The geological phenomena and processes that originate externally to the Earth’s surface derived through the strength of the earth. They are genetically related to the atmosphere, hydrosphere and biosphere. Comet and meteoroid impacts, the tidal forces of the moon, radiation from the sun etc. are the factors of exo-genetic forces.
• The processes which results in general the lowering of land surface through various factors are known as Denudation. Denudation includes all the processes that wear away or rearrange landforms on the surface of the earth.
• A general term which describes the general process by which rocks are broken down at the earth’s surface into sediments, clays, soils and substances that are dissolved in water.
• The process of weathering typically begins when the earth’s crust is uplifted by tectonic forces. After the physical breakup and chemical decay of exposed rocks by weathering, the loosened rock fragments and alterations products are carried away through the process of erosion.
• Erosion relies on transporting agents such as wind, rivers, ice, snow and downward movement of materials to carry weathered products away from the source area. As weathered products are carried away, fresh rocks are exposed to further weathering. Over time, that mountain or hill is gradually worn down.
• Weathering of rocks is influenced by
1. Rock Structure (folds, faults, joints, voids),
2. Climate (temperature, precipitation),
3. Topography (rainfall depends on topography),
4. Natural vegetation binds the rocks, decayed organic matter to the formation of carbon dioxide humid acids by penetrating its roots into the earth’s surface.
a) Physical Weathering
• When rock is broken and disintegrated without any chemical alternation, the process is called as physical weathering. Physical weathering is caused by:
1. Frost Action: The repeated action of water freezing in pores of cracks of rocks, or growth and melting of ice crystals in the pore spaces of soils and rocks fractures, disintegrates even the hardest rocks like granite. Frost action occurs predominantly in cold climates.
2. Temperature: In dry cloudless regions (deserts) a marked diurnal range of temperature leads to mechanical weathering (exfoliation – onion skin weathering). After sunset, sounds like ‘pistol-shots’ are commonly heard in deserts.
3. Salt Crystal Growth: In the arid climates, as the water evaporates, crystals form from dissolved minerals.
b) Chemical Weathering:
• It results from chemical reactions between minerals in rocks and external agents like air or water. Oxygen oxidizes minerals to alteration pro ucts whereas water can convert minerals to clays or dissolve minerals completely.
• It mainly occurs in the hot and humid climates where heat and moisture are in abundance.
• Chemical weathering processes :
1. Hydration: Once minerals have experienced hydration, they become more susceptible to chemical weathering, especially those of carbonation and oxidation.
2. Oxidation: Oxygen dissolved in water reacts with certain rock minerals, especially iron to form oxides.
3. Carbonation: Carbon dioxide (CO2) is introduced into a fluid. This is more prominent in karsts (limestone) regions.
4. Solution: Solid rocks like calcium, potassium, sodium magnesium, etc. get dissolved in water.
5. Hydrolysis: Water combines with rock-minerals to form an insoluble precipitate like clay mineral.
c) Biological or Organic Weathering:
• It is the disintegration of rocks as a result of the action by living organisms.
• Microscopic organisms like algae, moss, lichens and bacteria can grow on the surface of the rocks and produce chemicals that have the potential of breaking down the outer layer of the rock.
• They eat away the surface of the rocks. These microscopic organisms also bring about moist chemical micro-environments which encourage the chemical and physical breakdown of the rock surfaces.
d) Differential Weathering:
• The difference in rates of weathering due to different types of rocks,
textures or other characteristics is referred to as differential weathering forming unique landforms including pedestals, waterfalls and monadnocks.
• Climate can also produce differential weathering responses for the same rock type such as limestone weathers more quickly in wet climates than dry climates.
The process by which the surface of the earth is worn away by the action of water, glaciers, winds, waves etc. This is usually considered under four distinct categories:
• Water: erodes rocks and the landscapes by transporting weathered materials from their source to another location where they are deposited.
• Wind: erodes materials by picking them up and temporarily transporting them from their source to another location where they are deposited, and either stored or re-mobilized and transported to another location.
• Ice: erosion occurs when particles are plucked up or incorporated by moving ice, such as glaciers, and is transported downhill, or when friction between the ice and bedrock erodes materials and then transports them downhill.
• Gravity: facilitates the down slope transportation of loosened, weathered materials and enables them to move without the aid of water, wind, or ice. Gravity related erosion is a major component of mass-wasting events.
• It involves erosion by flowing water, shaping the landscapes by removing and transporting weathered materials from their source to another location where they are deposited or further transported. It has 3 distinct categories:
1. Rain Splash Erosion: when the rain drop loosens and mobilizes particles.
2. Sheet Erosion: when particles loosened by rain splash and are transported by runoff water down to the slope of a surface.
3. Rill Erosion: When water concentrates during sheet erosion and erodes small rills or gullys into the surface that channel flow down slope.
• Fluvial erosion can occur during rainfall events, from melt-water runoff, or ground water percolation. Materials being eroded and transported are either suspended in the water, bounced by saltation, or rolled along the ground by traction depending on a variety of conditions.
• The accumulation of fluvial erosion and associated processes over a large area forms pathways for surface and groundwater flow and carves v-shaped river valleys that continue to erode, transport, and deposit weathered sediments across the landscape.
• Wind erodes weathered rocks by picking them up and temporarily transporting them from their source to another location where they are deposited, and either stored or re-mobilized and transported to another location
• Erosion by wind is divided into two different categories: Deflation and Abrasion
• Deflation is the movement or transport of particles through the air or along the groun
• Abrasion is the process that occurs when wind-transported particles sculpt features in the landscape through a “sand-blasting” like process
• Aeolian erosion and deposition processes create a diversity of landforms including sand dunes, loess deposits, and yardangs.
• Periglacial and Glacial Erosion: It occurs when rock particles are incorporated into the glacial ice through a process referred to as plucking and they are transported down slope within the glacier.
• The friction and abrasion of the ice and rock moving across the bedrock, erodes the surface of the bedrock and often leaves scrapes, grooves, striae or polished rock surfaces.
• The cumulative effects of glacial erosion on a mountainous landscape can produce distinct u-shaped valleys which are a common glacial landform.
• Gravity facilitates the down slope transportation of loosened, weathered materials and enables them to move without the aid of water, wind, or ice. However, these agents can act as catalysts for gravity related erosion.
• Movements by gravity may be very slow or very abrupt.
• Gravity related erosion can be coherent or incoherent. Coherent refers to the erosion of a consolidated mass of materials that erode or move as a single unit, incoherent refers to the erosion or movement of a mass of unconsolidated individual fragments of materials.
• Unconsolidated materials tend to stabilize near an angle of 35° (referred to as the angle of repose) however this balance is easily disrupted by changes in environmental conditions, addition of weathered materials, and or other adjustments that may cause mass wasting.
• It is a rapid form of erosion that works primarily under the influence of gravity in combination with other erosional agents.
• Mass wasting occurs very quickly and can result in either small or large scale changes to the landscape depending on the type of event such as Rock falls, Landslides, Debris/Mud flows, Slumps and Creep.