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
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• 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.