Weathering: physical, chemical, and biological

Weathering weakens rock while erosion removes and carries it. Exam tip: focus on the word transport.

Direct answer: Option C, physical weathering. In areas with a large day-night temperature range, rocks heat during the day and cool at night. Heating causes expansion and cooling causes contraction. Repeated expansion and contraction produce stress, cracks, and eventually smaller pieces without changing the rock’s chemical composition. That is physical or mechanical weathering. Option A is less suitable because chemical weathering changes minerals through reactions with water, oxygen, or acids. Option B involves plants, animals, or organisms. Option D is deposition, the settling of material, not its breakdown. Cue: repeated heat and cold means physical breaking.

The direct answer is B, exfoliation. Exfoliation is a type of physical weathering in which outer rock layers peel away in curved or shell-like sheets. Step by step: the rock surface is repeatedly heated and cooled, or pressure is released when overlying material is removed; the outer part expands and contracts, or expands because of pressure release; parallel cracks develop near the surface; thin outer layers separate and peel off. Option B is correct because shell-like peeling is the defining visual clue for exfoliation. Option A, carbonation, is a chemical process in which carbonic acid helps dissolve carbonate rocks, especially limestone; it does not specifically mean peeling sheets. Option C, oxidation, involves reaction with oxygen, commonly producing rust-like colours in iron-bearing minerals. Option D, hydration, involves minerals taking in water and expanding or changing chemically; it is not the name for curved layer peeling. Heating and pressure release are common causes, although exfoliation can have more than one mechanism. Memory cue: outer shells peeling from a rock = exfoliation.

Correct answer: A, water expands on freezing. In a cold mountain environment, liquid water can enter small cracks in a rock. When the temperature falls below freezing, the water changes into ice and occupies more space than the same amount of liquid water. The expanding ice presses against the crack walls. Repeated freezing and thawing increases the pressure and gradually widens the crack until pieces of rock break away. This is called frost action or freeze-thaw weathering, a type of physical weathering because the rock's chemical composition need not change. B is wrong because freezing changes water into a solid, not a gas. C is wrong because freezing does not normally turn pure water into an acid. D is wrong because ice does not become soil simply by freezing. Memory cue: freeze, expand, press, crack—four steps of frost action.

Salt crystals grow in cracks and create pressure. Exam tip: remember arid areas and evaporation together.

When overlying load is removed rock expands and cracks form. Exam tip: this may also be called unloading.

Water and heat increase chemical reactions. Exam tip: link hot humid climate with chemical weathering.

Soluble minerals dissolve in water and weaken rock. Exam tip: remember limestone and salt as examples.

Carbonation is a chemical weathering process in which carbon dioxide combines with water to form weak carbonic acid, represented as \(\mathrm{H_2CO_3}\). This slightly acidic water can react with and dissolve minerals, especially carbonate minerals such as calcite in limestone. The process gradually weakens and removes parts of the rock, particularly where water can enter cracks and openings.

The important sequence is carbon dioxide plus water, formation of a weak acid, and chemical dissolution of susceptible minerals. It does not magnetise rocks, turn them into gold, or completely vaporise them. Because option A describes the actual chemical action of carbonated water, it is correct. The effect may be slow, but repeated contact with groundwater or rainwater can enlarge cracks and contribute to features such as cavities in limestone.

Carbonation can dissolve limestone and create underground cavities. Exam tip: connect it with karst landforms.

In oxidation oxygen combines with minerals like iron to form new compounds. Exam tip: reddish brown colour is a clue.

Iron minerals combine with oxygen to form rust-like material. Exam tip: link colour change with oxidation.

In hydration water enters minerals and may change their volume. Exam tip: treat it as chemical weathering.

The direct answer is A, hydrolysis. Hydrolysis is a type of chemical weathering in which water reacts with minerals and changes their chemical composition. Feldspar is a common rock-forming mineral. When water, often containing weak acids, reacts with feldspar, the original mineral structure breaks down and clay-like minerals are produced. The reasoning is: water enters cracks and mineral surfaces; it reacts with feldspar; the feldspar changes chemically; clay minerals and dissolved substances may result. Option A is correct because it names this exact chemical reaction. Option B, salt crystallisation, happens when dissolved salts grow as crystals in cracks and exert pressure; it mainly causes physical breaking, not feldspar-to-clay conversion. Option C, thermal fracture, occurs when repeated heating and cooling expand and contract rocks; it is mechanical weathering. Option D, frost action, occurs when water freezes, expands, and widens cracks; it is also physical weathering. The useful exam cue is: feldspar changing into clay means chemical weathering by hydrolysis. Do not confuse a chemical change in minerals with simple cracking of rocks.

The direct answer is A: water acts as a medium and active participant in many chemical reactions. Chemical weathering changes the minerals in rock rather than merely breaking the rock physically. Step by step: water enters pores and cracks; it dissolves some substances and carries dissolved ions; it also reacts with minerals through processes such as solution and hydrolysis; the original minerals may become weaker or transform into new minerals. Option A is correct because water enables and supports these reactions. Option B is wrong because water does not always burn rock. Option C is wrong because chemical weathering changes minerals rather than keeping them unchanged. Option D is wrong because reducing wind speed is not the main reason water matters in chemical weathering. Memory cue: water helps minerals dissolve, react, and change.

Root growth puts pressure on cracks. Exam tip: it can be linked with both biological and physical effect.

Small organisms can release chemicals that weaken rocks. Exam tip: do not ignore micro-biological action.

The direct answer is A: mining and excavation expose and weaken rocks. Weathering means the breakdown or alteration of rocks at or near Earth’s surface. Human beings can increase it by cutting hills, digging mines, making roads, removing vegetation, and exposing fresh rock surfaces. The reasoning is: excavation removes soil or covering material; the newly exposed rock meets water, air, temperature changes, salts, and organisms; cracks can widen; roots may enter cracks; therefore biological and mechanical weathering become easier. Option A is correct because mining and construction physically disturb rocks and expose them to weathering agents. Option B is wrong because farming does not stop Earth’s rotation; that claim is unrelated and physically unrealistic. Farming can sometimes increase erosion or expose soil, but it cannot stop planetary rotation. Option C is wrong because buildings do not make rocks invisible; construction may cover some surfaces but often involves cutting, drilling, and exposing others. Option D is wrong because roads do not always stop weathering. Road cutting, vibration, drainage changes, and exposed surfaces can increase weathering. Remember that humans can act as geomorphic agents: their activities change landforms and the conditions affecting rocks.

Cracks provide pathways for water and air. Exam tip: treat rock structure as a controlling factor.

Direct answer: Option A, surface area increases. When one rock breaks into many small pieces, the total exposed surface becomes much larger. Water, oxygen, carbon dioxide, and other reacting substances can then touch more mineral particles. Chemical reactions therefore occur at more places and chemical weathering can become faster. Option A is correct. Option B is wrong because gravity does not disappear. Option C is wrong because climate continues to control weathering. Option D is wrong because minerals do not become permanent or immune to change. The idea is similar to powder reacting faster than a large lump because more surface is exposed. Cue: smaller pieces, larger contact, faster reaction.

Limestone can dissolve easily in water containing carbonic acid. Exam tip: remember it for karst landforms.

Some minerals are more resistant to chemical weathering. Exam tip: remember the importance of mineral composition.

Physical weathering changes size and fragments but usually not chemical composition. Exam tip: this is a key difference.

Chemical weathering can form new minerals or dissolved substances. Exam tip: identify composition change.

Plants bacteria animals and humans can weaken rocks. Exam tip: remember examples of biological agents.