Tuesday, October 27, 2009

ANTICYCLONES, TORNADOES, MEASURING WIND

ANTICYCLONES This is the mass of air whose isobars also form an oval or circular 'shape but in which pressure is high at the centre, decreasing towards the outside. Winds in an anticyclone form a clockwise outspiral in the northern hemisphere, whereas they form an anticlockwise outspiral in the southern hemisphere.

TORNADOES
Tornadoes are violently rotating storms, characterised by a funnel shaped cloud, in which winds whirl around a small area of extremely low pressure. A
tornado differs from a tropical cyclone in that it forms over the land. The movement of air is, however, similar to that in a depression, moving in spirally from all directions. It is more destructive than a cyclone as the speed of the winds is very high, exceeding 320 km per hour. Tornadoes occur mostly in the Mississippi Valley, and are sometimes known as twisters. They also occur in Australia and occasionally in some mid-latitude places.

MEASURING WIND
Description. of wind requires measurement of two qualities: direction and speed. The direction of wind is measured by the wind vane. Wind direction is stated in terms of the direction from which the wind is coming. Wind speed is measured by the anemom­eter. For wind velocity of higher levels, hydrogen-filled pilot balloons called Rawinsondes are released, carrying a target that reflects radar waves and thus can be followed even when the sky is cloudy. Wind velocity is measured on a Beaufort scale, in miles per hour. But the Beaufort scale of numbers has now been largely replaced by a direct statement of wind velocity in knots.

Impact

Impact Severe tropical cyclones cause considerable damage to life, property and agricultural crops. The prin­cipal dangers posed are (a) fierce winds, (b) torrential rains and associated flooding, and (c) high storm tides (combined effect of storm surge and tides).

While gales and strong winds as well as torrential rain cause sufficient havoc to property and agriculture, loss of human life and cattle is mainly due to storm surges. If the terrain is shallow and shaped like a funnel, like that of Bangladesh-much of the exposed land is just about at the mean sea level or even less-storm surges get enormously amplified;.Coastal inundation due to a combination of high tide and storm surge can cause the worst disaster.

Tropical Cyclones in the Indian Ocean and Bay of Bengal Regions The frequency, intensity and coastal impact of cyclones vary from region to region. Interestingly, the frequency of tropical cyclones is the least in the north Indian Ocean regions of the Bay of Bengal and the Arabian Sea; they are also of moderate intensities. But the cyclones are deadliest when they cross the coast bordering north Bay of Bengal (coastal areas of Orissa, West Bengal and Bangladesh). This is mainly due to storm surges (tidal waves) that occur in this region inundating the coastal areas.

Tropical cyclones over the Bay of Bengal occur in two distinct seasons, the pre-monsoon months of April-May and the post-monsoon months of October-November. On an average, in fact, almost half a dozen tropical cyclones form in the Bay of Bengal and the Arabian Sea every year, out of which two or three may pe severe. Out of these, the stormiest months are May, June,' October and November. Compared to the pre-monsoon season of May-June, when severe storms are rare, the months of October and No­vember are known for severe cyclones.

'Tropical cyclones in the Bay of Bengal are more frequent than in the Arabian Sea. There could be three reasons fqr this-(i) waters in the Bay of Bengal are comparatively shallow; (ii) the coastline along the Bay of Bengal is more complicated; and (iii) more number of rivers
drain into the Bay of Bengal than into the Arabian Sea.

Major Breeding Grounds

Major Breeding Grounds The major areas affected by
cyclones are-­
(i) South-east Carribean region where they are called
hurricanes.
(ii) Philippines islands, eastern China and Japan where
they are called typhoons.
(iii) Bay of Bengal and Arabian Sea where they are
called cyclones.
(iv) Around south-east African coast and Madagascar­
Mauritius islands.
(v) North-west Australia.
Characteristics The main features of tropical cyclones
are as follows.
Size and Shape Tropical cyclones have symmetrical
elliptical shapes (2:3 ratio of length and breadth) with steep pressure gradients. They have a compact size-80 km near centre, which may develop upto 300 km to 1,500 km.

Wind Velocity all~ Strength Wind velocity, in a tropical cyclone, is more in poleward margins than at centre and is more over oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1,200 km per hour.

Orientation and Movement These cyclones start with a westward movement, but turn northwards around 20° latitude. They turn further north-eastwards around 25° latitude, and then eastwards around 30° latitude. They then lose energy and subside. Tropical cyclones follow a para­bolic path, their axis being parallel to the isobars.

Structure The centre is characterised by a patch of clear sky, called eye of cyclone. Here, because of the dry, descend­ing air, quiet conditions prevail. Outwards of the centre, there are cirrus clouds and still outwards, dark nimbus clouds, which cause occasional torrential downpour and thunderstorms. The right hand back comer of the cyclone gets heavy squalls and rainfall, but the left hand back comer gets clear weather as the trough passes. These cyclones are associated with destructive weather conditions, especially at the front of the cyclone. Also, these cyclones are highly unpredictable as the thermal effect over ocean and coastal areas changes very rapidly.

Associated Weather The arrival of a tropical cyclone at a particular place is marked by a sudden increase in air temperature and wind velocity, decrease in air pressure, appearance of cirrus or cirrostratus clouds in the sky, and emergence of high waves in the oceans. These clouds thicken and become cumulonimbus, and torrential rains begin. Giant waves spring up and the spray blows in continuous sheets which reduces visibility almost to zero.

This terrible storm continues for few hours and is abruptly followed by total calm and clear skies. This happens when the 'eye' of the cyclone arrives. Temperature rises, but the pressure is at its lowest. The eye is merely a hollow vortex produced by the rapid spiralling of air in the storm. The period of calm lasts for half an hour, and the weather suddenly changes with the arrival of the rear or the tail of the cyclone. The sky becomes overcast again; there is heavy downpour accompanied by lightning and thunder. Winds of high velocity again set in but this time in reverse direction to those of the first half of the storm. This situation continues for few hours, then gradually the winds abate, the clouds break and fair weather returns.

Tropical Cyclones

Tropical Cyclones Tropical cyclones, the most de­structive of nature's phenomena, are known to form over all tropical oceans, except South Atlantic and the South Pacific, during certain seasons. These cyclones have a thermal origin. They are believed to form in the Inter­Tropical Convergence Zone. (ITCZ), a narrow belt at the equator, where the trade winds of the northern and southern hemispheres meet.

ITCZ is a 'legion of high radiation energy which supplies the necessary heat Jor the vaporisation of sea water into the air. This moist unstable air rises, generates convective clouds and leads to an atmospheric disturbance with a fall in surface atmospheric pressure. This causes a convergence of surrounding air towards this region of low pressure.

