SIZE AND LOCATION
India with an area of 32,87,782 sq. km., is the seventh largest country after Russia, Canada, USA, China, Brazil and Australia in that order and the second most populous (next to China) country in the world. It extends between 8°4' and 37°6' north latitudes and 68°7' and 97°25' east longitudes, lying north of the equator and thus belongs to Northern Hemisphere.
The Tropic of Cancer (23°30' N) divides the country into almost two equal halves. While the southern half coinciding with peninsular India lies in the tropical zone, the northern half, somewhat continental in nature, belongs to the subtropical zone. Situated to the east of the Prime Meridian, India also belongs to the Eastern Hemisphere.
Fig: India is the 7th largest country
The country is of vast size. The north-south extension of India covers about 30°of latitude or measures about 3,214 km and the east-west extension covers almost 30° of longitude or measures about 2,933 km which is one-twelfth of the Earth’s circumference at the Equator.
India has a land frontier of 15,200 km and a coastline of about 6,100 km. India takes its standard time from the meridian of 82°30' E, which is 5 (1/2) hours ahead of Greenwich Mean Time (0° longitude). Pakistan time is 5 hours ahead of GMT and Bangladesh time is 6 hours ahead of GMT.
The Indian subcontinent is a peninsula, more or less triangular in shape, which adjusts southward from the mainland of Asia. The lofty mountain ranges of the Himalayas in the north form the base of the triangle while the apex runs far out into the Indian Ocean in the south.
It deserves to be called a subcontinent-meaning a large, relatively self-contained landmass forming a subdivision of a continent-both because of its large area and its large population.
India occupies the southern part of the Asian continent, which is not only the largest but also the most populous continent of the world. Such a location has its own geopolitical and economic advantages.
In ancient times its location helped in establishing cultural and other contacts with the Arab world in the west and the south-east Asia and the Far East in the east.
SIGNIFICANCE OF LOCATION
The country is isolated in a remarkable way from the rest of Asia, making it a distinct geographical unit. Barring the plateau of Baluchistan (which forms part of Pakistan), the two great ranges of Sulaiman and Kirthar cut it off from the west. Along the north, the great mountain wall formed by the Hindukush, Karakoram and the Himalayas, which is difficult to cross, cuts it off from the east of the continent.
Fig: Sulaiman and Kirthar Range
- Similarly, the southward offshoots of the Eastern Himalayas separate it from Russia. The mountain barriers thus secure the country from the rest of Asia.
- In the west, the Indian Ocean in the south and the Bay of Bengal in the east. These again perform the security function besides providing a very good infrastructural component for development of not only a transport system but also a defence system. The tropical monsoon climate of India, which ensures a fair supply of moisture and forms the basis of farming in India, is also a result of its location in the southern part of Asia.
- India’s strategic location at the head of Indian Ocean from where the entire Asia could be controlled had encouraged the British to make it the base of British empire in the east. India is favorably situated on the world’s highway to trade and commerce both to east and west.
- The oceanic routes serving East and South-east Asia and Australia in the east and the countries of West Asia and Africa in the west pass through both the routes—the Cape of Good Hope and Suez Canal. Since the opening of Suez Canal (1867) India’s distance from Europe has been reduced by 7000 Km. It thus bridges the space between the highly industrialized nation of the west and the semi-arid, and south-western Asia and the most fertile and populated regions in the south-east and far-east countries.
- The lack of islands in the Indian Ocean and the absence of any other country having such a large coastline this ocean as ours, with natural resources and population comparable to that of India, makes India even today, by far the most significant among the countries bordering the Indian Ocean.
The bending of rock starts due to compressional forces acting tangentially or horizontally towards a common point or plane from opposite directions is known as folding. In a simple series of folds, anticlines (up folds) may be distinguished from synclines (down folds).
Fig: Fold mountain
- The central line of either fold is known as axis, while the two sides are known as the limbs. If one side of the fold is steeper than the other, it is said to be asymmetrical.
- A drag fold is a minor fold, formed either subsidiary to the main fold, or along the side of a fault, where the vertical displacement has made flexures and puckers in the rocks on either side.
- The rest of a fold is usually structurally weak, and igneous material or masses of salt may be forced through the cracks and fissures; these intrusions are known as diapirs and the fold as diapiric or piercement folds.
