The Earth—Its Motions and their Effects
The earth has two motions,
viz., (1) Rotation around its axis or the daily motion. The axis of the earth is an imaginary line inclined at 66.5° to the plane of the orbit of the earth. The earth rotates round its axis from west to east once in 24 hours. Effects: Days and nights are caused. The sun, moon and other heavenly bodies appear to revolve round the earth from east to west. Direction of winds and currents is changed.
(2) Revolution round the sun on its orbit, or the annual motion: The earth revolves round the sun once in about 365.25 days. Effects: It causes seasons; days and nights are of unequal length at the same place.
Important elements in the earth’s crust
The five most abundant elements in the earth’s crust are: Oxygen, Silicon, Aluminium, Iron and Calcium. (The other three are Sodium, Potassium and Magnesium.)
Oceans—Their Importance
Oceans are the source of all water on earth as the evaporated water from over their surface is brought to earth by the winds passing over them. They are the highways of the world and most of the world trade is carried through the sea. Innumerable fish and other animals living in the oceans are a great source of food to mankind. Minerals like salt, iodine etc. are derived from the ocean waters and sea-weeds.
Ocean Currents: are rivers of warm or cold water flowing in an ocean. Their banks and beds also consist of water.
Natural Regions
A natural region is a large area in which the topography, climate and vegetation are largely similar, and therefore there is a certain uniformity in human activities.
Natural Regions of the World
(1) Equatorial Region (2) Hot-Grassland Region (3) Monsoon Region (4) Hot Deserts Region (5) Mediterranean Region (6) Steppe Region (7) Tundra Region (8) Warm Temperate Region (9) Cool Temperate Region.
Natural Regions of India
(1) The Himalayas and the adjacent mountains; (2) The Sutlej-Ganga plains; (3) The coastal plains of Western and Eastern ghats; (4) The Deccan plateau.
Factors Determining Climate of a Place
(1) Distance from the Equator (2) Height above sea-level (3) Distance from the sea (4) Winds (5) Direction of Mountains (6) Ocean currents (7) Slope of land (8) Nature of the soil (9) Forests.
Factors Determining Temperature
(i) sun rays, (ii) height above sea-level (iii) movements of atmospheric winds, (iv) ocean currents.
Rainfall
Two important conditions must be satisfied in order to have rain: (1) There should be moisture-laden air, (2) There should be some means whereby air is cooled and condensation takes place. The air obtains water vapours by evaporation from the surface of large bodies of water, usually from the sea.
Monsoons in India
Monsoons are periodic winds which blow from sea to land for six months in summer and from land to sea for six months in winter. Monsoon winds prevail over India at different seasons.
South-West Monsoons: These are rain-bearing winds which prevail from about the end of May to the end of September. During summer, the sun’s rays fall vertically on the Tropic of Cancer making the Indian plains intensely hot. But the rays of the sun fall obliquely over the Indian Ocean during this period. The land is hotter than the sea, there is, therefore, low pressure over the land and high pressure over the sea. The winds blow from high to low pressure i.e., from the sea to the land, and are therefore wet winds. Because of the rotation of the earth, the monsoon winds blowing over India deflect to the right after crossing the Equator and become south-west winds. These are, therefore, called south-west monsoons.
India depends largely on these rain-bearing south-west winds. These winds give to India about 90% of the total rainfall. During their prevalence, the chief crops cultivated are rice, cotton, tobacco, tea, jawar and bajra.
North-East Monsoons (or Winter Monsoons): During the months of November to January i.e., in winter, the sun’s rays fall vertically on the Tropic of Capricorn. The air over the Indian Ocean during this period thus becomes hot and light and there is low pressure. The sun’s rays fall obliquely on the plains of India during these months with the result that the air over these plains is cold and heavy and there is high pressure. The winds, therefore, blow from plains to the Indian Ocean. While crossing the Equator, they deflect to the left and are known as north-east monsoons.
The North-East Monsoons bring only about 10% of the total rain to India as they are chilly and dry land winds. But the moisture that they pick from the Bay of Bengal, little as it is, is very useful. Wheat, barley, oats, oilseeds and sugarcane are cultivated during this season.
Thus these monsoon winds have much importance for India.
