Exogenic Force

Introduction

The geomorphic processes undergoing in the Earth comprises the endogenic and exogenic forces. The endogenic forces result due to the various interactions taking place because of pressure arising from within the Earth, also called the internal forces. On the other hand, the exogenic process is known as the force which affects the Earth’s atmosphere resulting from the heat rays of the Sun causing erosions and weathering, also known as land wearing or external forces. Exogenic forces cause chemical and biological weathering using natural agents like water, wind, waves etc. which are collectively termed as geomorphic agents.

Causes of exogenic forces

The exogenic forces cause denudation which means removal or uncovering of layers. The elements responsible for these exogenic processes are called geomorphic agents (or exogenic geomorphic agents) e.g. waves, wind. The primary agents responsible for these exogenic forces are weathering, mass wasting, erosion, and deposition. The main causation of this force is via gravity and gradient. Gravitational force acts on the earth’s layers producing a sloping surface which in turn initiates the movement of matter downwards along the slope. As we know that stress is known as Force per unit area, this stress is produced in a solid by pushing or pulling which induces deformation.

The process of weathering can be described as the action of weather and climate over earth’s material i.e. the chemical decomposition and mechanical breakdown of rocks through various elements of weather and climate. When the rocks are subjected to weathering, some minerals are lost due to chemical/ physical percolation of groundwater and therefore the remaining (valuable) minerals increase in concentration.

• Weathering can be of three types: Physical, Chemical and Biological.

1)  Physical weathering :

Physical weathering, also known as mechanical weathering, depends upon applied forces. Those are as follows:

  (i) Forces of gravity leading to shear stress, tear and excess pressure 

 (ii) Often expansion forces develop due to changes in temperature, crystal formation and animal actions, which cause weathering and breakdown

 (iii) Pressure exerted by water during wet and dry cycles

Though the main reason that remains is the thermal expansion and pressure happening in the Earth’s environment.

2) Chemical weathering

Chemical weathering takes place when there is carbonation, oxidation/reduction, hydration or formation of solution.

Types of Chemical Weathering

1. A) SOLUTION :

• Combination of dissolved solute particles in water or acids is called a solution.
• Solubility of the mineral particles in water or weak acids becomes the deciding factor for the removal of solids in the solution.
• A suspension is formed after coming in contact with water which occurs due to the breakdown of solid particles after mixing in water.
• Minerals like sulphates,nitrates,potassium and common salt , also known as the rock-forming minerals are affected by this method as they are soluble in nature.
• This phenomenon takes place mostly in the rainy season.
• Limestones are a combination of calcium carbonate and magnesium bicarbonate which are soluble in water containing carbonic acid which are carried with water resulting into natural weathering.

B) CARBONATION :

• The reaction between carbonate and bicarbonate with minerals resulting in the breaking down of carbonate minerals and feldspar is called carbonation.
• Cave formation occurs when calcium carbonates and magnesium carbonates dissolve in carbonic acid leading to their removal in a solution without any residue.
• Carbonic acid is a weak acid which is formed when carbon dioxide in air and soil reacts with water.

C) HYDRATION:

• Hydration is the chemical reaction where the addition of water moiety takes place.
• Rocks possess many minerals which are hydrophilic in nature implying that they take in water. This causes an increase in their volume thereby leading to their expansion.
• This is a reversible and prolonged process. Due to the incessant continuation of this, rocks may disintegrate as fatigue sets in e.g. calcium sulphate reacts with water to produce gypsum, which is unstable than its parent salt.

D) OXIDATION AND REDUCTION :

• Oxidation is a reaction that involves the combination of a mineral (e.g. iron, manganese, sulphur ) with oxygen to form oxides or hydroxides.
• For example, Iron imparts the colour red in its mineral form but when oxidised it changes to brown-yellow.
• Rock breakdown becomes the result of the addition of oxygen and thereby disrupting its integrity.
• Reduction occurs when oxidised minerals are present in an environment in absence of oxygen.

(3) Biological weathering

• Biological weathering is associated with the growth or movements of organisms.
• The organisms cause the removal of minerals during their growth.
• It may also provide prospects for physical or chemical weathering.
• Bare rock surfaces are the most favourable places for the growth of lichens and mosses creating a humid chemical microenvironment.
• Sprouting of seedlings and roots of plants from cracks and crevices exert physical pressure thereby creating a path for infiltration by water and various chemicals.
• Organisms like earthworms, rats, mice, moles etc., expose new surfaces by burrowing and assist in the penetration of chemicals, moisture and air.
• Decaying plant and animal remains produce humic, carbonic and other acids which further enhance the process of decay and solubility of various elements.
• Algae use minerals as a part of their nutrition thereby helping in the concentration of mineral oxides (e.g. iron and manganese oxides).
• Ploughing by human beings and grazing of cattle and other animals too play a role in biological weathering.

Conclusion 

There are many geomorphic processes on Earth, but the two major processes are exogenic and endogenic forces. The endogenic force is what takes place within the earth whereas exogenic force occurs at the surface of Earth due to the predominant effect of the Sun. The exogenic processes further include weathering, erosion, deposition and mass movements. Weathering implies the breakdown of rocks by the effect of nature’s elements which are the wind, water, ice etc. The three main subtypes of weathering include the Physical, Chemical, and Biological processes. The physical process mostly involves gravitation, expansion and other mechanical forces. Chemical weathering involves the chemical reactions of oxidation and reduction, hydration, carbonation and solution. Biological weathering comprises the collective effects of plants, microorganisms and animals on the Earth’s surface leading to weathering. All these processes lead to both environmental relief and economic benefit (such as the production of mineral ores) thereby turning out to be an extremely important phenomenon. Exogenic processes are necessary for the stability of earth and its inhabitants and hence is a miracle of Mother Nature. We as human beings should protect our environment and preserve nature for a sustainable future for the upcoming generations.