GENERAL GEOLOGY

EROSION

Erosion is breaking- down or disintegration of rocks due to the physical forces associated with the natural agencies followed by removal of the dislodged rock-fragments and particles and the sum total of the process.

EROSION, SOIL EROSION

Concepts of Erosion Cycle:-

The concept of cycle of erosion was formulated by “W.M. Davis”. According to him in a cycle of erosion the surface features undergo changes as a result of the processes acting upon them. These changes are systematic and follow each other in a regular sequence. These sequences are termed stages.

  • Initial stage
  • Youth stage
  • Stage of maturity
  • Old stage

 

The cycle of erosion, operates through the three stages until the initial relief is almost fully reduced and the surface is again leveled. Most of the cycle of erosion does not reach the final stage as some time during their operation either climatic or tectonic disturbances take place. An incomplete or partial cycle results from this. Topography returns to a youthful stage. This phenomenon is known as Rejuvenation whereby a mature topography becomes young.

The various land-forms and other topographic features associated with the different geomorphic agents owe their origin to the geological action like erosion, transportation and deposition of the concerned geomorphic agents and are as follows.

  • Geological action of river:-this phenomenon which is associated with the geological action of river is usually known as the fluvial cycle of erosion.

The erosion caused by the running water is of two types:-

  1. Mechanical
  2. Chemical

 

   (1) Mechanical erosion

it is because of the physical forces associated with the running water and it takes place in four distinct manners like

(a)Hydraulic action:- forces inherent in the flow of running water can do a great deal of erosion of the bank and the bed-rock. It is mostly due to surface relief, i.e. gradient

(b)Abrasion:- the materials which are being carried always by the running water acts  as tools of destruction and during their transportation because of their rubbing against the surface of the bed rock they bring about a scraping of the surface. This process of erosion is also known as corrosion.

(c)Attrition:-materials during their transit often collide among themselves and in turn get themselves teared and this is the process through which big boulders are gradually reduced in size and finally reach the size grade of sand and silt.

(d)Cavitation:-this is because of the presence of the air bubbles which create a whirling action at the time of penetration of water through the existing pores and fissures and the small sand particles along with the air bubbles play a major role in widening the cavities.

 

Factors which help mechanical erosion:-

  • Hydraulic gradient
  • Climate- which determines precipitation and finally volume and velocity of water.
  • Nature of the bed rock whether it is hard or soft.
  • Hardness of the transported materials.

    (2) Chemical erosion

it is also known as solution or corrosion during which process the materials get dissolved in the water of the river and are transported in solution.

Factors which help chemical erosion:

  • Dissolving action of water due to the presence of carbon- dioxide.
  • Solubility of the river bed.

 

       Transportation

there are two methods of transportation

Mechanical transportation:-it takes place in three ways

  • Suspension
  • Traction(creeping and rolling)
  • Saltation
  • Velocity of the river
  • Nature of the river current
  • Density of the rock material to be transported

Chemical transportation:-  it is through the process of solution, usually in the form of carbonates, sulphates of calcium sodium, potassium , magnesium etc.

Deposition:- :-  it is the last geological action by the river whereby materials transported get accumulated in an appropriate site where the following factors play major roles:

  • Decrease in velocity of the transporting medium.
  • Decrease in slope.
  • Decrease in volume.
  • Changes in channels
  • Chemical precipitation

Example of erosion cycle:-

The main features of the ‘fluvial cycle of erosion’ are as follows:

The cycle begins on a recently uplifted landmass. It is initiated through the drainage system working on it.

(a)Initial stage:-during the initial stage a river is formed and it involves some of the characteristic processes like:

1:-channel deepening due to bed scouring.

2:-pot hole drilling

3:-tributaries are fast developed

4:-head ward erosion is maximum

5:-waterfalls, gorges and canyons are formed.

(b) Youth stage:-river system is to some extent established. River capture or river piracy takes place in which case one river reaches the course of another one and if the course of the second river is diverted because of the greater gradient of the earlier river this phenomenon will be known as river-piracy.  In this stage there is conspicuous formation of V –shaped valleys.

(c) Mature stage:-in this stage there is maximum erosion through lateral cutting. The landmass is fully dissected; ridges and valley develop strikingly.

     As a result of heavy erosion and deposition this stage is comprised of a large number of landforms such as hog-backs, cuestas, plateau table, meanders, ox-bow lakes, terraces etc.

(d) old- stage:-down- cutting stops at the base level of erosion. Which is the mean sea level produced inland. Frequently the river is over flooded and builds up flood- plains on both sides. The initial irregular surface has become practically flat at this stage. The river is mostly engaged in depositing and does little of erosion and transportation. This topography is characterized by a set of distributional features which comprise delta etc.

Also See Weathering

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