The converging mass of air gains a rotary motion because of what is known as the Coriolis force caused by the rotation of the earth. However, under favourable circumstances, such as high sea-surface temperatures, this low pressure area can get accentuated. The convective instability builds up into an organised system with high speed winds circulating around the low pressure interior. The net result is a well-formed cyclone.

The ideal conditions for the development of tropical cyclones are-(i) quiet air, (ii) high temperature, (iii) highly saturated atmospheric conditions.
Tropical cyclones never originate over land, although they often penetrate far into the margins of continents. But they soon lose their strength after crossing the coast and penetrating inland.

Characteristics Temperate cyclones

Characteristics Temperate cyclones have the following, characteristics.
Size and Shape The temperate cyclQnes are asymmetri cal and shaped like an inverted 'V'. They stretch from 50 km to 600 km. They may go upto 2,500 km over Norf America. They have a height of 8 to 11 km.

Wind Velocity and Strength These aspects of a temperat cyclone vary with season, location and from cyclone t cyclone, and the wind is directed a little to the right c the centre, rather than into it. The wind strength is mOl in eastern and southern portions, more over North Americ compared to Europe. The wind velocity increases with th approach but decreases after the cyclone has passed.

Orientation and Movement Since these cyclones mov with the westerlies, they are oriented east-west. If the storr front is east-west, the centre moves swiftly eastwards. : the storm front is directed northwards, the centre mov€ towards the north, but after two or three days, the pressuI difference declines and the cyclone dissipates. In case th storm front is directed southwards, the centre moves qui! deep southwards---even upto the Mediterranean regio (sometimes causing the. Mediterranean cyclones).

Structure The north-western sector is the cold sector and the north-eastern sector is the warm sector. As 011 moves eastwards in the northern sector, dark nimbI; clouds and altrostratus are foUowed by cirrostratus highl up with cirrus clouds finally at the storm front. In t~ eastern sector, the extent of cloudiness and rainfall limited. This sector is generally dominated by cumulonin bus which cause heavy downpour, thunderstorm, lightnil1 and hail-storm.
Associated Weather The approach of a temperate cyclor is marked by fall in temperature, fall in the mercury level wind shifts and a halo around the sun and the moon, and a thin veil of cirrus clouds.

A light drizzle follows which turns into a heavy downpour. These conditions change with the arrival of the warm front which halts the fall in mercury level and the rising temperature. Rainfall stops and clear weather prevails until the cold front of an anticyclonic character arrives which causes a fall in temperature, brings cloudiness and rainfall with thunder. After this, once again, clear weather is established.

The temperate cyclones experience more rainfall when there is slower movement and a marked difference in rainfall and temperature between the front and rear of the cyclone. These cyclones are generally accompanied by anti­cyclones.

DEPRESSIONS

DEPRESSIONS Depression is a mass of air whose isobars form an oval or circular shape, with low pressure at the centre. The air converges at the centre and rises to be disposed off. In a depression, the winds rotate anticlockwise in the northern hemisphere. While in the southern hemisphere, the circular movement of winds is in a clockwise direction. Depressions are rarely stationary and tend to follow definite tracks. They are most influential over the ocean spreads and they weaken over land areas. They are of two types.

1. Depression or Temperate Cyclones Also known as extra-tropical or wave cyclones, temperate cyclones are active over mid-latitudinal region between 35° and 65° latitudes in both hemispheres.
Origin and Development There are two theories of origin of temperate cyclones.
(i) Polar front theory by Bjerkenes According to this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus a polar front is formed as a surface of discontinuity. Such conditions occur over sub-tropical highs, sub-polar lows and along the
. tropopause. The cold air pushes the warm air upwards from underneath. Thus a void is created because of lessening of pressure. The surrounding air rushes in to occupy this void and, coupled with the earth's rotation, a cyclone is formed which advances with the westerlies.

(ii) Thermodynamic theory by Lampert and Shaw Accord­ing to this theory, in sub-tropical areas, an overcrowding of vertical currents releases the surplUs energy upwards which, after meeting the upper cool air, converts into an eddy. This eddy tends to settle down as an inverted 'V' shaped cyclone.
The temperate cyclones occur mostly in winter, late autumn and spring. They are generally associated with rainstorms and cloudy weather.
Distribution The favourite breeding grounds of temper­
ate cyclones are listed below.

1. Over USA and Canada, extending over Sierra Nevada, Colorado, Eastern Canadian Rockies and the Great Lakes region.
2. Mexican Gulf.
3. The belt extending from Iceland to Barents Sea and
continuing over Russia and Siberia.
4. Winter storms over Baltic Sea.
5. Mediterranean basin extending upto Russia and even
upto India in winters (called western disturbances).
6. The Antarctic frontal zone.
During summer, all the paths of temperate cyclone shift northwards and there is no temperate cyclone ove sub-tropics and the warm temperate zone, although a hig] concentration of storms occur over Bering Strait, USA ani Russian Arctic and sub-Arctic zone.

Mountain and Valley Winds

Mountain and Valley Winds These are local winds responding to local pressure gradients set up by heating or cooling of the lower air. During the day when the slopes are intensely heated by the sun, the air in contact with the slopes is also heated by conduction from the ground.

This warmed-up air rises and is replaced by the air moving up from the valley, thus creating an upslope wind during the day. It is particularly well developed when one side of thtt valley is heated much more than the other, for example<; the valleys which have an east-west trend in mid and high latitudes. When the same slopes have been cooled at night by radiation of heat from ground to air, the wind moves valleywards. The wind is at its strongest just before sunrise when radiational cooling is greatest. Valley winds are a kind of anabatic winds.

Katabatic Winds A cold downslope wind caused by the gravitational movement of cold dense air near the earth's surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley. Favourable conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.

The strongest katabatic winds are those that blow from an ice cap off the Greenland or Antarctic ice caps. In other parts of the world, these are known by various local names such as the bora in the northern Adriatic coast and the mistral in southern France. The Santa-Ana of southern California is of desert origin, carrying much dust and silt suspension.

Foehn and Chinook These result when strong regional winds passing over a mountain range are forced to descend on the ice side with the result that the air is heated and dried.

LOCAL WINDS


LOCAL WINDS
These winds affect only a limited area and blow for short periods of time, and are generated by immediate influences of the surrounding area. Most local winds are developed by depression: these are of consid­erable environmental importance as they may affect the plants and animals when dry and extremely hot or cold.

