Hydrosphere — Ocean Currents · Tides · Lakes · Rivers Soils — Types · Formation · Distribution Biomes — Natural Vegetation Zones · World Biomes
Earth's Interior Structure
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Layers of the Earth
Crust · Mantle · Core — composition and characteristics
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6,371 km
Earth's Mean Radius
5–70 km
Crust Thickness
2,900 km
Mantle Thickness
3,471 km
Core Radius
Three Main Layers
Crust (Sial + Sima): Outermost layer; 5–70 km thick; oceanic crust (5–10 km, basaltic, denser — Sima: Silica + Magnesium) and continental crust (30–70 km, granitic, lighter — Sial: Silica + Aluminium); boundary with mantle = Mohorovičić Discontinuity (Moho)
Mantle: 2,900 km thick; divided into Upper Mantle (includes asthenosphere — partially molten, plastic) and Lower Mantle (solid); rich in iron, magnesium, silicon; Gutenberg Discontinuity separates mantle from outer core
Core: Outer Core (liquid; iron-nickel; 2,270 km thick; generates Earth's magnetic field) and Inner Core (solid; iron-nickel; 1,220 km radius; hottest — ~5,100–6,000°C); Lehmann Discontinuity separates outer from inner core
Key Discontinuities — Most Tested
Conrad Discontinuity — within the continental crust (between upper granitic and lower basaltic crust)
Mohorovičić (Moho) Discontinuity — between crust and mantle
Repetti Discontinuity — within the upper mantle
Gutenberg Discontinuity — between mantle and outer core (~2,900 km depth)
Lehmann Discontinuity — between outer core and inner core
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Plate Tectonics, Earthquakes & Volcanoes
Continental Drift · Types of plate boundaries · Seismic waves
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Continental Drift Theory (Alfred Wegener, 1912): All continents were once joined as a single landmass called Pangaea, surrounded by Panthalassa Ocean. Pangaea broke up ~200 million years ago into Laurasia (northern continents) and Gondwanaland (southern continents). Modern Plate Tectonics Theory explains the mechanism through seafloor spreading and convection currents in the mantle.
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Convergent (Destructive) Boundary
Plates move towards each other. Results: fold mountains (when two continental plates collide — e.g., Himalayas from collision of Indian and Eurasian plates); ocean trenches and volcanic arcs (oceanic plate subducts under continental — e.g., Andes Mountains); deepest ocean trenches here.
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Divergent (Constructive) Boundary
Plates move apart. Results: mid-ocean ridges (e.g., Mid-Atlantic Ridge — largest mountain range in the world, submerged); rift valleys on land (e.g., East African Rift Valley); new oceanic crust formed; shallow earthquakes; basaltic volcanic activity.
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Transform (Conservative) Boundary
Plates slide horizontally past each other. Results: strike-slip faults; no new crust created or destroyed; major earthquakes but no volcanoes. Example: San Andreas Fault (Pacific and North American plates, California).
Seismic Waves — Types & Properties
P-waves (Primary/Compressional): Fastest; travel through solids, liquids, and gases; longitudinal; arrive first at seismographs; can travel through the core
S-waves (Secondary/Shear): Slower than P-waves; travel only through solids (NOT through liquids); transverse; do NOT travel through the outer core (liquid) — this is how we know the outer core is liquid
Surface waves (L-waves): Slowest; most destructive; travel along Earth's surface; Love waves and Rayleigh waves; cause most structural damage
Shadow Zone: Area on Earth's surface that receives no direct P-waves or S-waves from an earthquake; P-wave shadow zone: 103°–143°; S-wave shadow zone: beyond 103°
Types of Volcanoes
Shield Volcano: Broad, gently sloping; low-viscosity basaltic lava; effusive eruptions; e.g., Mauna Loa, Hawaii (largest volcano on Earth by volume)
Composite (Stratovolcano): Steep-sided, conical; alternating lava and ash; explosive; e.g., Mt. Fuji (Japan), Mt. Vesuvius (Italy), Mt. St. Helens (USA)
Caldera: Large depression formed when magma chamber collapses after eruption; e.g., Yellowstone Caldera (USA)
Pacific Ring of Fire: 75% of world's active volcanoes and 90% of earthquakes; horseshoe-shaped zone around the Pacific Ocean
⚑ Key Exam Facts — Plate Tectonics
Himalayas formed by collision of Indian Plate and Eurasian Plate — both continental → fold mountains, no subduction, no volcanoes
Deepest ocean trench: Mariana Trench (~11,034 m), Pacific Ocean — formed at convergent boundary (Pacific Plate subducting under Philippine Plate)
Mid-Atlantic Ridge = divergent boundary; explains why South America and Africa fit together
S-waves CANNOT travel through liquids — proof that outer core is liquid
Atmosphere — Layers, Composition & Pressure Belts
