World & Physical Geography Fundamentals

To find any place on Earth we use a grid of two kinds of lines. Latitudes (parallels) run east–west and tell you how far north or south you are. Longitudes (meridians) run north–south and tell you how far east or west you are. Latitude gives you climate; longitude gives you time. Hold that one sentence and this whole section becomes easy.

• Equator (0°) — the longest parallel; divides Earth into Northern and Southern Hemispheres.
• Tropic of Cancer (23½°N) and Tropic of Capricorn (23½°S) — the outer limits within which the Sun can ever be vertically overhead. Between them lies the Torrid (tropical) Zone.
• Arctic Circle (66½°N) and Antarctic Circle (66½°S) — beyond these lie the Frigid Zones, where the Sun can stay above or below the horizon for 24 hours (the "midnight sun").
• Between the tropics and the polar circles lie the Temperate Zones — the mildest belts, which is why most of the world's population and farming sit there.
• Prime Meridian (0°) — passes through Greenwich, London; the world reference for time (GMT / UTC). The 0° meridian and the 180° meridian together divide Earth into Eastern and Western Hemispheres.

Now the time arithmetic, which the paper loves. The Earth turns 360° in 24 hours. Divide: 15° = 1 hour, and therefore 1° = 4 minutes. Places to the east see the Sun first, so their local time is ahead. So if two towns are 30° apart in longitude, their local times differ by exactly 2 hours.

• Crust — the thin outermost solid shell. It is thicker under continents (roughly 30–40 km, more under mountains) and thinnest under the oceans (about 5 km). Continental crust is granitic and lighter (SIAL); oceanic crust is basaltic and denser (SIMA).
• Mantle — the thickest layer, extending to about 2,900 km depth. Its upper part behaves like a slow, plastic fluid (the asthenosphere) — this is the source of magma and the engine that drags the plates.
• Core — made mainly of iron and nickel (hence called NIFE). The outer core is liquid and generates Earth's magnetic field; the inner core is solid despite enormous heat, because the pressure is even more enormous.

The boundaries between these layers are called discontinuities, because seismic waves suddenly change speed there. Two names are compulsory: the Mohorovicic (Moho) discontinuity separates the crust from the mantle, and the Gutenberg discontinuity separates the mantle from the core. Inside the core, the Lehmann discontinuity marks the outer–inner core boundary.

These three are not separate boxes — they keep turning into one another. This is the rock cycle: magma cools into igneous rock; weathering and erosion break it into sediments that harden into sedimentary rock; heat and pressure convert either into metamorphic rock; and if the heat is extreme enough, everything melts back into magma. Nothing is wasted, only recycled.

The Earth's rigid outer shell (the lithosphere) is broken into about a dozen major plates — Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic — plus many minor ones. They float on the plastic asthenosphere and are dragged by convection currents in the mantle. Almost all earthquakes and volcanoes happen where two plates meet.

• Divergent (constructive) boundary — plates move apart, magma rises and new crust is born. Example: the Mid-Atlantic Ridge; on land, the East African Rift Valley.
• Convergent (destructive) boundary — plates move towards each other. One may subduct under the other, creating ocean trenches, island arcs and volcanoes; where two continents collide, fold mountains rise (the Himalayas).
• Transform (conservative) boundary — plates slide past each other; no crust is made or destroyed, but earthquakes are frequent. Example: the San Andreas Fault in California.

When rocks under strain finally snap, energy is released as an earthquake. The point inside the Earth where the rupture begins is the focus (hypocentre); the point on the surface directly above the focus is the epicentre — and it is here that damage is usually greatest. Magnitude (energy released) is measured on the Richter scale; the felt intensity of shaking on the Mercalli scale. The instrument is a seismograph.

• By activity, volcanoes are active (erupting or likely to, e.g. Etna, Stromboli — "the Lighthouse of the Mediterranean"), dormant (quiet but not dead, e.g. Fujiyama) and extinct/dead (e.g. Kilimanjaro).
• By shape, thin basaltic lava spreads far and builds gently sloping shield volcanoes; sticky, gas-rich lava builds steep composite (cone) volcanoes; a collapsed crater becomes a caldera.
• Most of the world's active volcanoes ring the Pacific Ocean along subduction zones — the famous "Pacific Ring of Fire". India's only active volcano is Barren Island, in the Andaman & Nicobar Islands.

The atmosphere is the blanket of gases held to Earth by gravity. By volume, dry air is about 78% nitrogen and 21% oxygen; the remaining 1% is argon, carbon dioxide (about 0.03–0.04%) and traces of others. Those tiny amounts matter enormously: carbon dioxide, water vapour and ozone are the gases that trap heat and filter ultraviolet rays.

