Notes Class 11 Chapter 3 Geography Maharashtra Board

Agents of Erosion

Introduction

Erosion is the process by which natural agents like rivers, sea waves, wind, groundwater, and glaciers wear away, transport, and deposit earth materials, shaping the Earth’s surface. These agents create distinct landforms through processes like erosion, transportation, and deposition. This chapter explores the mechanisms of these agents, their processes, and the resulting erosional and depositional landforms, with examples from India and globally.

1. Landforms and Place Names

Landforms often influence the names of places. For example:

• Revdanda, Raigad, Maharashtra: Associated with a sandbar, a depositional feature formed by sea waves.
• Ganpati Pule, Ratnagiri, Maharashtra: Linked to a beach, formed by wave deposition.
• Pravara Sangam, Ahmadnagar, Maharashtra: Named for the confluence of rivers, a point where rivers meet.

Activity: Identify similar place names in India linked to landforms using an atlas (e.g., Dehradun for a valley, Jaisalmer for dunes).

2. Processes of Erosion

Erosion involves the detachment, movement, and wearing down of rock particles by agents. Each agent employs specific processes, some common across agents, others unique.

Common Erosional Processes

1. Abrasion:
   • Definition: Scratching or polishing of a surface by particles moved by an agent, like sandpaper smoothing wood.
   • Agents: Glacier, sea waves, wind, river.
   • Effect: Smoothens surfaces (e.g., riverbeds, cliffs).
   • Example: Wave-cut platforms form due to wave abrasion.
2. Attrition:
   • Definition: Rocks and pebbles collide during transport, breaking into smaller, rounded fragments.
   • Agents: Wind, sea waves, river.
   • Effect: Reduces particle size, rounds edges.
   • Example: Pebbles in riverbeds become smooth.
3. Solution:
   • Definition: Dissolving of soluble rocks (e.g., limestone) by acidic water.
   • Agents: Underground water, river, sea waves.
   • Effect: Carries dissolved minerals, invisible load.
   • Example: Limestone coasts erode via solution.

Agent-Specific Erosional Processes

1. Plucking (Glacier):
   • Ice exerts pressure on rocks, uprooting them.
   • Creates rugged bedrock surfaces.
   • Example: Glacial valleys in the Himalayas.
2. Deflation (Wind):
   • Wind removes loose surface particles, creating depressions called deflation hollows.
   • Common in arid, vegetation-scarce areas.
   • Example: Qattara Depression, Egypt.
3. Drilling (River):
   • Circular water flow with trapped sediments deepens riverbeds, forming potholes.
   • Example: Potholes in Kukadi River, Nighoj, Maharashtra.
4. Downcutting (River):
   • Hydraulic action deepens stream channels by removing bed material.
   • Example: Gorges like those of the Narmada River.
5. Headward Erosion (River, Glacier):
   • Backward erosion at the river’s source, moving the source upstream.
   • Example: Rivers in mountainous regions.
6. Lateral Erosion (River, Glacier):
   • Erodes valley sides, widening valleys and floodplains.
   • Example: Wide valleys in the Gangetic plains.

3. Processes of Transportation

Transportation moves eroded materials via different mechanisms:

1. Traction:
   • Agents: All except groundwater.
   • Large boulders are rolled, pushed, or dragged along surfaces.
   • Example: Boulders in riverbeds.
2. Saltation:
   • Agents: River, wind, sea waves.
   • Pebbles and coarse sand bounce along the surface.
   • Example: Sand movement in deserts.
3. Suspension:
   • Agents: River, wind, sea waves.
   • Fine particles are carried in the upper layers without settling.
   • Example: Silt in river water.
4. Solution:
   • Agents: River, sea waves, groundwater.
   • Dissolved minerals (e.g., calcium carbonate) are carried in water.
   • Example: Limestone dissolved in groundwater.

4. Processes of Deposition

Deposition occurs when agents lose velocity or encounter obstacles, reducing their carrying capacity. Factors influencing deposition:

• Velocity: Slower movement leads to deposition of larger particles first.
• Slope: Flat or reduced slopes cause sediment settling.
• Obstacles: Trees or mountains slow agents, triggering deposition.
• Particle Size: Smaller particles settle later due to lower gravitational pull.

Example: Rivers deposit sediments in plains, forming floodplains.

5. Agents of Erosion and Their Landforms

Each agent creates unique erosional and depositional landforms, influenced by climate, rock type, slope, and erosion intensity.

5.1 Work of Rivers

• Erosional Landforms:
  • Gorges/Canyons: Formed by deep downcutting in mountainous areas (e.g., Narmada Gorge).
  • V-shaped Valleys: Result from vertical erosion in hilly areas.
  • Waterfalls: Occur where rivers flow over cliffs or hard rock layers (e.g., Jog Falls, Sharavati River).
  • Potholes: Formed by drilling in weak bedrock (e.g., Kukadi River, Nighoj).
• Depositional Landforms:
  • Alluvial Fans: Coarse sediments deposited at foothills in plains.
  • Meanders: Serpentine bends formed by erosion on outer banks and deposition on inner banks.
  • Oxbow Lakes: Isolated meander bends filled with water.
  • Floodplains: Extensive plains formed by sediment deposition during floods.
  • Natural Levees: Sediment deposits along riverbanks during floods.
  • Deltas: Triangular sediment deposits at river mouths with high sediment supply and shallow seas (e.g., Ganga Delta).
• Human Activities:
  • Agriculture thrives on fertile floodplains.
  • Hydropower from waterfalls and gorges.
  • Navigation and trade in delta regions.