- In some fold systems, the main anticlines and synclines appear to have numerous minor folds superimposed upon them. These are called anticlinorium and synclinorium.
If the folding movements are very intense, the asymmetrical anticline is pushed right over, and it becomes over fold. If it is pushed still further, it becomes a recumbent fold. In extreme cases, fracture may occur in the crust, so that the upper part of the recumbent fold slides forward over the lower part along a thrust plane, forming an over thrust fold. The over-riding portion of the thrust fold is called Nappe. The Klippe is a surviving portion of a nappe almost-destroyed by denudation.
Faulting: Faulting owes its origin to tension i.e. when forces act horizontally in opposite directions away from a given plane or a point. In a normal fault, usually the result of tension, the inclination of the fault plane and the direction of downthrow are either both to the left or both to the right. In a reverse fault, resulting from compressional forces, the beds on one side of the fault plane are thrust over the other.
Fig: Fault Mountain
- A tear or lateral fault is one in which rocks are displaced horizontally along a line of fracture.
- The amount of vertical displacement, if any, on a fault is referred to as the throw of the fault.
- The amount of lateral displacement is known as heave.
- The angle of inclination of the fault plane from the vertical is the head.
- The rock face on the upper side of the fault is the hanging wall, on the lower the foot-wall.
- An escarpment is caused by faulting.
- The land between the two parallel faults either rises forming block mountains or horsts or subsides into a depression termed as a rift valley or graben mountains.
- Igneous intrusions, sometimes make room for themselves by lifting up the overlying layers of the country rocks.
- Thus a dome-shaped structure is formed which are sometimes large enough to be described as dome mountains i.e. laccolith.
Example: Henry Mountain in Utah (USA).
- These rocks have been formed inside the earth, under great pressure and heat.
- They do not occur in layers and most of them are crystalline.
- Some rocks like granite, have cooled slowly and contain large crystals: others like basalt have cooled quickly and contain small crystals.
- Some rocks contain a high percentage of silica and are called acid rocks. Granite is a good example.
- Rocks like basalt contain a high percentage of iron, or aluminium or Magnesium oxides and are called basic rocks.
1. Igneous Rocks:
They do not contain fossils. Igneous rocks are parents of all other rocks and also known as ‘Primary rocks’. Basalt is fine-grained and granite is coarse grained.
There are two main groups of Igneous:
(a) Volcanic: Also called extrusive igneous rocks. Magma erupting and solidifying after escape of gases as lava on reaching the earth surface.
- Basalt is the typical example of extrusive type, other examples— Andesite, Lava-obsidian etc.
(b) Plutonic: Also called intrusive igneous rocks. This is formed by solidification of magma at moderate depths beneath the earth’s surface.
- Most common examples: granite, dolerite and Gabbro etc.
Fig: Igneous Rocks
Igneous Rock Bodies:
- Batholiths: Batholiths are the largest intrusive igneous rock bodies. Batholiths are great granite masses forming the cores of world’s mountain systems.
- Stocks: The batholiths covering small areas are called stocks, having somewhat rounded form and some general characteristics.
- Laccoliths: These are huge masses of igneous rocks that have been formed between horizontal or slightly tilted layers of sedimentary rock near the earth’s surface. These are dome shaped and appear like a mushroom or a loaf of bread. It may be regarded as inscarried volcano.
- Lopolith: When batholiths are deposited into a concave form it is called lopolith.
- Phacolith: When magma is deposited in a wave like form.
- Dykes: These are near vertical formations. They come into existence when liquid material passes through cracks in the country rocks gets hardened into a rock.
- Sills: The magma trying to reach the surface squares between two rocks layers and solidifies into a thin sheet in horizontal position parallel to the layers of already existing rocks.
2. Sedimentary Rocks:
"Sedimentary rocks constitute 75% of the earth’s surface but only 5% of the earth’s crust. All sedimentary rocks are non-crystalline. They contain fossils."
Fig. Sedimentary rocks
Mechanically formed sedimentary rocks:
- Wind-deposited sedimentary rocks—Loess.
- Glacier-deposited sedimentary rocks—Moraines, sands and gravels and boulder clay or sill.
- River-deposited—clays, alluviums etc. By compaction—sandstone (from sand).
