The Earth it’s Interior likeVolcano, Earthquake and manymoreIn this Article we are going to discuss about the the Earthits like layer of earth, structure of the earth, layers ofearth’s atmosphere, Volcano, Earthquake, distribution ofearthquake and volcano in the earth, Plate tectonic, Hotspring and Geyser. So let’s start

The Earth

Earth is the third closest planet to the Sun. In size, it isthe fifth-largest planet. It is slightly flattened at thepoles. That is why its shape is described as the geoid. Geoidmeans Earth-like shape.

Favourable conditions to support life are probably found onEarth. The Earth is neither too hot nor too cold. It has waterand air, which are very essential for our survival. There areliving gases like oxygen in the air. For these reasons, theEarth is a unique planet in the solar system. From outerspace, the Earth appears blue because two-thirds of itssurface is covered with water. Hence it is called the blueplanet.

Major Domains of Earth

The Earth’s surface is a complex region in which the threemain components of the environment meet, overlap and interact.

The solid part of the Earth on which we live is called thelithosphere. The gaseous layers around the Earth are theatmosphere where oxygen, nitrogen, carbon dioxide and othergases are found. Water covers a very large area of theEarth’s surface and this area is called the hydrosphere. Thehydrosphere has water in all its forms, that is, ice, water

and water vapour. The biosphere is a narrow area where we findland, water and air together, containing all kinds of life.

Lithosphere

The solid part of the Earth is called the lithosphere. Itconsists of rocks of the earth’s crust and thin layers of soilthat contain nutrients that sustain organisms. There are twomain parts of the Earth’s surface. Large landmasses are knownas continents and huge water bodies are called oceanic basins.All the oceans of the world are connected with each other

The level of seawater remains the same everywhere. The heightof the land is measured by the level of the sea, which istaken as zero. The highest mountain peak Mt. Everest is 8,848meters above sea level. The largest depth of 11,022 m has beenrecorded on the Mariana Trench in the Pacific Ocean. . Could

you imagine that depth of the sea is much more than thehighest point?

Hydrosphere

Earth is called the blue planet. More than 71 per cent of theEarth is covered by water and 29 per cent is with the land.The hydrosphere contains water in all its forms. As forrunning water in oceans and rivers and lakes, glaciers containice, underground water and Water vapour in the atmosphere, allhave hydrosphere. More than 97% of the Earth’s water is foundin the oceans and is too salty for human use. A largeproportion of the remaining water is in the form of an icesheet and glacier or below ground and a very small percentageis available as freshwater for human use. Therefore, despitebeing a ‘blue planet’ we have to face water scarcity !!

Biosphere – The Domain of Life

The biosphere is a narrow area of contact between land, waterand air. It is in this region that life, unique to thisplanet, exists. There are many species of organisms that varyin size from germs and bacteria to giant mammals. All livingorganisms, including humans, are connected to each other andthe biosphere to survive. Organisms in the biosphere can bebroadly divided into plant kingdom and animal kingdom. Thethree domains of the Earth interact with each other and affecteach other in some way. For example, cutting forests for ourfulfilment, the need for wood for agriculture, or clearing theland, can lead to the rapid removal of soil from the slopes.Similarly, due to natural disasters, the Earth’s surface canbe changed. Earthquake. For example, the submerging of land,as in the recent tsunami case. Andaman and parts of NicobarIslands were submerged under the water. Discharge of wastematerials in lakes and rivers makes water unsuitable for humanuse. It also harms other forms of life.

Emissions from industries, thermal power plants and vehiclespollute the air. Carbon dioxide (CO2) is an importantcomponent of air. But increasing the amount of CO2 leads to anincrease in global temperature. This is called global warming.Thus, there is a need to limit the use of Earth’s resources,to maintain the balance of nature between the domains of thelithosphere, atmosphere, and hydrosphere.

Atmosphere

The Earth is surrounded by a layer of gas called theatmosphere. This thin blanket of air is an integral andimportant aspect of the planet. It gives us the air we breatheand protects us from the harmful effects of the sun’s rays.

