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

Summary: Exogenetic Processes Part-1

External processes work on the surface using solar energy, gravity and the energy of moving things like water, wind and ice. Internal movements make primary and secondary landforms (continents, mountains, plateaus, plains); external processes — weathering, erosion, transportation and deposition — carve them into tertiary landforms such as valleys, dunes and deltas.

Weathering is the breaking or weakening of rocks where they lie. It is of three kinds: mechanical (physical), chemical and biological. Mechanical weathering is strongest in dry climates, chemical weathering in humid climates, and biological weathering is caused by living things.

Mechanical weathering is caused by temperature changes (exfoliation, block disintegration), frost, salt-crystal growth, release of pressure and water soaking into rocks (granular weathering). The rock breaks but its minerals do not change.

Chemical weathering changes the minerals. Carbonation (carbonic acid dissolving limestone), solution (minerals dissolving in water) and oxidation (iron rusting) are the main types. Biological weathering is done by tree roots, burrowing animals, lichens, moss and algae.

Mass movement is the downslope sliding of weathered material by gravity alone. It is rapid on steep, wet slopes (rockfall, landslide, slumping — e.g. Malin, Pune district) and slow on gentle, dry slopes (soil creep, solifluction).

Key points to remember

• External forces: solar energy, gravity and kinetic energy of moving agents
• Exogenetic processes: weathering, mass movement, erosion, transportation, deposition
• Weathering = breaking/weakening of rocks in place; three types
• Dry climate → mechanical weathering dominant; humid climate → chemical weathering
• Exfoliation: outer layers peel off because the surface heats more than the inside
• Frost action: water expands about 9% on freezing in cracks and splits rocks
• Salt crystals growing in pores make honeycomb holes in coastal rocks
• Granular weathering: sandstone grains loosen when soaked by water
• Carbonation: H2O + CO2 → H2CO3 (carbonic acid) dissolves limestone
• Oxidation: iron in rocks rusts and turns reddish
• Biological: roots, burrowing animals, lichens, moss and algae break rocks
• Mass wasting: rapid (landslide, rockfall, slumping) or slow (soil creep, solifluction)

Exogenetic Processes Part-1 – Notes

1. External forces and landforms

• Landforms are not only made from inside the Earth. Processes acting on the surface keep building up some landforms and wearing others down all the time.
• These exogenetic processes get their energy from the sun, from gravity and from the motion of agents like running water, wind, glaciers and waves.
• Internal movements produce primary landforms (continents, ocean basins) and secondary landforms (mountains, plateaus, plains).
• Weathering, erosion, transportation and deposition reshape these into tertiary landforms — valleys, U-shaped valleys, sand dunes, deltas and so on.
• The pictures in the book — peeling domes, split blocks, rusty rock, a worn statue, roots in rock — all show rocks being broken or weakened. This is weathering.

2. Mechanical (physical) weathering

• Rocks break into pieces without any change in their minerals. It is dominant in dry (arid) climates.
• Temperature: minerals swell in heat and shrink in cold. Different minerals expand by different amounts, so repeated heating and cooling sets up stress and cracks the rock. This is common in hot deserts, where days are very hot and nights cold (large diurnal range).
• Exfoliation: like peeling the skin layers of an onion, the sun-heated outer shell of a rock expands more than the cooler inside and comes away in curved sheets, leaving dome-shaped hills.
• Block disintegration: heating and cooling widen joints; water enters and the rock splits into big rectangular blocks.
• Frost action: in places where the temperature stays below 0 °C for long, water in cracks freezes, increases in volume and pushes the crack open until the rock shatters.
• Crystal growth: on rocky coasts, salty spray enters cracks and dissolves some minerals, leaving small holes. When the water evaporates, salt crystals grow and press on the rock, making honeycomb-like holes.
• Release of pressure: when upper layers are removed, the inner layers that were squeezed expand and crack.
• Water (granular weathering): sandstone and conglomerate are sand or pebbles held together. Water soaking in loosens the grains, which fall off one by one.

3. Chemical weathering

• Chemical weathering changes the minerals of the rock. Water is the key agent because it dissolves so many substances; it is strongest in humid, rainy areas.
• Carbonation: rain picks up carbon dioxide from the air and becomes weak carbonic acid (H2O + CO2 → H2CO3). This acid easily dissolves limestone — the chalk-in-water activity shows the idea.
• Solution: some minerals simply dissolve in water, making the rock brittle. Dissolved lime can be deposited again, forming features like the stalactite at Wadgaon Darya in Ahilyanagar (Ahmednagar) district.
• Oxidation: iron in rocks reacts with oxygen in the presence of water and forms a reddish-brown coating of rust — just like the iron nails left in a wet cloth that stain it brown.
• Everyday examples of chemical change: table salt getting damp in the monsoon, a cut apple turning brown.

4. Biological weathering

• Living things also break rocks.
• Roots of trees and plants grow into cracks; as they thicken they push the rock apart — you can see trees growing out of old fort walls and minarets.
• Burrowing animals — ants (anthills), rats, mice, rabbits, worms and insects — dig into the ground and loosen rock and soil.
• Algae, moss and lichens growing on rocks hold moisture and give out weak acids that slowly eat into the surface.
• Humans also weather rocks through mining, quarrying and construction.

5. Mass movements (mass wasting)

• Weathered material on slopes slides or creeps downhill under gravity alone and collects at the foot of the slope as a cone-shaped heap.
• Rapid movements — rockfalls, landslides and land subsidence — are sudden and destructive. They are most likely on steep slopes in humid climates where a thick weathered layer forms; rain soaks it, makes it heavy and it rushes down. Example: the 2014 mudslide that buried Malin village in Pune district.
• Slumping: the weathered mass sinks or rotates down where it is, instead of travelling far.
• Earthquakes can also trigger rapid mass movements.
• Slow movements happen in dry areas with gentle slopes. Soil creep is the very slow, almost unnoticed slipping of soil (seen in tilted fence posts and trees).
• Solifluction: in periglacial (near-ice) regions, soaked soil flows slowly down slopes in thin layers.
• Erosion is the next external process — wearing away by wind, rivers, glaciers, sea waves and groundwater. It is studied in the next lesson.

Types of weathering

Key terms

Exogenetic Processes Part-1 – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Exogenetic Processes Part-1 – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Exogenetic Processes Part-1

⏱ About 35 minutes

1. Understand (12 min)
   Learn the external forces and the difference between weathering, mass movement and erosion.
2. Sort the processes (13 min)
   Make a chart of mechanical, chemical and biological weathering with one example each; then learn rapid vs slow mass movements.
3. Check yourself (10 min)
   Answer the 'Try it yourself' questions, take the quiz and play 🎮 Play.