- By cementation—Shale (from clay) Conglomerate and Breccia.
- Organically formed sedimentary rocks:
(i) By animals—chalk, coral and limestone.
(ii) By Plants—Coal, peat, lignite
- Chemically formed sedimentary rocks-Rock-salt, borax, gypsum, nitrates, potash.
3. Metamorphic Rocks:
When the original character of rocks—their colour, hardness, texture and mineral composition is partly or wholly changed, it gives rise to metamorphic rocks, under favourable conditions of heat and pressure.
The formation of metamorphic rocks under the stress of pressure is known as dynamic metamorphism. In this case, granite is converted in genesis; clay and shale are transformed into schist. The change of form or recrystallisation of minerals of sedimentary and igneous rocks under the influence of high temperature prevailing within the earth’s crust is known as thermal or contact metamorphism.
Fig: Metamorphic Rocks
- Sandstone changes into quartzite
- Clay and shale into slate
- Coal turns into anthracite and graphite
- Limestone into marble slate may further change into phyllite (at 150°C to 200°C).
4. Quartzite Rock:
It is one of the most resistant to weathering, forming mountains and ridges. It is used in glass-making.
Fig: Quartzite Rock
A volcano is ‘Active’ when it is erupting intermittently or continuously. A volcano which has not erupted for a long time is known as ‘Dormant’, whereas an ‘Extinct’ volcano is one which has stopped eruption over a long time.
On the basis of mode of eruption as well as on the basis of nature of eruption, different types of volcanoes have been recognised.
Based on the mode of eruption, volcanoes are classified as
(1) Central types: where the products escape through a single pipe (or vent).
(2) Fissure types: where the ejection of lava takes place from a long fissure or a group of parallel or closed fissures.
Based on the nature of eruption, volcanoes may be of two types as
(1) Explosive Types: In which case the lava is of acidic (felsic) in nature and because of their high degree of viscosity, they produce explosive eruptions.
(2) Quiet Types: In this case, the lava is of basaltic composition (mafic lava), which is highly fluid and holds little gas, with the result that the eruptions are quiet and the lava can travel long distances to spread out in thin layers.
Besides the above, a number of other types of volcanoes have been identified. According to their degree of explosive activity and nature of eruption. They are as follows
- Hawaiian type: Silent effusion of lava without any explosive activity.
- Strombolian type: Periodic eruption, with a little explosive activity.
- Volcanion type: Eruption takes place at longer intervals and the viscous lava quickly solidifies and gives rise to explosions of volcanic ash.
- Vesuvian type: Highly explosive volcanic activity and eruption occurs after a long interval (measured in tens of years).
- Plinian type: The most violent type of vesuvian eruption is sometimes described as plinian. Here huge quantities of fragmental products are given out with little or no discharge of lava.
- Pelean type: This is the most violent type of all the eruptions. They are characterised by eruption of ‘nuess ardentes’.
It includes both positive as well as negative relief features. The high or elevated relief features comprising of hills, mountains, cones, plateaus or upland plains are some of the examples of positive relief feature, while the low lying relief features like craters, calderas, tectonic depression etc. represent the negative relief features.
These features are formed due to both quiet as well as explosive volcanic activity, and some of which are as follows:
- Hornitos: These are very small lava flows.
- Driblet cones: The most acid lavas often given rise to quite small conelets and are known as driblet cones.
- Cinder cone: These are volcanoes of central type of eruption, steep-sided with uniform slopes of 30° to 40°.
- Lava cone: These are built up of lava flows, due to heaping of lava during quiet type of eruption. It is also known as lava or ‘plug-dome’.
- Composite cone: These are made up alternatively of pyroclastic material and lava. Due to rude stratification, they are also known as ‘Strato-volcanoes’.
- Shield volcanoes: These are made up of lava alone and due to quiet type of eruption, whereby piling up of flow after flow of fluid lava, a rounded dome like mass is produced.
- Spatter cone: Small cones formed on lava flows where breaks occur in the cooled surface of the flow allowing hot gases and lava to be blown out.
- Volcanic plateau: These are formed because of fissure type of eruption.
- Crater: This is a depression located at the summit of the volcanic cone.
- Calderas: Sometimes because of violent volcanic explosion the entire central portion of the volcano is destroyed and only a great central depression, named a 'caldera' remains. The calderas may also be formed due to erosion and enlargement of the crater.
