of this it can cause disintegration or create conditions favorable for weathering. Its action is more pronounced on sea shores and in desert regions.

Wind causes disintegration by means of abrasion (i.e., breakdown of rocks exposed along the course of the wind) and attrition (i.e., breakdown of particles carried by the wind itself). Wind abrasion occurs when the loose particles (usually sand) carried by it strike against the exposed rocks. This action will be more pronounced when:

(i) the blown particles are hard,

(ii) the exposed rocks are soft, and

(iii) the velocity of the wind is high.

Wind attrition occurs in two different ways:

(i) during abrasion when sand particles strike the exposed rocks with force, they themselves receive an equal impact which results in their breakdown and consequent decrease in size,

(ii) further, during transit in the medium of air, the heterogeneous particles do not move uniformly, thereby leading to mutual collisions. This also leads to the breakdown of particles. A greater velocity and a greater length of transit favour the attrition process.

Deflation is another phenomenon caused by wind. Deflate means "to blow away". When a powerful blast of wind blows, it scours all loose weathered material on a surface and carries it along. This rips off the protective outer cover and lays bare the underlying rocks for weathering attack. Thus this process of deflation though by itself is not weathering. It creates conditions favourable for weathering.

River, Dashing waves and tides of the sea: Coastal erosion is the consequence of dashing tides and waves of the sea. The wind, when it blows over shallow parts of the sea, creates waves. The marine erosion is both by means of mechanical disintegration and chemical decomposition. Such wear and tear is done by hydraulic action, abrasion, attrition and corrosion. Broadly, river erosion and marine erosion resemble each other.

Gravity: The earth's gravitational attraction imparts enormous energy to falling bodies. When such bodies ultimately strike the ground. The rocks there are smashed into pieces. Thus waterfalls, landslides, avalanches or meteorites gain greater and greater momentum during their fall and cause extensive breakdown of rocks on the ground. Thus gravity contributes to mechanical disintegration of rocks. Further, along steep slopes loose weathered material rolls down due to gravity effect and thereby exposes underlying fresh rocks for weathering attack. Landslides, which are similar, also occur due to gravity along steep slopes and are of greater magnitude and are more destructive.

Exfoliation: Exfoliation is a type of mechanical disintegration of rocks that takes place due to frequent intense temperature changes. ln some deserts extreme hot and cold conditions prevail during day and night times respectively. (Example: In Kara-Qum desert daytime temperatures shoot up to 70-80°C  and at night fall drop to -10°C.) Such conditions lead to disintegration of rocks by reducing their internal cohesion as follows: during daytime when the scorching rays of the sun fall on the surface of a huge boulder, the surface becomes hot and expands. Since rocks are bad conductors of heat surface heat is transmitted slowly to the core of the boulder. It takes time to reach the centre. Meanwhile, the outer part of the boulder expands, while the interior remains unchanged in volume. In other words, the boulder as a whole is not heated up uniformly and at different parts different temperatures prevail leading to non-uniform expansion of the boulder. With nightfall, cold winds blow over the surface of the boulder, cooling it and causing contraction. But at this stage expansion in the interior of the boulder will be going on due to accumulated heat. Thus contraction at the surface and expansion in the interior will take place simultaneously, Because of this, internal cohesion or binding force of the rock decreases. When this process is repeated a number of times, the surface layers get detached from the main central mass and peel off as thin shells. These shells further break down i.n due course of time because, even within the shell, the constituent minerals due to differences in chemical composition undergo different degrees of expansion or contraction which result in breaking of the shell into smaller pieces. This process is appropriately called "exfoliation" (exepre-existing and folio=layers or sheets) because a number of layers are detached one after another from the original main boulder, with the passage of time.

Frost wedging and frost heaving: These are processes of mechanical disintegration of rocks which occur at high altitudes (near mountain tops) or high latitudes (near polar regions). Near the snowline (snowline is the lowest level up to which permanent ice exists) during daytime the snow melts into water and during the nights the water freezes into ice. This happens due to change in day and night temperatures. In such areas during daytime water percolates and fills the existing cracks and fractures in rocks. During the nights due to fall in temperature such trapped water freezes into ice. Then due to the consequent increase in volume (i.e., I/10th) tremendous pressure (equal to 140 kg/cm2 or 2000 tonnes/tt-) is exerted on the sides of the cracks. When this process is repeated, even hard rocks get shattered into pieces. This sort of disintegration is called frost action. Since frost acts like a wedge in this context in widening cracks, it is also called as frost wedging. Due to frost wedging not only do the existing cracks become widened but also innumerable new cracks (big and small) develop which, in tum, facilitate further frost' wedging. Ultimately, rocks are reduced to pieces.

