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
Two rocks sit under the same hot sun. One is wet and full of cracks; the other is dry and solid. Which weathers faster?
- The dry rock, because heat alone controls weathering.
- The wet, cracked rock.
- Neither, because weathering happens underground.
- Both equally, because the climate is the same.
Show the answer
The wet, cracked rock.
Water supplies what chemical reactions need, and cracks let it in and raise the surface area. A dry rock weathers slowly even in great heat.
Climate sets the conditions for weathering, but rock type, water, living things, time and people decide how fast it works.
Warmth speeds up chemical reactions, and water supplies what they need and washes products away. So hot, wet tropics often have deep chemical weathering. But a dry rock weathers slowly even in great heat. Waterlogging favours reduction instead of oxidation.
Rock type matters. Feldspar reacts differently from quartz, and limestone dissolves easily in weak acid. Joints, layers and pores let water in. Smaller pieces have more surface area, so they weather faster.
Slope and drainage matter. Gentle slopes keep thick layers of weathered material, called regolith. Steep slopes lose it to erosion and landslides, exposing fresh rock. A thick regolith may show a long time of weathering, not the fastest rate today.
Plants add carbon dioxide and acids to soil water, and roots widen cracks. Time matters too, because today's landscapes may reflect past climates.
People change these controls. Quarrying exposes fresh rock. Pollution can make water more acidic. Irrigation and drainage change how wet the ground is. Clearing vegetation reduces biological activity but exposes rock to bigger temperature swings. So one human change can speed up one process while slowing another.
- Climate
- Heat speeds reactions; water supplies and removes.
- Rock
- Minerals, joints and surface area.
- Regolith
- Loose weathered material above bedrock.
Step by step
Separate climate from rock
Moisture and heat versus minerals and cracks.
Add time and removal
Weathering makes material; erosion takes it away.
Trace a human change
How does it change exposure, chemistry or water?
Worked example: A fair comparison
Two equally wet rock samples differ only in minerals. Faster clay formation in the feldspar-rich one supports hydrolysis. If they also differ in cracks and exposure time, minerals alone cannot explain it.
Watch out for this
The deepest regolith shows weathering is fastest there today.
Regolith depth also reflects how long it built up and how slowly it is eroded.
Check your understanding
Which observation most weakens the claim that temperature alone controls weathering?
- Equally warm rocks weather differently, as one lacks water.
- Reactions are faster in warm conditions than in cool ones.
- The rock samples were measured on a warm afternoon.
Show the answer
Equally warm rocks weather differently, as one lacks water.
Yes. Other controls clearly matter.