Exogenic Forces

Exogenic Forces

Exogenic forces, also called external forces, originate from outside the Earth or within its atmosphere. These forces are mainly driven by solar energy (heat from the sun) and act on the Earth’s surface.

• These forces are responsible for the wearing down of landforms, so they are often described as land-degrading forces.
• They create stress in Earth materials, which leads to gradual changes in rocks and landforms.

Understanding Stress

• Stress means the force applied over a specific area.
• It is generated when materials are either pushed or pulled.
• This stress is the main cause behind processes like weathering, erosion, and deposition.

Climatic Control Factors
Two major climatic elements influence exogenic processes:

• Temperature: Causes expansion and contraction in rocks
• Precipitation: Influences moisture-related changes like chemical reactions and freezing

These factors together create stress in rocks, leading to their breakdown over time.

Exogenic Processes (or Denudation)

All external land-shaping processes are collectively known as denudation. The term ‘denude’ means to strip or expose.

Processes included under denudation:

• • Weathering – Breakdown of rocks
  • Mass movement – Downward movement of materials due to gravity
  • Erosion – Removal of materials
  • Transportation – Movement of materials from one place to another

Factors affecting denudation
Denudation depends largely on the nature and structure of rocks, such as:

• • Rock type (hard or soft)
  • Geological structures like folds and faults
  • Orientation and slope of rock layers
  • Presence of joints and cracks
  • Mineral composition and chemical reactivity
  • Permeability (ability to allow water through)

Although these processes act slowly and subtly, their long-term effect is significant, gradually weakening and reshaping the Earth’s surface.

Denudation:- 4 Phases

The process of denudation occurs in four main stages:

• • Weathering – Breakdown of rocks at their original location
  • Erosion – Removal of weathered material
  • Transportation – Carrying materials to new locations
  • Deposition – Settling of transported materials

Weathering

Weathering refers to the breaking down of rocks, soil, and minerals due to various natural factors.

Types of agents involved:

• • Physical agents: Heat and pressure
  • Chemical agents: Processes like oxidation, hydration, and leaching

Key characteristic:

• • Weathering occurs in situ, meaning the material does not move from its original position.
  • The broken material is later carried away by erosion.

Main types of weathering:

• • Mechanical (Physical)
  • Chemical
  • Biological

Mechanical Weathering

Mechanical weathering involves the physical breakdown of rocks without changing their chemical composition. This occurs due to environmental factors such as temperature changes and physical forces.

Common causes include:

• • Heating and cooling cycles
  • Freezing and thawing
  • Wetting and drying
  • Growth of salt crystals
  • Activity of plants and animals

Below are the major processes of mechanical weathering explained in detail:

Exfoliation due to pressure release or unloading

• • Rocks formed deep inside the Earth (especially igneous rocks) are under high pressure due to overlying layers.
  • When these upper layers are removed by erosion, the pressure reduces.
  • As a result, rocks expand and crack parallel to the surface.
  • Over time, outer layers peel off in sheets, a process called exfoliation or sheeting.

Exfoliation due to thermal stress weathering

• • Caused by repeated heating and cooling due to daily and seasonal temperature changes.
  • The outer surface of rocks expands more than the inner layers.
  • This leads to peeling off of surface layers, similar to onion skins.

Where it is most effective:

• • • • Dry regions (deserts)
      • High altitudes with large temperature variations

• • • • Moisture can increase the expansion effect, making the process more intense.
        

Granular Disintegration

• • Occurs in rocks made up of different mineral grains.
  • Dark-colored minerals absorb more heat than lighter ones.
  • This causes uneven expansion and contraction, leading to separation of grains.
  • Eventually, the rock breaks down grain by grain.
    
    

Frost weathering

• • Water enters cracks and pores in rocks during warmer conditions.
  • When temperature drops, the water freezes and expands in volume.
  • This expansion creates strong pressure, causing rocks to crack.
  • This process happens repeatedly through freeze-thaw cycles, gradually weakening rocks.
    