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Layers of the Atmosphere
Troposphere to Exosphere — altitudes and characteristics
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Layer
Altitude
Key Characteristics
Troposphere
0–12 km (avg)
All weather occurs here; temperature decreases with altitude (lapse rate ~6.5°C/km); tropopause = upper limit; thickest at equator (18 km), thinnest at poles (8 km)
Stratosphere
12–50 km
Contains ozone layer (15–35 km) — absorbs UV radiation; temperature increases with altitude (due to ozone absorption); calm air — ideal for jet aircraft
Mesosphere
50–80 km
Temperature decreases with altitude; coldest layer of atmosphere (~−90°C at mesopause); meteors burn up here; noctilucent clouds visible
Thermosphere (Ionosphere)
80–700 km
Temperature increases sharply; very thin air; aurora borealis and australis here; ionised layers reflect radio waves (enables long-distance radio communication); International Space Station orbits here
Exosphere
700+ km
Outermost layer; merges with outer space; hydrogen and helium; satellites orbit here
Atmospheric Composition (Dry Air)
Nitrogen (N₂): 78.09% — most abundant; inert; dilutes oxygen
Oxygen (O₂): 20.95% — supports combustion and life
Argon (Ar): 0.93% — noble gas; inert
Carbon Dioxide (CO₂): ~0.04% — greenhouse gas; critical for photosynthesis
Water vapour: Variable 0–4%; most important greenhouse gas; drives weather
Ozone (O₃): Trace amounts; concentrated in stratosphere 15–35 km; absorbs UV-B and UV-C
Equatorial Low Pressure Belt (ITCZ): 0°–5° N & S; intense solar heating; rising air; heavy convectional rainfall; calm winds (Doldrums)
Subtropical High Pressure Belt: ~30° N & S; descending air from Hadley cells; dry, hot; deserts located here (Sahara, Arabian, Atacama, Kalahari, Australian); Horse Latitudes
Sub-polar Low Pressure Belt: ~60° N & S; meeting of warm westerlies and cold polar air (Polar Front); rising air; cyclonic activity; temperate cyclones
Polar High Pressure Belt: 90° N & S; extremely cold, dense, descending air; very little precipitation; polar climate
Planetary Wind Belts
Trade Winds: Blow from subtropical high (30°) to equatorial low (0°); NE Trade Winds (N hemisphere), SE Trade Winds (S hemisphere); steady and reliable; used by early sailors
Westerlies: Blow from subtropical high (30°) to sub-polar low (60°); SW Westerlies (N hemisphere), NW Westerlies (S hemisphere); "Roaring Forties," "Furious Fifties," "Screaming Sixties" in Southern Ocean
Polar Easterlies: Blow from polar high (90°) to sub-polar low (60°); NE in N hemisphere, SE in S hemisphere; cold, dry winds
Jet Streams: Fast-moving air currents 9–16 km altitude in upper troposphere/lower stratosphere; affect aircraft routing and surface weather patterns
⚑ Ferrel's Law & Coriolis Effect
Due to the Coriolis Effect (Earth's rotation), winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is Buys Ballot's Law / Ferrel's Law. Cyclones rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Anticyclones rotate in the opposite direction.
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Clouds, Precipitation & Humidity
Cloud types · Types of rainfall · Humidity measures
Vertical clouds: Cumulus (fair weather, cauliflower tops), Cumulonimbus (thunderstorm cloud — extends from low to great heights; anvil top; lightning, hail, tornadoes)
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Orographic (Relief) Rainfall
Moisture-laden winds rise against a mountain barrier; cool, condense, and precipitate on the windward slope. The leeward side receives little rainfall — called the rain shadow zone. Example: Western Ghats (windward) vs Deccan Plateau (rain shadow); Cherrapunji/Mawsynram (India) — wettest places.
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Convectional Rainfall
Intense solar heating causes air to rise rapidly; cools at upper levels; condensation and heavy precipitation. Typical of equatorial regions (daily afternoon thunderstorms) and continental interiors in summer. Accompanied by thunder and lightning.
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Cyclonic (Frontal) Rainfall
Occurs along fronts where warm and cold air masses meet; warm air rises over cold air. Warm front: gradual rainfall before the front. Cold front: sudden heavy rainfall as cold air undercuts warm air. Common in temperate regions (westerly belt, 40°–60°).
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Tropical & Temperate Cyclones
Hurricanes · Typhoons · Nor'westers · Loo — formation and effects
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Tropical Cyclone
Forms over warm tropical oceans (sea surface temp >26°C); intense low pressure; strong spiralling winds; eye (calm centre, ~20–50 km wide); eye wall (most violent zone). Called: Hurricane (Atlantic/E Pacific), Typhoon (W Pacific), Cyclone (Indian Ocean/S Pacific), Willy-Willy (Australia). Anticlockwise in N hemisphere; clockwise in S hemisphere.