5.2 Work of Sea Waves

• Erosional Landforms:
  • Sea Cliffs: Formed by wave abrasion against steep coasts.
  • Sea Caves: Hollowed-out softer rock by wave erosion.
  • Sea Arches: Form when caves on opposite sides of a headland meet.
  • Sea Stacks: Collapsed arches leaving isolated rock pillars.
  • Wave-cut Platforms: Flat surfaces at cliff bases due to abrasion.
• Depositional Landforms:
  • Beaches: Sandy deposits between headlands (e.g., Marina Beach, Chennai).
  • Sandbars: Sediment deposits parallel to coasts (e.g., Revdanda, Maharashtra).
  • Lagoons: Brackish water bodies enclosed by sandbars (e.g., Chilka Lake, Orissa).
  • Sand Spits: Sandbars extending into the sea.
• Uniqueness: Waves work ceaselessly, with rapid erosion and deposition cycles, making coastal areas vulnerable to sea-level rise.

5.3 Work of Wind

• Conditions for Wind Erosion:
  • Aridity (high evaporation, low precipitation).
  • Sparse or no vegetation.
  • Loose, dry surface materials.
  • High wind velocity.
• Erosional Landforms:
  • Deflation Hollows: Depressions from removed loose sediments (e.g., Qattara Depression, Egypt).
  • Ventifacts: Polished, smooth rocks due to abrasion.
  • Mushroom Rocks: Rocks with eroded bases resembling mushrooms.
  • Yardangs: Ridges of harder rock after softer rock erosion.
• Depositional Landforms:
  • Sand Dunes:
    • Barchans: Crescent-shaped, gentle windward slope (e.g., Rajasthan deserts).
    • Longitudinal/Seif Dunes: Long dunes parallel to wind (e.g., Rub-al-Khali, Saudi Arabia).
  • Loess Plains: Fine silt deposits forming fertile plains (e.g., Northern China).

5.4 Work of Groundwater

• Conditions for Karst Topography:
  • Soluble rocks (e.g., limestone).
  • Carbon dioxide for acidic water.
  • Active groundwater movement.
  • Joints in rocks for water passage.
• Erosional Landforms:
  • Sinkholes: Circular depressions from dissolved rock.
  • Caves: Large underground cavities from solution (e.g., Bora Caves, Andhra Pradesh).
• Depositional Landforms:
  • Stalactites: Calcium carbonate deposits from cave ceilings.
  • Stalagmites: Deposits growing from cave floors.
  • Pillars/Columns: Stalactites and stalagmites meeting.
• Examples: Karst features in Meghalaya, Kanhur Plateau (Maharashtra).

5.5 Work of Glaciers

• Conditions: High altitudes or polar regions above the snowline, where snow accumulates and forms ice.
• Erosional Landforms:
  • Roche Moutonnees: Smooth, rounded bedrock hills from abrasion and plucking.
  • Cirques: Armchair-shaped depressions at glacier heads.
  • Arêtes: Narrow ridges between cirques.
  • Horns: Peaks from multiple cirque erosion (e.g., Matterhorn, Alps).
  • U-shaped Valleys: Broad, flat-bottomed valleys from lateral and vertical erosion.
  • Hanging Valleys: Tributary valleys eroded less than main valleys.
• Depositional Landforms:
  • Drumlins: Egg-shaped sediment heaps.
  • Eskers: Linear, zigzag sediment deposits.
  • Erratic Rocks: Large rocks deposited in areas of different geology.
  • Moraines:
    • Lateral Moraines: Sediments along glacier sides.
    • Medial Moraines: Merged lateral moraines of tributary glaciers.
    • End/Terminal Moraines: Sediments at the glacier’s front or farthest advance.

6. Factors Controlling Landform Formation

• Climate: Influences agent activity (e.g., arid climates favor wind).
• Rock Type: Soluble rocks (limestone) form karst; hard rocks resist erosion.
• Erosion Intensity: Stronger agents create deeper features.
• Slope: Steep slopes enhance erosion; flat slopes favor deposition.
• Obstacles: Slow agents, triggering deposition.

7. Key Observations

• Not all agents operate everywhere:
  • Wind dominates in arid regions.
  • Rivers are widespread.
  • Karst forms in limestone-rich areas.
• Common processes (e.g., abrasion) produce different landforms via different agents (e.g., floodplains by rivers, drumlins by glaciers).
• Coastal areas require management due to high population density and erosion risks.

8. Important Examples from India

• Rivers: Ganga Delta, Jog Falls, Narmada Gorge.
• Sea Waves: Marina Beach (Chennai), Chilka Lake (Orissa), Revdanda Sandbar.
• Wind: Rajasthan’s barchans.
• Groundwater: Bora Caves (Andhra Pradesh), Meghalaya caves.
• Glaciers: Himalayan U-shaped valleys, horns.

9. Summary Table of Agents and Landforms