• Insolation is the incoming solar radiation received at the Earth's surface. It is greatest near the Equator (Sun's rays are vertical, spread over a small area) and least at the poles (slanting rays spread over a large area). This single fact drives all pressure belts, winds and climate zones.
• Because of this, Earth has seven pressure belts: the Equatorial Low (the calm "doldrums"), two Sub-tropical Highs (about 30°, the "horse latitudes"), two Sub-polar Lows (about 60°) and two Polar Highs. Air always blows from high to low pressure.
• The three planetary (permanent) winds follow directly: Trade winds blow from the sub-tropical highs towards the Equator (North-East in the northern hemisphere, South-East in the southern); Westerlies blow from the sub-tropical highs towards 60° (fierce in the southern ocean — "Roaring Forties, Furious Fifties, Shrieking Sixties"); and Polar Easterlies blow from the poles towards 60°.
• Local winds are short-lived but heavily asked: the hot dry Loo of the north Indian plains (including Bihar) in May–June; Kalbaisakhi / Nor'westers, the violent pre-monsoon thunderstorms of Bihar, West Bengal and Assam in April–May; the warm Chinook of the Rockies and Foehn of the Alps; the cold Mistral of France; and the dusty Sirocco from the Sahara.

Two circulation systems complete the picture. A cyclone is a low pressure centre with winds spiralling inwards — anticlockwise in the Northern Hemisphere, clockwise in the Southern. It brings cloud, storm and rain. An anticyclone is the opposite: a high pressure centre with winds spiralling outwards, clockwise in the Northern Hemisphere, bringing clear, settled, dry weather. Tropical cyclones are called cyclones in the Indian Ocean, hurricanes in the Atlantic, typhoons in the China Sea and willy-willies near Australia.

• Convectional rainfall — hot air rises, cools, condenses. Happens almost daily in the equatorial belt, typically in the afternoon, with thunder.
• Orographic (relief) rainfall — moist wind is forced up a mountain barrier. The windward slope gets heavy rain; the leeward slope stays dry as a rain shadow. This is why the Western Ghats' coastal side is drenched while the Deccan interior is dry.
• Cyclonic (frontal) rainfall — warm and cold air masses meet, the lighter warm air is forced to rise along the front, and prolonged gentle rain follows. Typical of temperate latitudes; in India, the winter western disturbances work this way.

Oceans cover about 71% of the Earth's surface, which is why our planet is called the "Blue Planet". The Pacific is the largest and deepest, followed by the Atlantic, Indian, Southern and Arctic oceans. The Indian Ocean is the only major ocean named after a country — a useful reminder of India's maritime importance.

• Continental shelf — the shallow, gently sloping submerged edge of a continent. It is the richest fishing ground in the world because sunlight reaches the bottom and plankton thrive; most offshore oil also comes from here.
• Continental slope — the steep drop beyond the shelf; then the vast, flat abyssal (deep sea) plains, which cover most of the ocean floor.
• Trenches (ocean deeps) — the deepest features, formed at subduction zones. The Mariana Trench in the Pacific Ocean holds the deepest known point on Earth (the Challenger Deep, about 11,000 m).
• Salinity — the salt content of sea water, measured in parts per thousand (ppt/‰). The world ocean average is about 35 ppt. Salinity is higher where evaporation is strong and rivers are few (the Red Sea, and the famously salty Dead Sea), and lower where big rivers or melting ice pour in fresh water (the Bay of Bengal, the Baltic).

Ocean currents are rivers within the sea, driven mainly by prevailing winds and by differences in temperature and salinity. Warm currents flow from the equator towards the poles — the Gulf Stream and North Atlantic Drift (which keep north-west European ports ice-free in winter), the Kuroshio and the Brazil current. Cold currents flow from the poles towards the equator — the Labrador, Canary, Benguela, California and Peru (Humboldt) currents. Where a warm and a cold current meet, you get dense fog and superb fishing grounds, as at the Grand Banks of Newfoundland.

• Tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon (and, less strongly, the Sun). The Moon wins despite being far smaller because it is much closer.
• Spring tides are the highest tides, occurring on full moon and new moon, when the Sun, Earth and Moon are in a line and their pulls add up. Neap tides are the lowest, occurring at the first and third quarter moon, when Sun and Moon pull at right angles and partly cancel out.
• Coral reefs are built by tiny marine animals (polyps) and need warm, shallow, clear, salty water — so they occur mainly between the tropics. Three forms: fringing reef (hugging the shore), barrier reef (separated by a lagoon — the Great Barrier Reef off Australia is the world's largest) and atoll (a ring of coral around a lagoon). In India, reefs occur in the Gulf of Mannar, Gulf of Kachchh, Andaman & Nicobar and the Lakshadweep atolls. Rising sea temperature causes coral bleaching.