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Temperate Cyclone (Extra-tropical)
Forms along polar fronts between 35°–65° latitude; less intense; no eye; wider (~1,000 km); associated with westerlies; bring cold/wet conditions; cold/warm fronts. Responsible for rainfall in western Europe, NW India (winter Western Disturbances), and coastal temperate zones.
Local Winds — Frequently Tested
Loo: Hot, dry wind; North India plains (NW India) in summer (May–June); temperatures above 45°C
Nor'wester (Kalbaisakhi): Pre-monsoon thunderstorm wind; Bengal and Assam; accompanied by lightning; beneficial for jute and tea crops
Foehn: Warm, dry wind on leeward side of Alps (Switzerland/Austria)
Sirocco: Hot, dusty wind from Sahara Desert blowing towards Mediterranean
Mistral: Cold, dry wind; Rhône Valley, France; blows towards Mediterranean
Harmattan: Hot, dry, dusty wind; blows from Sahara towards West African coast (Gulf of Guinea)
Bora: Cold, dry NE wind; Adriatic coast (Slovenia, Croatia)
Hydrosphere — Ocean Currents & Tides
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Ocean Currents — Warm & Cold
Atlantic · Pacific · Indian Ocean — effects on climate
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How Ocean Currents Flow
Ocean currents in the Northern Hemisphere flow in a clockwise direction (gyre); in the Southern Hemisphere they flow anticlockwise. This is because of the Coriolis Effect. Warm currents flow FROM equator TOWARDS poles; cold currents flow FROM poles TOWARDS equator. Warm currents warm the coastal climates; cold currents cool them and cause fog, aridity, and fishing grounds (upwelling).
Ocean
Warm Currents
Cold Currents
Atlantic (N)
Gulf Stream → North Atlantic Drift (warms W Europe); Florida Current; Brazil Current (S)
Labrador Current (freezes NE Canada); Canaries Current (off W Africa); Benguela Current (S)
Atlantic (S)
Brazil Current (off E South America)
Benguela Current (off SW Africa)
Pacific (N)
Kuroshio (Japan Current — warms Japan); North Pacific Drift; Alaska Current
California Current (off W USA); Oyashio Current (off NE Japan)
Pacific (S)
East Australian Current
Humboldt/Peru Current (off W South America — upwelling; rich fishing; creates Atacama Desert)
Indian Ocean
Mozambique Channel Warm; Agulhas Current (S Africa)
Somali Current (seasonal); West Australian Current
Key Effects of Ocean Currents
Moderating effect: North Atlantic Drift keeps NW Europe (UK, Norway) warmer than expected for their latitude
Deserts on west coasts: Cold currents (Benguela → Namib Desert; Humboldt → Atacama Desert; Canaries → Sahara coast) cause aridity — cool, stable air, no rainfall
Fogs: Where warm and cold currents meet — e.g., Grand Banks (Labrador + Gulf Stream) — famous for dense fog and rich fisheries (cod)
Fishing grounds: Cold currents cause upwelling of nutrient-rich deep water — Grand Banks, Newfoundland; Humboldt Current (world's most productive fishery)
El Niño: Periodic warming of central/eastern Pacific — disrupts normal current patterns; causes droughts in SE Asia, Australia; floods in Peru; warm water replaces cold Humboldt Current
La Niña: Opposite of El Niño — stronger than normal cold current; intensified rainfall in SE Asia, drought in South America
Landforms — Formation & Classification
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Mountains, Plateaus & Plains
Types of mountains · Erosional vs depositional landforms
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Fold Mountains
Formed by compressional forces when tectonic plates collide; rocks buckle into folds. Examples: Himalayas (Asia), Alps (Europe), Rockies (N America), Andes (S America). Youngest and highest mountains. Anticlines (upfolds) and synclines (downfolds).
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Block (Fault-block) Mountains
Formed when crustal blocks are uplifted or tilted along faults. The uplifted block = Horst; the depressed block = Graben (rift valley). Examples: Vosges (France), Black Forest (Germany), Sierra Nevada (USA), Vindhyas, Satpura (India — relict block mountains).
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Volcanic Mountains
Built by accumulation of volcanic material (lava, ash). Examples: Mt. Kilimanjaro (Africa's highest — dormant), Mt. Fuji (Japan), Mt. Etna (Sicily — active, Europe's highest active volcano), Mt. Vesuvius (Italy). Hawaii = volcanic island chain.
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Plateaus (Tablelands)
Elevated flat-topped areas with steep sides. Lava plateaus: Deccan Plateau (India), Columbia Plateau (USA). Dissected plateaus: Chota Nagpur (India). Intermontane plateaus: Tibetan Plateau (highest in world — "Roof of the World," avg 4,500 m), Bolivian Plateau. Piedmont plateaus: Piedmont (USA), Patagonia (Argentina).
