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Predicted Questions with Model Answers
Q1 (5 marks — 50 words): What is the Mohorovičić discontinuity? State its significance.
Model Answer:
The Mohorovičić discontinuity (Moho), discovered by Croatian geologist Andrija Mohorovičić in 1909, is the boundary between Earth's crust and mantle at approximately 35 km depth under continents (10 km under oceans). Seismic P-wave velocity abruptly increases from ~6 km/s to ~8 km/s here, indicating a density and compositional change from lighter Sial/Sima crust to denser olivine-pyroxene mantle.
Q2 (5 marks — 50 words): Describe the main characteristics of the Carboniferous period.
Model Answer:
The Carboniferous period (359–299 Ma) was characterised by warm, humid conditions and dense coal-forming swamp forests of giant tree ferns (Lepidodendron) and horsetails. Atmospheric oxygen reached 35%, enabling giant insects. The first reptiles evolved. Massive organic accumulation formed the world's major coal deposits (including India's Gondwana coalfields). Pangaea was assembling during this period.
Q3 (5 marks — 50 words): Explain the difference between Sial and Sima with reference to Earth's crust.
Model Answer:
Sial (Silica + Aluminium) forms the continental crust: average density 2.7 g/cm³, thickness 30–70 km, composed of granites and gneisses. Sima (Silica + Magnesium) forms the oceanic crust: density 3.0 g/cm³, thickness 5–10 km, composed of basalts — hence denser and subductable. Sima also underlies the Sial layer beneath continents.
Q4 (10 marks — 150 words): Describe the internal structure of the Earth with reference to its major layers, compositions, and the discontinuities between them.
Model Answer:
Earth's interior, studied primarily through seismic wave behaviour, comprises four major layers:
Crust (0–35 km): The outermost solid layer — continental crust (Sial: Silica-Aluminium, density 2.7 g/cm³, 35 km thick) and oceanic crust (Sima: Silica-Magnesium, density 3.0 g/cm³, 5–10 km thick). The Mohorovičić discontinuity (Moho, ~35 km) marks its base.
Mantle (35–2,900 km): Comprises 84% of Earth's volume. Upper mantle contains the asthenosphere (100–350 km) — partially molten zone enabling plate tectonic movement. Lower mantle is solid perovskite. Temperature: 870–3,700°C. The Gutenberg discontinuity (2,900 km) marks the base.
Outer Core (2,900–5,100 km): Liquid iron-nickel, 3,700–4,300°C. Its convective circulation generates Earth's magnetic field (geodynamo). S-waves cannot travel through it. The Lehmann discontinuity (5,100 km) marks the inner boundary.
Inner Core (5,100–6,371 km): Solid iron-nickel, ~5,500°C, density ~13 g/cm³. Remains solid under extreme pressure (3.6 million atm). Rotates slightly faster than the overlying layers.
Heat sources: Primordial heat + radioactive decay (U-238, Th-232, K-40). Convection currents in the mantle drive plate tectonics — the primary force shaping Earth's surface.
Q5 (10 marks — 150 words): Trace the evolution of life on Earth through the major geological eras, highlighting key biological events and mass extinctions.
Model Answer:
Earth's 4.6-billion-year history records a remarkable evolution of life through four eons:
Precambrian (4,600–541 Ma): First life appeared ~3.7 Ga — prokaryotic bacteria and archaea. The Great Oxidation Event (~2,400 Ma) radically transformed atmosphere when cyanobacteria began photosynthesis, enabling complex life. First eukaryotes (~1,800 Ma); first multicellular organisms (~600 Ma).
Palaeozoic Era (541–252 Ma): Cambrian Explosion (~541 Ma) produced all major animal phyla within millions of years. Marine life diversified through Ordovician, Silurian (first land plants), Devonian (first amphibians). Carboniferous coal forests (359–299 Ma); first reptiles. Era ended with the Permian-Triassic extinction (252 Ma) — "Great Dying" eliminating 96% of marine species.
Mesozoic Era (252–66 Ma): Rapid recovery; dinosaurs dominated from Triassic onwards. First mammals (Triassic), first birds — Archaeopteryx (150 Ma), flowering plants (Cretaceous). K-Pg extinction (66 Ma) — Chicxulub asteroid — eliminated 75% of species including all non-avian dinosaurs.
Cenozoic Era (66 Ma–present): Mammal diversification; India–Asia collision (~50 Ma) built Himalayas. Pleistocene ice ages shaped modern landscapes; Homo sapiens evolved ~300,000 years ago; human civilisation in Holocene (last 12,000 years).