    

Types of frost weathering:

• • • Frost shattering
    • Frost wedging
    • Freeze-thaw action

Frost wedging

• • Water fills cracks in rocks.
  • When it freezes, it expands and forces the cracks wider.
  • Repeated cycles eventually break the rock apart.
    
    

Shattering

• • Intense freezing causes rocks to break into sharp, angular fragments.
  • These fragments accumulate as scree at the base of slopes or mountains.
    
    

Block Separation (freeze-thaw weathering)

• • Continuous freeze-thaw cycles weaken rocks along natural joints (cracks).
  • Rocks break into large angular blocks, a process called block disintegration.

Salt Weathering

• • Occurs when salt-rich water enters rock cracks and later evaporates.
  • This leaves behind salt crystals.
  • Salt crystals expand during formation and heating.
  • This creates pressure in rock pores, causing cracks and fragmentation.
  • Dry and desert areas where evaporation is high
    

Role of Physical Weathering

Physical weathering plays an important role in preparing rocks for further changes. Its key functions include:

• • It breaks large rock masses into smaller pieces, making them easier to move by natural agents like water, wind, or ice
  • It increases the surface area of rocks, which allows chemical and biological weathering to act more effectively

Chemical Weathering

• Chemical weathering refers to the decomposition and alteration of rocks and soils through chemical reactions. Unlike physical weathering, it changes the chemical composition of the materials.
• Main chemical processes involved:
• • Dissolution – Minerals dissolve in water
  • Carbonation – Reaction with carbon dioxide
  • Hydration – Addition of water molecules
  • Oxidation and Reduction – Reaction with oxygen or loss of oxygen
  • Solution – Complete mixing of minerals into water
• These processes often occur together and accelerate the breakdown of rocks.
• Factors that speed up chemical weathering:
• • Water (most important factor)
  • Air gases such as oxygen and carbon dioxide
  • Heat, which increases reaction rates
  • Organic acids released by plants and microorganisms
• Water is considered the primary agent because it acts as a medium for most chemical reactions.

Dissolution

• • Many minerals and compounds dissolve easily in water.
  • Elements such as calcium (Ca), sodium (Na), potassium (K), magnesium (Mg) and compounds like carbonates and sulphates are commonly affected.

Example reaction:

• • • Water + Carbon dioxide + Calcium carbonate → Calcium ions + Bicarbonate ions
  • This process gradually weakens and removes rock material.

Acid Reactions

• • Chemical weathering is often enhanced by acid formation.
  • Common acid reactions include:
    • Water + carbon dioxide forms carbonic acid
    • Water + sulfur forms sulfuric acid
    • Key point:Hydrogen ions (H⁺) in acids are highly effective in breaking down minerals.
      
      
• There are several types of Chemical Weathering:

Hydrolysis and oxidation:

• • Hydrolysis is the most significant chemical weathering process.
    
    • Water splits into H⁺ and OH⁻ ions
    • These ions react with minerals, leading to:
      
      • Breakdown of crystal structure
      • Formation of new minerals
    • Water behaves like a weak acid, especially on silicate minerals
  • Oxidation occurs when minerals react with oxygen
    
    • This process often affects iron-rich rocks, causing them to weaken and change color

Acid Action:

• • One of the most common forms of chemical weathering is caused by carbonic acid
  • Carbon dioxide dissolves in water to form this weak acid

Modern environmental impact:

• • Increased pollution (sulfur and nitrogen oxides from burning fuels) has led to acid rain
  • Acid rain accelerates the weathering process
  • Rocks most affected:Limestone and marble are highly vulnerable
  • Impact on human structures:Buildings and monuments made of marble are gradually damaged by acid rain

Biological Weathering

• Biological weathering is the breakdown of rocks by living organisms such as plants, animals, and microorganisms.
• It occurs in two main ways:
  • Physical action:
    
    • Plant roots grow into cracks and force rocks apart
    • Animal movement (like burrowing or trampling) causes mechanical stress
  • Chemical action:
    
    • Microorganisms and lichens release acids that dissolve minerals
    • Organic matter contributes to chemical changes in rocks
      

      

Mass Wasting

Mass wasting (also called mass movement) is the downward movement of soil, rock, and debris along a slope due to gravity.

