Weathering of Rocks: Types, Factors and Products | Geology

In this article we will discuss about:- 1. Types of Weathering 2. Role of Plants and Organisms in the Weathering of Rocks 3. Factors 4. Resistance 5. Products 6. Engineering Considerations.

Types of Weathering:

1. Mechanical (Physical) Weathering:

It is a natural process of in-situ disintegration of rocks into smaller fragments and particles through essentially physical processes without a change in their composition. A single rock block on a hill slope or a plain, for instance, may be disintegrated gradually into numerous small irregular fragments through frost action that in turn may break up naturally into fragments and particles of still smaller dimensions.

These loose fragments and particles may rest temporarily on the surface if it is a plain. On slopes, however, the end product fragments and particles may roll down under the influence of gravity and get accumulated at the base as heaps of unsorted debris. All these fragments and particles, however, have the same chemical composition as the parent rock.

Mechanical weathering is one of the very common geological processes of slow natural rock disintegration in all parts of the world. Temperature variations and organic activity are two important factors that bring about this change under specific conditions.

Temperature variations are held responsible for extensive mechanical weathering of rocks exposed on the surface.

This manifest in three different ways:

(a) Frost action in cold humid regions,

(b) Thermal effects (insolation) in hot arid regions and

(c) Unloading.

An outline of these processes is as follows:

(a) Frost Action:

Water on freezing undergoes an increase in its volume by about ten per cent. This expansion is accompanied by exertion of pressure at the rate of 140 kg/cm² (2000 lbs/in²) on the walls of the vessel containing the freezing water. In areas of intensive cold and humid climates, temperatures often fall below the freezing point of water repeatedly during winter months.

In such areas freezing of water in pots and pools, water pipes and taps and in cavities and cracks in concreted roads causing their bursting and disintegration in many cases is a matter of common observation. This process of freezing of water when happening within the pores, cracks, fractures and cavities of rocks affects them considerably.

The original openings are widened at the first stage of attack and thereby accommodate more and more water to come and freeze in subsequent cycles. A freezing cycle is often followed by a thawing cycle that means melting of ice formed within the cavities. Eventually, repetition of the freezing and thawing cycles over many years leads to gradual disintegration of the rocks because of internal stresses exerted in the process.

The frost formed fragments are angular, sub angular and irregular in outline and remain spread over the parent rock having flat surface or flat slopes. If the original surface forms a significant slope, as is commonly the case in the hilly and mountainous regions, these frost fragments get heaved up from the crevices and cavities and then roll down the slope under the influence of gravity.

Finally, the fragments accumulate at the base as heaps commonly called as Scree deposits. In some cases, especially when the slopes are stabilized and the pull of gravity is weaker, the fragments remain unevenly strewn over the surface of the slopes. Such slopes covered by frost formed scree are often referred to as talus slopes.

Exudation is a process similar to frost action but in this case disintegration takes place due to formation of crystals of sodium chloride, etc. within the cavities of rocks thereby causing their disintegration. This process is seen in good measure in porous rocks near seashore.

(b) Thermal Effects (Insolation):

In arid, desert and semi-arid regions where summer and winter temperatures differ considerably, rocks undergo physical disintegration by another phenomenon related to temperature. Rocks, like many other solids, expand on heating and contract on cooling.

They (rocks) are, of course classed as bad conductors of heat but even then prolonged exposure to direct heating by the Sun does induce appreciable volumetric changes in them. In arid and semiarid regions, the difference between day and night temperatures and also between average temperatures in summer and winter is quite considerable.

In some deserts, for instance Kara Qum, rocks are exposed to as high temperatures as 70-80 °C in summer and are then cooled down to -10 °C in winter. Such repeated variations in temperature experienced by a body of rock gradually break it into smaller pieces, especially in the top layers, by development of tensile stresses developing from alternate expansion and contraction.

Exfoliation:

In a thick rock body or where the rock is layered, these are the upper layers that get affected most due to the temperature variations. As a result, the upper layers may virtually peel off from the underlying rock mass. In many cases such a change is also accompanied by chemical weathering, especially at margins and boundaries of the separated layers, developing curved surfaces.