Land and Sea Breeze
When a region is hotter than the neighbouring region, air from the cooler region moves into the hot region to take the place of the hot air which has expanded and risen. During the day the land gets warmer than the sea producing low pressure over the land into which cooler air moves from over the sea. Thus the local wind that blows from sea to land during the day is called sea breeze. It reaches the maximum strength during the mid­afternoon when temperature difference between the land and the sea is greater, and dies out by the evening as the sun's heat diminishes. It is best developed in tropics when conditions are calm and the sky is clear, but can also be observed in temperate areas under similar conditions. It is greatly variable. During the night the land cools more quickly than the sea and a reverse process sets in LAnd breeze is a cool wind that blows from the land to the sea (or a large lake, etc.) during the night.

It occurs when night time radiation cools the land and the air in contact with the ground surface. The atmospheric pressure over the land is raised relative to that over the sea resulting in a difference of pressure and the flow of air towards the sea from land. Land breeze is most common in tropical areas but may also occur in high latitudes particularly when the weather is calm. Land breeze is not as strong as sea breeze.

MONSOON WINDS

MONSOON WINDS Derived from the Arabic word 'mausim', meaning season, 'monsoon' is applied to winds whose direction is reversed completely from one season to the next. Land masses of Asia and North America powerfully control the temperature and pressure conditions in the northern hemisphere. As pressure conditions control winds, these areas also develop wind systems, quite inde­pendent of the belted wind system in the southern hemi­sphere.

Summer Monsoon During summer, a 'thermal' or 'heat' low is developed over southern Asia in the lower levels of the atmosphere. It is a cyclone with a considerable air flow. From the Indian Ocean and the south-western Pacific warm, humid air moves northward and north­westward into Asia passing over India, Indo-China and China. This air flow accompanied by heavy rainfall consti­tutes the summer monsoon in southeast Asia.

Winter Monsoon Reverse air flow from that of sum­mer takes place in winter in Asia. The land area is dominated by a strong centre of high pressure from which there is an outward flow of air. Blowing southward and south-eastward towards the equatorial oceans, the winter monsoon brings dry, clear weather for several months. In North America, there is an alternation of temperature and pressure conditions between winter and summer, but the differences are not as remarkable as in southeast Asia. Australia also shows a monsoon effect but it reverses the conditions of Asia, being in the southern hemisphere.

GLOBAL CIRCULATION SYSTEMS The surface wind systems represent only a small part of the circulation pattern. There is an upper air flow also-mainly westerlies flowing in a complete circuit about the earth from latitude 25° almost to the poles, and equatorial easterlies between the high-pressure ridges.

JET STREAM
The term was introduced in 1947 by Swedish-born US meteorologist Carl-Gustaf Rossby, and it is used to describe a very strong steady westerly wind blowing at high altitudes (6,000 to about 1400 metres above the earth's surface) just below the tropopause. It is usually confined to a narrow band and its speed reaches up to 350­450 kmph. The highest streams occur during winter. There are two main jet streams: (a) polar front jet stream, irregular in its location and commonly discontinuous, (b) subtropical jet stream (between 20° and 30° latitudes, north and south)-it is fairly consistent for a given season. In the northern hemisphere, the strongest jets flow across Japan and the United States. The jets have an Important role in the weather changes. High altitude flying has to take them into account.

PLANETARY WINDS

PLANETARY WINDS The general, permanent circula­tion of surface winds throughout the world is.denoted by the term 'planetary winds'. The wind belts are basically controlled by the latitudinal pressure belts and by the forces produced by rotation of the earth.

Doldrums Doldrums is the name for the equatorial I of low pressure lying between 5° South and 5° Nc latitude. This zone has no pressure gradients to induc persistent flow of wind.

Trade Winds The word 'trade' comes from the Sa) word 'tredon' which means to tread and follow a reg\ path. Moving north and south of the equator, the m wind belts are trade winds, covering roughly the Zl between 5° and 30° North and South. They blow from subtropical high pressure area (Horse latituaes) towa equatorial low pressure areas (doldrums). Under the in ence of the Coriolis force, they blow from the north-E in the northern hemisphere (north-east trades) and fr the south-east in the southern hemisphere (SOUth-E trades). They are also called tropical easterlies.

Like pressure belts which cause them, trade winds and doldn; have a seasonal shifting tendency through several degI of latitudes. In summer these belts shift farther nortl1 the northern hemisphere than they shift in the south direction during winters because of the longer landmas: the northern hemisphere. Trade winds are upset in Indian Ocean due to the proximity of the great Asi landmass. They are best developed over the Pacific j Atlantic oceans. Trade winds are, however, not
favourable for navigation and flying. Though very const in strength and direction, they sometimes contain stn depressions.


Horse Latitudes
The subtropical belts of variable wi and columns that lie between the latitudes 25° and 35° Sc and North are called Horse Latitudes. They coincide v the sub-tropical high-pressure belts. (The high pressur probably caused by the rising air of equatorial latitu which descends here.)

Westerlies These winds blow from subtropical t pressure areas (Horse Latitudes) to subpolar low pres~ areas and lie between 35° and 60° Nand S latitudes. Vari, in direction and strength, westerlies contain depressiom the northern hemisphere, land masses cause considen disruption to the westerly wind belt. But between 40°
60° S lies the almost unbroken ocean belt. Westerlies strong and persistent here, giving rise to the marin expressions-"roaring forties", "the furious fifties" and' screaming sixties". In earlier times, this belt was extensi1 used for sailing vessels travelling eastward from Sc Atlantic ocean to Australia, Tasmania and New Zealand the Southern Pacific Islands.

It was easier to conti eastwards around the world from these places thar return to European ports. Today the westerlies are iml tant not in shipping but in long distance trans-oceanic transcontinental flights; in the easterly direction, tl require less fuel and a shorter time than the west... flights.

Polar Easterlies
These constitute the wind syst characteristic of the arctic and polar zones. They blow f] polar high pressure areas to subpolar low pressure ar In fact, winds in these regions take a variety of direction as dictated by local weather disturbances. Deflected to left in the southern hemisphere, the radial winds mov anticlockwise direction, producing a system of south-E erly winds. Perhaps in Antarctica, an ice-capped landn surrounded by vast oceanic expanse, the outward spiral flow of polar easterlies is a valid concept.

PRESSURE BELTS

PRESSURE BELTS Two factors leading to the formation of high and low pressure are thermal and dynamic.
(i) Thennal Factor Low pressure is caused by heating. Heating results in expansion of air, resulting in low density and-I thus 'leading to low pressure, e.g., equatorial lows in North America and North India during summer. High pressure is the result of contraction of cooled air and increase in density, e.g., polar highs which occur in North Asia and North America during w~nters. The main pro­cesses in the formation of thermal highs and lows are (a) latent heat of condensation, (b) conduction and radiation, and (c) advection of air masses (advection: horizontal movement of air or liquid transferring heat).

