UPSC CSE Prelims
Landform Development Previous Year Questions (PYQs)

Detailed Explanation:

Answer: Option 1 — Both Statement-II and Statement-III are correct and both of them explain Statement-I

Rainfall is a significant agent of weathering, particularly chemical weathering. Rainwater absorbs carbon dioxide from the atmosphere forming weak carbonic acid, and also dissolves atmospheric oxygen. Both these dissolved gases actively participate in breaking down rock minerals through chemical reactions.

✅ Statement I – Correct: Rainfall causes both physical and chemical weathering of rocks through impact and chemical reactions.
✅ Statement II – Correct: Rainwater dissolves atmospheric CO₂, forming carbonic acid (H₂CO₃) which chemically weathers rocks.
✅ Statement III – Correct: Rainwater contains dissolved atmospheric oxygen which causes oxidation of minerals, a form of chemical weathering.

📝 Short Notes: Chemical Weathering by Rainwater

• Carbonation: CO₂ in rainwater forms carbonic acid (H₂CO₃), which reacts with carbonate rocks (limestone, marble) and feldspars, dissolving them over time.
• Oxidation: Dissolved oxygen in rainwater reacts with iron-bearing minerals (like olivine, pyroxene) causing rust-like decomposition, weakening rock structure.
• Hydrolysis: Water molecules react with silicate minerals (feldspars, micas) to form clay minerals, contributing to rock breakdown.
• Solution: Rainwater can directly dissolve soluble minerals like halite and gypsum without additional chemical reactions.
• pH Factor: Natural rainwater has a pH of approximately 5.6 due to dissolved CO₂; acid rain (pH < 5.0) accelerates weathering rates significantly.
• Rate Factors: Weathering rate depends on rainfall intensity, temperature, rock composition, and vegetation cover which can either enhance or reduce weathering.
• Physical Impact: Besides chemical action, heavy rainfall causes mechanical weathering through raindrop impact and frost wedging in cold climates.

Detailed Explanation:

Answer: Option 3 — Phosphorus Cycle

The Phosphorus Cycle is unique among biogeochemical cycles as it lacks a gaseous phase and primarily relies on the weathering of phosphate-rich rocks as its main source. Geological weathering processes break down these rocks over time, releasing phosphate ions (PO₄³⁻) into the soil and water, which are then absorbed by plants and move through the food chain.

Why other options are incorrect:
• Carbon Cycle: Primarily driven by atmospheric CO₂ from respiration, combustion, and volcanic activity, not rock weathering.
• Nitrogen Cycle: Relies on atmospheric nitrogen with fixation by bacteria as the key process, not weathering of rocks.
• Sulphur Cycle: Main sources include volcanic eruptions, decaying organic matter, and fossil fuel combustion, not primarily rock weathering.

📝 Short Notes: Biogeochemical Cycles

Detailed Explanation:

✅ Statement 1 – Correct: Continental drift separated landmasses over millions of years, creating geographic barriers that isolated populations and led to allopatric speciation (e.g., unique species in Madagascar and Australia).

✅ Statement 2 – Correct: Glacial cycles caused repeated expansion and contraction of ice sheets, altered sea levels and habitats, and drove evolutionary adaptations, migrations, and extinctions (e.g., Ice Age megafauna evolution).