Weathering of Rocks: 3 Types | Soil Formation | Soil Science

Weathering of rocks leads to formation of sand, silt, and clay. On the basis of their mechanisms, following are the three types of weathering: 1. Physical Weathering 2. Chemical Weathering 3. Biological Weathering.

Type # 1. Physical Weathering:

Physical weathering of rocks is the breakdown of rocks into smaller size particles by pure mechanical processes without changing the chemical composition and mineralogy, except for the removal of some soluble components due to erosion. Many sedimentary rocks are composed of particles that have been weathered, eroded, transported, and terminally deposited in basins. Sandstone is formed from bonded sand-sized particles under water. Its porosity makes it vulnerable to the processes of physical weathering.

Physical weathering reduces the particle size and compactness, and increases the surface area and bulk volume. Physical weathering provides favorable conditions for chemical weathering by loosening the rock mass, decreasing the particle size, and increasing the surface area. Physical weathering is different from erosion or mass wastage, which involves the transport of material.

The processes involved in physical weathering are as follows:

i. Exfoliation.

ii. Frost wedging.

iii. Mineral crystallization.

iv. Slaking.

v. Pressure release or unloading.

vi. Action of vegetation.

Each of these processes is explained in the following subsections:

i. Exfoliation:

Exfoliation is the process of peeling the outer layers of the rock from the main body due to differential expansion and contraction between the outer and interior mass of the rock. It occurs in areas where there is extreme variation between day and night temperatures of the order of 25°C-30°C, for example in deserts.

During the day, when the temperature is high, the outer surface of the rock gets heated and undergoes expan­sion. As rock is a poor conductor of heat, the interior of the rock is not heated at the same rate, and this result in differential expansion between the outer and interior layers of the rock.

During the night, when the temperature drops to low values, the outer surface of the rock cools off quickly and undergoes contraction. As the interior of the rock is not cooled immediately, there is differential contraction of the outer and interior mass of the rock.

Repeated occurrence of this differential expansion and contraction of outer layer of rock with respect to the interior for long periods causes the outer layer to detach and peel off as thin shells from the interior of the rock. This process of physical weathering is known as exfoliation in which layer after layer peels off from the outer surface of the rock.

Exfoliation is also sometimes called spheroidal weathering, when spherical boulders are formed due to smoothening of sharp edges due to exfoliation. Exfoliation is also known as insolation weathering or thermal insolation.

Differential expansion and contraction may also occur at constant temperature due to the variation in the colors of mineral grains in rock. Dark-colored grains absorb more heat and expand much more than light-colored grains. Therefore, in a rock peppered with many different colored grains, rupturing can occur at different rates at the var­ious mineral boundaries.

Rocks are composed of different kinds of minerals. When heated up by solar radiation, different minerals expand and contract by a different amount at a different rate for the same surface-temperature fluctuations. With time, the stresses produced are sufficient to weaken the bonds along grain boundaries, causing rupture of fragments.

ii. Frost Wedging:

The physical disintegration of rocks by the wedging action of ice is called frost wedging. In polar regions, water is frozen in the form of ice. During the day time, ice melts due to higher temperature, percolates, and fills the cracks and fissures existing in the rocks. When the temperature falls during the night, the water filling the cracks freezes to form ice, which causes an increase in the volume by about 9% compared to the same mass of water. The formation of ice and the associated increase in its volume causes pressure on the walls of the cracks up to 100 kgf/cm². When the surface of fissures and cracks within the rock are continually subjected to this pressure, the rock is disintegrated and broken into pieces. This process is known as frost action or frost wedging.

Frost wedging also causes widening of existing cracks and formation of new cracks. Weathering by frost action is maximum in periglacial regions having temperature around 0°C and annual rainfall in the range of 100-1000 mm. Chemical weathering is low to moderate in this region.

iii. Mineral Crystallization:

The process of mineral crystallization is similar to that of frost wedging. When salts – occurring in the form of solu­tion in rock fissures and cracks – undergo crystallization, the volume of salt minerals increases, causing pressure on the walls of the fissures and cracks. The increase in temperature also causes expansion of the salts, resulting in additional pressure on the walls of the fissures and cracks, leading to disintegration of the rock.

The crystallization of salts exhibits volumetric changes from 1% to 5% depending on the temperature of the rock or mineral surface. The salts that cause intense weathering are sodium sulfate, magnesium sulfate, and calcium chloride, and some of them can expand up to three times the original volume due to the rise of temperature.