The atmosphere extends to an altitude of about 1,600

kilometers. The atmosphere is divided into five layers basedon structure, temperature and other properties. These layers,starting from the Earth’s surface, are called the troposphere,stratosphere, mesosphere, thermosphere, and exosphere.

The atmosphere is primarily composed of nitrogen and oxygen,which make up about 99 percent of clean, dry air. Nitrogen is78 per cent, oxygen 21 per cent and gases like carbondioxide,argon and others are 1 per cent by volume. Oxygen is thebreath of life while nitrogen helps in the growth of livingorganisms. Carbon dioxide, although present in minute amounts,is important because it absorbs the heat radiated by theEarth, keeping the planet warm. It is also essential for plantgrowth.

The density of the atmosphere varies with altitude. It ismaximum at sea level and decreases rapidly as it goes up. Youknow, climbers experience breathing problems due to thisdecrease in air density. To be able to breathe at highaltitude they have to carry oxygen cylinders with them. As wemove upwards, the temperature also decreases. The atmosphereincreases the pressure on Earth. It varies from place toplace. Some areas experience high pressure and some areasexperience low pressure. Air moves from high pressure to lowpressure. The moving air is known as the wind.

Structure of the Atmosphere

The atmosphere consists of different layers with differentdensities and temperatures. The density is highest near theEarth’s surface and decreases with increasing altitude. Thecolumn of the atmosphere is divided into five different layersdepending on the temperature conditions. They are:troposphere, stratosphere, mesosphere, thermosphere andexosphere

The troposphere is the lowest layer of the atmosphere. It hasan average elevation of 13 km and an elevation of about 8 kmnear the poles and about 18 km at the equator.

The thickness of the troposphere is greatest at the equator

because heat is transported to great heights by strongconvection currents. This layer contains dust mites and watervapor. All changes in climate and weather occur in this layer.The temperature in this layer decreases at a rate of 1 ° C forevery 165m height. It is the most important layer for allbiological activities

The area separating the troposphere from the stratosphere isknown as the tropopause. The air temperature at the tropopauseis about minus 800C at the equator and minus 45oC at thepoles. Here the temperature is almost constant, and hence, itis called the tropopause.

The stratosphere is found above the tropopause and extends toa height of 50 km. An important feature of the stratosphere isthat it contains an ozone layer. This layer absorbs ultra-violet radiation and shields life on Earth with intense,harmful forms of energy.

The mesosphere lies above the stratosphere, which extends toan altitude of 80 km. In this layer, once again, thetemperature starts to decrease with increasing altitude andreaches a temperature of 80 km to minus 100 ° C. The upperboundary of the mesosphere is known as the mesopause.

The ionosphere is located between 80 and 400 km above themesopause. It contains electrically charged particles known asions, and hence, it is known as the ionosphere. Radio wavestransmitted from the earth are reflected back to the earth bythis layer. The temperature here starts increasing with height

The uppermost layer of the atmosphere above the thermosphereis known as the exosphere. This is the highest layer but verylittle is known about it. Whatever contents are there, theseare extremely rarefied in this layer, and it gradually mergeswith the outer space. Although all layers of the atmospheremust be exercising an influence on us, geographers areconcerned with the first two layers of the atmosphere.

Inside Our Earth

Earth, our motherland is a dynamic planet. It is constantlyundergoing changes inside and out. Have you ever wondered whatis on the inside of the earth? What is the earth made of?

Interior of the Earth

Just like an onion, the earth is made up of several concentriclayers with one inside Another. The uppermost layer over theearth’s surface is called the crust. It is the thinnest of allthe layers. It is about 35 km. on the continental masses andonly 5 km. on the ocean floors. The main mineral constituentsof the continental mass are silica and alumina. It is thuscalled sial (si-silica and al-alumina). The oceanic crustmainly consists of silica and magnesium; it is thereforecalled sima (si-silica and ma-magnesium).

Just beneath the crust is the mantle which extends up to adepth of 2900 km. below the crust.

The innermost layer is the core with a radius of about 3500km. It is mainly made up of nickel and iron and is called nife(ni – nickel and fe – ferrous i.e. iron). The central core hasvery high temperature and pressure.

