Maarg Manthan · Topic 3.2

Exogenic Movements, Denudation and Weathering

Geography › Geomorphic Movements and Plate Tectonics · Topic 3.2

Exogenic processes are surface forces driven by the Sun and gravity that wear down the landscape, together called denudation. Denudation includes weathering, the breakdown of rock in place, mass movement, the downslope movement of material, erosion and transportation. Weathering is physical, chemical or biological and prepares material for soil formation and for the other processes.

5 min read · UPSC Prelims · Geography · Geomorphic Movements and Plate Tectonics

At a Glance

Energy source The Sun and gravity
Denudation Weathering, mass movement, erosion, transport
Weathering Breakdown of rock in situ
Types Physical, chemical, biological
Mass movement Downslope movement under gravity
Exam link UPSC Prelims, Geography: Geomorphic Movements and Plate Tectonics

Where Does It Fit?

This is the second topic of the category on geomorphic movements and plate tectonics. The previous topic described endogenic movements that build relief, and this one covers surface processes that wear it down. The next topic covers the continental drift theory. Previous topic: Endogenic Geomorphic Movements and Diastrophism. Next topic: Continental Drift Theory of Alfred Wegener.

What Are Exogenic Processes and Denudation?

In short: exogenic processes are the surface forces that wear down what endogenic forces build.

Exogenic processes take their energy from the Sun and from gravity. The Sun provides heat that drives the atmosphere and the water cycle, and gravity pulls material downslope. The action of these forces in lowering and levelling the land is called denudation, which literally means stripping off. The aim of the processes is to bring the surface to a common level, called gradation. Denudation has four parts: weathering, mass movement or wasting, erosion and transportation. The agents of erosion and transport are running water, groundwater, glaciers, wind and waves. These processes are examined in Soil Erosion – Water and Wind Erosion and in the topics on landforms that follow in this subject.

What Is Weathering?

Weathering is the mechanical disintegration and chemical decomposition of rocks in place, without much movement of the material. Rocks are broken into smaller pieces, and weathered material, called regolith, is the raw material for soil. It is therefore a precondition of erosion and soil formation. Weathering is controlled by the nature of the rock, climate, relief, vegetation and time. It is fastest in warm, humid climates for chemical processes, and in cold or dry climates with large temperature swings for physical processes.

Type	Main processes	Climate favouring it
Physical (mechanical)	Frost wedging, thermal expansion, exfoliation, salt crystal growth, pressure release	Cold and dry regions
Chemical	Solution, carbonation, hydration, oxidation, reduction, hydrolysis	Warm and humid regions
Biological	Root growth, burrowing animals, organic acids, human activity	Most places with vegetation

How Do the Main Weathering Processes Work?

Frost wedging: water enters cracks, freezes and expands by about 9 per cent, widening the cracks.
Exfoliation: in places with great temperature variation, outer layers of rock expand and contract and peel off like the skin of an onion, forming rounded domes and boulders.
Salt weathering: salts in cracks crystallise and exert pressure, common in deserts and coasts.
Solution: minerals such as rock salt dissolve in water.
Carbonation: carbon dioxide dissolved in rainwater forms a weak carbonic acid that dissolves limestone, creating caves and sinkholes.
Hydration and hydrolysis: minerals take up water and swell or are chemically altered; feldspar changes to clay.
Oxidation: iron minerals combine with oxygen and form rusty red-brown products.

Carbonation is the basis of the karst landscape discussed in Karst Landforms and the Cycle of Erosion.

What Is Mass Movement?

Mass movement is the downslope movement of rock, soil and debris under the force of gravity, without a carrying agent such as water or ice, although water usually makes it easier. It is controlled by the slope, the nature of the material and the amount of moisture. Mass movements are grouped by speed.

