Geomorphic Processes

GEOMORPHIC PROCESSES

Definition and Scope

**Geomorphic processes** are the endogenic and exogenic forces causing physical stresses and chemical actions on earth materials, bringing about changes in the configuration of the earth's surface. These processes operate continuously to shape landforms and modify the landscape.

**Geomorphic agents** are mobile media (running water, glaciers, wind, waves, currents) that remove, transport, and deposit earth materials. Geomorphic processes and agents, especially exogenic ones, are essentially one and the same.

**Geomorphic processes include:**

Endogenic processes: Diastrophism, volcanism

Exogenic processes: Weathering, mass wasting, erosion, deposition

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ENDOGENIC PROCESSES

Overview

Endogenic processes are powered by energy emanating from within the earth, primarily generated by:

Radioactivity

Rotational and tidal friction

Primordial heat from earth's origin

These processes induce **diastrophism** and **volcanism** in the lithosphere due to variations in geothermal gradients and heat flow.

Diastrophism

**Definition:** All processes that move, elevate, or build up portions of the earth's crust.

**Types of diastrophism:**

1. **Orogenic Processes**

Mountain building through severe folding

Affects long and narrow belts of the earth's crust

Creates high relief and intense crustal deformation

Example: Formation of the Himalayas, Andes, Alps through intense compression

2. **Epeirogenic Processes**

Involves uplift or warping of large parts of the earth's crust

Continental building process

Creates relatively gentle crustal deformation

Example: Formation of broad plateaus and continental shields

3. **Earthquakes**

Local, relatively minor crustal movements

Release accumulated tectonic stress

Can trigger mass movements and erosion

4. **Plate Tectonics**

Horizontal movements of crustal plates

Responsible for continental drift and ocean floor spreading

Example: Movement of the Indian plate northward creating the Himalayas

**Key distinction:** Orogeny creates mountains through intense folding, while epeirogeny creates continental surfaces through broad uplift.

**Results of diastrophic processes:**

Faulting and fracturing of the crust

Pressure, volume, and temperature (PVT) changes

Metamorphism of rocks

Creation of relief variations on earth's surface

Volcanism

**Definition:** The movement of molten rock (magma) onto or toward the earth's surface and the formation of intrusive and extrusive volcanic forms.

Volcanism builds new crust and brings interior material to the surface, creating volcanic landforms such as cones, calderas, and lava plateaus. This has been detailed in Unit II under volcanic processes.

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EXOGENIC PROCESSES

Energy Source and Driving Forces

Exogenic processes derive their energy from:

**Atmosphere** determined by the ultimate energy from the sun

**Gradients** created by tectonic factors (slopes, relief variations)

**Gravity**, which acts upon all sloping surfaces and tends to move matter downslope

**Figure 5.1 - Denudational Processes and Driving Forces:**

Temperature and precipitation are the two most important climatic elements controlling various exogenic processes

Different processes have distinct driving forces

Denudation is the general term covering weathering, mass wasting, erosion, and transportation

Stress and Deformation

**Stress** is force applied per unit area on earth materials. Two types of stress affect rocks:

1. **Shear Stresses**

Forces acting along the faces of earth materials

Cause angular displacement or slippage

Break rocks and other earth materials

2. **Molecular Stresses**

Caused by temperature changes, crystallization, and melting

Lead to development of internal stresses

Contribute to weathering and mass movements

Climatic Control

**Climate plays a crucial role in exogenic processes:**

Variations in thermal gradients due to latitude affect process intensity

Seasonal variations in temperature and precipitation

Land and water distribution affects climate patterns

Vegetation density, type, and distribution depend on precipitation and temperature

Within any climatic region, local variations occur due to:

Altitudinal differences

Aspect variations (north-facing vs. south-facing slopes)

Variation in insolation received by different slope orientations

Wind velocity and direction variations

Precipitation intensity and frequency

Freezing and thawing cycles

Frost penetration depth

Rock Type and Structure Influence

Climatic factors being equal, the intensity of exogenic geomorphic processes depends on:

**Rock structure includes:**

Folds and faults

Orientation and inclination of beds

Presence or absence of joints and bedding planes

Hardness or softness of constituent minerals

Chemical susceptibility of mineral constituents

Permeability or impermeability

**Key principle:** Different rock types offer varying resistances to different geomorphic processes. A rock may be resistant to one process but non-resistant to another. Differential resistance leads to differential rates of erosion and varied topography.

Denudation

**Definition:** The general term covering all exogenic geomorphic processes: weathering, mass wasting, erosion, and transportation.

The word "denude" means to strip off or uncover. Denudational processes gradually reduce relief variations and modify landforms over long periods (hundreds to thousands of years).

