Geomorphic Processes: Weathering & Erosion – NDA Geography Notes

Introduction

The Earth’s surface is never still. Mountains are being worn down. Valleys are being carved deeper. Cliffs are collapsing into the sea. Sand is being blown across deserts. Rivers are cutting into bedrock. All of this is happening around us — slowly enough that we do not notice it in a single lifetime, but fast enough that it has completely reshaped the Earth’s surface over millions of years.

This chapter is about the processes that break down, move, and deposit Earth’s material. These processes work from the outside — driven by the Sun, water, wind, gravity, and living organisms — rather than by the heat inside the Earth.

NDA tests this chapter through process identification questions — is a given example physical, chemical, or biological weathering? Is a boulder rolling downhill mass wasting or erosion? These questions require conceptual clarity more than memorisation.

Understand the Topic

The Big Picture — Endogenic vs Exogenic Processes

The Earth’s surface is shaped by two opposing sets of forces.

Endogenic (internal) processes are driven by heat from inside the Earth. They build up the surface — creating mountains, volcanoes, and plateaus through plate tectonics. These are creative forces.

Exogenic (external) processes are driven by energy from outside — mainly from the Sun, which drives water and wind. They break down and level the surface — wearing away mountains and filling valleys with sediment. These are destructive forces.

Denudation is the overall process of lowering and levelling the Earth’s surface through the combined action of weathering, erosion, mass wasting, and transportation. [NDA 2022-I]

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Weathering

Weathering is the process of breaking down rocks in place — without moving them. The key word is in place. Weathering happens at or near the Earth’s surface where rocks are exposed to the atmosphere, water, temperature changes, and living organisms.

Weathering is different from erosion. In weathering, the material does not move — it just breaks down where it is. In erosion, the broken material is carried away by a moving agent such as water, wind, or ice.

Rocks do not remain in their original form for long — they undergo transformation caused by weathering, erosion, and metamorphic action. [NDA 2022-I]

There are three types of weathering — physical, chemical, and biological.

1. Physical Weathering (Mechanical Weathering)

Physical weathering breaks rocks into smaller pieces without changing their chemical composition. The minerals in the rock remain the same — the rock just becomes smaller fragments.

Physical weathering is most effective in:

• Hot deserts (extreme temperature variation between day and night)
• Cold climates (freeze-thaw action)
• Coastal and arid areas with salt-rich water

The most prevalent form of weathering in hot tropical desert areas is mechanical weathering. [NDA 2022-II]

Processes of Physical Weathering:

Thermal Expansion:
In deserts, rocks heat up to very high temperatures during the day and cool sharply at night. This repeated expansion and contraction creates stress that eventually cracks and breaks rocks apart.

Freeze-Thaw Action (Frost Action):
Water seeps into cracks in rocks. When the temperature drops below 0°C, the water freezes and expands by about 9%. This expansion widens the crack. When the ice melts, water seeps deeper. The next freeze widens it further. Over time the rock splits apart.

Frost action is synonymous with freeze-thaw action. [NDA 2014-I]

Salt Crystal Growth (Haloclasty):
In coastal areas and arid regions, water containing dissolved salts seeps into pores and cracks. When the water evaporates, salt crystals form and grow. The growing crystals push on the surrounding rock, widening cracks.

Salt crystal growth is an example of physical weathering — not chemical. [NDA 2019-I]

Salt crystallisation is the most effective of all salt weathering processes. Salt crystal growth is more effective over sedimentary rocks than igneous rocks because sedimentary rocks are more porous. [NDA tested via CDS]

Exfoliation:
The outer layers of a rock peel off like the layers of an onion. This happens when the outer surface heats and expands while the inner rock stays cooler. The outer shell separates from the core. Exfoliation is classified as physical weathering. [NDA 2018-II]

Crystallisation:
When minerals crystallise inside rock pores, the growing crystals exert pressure on the surrounding rock. Crystallisation is a form of mechanical weathering. [NDA 2014-I]

Forces applied in mechanical weathering:
Gravitational force, expansion force, and force due to water pressure are all forces involved in mechanical weathering. [NDA 2025-I]

2. Chemical Weathering

Chemical weathering changes the chemical composition of rocks — converting minerals into new, weaker minerals through chemical reactions. It is most effective in hot and humid climates — because high temperature speeds up chemical reactions and water is essential for most processes. [NDA 2018-I]

Chemical weathering processes are more active in hot and humid environments because high temperature and rainfall help decomposition of rocks. [NDA 2014-I]

Processes of Chemical Weathering:

Solution (Dissolution):
Rainwater absorbs carbon dioxide from the atmosphere, forming weak carbonic acid. This acid reacts with rocks — particularly limestone — dissolving them. The dissolved minerals are carried away in solution.

Carbonation:
The reaction of carbonic acid with calcium carbonate (in limestone) breaks it down into soluble calcium bicarbonate, which washes away. This is especially important in karst (limestone) landscapes.

Oxidation:
Reaction of iron-bearing minerals with oxygen. Iron reacts with oxygen and water to form iron oxides (rust). This is why many rocks and soils appear red or reddish-brown.