(ii) Dynamic Factor It operates through a fractional drag and centrifugal force. The centrifugal force is very high
along the equator, where the velocity of rotation is high. Hence, the air masses tend to be thrown out, resulting in low pressure. Examples of dynamically produced pressure systems are sub-tropical highs and sub-polar lows.
Atmospheric pressure decreases with height. The dis­tribution of pressure is characterised by its zonal or belted nature. Each zone or belt constitutes elongated or circular cells of high or low pressure. There are seven pressure belts on the earth's surface.

The equatorial belt separates the three pairs of belts in northern and southern hemisphere, namely, the polar high, the sub-polar low and the sub­tropical high. Location of belts is based on the annual average. Pressure belts shift seasonally as the sun moves apparently from one hemisphere to the other, due to the relationship between insolation, heating, expansion, density and air pressure.

The pressure belts comprise cells in the northern hemisphere, where all belts shift a little north of their annual average locatio!) during summer and a little south of the annual average location in winters. In the southern hemisphere, the belts comprise isobaric bonds and opposite conditions prevail seasonally.

In both high and low pressure belts, the winds are light, blowing in all directions. As the air bodies of different properties meet, they rise up and become unstable, result­ing in rainfall. The air in high pressure belts sinks and spreads and is therefore stable and dry while in the low pressure belts, the air converges and is humid. The diagram: gives a representation of the planetary winds and pressurE:! belts.

PRESSURE GRADIENTS

PRESSURE GRADIENTS The change in baromet­ric pressure across a horizontal surface constitutes a pressure gradient. Where a pressure gradient exists, air molecules tend to drift in the same direction as that gradient. This tendency of mass movement of air is referred to as the pressure gradient force; on a weather chart, this is indicated by the spacing of isobars: the gradient being 'steep' if they are close together and 'gentle' if they are far apart. A steep gradient indicates strong winds and a gentle one slight winds. Wind is thus the horizontal motion of air in response to the pressure gradient force.

CORIOLIS FORCE
Winds would follow the direction of
the pressure gradient.. if the earth did not rotate on its axis. . But the earth's rotation produces another force called the'
Coriolis force which tends to turn the flow of air. It was named after the French mathematician, Gaspard de Corio­lis, who first described it in 1835. It is a 'fictitious' force, needed to relate the movement of air masses over the earth's surface to its rotating coordinate system (the grid).

The direction of action of the Coriolis force is stated in Ferrel's law: "any object or fluid moving horizontally in the northern hemisphere tends to deflect to the right of its path of motion regardless of the compass direction of the path." A deflection towards the left is experienced in the southern hemisphere. Coriolis force is absent at the equator and increases towards the poles. The force is responsible for the formation and direction of movement of the anticyclones and whirlpools.

AIR TEMPERATURE

AIR TEMPERATURE Air temperatures depend upon various geographical factors including elevation, aspect, proximity to sea, direction of prevailing winds and patterns of insolation.

Land-water Differences The absorption and radiatio properties of land and water differ. Land gets heated u rapidly and intensely under the sun's rays whereas tl' water surfaces get slowly and moderately heated. Land ge cooled off faster than water when solar radiation is cut oj These differing qualities result in greater temperatuJ contrasts over land areas, i.e., the middle of the continent while over the water areas (near the coasts), thesecontras are moderate. However, the air temperature over tr ocean has two features: (i) As water bodies heat and COI slower than land, maximum and minimum temperaturl are reached a month later than on land. (ii) The year] range of temperature in water is less than that over tl' land.

Annual Temperature
Patterns The air temperatw varies at different latitudes. Isotherms (lines. connectir places having the same air temperature) which run mOl or less parallel to the lines of latitudes in the east-west zonl reflect the general decrease of insolation from equator 1 poles. During the year, the isotherms follow the declinatio of the sun and change position north or south.

The patter of temperature changes also varies at different altitudes: ( In equatorial areas, annual temperature shows little sei sonal variation. as they receive constant amount of diurni insolation throughout the year. (ii) In mid and high-Iatituc areas, seasonal variation of annual temperature is muc more marked. (iii) In areas between the tropics and pole circle in each hemisphere, air temperatures and insolatio amounts have a marked seasonal pattern. This is due 1 the fact that the sun's path in these areas shifts throug a relatively large range of noon altitude and substanti, differences exist in the length of the days. (iv) In polar regions, there are large seasonal contrasts in air tempera­ture-very low in winter or during polar nights and extremely high during the summer. As the atmosphere is only indirectly heated by the insolation through the me­dium of ground, there exists a time-lag between the air temperatu.re pattern and the insolation pattern despite a strong correlation. In winter, in the northern hemisphere, the coldest air temperatures are experienced in January,
while the winter solstice is in December because the ground continues to lose heat even after insolation has begun to increase.

Daily Cycle of Temperature
The daily pattern of temperature changes illustrates energy changes on a small time-scale. Air temperature falls if the ground is cooler than air owing to the fact that the atmosphere is largely heated from the earth's sur­face. On a calm day with little cloud, air tempera­tures fall to the lowest; as the ground becomes cold during the night, the air above it is cooled by conduction. As the ground temperature rises after sunrise due to insolation, air temperature also begins to rise with the lag of about an hour. Maximum insolation received is at midday, but the maximum air temperature is usually at about 1400 hours. After this time, temperature drops as the convection mixes cooler upper air with warm air near the ground. After sunset, the air remains warm for some time on still being heated by radiation from the ground before the temperature drops eventually.

Vertical Changes of Temperature Air tempera­ture also varies according to the altitude. At higher altitudes as air becomes less dense, it is unable to absorb heat, resulting in colder air temperature. The normal drop of temperature with height is known as normal lapse rate, which is 6.4°C per km on an average. But this can vary according to geographic position, season and time of the day. Temperature inversion is the situation where there is increase in temperature with height, before beginning to drop. into the normal lapse rate. In cases where the temperature remains the same with increase in altitude, the layer of atmosphere is called isothermal.

GLOBAL RADIATION AND HEAT BALANCES

GLOBAL RADIATION AND HEAT BALANCES
The flow of energy from sun to earth and then into space is a complex system but of vital importance to us.

INSOLATION
Insolation is the radiant energy that reaches the surface of the earth from the sun. The sun emits a wide variety of energy waves from very short X-rays to longer infrared rays. Only about 1 part in 2,bOO million of the total of sun's radiation reaches the earth; it is essential for the sustenance of life here. Insolation is the most important single source of atmospheric heat.

At any par­ticular place on earth, the amount of insolation received each day depends upon (i) the angle at which the sun's rays strike the earth, and (il) the length of time of exposure to the rays. Thus, the greatest amount is received at places where the skies are generally clear,.the angle of sun is high and the days relatively long, e.g., around the tropics. On an average, throughout the year, the amount of insolation received by the surface decreases from the equator towards the poles and while the equatorial areas have little variation throughout the year, there is great variation near the poles. The unit of measurement of solar insolation is langely, one langely being equal to one gram-calorie per square centimetre. Measurement by satellites indicates that the
radiation rate from the sun is two langleys per minute.