Weather and Climate
Weather means the atmospheric conditions e.g., temperature, rainfall, humidity, winds, sunshine and cloudiness of a particular place on a particular day. Climate, on the other hand, is the average condition of weather obtaining in a country or a place for a considerable period.
India has a great diversity of climatic conditions. Lying largely within the tropics and in the great Asiatic Continent and the vast expanse of the Indian Ocean to the South, the climate of India is essentially the tropical monsoon type. The average annual rainfall in India is 42 inches.
Types of Soil in India
The main categories of soils in India are: (i) Alluvial soils (ii) Black soils (iii) Red soils (iv) Laterite soils (v) Mountain and hill soils (vi) Terai soils (vii) Desert (or Arid) soil and (viii) Peat soils.
Alluvial soil and Black soil
Alluvial soil is that soil which is formed by deposition of silts brought down by the rivers. It is rich in hydrated oxides of iron and is very fertile. Black soil or the black cotton soil has a good water-holding capacity and is best suited for deep-rooted crops like cotton. The black soil in wet condition is compact and sticky.
The most extensive soil cover of India comprises alluvial soils.
Soil Erosion: The soils are usually six to twelve inches in depth. In course of time, the fertility level of the soil is depleted with the result that the soil no longer remains suitable for agriculture. Soil conservation is, therefore, necessary for continued agricultural prosperity.
The agencies of erosion are winds, water and waves of which the water erosion is most common. Rain water removes soil from the surface of sloping lands. Winds remove top soil of lands.
Laterite soils are formed by the weathering of laterite rocks. These can be distinguished from other soils by their acidity. Laterite soils are generally poor on the higher levels and cannot retain moisture. In the plains, however, they consist of heavy loams and clay and can retain moisture.
Laterite soils occur in Madhya Pradesh, Assam and along the Eastern and Western Ghats. Tea plantation requires acidity which is there in the laterite soil. It is, therefore, common in these areas.
Star and Planet
Star is the name given to a fixed celestial body which has its own light whereas Planet is the name given to a celestial body which revolves round the sun in elliptical (regular oval shape) orbit. A planet has no light of its own but reflects the light of the sun.
Rocks
Three main groups of rocks: Igneous, sedimentary and metamorphic.
Classification of rocks
Igneous rocks: granite.
Sedimentary rocks: sandstone; limestone; shale; coal.
Metamorphic rocks: marble.
Phyllite: This rock is formed by deposits of animal shells and skeletons.
Land Breeze and Sea Breeze
Land Breeze: At night, land masses cool quicker than the sea. Therefore, in calm, cloudless weather, an air-stream passes from the land to the sea. This breeze carries no moisture, and is a little warm.
Sea Breeze: In day-time, the land is hotter than the sea. The air over it rises, and is replaced by a cool breeze from the sea carrying some moisture.
Tides
Alternate rise and fall of waters of the ocean twice in the course of nearly twenty four hours is termed as “tides”. The tides are caused by the gravitational force exerted by the moon and to a lesser degree by the sun, on the earth. The tides do not always rise to the same height. At the time of the new and full moon, when the sun and moon are in a straight line with the earth, the tides rise higher and are known as Spring Tides. Midway between new and full moon when the sun and the moon are at right angles as to their direction from the earth, tides are at the lowest height and are called Neap Tides.
Spring Tides and Neap Tides
When a high tide is caused twice a month at new moon and again when the moon is full, spring tide is caused as a result of combined attraction of the sun and the moon.
When the high tide is not so high, nor the low tide so low, neap tides are caused as a result of the difference of attraction of the sun and the moon.
Seasons
The change of seasons is due to (i) revolution of the earth round the sun (ii) inclination of earth’s axis at 66.5° to the plane of its orbit and always pointing to the same direction. On the 21st June, the North Pole is inclined towards the sun and the South Pole is inclined away from it. The rays of the sun fall perpendicularly at the Tropic of Cancer (23.5° North) and fall comparatively slanting in the southern hemisphere. Hence the days are longer than nights in the northern hemisphere and it is summer there. Just opposite is the case in the southern hemisphere where the nights are longer at that time and it is winter there.
Latitudes and Longitudes
India lies entirely to the north of the Equator, between latitudes 8°-4´ and 37°-6´ north and longitude 68°-7´ and 97°-25´ east.