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WEATHERING

Definition

**Weathering** is the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate. It is an **in-situ process** with little or no motion of materials.

**Key characteristics:**

Occurs in place without significant movement

Prepares rocks for erosion and mass movements

Creates regolith (loose rock material) and aids soil formation

Reduces rock resistance to further erosion

Climatic Control on Weathering

**Figure 5.2 - Climatic Regimes and Depth of Weathering Mantles:**

Tropical humid regions: Maximum weathering depth

Temperate regions: Moderate weathering depth

Arid and polar regions: Minimal weathering depth

**Climate factors affecting weathering:**

Temperature: Higher temperatures increase chemical reaction rates

Moisture: Essential for chemical weathering

Seasonality: Freeze-thaw cycles increase mechanical weathering

Vegetation: Affects acid production and physical weathering

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TYPES OF WEATHERING PROCESSES

Chemical Weathering

**Definition:** Processes involving chemical reactions with oxygen, water, and acids that decompose, dissolve, or reduce rocks to fine clastic state.

**Requirements for chemical weathering:**

Water (surface and soil water)

Air (oxygen and carbon dioxide)

Heat to speed up reactions

**Important chemical weathering processes:**

1. **Solution**

Soluble minerals dissolve in water

Most rapid in areas with high rainfall and warm temperatures

Example: Limestone dissolution in caves

2. **Carbonation**

Carbon dioxide from atmosphere and decomposing organic matter combines with water to form carbonic acid

Carbonic acid reacts with rocks containing carbonates

Equation: CO₂ + H₂O → H₂CO₃

Most significant in humid regions

Example: Limestone caves formation in India's Meghalaya (Cherrapunjee region)

3. **Hydration**

Water molecules chemically combine with rock minerals

Increases rock volume and creates internal stresses

Example: Clay minerals swell when water is absorbed

4. **Oxidation and Reduction**

Oxygen combines with minerals, changing their chemical composition

Common in iron-rich rocks (iron oxides form rust)

Example: Weathering of iron pyrite to iron oxide

Creates brown or reddish coloration in soils

**Factors accelerating chemical weathering:**

Decomposition of plants and animals increases CO₂ concentration underground

Humid tropical climates accelerate all chemical reactions

Active vegetation cover enhances acid production

Physical or Mechanical Weathering

**Definition:** Weathering processes depending on applied mechanical forces that fracture and disintegrate rocks without chemical changes.

**Applied forces include:**

1. **Gravitational Forces**

Overburden pressure from overlying materials

Load stress

Shearing stress

Cause exfoliation and rock disintegration

2. **Thermal Expansion and Contraction**

Temperature changes cause rocks to expand and contract

Creates internal stress and fatigue

Most significant in deserts with large diurnal temperature ranges (up to 50°C)

Repeated cycles cause gradual rock breakdown

Example: Desert rock disintegration in Thar Desert

3. **Pressure Release (Unloading)**

Removal of overlying material reduces pressure

Rock responds by expanding perpendicular to exposed surface

Creates curved fracture patterns

Example: Quarry floors fracturing after excavation

4. **Frost Weathering (Freeze-Thaw)**

Water enters rock joints and cracks

Freezes and expands with ~9% volume increase

Creates pressure exceeding rock strength

Repeated cycles progressively fragment rocks

Most significant in mountain regions with freeze-thaw cycles

Example: Himalayan mountain fragmentation in winter

5. **Salt Weathering (Haloclasty)**

Salt crystallization in pores creates internal pressure

Particularly effective in arid and coastal regions

Example: Coastal rock disintegration in India

**Characteristics of physical weathering:**

Small and slow but causes great damage through repeated fatigue

Continued repetition of stress cycles weakens rocks progressively

Creates angular fragments (unlike chemical weathering)

Biological Weathering

**Definition:** Contribution to or removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms.

**Biological weathering mechanisms:**

1. **Mechanical Weathering by Organisms**

Burrowing animals: Earthworms, termites, rodents expose new surfaces

Wedging by organisms breaks apart rock

Exposes fresh surfaces to chemical and mechanical attack

2. **Plant Activity**

Plant roots exert tremendous mechanical pressure

Physically break apart earth materials

Penetrate cracks and expand rocks

Example: Tree roots breaking pavements and walls

3. **Acid Production**

Decaying plant and animal matter produces humic acid

Carbonic acid production enhances weathering

Dissolves certain minerals and cements

Creates acidic soil conditions

4. **Moisture and Air Penetration**

Biological activity creates channels for water and air penetration

Increases chemical weathering rates

Example: Increased weathering in vegetated vs. barren areas

5. **Human Activity**

Disturbance of vegetation increases weathering

Ploughing and cultivation mix materials

Creates new contacts between air, water, and minerals

Increases soil aeration and chemical weathering

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SPECIAL EFFECTS OF WEATHERING

Exfoliation

**Definition:** Flaking off of more or less curved sheets or shells of rock from bedrock surfaces resulting in smooth, rounded forms.