Frost heaving is a similar phenomenon which occurs along steep slopes. In such places rocks are upheaved (uplifted) by the expansion of trapped water below on freezing. The upheaved loose rocks roll down the slopes due to the gravity effect.

Chemical Factors:

Such factors refer to the decay or decomposition effect in rocks due to various natural causes. This is a relatively slow process but very effective in the weathering of rocks. This process weakens the rocks to the extent of offering no resistance to natural forces to break them down. Here, the size and shape of bodies may remain nearly the same, but on decay, they completely lose internal cohesion and are reduced to a powdery mass.

Water: Water in the form of river, glacier, frost and sea is very active in bringing about the disintegration of rocks. Water directly affects rocks by way of dissolution (complete disappearance of rocks), leaching (making porous, i.e., partial disappearance of rocks), hydration and hydrolysis (no disappearance of rock material).

Dissolution: This happens in the case of carbonate rocks, particularly limestones. Carbon dioxide of the atmosphere has the natural tendency to dissolve in water. The extent to which it dissolves depends on the temperature, pressure and chemical environment.

CaCO₃ + H₂O + CO₂ = Ca(HCO₃)₂

Marbles, dolomites, marls and other calcareous rocks are similarly affected

Leaching: Water, among different liquids, is the most powerful corroding and leaching agent. Only very few materials are totally unaffected by water. Most of the minerals are decayed and leached due to prolonged contact with water. Under a tropical climate rocks are thoroughly leached (leaching means removal of soluble content from the body by water) and made porous. Laterite is a typical example. Such porous rock is naturally very weak compared to its compact and fresh parent rock and easily breaks down when attacked by natural forces.

Hydration: This is the most powerful attack which water makes on rocks in bringing about decomposition. Hydration is the process wherein water or hydroxyl molecules are injected into the molecular structures of minerals, thereby bringing about the decomposition of these minerals. This phenomenon is extremely important because it affects more than 80% of the minerals of an average rock. This is so because feldspars (making 60% of an average rock), ferromagnesium minerals (i.e., pyroxenes, amphiboles and olivines making nearly 19-20 % of an average rock) and mica (particularly biotite which makes up nearly 4% of an average rock) are subjected to hydration.

Feldspars which are the most abundant rock-forming minerals are compositionally aluminum silicates of potassium, sodium and calcium. Potassium and sodium are extremely active elements and have the tendency to leave the mineral structure. This takes place because of hydration and, as a consequence, feldspar minerals are decomposed. Incidentally, some other radicals too may take part in reactions. This may be illustrated by the following example:

K₂Al₂O₃.6SiO₂ + H₂O + CO₂ = 4SiO₂ + Al₂O₃.2SiO₂.2H₂O + K₂CO₃ (orthoclase  turns to kaoline)

Ultimate products of weathering of feldspars are the different types of clays, which are basically hydrous aluminium silicates in composition. Similarly, ferromagnesium minerals also undergo hydration. As a result, anhydrous pyroxenes are changed over to amphiboles (uralitization). Amphiboles may be altered to biotite which, in turn, may change over to chlorite, etc. Thus, water brings about a series of compositional changes.

All the preceding changes are cases of decomposition which lead to the weakening of rocks. Since the bulk of an average rock is decayed in this way, hydration is a unique process in bringing about weathering of rocks.

Hydrolysis: In some cases of decomposition of minerals, instead of the water molecule, only the hydrogen part of water enters into the mineral structure. This is called hydrolysis. The following is an example

KAlSi₃O₈ + H+ = HAlSiO₈ + K+

Atmospheric gases: Carbon dioxide, oxygen and nitrogen are some atmospheric gases which take part in the weathering of rocks.

Carbon dioxide: Chemical combination with carbon dioxide is called carbonation. Its importance in the· dissolution of limestone and the production of clay from feldspars has already been referred to. Indirectly it supports vegetation which produces humus and thereby brings about chemical weathering.