The interior of the earth can be understood only by indirectevidences as neither any one has nor any one can reach theinterior of the earth. The configuration of the surface of theearth is largely a product of the processes operating in theinterior of the earth. Exogenic as well as endogenic processesare constantly shaping the landscape. A proper understandingof the physiographic character of a region remains incompleteif the effects of endogenic processes are ignored. Human life

is largely influenced by the physiography of the region.Therefore, it is necessary that one gets acquainted with theforces that influence landscape development. To understand whythe earth shakes or how a tsunami wave is generated, it isnecessary that we know certain details of the interior of theearth.

Layers of Earth with its Discontinuity

CRUST: It is the outermost solid part of the earth, normallyabout 8-40 kms thick.It is brittle in nature.Nearly 1% of theearth’s volume and 0.5% of earth’s mass are made of thecrust.The thickness of the crust under the oceanic andcontinental areas are different. Oceanic crust is thinner(about 5kms) as compared to the continental crust (about30kms).

Major constituent elements of crust are Silica (Si) andAluminium (Al) and thus, it is often termed as SIAL (SometimesSIAL is used to refer Lithosphere, which is the regioncomprising the crust and uppermost solid mantle, also). Themean density of the materials in the crust is 3g/cm3.Thediscontinuity between the hydrosphere and crust is termed asthe Conrad Discontinuity

MANTLE: The portion of the interior beyond the crust is called

as the mantle. The discontinuity between the crust andmantle is called as the Mohorovich Discontinuity or Mohodiscontinuity. The mantle is about 2900kms in thickness Nearly84% of the earth’s volume and 67% of the earth’s mass isoccupied by the mantle. The major constituent elements of themantle are Silicon and Magnesium and hence it is also termedas SIMA. The density of the layer is higher than the crust andvaries from 3.3 – 5.4g/cm3. The uppermost solid part of themantle and the entire crust constitute the Lithosphere.The asthenosphere (in between 80-200km) is a highly viscous,mechanically weak and ductile, deforming region of the uppermantle which lies just below the lithosphere. Theasthenosphere is the main source of magma and it is the layerover which the lithospheric plates/ continental plates move(plate tectonics). The discontinuity between the upper mantleand the lower mantle is known as Repetti Discontinuity. Theportion of the mantle which is just below the lithosphere andasthenosphere, but above the core is called as Mesosphere.

CORE: It is the innermost layer surrounding the earth’scentre. The core is separated from the mantle by Guttenberg’sDiscontinuity. It is composed mainly of iron (Fe) and nickel(Ni) and hence it is also called as NIFE. The core constitutesnearly 15% of earth’s volume and 32.5% of earth’s mass. Thecore is the densest layer of the earth with its density rangesbetween 9.5-14.5g/cm3. The Core consists of two sub-layers:the inner core and the outer core. The inner core is in solidstate and the outer core is in the liquid state (or semi-liquid). The discontinuity between the upper core and thelower core is called as Lehmann Discontinuity. Barysphere issometimes used to refer the core of the earth or sometimes thewhole interior. The temperature here is estimated to be ashigh as 3,500°F, and the core is subject to extremely highpressure. Under such conditions, the core could be expected tobe in a liquid state. But recent studies through earthquakew’aves have suggested that the innermost part of the core is

probably a crystalline or solid mass.

Parts of the earth’s crust are immersed by oceans and seas.These form the hydrosphere. Extending skywards for overfifteen miles, the earth is enveloped by a mass of gases whichmake up the atmosphere.

Sources of Information about theInterior

The earth’s radius is 6,370 km. No one can reach the centre ofthe earth and make observations or collect samples of thematerial. Under such conditions, you may wonder how scientiststell us about the earth’s interior and the type of materialsthat exist at such depths. Most of our knowledge about theinterior of the earth is largely based on estimates andinferences. Yet, a part of the information is obtained throughdirect observations and analysis of materials.