**Mechanisms producing exfoliation:**

1. **Thermal Exfoliation**

Temperature changes cause outer layers to expand and contract

Inner material remains relatively stable

Creates curved stress patterns

Most common in granite in arid regions

2. **Unloading Exfoliation (Pressure Release)**

Removal of overburden allows rocks to expand

Creates concentric curved fractures

Sheets parallel to exposure surface

3. **Salt Weathering**

Salt crystallization beneath surface

Pressure causes flaking of outer layers

**Resulting Features:**

1. **Exfoliation Domes**

Large rounded dome-shaped hills

Result from pressure release and unloading

Example: Half Dome in Yosemite, similar formations in Indian Deccan

2. **Tors**

Isolated rock masses with rounded tops

Result from selective weathering along joints

Curved surfaces from thermal expansion

Example: Granite tors in Western Ghats

**Figure 5.3 - Exfoliation and Granular Disintegration:**

Shows curved fracture patterns in exfoliation

Illustrates granular breakdown of crystalline rocks

Demonstrates transition from intact rock to regolith

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SIGNIFICANCE OF WEATHERING

Geomorphological Significance

1. **Breaking Down Rocks**

Reduces large rocks to smaller fragments

Prepares materials for transportation by agents

2. **Regolith and Soil Formation**

Creates regolith (unconsolidated rock fragments)

Essential for soil formation and agriculture

Depth of weathering mantle determines soil thickness

3. **Enabling Mass Movements**

Weathered materials are more mobile

Mass movements more active on weathered slopes

Creates conditions for landslides and creep

4. **Facilitating Erosion**

Weathered materials easily eroded

Erosion cannot be significant without prior weathering

Link: Weathering → Mass movement → Erosion → Relief reduction

5. **Landform Changes**

Weathering processes change landforms

Differential weathering creates varied topography

Works in conjunction with erosion to modify relief

Economic Significance

1. **Ore Enrichment**

Chemical weathering concentrates valuable minerals

Leaching removes unwanted materials by groundwater

Increases concentration of valuable minerals

Makes ore economically viable for extraction

**Example: Iron Ore Enrichment**

Weathering of iron-rich rocks concentrates iron oxides

Surface enrichment creates minable deposits

India's iron ores in Chhattisgarh, Jharkhand

2. **Valuable Ore Concentrations**

Manganese ore deposits

Aluminium ore (bauxite) deposits

Copper ore deposits

Example: Bauxite deposits in Odisha, Jharkhand

Ecological Significance

1. **Biodiversity and Biomes**

Vegetation depends on soil depth

Weathering depth determines forest cover

Forests depend on weathering mantles for rooting depth

Biodiversity is consequence of forest ecosystems

2. **Soil Quality and Fertility**

Weathering creates diverse mineral compositions in soils

Affects soil fertility and agricultural productivity

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MASS MOVEMENTS

Definition and Characteristics

**Mass movements** (also called mass wasting) are the transfer of rock debris down slopes under the direct influence of gravity. Key characteristics:

**Gravity-driven:** Direct influence of gravity; no geomorphic agent carries debris

**In-place particle movement:** Unlike erosion, agents (water, ice, wind) do not carry debris away; debris may carry water or air with it

**Not erosion:** No removal by geomorphic agents; shift of materials is gravity-aided

**Rapid to slow:** Movements range from imperceptible creep to rapid falls

**Variable depth:** Affect shallow to deep columns of earth materials

**Distinction from erosion:**

Mass movements: Gravity-driven, no agent participation

Erosion: Agent (water, wind, ice) removes and transports material

Conditions Favoring Mass Movements

**Weak Material Conditions:**

Weak unconsolidated materials

Thinly bedded rocks

Presence of faults and fractures

Steeply dipping beds

**Topographic Factors:**

Vertical cliffs or steep slopes

High relief and gradient

**Climatic Factors:**

Abundant and heavy precipitation

Torrential rains causing saturation

Freeze-thaw cycles

**Vegetation Factors:**

Scarcity or absence of vegetation

Removal of natural vegetation

Loss of root binding effect

Activating Causes of Mass Movements

**Natural and human-induced factors that trigger mass movements:**

1. **Removal of Support from Below**

Undercutting by rivers or waves

Erosion of slope base

Removal of underlying material (natural subsidence)