This phenomenon of pealing off of curved shells from rocks under the influence of thermal effects in association with chemical weathering is often termed as exfoliation. It is a large-scale phenomenon resembling in some details with spheroidal weathering that results from predominantly chemical weathering on smaller rock blocks.

(c) Unloading:

This is another process of mechanical weathering where large-scale development of fracturing in confined rock masses is attributed to removal of the overlying rock cover due to prolonged erosional work of other agencies. These rock masses remain confined from sides but due to relief of pressure from above, they expand upwards; consequently joints develop in them parallel to the uncovered surface dividing them into sheets.

This rupturing or jointing in itself is a mechanical breakdown of rocks and makes them available for further weathering or decay along the joint planes. It is believed by many that unloading of deeply buried plutons is often the cause of development of concentric joints in them. Further mechanical weathering along these joints leads to pealing off of slabs and converting the pluton into an exfoliation dome.

2. Chemical Weathering:

It is a process of alteration of rocks of the crust by chemical decomposition brought about by atmospheric gases and moisture. The chemical change in the nature of the rock takes place in the presence of moisture containing many active gases from the atmosphere such as carbon dioxide, nitrogen, hydrogen and oxygen.

Rocks are made up of minerals, all of which are not in chemical equilibrium with the atmosphere around them. Chemical weathering is, essentially a process of chemical reactions between the surfaces of rocks and the atmospheric gases in the direction of establishing a chemical equilibrium. The end product of chemical weathering has a different chemical composition and poorer physical constitution as compared to the parent rock.

Chemical weathering eats up the rocks in a number of ways depending upon their mineralogical composition and the nature of chemical environment surrounding them.

Following are some of the main processes of chemical weathering:

(a) Solution,

(b) Hydration and Hydrolysis,

(c) Oxidation and Reduction,

(d) Carbonation

(e) Colloid formation.

(a) Solution:

Some rocks contain one or more minerals that are soluble in water to some extent. Rock salt, gypsum and calcite are few common examples. It is also well known that though pure water is not a good solvent of minerals in most cases, but when it (the water) is carbonated, its solvent action for many common minerals is enhanced. Thus, limestone is not easily soluble in pure water but carbonated water dissolves the rock effectively. Limestone gets pitted and porous due to chemical weathering.

(b) Hydration and Hydrolysis:

These two processes indicate the direct attack of atmospheric moisture on the individual minerals of a rock that ultimately affect its structural make up. It is believed that though the interior of many minerals is in electric equilibrium, the surfaces of many crystals are not; they may have partially unsatisfied valences.

When polarized water molecules come in contact with such crystals, it may cause any one of the following two reactions:

First:

The ions tend to hold the polarized side of the water molecule and form a hydrate. This process of addition of the water molecule is termed as hydration. Examples are provided by hydration of iron oxides and calcium sulphate crystals. In some minerals with ferrous iron, the Fe++ ion holds the water molecule and forms water-iron complex or a hydroxide.

Similarly, CaSO₄ or anhydrite gets slowly converted to gypsum by hydration:

Second:

Ions may be exchanged whereby some ions from water may enter into the crystal lattice of the mineral. This process of exchange of ions is called hydrolysis. It is a very common process of weathering of silicate minerals (which are quite abundant in rocks) and is best explained with reference to weathering of mineral Orthoclase, a felspar.

(c) Oxidation and Reduction:

Iron is a chief constituent of many minerals and rocks. The iron bearing minerals (and hence rocks) are especially prone to chemical weathering through the process of oxidation and reduction.

Oxidation:

Ferrous iron (Fe++) of the minerals is oxidized to ferric iron (Fe+++) on exposure to air rich in moisture.

