Physical Geography: Earth, Rocks, Atmosphere and Oceans

Latitude and longitude

Latitude and longitude form the geographical grid by which every point on the Earth's surface is located. Latitude is the angular distance of a place north or south of the Equator, measured from the Earth's centre, and it ranges from 0^\circ at the Equator to 90^\circ at the poles. Lines of latitude, called parallels, run east to west around the globe. Longitude is the angular distance of a place east or west of the Prime Meridian, which is at 0^\circ, and it ranges up to 180^\circ. Lines of longitude, called meridians, run north to south from the North Pole to the South Pole.

Each parallel is a circle whose radius shrinks toward the poles, so parallels become shorter as latitude increases. The Equator is therefore the longest parallel, and the 90^\circ latitude shrinks to a single point of zero length at the North Pole. For the same reason, the distance between meridians is greatest at the Equator and falls to zero at the poles, where all meridians meet. Counted in degrees, there are 360 meridians but only 180 degrees of latitude from pole to pole, so there are more longitude lines than latitude lines.

A single coordinate fixes only a line on the surface, either a parallel or a meridian. Only latitude and longitude taken together fix a unique point, and altitude is not needed to plot a place on a flat map.

The Earth turns through 360^\circ in about 24 hours, so every 15^\circ of longitude corresponds to one hour of time, and every degree corresponds to four minutes. A place east of another has a later local time. To find a time difference, divide the longitude difference by 15. For example, if a meridian at 82.5^\circE has a given time, a meridian 75^\circ further west, which is five hours behind, lies at 7.5^\circE.

Standard time zones are offsets from Greenwich Mean Time (GMT). India and Sri Lanka both use GMT + 5{:}30, Nepal uses GMT + 5{:}45, and Bhutan uses GMT + 6{:}00, so Bhutan has the largest offset among these four. The time difference between two places is the difference between their standard offsets. Since India is at GMT + 5{:}30 and London uses GMT, when it is 12 noon in New Delhi, it is 6:30 A.M. in London. Daylight saving changes the London offset in summer, so an examination question must always state its reference.

A higher latitude means a place lies farther north. Among the cities, New Delhi, at about 28.6^\circN, lies farther north than Kathmandu and Thimphu, which are at about 27.5^\circN to 27.7^\circN, and Dhaka, at about 23.8^\circN.

The Tropic of Cancer lies at about 23.5^\circN. In India it passes through Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. Manipur lies entirely north of this latitude, so the Tropic of Cancer does not pass through it.

The International Date Line follows roughly the 180^\circ meridian, but it bends in places to avoid land. Its deviations occur in the Pacific Ocean, for example near the Bering Strait and Fiji, not in the Atlantic. Crossing the line eastward moves the calendar back by one day, so a full 24 hours are lost. Crossing it westward moves the calendar forward by one day, so 24 hours are gained. The date to the east of the line is therefore one day earlier than the date to the west.

The Prime Meridian, at 0^\circ longitude, crosses western Africa. It passes through Algeria, Burkina Faso, Mali, Ghana and Togo. Morocco lies entirely west of the meridian, so the Prime Meridian does not pass through it.

Earth's interior and earthquakes

The Earth consists of the crust, the mantle, the outer core and the inner core. The lithosphere is the rigid outer shell, made of the crust together with the uppermost part of the mantle, and it lies above the weaker, partly plastic asthenosphere. The lithosphere is thickest beneath large continental mountain belts, where the crust and its root are thickened by collision, so the Himalayas have the greatest lithospheric depth among common examples. Oceanic lithosphere is much thinner than continental lithosphere.

The crust is a brittle solid that fractures under stress, and this brittleness is the basis of faulting and earthquakes. The mean thickness of the oceanic crust is only about 7 km, whereas the continental crust is much thicker, averaging roughly 30 to 40 km. Oceanic crust is basaltic and denser than the granitic continental crust, so it is heavier. The mantle makes up most of the Earth's volume and holds most of its internal heat. Convection cells operate in the mantle and not in the crust, and they drive plate-tectonic movement.

A discontinuity is a boundary inside the Earth where seismic velocities change sharply. The Mohorovičić discontinuity, or Moho, separates the crust from the upper mantle. The Gutenberg discontinuity lies at the mantle-outer core boundary, at about 2,900 km depth. The Conrad discontinuity lies within the continental crust. The Lehmann discontinuity lies at about 5,150 km and separates the outer core from the inner core, which is the innermost part of the Earth.

