Exogenic processes: stress, denudation and what controls it

Detail

1. Where exogenic processes get their energy

• Exogenic processes are processes that act on or near the Earth's surface. Their force comes from outside the crust, mainly from solar energy and gravity [4].

• This is the opposite of endogenic processes. Those are driven by the Earth's inner heat and they build relief through folding, faulting and volcanism.

• Energy chain:

• The sun heats the atmosphere unevenly. This creates differences in temperature and pressure.
• These differences drive winds, rain, running water, glaciers and waves.

• The tectonic forces of endogenic processes have already raised the land into slopes. Exogenic agents work on these gradients (slopes).

• Gravity pulls on all material lying on a slope. Steeper slopes mean a stronger downhill pull.

• Key exam fact: the only ultimate driver of all exogenic processes is solar energy. Gravity and the slopes made by tectonics are helpers, not the ultimate source.

2. Stress: the basic cause

• Stress is force per unit area. It is the push or pull felt by each square unit of a rock surface.
• Deformation: pushing or pulling stress changes the shape of a rock (it bends, squeezes or stretches).
• Shear stress is a separating force that acts along a surface, parallel to it, not straight into it.
• Result: rocks break, and pieces shift sideways at an angle or slide over each other (angular displacement or slippage).

• Seen in: landslides on Himalayan slopes, where rock blocks slide along joints and bedding planes.

• Molecular stresses act inside the rock and come from three sources:

• Temperature change. Rock grows by day and shrinks by night. Repeated many times, this tires the rock.
• Crystallisation. Ice or salt crystals grow inside cracks and push the walls apart.

• Melting. A change of state alters the volume and the pressure inside pores.

• Freeze-thaw (frost wedging or cryofracturing) shows molecular stress at work [3]:

• Water seeps into cracks in a rock.
• The temperature falls below 0 °C. The water freezes and expands.
• The ice acts like a wedge, slowly widening the crack until the rock splits [3].

• Seen in: the cold deserts of Ladakh, the higher Himalaya, the Alps and the Rockies.

• Chemical stress:

• Chemical processes loosen the bonds between grains.
• They also dissolve soluble minerals and the cements that hold grains together.

• Result: the rock becomes weak and crumbly. Example: limestone dissolving in rainwater that carries CO₂.

• Core statement: stress built up in earth materials is the basic cause of weathering, mass movement and erosion.

• Fatigue: each single stress event is small and slow. But the effects add up over time because the rock keeps getting tired (continued fatigue). In the end the rock fails.

3. Denudation: the umbrella term

• Denudation comes from a word meaning "to strip off" or uncover. It is the general term for all exogenic processes that wear down the land and lay bare the rock beneath.
• It has four parts: weathering, mass wasting/movements, erosion and transportation.
• Each part has its own driving force (Figure 5.1):

Process	What it does	Driving force
Weathering	Breaks or dissolves rock where it lies (in situ); no movement	Gravity; temperature change; chemical action
Mass movement / mass wasting	Moves rock and soil downslope as a mass	Gravity (helped by temperature and chemical action)
Erosion	Removes and carries rock material	Kinetic energy (energy of motion) of running water, ice, wind and waves
Transportation	Carries the eroded load away	Same moving agents: running water, glaciers, wind, waves

• Order of work: weathering loosens the rock → mass movement moves it downslope → erosion and transport carry it away. Erosion means the broken bits of rock and mineral are carried off after weathering [3].
• Trap: weathering needs no transport. Erosion always involves movement by an agent.

4. Climatic controls

• With all else equal, climate decides which process wins and how strong it is. Temperature and precipitation are the two key elements.
• Climate itself changes with latitude, season and the distribution of land and water (inland places have hotter days and colder nights than coastal places).

Climatic factor	How it controls exogenic processes	Example
Temperature and rainfall	Hot and wet: strong chemical weathering. Hot and dry, or cold: physical weathering	Kerala and the Western Ghats (chemical); Thar Desert, Rajasthan (physical)
Vegetation (density, type, spread)	Roots bind soil and slow erosion. Bare slopes erode fast	Deforested Himalayan slopes vs forested slopes
Altitude	Higher places are colder and have more frost action	Upper Himalaya, Ladakh
Aspect (the direction a slope faces)	Slopes facing different directions get different insolation (incoming solar radiation). North- and south-facing slopes differ from east- and west-facing slopes in heat, moisture and snow cover	In the northern hemisphere, south-facing slopes are sunnier and drier. North-facing slopes are cooler and moister
Wind speed and direction	Fast, steady winds erode and transport more, especially in dry areas	Wind erosion in western Rajasthan
Precipitation type and intensity	Short, intense bursts of rain cause more runoff, more erosion and more landslides than steady drizzle. Snow feeds glaciers	Monsoon cloudbursts in Uttarakhand and Himachal Pradesh
Precipitation–evaporation balance	Decides how much water stays in the rock and soil for chemical action	Humid Northeast vs arid north-west India
Daily temperature range	A large range means repeated expansion and contraction, which tires the rock	Hot deserts, which have scorching days and cold nights
Freeze-thaw frequency and frost depth	More cycles and deeper frost mean more frost wedging [3]	High mountains and periglacial zones

5. Rock controls: type and structure

• In the same climate, the strength of exogenic processes depends on rock type and structure:
• Folds and faults crush and weaken rock along zones of stress.
• The orientation and dip of beds matter. Beds dipping towards the valley make sliding easy.
• Joints and bedding planes let water in and act as surfaces along which rock breaks and slips.
• Mineral hardness: quartz-rich rocks resist wear more than soft shales.
• Chemical susceptibility: limestone dissolves easily, while granite resists solution.

