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Geography 3% exam weight

Earth's Interior and the Lithosphere

Part of the TNPSC Group 1 study roadmap. Geography topic geogra-001 of Geography.

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Earth’s Interior and the Lithosphere

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

  • Physical Geography covers four spheres: lithosphere (solid Earth), hydrosphere (water), atmosphere (air), and biosphere (life).
  • Earth’s surface area A = 4πR² ≈ 510.1 million km², where land covers ~29.2% and water ~70.8%.
  • Solar constant S ≈ 1361 W/m² — mean insolation at the top of the atmosphere, perpendicular to the solar beam.
  • Coriolis acceleration a = 2Ωv sin(φ) (Ω = 7.292×10⁻⁵ rad/s) deflects winds right in the Northern Hemisphere and left in the Southern Hemisphere.
  • Pressure belts sit at the Equator (Low), 30° (Subtropical High), 60° (Subpolar Low), and Poles (High); winds between them form the Trade Winds, Westerlies, and Polar Easterlies.
  • Average ocean salinity is ~35‰, peaking near 30° N/S under subtropical highs.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Earth’s Interior and the Lithosphere

The Earth is layered by composition and seismic behaviour. The crust (5–70 km) sits above the mantle (~2900 km thick), which surrounds the core (~3480 km radius). P-waves travel through solids and liquids; S-waves travel only through solids — their shadow zones revealed the liquid outer core.

Wegener’s Continental Drift Theory (1912) was supported by the jigsaw fit of Atlantic coasts, matching Glossopteris fossils across South America, Africa, India, and Antarctica, and identical rock strata. Plate Tectonics extended this idea: seven major plates (Pacific, North American, Eurasian, African, South American, Indo-Australian, Antarctic) interact along divergent, convergent, and transform boundaries.

Atmosphere and Circulation

The atmosphere is divided by temperature profile: troposphere (0–12 km, weather layer), stratosphere (12–50 km, hosts the ozone layer), mesosphere, and thermosphere. Temperature drops at the environmental lapse rate of −6.5 °C/km through the troposphere.

The Coriolis effect, governed by a = 2Ωv sin(φ), deflects moving air: rightward in the Northern Hemisphere, leftward in the Southern Hemisphere. This shapes the Trade Winds (0–30°), Westerlies (30–60°), and Polar Easterlies (60–90°).

Pressure BeltLatitudeWind Deflection (NH)
Equatorial Low0°Calm / ITCZ rain
Subtropical High30°Trade Winds diverge
Subpolar Low60°Westerlies converge
Polar High90°Polar Easterlies diverge

Hydrosphere

Salinity (‰) = (mass of dissolved salts in g / mass of seawater sample in kg) × 1000, averaging ~35‰ globally. Salinity peaks in subtropical high-pressure belts (~30° N/S) where evaporation dominates, and falls near river mouths and polar melt zones. In the Indian Ocean, surface circulation reverses seasonally: the South-West Monsoon Current (summer) and North-East Monsoon Current (winter) — both shaped by the monsoon, not simple rainfall.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanisms and Edge Cases

Seasons arise from Earth’s 23.5° axial tilt, not from changing Sun–Earth distance — aphelion (farthest) actually occurs during Northern Hemisphere summer. Insolation therefore varies with latitude, season, and day length, not with orbital radius alone.

The monsoon is a seasonal reversal of wind direction, driven by differential heating between land and sea. In summer, the Asian landmass heats faster than the Indian Ocean, drawing in moist south-westerly winds that deliver ~75% of India’s annual rainfall. Calling monsoon merely “rainfall” misses the wind-reversal mechanism.

Common Mistakes

  • Misordering atmospheric layers by altitude instead of by temperature gradient — students often swap mesosphere and stratosphere.
  • Treating salinity as uniform: it varies with latitude, evaporation, precipitation, and river inflow.
  • Confusing continental-drift evidence (fossils, coast fit) with plate-tectonic evidence (mid-ocean ridges, seafloor magnetism, GPS plate motion).
  • Stating that Coriolis force deflects winds only in one hemisphere.
  • Equating monsoon with rainfall alone, ignoring the land–sea thermal contrast and pressure shift over Tibet.

Exam Strategy for TNPSC Group 1

Geography contributes ~3% of Prelims (2–4 MCQs) and a larger slice of GS Mains Paper I. High-yield clusters: Himalayan ranges (Greater, Lesser, Shiwaliks), Western and Eastern Ghats, monsoonal rainfall, pressure belts, and Indian Ocean currents. Map-based questions frequently ask identification of rivers, passes, or currents — practice alongside NCERT Class 11 Fundamentals of Physical Geography.

Practice Prompts

  1. Explain why the Subtropical High at 30° produces deserts like the Sahara, linking it to descending air, Coriolis deflection, and evaporation-driven salinity maxima.
  2. Compare Wegener’s evidence for continental drift with post-1960s plate-tectonic evidence, and state why each line of evidence supports a different timescale of Earth behaviour.

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