Ferric iron is not stable and is further oxidized to a stable ferric hydroxide:

(i) 4Fe + 3O₂ → 2Fe₂O₃

(ii) Fe₂O₃ → Fe₂O₃.H₂O

Similarly, Pyrite (FeS₂), a natural and common iron mineral present in many rocks in small amounts (e.g. in limestone), may undergo oxidation and hydration in a sequence forming sulphuric acid in the process that may further corrode the carbonate rock (limestone)-

Reduction:

In specific types of environment, such as where soil is rich in decaying vegetation (swamps), minerals and rocks containing iron oxide may undergo a reduction of the oxides to elemental iron. In this case the decaying vegetation supplies the carbonaceous content causing reduction.

The effects of oxidation (and to some extent of reduction) weathering are easily observed from the colour changes produced in iron bearing rocks. Those rocks in which the iron has been oxidized to ferric state show a marked brown colour, especially in oxides, hydroxides and hydrates. But where the oxidation has reached only the ferrous state, the typical colours developed in the rocks are various shades of green, blue and grey.

(d) Carbonation:

It is the process of weathering of rocks under the combined action of atmospheric carbon dioxide and moisture, which on combination form a mildly reacting carbonic acid. The acid so formed exerts an especially corrosive action over a number of silicate bearing rocks. The silicates of potassium, sodium and calcium are particularly vulnerable to decay under conditions of carbonation.

A typical example is that of felspar orthoclase, a very common and important constituent of many igneous, sedimentary and metamorphic rocks, which decomposes according to following reaction-

The end products in the above reaction are a clay mineral, a soluble bicarbonate and silica. Further, in the above equation, Na or Ca may be present instead of K if the mineral in question is another type of felspar. The main end product, Kaolinite, is formed in all such cases.

Only the soluble carbonate differs in accordance with the metallic ions of the felspar type. This chemical change in the rock produces definite alteration in the physical constitution of the rock- a soft (H = 1) clay mineral is formed in place of a hard mineral (felspar, H = 6), thereby affecting the strength of the rock very significantly.

Carbonates are removed in solution and silica forms colloids; this may result in partial or total conversion of a strong igneous rock like granite into a mass of soft clay like product in the zone of weathering. Many igneous rocks like granites, granodiorites, syenites, basalts and porphyries suffer this type of weathering on a massive scale, as felspars are their chief constituent minerals.

(e) Colloid Formation:

The processes of hydration, hydrolysis, oxidation and reduction operating on the rocks and minerals under different atmospheric conditions may not always end in the formation of stable end products.

Often they result in splitting of particles into smaller particles – the colloids – characterized by atoms with only partially satisfied electrical charges. Formation of colloidal particles is especially common in the weathering of clay minerals, silica and iron oxides. The colloids of these minerals are, however, soon precipitated as their charges are satisfied and they form stable products.

Spheroidal Weathering:

It is a complex type of weathering observed in jointed rocks and characterized with the breaking of original rock mass into spheroidal blocks. Both mechanical and chemical weathering is believed to actively cooperate in causing spheroidal weathering.

The original solid rock mass is split into small blocks by development of parallel joints due to thermal effects (insolation). Simultaneously, the chemical weathering processes corrode the borders and surfaces of the blocks causing their shapes roughly into spheroidal contours.

Role of Plants and Organisms in Weathering:

It is a well-known fact that plants and organisms also contribute towards mechanical disintegration and chemical decomposition of rocks of the crust.

Plants:

Hydrogen ions (H⁺) are known to be released at the roots of plants during their growth and metabolism. These ions are capable of replacing K+, Ca++ and Mg++ ions from the minerals and rocks surrounding the root system and make them available for use in plant growth.

But in this process, the original minerals and rocks around the root system start undergoing decomposition and disintegration. Root systems of conifers and other big trees creep into pre-existing cracks in the nearby rocks. Often this results in loosening apart of the stone fragments followed by their rolling down slope.

Man himself is known to be the greatest destroyer of rocks. He has been breaking them since the very beginning for one purpose or other and making their use in a variety of ways. All the above processes of decay and disintegration of rocks by living things are sometimes grouped as organic weathering.