The latitude of the South Pole is 90°. South Pole has no longitude.
Longitude of a place is its distance east or west of a fixed meridian. The distance of any place north or south of the Equator is called the Latitude of that place.
Parallels of latitude: are lines drawn on a map (or globe) showing the latitude of a place.
Meridians (or lines) of longitude: These are lines drawn on a map (or globe) showing the longitude of a place. These lines join the north and south pole cutting the Equator at right angles.
(Latitudes and Longitudes should be clearly distinguished from Parallels of Latitude and Meridians of Longitude respectively.)
By knowing these lines, we can find out exact location of a place. By knowing the latitude of a place we can find out its average temperature, as also its distance from the Equator. By knowing the longitude of a place, we can calculate its local time.
Longitude is the angular distance of a place east or west of the prime meridian. The earth rotates upon its axis once in 24 hours and covers 360° in 24 hours. Thus it takes 60 x 24/360 or 4 minutes to cover a degree of longitude or we may say that in four minutes, the earth moves through 1°. There is thus a difference of 4 minutes for each degree of longitude. This fact is used for determining the longitude of a place. All longitudes are measured from the meridian of Greenwich.
We can
determine the latitude of a place in the northern hemisphere by measuring the altitude of the Pole Star. The altitude of the Pole Star is the latitude of that place. For example, if the altitude of Pole Star at Delhi is 28.5° North, its latitude will also be 28.5°N. The altitude of Pole Star is measured by an instrument called Sextant.
Solar Eclipse and Lunar Eclipse
Solar Eclipse: is the partial or complete obscuration of the sun because of the passage of the moon in front of it i.e., when the moon comes in between the sun and the earth.
The moon then appears as a dark object obscuring the sun. Over a small portion of the earth’s surface, the moon is seen to blot out the sun completely and a total eclipse is seen by the people in that particular area. But over most of the earth’s surface, the eclipse seen is partial because only a portion of the sun’s face remains covered by the moon.
Lunar Eclipse: is the partial or complete obscuration of the moon’s surface when the earth comes in between the sun and the moon. The moon, when it moves through the shadow of the earth, loses its bright direct illumination by the sun, although its disc still remains faintly visible.
An eclipse of the moon is visible and presents the same features at all places on the earth where the moon is above the horizon. The lunar eclipse can be seen with the naked eye, field glass or a small telescope.
The lunar eclipse occurs at full moon only when the earth comes in between the sun and the moon which phenomenon does not occur at every full moon.
Indian Standard Time
The Indian Standard Time is a uniform time adopted by all palces in India without regard to their local time. It is usual for each country to have its standard time for use over the whole country as it would be very difficult if every town or village had its own local time and whenever we moved from one place to another, we should have to alter our watches.
Indian Standard Time is the local time of a place near Allahabad situated at 82.5° East longitude.
Winds
Air moving from one direction to another horizontally is called wind. It is the air in motion.
Cause of Wind: The chief cause of winds is difference in pressure. Air always moves from region of high pressure to a region of low pressure to equalize the pressure. For example, the low pressure belt round the Equator is a region of calm known as the doldrums. Although there are no regular winds there, violent squalls and thunderstorms are frequent which come from high pressure areas north and south of the Equator.
Direction of Winds: As the earth is rotating daily on its axis from west to east, all winds are deflected. According to Ferrel’s Law, winds are deflected to the right in Northern Hemisphere and to the left in the Southern Hemisphere.
Characteristics of the important Wind Systems of the World:
1. Trade Winds: The rays of the sun fall almost vertically at the Equator and the air there becomes hot and the pressure is low. The air rises towards the Poles and descends near 30°N and 30°S. The pressure is high near 30°N and 30°S. Because winds blow from high-pressure to low-pressure areas, winds from over these altitudes blow towards the Equator and Trade Winds are caused.
2. Westerlies (or Anti-Trade Winds): are winds which blow from about 40 degrees N to the Arctic Circle and from about 35 degrees S to the Antarctic Circle throughout the year. They derive their name from the direction in which they blow. In the northern hemisphere they blow in the south-westerly direction and bring winter rain to the Mediterranean regions etc. In the southern hemisphere, they blow in a north-westerly direction.