Oxygen: Chemical combination with oxygen is called oxidation. Ferruginous minerals show conspicuous colour changes due to oxidation. Combination with water and oxygen produces hydroxides of various elements, which are then leached out. Reduction is another similar process of chemical weathering. It takes place in the zones where the soil is rich in decaying vegetation. This provides carbonaceous material which causes the reduction reaction.

Most of the rocks are iron bearing in nature. When such rocks undergo weathering either due. to oxidation or reduction, specific colour changes are produced. Those rocks in which the iron has been oxidized to the ferric state show a marked brown colour, in oxides, hydroxides and hydrates. On the other hand, the presence of ferrous iron is indicated by green, blue and grey shades of ferruginous rocks.

Nitrogen: The greater part of the atmosphere consists of nitrogen. It gets transformed into nitric acid in nature due to bacteria and lightning. Nitric acid is a good dissolving and powerful oxidizing agent. Therefore it is effective in causing weathering.

Biological Factors

Plants, animals, man and even bacteria help in disintegration and decomposition of rocks.

Trees and plants: The developing roots of growing trees and plants, sometimes, penetrate into the cracks, widen them and ultimately the rocks disintegrate.

Animals: Some animals make burrows underground and help in the weathering of rocks. The major contribution towards weathering from animals and plants is that when they die their soft parts decay and decompose, releasing toxic products which are highly potent in causing decomposition of rocks, particularly reduction.

Bacteria: Some bacteria also take part in the decomposition of rocks. These help in decay of organic material and produce humic, carbonic and other acids.

Man: Man ranks top in the list of various factors responsible for forced unnatural weathering of rocks. He is all powerful and to satisfy his various requirements he undertakes large-scale construction of buildings, dams, bridges, roads, etc. For all these, enormous quantities of construction material are needed which he gets by quarrying. This results in the rapid disappearance of hills and mountains. To suit him, when required, man undertakes tunnelling too which means' large-scale blasting of rocks. Further, to feed minerals into his industries and factories he resorts to extensive mining - open cast and underground - which again means breakdown of rocks. Thus, man in a unique way causes the disintegration of rocks.

Recognition of Weathered Minerals or Rocks .

Weathered. i.e., decomposed minerals or rocks can be easily distinguished in hand specimens from their originals with the help of the following clues:

I. Weathered minerals exhibit leached out appearance, i.e., change in colour intensity or altogether a different colour.

2. They will be less compact; therefore, their specific gravity will be less.

3. Their hardness will decrease, i.e., they become softer and weaker. A weathered mineral is easily scratched by its fresh counterpart but not vice versa.

4. They become less transparent or tend to become opaque.

5. They lose their natural original shine and fresh appearance, i.e., they exhibit a dull lustre.

6. Weathered minerals or rocks lose internal cohesion; therefore, they are easily powdered.

7. Weathered rocks usually develop brown, red and yellow colours on the surface.

 

Importance of Weathering

Some useful effects of weathering are:

I. Weathering produces soil which is vital for agriculture and for the production of different crops.

2. Weathering makes rocks porous and permeable. This is very important from ground. water occurrence point of view in the case of hard rocks like granites and gneisses. These acquire acquifer characteristics because of weathering. Of course, the presence of joints, faults, shear zones in them also contributes to this phenomenon.

3. Cheap building stones like laterites develop due to weathering.

4. Economic mineral deposits like bauxites are formed due to weathering.

5. Oxidation and supergene enrichment are important phenomena in the formation of some ore deposits, particularly sulphides.

However, from the civil engineering point of view, weathering is not a welcome process, because it reduces the strength, durability and good appearance of rocks.

l. Therefore, all thoroughly weathered rocks, irrespective of their original competence, become unfit to be at the site of foundation of important civil structures like dams and bridges. To make such sites fit, either intensive grouting or digging the weathered zone and refilling with concrete are resorted to. These increase the cost.

2. Since weathered rocks lose characters of strength, durability and good appearance, they also become unfit to be used as construction material, either in the form of building stones, or railway ballast or road metal or concrete aggregate.

3. Weathering due to sea waves results in coastal erosion, which poses a difficult problem for civil engineers.

4. Weathered rocks being weak are unsuitable for tunnelling.

5. Enormous loose soils (formed out of weathering) along steep slopes may tum out to be landslides, a civd engineering hazard.

6. Occurrence of a thoroughly weathered zone in the upstream side creates silting problems in case of reservoirs. Rapid silting reduces the capacity of the reservoir; in other words, the life of the reservoir. Thus, weathering poses many problems for civil engineers.