Weathering

Weathering is the process of gradual disintegration of rocks at or near the earth’s surface through physical, chemical and biological processes caused by wind, water, climate change etc.

Unlike erosion (which moves materials), weathering happens in place (in-situ or on-situ process).

🌍 Example: The crumbling of old monuments and buildings due to rain and wind.

Types of Weathering

Weathering is classified into three main types:

1️⃣ Chemical Weathering
2️⃣ Physical (Mechanical) Weathering
3️⃣ Biotic (Biological) Weathering

Note that although, it is divided into these types but they are so intimately interrelated that it is practically difficult to isolate one process from the other.

1️⃣ Chemical Weathering: The Breakdown by Reactions

Chemical weathering changes the chemical composition of rocks through various reactions. It happens faster in warm, humid climates.

🔹 Solution: Minerals dissolve in water or weak acids.
📍 Example: Limestone dissolving in acidic rain to form caves.

🔹 Carbonation: Carbon dioxide in water forms carbonic acid, which dissolves carbonate rocks.
📍 Example: Formation of karst landscapes like the Guilin hills in China.

🔹 Hydration: Minerals absorb water, expand, and weaken.
📍 Example: Calcium sulfate turns into gypsum, making it softer.

🔹 Oxidation & Reduction: Oxygen reacts with minerals to form oxides (rusting). In oxygen-poor conditions, the reverse happens (reduction).
📍 Example: The reddish color of some rocks due to iron oxidation.

2️⃣ Physical (Mechanical) Weathering: The Breakdown by Force

Physical weathering happens without changing the rock’s composition but breaks it into smaller pieces.

🔸 Block Disintegration: Rocks expand in heat and contract in cold, causing cracks.
📍 Example: Happens in deserts with extreme day-night temperature differences.

🔸 Granular Disintegration: Different minerals in the rock expand and contract at different rates, breaking the rock into small grains.

🔸 Exfoliation (like Onion-Skin Weathering): With rise in temperature, every mineral expands and pushes against its neighbour and as temperature falls, a corresponding contraction takes place. Because of diurnal changes in the temperatures, this internal movement among the mineral grains takes place regularly.

• This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic. The surface layers of the rocks tend to expand more than the rock at depth and this leads to the formation of stress within the rock resulting in heaving and fracturing parallel to the surface.
• Exfoliation results in smooth rounded surfaces in rocks.

📍 Example: Domes of granite rocks in Rajasthan, India.

🔸 Frost Weathering: Mainly at high altitudes & cold climates where during day cracks & joints inside rock fill with water & during night they get frozen. This leads to increase in volume of water in rock approx. by 9 %
📍 Example: Formation of jagged peaks in cold mountain regions.

🔸 Spalling: Sudden rain on hot desert rocks creates cracks due to thermal shock.

🔸 Salt Weathering: Salts in rocks expand due to heat and moisture, breaking them apart.
📍 Example: Honeycomb weathering in coastal cliffs.

3️⃣ Biotic (Biological) Weathering: The Breakdown by Living Organisms

Plants, animals, and humans contribute to rock weathering in various ways.

🌱 Floral Weathering (Plants)

• Physical: Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.
• Chemical: Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements.

🐜 Faunal Weathering (Animals)

• Burrowing animals (earthworms, rodents) expose rocks to air and moisture, speeding up weathering.

👷 Anthropogenic Weathering (Humans)

• Mining, construction, and pollution accelerate rock breakdown.
  📍 Example: Acid rain caused by pollution eroding buildings.

Are Physical and Chemical Weathering Linked?

Yes! They often work together, enhancing each other.

📌 Example:
1️⃣ Exfoliation (Physical) creates cracks in rocks.
2️⃣ Rainwater (Chemical) seeps in and reacts with minerals.
3️⃣ Plant roots (Biotic) grow into cracks and break them further.

This shows that weathering is a combined process, constantly shaping landscapes.

Why is Weathering Important?

🌍 Weathering is crucial for Earth’s surface evolution.

✔ Soil Formation – Weathering breaks down rocks into soil.
✔ Supports Life – Provides minerals for plants and animals.
✔ Shapes Landforms – Mountains, valleys, and caves are created.
✔ Enables Erosion & Mass Movement – Weathering weakens rocks, making them easier to transport by rivers, wind, and glaciers.
✔ Influences Agriculture & Forests – Soil fertility depends on weathering depth.