Most salt weathering occurs in hot arid regions or in coastal areas due to high salinity of the sea environment. Salt crystallization is also known as haloclasty, and it may also occur in cold climates. The presence of salts and their mineral crystallization enhance weathering by frost wedging considerably.

iv. Slaking:

Slaking is alternate wetting and drying of rocks and can be a very important factor in weathering. Slaking occurs by the mechanism of “ordered water,” which is the accumulation of successive layers of water molecules in between the mineral grains of a rock. The increasing thickness of the water pulls the rock grains apart with high tensile stress. Slaking, in combination with dissolved sodium sulfate, can disintegrate a rock in only 20 cycles of wetting and drying.

v. Pressure Release or Unloading:

Rocks deep below ground surface are subjected to large external pressures, which create internal stress within the body of the rocks. When the pressure in the rock is removed suddenly, due to erosion, tectonic activity, moving gla­cier, mining, etc., the internal stress causes bursting of the rocks. If rock masses are uplifted due to tectonic forces, it causes a reduction in pressure and sudden expansion of the rock, thereby leading to its deformation.

The majority of igneous rocks were created deep under the Earth’s surface at much higher pressures and tem­peratures. As erosion brings these rock formations closer to the surface, they become subjected to less and less pressure.

This unloading or pressure release causes the rocks to fracture horizontally with an increasing number of fractures as the rock approaches the Earth’s surface. Spalling, which is the development of vertical fractures, occurs because of the bending stresses of unloaded sheets across a three-dimensional plane.

vi. Action of Vegetation:

Growth of vegetation in rocky terrain, causes the roots of trees and plants to enlarge and extend through weak planes of the rocks. This leads to widening the existing cracks and creating new cracks in the rock mass.

Type # 2. Chemical Weathering:

Chemical weathering is the decomposition of rocks by a change in the chemical and mineralogical composi­tion, through a combination of several chemical processes. It is a slow but more intense process than physical weathering.

Most of the chemical weathering processes occur in the presence of water. Chemical weathering takes place mainly at the surface of rocks and minerals, leading to disappearance of certain minerals and formation of new products and secondary minerals.

For example, feldspar undergoes chemical weathering and it results in the for­mation of clay minerals and soluble cations and anions. Since the chemical reactions occur largely on the surface of the rocks, the smaller the fragments, the greater is the surface area per unit volume available for the reaction and the more intense will be the chemical weathering.

Thus, chemical weathering is more intense in areas where it is preceded by physical weathering which causes decrease in particle size and increase in surface area. Chemical weathering is therefore aided and abetted by physical weathering.

The intensity of chemical weathering is closely related to the mineral composition of rocks. Quartz responds far slowly to the chemical attack than olivine or pyroxene. The arrangement of minerals according to the susceptibility to chemical weathering is called weathering series. The minerals in the weathering series are essen­tially in the same order as Bowen’s reaction series, the order in which minerals crystallize from molten magma.

During the process of chemical weathering, one or more of the following components are formed:

a Minerals in solution (cations and anions).

b. Oxides of iron and alumina (sesquioxides Al₂O₃, Fe₂O₃).

c. Various forms of silica (silicon oxide compounds).

d. Stable wastes such as very fine silt (mostly fine quartz) and sand (coarser quartz).

In sedimentary rocks, which are made up of primary and secondary minerals, weathering acts initially to destroy any relatively weak bonding agents (FeO) and the particles are freed and can be individually subjected to weathering.

Chemical weathering of plagioclase feldspars by carbonation or hydrolysis results in the formation of calcite, clay minerals, or silica, which are finally deposited in the form of limestone, shale, or chert. Chert is a fine-grained silica-rich microcrystalline or microfibrous sedimentary rock that may contain small fossils. Ferromagnesium minerals in addition to forming clay minerals and silica by carbonation and hydrolysis also undergo oxidation forming hematite and limonite.

Following are the most common chemical weathering processes:

i. Carbonation.

ii. Solution.

iii. Hydrolysis.

iv. Hydration.

v. Oxidation.

vi. Reduction.

vii. Complexation

Bowen’s series is explained in the following subsection before considering the processes of chemical weathering.

Bowen’s Series:

Bowen’s series indicates the order in which minerals crystallize by cooling and solidification of molten magma with declining temperature during the formation of igneous rocks. Olivine has the highest melting/freezing point at nearly 1600°C. Pyroxene and Ca plagioclase form at somewhat lower temperatures, amphibole and mixed plagioclase somewhat lower still, biotite and Na plagioclase even lower, and K feldspar, muscovite, and quartz at the lowest temperatures of all the common igneous minerals.

Bowen’s series also indicates the two separate processes that accomplish the sequence for the feldspars and ferromagnesian minerals. Each of these pathways is called a “reaction series.” In the “discontinuous” series the olivine atomic arrangement becomes unstable at some specific temperature and that of pyroxene stable. At this point, the olivine re-dissolves the melt and, simultaneously, pyroxene begins to form from the ions thus liber­ated. With further temperature decrease, the same thing happens to cause amphibole to replace pyroxene and finally happens again to cause biotite to replace amphibole. At this point, the discontinuous series stops because biotite is the stable ferromagnesian mineral at this and all lower temperatures.