Glacial Landforms
Cirque (Corrie/Cwm): Amphitheatre-shaped depression at head of glacier — formed by glacial erosion
Arête: Sharp rocky ridge between two cirques
Horn (Pyramidal Peak): Pointed peak formed when three or more cirques erode a mountain from multiple sides; e.g., Matterhorn (Alps)
U-shaped valley: Carved by glacier (vs V-shaped = river erosion)
Fjord: Drowned U-shaped valley; found in Norway, NZ, Canada
Moraine: Accumulation of rock debris by glaciers; terminal, lateral, medial, and ground moraines
Drumlin: Elongated oval hill formed under ice sheets; streamlined in direction of ice flow
Esker: Sinuous ridge of sediment deposited by meltwater rivers under a glacier
Seasonal rainfall; tall grasses with scattered trees (acacia); Llanos (Venezuela), Campos (Brazil), African Savanna; wildlife: elephants, lions, zebra
Hot Desert
Subtropical high pressure (~30°)
Arid; Aeolian (wind) processes; Sahara (largest hot desert), Arabian, Thar, Atacama (driest), Australian; sandy (erg) and rocky (hamada)
Mediterranean
Subtropical (30°–45° W coasts)
Dry summer, wet winter; scrub vegetation (maquis/chaparral); terra rossa soils; olives, citrus, grapes; Mediterranean, California, SW Australia
Chernozem vs Laterite — Key Contrast
Chernozem (Black Earth): Temperate grasslands; formed by accumulation of organic matter (humus); most fertile soil in the world; found in Prairies (USA, Canada) and Steppes (Ukraine, Russia); ideal for wheat cultivation
Laterite Soil: Tropical humid regions; heavy leaching removes silica and bases; iron/aluminium oxides concentrated; reddish colour; LOW fertility (paradox — dense forest but poor soil); hardens on exposure (used as building material)
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Köppen Climate Classification
A, B, C, D, E groups — the most widely used system
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Group
Type
Key Characteristics & Examples
A — Tropical
Af, Am, Aw
No cold season; all months >18°C; Af = Tropical rainforest (year-round rain); Am = Monsoon; Aw = Tropical savanna (dry season). Amazon, Congo, SE Asia, India
B — Arid/Dry
BWh, BWk, BSh, BSk
Evaporation > Precipitation; BW = desert, BS = steppe; h = hot, k = cold. Sahara (BWh), Arabian, Gobi (BWk), Thar
C — Temperate/Mesothermal
Cfa, Cfb, Csa, Csb
Warmest month >10°C; coldest -3° to 18°C; Cf = no dry season; Cs = dry summer (Mediterranean); Csa = Mediterranean hot summer (Spain), Cfb = oceanic (UK, NW Europe)
D — Continental/Microthermal
Dfa, Dfb, Dfc, Dfd
Coldest month <-3°C; warmest >10°C; severe winters; continental interiors; Df = no dry season; Taiga (Dfc), Chicago (Dfa). Russia, Canada, NE USA
E — Polar/Arctic
ET, EF
No warm season; ET = Tundra (warmest month 0–10°C); EF = Ice cap (all months below 0°C). Greenland, Antarctica, Arctic Canada
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Types of Rocks
Igneous · Sedimentary · Metamorphic — the rock cycle
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Igneous Rocks
Formed by cooling and solidification of magma/lava. Intrusive (plutonic): cooled slowly underground; coarse-grained; granite (most common continental rock), diorite, gabbro. Extrusive (volcanic): cooled rapidly at surface; fine-grained; basalt (most common oceanic rock), rhyolite, obsidian, pumice. No fossils.
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Sedimentary Rocks
Formed by accumulation and lithification of sediments. Arranged in strata (layers). Contain fossils — most important for palaeontology. Examples: limestone (most abundant sedimentary rock — karst topography), sandstone (Red Fort material), shale, coal (sedimentary), conglomerate. Cover ~75% of Earth's surface.
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Metamorphic Rocks
Formed by transformation of existing rocks under extreme heat and pressure. Examples: Marble (from limestone — Taj Mahal), Quartzite (from sandstone), Slate (from shale), Schist, Graphite, Diamond (metamorphic forms of carbon). Gneiss (from granite). Typically found in Himalayan and Alpine zones.
⚑ Rock Facts — Common Exam Traps
Coal is a sedimentary rock (NOT igneous or metamorphic)
Diamond and Graphite are both forms of carbon — diamond is metamorphic; graphite is also metamorphic
Marble is metamorphic (from limestone) — NOT sedimentary
Most fossils found in sedimentary rocks only
Granite is intrusive igneous; Basalt is extrusive igneous
Oceanic crust is mainly basaltic (denser); Continental crust is mainly granitic (lighter)