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Weathering - Definition and Types of Weathering

Weathering
 
 

Definition

It is a general term and applied to the combine action of all processes causing rocks disintegration physically and decomposition chemically.

Or

It is the process of weakening rock and producing a rock waste or soil cover. Weathering occurs when rocks and minerals comes in contact with the atmosphere, organic life and suicidal water. Essential feature of weathering is that it affects rocks in-situ (in place) and no transportation of material is involved.

Also Read: Tip Physical Weathering | Tip Chemical Weathering | Tip Rock Faults

Factors affecting weakening of rock are:

  1. A part of rainfall always snips into the soil on the covered rocks and looses the rock particle by dissolving it.
  2. Frost action also takes place in the for-spaces of rocks and causes disintegration and splitting of rocks into fragments.
  3. Chemical reactions are also taking place in rocks internally and causing rock-loosening.
  4. Slow geological process of construction and destruction is going on the surface of earth for millions of years because earth is old enough and has been completely leveled by weathering and erosion. Factors affecting rock weathering.

Rock weathering is greatly affected by the following parameters.

Types of Weathering

There are two types of weathering:

  1. Physical weathering
  2. Chemical weathering

1. Physical weathering:

In the mechanical disintegration and breaking of the rocks to form particles of smaller size. In physical weathering, the composition of the weathered products remains unchanged. Following are the different agents of physical weathering:

  1. Frost action
  2. Heating and cooling
  3. Organisms

a. Frost action:

Freezing of water in the cracks of rocks tends to disintegrate them because volume of water increases 1-11 times of its actual volume. It exerts a great pressure on the wall having cracks. By this process, angular fragments of rocks are broken off from the main body of the rock, causing physical weathering.

b. Heating and cooling:

Heating and cooling of rock masses occur due to daily changes in temperature. Heating of rocks causes expansion and cooling cause contraction of rocks. This repeated heating and cooling helps to develop cracks in rocks and the rock will be disintegrated.

c. Organisms:

Plants and insects like earthworms, ants, termites and snakes play an important role in physical weathering because they convert rock into soil to get their food. Plants also grow in joints and cracks of the rocks and push them further apart. Similarly, man also breaks rocks by making roads, constructing dams and reservoirs by drawing tunnels and mines etc.

2. Chemical Weathering:

It is the type of weathering in which the rock minerals are attacked by water, oxygen, or by alkaline or acid materials dissolved in the water. Chemical structure of the rock minerals is altered and Clay minerals are formed in this way.

3. Mechanical Weathering

It is the breakdown of the rock by physical or mechanical forces.

Frost wedging and heaving

water enters cracks and expands on freezing, pressure splits rock. alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks (water expands about 9% upon freezing)

Exfoliation

rock layers peel like layers of an onion Due to pressure release on igneous rocks (formed in Earth’s crust)

Thermal expansion

repeated heating and cooling of rocks will induce stress and breakage - daily cycle of temperature change weakens rock

Organic activity

physical breakdown and/or transport of material by plants and animals, e.g. plant roots growing in cracks, burrowing animals

Minerals in the parent rock are unchanged

Result is a primary rock mineral, which is the constituent of most sands & gravels

Weathering of rock is a complex process and depends on many factors including:

  • Climate – wind, rain, frost & temperature
  • Topography (e.g. waterlogged valleys are subjected to deeper weathering than plateaus)
  • Rock structure (e.g. presence of faults, joints etc dictate rock fracturing due to freeze-thaw cycles)
  • Rock types (e.g. dominance of various minerals dictate resilience to weathering)

Looking at climate

  • Mechanical weathering (due to freeze-thaw & heat-cool cycles) obviously depends on climate
  • Chemical weathering is also strongly climate dependent as this form of weathering dominates in regions of relatively high humidity and rainfall
 

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