Exogenic Forces

By Sud
Apr 29, 2023

Exogenic (Exogenetic) Processes

  • Exogenic forces are forces that come from the earth’s exterior or atmosphere.
  • These forces cause land wearing down and are known as land wearing forces.
  • Exogenic processes result from stress induced by various forces caused by the sun’s heat.
  • Stress is the force applied per unit area, which can cause weathering, erosion, and deposition.
  • Temperature and precipitation are two important climatic elements that induce stress in earth materials and control various processes.

Exogenic Processes (or Denudation)

  • Exogenic processes are also known as land wearing forces.
  • These processes result in wearing down of the earth’s surface.
  • They are a direct result of stress induced in earth materials due to various external forces.
  • Denudation is a term used to describe all exogenic geomorphic processes.
  • Denudation includes weathering, mass wasting/movements, erosion, and transportation.
  • Rock type and its structure determine the extent of denudation.
  • The effects of exogenic processes may be small and slow, but they will ultimately affect the rocks severely due to continued fatigue.
  • Denudation has 4 Phases:
    • Weathering
    • Erosion
    • Transportation
    • Deposition

Weathering

  • Weathering is the disintegration of rocks, soil, and minerals under the influence of physical and chemical agents.
  • It is an in situ process where the rock material is broken down into smaller pieces.
  • The weathered material is carried away by erosion.
  • There are three major groups of weathering processes: physical or mechanical, chemical, and biological.

Mechanical Weathering:

  • Physical weathering involves mechanical disintegration of rocks due to temperature changes, freeze-thaw cycles, wet-dry cycles, crystallization of salts, animal and plant activity, etc.
  • Various mechanisms of mechanical weathering are explained below.
  • Exfoliation due to pressure release or unloading:
    • Intrusive igneous rocks are formed deep beneath the Earth’s surface and are under tremendous pressure due to overlying load.
    • Removal of overlying load due to erosion causes vertical pressure release.
    • The upper layers of the rock expand and fracture parallel to the surface.
    • Sheets of rock break away from the exposed rocks along the fractures.
    • The process of sheeting is also known as exfoliation.
  • Exfoliation due to thermal stress weathering:
    • Thermal stress weathering is caused by expansion and contraction of rocks due to temperature changes.
    • Surface layers of rocks expand more than the rock at depth, causing the outer layers to peel off or exfoliate.
    • This process is most effective in dry climates and high elevations with drastic diurnal temperature changes.
    • Moisture can enhance thermal expansion in rock, but temperature changes are the primary driver.
  • Granular Disintegration:
    • Granular disintegration happens in rocks composed of different types of coarse-grained minerals.
    • Dark-colored minerals absorb more heat than light-colored minerals.
    • This leads to differential expansion and contraction of mineral grains resulting in grain by grain separation from the rock.
  • Frost weathering:
    • Water penetrates the pore spaces or fractures in rocks during warm season
    • Water freezes into ice during the cold season
    • The expansion of ice due to freezing exerts tremendous pressure on rock walls
    • This pressure can tear apart even massive rocks
    • Frost weathering occurs due to growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting
    • Frost weathering includes several processes where ice is present such as frost shattering, frost-wedging, and freeze-thaw weathering
    • ​​
  • Frost wedging:
    • Freeze wedging is caused by the repeated freeze-thaw cycle.
    • Cracks filled with water are forced further apart with subsequent freezing and thawing.
  • Shattering:
    • Severe frost can disintegrate rocks along weak zones to produce highly angular pieces with sharp corners and edges through the process of shattering.
    • Shattering piles up rock fragments called scree at the foot of mountain areas or along slopes.
  • Block Separation (freeze-thaw weathering):
    • Repeated freeze-thaw cycles weaken the rocks which, over time, break up along the joints into angular pieces. 
    • The splitting of rocks along the joints into blocks is called block disintegration.
  • Salt Weathering:
    • Salt weathering occurs when saline solutions seep into cracks and joints in rocks and evaporate, leaving salt crystals behind.
    • Salt crystals expand during the crystallization process and when they are subjected to above-normal temperatures.
    • Expansion in near-surface pores causes splitting of individual grains within rocks.
    • Eventually, the split grains fall off (granular disintegration or granular foliation).
    • Salt weathering is associated with arid climates where strong heating causes strong evaporation and crystallization.
  • Role of Physical Weathering:
    • Reduces rock material to smaller fragments that are easier to transport
    • Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering

