Physical Geography

Earth has three concentric layers: Crust, Mantle, Core. Crust is thin (5-30 km; oceanic crust ~5-10 km made of SIMA—silica+magnesia, basaltic; continental crust ~30-70 km of SIAL—silica+alumina, granitic). Mantle extends to 2900 km. Core extends to 6371 km centre; outer core is liquid, inner core solid (iron-nickel, NIFE). Memory aid for discontinuities (top to bottom): 'Con-Moho-Rep-Gut-Lehmann.' Conrad (within crust), Mohorovicic/Moho (crust-mantle), Repetti (upper-lower mantle), Gutenberg (mantle-core), Lehmann (outer-inner core). The asthenosphere (upper mantle, 100-400 km) is the source of magma. S-waves cannot pass through the liquid outer core, proving its liquid state—a shadow zone forms between 105 and 142 degrees.

Earthquake (body) waves: Primary (P) and Secondary (S). P-waves are longitudinal/compressional, fastest, travel through solids, liquids and gases. S-waves are transverse, travel ONLY through solids—this is the key UPSC fact. Surface waves (L-waves) are most destructive. Shadow zones: P-wave shadow zone lies between 105 and 142 degrees from the epicentre; S-wave shadow zone is everywhere beyond 105 degrees (a much wider zone). The S-wave shadow zone existence proves the outer core is liquid; P-waves bend (refract) on entering the liquid core, creating their narrower shadow zone. Mnemonic: 'S Stops in Solids only.'

Temperature increases with depth (geothermal gradient ~1 degree C per 32 m near surface, but the rate slows sharply deeper). Sources of internal heat: radioactive decay (uranium, thorium, potassium), primordial heat from Earth's formation, and gravitational/tidal friction. Density rises with depth—from ~2.7 g/cc (crust) to ~13 g/cc (inner core). Pressure also rises with depth. Despite high temperature, the inner core stays solid due to immense pressure. Example: deepest mine (Mponeng, South Africa, ~4 km) and deepest borehole (Kola Superdeep, Russia, ~12.3 km) only scratched the crust, confirming our interior knowledge relies mainly on indirect (seismic) evidence.

Three boundary types: Divergent (constructive)—plates move apart, new crust forms; e.g., Mid-Atlantic Ridge, East African Rift, Iceland. Convergent (destructive)—plates collide; ocean-continent collision = subduction forming trenches + volcanic mountains (Andes); ocean-ocean = island arcs (Japan, Philippines); continent-continent = fold mountains without volcanism (Himalayas from Indo-Australian + Eurasian plates). Transform (conservative)—plates slide past horizontally, crust neither created nor destroyed; e.g., San Andreas Fault. Memory aid: 'DCT — Divergent Creates, Convergent Consumes, Transform Conserves.' Wegener's Continental Drift (1912) proposed Pangaea + Panthalassa; Harry Hess's Sea-Floor Spreading (1960s) and palaeomagnetism provided the mechanism that led to Plate Tectonics theory (1960s, Morgan, McKenzie, Parker).

There are 7 major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic. The Pacific Plate is the largest. The Pacific 'Ring of Fire' is a horseshoe-shaped belt around the Pacific where ~75% of the world's active volcanoes and ~90% of earthquakes occur, caused by subduction along convergent boundaries. The Himalayas formed (and continue rising) due to the northward collision of the Indo-Australian Plate into the Eurasian Plate, closing the Tethys Sea—this is why marine fossils are found in the high Himalayas. Continent-continent collisions produce earthquakes but generally no active volcanism, which is why the Himalayas lack volcanoes.

Wegener's key evidences: (1) Jig-saw fit of South America's east coast and Africa's west coast. (2) Matching geological structures—Caledonian/Appalachian mountains across the Atlantic. (3) Fossil distribution—Mesosaurus (freshwater reptile) in Brazil and South Africa; Glossopteris flora across all southern continents. (4) Placer gold deposits in Ghana traced to Brazil. (5) Tillite deposits (glacial) indicate former Gondwana. Sea-floor spreading evidence: symmetrical magnetic stripes (palaeomagnetism) on either side of mid-ocean ridges, and youngest rocks at the ridge with age increasing away from it. Mnemonic for southern supercontinent: 'Gondwana = South (India, Australia, Antarctica, Africa, South America).' Laurasia = northern.

