Weathering: physical, chemical, and biological

Minerals expand and contract at different rates creating stress. In exams connect it with thermal weathering.

Water freezes in cracks expands and enters again after thawing. Repeated freeze-thaw is the key exam clue.

The direct answer is B, salt forms crystals in cracks and exerts pressure. In dry and semi-dry regions, water containing dissolved salts may enter tiny cracks in a rock. When the water evaporates, salt remains. The salt crystals grow as more salt is deposited, and their growth pushes against the crack walls. Repeated cycles widen the cracks, loosen grains, and weaken or break the rock. Option A is wrong because salt does not melt ordinary rock; melting requires very high temperature. Option B is correct because crystal growth produces mechanical pressure inside cracks. Option C is wrong because salt does not make all minerals magnetic; magnetism is a different physical property. Option D is incomplete and therefore wrong: high salt concentration can affect vegetation, but that is not the mechanism of salt-crystal weathering of rocks. The key chain is evaporation, salt deposition, crystal growth, pressure, and cracking. Memory cue: in deserts, evaporation leaves salt behind; growing crystals push rocks apart.

Corners weather faster because of greater exposed surface area. Link rounded shapes with uneven surface weathering.

Direct answer: Option C, red or yellowish brown. Oxidation occurs when oxygen reacts with minerals, especially iron-bearing minerals. Iron oxides and hydroxides resemble rust, so weathered rock commonly develops reddish, brown, or yellowish-brown colours. Option C is correct. Option A is wrong because blue or purple is not the usual colour produced by iron oxidation. Option B is wrong because oxidation does not generally make iron-bearing rock white or transparent. Option D is wrong because green or shiny black is not the standard rust-like result. The exact shade can vary with minerals and moisture, but the school-level clue is iron plus oxygen equals rust colour. Cue: oxidation means rust-like red-brown.

Water with carbon dioxide forms carbonic acid and dissolves calcite. Link limestone and karst with carbonation.

Answer: A, internal pressure and breakage increase. Hydration is a chemical weathering process in which certain minerals absorb water into their structure. The added water can make the mineral swell, so its volume becomes larger. When this swelling occurs inside a solid rock, it produces pressure against neighbouring mineral grains and existing cracks. Repeated wetting and drying, or continued expansion, weakens the rock and may cause splitting or crumbling. B is wrong because swelling does not automatically close every crack; it can enlarge or create cracks. C is wrong because hydration does not turn rock into metal. D is wrong because gravity continues to act on the rock. Memory cue: water added to a mineral may mean swelling, pressure, and eventual breakage.

Feldspar can alter into clay minerals through water and weak acidic water. Remember feldspar to kaolinite for exams.

Heat and moisture increase reaction rates. Link warm humid climate with intense chemical weathering.

Water is important for chemical weathering and deserts have little water. Link dryness with dominance of physical weathering.

Roots grow and exert mechanical pressure. Link root pressure with the physical side of biological weathering.

Direct answer: Option A, lichens release organic acids. Lichens are partnerships between a fungus and an alga or similar organism. They grow on bare rock and produce weak organic acids. These acids react with and slowly dissolve or weaken minerals, helping chemical weathering. Their attachment can also help open tiny spaces, but the key answer is acid release. Option B is wrong because lichens do not instantly freeze rocks. Option C is wrong because their role is not merely cleaning dust. Option D is wrong because they do not make rocks magnetic. This is biological activity causing chemical change, so it is a useful example of biological chemical weathering. Cue: lichen acids loosen minerals.

Carbon dioxide combines with water to form carbonic acid. Link groundwater and limestone with carbonation.

Differences in mineral composition change weathering rates. Link hard and soft rock contrast with differential weathering.

Cracks allow water and air to enter and increase weathering. Link joints with surface area and pathways.

Weathering breaks parent rock and forms the basis of soil. Treat soil formation as a major result of weathering.

Removal of outer layers due to unloading or thermal effects is exfoliation. Link onion-like sheets with exfoliation.

High moisture and temperature alter rocks deeply. Link deep weathering mantle with humid tropics.

Low temperature reduces reaction rate and lack of water also limits it. Link cold dryness with slow chemical weathering.

Oxidation of iron-bearing minerals in basalt can produce reddish colour. Link red colour with iron oxides.

Direct answer: Option A, soluble minerals and sufficient water. Solution weathering is a chemical process in which water dissolves minerals from rock. First, water must be present. Second, the mineral must be able to dissolve in that water. When both conditions continue for a long time, the rock gradually loses material. Salts and some limestone minerals are familiar examples. Option A is correct because it contains both necessary conditions: a soluble substance and enough water. Option B is wrong because insoluble minerals do not easily dissolve, and complete dryness removes the main agent, water. Option C is wrong because an earthquake breaks or moves rocks suddenly; it is not solution weathering. Option D is wrong because low gravity alone does not dissolve minerals and is not the controlling requirement. The word “solution” should remind you of a substance going into water. Memory cue: soluble mineral plus water equals solution weathering.

In permeable rocks water can enter and deepen weathering. Link permeability with depth of chemical action.

Microorganisms can produce acidic and biochemical substances. Link them with biological chemical weathering.

Quartz is generally a hard mineral and is relatively stable under many common weathering conditions. A rock containing a high proportion of quartz therefore tends to resist both physical breakdown and many chemical changes better than a rock dominated by less stable minerals. Its resistance does not mean that the rock can never weather; climate, fractures, water, and exposure time still matter.

The question asks for the general effect of high quartz content, so “more resistant” is the best choice. Quartz is not normally described as the most soluble mineral, and the presence of quartz does not imply instant hydration or inevitable biological destruction. Weathering may attack other minerals in the same rock first, leaving quartz-rich grains behind. Therefore option A follows from quartz's hardness and relative chemical stability.

Water expands on freezing and widens cracks. Treat volume expansion as the basis of frost action.