• • It often occurs without the direct involvement of transporting agents like rivers or wind
  • However, water content can significantly influence the movement
    

Landslide

• A landslide is a type of mass wasting where rock, soil, or debris moves down a slope due to gravitational force.
  
  • It happens when the slope material becomes unstable
  • Also known as landslips or slope failure
• Types of landslide Movements:
• Different types of landslide movements are classified based on how the material moves:
  • Falls
    
    • Rocks drop freely from steep slopes
    • Movement is very fast, involving bouncing or rolling
  • Topples
    
    • Forward rotation of rock around a pivot point
  • Lateral spreads
    
    • Horizontal movement due to cracks and fractures
    • Often triggered by earthquakes
  • Slides
    
    • Movement along a defined surface
    • Two types:
      
      • Rotational slides: Movement along a curved surface
      • Translational slides: Movement along a flat or gently sloping surface
  • Flows
    
    • Material behaves like a thick fluid, moving without a clear fracture surface
      
      

Note: Some landslides may involve multiple types of movement at once.

Causes of Landslides:

Natural Causes:

• • Water pressure within the soil
  • Loss of vegetation cover
  • Heavy rainfall or glacier melting
  • Earthquakes
  • Volcanic activity

Human Causes:

• • Vibrations from heavy machinery
  • Mining and blasting activities
  • Construction and land modification
  • Agricultural and forestry practices affecting water flow

Prevention of Landslides:

• The best way to prevent landslides is to avoid high-risk areas, but when that is not possible, several methods can help:
  • Improving drainage
    
    • Divert surface and underground water away from slopes
    • Prevent water accumulation
  • Removing material from the top (head)
    
    • Reduces pressure on the slope
  • Strengthening the base (toe)
    
    • Adding support materials to prevent downward movement
  • Building retaining structures
    
    • Use strong materials like concrete or metal piles
  • Replacing weak materials
    
    • Substitute unstable soil with stronger alternatives
  • Maintaining vegetation
    
    • Plants reduce water infiltration and hold soil together
  • Rockfall protection
    
    • Use barriers like fences, ditches, or walls to stop falling rocks

Erosion

• Erosion is the process by which Earth’s surface is worn away and materials are removed by natural agents such as water, wind, and ice.
  
  • It is an ex-situ process, meaning materials are moved from their original location
  • Movement occurs due to external forces (kinetic energy)
• Key difference from weathering:
  • Weathering breaks rocks in place
  • Erosion involves removal and transport
• Erosion is one of the most powerful processes shaping landforms, especially in forming valleys, plains, and other features.

Agents of Erosion

• Five main agents are responsible for erosion, each creating unique landforms:
  • Running water
    
    • Forms fluvial landforms like valleys and floodplains
  • Wind
    
    • Dominant in dry regions, forming aeolian landforms like dunes
  • Glaciers
    
    • Shape landscapes in cold regions, especially mountains
  • Waves
    
    • Act along coastlines, forming cliffs and beaches
  • Underground water (Karst processes)
    
    • Create features in limestone regions such as caves

Deposition

• Deposition is the final stage of the erosion process, where transported materials are laid down or settled.
  • It occurs when the energy and speed of transporting agents decrease
  • Common in areas with gentle slopes or calm conditions

• Deposition is not an independent process
• It is the result of erosion and transportation

In simple terms:

• • Weathering breaks rocks
  • Erosion removes them
  • Transportation carries them
  • Deposition settles them in a new location

This continuous cycle plays a crucial role in shaping the Earth’s surface over time.