Direct Sources

The most easily available solid earth material is surface rockor the rocks we get from mining areas. Gold mines in SouthAfrica are as deep as 3 – 4 km. Going beyond this depth is notpossible as it is very hot at this depth. Besides mining,scientists have taken up a number of projects to penetratedeeper depths to explore the conditions in the crustalportions. Scientists world over are working on two majorprojects such as “Deep Ocean Drilling Project” and “IntegratedOcean Drilling Project”. The deepest drill at Kola, in the

Arctic Ocean, has so far reached a depth of 12 km. This andmany deep drilling projects have provided a large volume ofinformation through the analysis of materials collected atdifferent depths.

Volcanic eruption forms another source of obtaining directinformation. As and when the molten material (magma) is thrownonto the surface of the earth, during volcanic eruption itbecomes available for laboratory analysis. However, it isdifficult to ascertain the depth of the source of such magma.

Indirect Sources

Analysis of properties of matter indirectly providesinformation about the interior. We know through the miningactivity that temperature and pressure increase with theincreasing distance from the surface towards the interior indeeper depths. Moreover, it is also known that the density ofthe material also increases with depth. It is possible to findthe rate of change in these characteristics. Knowing the totalthickness of the earth, scientists have estimated the valuesof temperature, pressure and the density of materials atdifferent depths.

Another source of information is the meteors that at timesreach the earth. However, it may be noted that the materialthat becomes available for analysis from meteors, is not fromthe interior of the earth. The material and the structureobserved in the meteors are similar to that of the earth. Theyare solid bodies developed out of materials the same as, orsimilar to, our planet. Hence, this becomes yet another sourceof information about the interior of the earth.

The other indirect sources include gravitation, magneticfield, and seismic activity. The gravitation force (g) is notthe same at different latitudes on the surface. It is greaternear the poles and less at the equator. This is because of thedistance from the centre at the equator being greater thanthat at the poles. The gravity values also differ according tothe mass of material. The uneven distribution of mass ofmaterial within the earth influences his value. The reading ofgravity in different places is influenced by many otherfactors. These readings differ from the expected values. Sucha difference is called gravity anomaly. Gravity anomalies giveus information about the distribution of mass of the materialin the crust of the earth. Magnetic surveys also provideinformation about the distribution of magnetic materials inthe crustal portion, and thus, provide information about thedistribution of materials in this part. Seismic activity isone of the most important sources of information about theinterior of the earth. Hence, we shall discuss it in somedetail.

Let’s Study Volcanism and Earthquakes

The lithosphere is broken into a number of plates known as theLithospheric plates. You will be surprised to know that theseplates move around very slowly – just a few millimetres eachyear. This is because of the movement of the molten magmainside the earth. The molten magma inside the earth moves in acircular manner as shown in the activity.

The movement of these plates causes changes on the surface ofthe earth. The earth movements are divided on the basis of theforces which cause them. The forces which act in the interiorof the earth are called Endogenic forces and the forces that

work on the surface of the earth are called Exogenic forces.

Endogenic forces sometimes produce sudden movements and at theother times produce slow movements. Sudden movements likeearthquakes and volcanoes cause mass destruction over thesurface of the earth.

A volcano is a vent (opening) in the earth’s crust throughwhich molten material erupts suddenly.

Similarly, when the Lithospheric plates move, the surface ofthe earth vibrates. The vibrations can travel all around theearth. These vibrations are called earthquakes. The place inthe crust where the movement starts is called the focus. Theplace on the surface above the focus is called the epicentre.Vibrations travel outwards from the epicentre as waves. The

greatest damage is usually closest to the epicentre and thestrength of the earthquake decreases away from the centre.

In order to understand the geography of the external landformsof the earth, it is essential that we have some idea of whatlies within the earth’s crust. It is not possible to knowexactly how the earth was formed about 4.500 million yearsago. but from the evidence of volcanic eruptions, earthquakewaves, deep-mine operations and crustal borings the followingfacts are quite clear.

Isn’t it’s interesting to know more about Volcanoes andEarthquakes. Let’s discuss more in detail…

Volcanoes

A volcano is a place where gases, ashes and/or molten rockmaterial – lava – escape to the ground. A volcano is called anactive volcano if the materials mentioned are being releasedor have been released out in the recent past. The layer belowthe solid crust is the mantle. It has a higher density thanthat of the crust. The mantle contains a weaker zone calledthe asthenosphere. It is from this that the molten rockmaterials find their way to the surface. The material in theupper mantle portion is called magma. Once it starts movingtowards the crust or it reaches the surface, it is referred toas lava. The material that reaches the ground includes lavaflows, pyroclastic debris, volcanic bombs, ash and dust andgases such as nitrogen compounds, sulphur compounds and minoramounts of chlorine, hydrogen and argon.

