Physical Geography: Earth, Landforms, Climate and Oceans

The earth's interior

Layer	Depth (approx.)	Features
Crust	0–35 km (oceanic about 5 km, continental about 30–70 km)	SIAL (continents) above SIMA (ocean floor)
Mantle	to 2,900 km	Asthenosphere (partly molten) is the source of magma
Outer core	2,900–5,100 km	Liquid iron–nickel
Inner core	to 6,371 km	Solid, very hot (NIFE)

• Discontinuities: Moho (crust–mantle), Gutenberg (mantle–core) and Lehmann (outer–inner core).
• Seismic waves:
  • P waves travel through solids and liquids and are the fastest.
  • S waves travel through solids only. Their shadow zone shows the outer core is liquid.
  • Surface waves do the most damage.

Plate tectonics, earthquakes and volcanoes

• Theory: Wegener proposed continental drift (1912), with the supercontinent Pangaea and the ocean Panthalassa. Sea-floor spreading was described by Hess (1960s). Together these led to plate tectonics, with 7 major plates.
• Plate boundaries:
  • Divergent: mid-ocean ridges, the East African Rift.
  • Convergent: ocean–continent forms the Andes; ocean–ocean forms the Japanese arcs; continent–continent forms the Himalayas.
  • Transform: the San Andreas Fault.
• Earthquakes:
  • The focus is called the hypocentre; the point above it on the surface is the epicentre.
  • They are measured on the Richter and moment-magnitude scales.
  • Seismic zones of India: zones II to V. Zone V covers the north-east, J&K, Himachal, Uttarakhand, Rann of Kutch, north Bihar and the Andaman and Nicobar Islands.
• Tsunami: caused by undersea earthquakes, as in the Indian Ocean, 26 December 2004. India's Tsunami Early Warning Centre is at INCOIS, Hyderabad.
• Volcanoes:
  • The Pacific Ring of Fire has about three-quarters of the world's active volcanoes.
  • Types: shield (Hawaii), composite (Fuji, Vesuvius) and caldera.
  • India's only active volcano is Barren Island (Andamans).
  • The Deccan Traps were formed by fissure eruptions.

Landforms

Agent	Erosional landforms	Depositional landforms
River	V-shaped valley, gorge, waterfall, meander	Alluvial fan, flood plain, levee, oxbow lake, delta
Glacier	Cirque, U-shaped valley, hanging valley, arête	Moraine, drumlin, esker
Wind	Mushroom rock, yardang	Sand dunes (barchan crescent shape), loess
Sea waves	Cliff, sea cave, arch, stack	Beach, spit, bar, lagoon
Groundwater (karst)	Sinkhole, cave	Stalactite, stalagmite

Atmosphere and climate

• Composition: nitrogen 78%, oxygen 21%, argon 0.93%, carbon dioxide about 0.04% (about 420 ppm).
• Layers:
  • troposphere: weather happens here; temperature falls by 6.5 °C per km (the normal lapse rate);
  • stratosphere: ozone layer, about 15–35 km;
  • mesosphere: meteors burn up;
  • thermosphere / ionosphere: radio waves are reflected;
  • exosphere.
• Insolation and heat balance: the earth's albedo is about 30%. Temperature inversion causes valley fog in winter.
• Pressure belts and winds:
  • The belts are the equatorial low (the doldrums or ITCZ), subtropical highs (horse latitudes), subpolar lows and polar highs.
  • The planetary winds are the trade winds, westerlies ("roaring forties") and polar easterlies.
  • Coriolis force deflects winds to the right in the northern hemisphere. It is zero at the equator.
• Jet streams: the subtropical westerly jet and the tropical easterly jet both affect the Indian monsoon.
• Cyclones:
  • Tropical cyclones form over warm seas (above 27 °C), at 5–30° latitude. In the northern hemisphere they turn anticlockwise. The calm eye is at the centre.
  • Names: hurricanes in the Atlantic, typhoons in the West Pacific, and cyclones in the Indian Ocean (names given by WMO/ESCAP panel countries, including India).
  • Temperate (extratropical) cyclones form along fronts. Western disturbances bring winter rain to north India.
• Climate drivers of the monsoon:
  • El Niño: the eastern Pacific warms, usually weakening the monsoon.
  • La Niña: the opposite.
  • Indian Ocean Dipole: a positive IOD helps the monsoon.
  • The MJO.
• Köppen climate classification: India has Amw, Aw, BWhw, Cwg and other types.

Oceans

• Relief of the ocean floor: continental shelf, slope, abyssal plain and trenches. The Mariana Trench (Challenger Deep, about 11 km) is the deepest point.
• Salinity averages 35‰. The Dead Sea and Lake Van are very salty; the Baltic Sea has low salinity.
• Ocean currents:
  • Warm: the Gulf Stream and North Atlantic Drift (which warm north-west Europe), Kuroshio, Agulhas, Brazil.
  • Cold: Labrador, California, Humboldt/Peru, Benguela, Canaries.
  • Where cold and warm currents meet, there are rich fishing grounds (the Grand Banks off Newfoundland).
• Tides:
  • Spring tides happen at new and full moon, when the Sun, Moon and Earth are in a line.
  • Neap tides happen at the quarter moons.
  • The Gulf of Khambhat and the Gulf of Kutch have India's highest tidal range, which gives them tidal energy potential.
• Coral reefs: need warm (above 20 °C), clear, shallow water. Coral bleaching comes from heat stress. India has reefs in the Gulf of Mannar, Gulf of Kutch, Lakshadweep and the Andamans.

Exam tip: UPSC likes "why" questions: why the outer core is liquid, why cyclones avoid the equator, why west coasts in the trade-wind belt have deserts (cold currents plus offshore winds). Learn the mechanism, not just the name.