Physical and biological weathering

Detail

1. What physical weathering is

• Physical weathering (also called mechanical weathering): rock breaks into smaller pieces, but its chemical make-up stays the same.
• Britannica gives three main causes: heating and cooling, the growth of foreign crystals (such as ice or salt) inside the rock, and rock pieces hitting each other as they move [3].
• It happens in situ, meaning in place. Very little rock is carried away while it happens.
• The pieces it makes give a larger surface area. This lets chemical weathering work faster later on.

2. The three forces behind physical weathering

Force	What it is	How it breaks rock
Gravitational forces	Overburden pressure (the weight of rock lying on top), load, shearing stress (a force that makes one part of a rock slide past another)	Weight squeezes or pulls the rock. When the weight is removed, the rock cracks.
Expansion forces	Caused by temperature change, crystal growth (ice or salt) or animal activity	Something inside the rock swells and pushes outwards.
Water pressure	Repeated wetting and drying	The rock swells when wet and shrinks when dry. This repeats again and again.

• Most physical weathering comes from two causes:
• thermal expansion (rock swelling when heated);

• pressure release (rock swelling when the load on top is removed).

• Fatigue: each single change is tiny. The same stress repeated thousands of times weakens the rock, like a wire bent back and forth until it snaps. The result is great damage over a long time.

3. Unloading and expansion (pressure release)

• Unloading: erosion removes the rock on top, so the buried rock below is "unloaded".
• Deep granite (an igneous rock that cools slowly underground) forms under very high pressure.
• Erosion strips away the cover → the pressure drops → the rock expands upwards.

• Cracks form in curved sheets parallel to the ground surface. This is called sheeting.

• Exfoliation: these outer sheets peel off like the layers of an onion.

• National Geographic describes it as whole outer layers of rock flaking off in sheets [2].

• Exfoliation dome: a smooth, rounded, dome-shaped rock hill that is left after sheets peel off again and again.

• Half Dome, Yosemite National Park, California (USA), was formed by exfoliation [2].
• Exfoliating granite domes are found on Ryan Mountain, Joshua Tree National Park, California (USA) [4].

• In India they are seen on the exposed granite and gneiss of the Peninsular plateau, for example in Karnataka and Tamil Nadu.

• Exam point: exfoliation is mainly caused by unloading. Heating and cooling only helps.

4. Thermal expansion and contraction

• Cause: rock gets hot by day and cold by night.
• Process:
• Rock conducts heat poorly. So the outer layer heats up and cools down much more than the inside.
• The outer layer expands and contracts more than the core. Stress builds up where the two meet.

• Rocks are made of minerals of different colours, and each mineral expands by a different amount.

• Result:

• Flaking: thin outer shells peel off.

• Granular disintegration: the rock crumbles grain by grain into coarse sand. This is common in coarse-grained rocks like granite.

• Where: most important in hot deserts with a large daily range of temperature (a big gap between day and night temperatures).

• India: the Thar Desert (Rajasthan).
• World: the Sahara and the Arabian Desert.

5. Freezing, thawing and frost wedging

• Frost weathering: rock breaking because water freezes inside it.

• It is also called frost wedging or cryofracturing [2].

• Process:

• Water seeps into cracks and joints in the rock.
• When it freezes, its volume grows by about 9%.
• The ice acts like a wedge and widens the crack [2].

• The ice melts by day and refreezes at night. Each freeze–thaw cycle pushes the crack further open.

• Result: sharp, angular rock fragments. They pile up at the foot of slopes as scree or talus.

• Where:
• High mountains: the Himalaya, and Ladakh (a cold desert).

• Cold, high-latitude regions where the temperature often swings around 0 °C.

• Trap: the temperature must cross freezing point many times. Places that stay frozen all year have less frost wedging.

6. Salt weathering (haloclasty)

• Salt weathering, or haloclasty: salt crystals growing in rock pores break the rock [2].
• Process:
• Salty water enters the pores and cracks of the rock.
• The water evaporates and leaves salt crystals behind [2].
• The crystals grow and push the grains apart.

• Salts such as calcium, sodium, magnesium, potassium and barium salts also expand when heated. Hot sun adds even more pressure.

• Result:

• Grains break loose and the rock crumbles.