Landforms associated with VulcanicActivities

Vulcanic activities have a profound influence on the earth ‘slandforms. Solid, liquid or gaseous materials may find theirway to the surface from some deepseated reservoir beneath.Molten magma is mobile rock that forces its way into theplanes of weakness of the crust to escape quietly orexplosively to the surface. The resultant landforms depend onthe strength and fluidity of the magma , the types of cracks,faults and joints that it penetrates, and the manner in whichit escapes to the surface. Magma while thrusting its way up tothe surface may cool and solidify within the crust as plutonicrocks resulting in intrusive landforms. Magmas that reach thesurface and solidify, form extrusive landforms. Rocks formedby either plutonic or volcanic activity are called igneousrocks.

Volcanic Landforms

Intrusive Forms:

The lava that is released during volcanic eruptions on coolingdevelops into igneous rocks. The cooling may take place eitheron reaching the surface or also while the lava is still in thecrustal portion. Depending on the location of the cooling ofthe lava, igneous rocks are classified as volcanic rocks(cooling at the surface) and plutonic rocks (cooling in thecrust). The lava that cools within the crustal portionsassumes different forms. These forms are called intrusiveforms.

Perhaps the commonest intrusive landforms are sills and dykes.When an intrusion of molten magma ismade horizontally alongthe bedding planes of sedimentary rocks, the resultantintrusion is called a sill. Denudation of the overlyingsedimentary strata will expose the intrusion which willresemble a lava flow, or form a bold escarpment like the GreatWhin Sill of N.E. England. Similar intrusions when injectedvertically as narrow walls of igneous rocks within thesedimentary layers are termed as dykes.

Because of their narrowness, dykes seldom dominate thelandscape. When exposed to denudation they may appear asupstanding walls or shallow trenches, depending on whetherthey are more or less resistant than the rocks in which theyare emplaced . Examples of dykes are the Cleveland Dyke ofYorkshire, England and hundreds of others in the Isles of Mull

and Arran in Scotland. A large, very resistant dyke ofquartzite forms a long ridge to the north of Kuala Lumpur.

Igneous intrusions on a larger scale are the various types of‘— liths’: laccoliths, lopoliths, phacoliths and batholiths.The names may sound difficult; they are, in fact, allvariations of igneous intrusions placed differently in theearth’s crust, and solidifying within the upper layers of thecrust. A laccolith is a large blister or igneous mound with adome–shaped upper surface and a level base fed by a pipe-likeconduit from below. It arches up the overlying strata ofsedimentary rocks, e.g. the laccoliths of the Henry Mountains,in Utah U.S.A.

A lopolith is another variety of igneous intrusion with asaucer shape. A shallow basin is formed in the midst of thecountry rocks. The Bushveld lopoliths of Transvaal, SouthAfrica are good examples.

A phacolith is a lens-shaped mass of igneous rocks occupyingthe crest of an anticline or the bottom of a syncline andbeing fed by a conduit from beneath . An example of aphacolith is Corndon Hill in Shropshire, England.

A batholith is a huge mass of igneous rocks, usually granite,which after removal of the overlying rocks forms a massive andresistant upland region such as the Wicklow Mountains ofIreland, the uplands of Britanny, France and the Main Range ofWest Malaysia. Their precise mode of origin is still a matterof controversy. It is generally believed that large masses ofmagma rising upwards metamor¬phosed the country rocks with

which they came into contact. These metamorphosed rockstogether with the solidified magma give rise to extensivebatholiths, sometimes hundreds of miles in extent . They arethe most spectacular of the intrusive landforms.