HEAT BALANCE The moving solar radiation is bal­anced by some sort of equal energy losses in the atmo­sphere which prevent the earth from becoming intolerably hot. The balance is achieved by a complex series of energy transfers involving three common types. (a) Radiation is the transference of heat by electromagnetic waves including X­rays, heat rays and radiowaves. The sun, having a very hot surface, radiates short wavelengths while earth, having a cool surface; re-radiates heat at much larger wavelengths. (b) Convection involves the mass movement of gases or liquids, the heat acquired by the liquid or gas being transported with the medium. (c) Conduction is the trans­ference of heat by actual contact.

As insolation arrives in different wavelengths, different diversions by the atmosphere result. The most significant changes are as follows: (a) Absorption of insolation takes place in the lower layers of atmosphere by carbon dioxide and water vapour, and in the upper layers by oxygen and ozone. Absorption leads to a rise in the air temperature. (b) Scattering takes place by gas molecules and dust particles in all directions. (c) Radiation takes place by clouds and water droplets. Presence or absence of clouds is an important determinant in the amount of radiation reaching the earth; thick clouds are capable of reflecting (back into space) up to 60 per cent of insolation. (d) Reflection of radiation takes place from earth's surface also, varying in amount according to the nature of the ground.

The percentage of radiant energy reflected back by a surface is called the albedo. While water has a low albedo, land surfaces have a much higher albedo. The total amount of energy lost by scattering and reflection of various kinds and returned to space is called earth's albedo, which amounts to about 36 per cent of insolation while the absorption that takes place directly by the atmosphere is about 17 per cent of insolation. Together, they constitute about 53 per cent of solar radiation which gets reflected back into space. Thus, only about 47 per cent of the original insolation received at the top of the atmosphere actually reaches the ground.

Terrestrial Radiation
is the energy which is re-radiated by the earth into atmosphere at long wavelengths. Some of it is directly lost into space and a great deal is absorbed by the atmosphere, especially the clouds. The atmosphere, in turn, radiates or reflects much of this heat back again to the earth and thus a continuous interchange of energy exists with the ground. Further heat is lost from earth through evaporation. There is also a small amount of conduction of heat between the ground and the atmo­sphere. The terrestrial radiation equals and balances with the incoming radiation. The most significant fact in the energy budget is that atmosphere is largely heated from below: while it reflects or lets through the short-wave incoming solar radiation, it absorbs a great amount of the outgoing terrestrial energy. This results in warming up of the atmosphere.

A similar principle is applied in the greenhouse effect where the glass lets in insolation but does not allow the warm air inside to escape readily. While equatorial areas have a positive heat budget (surplus heat), the poles have a negative budget. But the mean tempera­ture of both areas remains fairly constant owing to the presence of horizontal circulation systems. The excess heat received at low latitudes is transferred to poles through various media, including wind systems and ocean currents.

PHENOMENA OF OUTER ATMOSPHERE

PHENOMENA OF OUTER ATMOSPHERE
Certain physical phenomena of the outer atmospheric
region are of importance in the broad framework physical geography. Of particular interest is the devel ment of radio communication on a global scale in a la known as the ionosphere, located in the altitude rang~ 80 to 400 kIn (i.e., identical with the lower thermosphere.)

It is so called because the incoming solar radiation ionises the gases within it, forming ionised layers. These layers reflect radio signals and other electromagnetic waves back to the earth. Most of this reflection of long-wave radio.
.waves takes place in the lower part of ionosphere which bears the name of Kenelly-Heaviside layer. The process of ionisation, being based on solar radiation, takes place on the sunlight-side of the earth.

OZONE LAYER A zone within the atmosphere between 20 and 80 km, extending from upper stratosphere into the mesosphere, called the ozone layer, is of vital importance to all living forms on earth. The ozone layer is a region of concentration of the form of oxygen molecule called ozone (°3)' It results from the splitting of oxygen (°2) molecules by the ultraviolet radiation from the sun to form atomic oxygen (0), which then combines with other oxygen (°2) molecules to give ozone (°3)' The ozone layer serves as a shield preventing most of the potentially damaging ultraviolet radiation of the sun from reaching the tropo­pause and earth's surface. If the ultraviolet rays were to reach the earth in full intensity, all exposed bacteria would be destroyed and animal tissue severely burnt.

VAN ALLEN RADIATION BELTS
A region of intense radioactivity. within the magnetosphere was reported to be existing by the satellites. The mngnetospherf? is the area surrounding the earth extending to about 60,000 km on the side facing the sun and more on the opposite side. It was discovered that two ring-shaped belts of radiation, one lying within the other, existed and were named Van Allen Radiation belts after the physicist who first described them. The inner belt lies about 2600 km from earth's surface, while the outer lies at about 13,000 to 19,000 km from it. These belts represent concentrations of highly charged particles, protons and electrons from the sun, trapped within lines of force of the earth's external magnetic field­the magnetopause.

AURORA The aurorae are produced by the charged particles from the sun captured by earth's magnetic field at heights of about 100 km. It is a luminous phenomenon seen in the sky at night in high latitudes. It may be visible as arcs of light or as coloured curtains, streamers and rays. Aurorae occur most frequently during the intense periods of the ll-year sunspot cycle. In the northern hemisphere, they are called aurora borealis (or northern lights); in the southern hemisphere, as aurora australis (or southern lights). Areas experiencing aurorae most frequentfy Ii!'! about 20° from geomagnetic poles; the belt to the north of Norway, south of Iceland and Greenland, over north Canada and across northern Russia is the area of aurorae in northern hemisphere.

MAGNETIC STORMS
These are world-wide temporary disturbances of the earth's magnetic field that appear to be associated with the occurrence of solar flares and sunspots. The aurora is also usually visible and extensive during such a storm but whether this is a cause or an effect is not yet fully understood. The main effects are the disruption of radio and telegraph communications and magnetic surveys.

COSMIC PARTICLES These are elementary particles­protons-travelling through space at speeds approaching that of light. Most are nuclei of hydrogen atoms, others are helium and a few of other heavier atomic nuclei. Also called the cosmic rays, their penetrating power is enormous and they can reach earth's surface.

ATMOSPHERIC PRESSURE

ATMOSPHERIC PRESSURE
Air has weight and thus exerts pressure on eartl1 surface. The pressure exerted by atmosphere as a rest of its weight above a unit area of the earth's surface called the atmospheric pressure.

