Weathering Notes

Weathering is the breaking down or disintegration of substances like rock or minerals by physical, chemical, and biological processes.
Weathering can affect natural formations like rocks, as well as human-made structures such as buildings and statues.

Physical/mechanical weathering involves the breaking down of rock without changing its chemical composition.
Temperature and water are the main causes of mechanical weathering.
Different mechanical weathering processes:
- Freeze-thaw
- Thermal stress/fracturing

Freeze-thaw occurs with cycles of cold and hot temperatures, affecting water or moisture around rocks.
When it rains, moisture enters cracks and holes in rocks.
If the temperature drops below 0ºC, the water freezes.
This causes the cracks to widen, eventually breaking the rock apart.
The term 'Frost Shattering' is incorrect.

Also termed expansion and contraction.
Rocks expand when heated and contract when cooled.
The particles in the rock expand and contract at different rates.
This leads to cracks and granular disintegration, where small pieces or grains of rock fall off.
During a hot, sunny day, the rock heats up on the outside, and at night, it cools and contracts, causing the outer layer to peel off, known as exfoliation.
Exfoliation is the removal of the outer layers of a rock after repeated contraction and expansion.

Chemical weathering occurs when rocks are weathered due to chemical processes.
It involves the breaking down of rock by changing its chemical composition.
It is common in warm, moist areas but can occur in colder areas as well, including deep soil.
Types of chemical weathering discussed:
- Oxidation
- Carbonation
- Hydrolysis

Occurs when oxygen mixes with minerals in a rock to form oxides.
For example, rust forms when rocks containing iron are exposed to air, causing the iron to oxidize.
This weakens the rock and causes it to crumble.
Uluru is a sandstone formation that contains iron minerals.
Oxidation can affect both rocks and human-made materials.

Some rocks, like limestone and chalk, are easily weathered by chemicals.
Limestone and chalk are made of calcium carbonate.
Acidic water (containing carbon dioxide) dissolves these rocks.
Acidic rain goes into the ground, forming groundwater.
Limestone caves form because of this acidic water, which eats away at the rock over time, creating underground caves.
Dissolving means that a substance integrates into a liquid, thereby forming a solution.
Rain contains carbon dioxide, making it carbonic acid.
When rocks containing calcium carbonate (limestone) mix with rain, a chemical reaction occurs.
The rain transforms the calcium carbonate into calcium bicarbonate.
Calcium bicarbonate is water soluble and is thus washed away.
This process is referred to as carbonation.
Soluble means that a substance will dissolve in water.

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Minerals in the rock and hydrogen in rainwater form silicates.
Silicates are found in many types of rocks, specifically in sandstone and granite.
Silicate minerals absorb water and change into clay.
Clay is soluble and can be washed out of the rock.
This makes the rock unstable and crumble apart.

Living organisms like insects, animals, and plants can cause weathering.
Burrowing animals such as rabbits and moles dig up and break up soil and rocks, exposing them to the atmosphere.
Plants, such as trees, grow in cracks of rocks and widen them.
Plants, such as lichen, grow on rocks and produce a dilute acidic solution that breaks down the rocks. Smaller pieces of the rocks then start to fall off.
The decaying of animals and plants can release chemicals into the surrounding soil and water, leading to erosion of nearby rocks.

Physical: Humans alter the landscape through mining, clearing vegetation, digging, building dams, and construction, leading to the movement and breakdown of rocks and their exposure to the atmosphere. Farmers clear and burn vegetation, which causes the rocks to break up.
Deforestation, the removal of vegetation, reduces biological and chemical weathering.
Chemical: Pollution enters the water system and atmosphere, creating acid rain.
Biological: Repeated human foot traffic over rocks can introduce plants and other elements that break down rock. Humans also smooth and polish rocks through contact.