WEATHERING AND MASS MOVEMENT (WASTING)

WEATHERING AND MASS MOVEMENT (WASTING)

Weathering is the physical disintegration and chemical decomposition of rocks without removing the rock broken pieces from the location. Rocks that undergo the process of weathering are at the surface or near the surface of the earth. The physical forces acting on such rocks tend to break the igneous rocks into small fragments and separate the component minerals, grain from grain. The fragmentation increases the mineral surface area that is exposed to the chemical active solutions.

Weathering process marks the beginning of changes in the landscape after its construction through rock formation. It changes the rocks from the massive form to the elastic state (broken into smaller particles). In other words, it is the process of rock reduction since the rocks are broken into particles. It is also a process of rock alteration because the rock minerals decompose and change their state.

Looking at the state in which the rocks are fragmented and the manner in which the rock minerals and other constituents get rotten, we always talk of two major types of weathering. These are: Physical weathering and Chemical weathering.

Some writers add a third type as Biological weathering because they want to recognize the work of living organisms and plants in the process of rock breakdown. It is true that that living things (animals and plants) play active roles in the breakdown of rocks, but such roles may be either physical or chemical in nature. Thus, emphasis is on the two major types stated above.

Physical weathering

This is a process in which rocks breakdown into smaller fragments due to application of physical forces. Some writers prefer to name physical weathering as mechanical weathering because both refer to application of forces that lead to the fragmentation of rocks. Other writers have gone further to identify two types of physical weathering as Thermal Weathering in which the force that disintegrate or breakdown the rocks is generated within the rocks, and Mechanical Weathering in which the force responsible for the disintegration or fragmentation of rocks is generated externally and applied to the rocks.

A detail discussion of the two types of the physical weathering reveals which forces are internally generated that lead to rock disintegration and the forces externally applied leading to rock fragmentation.

The thermal weathering

According to Odemerho, F.O. and Onokerchoraye, A. G. (1994), thermal weathering occurs when the force of disintegration is generated within the rocks as its mineral constituents expand or contract, due to either the loss or gain of heat or even by the interaction of the electrical double layers at the rock — water interface. Rock minerals expand and contract at different rates and this causes internal stress and strains within the rock, forcing the rock to disintegrate. Exposed rock surfaces especially in arid zones, undergo differential heating which causes strain leading to the top layers of the rock to peal in a process called exfoliation.

The expansion of interstitial air (the air between the rock mineral spaces) is greater than that of the rock minerals, and this interstitial air is responsible for the granular disintegration of most rocks that contain minerals.

During the initial formation of rocks, water is trapped within the rock and this water plays a vital role in rock breakdown. This is another form of thermal weathering.

Mechanical weathering

Having seen what mechanical weathering is, that is, the application of external forces to the rocks and this force it to break, the mechanical weathering the task now is to identify its processes. These processes include:

1. Frost action

When water collects in the crevices, pores, cracks and joints of rocks, this water freezes into ice; and when the temperature drops below the freezing point; this frozen water increases in volume. The increase is calculated to be about 10% once water changes to ice. This voluminal increase exerts pressure on the walls of the rock crevices and this forces the rock to break. This process of mechanical weathering is common in desert regions during the nights and in temperate regions during the winter seasons. Both the above mentioned periods are responsible for sharp drops in temperature and which lead to the process of rock breakdown.

1. Frost heaving

This process of mechanical weathering occurs on fine grained rock particle deposits that are unconsolidated. When the deposits soak in rainwater or snow during winter, the deposits freeze. This leads to a gradual increase of the freezing zone due to the additions from snow accumulations above it and an upward movement of water from the unfrozen ground below it. Therefore, the soil above the freezing zone is heaved upwards. This process of mechanical weathering is common in the temperate regions of the world.