Factors Affecting Weathering:

(i) Nature of the Rock:

Rocks vary in chemical composition and physical constitution. Some rocks are easily affected by weathering processes in a particular environment whereas others may get only slightly affected and still others may remain totally unaffected under the same conditions.

Thus of granite and sandstones exposed to atmosphere simultaneously in the same or adjoining areas having hot and humid climate, the sandstone will resist weathering to a great extent because they are made up mainly of quartz (SiO₂) which is highly weathering resistant mineral.

Granites, on the other hand, are likely to undergo a lot of chemical decay due to carbonation, hydration and hydrolysis etc. Hence, chemical composition of the rock is an important factor in determining the stability or otherwise of a rock in a given environment.

Further, of any two granite masses, one massive, compact and of dense structure and the other of fractured type, that are exposed close by, the first type will weather at much slower rate compared to the second type. This is because gases and moisture find easy pathways into the body of rock through the fractures and act from many places.

(ii) Climate:

The process of weathering is intimately related to the climatic conditions prevailing in an area. Same types of rocks exposed in three or more types of climates may show entirely different trends of weathering.

Thus cold and humid conditions favour both chemical and mechanical types of weathering, whereas in totally dry and cold climates, neither chemical nor mechanical weathering may be quite conspicuous (due to absence of moisture). Similarly, in hot and humid climates, chemical weathering processes predominate whereas in hot and dry climates (the arid areas) mechanical breakdown due to expansion and contraction of the rocks at the surface may be more pronounced.

(iii) Physical Environment:

The topography of the area where rocks are directly exposed to the atmosphere also affects the rate of weathering to a good extent. Rocks forming bare cliffs, mountain slopes devoid of vegetation and valley sides are more prone to weathering than same rocks exposed in level lands in similar climates and/or under vegetable cover.

This is because in the first case the slopes assist in removal of the weathering end product comparatively faster and make fresh rock surface available for weathering. In the second case the weathered product accumulates over the parent rock and slows down the further destruction.

Resistance to Weathering:

Mineral constituents of a rock show remarkable variation in their susceptibility to weathering. Some minerals get decomposed and disintegrated easily whereas others may remain intact for considerable length of time with little or no alteration.

It has been observed that in igneous minerals, the resistance to weathering is broadly related to the stage of their formation from a composite igneous melt. Thus, felspars, which are alumino-silicates and are believed, to crystallize from the melts in the initial stages of the process are easily weathered compared to the mineral quartz (SiO₂), which is formed towards the last stage.

In fact, quartz is one of the most resistant minerals as far as chemical and mechanical weathering are concerned. Within the felspars, the calcic felspars are less resistant to weathering than the sodic and potash felspars because the former crystallize out earlier in a magmatic crystallization process.

For most common rock forming minerals, resistance to weathering increases in the following order:

Dark Coloured Minerals- Olivine, Augite, Hornblende; Biotite

Light Coloured Minerals- Calcic felspars, Sodic felspars, Potash felspars, Potash mica (muscovite); quartz.

Products of Weathering:

Weathering is a global process of rock decay and disintegration. Rocks exposed anywhere in the world are subject to change, though very slowly, into smaller fragments and new products.

The weathering products are commonly classified into two main types:

(i) Eluvium:

It is the end product of weathering that happens to lie over and above the parent rock. It may consist of fragmentary material resulting from rock disintegration or fine powdered material resulting from chemical decomposition or a mixture of both. The eluvium may form a thin or thick layer depending on the duration for which weathering has been operative on the parent rock. When the cover is sufficiently thick, the parent rock is always traceable at some depth below. Regolith is another term for eluvium.

(ii) Deluvium:

It is that category of end product of weathering that has been moved to some distance after its formation due to weathering processes. It is invariably associated with weathering of slopes and forms heaps of various thickness and grade at the base of slopes. Gravity and rain-wash are the main agents involved in removal of the weathering products after their formation.

Regolith:

The term regolith has been broadly used to express all the weathered material, eluvium or deluvium that covers the parent rock or is lying close to it. It forms deposits of huge thickness in suitable environment. In most cases, weathering of rocks becomes slow after the formation of weathered layers at the top.