An earthquake originates at the focus, also called the hypocentre, which is a point inside the Earth. The point on the surface directly above the focus is the epicentre. Seismic waves radiate outward from the focus. Shallow-focus earthquakes release their energy close to the surface and cause more damage than deep-focus earthquakes. The speed of seismic waves increases with the density and elasticity of the medium, so waves travel faster in denser materials.

P waves are longitudinal, the fastest seismic waves, and the first to reach the surface. They can travel through solids, liquids and gases. Seismographs located beyond about 145^\circ from the epicentre can record P waves. The P-wave shadow zone is a ring-shaped band at intermediate distances from the epicentre, roughly between 105^\circ and 145^\circ. S waves cannot pass through the liquid outer core, so the S-wave shadow zone begins at about 105^\circ and covers the entire far side of the Earth, which makes it much larger than the P-wave shadow zone. The area between 105^\circ and 145^\circ therefore lies in the shadow zone of both wave types.

Rock and mineral processes

Rocks are continually transformed over time. Weathering breaks them down, erosion removes and transports the debris, and metamorphism alters rocks under heat and pressure, so rocks do not stay in their original form for long.

Several chemical and physical processes change minerals and rocks, and each has a distinct name. Hydration is the chemical addition of water to a mineral's structure. Some minerals take up water, expand and change form, as anhydrite does when it converts to gypsum. Oxidation is the combination of a substance with oxygen, hydrolysis is the breakdown of a compound by reaction with water ions, and carbonation is the reaction of a mineral with carbon dioxide. None of these describes simple uptake of water with expansion, so hydration is the term to use for that process.

Igneous intrusions are named by shape and size. A batholith is a very large, deep-seated mass of intrusive igneous rock that solidifies slowly within the crust. Its upper portion is often dome-shaped, and it is exposed only after long erosion. A laccolith is a smaller intrusion with a flat base and a domed top, formed between sedimentary layers. A lopolith is saucer-shaped and sinks downward like a basin, and a phacolith is a lens-shaped body that lies along the crests or troughs of folded rock layers.

Metamorphic rocks develop foliation when platy or elongated minerals are segregated into bands or layers under directed pressure. A foliated rock with broad mineral bands, such as gneiss, is strong and resistant. A rock with thin foliation, such as slate, splits along cleavage planes into thin flakes. Rocks made of a single dominant mineral, such as quartzite from sandstone and marble from limestone, recrystallize into interlocking granular mosaics without a preferred orientation, so they are generally not foliated.

Igneous rocks form by the cooling and solidification of magma or lava. Granite, basalt and gabbro are all igneous. Slate is metamorphic, formed from shale under low-grade regional metamorphism, so it is not an igneous rock.

Sedimentary rocks are classified by the process that forms them. Chert forms by the chemical precipitation of silica, and shale forms by the mechanical compaction of fine clay and silt. Geyserite is siliceous sinter deposited from hot spring water, and in the classification used for this question it is grouped under the organic process, because its silica is built up with the help of microbial and algal mats.

Atmospheric layers and composition

The atmosphere is divided into concentric layers on the basis of how temperature changes with altitude. Counting upward from the surface, the layers are the troposphere, stratosphere, mesosphere and thermosphere, and the exosphere lies beyond them as the outermost layer. This order must be memorised exactly, because the layers are defined by temperature behaviour, not by composition.

The troposphere is the lowest layer. It extends to about 8 km over the poles and about 16 to 18 km over the equator, and it holds nearly all of the atmosphere's water vapour and aerosols. Clouds, rain, snow and winds all form here, so every weather phenomenon occurs within the troposphere. Temperature falls with height in the troposphere at an average rate of about 1°C per 165 m, which is the same as about 1°F per 300 ft. For example, air at 30°C at the surface would be near 12°C at 3 km under this average rate. The tropopause, the upper boundary of the troposphere, is thickest and coldest over the equator and comparatively warmer over the poles. A common error is to pair the figure 165 m with 1°F; the 165 m figure belongs to 1°C.