The “continuous” series of plagioclase feldspars behaves differently. At the highest temperature that any feld­spar is stable (approximately the same as for pyroxene), only Ca ions are taken into the crystals. With continued temperature decrease, Na ions replace Ca ions in the structure in an essentially one-by-one fashion.

Thus, the tran­sition from Ca plagioclase at high temperatures to Na plagioclase at low temperatures is gradual and can produce a feldspar with any proportion of the two ions. There is no rapid and instantaneous change from one mineral to another as in the discontinuous series. Pure Na plagioclase is the stable composition at about the same temperature that biotite forms and all lower temperatures.

The last minerals on the diagram, K feldspar, muscovite, and quartz all form at slightly lower temperatures from the remaining liquid not yet used up in the minerals of the two reaction series.

i. Carbonation

Carbonation is the process in which the carbonic acid and other acids are responsible for chemical weathering

Carbonic acid (H₂CO₃) is formed when carbon dioxide in the atmosphere dissolves in rain water, as shown by the following chemical reaction –

CO₂ + H₂O → H₂CO₃

Carbonation is the process in which carbonic acid reacts with the calcium carbonate in rocks and forms calcium bicarbonate, which is soluble in water –

CaCO₃ + H₂CO₃ → Ca (HCO₃)₂

Carbonation occurs in dolomite by combining with carbonates. As water contains more CO₂ at low temperatures carbonation occurs faster in colder climates such as in glacial weathering.

Carbonic acid and other acids may also be formed by the roots of plants, insects living in the soil, and the bacteria. These acids, although weak in concentration, can considerably accelerate the chemical weathering depending on the solubility of different constituents. Alumina is soluble at pH of 4 and 10, whereas silica is soluble when pH > 7. Chelation is the formation of soluble compounds in acidic environment and helps in accel­erating hydrolysis, thereby causing’ removal of iron.

ii. Solution:

Solution is one of the processes of chemical decomposition of rocks in which the water dissolves and removes soluble cementing materials such as calcium carbonate. When rocks are continually exposed to water or subject to action of water over long duration, the water soluble substances are removed from the rock. The rock no longer remains solid and forms holes or rills, and ultimately breaks into pieces or decomposes.

Most of the rock minerals are affected by this process including limestone, marl, calcareous shale, dolomite, quartz, etc.

The effectiveness of solution to cause weathering is considerably increased when water is acidified by the dissolution of organic and inorganic acids, for example, halites –

NaCl + H₂O → Na⁺, CI^–, H₂O (dissolved ions with water)

Solution also entails the effects of a number of other dissolved compounds on a mineral or rock surface. Molecules can mix in solution to form a great variety of basic and acidic decompositional compounds. Solution tends to be most effective in areas that have humid and hot climates.

iii. Hydrolysis:

Hydrolysis is the most important process in chemical weathering. It occurs due to the dissociation of water mole­cules (H₂O) into hydrogen (H⁺) and hydroxyl (OH)^– ions, which chemically combine with minerals and bring about changes, such as ion-exchange, decomposition of crystalline structure, and formation of new compounds. Water acts as a weak acid on silicate minerals.

Hydrolysis is one of the processes responsible for the formation of clay minerals. The hydroxyl ions of water com­bine with aluminum (Al³⁺) ions, forming Al—OH octahedral layers, which combine with the silica (Si—O) tetrahedral layers, leading to the formation of clay minerals. The effect of hydrolysis increases with increase in temperature and acidity of the environment.

Hydrolysis increases the pH of the solution through the release of hydroxide ions. It is especially effective in the weathering of common silicate and alumino-silicate minerals because of their electrically charged crystal surfaces.

Kaolinite is produced by hydrolysis in feldspar. Other clay minerals are also produced by hydrolysis in silicate minerals when the ions in their minerals such as Si, Na, K, Ca, and Mg are removed. As in the case of corrosion, the continuation of hydrolysis reaction also requires the removal of the reaction production by leaching.

When hydrogen ion (H⁺) reacts with orthoclase feldspar, silicic acid and potassium hydroxide are produced, leaving a residue of clay mineral illite.