Chemical Weathering

  • Chemical weathering decomposes rocks and soil through various processes.
  • The processes include dissolution, solution, carbonation, hydration, oxidation, and reduction.
  • Chemical weathering processes are interrelated and work together to speed up the weathering process.
  • Acids from microbial and plant-root metabolism, water, air, and heat speed up chemical reactions.
  • Water is the main operator in chemical weathering, and many ionic and organic compounds dissolve in it.
  • Examples of dissolved compounds include silica, K, Na, Mg, Ca, Cl, CO3, and SO4.
  • Acid reactions occur in chemical weathering, including carbonic acid from water and carbon dioxide and sulfuric acid from water and sulfur.
  • H+ ions are effective at breaking down minerals.
  • There are several types of Chemical Weathering:
  • Hydrolysis and oxidation:
    • Hydrolysis and oxidation are important processes in chemical weathering.
    • Hydrolysis occurs when water dissociates into H+ and OH- ions, which chemically combine with minerals and bring about changes.
    • Water acts as a weak acid on silicate minerals.
    • Oxidation is the reaction of a substance with oxygen.
  • Acid Action:
    • Acid action, most commonly carbonic acid, is another form of chemical weathering.
    • Carbon dioxide dissolves in water to form a weak acid.
    • Increasing concentrations of sulfur oxides and nitrogen oxides in the atmosphere have led to increased acidity of rain.
    • Carbonate sedimentary rocks, especially limestone and marble, are highly susceptible to this type of weathering.
    • Acid rain is also harmful to architectural structures made of marble.

Biological Weathering

  • Biological weathering weakens and breaks down rock by the actions of living organisms.
  • The organisms responsible for biological weathering include plants, animals, and microbes.
  • Physical stress caused by plant roots or animal hooves can contribute to biological weathering.
  • Chemical changes caused by organisms such as worms or lichens can also cause biological weathering.

Landslide

  • A landslide is the movement of rock, debris, or earth down a slope driven by the force of gravity.
  • Landslides are also known as landslips, slumps, or slope failures.
  • Types of landslide movements:
    • Falls are masses dislodged from very steep slopes that move extremely rapidly.
    • Topples are a forward rotation around a pivot point low or below one or more masses.
    • Lateral spreads are the result of movement involving lateral extension accommodated by shear or tensile fractures, often earthquake-induced.
    • Slides displace masses along one or more discrete planes and may be rotational or translational in their movement.
    • Rotational movement is where the plane is curved, and the mass rotates backward around a common point with an axis parallel to the slope.
    • Translational movement is where the plane is more or less planar or gently undulating, and the mass moves roughly parallel to the ground surface.
    • Flows are masses moving as a deforming, viscous unit without a discrete failure plane.
    • Some landslides may represent more than one form of movement.
  • Causes of Landslides:
    • Natural Causes:
      • Groundwater pressure acting on the slope.
      • Loss of vegetation
      • Weakening of slope due to melting of the glacier or heavy rainfall
      • Earthquakes
      • Volcanic eruptions
    • Human Causes:
      • Vibrations from machinery
      • Blasting of mines
      • Earthwork which alters the slope
      • Construction, agriculture or forestry activities which can affect the amount of water entering the soil
  • Prevention of Landslides:
    • Avoiding landslide-prone areas is the best solution
    • Improving surface and subsurface drainage can increase slope stability
    • Excavating the head of the landslide can decrease driving pressure and slow or stop a landslide
    • Buttressing the toe of the landslide with fill can increase resisting forces along the failure surface in the toe area
    • Constructing piles and retaining walls can be used to reinforce the slope
    • Removal and replacement of landslide-prone soil and rock with stronger materials is an option
    • Preserving vegetation can minimize water infiltration and slow erosion
    • Rockfall protection measures include ditches, heavy-duty fences, and concrete catch walls.

Erosion

  • Erosion is the process of wearing away the earth’s surface by external agents like water, wind, or ice.
  • It involves the breaking of rocks due to kinetic energy from external agents and the transport of broken particles to other locations.
  • Erosion is the most destructive process shaping the earth’s surface.
  • There are five agents of erosion: running surface water, wind, glaciers, waves, and karst.
    • Running surface water creates fluvial landforms.
    • Wind creates Aeolian landforms in arid and semi-arid regions.
    • Glaciers carve landforms in high alpine mountains.
    • Waves form landforms on the edge of the continent.
    • Karst landforms are created by the action of underground water on karst or limestone regions.

Deposition

  • Erosion is the process of acquiring and transporting rock debris by agents like water, wind, and waves.
  • Weathering is not a pre-condition for erosion to occur, as erosion can happen in unweathered conditions too.
  • Deposition is a consequence of erosion, where the erosional agents lose their velocity and energy on gentle slopes, causing materials to settle.
  • Deposition is not caused by any agents, but rather the end result of erosion.

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