3. Polar Winds: The winds which blow from the high pressure area around the poles towards the temperate regions are known as polar winds. They are extremely cold. They rise from the North West in the Northern Hemisphere and from the South East in the Southern Hemisphere.
4. Periodical Winds: These are (i) Land and Sea Breezes and (ii) Monsoons which blow in one direction at a particular time or during a particular season. In the hot season in India, the sun shines vertically over the Tropic of Cancer, i.e., roughly over the great plains of the Ganges and Brahmaputra so that the air over the plains becomes very hot by about the month of May. At this time, South West Monsoon commences to blow. They bring heavy rains. Monsoon winds prevail over India at different seasons. India depends on the rain-bearing south-west winds which prevail from about the end of May to the end of September. These winds bring to India about 90% of all the rain that falls there.
5. Variable Winds: are the irregular winds as Cyclones and Anti-Cyclones.
Rainfall
There is heavy rainfall on the West coast because the Western Ghat ranges receive the full force of the monsoons from the Arabian sea and there is heavy rainfall (about 100 inches). On the other hand, the Deccan Plateau gets very scanty rainfall because it falls within the rain-shadow area.
Chennai gets winter rainfall as the north-east monsoons which blow in winter pick up moisture from the Bay of Bengal and bring rain to that city.
The Bay of Bengal monsoons first bring rain to the eastern parts and then turn westwards. As Kolkata is in the east, it receives more rainfall. As the monsoons blow westwards they become drier and cause less rainfall. So Delhi does not get as much rainfall as Kolkata.
In the northern region, the Bay of Bengal monsoons first bring rain to eastern parts and then turn westwards. As the monsoons blow westwards, they go on losing moisture and cause decreasing rainfall.
In the southern region, the Arabian Sea monsoons first strike the western ghats and the moisture is drained on the western side whereas rainfall goes on decreasing towards eastern region.
Two important conditions must be satisfied in order to have rain: (1) There should be moisture-laden air, (2) There should be some means whereby air is cooled and condensation takes place. The air obtains water vapours by evaporation from the surface of large bodies of water, usually from the sea.
The moisture-laden air is cooled in two ways: (i) by rising upward into colder upper regions of the atmosphere, (ii) by blowing as wind to colder regions.
Thus we see: (a) Moist air is lighter than dry air and so it readily rises, expands in a short time, cools and falls. (b) When warm winds blow towards cooler regions, it is condensed by cooling effect and rain falls. (c) The land masses or mountains also tend to condense water vapours. When moisture-laden wind is obstructed by mountains, it is forced to rise. As it rises, it becomes cool and rainfall results.
Rivers
The work of a river is three-fold:
(i) The Mountain Stage: The mountain or upper course of a river is swift as the slope at this stage of a river is steep. The main work of a river at this stage is denudation (wearing away). In this swift upper course, the rivers carry big stones, pebbles etc. which go on eroding the sides and beds of the valleys. As time goes on, the river cuts away the spurs on both sides and the valleys become wider and deeper. The mountain stage of the Ganges in India extends from its source up to Hardwar.
(ii) The Plain Stage: In this stage the river moves slowly as the slope is gradual and its main work is transportation (navigation) and irrigation. The plain stage of the Ganges extends from Hardwar to Bhagalpur.
(iii) The Delta Stage: This is the last stage and the rivers are very slow at this stage. In this slow lower or deltaic course, the main work of the river is deposition. The level of the bed at this stage rises due to mud and silt brought by it and deposited into several channels before falling into the sea. The Ganges forms her delta from Bhagalpur up to the sea.
The deltas are not formed at the mouths of rivers where tides carry away all the mud and silt deposited (at the mouth) e.g., the Narbada and the Tapti do not form any delta. Also rivers which deposit all their mud into the lakes through which they pass do not form delta e.g., the St Lawrence in Canada.
Estuary is formed at the mouth of a river where tidal effects are evident and where fresh water and sea water mix. In most cases it is due to subsidence of coastal low-land.
Delta is the triangular piece of land formed by the deposition of mud and silt near the mouth of a river. In the case of delta formation, more solid material is deposited which cannot be removed by tidal or other currents.
