Geography Notes Chapter 3 Exogenetic Processes Part-1 Class 9 Maharashtra Board

Notes For All Chapters – Geography Class 9th

1. Introduction to Exogenetic Processes

What are Exogenetic Processes?

• These are processes that happen on the Earth’s surface.
• They form or break down landforms continuously.
• Examples: weathering, erosion, transportation, and deposition.

Forces Behind Them:

• Solar energy (sun’s heat).
• Gravitational force (pulls things down).
• Kinetic energy (energy from moving objects like wind or water).

2. Types of Landforms

Primary and Secondary Landforms:

• Formed by internal movements inside the Earth.
• Examples: continents, mountains, plateaus, plains.

Tertiary Landforms:

• Formed by external processes acting on primary/secondary landforms.
• Examples: valleys, sand dunes, deltas, U-shaped valleys.

3. Weathering

What is Weathering?

• Breaking down of rocks into smaller pieces by external forces.
• Three main types: Mechanical, Chemical, and Biological.

A. Mechanical Weathering

Definition:

• Rocks break into smaller pieces without changing their chemical makeup.

Causes: Temperature:

• Rocks heat up in the day and cool at night.
• This makes them expand and contract, causing cracks.
• Common in hot deserts with big temperature changes.

Frost:

• Water enters rock cracks and freezes below 0°C.
• Frozen water expands, putting pressure on rocks and breaking them.
• Example: Shattering (Fig 3.1 C).

Crystal Growth:

• Alkaline seawater enters rock cracks on coasts.
• Water evaporates, leaving crystals that grow and make holes (like a honeycomb).
• Example: Alkaline crystallization (Fig 3.3).

Release of Pressure:

• Outer rock layers press on inner layers.
• When outer layers break off, inner layers expand and crack.

Water:

• Rainwater soaks rocks like sandstone, loosening particles.
• Example: Granular weathering (Fig 3.4).

Special Types:

• Exfoliation: Outer layers of rocks peel off like onion skin due to heat (Fig 3.2).
• Block Disintegration: Big rock blocks separate due to temperature and water (Fig 3.1 B).

B. Chemical Weathering

Definition:

• Rocks break down by changing their chemical structure, usually with water.

Main Types:

1. Carbonation:
   • Rainwater mixes with carbon dioxide to form carbonic acid (H₂O + CO₂ = H₂CO₃).
   • This acid dissolves rocks like limestone.
   • Common in wet areas.
2. Solution:
   • Water dissolves minerals in rocks, making them brittle.
   • Example: Limestone at Wadgaon Darya (Fig 3.7).
3. Oxidation:
   • Iron in rocks reacts with water and oxygen, forming rust (reddish layer).
   • Example: Rusty rocks (Fig 3.1 D).

Examples in Daily Life:

• Table salt gets wet in the rainy season.
• Apple slices turn brown.

C. Biological Weathering

Definition:

• Living things like plants and animals break rocks.

How It Happens: Plant Roots:

• Roots grow into rock cracks and split them as they get bigger.
• Example: Trees on forts (Fig 3.8).

Burrowing Animals:

• Animals like ants, rats, and rabbits dig into the ground, loosening rocks.

Flora:

• Algae, moss, and lichens grow on rocks and break them slowly.
• Example: Lichen (Fig 3.9).

4. Mass Movements (Mass Wasting)

What is Mass Wasting?

• Weathered rock materials move down slopes due to gravity.
• Forms heaps at foothills.

Types: Rapid Mass Movements:

• Happen fast on steep slopes in wet climates.

Examples:

• Rockfalls: Rocks fall quickly (Fig 3.11).
• Landslides: Big slides of material (Fig 3.12).
• Slumping: Material sinks in place (Fig 3.10).

Example Event: Mudslide at Malin Village, Pune.

• Can be triggered by rain or earthquakes.

Slower Mass Movements:

• Happen slowly on gentle slopes in dry climates.

Examples:

• Soil Creep: Soil moves down slowly.
• Solifluction: Soil layers move in cold regions (Fig 3.13).

5. Erosion

What is Erosion?

• Wearing away of rocks by external agents.

Agents of Erosion:

• Wind, running water, glaciers, seawater, groundwater.

Note:

• More details about erosion and its landforms will be in the next lesson.