Chemical weathering

This is the gradual decomposition of rocks when they are exposed to air and water. The term “decomposition”, when applied to rocks means many things. For example, it means total alteration of the rock mineral constituents by transferring them into new secondary minerals and this eventually leads to the breakdown of the rocks, into smaller fragments. Odemerlio and Onokerhoraye (1994) have added that both the decomposition, transformation and the breakdown of the rock into fragments can only take place when air and water containing reactive chemical elements get into the rock cracks and crevices. The chemical reactive elements are mostly in small quantities and their main function is to set up chemical reaction in the surface layers of the exposed rocks. It is this reaction together with some dissolved rock constituents that weaken the whole rock surfaces, leading to a final breakdown of the rock. Therefore, from the explanation and the opinions of researchers as expressed above, various processes are responsible for the decomposition of the rocks.

The processes of chemical weathering

There are many arid different types of processes of chemical weathering. The processes operate in different forms and this leads to the decomposition of the affected rocks and the rocks finally breakdown into pieces. These processes are as follows:

Carbonation

The process of carbonation in chemical weathering of rocks is illustrated in the following equation:

2KOH + H₂C0₃      K₂CO₃ + 2HOH

(Potash) + (Carbonic Acid)     (Potassium Carbonate) + (Ionized water). Meaning that water will react with carbon dioxide to form weak carbonic acid, which combines with potash to form a highly soluble compound — potassium carbonate. In this form, the potassium and magnesium are carried away in solution and lost to the regolith.

Hydration

The equation illustrating the process of hydration is as follows:

CaSO₄ + 2H₂0     CaSO₄.2H₂0

(Anhydrite) + (water)     (Gypsum).

From the equation above, hydration is a simple process that involves absorption of water. It is an exothermic reaction. The process is accomplished when anhydrite is converted to gypsum.

Hydrolysis

This process starts with the dissolution of water into its constituent ion in rock. This means that water contains two elements of hydrogen, these are hydrogen ions (H₊) and hydroxyl ions (H). In the dissociation form, the ions react with the mineral elements of the rocks thereby leading to the complete dissolution of the rock minerals and these breaks down the structure of the rock. The chemical weathering of some rocks such as orthoclase (potash feldspar) for example; though the process of hydrolysis leads to total alteration of the orthoclase mineral by hydroxyl ions in the dissociated water

molecule to form kaolimite, silica and soluble potash. The process of hydrolysis is illustrated in an equation as follows:

2KALSi₃O₈ + ₃Ho          A1₂SiO₅ (OH)₄ + 4SiO₂ + 2KOH

(Orthoclase) + (water)         (Kaolinite) + (Silica) + (Potash)

In warm humid climates of tropical and equatorial zones, hydrolysis and oxidation can result in the decay of igneous and metamorphic rocks. The functions of decomposition by hydrolysis and oxidation is to change a very strong rock into a very weak surface layers and allow erosion process to operate with great effectiveness.

Oxidation

The process of oxidation explains how oxygen in the air or water reacts with the rock minerals and the reaction dissolves some of the minerals. Where the rocks contain iron, the iron readily reacts with oxygen to form iron oxide. The iron oxide is weakened and the rock surface is loosened and become decomposed. The oxidation process is illustrated in an equation as follows:

2Fe + 2H₂O          2FeO +2H₂

(iron) (water)        (Iron oxide) (Hydrogen)

Solution

The process of solution in chemical weathering is illustrated in the following equation:

H20+ O3       H2C03

(water) (Oxygen)         (Carbonic Acid) showing that:

The process of solution takes place when rain water gets into contact with rocks containing carbonates of calcium or solution. Since the rainwater and oxygen react in the air to form carbonic acid, the reaction is capable of dissolving some of the minerals of the rocks especially when the rocks react with the carbonic acid. The rocks become weaken and are easily decomposed and dissolved in solution.

The Process of decomposition

Organisms such as bacteria present in the soil are solely responsible for decomposition. These bacteria cover some rocks and live on decaying plants and animals and produce some organic acids. When these acids get into contact with water, the acids dissolved and accelerate up the process of rock decomposition.