Indian Geography- 2 | Additional Study Material for UPSC PDF Download

Type Volcanoes

• 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:

• Central types, where the products escape through a single pipe (or vent).
• 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:

• 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.
• 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.
• Volcanian 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’.

Volcanic Topography

• 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.
• Positive-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.

Negative-relief-features

• 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 of solid crust while the interior may still remain fluid. 
  • Under such conditions the enclosed fluid lava drain 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.

Cause of Earthquakes

• Tectonic earthquakes. 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.

Volcanic earthquakes. Usually, earthquakes associated with volcanoes are more localised both in extend of damage and in 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 shockwaves originate at a depth between 30 to 150 miles.
• Deep-focus earthquake. 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—tectonic, volcanic and 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,   Mercalli-scale, and 
Richter scale of earthquake magnitude etc.

• In the Rosi-Forrel scale, the intensity has been classified into severe, catastrophic and disastrous.
• 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.
• 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, Gibralatar and the Azores Islands.
• A belt also extends from the Gulf of Aden, between Seychelles and Maladive 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.

Types of Weathering

• There are three main types of weathering: 1. Physical weathering/Mechanical weathering, 2. Chemical weathering, 3. Biological weathering.

Physical Weathering Processes

• This process refers to the mechanical disintegration of rocks in which their mineralogical composition is not changed.
• This is brought about chiefly by temperature changes, e.g., thermal expansion and contraction.

The following are some of the important processes of physical weathering.

• Exfoliation. In this case thin sheets of rock split off owing to differential expansion and contraction during heating and cooling over the diurnal temperature range.
• Crystal growth. The soluble constituents of the rocks or minerals, enter the rocks through fractures and joints, along with water.
  • With the evaporation of water the solution is precipitated to form crystals or crystalline aggregates and as they grow, they exert large expansive stresses, which help in breaking up some rocks.
• Freezing of water. Water, as we know, expands by about 9.05 per cent in volume when it freezes.
  • The water seeps down into the fracture and under suitable climatic condition, begins to freeze at the top of the fracture first.
  • As freezing continues, the pressure exerted on the walls becomes more and more intense, which results in widening the existing fracture and new fractures form.
  • This is the dominant mode of weathering, in climates where there is repeated freezing and thawing. This is also known as Frost action.
• Differential expansion. Rock-forming minerals expand when heated, but contract when cooled.
  • Where rock surfaces are exposed daily to intense heating by direct solar rays, alternating with intense cooling by long wave radiation at night, the resulting expansion and contraction of mineral-grains tend to break them apart.
  • The intense heat of forest and bush fires is known to cause rapid flaking and scaling of exposed rock surfaces.

Chemical Process of Weathering

• It is also known as mineral alteration and consists of a number of chemical reactions.
• All these reactions change the original silicate minerals of igneous rock, the primary minerals, into new compounds, the secondary minerals, that are stable in the surface environment.
• Besides, sedimentary and metamorphic rocks are also substantially affected by the chemical processes of weathering. Chemical weathering is more important than mechanical weathering in almost all the climatic regions.

Following processes are notably responsible for chemical weathering:
Oxidation,  Hydration,   Carbonation

• Oxidation. The presence of dissolved oxygen in water in contact with mineral surfaces leads to oxidation; which is the chemical union of oxygen atoms with atoms of other metallic elements.
  • Oxygen has a particular affinity for iron compounds and these are among the most commonly oxidised materials.
• Hydration. The chemical union of water with a mineral is called hydration. It is sometimes confused with ‘hydrolysis’, the reaction between water and a compound. 
  • The process of hydration is particularly effective on some aluminium bearing minerals, such as feldspar.
• Carbonation. Carbon-dioxide is a gas and is a common constituent of the earth’s atmosphere. Rain water in course of its passage through the atmosphere, dissolves some of the carbon dioxide present in the air.
  • It thus turns into a weak acid called carbonic acid, H₂CO₃ and is the most common solvent acting on the crust. 
  • The effect of this process is well noticed in the limestone or chalk areas in the humid regions of the world.
  • Besides the above, another process known as “Solution” is quite significant in bringing about the chemical weathering of rocks.
  • In this case, some of the minerals get dissolved by water and thus removed in solution, for example gypsum, halite etc.

Biological Weathering

• This process of weathering is mainly related to the activities of various organisms. Organisms, mainly plants and bacteria, take part in the transformation of rocks at the surface, in the following ways: F Bio-physical processes,  F Bio-chemical processes.

Bio-physical processes

• Plant-roots, growing between joint blocks and along minute fractures between mineral grains, exert an expansive force tending to widen those opening and sometimes create new fractures.
• Insects like earth-worm, snail etc. loosen the soil cover and create suitable conditions for the various external agencies to have their own action on the underlying rocks, which ultimately lead to rock weathering.

Bio-chemical processes of weathering

• Sometimes, certain groups of bacteria, algae and mosses break rock-forming silicates down directly, removing from them elements like silicon, potassium, phosphorous, calcium, magnesium, that they need as nutrients.
• This transformation sometimes occurs on a large scale and is decisive in the alteration of parent rocks and facilitate rock weathering.

• After the death of animals or plants, with their subsequent decay and degeneration, chemically active substances are produced, which are capable of bringing about rock weathering.
• For example, humic acid which is formed during decay and degeneration of plant life is capable of bringing about rock weathering effectively, to some extent.