It is expressed in mil bars (mb) and measured with a mercury barometer. 11 average atmospheric pressure at sea level is 1013.25 m
Pressure varies with both temperature and altituc decreasing logarithmically with height. Two princiF types of barometer are Mercury barometer and Anerc barometer. Mercury barometer is a very accurate instr ment, though large and cumbersome. Atmospheric presssure is read in inches and centimetres; thus 29.92 incl1
(76 em) of mercury are equivalent to 1013 mb at sea lev one inch of mercury is equivalent to 33.2 mb. Anerl barometer is an instrument consisting of a metal box fr( which the air is virtually exhausted. The sides are flexil and expand and contract with changes in air pressure. The movements are amplified and registered by a needle a calibrated circular dial. The instrument is light a portable.
DISTRIBUTION OF PRESSURE Air pressure decrea: with altitude. For about every 275 metr:es of rise in elevation, pte mercury column falls 1/30 of its height. 1 rate of drop of mercury becomes less and less w increasing height and beyond 50 kIn, decrease is extrem slight.

STRUCTURE OF ATMOSPHERE

STRUCTURE OF ATMOSPHERE
From the earth's surface upward to an altitude of about 80 km, the chemical composition of atmosphere is uniform in terms of the proportions of its component gases. This layer is the homosphere. The homosphere can be divided into layers on the basis of temperatures and zones of tempera­ture change. Above 80 kIn, atmospheric composition tends to be independent of height.
TROPOSPHERE It is the lowermost atmospheric layer extending from about 8 km ,at the poles and 16 kIn at equator. It is characterised by almost uniform decrease of
temperature with a rise in altitude (about 1°C per 165"
metres).

All phenomena of weather and climate which physically affect man take place within this layer. Tropo­sphere contains water vapour which mixes perfectly with air and in its various proportions gives rise to the different phenomena like ,rain, snow, hail or sleet. Dust particles present in this layer hold the water vapour and contribute to the occurrence' of twilight and the red colours of sunrise and sunset and distribute insolation.

STRATOSPHERE The second layer of atmosphere is called the stratosphere. The level at which the troposphere gives way to stratosphere is called tropopause. (At this level, the fall in temperature stops.) Within the stratosphere, the increase in temperature with altitude is slow and constant at lower sections but becomes rapid at higher altitudes The upper limit of this layer is called stratopause. Withir the stratosphere, temperature increases fropt about -600( at tropopause to about O°C at stratopause. Little weathe is generated here as there is very little water vapour anc virtually no dust present. Ozone is produced in tropica and mid-latitudes of stratosphere. The stratosphere pro vides ideal conditions for flying aeroplanes.

MESOSPHERE Mesosphere is the atmospheric laye extending between the stratopause (at an altitude c about 50 kIn) and mesopause-the upper limit of mesc sphere (at about 80-90 km). Within mesosphere, th temperature decreases with altitude from about OOC "
stratopause to about -100°C at mesopause. Vertical ai currents are not strongly inhibited here and formatio of ice crystal clouds called the noctilucent clouds tak~ place occasionally in the upper regions of the layer.

THERMOSPHERE AND EXOSPHERE Thermosphere: the uppermost layer of the atmosphere, extending frO! the mesosphere at an altitude of about 85 km to 400 kJ of the atmosphere. Within it, the temperature increaSE with altitude from about -100°C at the mesosphere f over 150°C. Exosphere is the boundary between tl earth's atmosphere and the interplanetary space. It e: tends from about 400 km above the earth's surface.

Atmosphere

WHAT IS THE ATMOSPHERE?
The earth's atmosphere is of immense importance to human beings. The atmosphere is a layer of a mixture of gases enveloping the earth, held to it by gravitation force. Almost all of the atmosphere (97 per cent) lies within 29 km of the earth's surface. The upper liqrit of the atmo­sphere can be drawn approximately at a height of 10,000 km. Beyond about 100 km, recent data from satellites suggest that the lightest gases separate out, forming several concentric layers around the earth.

The innermost of these is the nitrogen layer (between 100-200 km); then comes oxygen (200-1100 km); helium (1100-3500 km) and then hydroge~ only, to which there is really no clearly defined upper limit~ The composition of gases remains more or less constant in the lower layers of the atmosphere, though its temperature and pressure may vary in time and place. This composition commonly known as 'air' tends to act as a single gas. About 99 per cent of dry air is constituted by oxygen and nitrogen. Carbon dioxide, though apparently a minor gas, is important due to its ability to absorb heat, enabling the layers of the atmosphere to be warmed up.

MINERALS

MINERALS
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Minerals are naturally occurring, inorganic substances, usually possessing a definite chemical composition and a characteristic atomic structure. About 2000 minerals are known to be existing. Rocks are composed of various combinations of minerals. Some minerals form a series in which there is a gradual replacement of one element by another. The most common mineral in rocks is quartz. Silica often combines with other oxides to form silicates, the most common of which are feldspars. Mica is a common silicate. Feldspars are silicates of aluminium, potassium, sodium and calcium. Clay minerals are complex silicates derived from weathered minerals. The term mineral is also used for any naturally occurring material which is mined and is of economic value.

PHYSICAL PROPERTIES
Crystal structure, cleavage and fracture, specific gravity, hardness, lustre, colour and streak are important aspects for understanding physical properties of rocks.

Crystal Structure Mineral crystals fall into six catego­ries each of which is defined in terms of its crystallographic axes: (i) isomeric system (halite mineral); (ii) tetragonal system (zircon material); (iii) hexagonal system (beryl mineral); (iv) orthorhombic system (staurolite mineral); (v) monoclinic sys­tem (gypsum mineral); and (vi) triclinic system (albite mineral).

Specific Gravity Specific gravity of minerals deter­mines the density of a given rock and rock density in turn determines the gross layered structure of the earth.

Hardness
It determines how easily a mineral is worn away by the abrasive action of streams, waves, wind and glaciers in the processes of erosion and transportation. Ten standard minerals constitute the Mohs Scale of hardness ranging from the softest to the hardest.

Lustre The appearance of a mineral surface under reflected light is referred to as its mineral lustre. It is described by several descriptive adjectives, such as metallic (metal like), adamantine (diamond like), vitreous (glass like), resinous (oil like), pearly or silky (pearl or silk like).

Colour Certain minerals possess a distinctive mineral colour that facilitates recognition. The impurities present in the mineral provides shades of colour to a mineral.

Streak
When a mineral is rubbed across the white porceline plate, known as streak plate, it may leave a streak of mineral powder of distinctive colour. The colour of powder of mineral may sometimes be different than the colour of the mineral.
(Note: Regional distribution and economic importance of minerals are dealt separately in different chapters.)

