1.5 - Geography Notes 17

Elaborate Notes

Exogenic Movements

Exogenic movements, also known as exogenetic processes, are geomorphic processes that originate from forces acting on or above the Earth’s surface. These processes are fundamentally driven by energy from outside the Earth’s lithosphere, primarily solar energy and gravity. The term was popularised by geomorphologists in the early 20th century to contrast with endogenic (internal) processes like volcanism and tectonics.

• Energy Sources:

  • Solar Energy: The sun is the ultimate source of energy for most exogenic processes. It drives the hydrological cycle (evaporation, precipitation), which powers rivers and glaciers. Differential heating of the Earth’s surface creates pressure gradients, which in turn generate winds.
  • Gravity: Gravity is a constant force that acts on all materials on the Earth’s surface, pulling them from higher elevations to lower ones. It is the primary driving force behind mass movements and is crucial for the flow of water and ice.

• Agents of Exogenic Movements: These are the media that carry out the work of denudation.

  • Running Water (Fluvial): Rivers and streams are the most widespread and effective agents of erosion and transportation.
  • Groundwater: Water that seeps into the ground can dissolve rocks (especially carbonates) and transport material in solution.
  • Glaciers (Glacial): Moving masses of ice are powerful agents of erosion, capable of carving deep valleys and transporting vast amounts of debris.
  • Wind (Aeolian): Wind is a significant agent in arid and coastal regions, transporting sand and dust.
  • Waves and Currents (Marine/Coastal): These shape coastlines through erosion and deposition.

• Fundamental Processes: Aggradation and Degradation

  • Degradation (Denudation): This refers to the overall lowering of the Earth’s surface through the combined action of weathering, mass movement, and erosion. The concept of a “base level of erosion,” introduced by John Wesley Powell in his “Exploration of the Colorado River of the West” (1875), is central here. It is the lowest level to which a land surface can be eroded by running water, which is ultimately sea level.
  • Aggradation: This is the process of building up the land surface through the deposition of sediments by agents like rivers, wind, or glaciers. It occurs when the energy of the transporting agent decreases, for example, when a river enters a plain, forming an alluvial fan, or flows into a lake or sea, forming a delta. The goal of both processes, in the long run, is to achieve a state of equilibrium or “grade,” a concept developed by G.K. Gilbert in his monograph “Report on the Geology of the Henry Mountains” (1877), where he described a graded stream as one that has achieved a balance between erosion and deposition.

Weathering

Weathering is the in-situ (on-site) breakdown of rocks, soil, and minerals at or near the Earth’s surface through contact with the atmosphere, water, and biological organisms. It is a preparatory process for erosion as it weakens the rock, making it more susceptible to removal by erosional agents.

• Factors Affecting Weathering:
  • Rock Type and Structure: The mineral composition, texture, and presence of joints, fractures, and bedding planes profoundly influence weathering. For instance, granite, a crystalline igneous rock, is resistant to weathering but its joint patterns can be exploited by frost action and hydrolysis. Conversely, sedimentary rocks like limestone are highly susceptible to chemical weathering (carbonation), while shale, being less permeable, weathers more slowly.
  • Slope and Aspect: Steep slopes facilitate the removal of weathered material (regolith) by gravity, constantly exposing fresh rock surfaces to weathering. The aspect, or the direction a slope faces, determines its exposure to sunlight, wind, and rain. In the Northern Hemisphere, south-facing slopes (adret slopes) receive more solar radiation, leading to greater diurnal temperature ranges and thus more intense physical weathering (e.g., thermal expansion). The south-facing slopes of the Himalayas are a prime example.
  • Temperature: Fluctuation in temperature, especially the diurnal range in hot deserts, causes rocks to expand and contract. As different minerals expand and contract at different rates, this differential stress leads to granular disintegration. High temperatures also accelerate the rate of chemical reactions, following Van’t Hoff’s rule, which states that for every 10°C rise in temperature, the rate of reaction doubles.
  • Water: Water is the most crucial agent in both physical and chemical weathering. In physical weathering, the freeze-thaw action of water in cracks is a powerful force. In chemical weathering, water acts as a universal solvent and a medium for chemical reactions like hydrolysis and carbonation.
  • Biological Agents:
    • Flora: Plant roots can penetrate cracks in rocks, exerting mechanical pressure (root wedging). Decomposing plant matter releases humic acids, which enhance chemical weathering. Lichens and mosses produce weak acids that can dissolve rock minerals.
    • Fauna: Burrowing animals like rabbits, earthworms, and termites mix the soil and regolith, exposing new surfaces to weathering agents.
    • Anthropogenic: Human activities such as mining, quarrying, and agriculture drastically alter the landscape and accelerate weathering rates.