The stratosphere lies above the troposphere, extends to about 50 km, and is largely free of weather. Its temperature rises with height because the ozone layer absorbs ultraviolet radiation. The mesosphere lies above it, reaches up to about 80 to 85 km, and is the layer where temperature falls again with height. The thermosphere lies above the mesosphere, has very thin air and high temperatures, and contains the ionosphere. The exosphere is the outermost layer, with extremely low gas density, and its gases are dominated by the lightest elements, hydrogen and helium, which can escape into space.

Near the surface, dry air is about 78 per cent nitrogen and 21 per cent oxygen, with argon near 0.9 per cent. Trace gases such as neon and carbon dioxide make up the remainder. Greenhouse gases absorb infrared radiation emitted by the Earth's surface and re-emit it, which warms the lower atmosphere. The main greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone. Oxygen is a symmetric diatomic molecule that does not absorb infrared radiation appreciably, so it is not a main greenhouse gas.

Aurora forms when charged particles of the solar wind are guided along the Earth's magnetic field lines towards the polar regions, where they collide with atmospheric gases. Oxygen emits green and red light and nitrogen emits blue and red light, so each gas produces its own colour. The display is therefore seen mainly at high latitudes, and its causes can be explained entirely by the magnetic guidance of the particles.

Tides and ocean movements

Tides are the periodic rise and fall of sea level caused mainly by the gravitational attraction of the Moon and the Sun on the oceans. They must be distinguished from ocean currents, which are continuous horizontal movements of water, from waves, which are surface disturbances driven mainly by wind, and from tsunamis, which are long waves generated by undersea earthquakes or landslides.

When the Sun, Earth and Moon lie approximately in a straight line, the arrangement is called syzygy. Syzygy occurs at conjunction, when the Moon lies between the Sun and Earth at new moon, and at opposition, when the Earth lies between the Sun and Moon at full moon. In both cases the pulls of the Sun and Moon reinforce each other and produce spring tides with the largest tidal range. The word spring here refers to the rising water, not to the season.

When the Moon is at right angles to the Sun as seen from the Earth, which corresponds to the first and third quarter phases, the two tide-generating forces partly cancel. These quadrature positions produce neap tides with a small tidal range. Neap tides recur about every 14 to 15 days, which is half a lunar cycle, and both neap and spring tides are linked to the phases of the Moon.

The height of wind-generated waves depends on three factors: wind speed, the duration for which the wind blows, and the fetch, which is the distance of open water over which the wind acts. Wave energy per unit crest length is given by E=\frac{1}{8}\rho g H^2, so energy is proportional to the square of the wave height H. Waves lose little energy in deep water, which allows swell to travel thousands of kilometres across an ocean.

Ocean currents are classed as warm or cold. Cold currents flow along the western margins of continents in tropical and subtropical latitudes and carry cold water towards the equator. Examples are the Peru (Humboldt), Benguela and West Australian currents. Warm currents, such as the East Australian Current, flow poleward. The East Australian Current is a warm western boundary current in the South Pacific, and it flows southward along the eastern coast of Australia.

Cyclones and anticyclones

A cyclone is a low-pressure system in which winds converge towards a central low and the air rises, while an anticyclone is a high-pressure system. At the centre of an anticyclone the air subsides, warms adiabatically and suppresses cloud formation, and the winds diverge outward from the centre. Because rising air cools and condenses, cyclones bring cloudy and rainy weather, whereas anticyclones are associated with fair weather.

Because of the Coriolis force, moving air is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In the Northern Hemisphere, cyclones rotate anticlockwise and anticyclones rotate clockwise. The sense is reversed in the Southern Hemisphere, where cyclonic circulation is clockwise.

Tropical cyclones form over warm tropical oceans, mainly in summer and autumn, in all tropical oceans except the South Atlantic and the eastern South Pacific. Their regional names depend on the ocean basin. They are called hurricanes in the North Atlantic and eastern North Pacific, typhoons in the western North Pacific, and cyclones in the Indian Ocean and the South Pacific.

Several conditions favour the formation and intensification of tropical storms. These include warm sea surface temperatures, a Coriolis force that is large enough to start cyclonic rotation (which is why storms do not form at the equator), upper-level divergence that draws air away from the top of the column, and low vertical wind shear. High variation of wind speed with height disrupts the storm and prevents its growth.