Speed	Type	Example
Slow	Soil creep, solifluction	Gradual shifting of surface soil on slopes; flow of saturated soil in cold regions
Rapid	Mudflow, earthflow, debris avalanche	Water-saturated debris flowing down valleys
Sudden	Landslide, rockfall, slump	Large masses sliding or falling along steep slopes

A landslide is the rapid sliding of a mass of rock or soil. The Himalayas, the Western Ghats and the northeastern hills of India are especially prone to landslides because of steep slopes, young and weak rocks, heavy monsoon rains, earthquakes and construction on slopes.

Why Is Weathering Significant?

Weathering is the first step in the formation of soil and thus supports agriculture and vegetation. It enriches the concentration of some ores: bauxite, iron ore, manganese and copper accumulate when weathering removes other material. It breaks rock into material that can be eroded and shapes the surface, creating features such as tors, caves and rounded boulders. As an analytical point, the same processes are also hazards, since weathered slopes fail in landslides and heavy rainfall on loosened rock raises risk, so land-use planning and slope management in the hills are as important as the science itself. The relation of weathering to soil is developed in Soil Types and Soil Profile.

Why Is This Topic Important?

Process Basis of surface change
Hazards Landslides in current affairs
Soil Weathering is the first step

What Came Next?

The next topic describes the continental drift theory of Alfred Wegener. Next topic: Continental Drift Theory of Alfred Wegener.

Key People and Terms

Denudation Lowering of land by surface processes
Regolith Weathered loose material
Gradation Levelling of the surface
Frost wedging Splitting by freezing water
Solifluction Flow of saturated soil
Creep Slow downslope soil movement

Exam Corner

Points to Remember

Exogenic energy comes from the Sun and gravity.
Weathering is in place; erosion involves movement.
Carbonation dissolves limestone.
Exfoliation peels rock like an onion.
Mass movement needs no transporting agent.

Do Not Confuse With

Weathering and erosion In place versus with transport
Endogenic and exogenic Internal versus surface forces
Creep and landslide Slow versus sudden

Memory Hook

Weather breaks, gravity moves, water and wind carry; Sun and gravity run the show.

Mains Angle

Questions ask about weathering types and landslides. These points add depth.

Climate Controls the type of weathering
Landslides Slope, rainfall and human activity
Soil Weathering is the base

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between weathering and erosion and discuss the factors that cause landslides in the Himalayan region.”

How to Answer

Define Weathering and erosion.
Types Physical and chemical.
Landslides Slope, rain, tremors.
Management Land-use planning.

GS Relevance

Prelims: weathering types, mass movement, landslides. GS Paper 1: geomorphology; GS Paper 3: disaster management.

Frequently Asked Questions

What is denudation?

Denudation is the wearing down and lowering of the land surface by exogenic processes. It consists of weathering, mass movement, erosion and transportation, and its agents are running water, groundwater, glaciers, wind and waves, driven by solar energy and gravity.

What is the difference between weathering and erosion?

Weathering is the breakdown of rock in place, with little or no movement of the broken material. Erosion is the removal and transport of that material by agents such as water, ice, wind and waves, so it involves movement.

What are the types of weathering?

Weathering is of three types: physical or mechanical, which breaks rock without changing its chemistry, as in frost wedging and exfoliation; chemical, such as carbonation and oxidation; and biological, caused by roots, animals, organic acids and human activity.

What is exfoliation?

Exfoliation is a physical weathering process in which outer layers of rock peel off like the layers of an onion. It occurs because of repeated heating and cooling, especially in dry regions with large temperature variation, and produces rounded domes and boulders.

What is mass movement?

Mass movement is the downslope movement of rock, soil and debris under the force of gravity, with no flowing agent such as a river. It ranges from slow creep to sudden landslides and rockfalls, and it is encouraged by steep slopes and heavy rainfall.

PYQ Practice — Statement Analysis

1 Weathering involves the transport of broken rock by an agent.

False

That is erosion.

2 Carbonation is an example of chemical weathering.

True

It dissolves limestone.

3 Gravity is the force behind mass movement.