Types of Weathering

Physical (Mechanical) Weathering

This process involves the disintegration of rocks into smaller fragments without any change in their chemical composition. It is dominant in cold and arid climates.

• Crystal Growth:

  • Frost Action (Freeze-Thaw): When water in rock joints freezes, it expands in volume by about 9%. This expansion exerts immense pressure (up to 2100 kg/cm² at -22°C), widening the cracks. Repeated cycles of freezing and thawing cause angular fragments of rock to break off, a process known as frost shattering or frost wedging. This is highly effective in periglacial and high-altitude environments, leading to the formation of scree or talus slopes.
  • Haloclasty (Salt Crystal Growth): In arid and coastal regions, saline water seeps into rock pores. As the water evaporates, salt crystals (e.g., sodium chloride, gypsum) form and grow, exerting pressure on the pore walls and causing the rock to disintegrate grain by grain. This is a significant process in the weathering of sandstones and can be observed on ancient monuments in coastal areas.

• Pressure and Thermal Expansion:

  • Exfoliation (Onion-peeling): This occurs due to a combination of thermal stress and pressure release. Rocks heat up and expand during the day and cool and contract at night. The outer layers experience the greatest temperature fluctuations, causing them to expand and contract more than the interior. This differential stress leads to the peeling off of concentric shells of rock, much like the layers of an onion. It creates rounded landforms known as exfoliation domes, such as the Half Dome in Yosemite National Park, USA.
  • Pressure Release (Unloading): Intrusive igneous rocks like granite form deep beneath the surface under immense pressure from overlying rock (overburden). As denudation removes this overburden, the confining pressure is released, causing the rock mass to expand upwards and develop horizontal cracks or joints parallel to the surface. This is known as sheeting.
  • Block Disintegration: This occurs in well-jointed rocks, such as granite or sandstone, where weathering agents penetrate along joints and bedding planes, causing the rock to break down into large, rectangular blocks.
  • Granular Disintegration: This is common in coarse-grained rocks like granite, where different minerals expand and contract at different rates due to temperature changes, causing the rock to crumble into individual mineral grains.

Chemical Weathering

This process involves the decomposition of rocks through chemical reactions, altering the original minerals into new, more stable mineral compounds. It is most effective in warm, humid climates.

• Hydrolysis: The chemical reaction between water molecules (H⁺ and OH⁻ ions) and silicate minerals. For example, feldspar, a common mineral in granite, reacts with water to form kaolinite (clay mineral) and silicic acid. This process weakens the rock structure, making it susceptible to erosion. 2KAlSi₃O₈ (Orthoclase Feldspar) + 2H₂O → Al₂Si₂O₅(OH)₄ (Kaolinite) + 4SiO₂ (Silica) + 2K⁺(aq)
• Hydration: The absorption of water molecules into the crystal structure of a mineral, without changing its chemical composition. This causes the mineral to expand in volume, creating physical stress within the rock. For instance, anhydrite (CaSO₄) hydrates to form gypsum (CaSO₄·2H₂O), which involves a significant increase in volume.
• Oxidation and Reduction:
  • Oxidation: The reaction of minerals with oxygen, usually in the presence of water. It is most common in iron-bearing minerals (e.g., pyroxene, amphibole, biotite). The oxidation of iron results in the formation of iron oxides (like hematite) and hydroxides (like limonite), which gives weathered rocks and soils a reddish-brown colour (rusting).
  • Reduction: Occurs in waterlogged, oxygen-deficient (anaerobic) environments, where oxidised minerals are chemically reduced, often changing the colour of the soil from red to grey or bluish-green.
• Carbonation: The reaction of carbonic acid (H₂CO₃) with carbonate rocks. Rainwater combines with atmospheric carbon dioxide to form this weak acid. It is particularly effective on rocks like limestone (calcium carbonate) and dolomite, dissolving them to form calcium bicarbonate, which is soluble in water. This process is responsible for the formation of karst topography, including features like caves, sinkholes, and stalactites. CaCO₃ (Calcite) + H₂CO₃ (Carbonic Acid) → Ca(HCO₃)₂ (Calcium Bicarbonate)

Biological Weathering

This involves the contribution of living organisms to the weathering process. It can be both physical and chemical.

