How the Earth works

The most popular explanation of the universe’s origin, also called the expanding universe hypothesis; the big bang is generally dated to 13.7 billion years before the present. Edwin Hubble gave evidence in 1920 that the universe is expanding.

Put forward by the German philosopher Immanuel Kant and revised by the mathematician Laplace in 1796: the planets formed from a cloud of material around a young, slowly rotating sun.

The oceanic crust averages 5 km thick and the continental crust about 30 km — as much as 70 km in the Himalayan region.

Extends from the Moho discontinuity to a depth of 2,900 km. Its upper part, the asthenosphere (‘astheno’ means weak), reaches about 400 km and is the main source of magma.

Begins at 2,900 km. The outer core is liquid and the inner core solid; mostly nickel and iron, it is sometimes called the nife layer.

Earthquake waves are not recorded between 105° and 145° from the epicentre; beyond 105° no S-waves arrive, so the S-wave shadow zone is much larger than the P-wave one.

Alfred Wegener, a German meteorologist, set out the continental drift theory in 1912. Around 200 million years ago the supercontinent Pangaea began to split, first into Laurasia (north) and Gondwanaland (south).

Hess proposed sea floor spreading in 1961. Plate tectonics divides the lithosphere into seven major and some minor plates; the Mid-Atlantic Ridge is the best-known divergent boundary.

India began moving north about 200 million years ago when Pangaea broke, and collided with Asia about 40–50 million years ago, rapidly lifting the Himalayas.

On gentle gradients streams meander over wide floodplains with natural levees and oxbow lakes; meanders cut into hard rock are called incised or entrenched meanders.

Cirques — deep basins cut by ice at the heads of glacial valleys — are the most common landforms of glaciated mountains.

Barchans are crescent-shaped dunes whose points face downwind. Karst topography takes its name from limestone country in the Balkans by the Adriatic Sea.

Troposphere, stratosphere, mesosphere, thermosphere and exosphere. The stratosphere (to 50 km) holds the ozone layer, which absorbs ultra-violet radiation; the ionosphere (80–400 km) reflects radio waves back to Earth.

The lowest layer: about 13 km high on average — roughly 8 km near the poles and 18 km at the equator, where strong convection carries heat highest.

Incoming solar radiation. The Earth is nearest the sun (147 million km, perihelion) on 3 January and farthest (152 million km, aphelion) on 4 July.

What the Earth absorbs it radiates back, so it neither warms up nor cools down overall; the radiation reflected straight back is the Earth’s albedo.

When air near the ground is cooler than the air above; over polar areas it is normal throughout the year.

The Earth’s rotation deflects winds — to the right in the northern hemisphere. It is named after the French physicist who described it in 1844.

General circulation runs in three cells: the Hadley cell in the tropics, the Ferrel cell (surface westerlies) in middle latitudes and the polar cell (polar easterlies). Air rises at the ITCZ.

Warm water appearing off the coast of Peru in place of the cool Peruvian current; it is linked to pressure changes over the Pacific called the southern oscillation.

Cirrus, cumulus, stratus and nimbus. Cirrus form high up (8,000–12,000 m), thin and feathery; cumulus look like cotton wool.

By origin, rainfall is convectional, orographic (relief) or cyclonic (frontal). Relative humidity is greater over the oceans and least over the continents.

V. Koeppen related values of temperature and precipitation to the distribution of vegetation. His groups: A tropical humid, B dry, C warm temperate, D cold snow-forest and E cold climates.

The rising concentration of greenhouse gases is the most important human effect on climate and is likely to cause global warming.

Four divisions: the continental shelf (the shallowest part), the continental slope, the deep sea plain and the oceanic deeps or trenches, some 3–5 km deeper than the floor around them.

The Indian Ocean averages 35 parts per thousand; the Bay of Bengal is less salty (river water) and the Arabian Sea more (high evaporation). The saltiest: Lake Van (330), the Dead Sea (238), the Great Salt Lake (220).

Spring tides come when the sun, moon and Earth are in a straight line — twice a month, at full and new moon; neap tides about seven days later. The world’s highest tides are in the Bay of Fundy, Canada (15–16 m).

Cold currents usually flow along west coasts in low and middle latitudes; warm currents along east coasts in low and middle latitudes.

Biodiversity — the result of 2.5–3.5 billion years of evolution — is seen at three levels: genetic, species and ecosystem. Areas rich in species are called hotspots.

The IUCN classifies threatened plants and animals into three categories for their conservation.