When a tropical cyclone makes landfall, its destructive winds damage houses and other structures, its storm surge and heavy rain cause flooding, and the destruction of crops can lead to food shortages in the affected region.

Tectonic plates and tectonic activity

The lithosphere is divided into seven major tectonic plates: the Pacific, Indo-Australian, Eurasian, African, North American, South American and Antarctic plates. Several smaller plates, such as the Nazca, Cocos, Caroline, Philippine and Arabian plates, are classed as minor plates. A plate that appears on the major list is therefore not a minor plate.

Tectonic activity produces benefits as well as hazards. Iceland lies on the Mid-Atlantic Ridge, where volcanic and hydrothermal heat is harnessed as geothermal energy for electricity and space heating. Volcanic eruptions there have built new land, as with the island of Surtsey, which rose from the sea. Geysers, hot springs and volcanic landscapes also attract tourists. All three of these are advantages of tectonic activity.

Fold mountains, such as the Alps, the Andes and the Rockies, form at convergent plate boundaries, where compression crumples sedimentary layers into folds. Volcanic mountains are built by the accumulation of erupted lava and ash around a vent. Mount Fuji is a volcanic cone and is not a fold mountain. The two categories must be kept separate in answering questions on mountain types.

Oceanic trenches are the sites of subduction, where one oceanic plate descends beneath another. This makes trenches essential for studying plate movements. Subduction zones are linked with active volcanic arcs and with strong earthquakes, including deep-focus earthquakes along the descending slab.

Cloud types and condensation

Condensation is the change of water vapour into liquid water. It produces dew on cool surfaces, fog near the ground, and the droplets that form clouds. Frost forms when water vapour turns directly into ice on a surface below freezing point, and Indian textbooks group it with the forms of condensation. Sleet, by contrast, is a form of precipitation: falling snow partly melts and refreezes into ice pellets, so it is not a form of condensation.

Clouds are classified by their altitude and by their form. By altitude, they are high, middle or low clouds. High clouds include cirrus, cirrostratus and cirrocumulus, middle clouds include altocumulus and altostratus, and low clouds include stratus and stratocumulus. Nimbostratus usually extends from low to middle levels. Stratus, stratocumulus and nimbostratus are therefore not high clouds.

The form-based names are useful for memorising cloud types. Cirrus means a wispy, fibrous cloud made of ice crystals, cumulus means a heap cloud, stratus means a layer, and nimbus means a rain-bearing cloud. Nimbostratus and cumulonimbus are the main rain-bearing clouds, whereas cirrus and cumulus clouds generally do not produce rain.

Weathering and denudation processes

Weathering is the breakdown of rock in situ, and it occurs in three forms. Mechanical (physical) weathering breaks rock into fragments without changing its chemistry. Chemical weathering alters minerals through dissolution, oxidation and hydrolysis. Biological weathering is the breakdown of rock by plants and animals. Chemical weathering requires abundant water, so it is slow in dry regions.

In hot deserts the dominant form is mechanical weathering. The large daily temperature range causes rocks to expand by day and contract by night, which produces cracks, exfoliation and granular disintegration. Because water is scarce, chemical weathering is limited, so mechanical weathering is the most prevalent process.

Leaching is the removal of soluble substances from a layer of soil or rock by percolating water. It is a soil-forming process and is not a major weathering process in dry deserts.

Denudation is the collective term for all processes that lower and wear down the land surface. It includes weathering, mass wasting (mass movement), erosion and transportation. These are exogenetic processes driven by external agents such as water, wind, ice and gravity. Endogenetic processes, diastrophism and mountain building are internal forces that deform or build up the crust, so they are not denudation.

Tors are isolated, residual rocky hills or boulder piles that stand above a weathered surface. They form mainly in massive granite. Water and chemical weathering work along the joints of the granite, rounding the blocks, and the weathered material is removed, leaving the resistant core standing as tors. Limestone is linked with karst landforms rather than tors, and alluvial deposits are unconsolidated river sediments.

Geologic time and paleogeography

The geologic time scale is divided into eons, eras, periods and epochs. The Precambrian is the oldest and longest division, covering the period from Earth's formation to about 541 million years ago, and it contains the oldest known rocks. The Phanerozoic eon, which follows it, is divided into the Paleozoic, Mesozoic and Cenozoic eras.