• Honeycomb weathering: rock surfaces with hundreds or thousands of small pits made by growing salt crystals. It is common on coasts hit by sea spray [2].

• Where:

• Deserts: high evaporation draws salty water up to the surface. India: Thar and the Rann of Kutch (Gujarat).
• Coasts: sea spray brings salt.

7. Biological weathering

• Biological weathering: weathering caused by living things [5].
• Living things work in two ways:
• physical changes: they break or move rock by growing or moving;
• chemical changes: they add or remove minerals and ions (charged particles) in the weathering zone.

Agent	What it does	Effect
Burrowing animals: earthworms, termites, rodents, and also moles and prairie dogs [2]	Dig, wedge and turn over soil and rock	Expose fresh surfaces and let air and moisture reach deeper
Humans	Clear vegetation, plough and cultivate	Mix the soil and create new contacts between air, water and minerals
Decaying plant and animal matter	Releases humic acid, carbonic acid and other acids	Speeds up the chemical decay of minerals
Plant roots	Grow into cracks and press outwards with tremendous pressure	Split rocks apart
Mosses and lichens	Grow on rock surfaces or inside cracks	Make tiny cracks bigger [2] and wear down or etch patterns on rock [5]

• Root wedging, step by step:
• Soil gathers in a crack, and a tree seed sprouts there [2].
• The root thickens as it grows and pushes the crack walls apart.

• Over years, the crack widens until the rock breaks [2][5].

• Lichen: a living partnership of a fungus and an alga. It is often the first living thing to settle on bare rock.

• Exam point: biological weathering is both physical and chemical. Roots and burrows are physical. Organic acids are chemical.

8. Physical weathering processes compared

Process	Main trigger	Typical result	Typical region
Unloading	Erosion removes the rock on top	Curved sheets, exfoliation domes	Granite areas: Peninsular India, Yosemite
Thermal expansion and contraction	Large daily temperature range	Flaking, granular disintegration	Hot deserts: Thar, Sahara
Frost wedging	Repeated freeze–thaw cycles; ice grows about 9% in volume	Angular fragments, scree	Himalaya, Ladakh, cold regions
Salt weathering	Evaporation and salt crystal growth	Crumbling grains, honeycomb weathering	Deserts, coasts
Biological weathering	Roots, burrowing animals, organic acids	Split rock, mixed soil, faster decay	Everywhere; strongest where vegetation is dense

Prelims Hooks

• Water expands by about 9% on freezing. Frost wedging leaves angular fragments, not rounded ones.
• Frost wedging is also called cryofracturing. Salt weathering is also called haloclasty.
• Exfoliation domes form mainly by unloading (pressure release), not by chemical weathering. Example: Half Dome, Yosemite (California).
• Granular disintegration is linked to hot deserts with a large daily temperature range.
• Honeycomb weathering (pitted rock surfaces) comes from salt crystal growth. It is common on coasts with sea spray.
• Physical weathering does not change a rock's chemical make-up. Chemical weathering does.
• Biological weathering works both physically (root wedging, burrowing) and chemically (humic and carbonic acids from decay).
• The three forces behind physical weathering are gravitational forces, expansion forces and water pressure (wetting–drying).
• Most physical weathering comes from thermal expansion and pressure release.
• Pairs to match: Ladakh → frost wedging; Thar → granular disintegration and salt weathering; Rann of Kutch → salt weathering; Peninsular granite → exfoliation.

Mains Points

• Climate decides which process leads (GS-I):
• Hot deserts: thermal expansion and salt weathering.
• High mountains and cold regions: frost wedging.
• Humid, forested tropics: biological and chemical weathering.

• So weathering patterns roughly follow climatic zones.

• Links to hazards (GS-III, disasters):

• Frost wedging makes loose angular debris on Himalayan slopes. This feeds rockfalls and landslides on hill roads and highways.

• Unloading joints in granite cut slopes can lead to slab failures.

• Weathering creates soil and resources:

• Physical and biological weathering break rock into regolith (the loose layer of broken rock over solid bedrock). Regolith is the parent material of soil.

• Root and burrow activity improves soil aeration and drainage. Human ploughing speeds this up but also exposes soil to erosion.

• Heritage and infrastructure:

• Salt weathering and honeycomb weathering damage coastal monuments, buildings and bridges.
• This calls for salt-resistant materials and regular conservation in coastal and arid India.