This is because the atmospheric agencies, the moisture and gases, cannot penetrate effectively down into the rocks through the overlying cover. The exact depth at which the effect of weathering stops varies from place to place depending on the topography, the climate and rock composition.

But it is generally limited to a few meters only. The term soil refers to the upper part of regolith that has further undergone biochemical decomposition and modifications reducing it to a uniformly pulverized state. Soils of almost all types are capable of supporting some vegetation.

Soil Profile:

It is defined as the record of behaviour of the material with depth below the surface up to which the effects of weathering can be easily established. The emphasis is both on the depth as well as quality of effects.

In a typical Soil Profile, following four weathering zones are commonly recognized:

Zone A:

It is made up entirely of completely weathered soil that may be supporting a vegetable cover. This zone may or may not contain enough humus (organic acid produced due to decay of vegetation).

Zone B:

It is a zone of mixed composition, partly of soil and partly of weathered rock, the latter becoming more dominating with depth.

Zone C:

This is practically soil free zone and has enough evidence that rocks at this level are already under attack by weathering although the effects have yet not become pronounced and the rocks are not disintegrated or decayed.

Zone D:

It is the zone of the so far intact rock, the parent rock, the overlying part of which has already been attacked by weathering processes. This zone starts from the base of zone C and extends downwards indefinitely.

As said above, the thickness of different zones in a soil profile is quite variable ranging from a fraction of meter to many meters. At places, zone A may be completely absent, having been removed after its formation, as on slopes. At other places this zone may extend downwards for quite considerable depths, as in level lands or valleys.

Mineral and Rock Formation:

Weathering results in the formation of quite a few minerals and rocks.

Two groups deserve special mention:

(a) Clay Minerals:

Montmorrilonite, Kaolinite and Illite are formed as a by-product of weathering of pre-existing silicate rocks under humid climatic conditions. Thus hydration of volcanic dust in semi dry climates results in the formation of montmorrilonite. Kaolinite is formed from hydration and carbonation of igneous rocks under humid climates.

(b) Ores of Aluminium:

The well-known ores of aluminium, namely bauxite (Al₂O_3.nH₂O) and laterite are products of advanced stages of weathering of clay rocks. Weathering leaches out silica and many cations from the clay rocks in humid climates like those of rain forests leaving behind rich concentration of aluminium oxides and hydroxides.

Engineering Considerations for the Weathering of Rocks:

Engineering Projects are built either on soil or on rocks. Soil is the ultimate end product of weathering of rocks. As such, for a better understanding of the engineering properties of soils, the engineer will benefit a lot if he has a clear knowledge of genetic background of the soils.

Similarly, when foundations are to be carried down to the bed rock, the depth of weathered cover, degree of weathering and most important of all, the trend of weathering in that area have very important bearing on the ultimate safety of the project.

For the construction engineer, it is always necessary to find out:

(a) To what extent the area for a proposed project has already been physically deteriorated by cumulative effects of weathering processes operative in that area;

(b) What would be the likely effect of the weathering on the construction materials proposed to be used in the project? Once these aspects become clear, the problem is simplified. It may be necessary to remove the loose weathered material and carry the foundation to the solid rocks. Further, this will also help in selecting the right type of construction material that will be more durable against the weathering processes typical of that area.

Weathering processes are important for a civil engineer from yet another aspect. It is now well established that weathering is the main cause of instability of slopes in many areas. The chemical weathering in particular breaks the bonds between the various rocks making the slopes.

When bonds between the minerals are weakened or totally removed, the slope rocks lose shearing strength and become prone to failure. Hence any process of slope stability must also ensure protection of slope rocks from attacks by weathering agencies in and around the zone of slope failure.

Similarly, it is very important for the construction engineers and architects before recommending use of some special type of stones in major constructions (such as marble, limestone, granites) to determine possible response of such stones towards the chemical-environment of the area. Disfiguring, pitting, honeycombing and loss of surface appearance are quite common effects of chemical weathering on stones used irrationally without due regard to the local environment.