3KAl⁴ + Si₃O_g + 14H₂O → K (AlSi₃)⁴Al₂⁴O₁₀(OH)₂ + 6Si(OH)₄ + 2KOH

iv. Hydration:

Hydration is the adsorption of water on rock surface. It generally precedes all other processes in chemical weath­ering. When the rock is in contact with water for a long duration, the disintegration of rock takes place due to hydration. Soil-forming minerals in rocks undergo hydration (wetting with water), when exposed to humid condi­tions. Hydration causes swelling and an increase in the volume of minerals. Water enters the mineral structure of anhydrous mineral due to hydration and becomes a part of its chemical composition. The minerals lose their luster and become soft. It is one of the most common processes in nature and works with secondary minerals such as aluminum oxide and iron oxide minerals and gypsum.

Examples of hydration are as follows:

Similarly, ferromagnesium minerals such as pyroxenes, amphiboles, and olivines, which constitute about 20% of minerals of rocks, are also affected by hydration, resulting in chemical weathering. Thus, amphiboles are converted ultimately to chlorite by hydration.

Weathering by hydration also occurs in arid environments where salts are present. For example, chlorides and sulfates weather due to hydration. In general, ions with the same charge but smaller ion radius have a larger layer of H₂O ions and therefore do not tend to adsorb strongly. The small lithium (Li⁺) ion tends to remain hydrated at the surface, whereas the large aluminum (Al³⁺) ion tends to dehydrate and become strongly adsorbed.

The strength of adsorption increases in the following sequence:

Li⁺ < Na⁺ < K⁺ < Mg²⁺ < Ca²⁺ < Al³⁺

v. Oxidation:

Oxidation is the process in which the oxygen ions combine with the minerals in rocks, causing the removal of one or more electrons from a compound. This weakens the mineral structure and makes it less rigid and unstable, causing decomposition of minerals. Oxidation of rocks is similar in process to the corrosion of steel.

Iron oxides formed by oxidation give the red color to the red soil. The most common oxides are those of iron and aluminum, and their respective red and yellow staining of soils is quite common in tropical regions, which have high temperatures and precipitation.

Oxidation converts Fe²⁺ and Mn²⁺ present in several primary minerals into Fe³⁺ and Mn³⁺ or Mn⁴⁺, respectively. This conversion results in increase of positive charge and the mineral becomes unstable. This charge imbalance is neutralized by loss of some oxidized iron and manganese ions, and some cations may also dissociate from the mineral.

The precipitate may form a coating over the mineral surface, which slows down the subsequent rate of hydrolysis:

Fe²⁺ + 2H₂O + ½ O₂ ↔ Fe (OH)₃ + H⁺

The H⁺ ions produced by this reaction increase the acidity and accelerate the rate of hydrolysis.

vi. Reduction:

It is the process of removal of oxygen and is the reverse of oxidation. It is equally important in changing soil color to gray, blue, or green as ferric iron is converted to ferrous iron compounds. Reduction takes place under the con­ditions of excess water or waterlogged condition with little or no oxygen.

2Fe₂O₃ (Hematite) – O₂ → 4FeO (Ferrous Oxide) – Reduced form

vii. Complexation:

Metals released from primary minerals such as Fe, Mn, and Al build complexes with organic components, such as fulvic acid and humic acid, which are very stable. Weathering of primary minerals produces secondary minerals. Elements released from primary minerals are prone to leaching if they do not form complexes.

Type # 3. Biological Weathering:

Biological weathering is the process in which the rocks are weathered by the organic acids released by living organ­isms. The types of organisms that can cause weathering range from bacteria and fungi to plants and animals. These release oxalic acid, phenolic acid, fulvic acid, humic acid, etc.

Biological weathering may also take place through one or more of the following processes:

i. Breaking and fracture of particles because of animal burrowing or by the pressure put forth by growing roots.

ii. Consumption of soil particles by plants and animals as nutrients.

iii. Movement of soil particles caused by many large soil organisms. This movement can introduce the materials to different weathering processes found at distinct locations in the soil profile.

iv. Decomposition of minerals by chelates. Organisms produce organic substances known as chelates that have the ability to decompose minerals and rocks by the removal of metallic cations. This process is called chelation.

v. Weathering by lichens. Lichens are rich in chelating agents, which trap the elements of the decomposing rock in organometallic complexes. Some of the lichens are epilithic, living on the rock surface; some are endolithic, actively boring into the rock surface; and others are chasmolithic, living in hollows or fissures within the rock.

vi. Changes in moisture regimes in soils by organisms. Water has an important role in initiating and accelerating both physical and chemical weathering. The availability of water in soils is increased by shade from aerial leaves and stems, roots, mass, and humus.

vii. Carbonation due to plant roots. Weathering may be caused by carbonic acid, which is formed when carbon diox­ide released during respiration by plant roots combines with water.

viii. Cation exchange reactions. Plants absorb nutrients from the soil through cation exchange that can cause pH changes, leading to favorable conditions for weathering. The absorption processes often also involve the exchange of basic cations for hydrogen ions.