Mining and quarrying (artificial)

Example: Bank erosion creating unstable slopes

2. **Increase in Gradient and Height**

Steeper slopes increase gravitational stress

Higher elevation increases potential energy

Mountain building increases slope instability

3. **Overloading of Slopes**

Addition of materials through natural processes

Artificial filling and construction

Increased weight reduces stability

4. **Overloading Due to Heavy Precipitation**

Rainfall saturation increases weight

Water acts as lubricant reducing friction

Reduces shear resistance of materials

Example: Monsoon-triggered landslides in Himalayas, Western Ghats

5. **Removal of Material from Slope**

Quarrying and excavation reduce confining pressure

Erosion of overlying materials

Reduces stability by removing support

6. **Earthquakes and Explosions**

Seismic shaking destabilizes slopes

Sudden vibrations overcome friction

Can trigger multiple landslides

Example: 2015 Nepal earthquake triggered massive landslides

7. **Machinery Vibrations**

Construction equipment vibrations

Blasting operations

Heavy traffic on slopes

8. **Excessive Natural Seepage**

Groundwater reduces friction

Creates water pressure within slopes

Increases weight of materials

9. **Drawdown of Water Levels**

Rapid emptying of lakes, reservoirs, rivers

Removes water support from slopes

Creates outflow from beneath slopes

Example: Dam release creating downstream slope failures

10. **Indiscriminate Removal of Vegetation**

Deforestation removes root binding

Reduces slope stability

Increases erosion and saturation

Example: Clear-cutting on steep slopes triggering mudslides

Material Resistance and Shear Forces

**Principle:** Materials have inherent resistance to disturbing forces. They yield only when applied force exceeds shearing resistance.

**Factors affecting resistance:**

Rock type and cohesion

Degree of weathering

Saturation conditions

Vegetation cover

Slope material composition

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FORMS AND TYPES OF MASS MOVEMENTS

Classification Framework

**Figure 5.5 - Mass Movement Types, Rates, and Moisture Limits:**

Relates different movement types to rates of movement

Shows relationship between movement types and moisture content

Distinguishes slow, intermediate, and rapid movements

Three Major Forms of Mass Movement

**1. Heave**

Heaving upward of soils due to frost growth

Expansion of soil by ice crystal formation

Caused by other physical and chemical changes

Creates vertical displacement of surface

**2. Flow**

Continuous deformation of material

Like movement of viscous liquid

Typically with high water content

Slower movements include creep; faster include mudflows

**3. Slide**

Slipping of rock or debris along failure surface

Materials relatively intact

Relatively rapid movement

Along pre-existing weaknesses (faults, joints)

Landslides

**Definition:** Relatively rapid and perceptible movements of rock and debris masses down slopes.

**Characteristics of landslides:**

Materials relatively dry compared to flows

Rapid movement (perceptible)

Size and shape depend on:

Nature of rock discontinuities (joints, faults)

Degree of weathering

Steepness of slope

**Types of landslides based on movement:**

**Slump**

Slipping of rock debris units with backward rotation

Materials move relative to slope (rotational movement)

Creates distinctive curved slip surface

Often forms crescent-shaped scarps

Example: Bank failures in river valleys

**[NOTE: Text cuts off here, but comprehensive coverage continues in original NCERT text with additional landslide types including slides, debris avalanches, and rockfalls that would be covered in full chapter notes]**

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KEY EXAMINATION POINTS

1. **Differentiate endogenic and exogenic processes** with examples and their respective roles in shaping earth's surface

2. **Weathering processes and their climatic controls** — importance of climate in determining depth and type of weathering

3. **Chemical vs. physical weathering** — conditions required, processes involved, and resulting landforms

4. **Mass movements classification** — understand triggers, activating causes, and conditions favoring different types

5. **Significance of weathering** — geomorphological, economic (ore enrichment), and ecological aspects

6. **Rock structure and resistance** — differential weathering rates in different rock types

7. **Integration concept:** Weathering → Mass movement → Erosion → Transportation → Deposition → Landform development

8. **Indian examples:** Himalayan landslides, bauxite enrichment in Odisha, granite tors in Deccan, carbonation in cave systems of Meghalaya

9. **Human-environment interaction:** Human-induced mass movements through deforestation, mining, construction

10. **Map-based questions:** Relating climatic regions to weathering intensity, landslide-prone zones in India, river valley failures

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This comprehensive chapter coverage on geomorphic processes establishes foundational knowledge for understanding soil formation, erosion processes (fluvial, glacial, aeolian), and landscape development that follow in subsequent chapters.