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
- Natural Causes:
- 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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