What this lesson teaches
I can explain the chemical and physical weathering processes and what controls them.
Syllabus 9173, Topic 2.2, 1. Geomorphic Processes. What influences the weathering of rocks?
Make a guess
A granite boulder sits in the same spot for thousands of years in a hot, wet climate. It slowly crumbles into clay and sand. Was it eroded?
- No. It was weathered in place.
- Yes, because the rock changed.
- No, because granite cannot change.
- Yes, because rain fell on it.
Show the answer
No. It was weathered in place.
Weathering breaks rock down where it sits. Here water reacted with feldspar, turning it to clay and leaving quartz grains. Erosion would carry the material away.
Chemical weathering changes or dissolves minerals where the rock sits, and the reaction depends on the water and the minerals.
Geomorphic processes shape the Earth's surface by changing and moving material. Weathering is the breakdown of rock in place. Chemical weathering changes its minerals. Erosion is different: it removes material. Several reactions can happen together, so name the mineral, what reacts with it and what forms.
Carbonation starts when carbon dioxide dissolves in water, forming weak carbonic acid. This reacts with calcium carbonate in limestone. The products dissolve and are carried away. Soil often adds more carbon dioxide than the air.
Hydrolysis is when water reacts with silicate minerals. Feldspar in granite turns into clay. So granite weakens, while hard quartz grains remain.
Oxidation is when minerals lose electrons, often by reacting with oxygen. Iron minerals form red or brown iron oxides. Reduction is the opposite: gaining electrons. In waterlogged soil with little oxygen, iron becomes soluble and is washed out, leaving grey patches.
Solution is minerals simply dissolving in water, like rock salt. Carbonation also dissolves rock, but through a specific acid reaction. Hydrolysis is different again: it creates new minerals, unlike physical break-up.
- Carbonation
- Carbonic acid dissolves limestone.
- Hydrolysis
- Water turns feldspar into clay.
- Oxidation and reduction
- Iron rusts red, or is removed leaving grey.
Step by step
Name the mineral
Limestone, feldspar and iron minerals react differently.
Name the conditions
Water, acidity, oxygen and temperature.
State the change
What dissolves, what forms, or what moves.
Worked example: One soil, two colours
A well-drained upper slope has red iron oxides. A waterlogged hollow has grey patches, where low oxygen lets iron be washed out. Drainage, not just rainfall, explains the difference.
Watch out for this
Reduction just means a rock gets smaller.
Reduction is a chemical change where minerals gain electrons. Smaller pieces without mineral change is physical weathering.
Check your understanding
Feldspar turns into clay after reacting with acidic water. Which process is this?
- Hydrolysis.
- Freeze-thaw.
- Traction.
Show the answer
Hydrolysis.
Yes. Water reacts with the silicate mineral to form clay.
Check your understanding
Why is chemical weathering often deep in the humid tropics?
- Heat speeds reactions, and plentiful water supplies and removes the products.
- Freeze-thaw is common in tropical lowlands at night.
- There is little vegetation, so rock lies bare to the rain.
Show the answer
Heat speeds reactions, and plentiful water supplies and removes the products.
Warm, wet conditions favour deep weathering over long periods.