The Quaternary is the most recent geological period. It consists of two epochs, the Pleistocene and the Holocene. The Pliocene and Miocene are epochs of the Neogene period, which precedes the Quaternary, and the Eocene belongs to the Palaeogene period, which is older still.

Paleogeography is the branch of geology that reconstructs the positions of continents, oceans, mountains and basins in past geological times, and it produces historical maps of the Earth. It draws on stratigraphic evidence, fossils and plate-tectonic reconstructions. Related terms refer to narrower fields: paleoclimatology studies past climates, paleogeomorphology studies ancient landforms, and paleolithology studies the lithology of ancient rocks.

Soil-forming processes and regimes

Soil-forming processes are commonly grouped into additions, removals, translocations and transformations. Translocation moves material within the soil profile. In translocation, eluviation carries fine particles and dissolved substances downward out of the upper, eluvial horizon, and illuviation deposits them in the lower, illuvial horizon.

The dominant soil regime is controlled by climate, especially the balance between precipitation and evapotranspiration. Where evapotranspiration exceeds precipitation, as in semi-arid and arid regions, soluble salts and calcium carbonate are not leached out of the profile. Calcium carbonate therefore accumulates in the subsoil, and this process is called calcification.

Laterization occurs in hot, humid tropical regions where intense leaching removes silica and bases and leaves iron and aluminium oxides. Podsolization occurs under cool, humid conditions, where coniferous litter produces acidic humus. Bases, iron and aluminium are leached from the upper horizon, leaving a grey, ash-like layer above an enriched subsoil. Podsolization is typical of the boreal taiga forest belt. Gleization, by contrast, occurs under waterlogged conditions and is not a process of dry regions.

Glacial landforms

Glacial landforms are produced by glacial erosion and deposition. Moraines are accumulations of glacial debris, including lateral, medial, terminal and ground moraines. Drumlins are elongated, streamlined hills of glacial drift. When many drumlins occur together they form a drumlin field, which produces basket-of-eggs topography, an irregular landscape of closely spaced rounded hillocks.

Eskers are sinuous ridges of sand and gravel deposited by meltwater streams flowing in tunnels within or beneath a glacier. Outwash plains are gently sloping sheets of stratified sediment spread by meltwater beyond the glacier's front. Cirques are armchair-shaped hollows formed by glacial erosion at the head of a valley.

Moraines, eskers and outwash plains are found where glaciers exist or once existed, such as in the high-altitude Himalayan region. Ladakh, a Union Territory in the Himalaya, has extensive glaciated terrain and is therefore the region where these features can be observed. Tropical island groups and coastal plains contain no glacial landforms.

Planetary density comparison

Density is mass divided by volume, \rho = \frac{m}{V}, and planetary densities are usually expressed in \text{g/cm}^3. The planets fall into two groups. The terrestrial planets (Mercury, Venus, Earth and Mars) are rocky and dense, whereas the giant planets (Jupiter, Saturn, Uranus and Neptune) are composed largely of hydrogen and helium and are much less dense. Earth has the highest mean density among all the planets, about 5.51\ \text{g/cm}^3, followed by Mercury at about 5.43\ \text{g/cm}^3 and Venus at about 5.24\ \text{g/cm}^3. Jupiter has a mean density of about 1.33\ \text{g/cm}^3, and Saturn is the least dense planet at about 0.69\ \text{g/cm}^3, which is less than that of water. To arrange planets in descending order of density, place the rocky planets first in the order Earth, Venus, and then the gas giants, with Jupiter denser than Saturn. For example, a body of mass 11 g and volume 2 cm^3 has density \rho = 11/2 = 5.5\ \text{g/cm}^3.

Solar system planets

The eight planets orbit the Sun in the order Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Among the inner terrestrial planets, Earth has the largest diameter, followed by Venus, Mars and Mercury, which is the smallest planet of the solar system. Jupiter is the largest planet overall. The heliocentric model, which places the Sun at the centre and holds that the Earth and other planets revolve around it, was proposed by Nicolaus Copernicus in his work De revolutionibus orbium coelestium published in 1543. Johannes Kepler later described the elliptical orbits and the laws of planetary motion, Galileo Galilei provided telescopic observations that supported the model, and Isaac Newton explained planetary motion through universal gravitation. The key point for examination is that Copernicus proposed the Sun-centred system, while the other three scientists developed or supported it.