Exfoliation of rock is a result of

1. Analyze the question The question asks what process causes exfoliation, which is the peeling off of rock in thin sheets or layers, like the skin of an onion.

2. Explain the concept Exfoliation (also called onion-skin weathering) happens mostly in hot desert and tropical areas with large daily temperature ranges, such as parts of northern Nigeria.

• During the day, the sun heats the outer surface of the rock, which expands.
• At night, the temperature drops sharply and the outer surface cools and contracts.
• Rock is a poor conductor of heat, so the outer layer expands and contracts more than the inside.
• Repeated stress eventually causes the outer layers to crack and peel off in curved sheets.

Exfoliation can also be helped by pressure release (unloading), when overlying rock is removed and the rock expands outward. Both are physical processes.

3. Solution: checking each option

• A. Oxidation is a chemical process, a reaction with oxygen that produces rust-coloured iron compounds. It is not the main cause of exfoliation.
• B. Solution is a chemical process where minerals dissolve in water, for example limestone dissolving in carbonic acid. It does not cause peeling.
• C. Mechanical (physical) weathering breaks rock apart without changing its chemical composition. Temperature changes and pressure release fit this exactly.
• D. Biological weathering involves plants, animals or humans, such as roots prying rocks apart. It is not the cause here.

4. Answer Exfoliation is a result of mechanical weathering, which is option C.

5. Exam tip Link exfoliation with "heating and cooling". Whenever you see exfoliation, onion-skin weathering, or granular disintegration, think physical/mechanical weathering caused by temperature change. Remember: Oxidation and solution = chemical; roots and animals = biological; temperature change = mechanical.