1.5 - Geography Notes 10

Elaborate Notes

Geomorphic Processes: Shaping the Earth’s Surface

The configuration of the Earth’s surface is in a constant state of flux, shaped by processes known as geomorphic processes. These processes are driven by forces originating from both within the Earth’s interior (endogenetic) and on its surface (exogenetic). The interplay of these forces induces physical stress and chemical actions on earth materials, leading to the creation, modification, and destruction of landforms. The study of these landforms and the processes that shape them is known as geomorphology.

• Endogenetic Forces: These are internal forces that operate from beneath the Earth’s crust. Their primary energy source is the Earth’s internal heat, which originates from two main sources:

  1. Primordial Heat: Residual heat left over from the planet’s formation approximately 4.6 billion years ago.
  2. Radiogenic Heat: Heat produced by the radioactive decay of elements like uranium, thorium, and potassium within the Earth’s mantle and crust. This internal energy drives processes like mantle convection, which is the foundational mechanism of plate tectonics as proposed in the 1960s, building upon the earlier works of Alfred Wegener’s Continental Drift (1912) and Harry Hess’s Seafloor Spreading (1960). Endogenetic forces are fundamentally constructive, responsible for creating the major relief features of the planet, such as continents, ocean basins, mountain ranges, plateaus, and rift valleys. Their effects manifest as large-scale uplift, subsidence, folding, and faulting.

• Exogenetic Forces: These are external forces that act upon the surface of the Earth. Their energy is derived primarily from the sun (insolation) and gravity.

  1. Solar Energy: Drives the climatic systems, including wind patterns, precipitation, and temperature variations, which in turn power the agents of erosion like running water (rivers), wind, glaciers, and sea waves.
  2. Gravity: Acts as a constant force pulling materials downslope, driving mass wasting events (landslides, soil creep) and enabling the flow of rivers and glaciers. These forces are primarily responsible for denudation, a combined process of weathering (in-situ breakdown of rocks), erosion (removal of material), and transportation. They work to wear down the landforms created by endogenetic forces, creating smaller, secondary topographical features like valleys, canyons, deltas, sand dunes, and caves. The concept of the ‘Cycle of Erosion’, famously articulated by William Morris Davis (1899), provides a model for how these forces sculpt landscapes over geological time.

Endogenetic Movements: Diastrophism and Catastrophism

Endogenetic movements are broadly classified based on their velocity and intensity into two categories:

• Catastrophic (Sudden) Movements: These are abrupt and high-intensity events that occur over a very short duration, often with destructive consequences. Their effects are observable in human timeframes.

  • Earthquakes: A sudden release of stored strain energy in the Earth’s crust, propagating as seismic waves. They occur primarily along fault lines where tectonic plates interact. The historic earthquake of Bhuj (2001) in Gujarat is a stark example of catastrophic movement along a fault line.
  • Volcanic Eruptions: The extrusion of magma, ash, and gases from a vent in the Earth’s crust. They provide direct evidence of the molten nature of the Earth’s asthenosphere. The eruption of Mount Vesuvius in 79 AD, which buried the Roman cities of Pompeii and Herculaneum, is a classical archaeological and geological example.

• Diastrophic Movements: This term, coined by American geologist G.K. Gilbert (1890), refers to all processes of crustal movement and deformation that operate very slowly, over vast geological timescales. These are the fundamental building processes of the Earth’s crust, leading to the formation of primary landforms like continents and mountains. Their effects are not perceptible in a human lifetime. They are further classified into tectonic, isostatic, and eustatic movements.

  • Tectonic Movements: The term ‘tectonic’ is derived from the Greek word ‘tekton’, meaning ‘builder’. These movements involve the deformation of the Earth’s crust due to internal forces, leading to the creation of large-scale structural features. They are categorised based on the direction of the force.

    • Epeirogenic Movements (Continent-building): Derived from Greek ‘epeiros’ (continent) and ‘genesis’ (origin). These are large-scale vertical movements caused by radial forces acting along the Earth’s radius. They result in broad, gentle uplift (emergence) or subsidence (submergence) of continental masses without significant internal deformation or folding.

      • Example of Uplift: The gradual uplift of the Deccan Plateau in Peninsular India. Another significant example is the post-glacial rebound of Scandinavia and North America, where the landmasses are still slowly rising after the removal of the immense weight of ice sheets from the last Ice Age.
      • Example of Subsidence: The slow sinking of the land along the northern coast of the Gulf of Mexico or the subsidence of the ‘Gateway to India’ in Mumbai.