SEDIMENTARY ROCKS and METAMORPHIC ROCKS

SEDIMENTARY ROCKS Compaction and cementation of layers of sediment leads to formation of sedimentary rocks. A characteristic feature of these rocks is their layered
arrangement, the layers being collectively called strata. .
Layers of different textural composition are alternated or inter-layered. The planes of separation between these layers are called bedding planes. All sedimentary rocks are non­crystalline and contain fossils. Sedimentary rocks fall into three main groups.
(i) Mechanically formed These are called clastic sedi­mentary rocks; the sediments are largely derived from pre­existing rocks that have been broken down and then transported by water (e.g. loess), wind (e.g. clay) or ice (e.g. moraines, gravel) to form rocks. They are further divided into three groups on the basis of the size of the constituent particles: (a) ludaceous or pebbly, e.g., conglomerate, (b) arenaceous or sandy, e.g., sandstone, (c). argillaceous or clayey, e.g., shale.
(ii) Organically formed These rocks are derived from remains of plants (e.g., peat, lignite, coal), or animals (e.g., chalk and coral).
(iii) Chemically formed Some examples are evaporitics, which are formed from sediments precipitated from a saturated solution and dried out by evaporation, such as rock salt, borax, gypsum, nitrates, potash and certain limestones. Shale, sandstone and limestone are the most abundant of sedimentary rocks.
Sedimentary rocks contain certain essential resources for man's industrial society: (i) they provide materials to build structures and highways; (ii) they supply compounds for induStrial processes that yield chemicals such as fertilisers and acids; (iii) they are the source of hydrocarbon com­pounds, e.g., coal, petroleum and natural gas that turn the industrial wheel and provide the fuel for heat and trans­portation.

METAMORPHIC ROCKS
These rocks are formed when pre-existing sedimentary or igneous rock is altered as a result of changes in physical or chemical conditions. This process of metamorphism may be through intense pressure or stress caused by earth movements, increased tempera­ture caused by volcanic activity, or the action of gases and liquids of magmatic origin. There are three main types of metamorphism.
(i) Contact (thermal) metamorphism results from the intrusion of a mass of molten rock.
(ii) Dynamic (regional) metamorphism when rock 'layers undergo strong structural during mountain building.
(iii) Dislocation metamorphism occurs when pre­existing rocks undergo localised deformation along a fault plane or thrust plane. The mineral composition and the structure of the rock can be altered; for example, under stress some minerals like mica take on a parallel arrange­ment.

Each igneous and sedimentary rock has a metamorphic equivalent. Metamorphic rocks, generally speaking, are harder and more compact than their original types, except when derived from igneous rocks. Examples of metamor­phic rocks are marble (from limestone); slate (from shale); quartzite (from sand); graphite (from coal); gneiss (from granite).

ROCKS

ROCKS:
Any aggregate of mineral particles that forms part of the earth's crust is called a rock. Composed of mineral matter in solid state, it is present in a wide range of compositions, physical characteristics and ages. While a few rocks are constituted almost by a single mineral, there are others composed of two or more minerals. Rock formation is a continuous process. Though most rocks of the earth's crust are very old, the times of formation range back to millions of years. Rocks are classified into three major groups-igneous, metamorphic and sedimentary; each group is further subdivided into different types each with its own composition, character and age.

IGNEOUS ROCKS
Igneous rocks are rocks that have solidified from molten magma at considerable depth in the earth under conditions of very high temperatures and pressure. They do not occur in layers. Most of them are crystalline and do not contain fossils. Igneous rocks are classified on the basis of chemical composition as well as on grades of sizes of the component crystals:
1 2
(i) Extrusive rocks These are rocks formed out onto the surface of the earth as magma before cooling and are generally glassy or fine-crystalled. These are also called volcanic rocks, or lava, e.g., basalt.
(ii) Intrusive rocks These cool and solidify within the earth's crust and reach the earth's surface only by being exposed by erosion. These may be either (a) plutonic which cool deep within the crust and have large crystals, e.g., granite, or (b) hypabyssal which cool at intermediate depths and contain moderate-sized crystals.
Igneous rocks are also classified on the basis of chemical composition. For example, one classification is based on silica content and has the following divisions: acid rocks (over 66 per cent silica); intermediate rocks (55-66 per cent silica); basic rocks (45-55 per cent silica; e.g., basalt); ultra basic rocks (less than 45 per cent silica).

STRUCTURE OF THE EARTH

STRUCTURE OF THE EARTH
The nature of earth's interior is understood by scien­tists through studying the lavas emitted by volcanoes and studying the behaviour of earthquake waves. These obser­vations have led to the conclusion that earth's structure has three parts--the core, the mantle and the crust. Until recently it was thought that the earth's outer crustal layer was composed of rafts of sial which floated on a sea of sima. But recent studies have revealed that large areas of the outer crustal layer are made of basaltic rocks similar to the sima. The earth's crust is now regarded as a series of plates which are gradually being pushed apart. The sial rocks carried on some of the plates from the continents.

CORE
The centre Qf the earth is occupied by a spherical zone called core, about 3475 km in radius. The innermost part of the core may be solid or crystalline, with a radius of about 1255 km, while the outer core has properties of a liquid. The liquid core is considered to comprise iron and a small. proportion of nickel. The temperatures in the earth's core lie between 22000C and 2750°C. Pressures are as high as three to four million times the pressure of annosphere at sea level.

MANTLE
The mantle lies outside the core, a layer about 2895 km thick, composed of mineral matter in a solid state. It is probably composed largely of magnesium iron silicate, which comprises an ultramafic rock called dunite. This rock exhibits great rigidity and high density in response to earthquakes that pass through it. It can also adjust to unequal forces acting over great periods of time.

THE CRUST It is the outermost and thinnest layer of the earth's surface, about 8 to 40 km thick.I The base of the crust is sharply defined where it contacts the mantle. This surface of separation between the mantle and the crust is called Moho (Mohorovicic Discontinuity). The crust varies greatly in thickness and composition-as small as 5 km thick in some places beneath the oceans, while under some moun­tain ranges it extends upto 70 km in depth.

The rocks of this layer can be sub-divided into (i) basaltic rocks, under­lying the ocean basins, containing much iron and magne­sium, and (ii) the rocks that make up the continents which are rich in silicon and aluminium and are lighter in colour and density.

Ccmposition of Earth's Crust The earth's crust is the most significant zone of the solid earth. With an average thickness of 17 km, this mineral skin contains the continents and ocean basins and is the source of soil and other sediments vital to life, of salts of the sea, of gases of the atmosphere and of all free water of the oceans, atmosphere and lands. Oxygen is the predominant element accounting for 46,6 per cent of the weight. It occurs in combination with silicon (27.7 per cent), aluminium (8.1 per cent), iron (5 per cent), calcium (3.6 per cent) and other elements.