• Physical: Plant roots growing into fractures pry rocks apart. Burrowing animals expose fresh rock surfaces.
• Chemical: Decomposing organic matter produces humic and fulvic acids. Organisms like lichens secrete chelating agents that can extract mineral ions from rocks, causing decomposition.

Erosion

Erosion is the process of wearing away and transporting the Earth’s surface materials (weathered rock, soil) by dynamic agents such as running water, wind, glaciers, and waves. Unlike weathering, erosion involves movement or transportation.

• Types of Erosional Processes:
  • Abrasion (or Corrasion): The mechanical scraping, grinding, and wearing away of rock surfaces by friction and impact of rock particles carried by the erosional agent. Wind-borne sand blasts rock surfaces (creating ventifacts), and pebbles carried by a river grind against its bed and banks. Corrasion is a specific term for abrasion by running water.
  • Attrition: The process where the transported sediment particles themselves are worn down as they collide with each other. This results in the particles becoming smaller, smoother, and more rounded as they move downstream in a river or are worked by waves on a beach.
  • Hydraulic Action: The sheer force of moving water dislodging and quarrying loose material from the bed and banks of a river or from a coastline. The compression of air in cracks by advancing waves can also weaken and shatter rock.
  • Corrosion (Solution): The chemical or solvent action of water on soluble or partly soluble rocks with which it comes into contact. This is essentially chemical weathering in action, but considered erosion when the dissolved material is carried away by the agent.
  • Cavitation: Occurs in fast-flowing water. As water velocity increases, pressure drops, leading to the formation of water vapour bubbles. When these bubbles move into zones of higher pressure, they collapse violently, generating powerful shockwaves that can shatter rock surfaces. This is significant in waterfalls and rapids.
  • Deflation: The erosional process exclusive to wind (aeolian), where loose, fine-grained particles like sand and silt are lifted and removed from a surface, leaving behind coarser material. This can lead to the formation of desert pavements and blowouts (deflation hollows).
  • Plucking (or Quarrying): A glacial erosion process. As a glacier flows over bedrock, meltwater seeps into cracks, freezes, and attaches to the glacier. As the glacier moves forward, it pulls or ‘plucks’ out large blocks of rock from the bedrock.

Mass Movement

Also known as mass wasting, it is the downslope movement of rock, regolith (weathered material), and soil under the direct influence of gravity. While water can act as a lubricant and add weight, it is not a transporting agent in the way it is in a river. Gravity is the sole driving force. Mass movements are a crucial link between weathering and erosion, delivering weathered debris from slopes to the valley floors where it can be picked up by rivers or glaciers. It represents a large-scale, often catastrophic, form of denudation.

Prelims Pointers

• Exogenic Forces: External forces powered by solar energy and gravity that operate on the Earth’s surface.
• Denudation: A comprehensive term for the combined processes of weathering, mass movement, and erosion.
• Aggradation: The building up of a land surface by deposition of material.
• Degradation: The lowering of a land surface by weathering and erosion.
• Base Level of Erosion: The lowest point to which a river or stream can erode its valley; ultimate base level is sea level.
• Weathering: The in-situ (in place) disintegration and decomposition of rocks.
• Physical Weathering: Breakdown of rocks without chemical change; dominant in arid and cold climates.
• Chemical Weathering: Decomposition of rocks with chemical change; dominant in warm and humid climates.
• Freeze-Thaw: A type of physical weathering caused by the repeated freezing and expansion of water in rock cracks.
• Haloclasty: Physical weathering caused by the growth of salt crystals in rock pores.
• Exfoliation: The peeling away of outer rock layers, often due to pressure release and thermal expansion.
• Hydrolysis: Chemical weathering where water reacts with minerals (e.g., feldspar turning into clay).
• Carbonation: Chemical weathering of carbonate rocks (e.g., limestone) by carbonic acid. This process forms Karst topography.
• Oxidation: Chemical reaction of minerals with oxygen, causing ‘rusting’ (e.g., iron-bearing minerals).
• Erosion: The removal and transportation of weathered material by agents like water, wind, ice, and waves.
• Abrasion: Mechanical erosion by scraping and grinding of particles carried by an erosional agent.
• Corrasion: The specific term for abrasion by running water.
• Attrition: The wearing down of transported particles as they collide with each other.
• Deflation: The lifting and removal of loose material by wind.
• Plucking: Glacial erosion process where rock blocks are torn away from the bedrock.
• Mass Movement (Mass Wasting): The downslope movement of rock and regolith under the direct influence of gravity.