    • Orogenic Movements (Mountain-building): Derived from Greek ‘oros’ (mountain) and ‘genesis’ (origin). These are caused by tangential forces acting horizontally or parallel to the Earth’s surface. These forces induce immense compression or tension, leading to intense folding and faulting of narrow, elongated belts of the crust, thereby forming mountain ranges.

      • Compressional Forces: These forces push crustal rocks together, causing them to buckle and fold. This is characteristic of convergent plate boundaries where two plates collide.

        • Folding: The bending of rock strata into wave-like structures without breaking.
          • Anticline: An up-fold, arched shape where the rock layers dip away from the central axis. In a simple anticline, the oldest strata are found at the core.
          • Syncline: A down-fold, trough-like shape where the rock layers dip towards the central axis. The youngest strata are found at the core.
          • Limbs: The two sides or flanks of a fold.
          • Types of Folds:
            • Symmetrical Fold: Both limbs have an equal and gentle inclination.
            • Asymmetrical Fold: One limb is steeper than the other due to unequal compressional force.
            • Overfold (Overturned Fold): Due to intense compression, one limb is pushed over the other, causing the strata in the inverted limb to be overturned.
            • Recumbent Fold: The compressional force is so strong that the axial plane of the fold becomes nearly horizontal, causing the fold to lie on its side.
            • Nappe: A large-scale recumbent fold that is broken at its root by a thrust fault and has been moved a considerable distance from its original position. The study of nappes was pioneered by geologists like Albert Heim in the Swiss Alps during the late 19th century. The Himalayas are a classic example of a complex mountain system with extensive recumbent folds and nappes, formed by the collision of the Indian and Eurasian plates.

      • Tensional Forces: These forces pull the crustal rocks apart, causing them to fracture and break. This is characteristic of divergent plate boundaries.

        • Faulting: A fracture or zone of fractures between two blocks of rock, along which there has been significant displacement.
          • Types of Faults:
            • Normal Fault: Caused by tensional forces. The block above the fault plane (hanging wall) moves down relative to the block below (footwall). This leads to an extension of the crust. The East African Rift Valley is a major geological feature formed by a series of normal faults.
            • Reverse Fault (or Thrust Fault): Caused by compressional forces. The hanging wall moves up relative to the footwall. This leads to a shortening of the crust. Thrust faults are low-angle reverse faults and are very common in mountain-building zones like the Himalayas.
            • Strike-Slip Fault (or Transform Fault): Caused by shear forces where blocks slide past each other horizontally with little to no vertical movement. The San Andreas Fault in California is a world-renowned example of a right-lateral strike-slip fault.
          • Associated Landforms:
            • Horst: An uplifted block of land between two parallel normal faults. The Black Forest in Germany and the Vosges mountains in France are classic examples of Horsts. The Vindhya and Satpura ranges in India are considered block mountains.
            • Graben: A down-dropped block of land between two parallel normal faults, forming a rift valley. The Rhine Valley in Europe and the Narmada-Tapti rift valleys in India are prominent examples.

Comparison of Fold and Fault-Block Mountains

Feature	Fold Mountains	Fault-Block Mountains
Formation Process	Formed by the folding of crustal layers due to intense compression. It is a process of ductile deformation.	Formed by the vertical displacement of crustal blocks along faults due to tensional or compressional forces. It is a process of brittle failure.
Dominant Force	Primarily caused by compressional tangential forces at convergent plate boundaries.	Can be caused by both tensional (leading to Horsts and Grabens) and compressional (leading to uplifted blocks via reverse faults) forces.
Dimensions	Typically much greater in length than in width, forming long, linear chains or arcs. They cover vast areas.	Generally have a more limited extent, being greater in width relative to their length, and are characterised by steep, faulted scarps.
Examples	The Himalayas (Asia), the Alps (Europe), the Andes (South America), the Rockies (North America).	The Vindhya and Satpura Ranges (India), the Black Forest (Germany), the Sierra Nevada (USA), the Vosges (France).