From surface upward: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere. Memory aid: 'The Strong Man Travels Easily.' Troposphere (0-12 km avg; thicker at equator ~18 km, thinner at poles ~8 km)—all weather occurs here; temperature falls with height (normal lapse rate ~6.5 degrees C per km). Stratosphere (up to ~50 km) contains the ozone layer; temperature RISES with height here (ozone absorbs UV); ideal for jet aircraft. Mesosphere (up to ~80 km)—coldest layer, meteors burn up; temperature falls again. Thermosphere/Ionosphere—reflects radio waves; temperature rises sharply; auroras occur. Tropopause, stratopause, mesopause are transition zones. The boundary where temperature stops falling above the troposphere is the tropopause.

Dry air by volume: Nitrogen 78%, Oxygen 21%, Argon 0.93%, Carbon dioxide ~0.04%, plus trace gases. Memory: 'Nitrogen rules, Oxygen follows.' Most gases concentrate below 32 km. Ozone is concentrated in the stratosphere (15-35 km). Insolation = incoming solar radiation, received as short waves; Earth re-radiates as long-wave (terrestrial/infrared) radiation. The atmosphere is heated more by terrestrial radiation from below than directly by the Sun—hence air near the ground is warmer. Albedo = reflectivity (Earth's average 30%). Factors affecting insolation: angle of incidence (latitude), day length, atmospheric transparency, and solar constant (1.94 cal/cm2/min). Heat budget: incoming = outgoing, keeping Earth's temperature balanced.

Temperature controls: latitude, altitude (decreases ~6.5 degrees C/km), distance from sea (continentality—land heats/cools faster than water), ocean currents, prevailing winds, slope/aspect and cloud cover. Isotherms are lines joining equal temperature. Temperature Inversion: normally temperature decreases with height, but in inversion it INCREASES with height. Conditions favouring inversion: long winter nights, clear cloudless skies, dry air, calm/still air, and snow-covered ground. Valleys experience inversion as cold dense air drains downslope (air drainage)—frost forms in valley bottoms while slopes stay warmer (why orchards/tea are grown on slopes, not valley floors). Inversion traps pollutants, causing smog. Mnemonic for inversion conditions: 'CLeaN DCS — Clear, Calm, Night, Dry, Cold, Snow.'

Seven pressure belts (alternating, from equator to pole): Equatorial Low (doldrums), Sub-tropical Highs (horse latitudes 30 deg), Sub-polar Lows (60 deg), Polar Highs. Equatorial and polar belts are thermally induced; sub-tropical and sub-polar belts are dynamically induced. Planetary/permanent winds blow from high to low pressure: Trade winds (sub-tropical high → equatorial low; NE in N hemisphere, SE in S), Westerlies (sub-tropical high → sub-polar low; the 'Roaring Forties, Furious Fifties, Shrieking Sixties' in S hemisphere), and Polar Easterlies. Coriolis force deflects winds RIGHT in the Northern Hemisphere and LEFT in the Southern (Ferrel's Law), and is zero at the equator, maximum at poles. Winds are named after the direction they blow FROM.

Three main types. (1) Convectional—intense surface heating causes air to rise, cool and condense; common in equatorial regions and summer afternoons; produces heavy thunderstorms (afternoon '4 o'clock rain'). (2) Orographic (relief)—moist air forced up a mountain barrier rises, cools and rains on the WINDWARD side; the leeward side gets a dry 'rain shadow' (e.g., low rainfall in interior Maharashtra leeward of Western Ghats; Mahabaleshwar wet vs. Pune drier). (3) Cyclonic/Frontal—warm and cold air masses meet; warm air rises over cold; common in temperate latitudes. Memory aid: 'COC — Convection rises, Orographic climbs, Cyclonic meets.' The rain shadow effect is a favourite UPSC concept.

Local winds (know their nature—hot or cold): Loo (hot dry, N India summer), Foehn (warm dry, Alps leeward), Chinook ('snow-eater', warm dry, Rockies leeward—beneficial to ranchers), Mistral (cold, France/Spain Rhone valley), Sirocco (hot dusty, Sahara to Mediterranean, 'blood rain'), Bora (cold, Adriatic), Harmattan (dry dusty, West Africa, called 'the Doctor' for relief from humidity). Jet streams are fast, narrow, high-altitude westerly air currents in the upper troposphere; the Sub-tropical Jet and Polar Front Jet steer weather and aircraft routes. The withdrawal of the sub-tropical westerly jet and onset of the Tropical Easterly Jet is linked to the burst of the Indian SW monsoon. Sea breeze blows day (sea→land); land breeze blows night (land→sea).