The rivers of Northern India are more important than those of Southern India because they have a flow of water throughout the year. Even in summer these rivers receive water from the melting of Himalayan snow. Flowing through broad basins, they form large tracts or rich alluvial soil on either side. It is no wonder, therefore, that their fertile basin are the natural grannaries of the country. Further, the Ganga and the Brahmaputra are navigable and provide excellent waterways for commerce. The Peninsular rivers, on the other hand, have water during the monsoons but shrivel into muddy pools in the dry season. These rivers are of little use for navigation on account of their torrential nature in the upper course, and the rapids that occur where they descend into deep gorges from the table land to the coastal plains.
Climate and Vegetation
The
Equatorial type climate, in which the temperature remains high all the year round but does not vary much, produces hot, wet forests.
The
Tropical type climate produces grasslands which are found on either side of the equatorial belt where the rainfall usually occurs soon after the sun has been shining vertically while the dry season occurs in the colder part of the year.
The
lowlands along the Tropic of Cancer lie mainly in the high-pressure belt just outside the Tropics. The Trade Winds blow away from these lowlands towards the Equator and the Westerly winds blow away from them towards the Poles. There are, therefore, no winds to bring rain to this region. Some of these lowlands are dry because these are very very far from the sea, like centre of Asia. There are few clouds and very little rain with the result that the sun’s rays strike straight on the ground and make the days very hot.
The temperature of the ocean varies much less than that of land because (i) water has a higher specific heat than land with the result that it both absorbs and loses heat slowly as compared to land; and (ii) due to large surface of water at sea more evaporation occurs than on land. Evaporation causes cooling and this results in the sea having a lower temperature than that on land.
We may divide India into two parts for the purpose of climatological studies: (1) peninsular India and (2) Northern India. Peninsular India has the characteristic of tropical climate where “the temperature is uniformly high and seasonal variation relatively low”.
The climatic conditions in Northern India have no general similarity. This region lies beyond the Tropic of Cancer. The Western part of it includes East Punjab and Rajasthan where air is devoid of moisture and it is hot in summer and very cold in winter. The eastern part of this region includes U.P., Bihar, Assam and West Bengal. Here winter is mild and summer is very hot with plenty of moisture in the air.
These climatic conditions are however, disturbed by two Monsoon Currents—the South West Monsoon and the North-East Monsoon. The South-West Monsoon causes heavy rainfall in Assam, West Bengal and U.P. It begins to retreat from Northern India in early October and this retreat is completed by mid-December. During this retreat period the weather in Northern India becomes dry.
The North-East monsoons begin in January and last till March. These winds cause light rain in Northern India, particularly in the Punjab plains. This scanty rainfall is very important for Rabi crops.
Vegetation: Agriculture is the most important occupation of the people of India. In Northern India, typical monsoon land crops are grown such as rice in Bengal with its warm and humid climate; wheat and maize in Northern plains, Punjab and U.P.; jute in Bengal and Assam and tea in Assam.
In Peninsular India where regur or black cotton soil is found and sufficient moisture available, cotton is grown. It is the chief crop of the Deccan Peninsula—Mumbai and Berar being the chief producers. Coffee is grown on the Nilgiris in the South.
Climatic Effect: India has on the whole monsoon-tropical climate: ‘Monsoon—lands are dominated by the winds from sea to land in summer—the wet season and by winds from land to sea in winter—the dry season.’ This type of climate is not very conducive to health and vigour. Man’s well-being in such a climate depends largely on rainfall. The agricultural products do not grow if the monsoon fails and famine conditions break out. This dependence on rain, however, is not absolute owing to development of irrigation by means of projects, canals, wells etc.
The desert type climate is hot and dry. The rainfall is scanty, not more than 10 inches a year. The day and night temperatures vary to much extent. The evenings and afternoons are marked by hot dust storms.
The regions lie mainly in the high-pressure belts just outside the Tropics. The Trade Winds blow away from them towards the Equator and the Westerly Winds blow away from them towards the Poles. There are no winds which bring rain to this region and the climate remains hot and dry.
Mediterranean Climate
It is the type of climate experienced by the lands bordering the Mediterranean Sea, and also by other regions, in both hemispheres, situated in a similar geographical position. The characteristic features are warmth of the summer, mildness of the winter, and ample sunshine.