Mains Insights

Historiographical Debates in Geomorphology

• A central debate in classical geomorphology revolves around the models of landscape evolution proposed by William Morris Davis and Walther Penck.
  • Davisian Cycle of Erosion (1899): W.M. Davis proposed a model where landforms evolve through a sequential series of stages: ‘youth,’ ‘maturity,’ and ‘old age,’ culminating in a low-relief plain called a ‘peneplain.’ His model assumed a rapid period of tectonic uplift followed by a long period of tectonic stability during which denudation occurs. It is a time-dependent model summarized by his famous triad: “Structure, Process, and Stage.”
  • Penck’s Model of Morphological Analysis (1924): In contrast, Walther Penck argued that uplift and denudation occur simultaneously. The shape of slopes (convex, straight, or concave) is a reflection of the rate of uplift versus the rate of denudation. His model is more dynamic and does not rely on distinct stages, leading to the formation of an ‘endrumpf’ (final trunk) rather than a peneplain.
  • Relevance: This debate highlights the different ways of interpreting landscape evolution—one as a historical sequence (Davis) and the other as a dynamic interplay of forces (Penck). Modern geomorphology incorporates elements of both, recognizing that tectonic activity and denudation are concurrent processes.

Human-Environment Interaction and Accelerated Denudation

• Cause-Effect Relationship: Human activities are significantly accelerating the natural rates of exogenic processes, leading to environmental degradation.
  1. Deforestation and Agriculture: Removing vegetation cover exposes soil to direct raindrop impact (splash erosion) and surface runoff (sheet and rill erosion), drastically increasing soil erosion rates. This leads to loss of fertile topsoil, siltation of reservoirs, and increased flood risk. The creation of badland topography in regions like the Chambal Valley is partly an outcome of natural processes exacerbated by human activities.
  2. Urbanization and Construction: Impermeable surfaces (roads, buildings) increase surface runoff, leading to urban flooding and channel erosion in rivers. Construction activities on slopes, such as road cutting in the Himalayas, destabilize the terrain and trigger mass movements (landslides).
  3. Mining and Quarrying: These activities involve the removal of large volumes of rock and soil, directly altering landforms and increasing the load of sediment in rivers, which affects aquatic ecosystems and channel morphology.

Climate Change and Exogenic Processes

• Climate change is a critical driver altering the intensity and frequency of exogenic processes, creating a feedback loop with significant consequences.
  • Intensified Weathering: Rising global temperatures accelerate chemical weathering rates. In high-latitude and high-altitude regions, warming leads to the thawing of permafrost, causing ground subsidence and increasing susceptibility to mass movement (solifluction).
  • Changes in Hydrological Cycle: Increased frequency of extreme precipitation events leads to more powerful floods and higher rates of fluvial erosion. Conversely, prolonged droughts in other areas can lead to vegetation loss and increased aeolian erosion (deflation), contributing to desertification.
  • Glacial Retreat: The rapid melting of glaciers due to global warming exposes vast areas of unstable morainic debris, increasing the risk of landslides and glacial lake outburst floods (GLOFs), a major hazard in Himalayan regions.

Linkage to Disaster Management (GS Paper III)

• An understanding of exogenic processes, particularly mass movements, is fundamental to disaster risk reduction.
  • Hazard Zonation: Geologists and geographers can map areas prone to landslides, debris flows, and avalanches by studying slope stability, rock structure, and triggers like rainfall intensity. This information is vital for land-use planning and restricting construction in high-risk zones.
  • Mitigation Strategies: Engineering solutions like building retaining walls, rock bolting, and creating effective drainage systems can help stabilize vulnerable slopes. Non-structural measures, such as afforestation on slopes and developing early warning systems based on rainfall thresholds, are equally crucial. The National Landslide Susceptibility Mapping (NLSM) project by the Geological Survey of India is a key initiative in this direction.