- Lava-tunnels: The more mobile lavas of basic composition, when erupted on the surface in the form of flows, quickly consolidate and form solid crust while the interior may still remain fluid. Under such conditions the enclosed fluid lava drains out through some weak spots lying at the periphery of the flow, forming what is known as ‘lava tunnel’.
- Cone-in-cone topography: After an explosion destroys an existing crater, a new-built smaller cone with its own crater is built up. This is known as cone-in-cone topography.
- Explosion-pits are also negative relief features of volcanoes.
Earthquakes are produced by sudden movements along faults, and are mostly, therefore of tectonic origin. The concept of possible mode of origin of tectonic earthquakes is known as ‘elastic-rebound theory’.
Such earthquakes generally result from sudden yielding to strain produced on the rocks by accumulating stresses. This causes the breaking of rocks and produces relative displacement of rocks. Such faulting causes shaking because displacement of rocks can only be possible by overcoming frictional resistance against the walls of the fault-plane. The association of earthquakes with faultiness is an established fact.
Usually, earthquakes associated with volcanoes are more localised both in extend of damage and in the intensity of the waves produced in comparison to those which are associated with faulting motions. A shock may be produced by any of the following mechanisms: explosion of the volcano upon the release and expansion of gases and lavas, faulting within the volcano resulting from pressures in the chamber of molten rocks, and collapse of the centre of the volcano into the space formed by the extrusion of gases and molten matter.
TYPES OF EARTHQUAKES
Natural earthquakes are of three types, according to the depth of their origin. They are as follows:
- Shallow-focus earthquakes: In this case, the seismic shocks originate at a depth of about 30 miles or less, below the earth’s surface.
- Intermediate-focus earthquakes: In this case, the shock-waves originate at a depth between 30 to 150 miles.
- Deep-focus earthquakes: Here the point of origin of the shock is at a depth between 150 to 450 miles.
According to the origin of the earthquakes, they are also of three types like:
(1) Tectonic earthquakes
(2) Volcanic earthquakes
(3) Submarine earthquakes
The submarine shocks often generate very large waves on the surface of the seas and destroy the coastal tracts. These submarine earthquakes are known as ‘Tsunamis’.
SCALE OF INTENSITY AND DISTRIBUTION
Various scales have been proposed to estimate the intensity of earthquake from the amount of damage caused. These scales are:
- Rosi-Forrel scale: In the Rosi-Forrel scale, the intensity has been classified into severe, catastrophic and disastrous
- Mercalli-scale: The Mercalli-intensity scale has devised twelve numbers with the increase of intensity. In this case—number 1 detected only by seismographs. Gradually the number increases when the earthquake intensity becomes feeble, slight, moderate, strong etc. At number ‘8’ it is “destructive”. Similarly, it becomes “disastrous” at number ‘10’, and at number ‘12’, the effect is totally catastrophic, where there is total destruction and objects thrown into air.
- The richter scale of earthquake magnitude: In the Richter-scale, the scale number ranges from ‘0’—‘9’. Here it is particularly important to notice that a magnitude—‘8’ earthquake is 10 times larger than a magnitude—7 earthquake, 100 times larger than a magnitude-6 earthquake, and 1000 times larger than a magnitude—5 earthquake.
The instrument used for recording of seismic shocks is known as ‘Seismograph’, and the records of seismic shocks prepared and presented by seismographs are known as ‘Seismograms’.
DISTRIBUTION OF EARTHQUAKES
- The destructive earthquakes are concentrated in a ring surrounding the Pacific Ocean-This ring coincides with the Circum-Pacific Ring of Fire.
- The second chain is termed as East-Indian, which extends over Indonesia, Andaman-Nicobar Islands and Burma.
- The third belt extends over Himalayas, Kun-Lun, Tien Shan and Altai Range up to the Lake Baikal.
- Another belt extends from the Pamir Knot to Afghanistan, Iran, Turkey, Greece, Rumania, Atlas Mountains, Gibraltar and the Azores Islands.
- A belt also extends from the Gulf of Aden, between Seychelles and Maldive Islands, turns to the West-South of Africa and goes up to the Falkland Islands.
- Another belt also runs along the Great Rift Valley of East Africa.