Hydration:
Some minerals absorb water molecules and expand. This absorption changes their chemical structure and weakens the rock. The process whereby minerals absorb water, expand and change is called hydration. [NDA 2021-I]

Hydration causes chemical change in minerals — it does NOT cause granular disintegration. Granular disintegration is a physical process. [NDA 2014-I]

Hydrolysis:
Water chemically reacts with minerals, breaking them down into new compounds. Feldspars break down through hydrolysis into clay minerals. This process is effective in humid tropics because high temperature and humidity both accelerate the reaction. [NDA 2014-I]

High diurnal range in temperature is NOT a reason why hydrolysis is effective in humid tropics — the tropics have LOW diurnal range. The real reasons are high temperature and high humidity. [NDA 2014-I]

Exfoliation is NOT a process of chemical weathering — it is physical. [NDA 2018-II]

3. Biological Weathering

Biological weathering is weathering caused by living organisms — plants, animals, bacteria, and fungi.

Plant roots grow into cracks in rocks and push them wider over time — this is physical biological weathering.

Lichens grow on bare rock surfaces and secrete acids that dissolve the rock surface — this is chemical biological weathering.

Bacteria break down minerals in rock through chemical processes.

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Summary — Three Types of Weathering:

Type	Changes Chemistry?	Most Effective Climate	Key Processes
Physical	No	Hot deserts; Cold climates	Freeze-thaw, Salt crystals, Exfoliation, Thermal expansion
Chemical	Yes	Hot and humid	Solution, Carbonation, Oxidation, Hydration, Hydrolysis
Biological	Both	Varies	Plant roots (physical); Lichens (chemical)

In the multi-statement NDA 2014-I question:

1. Crystallisation = mechanical weathering — TRUE
2. Hydration causes granular disintegration — FALSE (hydration causes chemical change)
3. Frost action = freeze-thaw action — TRUE
   Answer: (c) 1 and 3 only

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Soil Formation

Weathered rock material, mixed with organic matter, forms soil. The process is very slow — it takes hundreds to thousands of years to form just a few centimetres of soil.

Classes of soil forming processes:
Translocation, enrichment, removal, and transformation are all classes of soil forming processes. [NDA 2025-II]

In translocation, fine particles are transported downward by eluviation and accumulate in lower horizons by illuviation. [NDA 2025-II]

Erosion

Erosion is the wearing away of the Earth’s surface and the transportation of the loosened material by agents like water, wind, ice, and waves.

Erosion differs from weathering in one key way — in erosion, the material is carried away from where it was broken down. In weathering, the material stays in place.

The main agents of erosion are:

• Running water (rivers) — the most powerful erosion agent overall
• Wind — important in deserts and dry areas
• Glaciers (ice) — powerful in polar and high mountain regions
• Waves — in coastal areas
• Underground water — in limestone regions

The specific landforms created by each agent are covered in Chapter 11.

Mass Wasting (Mass Movement)

Mass wasting is the downward movement of rock, soil, and debris under the direct influence of gravity alone — without a fluid agent (like water or wind) carrying the material.

The key distinction: in mass wasting, gravity does the work directly. In erosion, a medium (water, wind, ice) carries the material.

A boulder loosened by heavy rains and moving downhill is an example of mass wasting — not erosion. [NDA 2022-I]

Weathering, mass wasting, erosion, and transportation together constitute denudation. [NDA 2022-I]

Types of Mass Wasting:

Creep:
The slowest form of mass wasting. Soil and rock fragments move downhill extremely slowly — millimetres per year. Evidence includes fence posts leaning downhill and telegraph poles tilting.

Creep is characterised by slow speed with soil. [NDA tested via CDS]

Earthflow:
Water-saturated soil and soft rock flowing slowly downslope. Faster than creep.

Mudflow:
Fast-moving flow of water-saturated soil and rock after heavy rainfall on unstable slopes.

Slump:
A rotational movement where a block of earth and rock slides along a curved slip surface.

Rockslide:
A mass of rock fragments sliding rapidly down a steep slope.

Rockfall:
Individual rocks or groups of rocks falling freely from steep cliffs. The fastest type.

Landslide:
A general term for rapid downslope movement of rock, soil, or debris.

Landslides generally occur in clay-rich soil and are triggered by earthquakes and heavy rainfall. They do NOT occur only on gentle slopes — they occur on steep slopes. [NDA 2022-I]

Avalanche:
A sudden, rapid movement of snow and ice down a steep mountain slope. It involves falling, rolling, sliding, and flowing of materials and is hazardous to skiers and mountaineers. [NDA 2022-I]

Karst Topography — Brief Introduction

In areas underlain by limestone, slightly acidic rainwater dissolves the rock over time. This creates a distinctive landscape called karst topography — characterised by sinkholes, caves, and underground streams.

The most common topographic form in karst terrain is the sinkhole — a circular or funnel-shaped depression formed when the roof of an underground limestone cavern collapses. A sinkhole is a depression, NOT formed by upliftment of land. [NDA tested via CDS]