In geomorphology, which one of the following processes of weathering is considered to belong to both mechanical (physical) and chemical weathering?

Introduction / Context:
Weathering is the process by which rocks are broken down at or near the Earth surface due to physical and chemical agents. Some processes are mainly mechanical, others mainly chemical, but a few can operate through both mechanical and chemical effects. This question asks about a weathering process that is considered to belong to both mechanical and chemical categories in many geography texts.

Given Data / Assumptions:
• The processes listed are hydration, exfoliation, carbonation and crystallization. • We need to identify which process has both mechanical and chemical aspects. • Standard school geography classification distinguishes physical and chemical weathering, with some overlap. • We rely on the way competitive exam question banks categorise these processes.

Concept / Approach:
Hydration and carbonation are typically grouped as chemical weathering processes because they involve chemical combination of water or carbon dioxide with minerals. Exfoliation is a mechanical process where layers peel off due to pressure release or thermal expansion. Crystallization, especially of salts in rock pores, can be seen as both mechanical and chemical, because chemical growth of crystals leads to physical stress and fracturing. Many exam keys therefore identify crystallization as belonging to both mechanical and chemical weathering.

Step-by-Step Solution:
Step 1: Recall that hydration involves the addition of water molecules to mineral structures, changing their chemical composition, so it is mainly chemical. Step 2: Remember that carbonation involves reaction of carbon dioxide dissolved in water with minerals like limestone, again a chemical process. Step 3: Note that exfoliation refers to peeling of rock layers due to unloading or temperature changes, which is a physical or mechanical effect. Step 4: Consider crystallization of salts in rock joints and pores. Here, chemical solution and precipitation produce solid crystals, and the growth of those crystals exerts mechanical pressure that breaks the rock. Step 5: Because crystallization combines chemical formation of crystals with mechanical stress, it is commonly classified as a process belonging to both mechanical and chemical weathering in exam literature.

Verification / Alternative check:
When we consult multiple geography MCQ sources for this exact question, crystallization is consistently given as the correct answer. Explanations emphasise how salt crystal growth results from chemical processes and at the same time generates mechanical disintegration. This dual nature justifies its placement in both categories.

Why Other Options Are Wrong:
Option A: Hydration is a chemical process because minerals absorb water and change their composition and volume, but it is not generally described in school texts as both mechanical and chemical for this question pattern. Option B: Exfoliation is purely mechanical, involving physical peeling of rock sheets due to temperature or pressure changes, without chemical reaction as the main driver. Option C: Carbonation is chemical weathering, where carbonic acid reacts with minerals, especially carbonates, to dissolve or weaken them, with no primary mechanical effect highlighted in basic courses.

Common Pitfalls:
A frequent mistake is to pick hydration because it may cause physical expansion, but exam keys for this specific question emphasise crystallization instead. Another pitfall is to treat any process that changes rock volume as mechanical, overlooking the essential chemical reactions involved. Students should memorise the classification of major weathering processes as presented in standard school geography texts.

Final Answer:
The process of weathering that belongs to both mechanical and chemical weathering is crystallization.