The entire west coast of the United States has Mediterranean type of climate because this region gets winter rainfall from “Westerlies” winds.
Irrigation
Methods of Irrigation: The various systems of irrigation used in India are: (1) Canals; (2) Wells; (3) Tube-wells; and (4) Tanks.
Canals: Canals are the most important of the systems of irrigation in India because:
(i) the rivers are snow-fed and never run dry; (ii) the plain has a soft and alluvial soil, so canals can be easily dug; (iii) the rainfall is insufficient for irrigation and wells alone cannot satisfy the needs of agriculturists.
Of the total irrigated land in India, 40 per cent is irrigated by canals.
Wells: Wells are found all over India but these are largely used in Uttar Pradesh, Punjab, Haryana and Bihar. They are also used in Tamil Nadu, Maharashtra and Rajasthan. The reason for irrigation by wells is that the soil is porous and after a rainfall, water is stored up below the soil, and wells can be easily sunk.
Tube-wells: Irrigation by tube-wells has become very popular these days. Tube-wells are worked by electric power. These are much deeper than the ordinary wells. Due to shortage of power, the agriculturists do face the difficulty in running the tube-wells as and when they require but the prosperous ones are making use of the diesel engines for the purpose.
Tanks: Tanks are used in the Deccan plateau—especially in Tamil Nadu, Andhra Pradesh, Karnataka and in some parts of Madhya Pradesh. They are made by filling natural hollows with water or by building dams across the river valleys. As the soil is rocky in these areas, it is not easy to sink wells. The soil is not porous and the rain water flows off; Canals cannot be constructed as the rivers are not snow-fed. So the tanks are the chief means of irrigation in the Deccan plateau.
Volcanoes and Earthquakes
Volcanoes: By the pressure of the earth’s crust the hot matter or lava in the interior of the earth is pressed down. It gushes out through a crack or a hole when it finds a weak spot in the crust and begins to accumulate round it. By and by it cools down and solidifies and in the course of several years these accumulated layers of lava build up a conical mountain. Such lava mountains are called volcanoes.
Volcanoes are also formed when rain or sea water percolates in the soil and sinks deep down into the earth where it is converted into steam by the internal heat and forces its way out of the crust bringing with it large quantity of lava etc.
Earthquakes: (i) When an active volcano bursts with great force or when a dormant volcano erupts into activity, the surrounding areas feel tremors and earthquake is caused. (ii) When the interior part of the earth cools down and contracts, the outer crust cracks or a part of it actually drops down causing earthquake. (iii) Sometimes water percolates deep down into the earth and is converted into steam on account of internal heat. This steam forces its way out by expanding and thus causes earthquake shocks.
Fold and Block Mountains
Fold Mountains: These are formed as a result of series of earthquakes by which in course of a long time, rocks are folded up above the general level and the agents of denudation start to wear them away. The Himalayas, the Andes, the Alps are example of Fold Mountains.
Block Mountains: the formation of mountains when a mass of land is pushed up between several cracks, is known as Block Mountains as shown in the figure below. The narrow piece of the crust led down between two parallel cracks forms what is called “Rift Valley”.
Mountain Ranges
Himalayas: The Himalayan ranges stretch for about 2400 km from the eastern extremity of Assam to the western limit of Kashmir. Their width varies from 150 km to 450 km. These are fold mountains and consist of long lines of folded ranges.
Arvalies: It stretches from Gujarat in the west to Delhi in the north.
Indian Plateau: It is the table-land region of the Deccan lying south of the Indo-Gangetic Plain. It is bounded on the north by the ranges of Vindhyas and the Satpuras running east to west.
Vindhyas and Satpura: The Vindhyas lie north of the Narbada Valley, whereas the Satpuras Range lies south. Satpura ranges are an example of Volcanic mountains.
Western Ghats: In the west, the plateau is margined by the Western Ghats which rise abruptly from the Malabar and the Konkan coasts and run parallel to the sea coast with an average height of 1200 metres.
Eastern Ghats: Towards the east are broken Eastern Ghats which descend to the low-lands of the Coromandal coast and are broken by a number of rivers, the most important of which are the Mahanadi, Godavari, Krishna, Penner and Cauvery. These rivers flow south-east across the plateau to the Bay of Bengal.