• Permeability: porous, permeable rocks let water deep inside for chemical attack and freezing.

• Selective resistance: a rock may resist one process but give way to another.

• Example: limestone holds up well against mechanical (physical) breakdown in dry climates. In humid climates it dissolves fast into karst topography: landforms made by solution, such as caves and sinkholes (e.g. the Borra Caves in Andhra Pradesh, and the limestone caves of Meghalaya).

• Result: different processes work at different speeds on different rocks. This differential erosion creates differences in topography (relief), such as hard-rock ridges standing above soft-rock valleys.

6. Present-day example: climate and rock controls together drive landslides in India

• Extent: about 4.2 lakh km² of hilly and mountainous land in parts of 19 States/UTs is open to landslides. This affects parts of 179 districts (GSI, 2024) [2]. That is about 12.6% of India's land area, not counting snow-covered areas [2].
• Mapping: under the National Landslide Susceptibility Mapping (NLSM) programme, the GSI has finished 1:50,000-scale susceptibility maps for the whole landslide-prone area. This covers about 4.34 lakh km² (433,863 km²) across the Himalaya, the Northeast and the Western Ghats [2].
• Inventory: the GSI has recorded 87,474 active landslides (2024) [2].

State/UT	High-susceptibility area (%)	Landslides recorded	Landslides per km²
Himachal Pradesh	26 (highest)	17,102	0.41
Uttarakhand	22	14,780	0.38
Ladakh (UT)	21	833	0.02
Sikkim	18	3,378	0.68 (highest density)
Arunachal Pradesh	15	26,213 (highest number)	0.37
Kerala	13	1,395	0.07

Source: GSI, 2024 [2]

• Why the Himalaya leads (rock and climate controls together):
• The rock is young, tectonically active, folded and faulted, with many joints. This is the rock control.
• Heavy, intense monsoon rain falls on it. Most landslides happen during the monsoon [2]. This is the climatic control.

• Shear stress then exceeds the strength of the slope, and the slope fails.

• Response: the National Landslide Forecasting Centre (NLFC) opened on 19 July 2024. Operational forecast bulletins now cover Darjeeling and Kalimpong (West Bengal) and the Nilgiris (Tamil Nadu). Experimental bulletins go to 13 more districts in Sikkim, Uttarakhand, Himachal Pradesh and Kerala [2]. The aim is a Landslide Early Warning System (LEWS) for all 19 landslide-prone States/UTs [2].

Prelims Hooks

• The sole ultimate source of energy for all exogenic processes is solar energy. Gravity and tectonic slopes only help.
• Stress = force per unit area. Shear stress acts along a surface and causes breaking and slipping.
• Denudation ("to strip off") = weathering + mass wasting + erosion + transportation.
• Weathering and mass movement are driven by gravity, temperature and chemical action. Erosion and transport are driven by the kinetic energy of water, ice, wind and waves.
• Weathering happens in place (in situ), with no transport. Trap: "weathering involves the movement of material" is wrong.
• Aspect = the direction a slope faces. It controls insolation, and so the rate of weathering.
• Frost wedging (cryofracturing): water expands when it freezes and splits the rock [3].
• India's landslide-prone area is about 4.2 lakh km² (about 12.6% of land, excluding snow-covered areas) in 19 States/UTs [2].
• Himachal Pradesh has the highest high-susceptibility share (26%). Arunachal Pradesh has the most landslides recorded (26,213). Sikkim has the highest landslide density (0.68 per km²) [2].
• The NLFC was inaugurated in July 2024 by the Ministry of Mines/GSI. The GSI is the nodal agency for landslides [2].

Mains Points

• Climate-rock interaction: the same rock weathers differently in different climates. Limestone dissolves fast in humid Meghalaya but resists in arid Rajasthan. Granite breaks up through temperature change in deserts. This explains regional landform differences across India (GS-I).
• Differential erosion: a rock resists one process but not another. So selective wear creates relief: ridges, valleys, scarps and karst. Small, slow stresses add up through continued fatigue over geological time.
• Landslides as a GS-III disaster issue: young, folded and jointed Himalayan rocks plus intense monsoon rain lead to shear failure. About 12.6% of India's land is prone [2]. Responses: NLSM 1:50,000 mapping, the 87,474-landslide inventory, the NLFC and regional LEWS [2]. Gaps remain: hill roads and urbanisation on fragile slopes, and the need for larger-scale maps for local planning.
• Human role: clearing vegetation removes a climatic-biotic control. This speeds up denudation, soil loss and slope failure. Afforestation, slope stabilisation and aspect-sensitive land use help reduce it.