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Prelims Pointers

• Geomorphic Process: The physical and chemical interactions between forces that produce and alter landforms.
• Endogenetic Forces: Internal forces originating from within the Earth.
  • Energy Source: Primordial heat and radioactive decay.
  • Also known as constructive forces.
• Exogenetic Forces: External forces originating from above the Earth’s surface.
  • Energy Source: Solar energy (insolation) and gravity.
  • Also known as destructive forces (part of denudation).
• Diastrophism: Very slow, long-term movements of the Earth’s crust (e.g., continent and mountain building).
• Catastrophic Movements: Sudden, short-duration movements (e.g., earthquakes, volcanic eruptions).
• Epeirogenic Movements: Vertical, continent-building movements (uplift or subsidence).
• Orogenic Movements: Horizontal, mountain-building movements (folding and faulting).
• Tangential Forces: Forces acting parallel to the Earth’s surface.
  • Compression → Folding, Reverse Faulting.
  • Tension → Normal Faulting.
• Folding Terminology:
  • Anticline: Up-fold.
  • Syncline: Down-fold.
  • Recumbent Fold: A fold lying on its side.
  • Nappe: A large-scale recumbent fold detached from its roots and thrust forward. A key feature of the Alps and Himalayas.
• Faulting Terminology:
  • Normal Fault: Hanging wall moves down; caused by tension.
  • Reverse Fault: Hanging wall moves up; caused by compression.
  • Strike-Slip Fault: Blocks move horizontally past each other.
  • Horst: An uplifted block between two normal faults (Block Mountain). Example: Black Forest.
  • Graben: A down-dropped block between two normal faults (Rift Valley). Example: Rhine Valley.
• Mountain Type Examples:
  1. Fold Mountains: Himalayas, Alps, Andes, Rockies.
  2. Block Mountains (Horst): Vindhyas, Satpuras, Sierra Nevada (USA).

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Mains Insights

GS Paper I (Geography)

1. Interplay of Forces: Endogenetic forces act as the primary ‘constructors’ of relief, creating high mountains and deep basins. Exogenetic forces immediately begin their work as ‘destroyers’ or ‘modifiers’, sculpting and wearing down these features. This dynamic equilibrium or conflict is the central theme of geomorphology. The majestic height of the Himalayas is a result of ongoing orogenic uplift (endogenetic) being greater than the rate of denudation by rivers and glaciers (exogenetic).

2. Evolution of Geotectonic Theories: The understanding of these movements has evolved significantly. Early theories (like the Contraction Theory) were replaced by Alfred Wegener’s Continental Drift Theory (1912), which provided evidence but lacked a mechanism. Post-WWII studies of the ocean floor led to the Sea Floor Spreading Theory (Hess, 1960s). The synthesis of these ideas culminated in the Theory of Plate Tectonics, which provides a comprehensive framework explaining orogeny, epeirogeny, earthquakes, and volcanism as manifestations of plate interactions at their boundaries (convergent, divergent, transform).

3. Significance for Indian Geography:

   • Himalayan Orogeny: The compressional forces from the collision of the Indian and Eurasian plates created the Himalayas, influencing India’s climate (monsoon), river systems (perennial rivers like Ganga, Indus), and biodiversity.
   • Deccan Traps: An example of large-scale volcanism (catastrophic movement) leading to the formation of the Deccan Plateau with its black soil, crucial for agriculture.
   • Rift Valleys: The Narmada and Tapti rivers flow through grabens (rift valleys) formed due to faulting, explaining their unique westward flow compared to other peninsular rivers.

GS Paper III (Disaster Management & Economy)

1. Hazard Zonation and Risk Mitigation: Understanding endogenetic movements is fundamental to disaster management. The knowledge of fault lines and plate boundaries allows for the seismic zonation of India (Zones II to V). This informs building codes (e.g., National Building Code of India), infrastructure planning, and disaster preparedness strategies to mitigate the impact of earthquakes (catastrophic events). Similarly, monitoring volcanic activity is crucial for areas with active volcanoes.

2. Economic Significance of Geomorphic Processes:

   • Mineral and Energy Resources: Orogenic belts are often sites of significant metallic mineral deposits (copper, lead, zinc) due to magmatic activity and metamorphism. The folding and faulting processes create structural traps where petroleum and natural gas accumulate. The Gondwana grabens in Peninsular India, for instance, are the primary repository of India’s coal deposits.
   • Soil and Agriculture: Volcanic eruptions, like those which formed the Deccan Plateau, result in highly fertile soils (black cotton soil). The deep alluvial deposits in the Indo-Gangetic plains are a direct consequence of erosion (exogenetic process) of the Himalayas (formed by endogenetic process).
   • Geothermal Energy: Rift valleys and volcanic zones, areas of crustal thinning and high geothermal gradients, are potential sites for harnessing geothermal energy.

GS Paper IV (Ethics - Not directly applicable but can be linked through analogy)

• The concept of ‘Diastrophism’ (slow, imperceptible change leading to massive transformations) can be used as an analogy for social or ethical reforms, which are often gradual and slow but can fundamentally reshape a society’s values and structure over time. Conversely, ‘Catastrophism’ can be an analogy for sudden, disruptive events that force rapid societal change.