GEOLOGICAL HISTORY OF INDIA

GEOLOGICAL HISTORY OF INDIA

The present physical form of the Indian subcontinent is the result of a vast geological formation. India is mainly composed of three geological units: (a) the peninsular plateaus, (b) the Himalayan mountains, and (c~ the Indo­Gangetic plains. Most of the geologists believe that the Indian peninsula, the oldest of the three geological forma­tions, was a part of the global Gondwanaland (continent), which drifted northwards and striking with the Central Asiatic plates raised up to form the high Himalayas out of the Tethys sea.
Geological' formations of India may be divided into four groups: (i) the Archean (the earliest), (ii) the Purana, (iii) the Dravidian and (iv) the Aryan (the youngest). The Archean of India corresponds to the first half of the Pre­Cambrian era, and the Pur ana to the second half of the Pre-Cambrian. The Dravidian covers the period from Cambrian to middle Carboniferous, while the Aryan from the Carboniferous to the Pleistocene (see table on major geological formations of India).

INDIAN ROCK SYSTEMS AND THEIR OCCURRENCE Important Indian rock systems and their characteristics are listed as under:
(i) The Archean System contains the first formed rocks of the earth. These rocks in the peninsula are found primarily in Tamil Nadu, Andhra Pradesh, Chhattisgarh, Jharkhand and Rajasthan. The rocks are primarily gneisses and granites, having no marks of fossils.
(ii) The Dharwar System of rocks are the earliest formed sedimentary rocks, found today in metamorphic forms. These rocks do not contain fossils and are found in Karnataka, Madhya Pradesh, Jharkhand, Meghalaya and Rajasthan. They occur also in the central and northern Himalayas. Schists, slates, quartzites and conglomerates are some of the rocks. This system carries minerals like gold, manganese ore, iron ore, chromium, copper, uranium, thorium, mica and building materials like granites, marbles, quartzites and slates.
(ill) The Cuddapah System of rocks are found in Rajasthan, Tamil Nadu, Andhra Pradesh, Madhya Pradesh and Chhattisgarh. These rocks contain iron ore, manganese, ore slate and marble.
(iv) The Vindhyan System of rocks stand over the
Cuddapah rocks and cover large areas in Madhya Pradesh, Chhattisgarh, Uttar Pradesh and Rajasthan. This system contains rocks like limestones, sandstones, shales and slates which are useful as building materials.
(v) Gondwana System of rocks contain coal deposits and have marks of climatic changes­from arctic cold to tropical and desert conditions. These rocks are found mainly in the Damodar, the Mahanadi and the Godavari val­leys of the peninsula.
(vi) The Deccan Traps Sys­tem of rocks, volcanic in nature, are found in Maharashtra and other parts of the Deccan. These volcanic rocks also contain some thin fossiliferous sedimentary lay­ers found between the lava flows. The volcanic activity in the region led to two great events: (i) break up of the Gondwana landmass, and (ii) uplift of the Himalayas out of the Tethys sea.
(vii) The Tertiary System of rocks are found mostly in the Himalayas. In the peninsula, they occur in coastal areas of Gujarat, Kerala and Tamil Nadu. The Ter­
tiary rocks contain brown coal, rock salt, gypsum and limestone.
(viii) Quaternary System The
important quarternary forma­
tions are Ice Age deposits in Kash­mir, formation of alluvial plains in north India, creation of Rajasthan deserts, Rann of Kachchh, laterite

Origin of the Earth

ORIGIN:
The origin of the earth is currently believed to have been about 4,600,000,000 years ago. This beginning, or zero time, in geological history is marked by the formation of a solid crust. The age of earth is judged by the age of the oldest moon rocks, meteorites, tenestrial lead and the rate of retreat of galaxies.

There are, however, different theories with regard to the origin of the earth: Georges De Button's theory based on the assumption of a collision between a huge comet and the sun; Emanuels Kant's theory of gaseous mass; nebular theory of Laplace; Chamberlain-Moulton's planetismal hypothesis; tidal hypothesis of Jeans and Jeffreys; inter­stellar dust hypothesis of Otto Schmidst; Fasenkov's hy­pothesis; Binary star hypothesis by Russel and Littleton; and nova hypothesis by Hoyle and Littleton.

While most theories believe the earth to have evolved into a partly solid structure, beginning in a gaseous state and going through a liquid state, some other theories propose earth's origin to have been in form of a dust cloud, containing aggre­gation of dust particles-a process perhaps furthered by gravitational force. With the progress of aggregation, higher terrestrial elements moved towards the top while the heavier ones gravitated below. Transformation of earth into a cooling body from the original hot state and, on the other hand, warming of the earth eventually, beginning as a cold body, are yet other issues in the debate about the origin of earth.

GEOLOGICAL HISTORY Geology deals with the origin, composition and history of the earth. The various stages of earth's history are classified by using terms for division of time and/or terms applying to strata accumulated during the various periods. The 'time terms' are Era, Period, Epoch and Age. Their corresponding 'strata terms' are Group, System, Series, Stage or Formation.

An outline of the earth's history is given in the form of a chart on the next page.
The Archaeozoic and Proterozoic eras, the earliest and longest period of earth's history, are also called Precam­brian time. In the Archaeozoic era, continents are believed to have taken shape and grown, even as oceans and' atmosphere were formed. Rocks of this era contain the earliest fossils, some three-and-a-h.alf billion years old. These fossils are of primitive bacteria and cyanobacteria. Fossils of the first animals-worms, jellyfish, corals-appear in rocks about 700 million years old, i.e., in the Proterozoic era.

Towards the end of the Palaeozoic era, the Appalachian geosyncline disappeared and the Appalachian Mountains were built up as a result of the collision of the North American plate with the Eurasian, African and South American plates to form Pangaea. This supercontinent included India, Australia and Antarctica. In the Ordovician Period occurred a great flood which covered vast areas of North America, besides creating large shallow seas.
The cretaceous period of the Mesozoic era gets its name from creta, the Latin word for chalk.

The Mesozoic era began with most continents exposed as land.
During the Triassic and Jurassic periods, Pangaea brok. apart to form Laurasia and Gondwanaland. In the Creta. ceous period, the continents we know today were bein~ formed as Laurasia broke up to form Eurasia and Nortl
America, while Gondwanaland broke up into Africa, Ant arctica, Australia, India and South America.
In the Cenozoic era, the European Alps, Andes ani Himalaya mountain ranges were formed. Many volcanoe erupted across western North America, Greenland ani India, forming coverings of lava. The individual continents in the Pleistocene epoch, glaciers covered much of , earth and then melted. The wide variety of plants a animals that we know today came into existence dwi the Cenozoic era.

The earliest fossils of human-like creatures are tl found in rocks of the Pliocene epoch.