Extrusive Landforms:

Extrusive landforms are determined by the nature andcomposition of the lava and other ejected material that reachthe surface of the earth. The fluid basic lava flowing forlong distances produces extensive Lava Plains and Balastplateaux, such as the great lava plains of the Snake Basin.U.S. A. The basalt plateaux are found in many continents, e.g.the north, western part of the Deccan Plateau and in IcelandVolcanic cones are most typical of the extrusive

Classification of Volcanoes

Volcanoes are classified on the basis of nature of eruptionand the form developed at the surface. Major types ofvolcanoes are as follows:

Shield Volcanoes:

Barring the basalt flows, the shield volcanoes are the largestof all the volcanoes on the earth. The Hawaiian volcanoes arethe most famous examples. These volcanoes are mostly made upof basalt, a type of lava that is very fluid when erupted. Forthis reason, these volcanoes are not steep. They becomeexplosive if somehow water gets into the vent; otherwise, theyare characterised by low-explosivity. The upcoming lava movesin the form of a fountain and throws out the cone at the topof the vent and develops into a cinder cone.

Composite Volcanoes:

These volcanoes are characterised by eruptions of cooler andmore viscous lavas than basalt. These volcanoes often resultin explosive eruptions. Along with lava, large quantities ofpyroclastic material and ashes find their way to the ground.This material accumulates in the vicinity of the vent openingsleading to the formation of layers, and this makes the mountsappear as composite volcanoes.

Caldera:

These are the most explosive of the earth’s volcanoes. Theyare usually so explosive that when they erupt they tend tocollapse on themselves rather than building any tallstructure. The collapsed depressions are called calderas.Their explosiveness indicates that the magma chamber supplyingthe lava is not only huge but is also in close vicinity.

Flood Basalt Provinces:

These volcanoes outpour highly fluid lava that flows for long

distances. Some parts of the world are covered by thousands ofsq. km of thick basalt lava flows. There can be a series offlows with some flows attaining a thickness of more than 50 m.Individual flows may extend for hundreds of km. The DeccanTraps from India, presently covering most of the Maharashtraplateau, are a much larger flood basalt province. It isbelieved that initially, the trap formations covered a muchlarger area than the present.

Mid-Ocean Ridge Volcanoes:

These volcanoes occur in oceanic areas. There is a system ofmid-ocean ridges more than 70,000 km long that stretchesthrough all the ocean basins. The central portion of thisridge experiences frequent eruptions.

The Distribution of Volcanoes in theworld

Volcanoes are located in a fairly clearly –defined patternaround the world, closely related to the region that has beenintensely folded or faulted. There are well over 500 activevolcanoes and thousands of dormant and extinct ones. Theyoccur along with coastal mountain ranges, as off-shore islandsand in the midst of oceans, but there are few in the interiorsof continents. The greatest concentration is probably that inthe Circum-Pacific region, popularly termed the ‘ Pacific Ringof Fire’, which has been estimated to include two-thirds ofthe world’s volcanoes. The chain of volcanoes extends foralmost 2.000 miles from the Aleutian Islands into Kamchatka.Japan, the Philippines, and Indonesia ( Java and Sumatra inparticular), southwards into the Pacific islands of Solomon,

New Hebrides. Tonga and North Island, New Zealand. On theother side of the Pacific, the chain continues from the Andesto Central America ( particularly Guatemala, Costa Rica andNicaragua), Mexico and right up to Alaska. It is said thatthere are almost 100 active volcanoes in the Philippines, 40in the Andes, 35 in Japan, and more than 70 in Indonesia. Incontrast, the Atlantic coasts have comparatively few activevolcanoes but many dormant or extinct volcanoes, e.g. Madeira,Ascension, St. Helena. CapeVerde Islands and the CanaryIslands, but those of Iceland and the Azores are active.Volcanoes of the Mediter ¬ ranean region are mainly associatedwith the Alpine folds, e.g. Vesuvius, Etna. Stromboli, Vulcanoand those of the Aegean islands. A few’ continue into AsiaMinor ( Mt. Ararat, Mt. Elbruz). The Hima¬layas have,surprisingly, no active volcano at all. In Africa somevolcanoes are found along the East African Rift Valley, e.g.Mt . Kilimanjaro and Mt. Kenya, both probably extinct. Theonly active volcano of West Africa is Mt. Cameroon. There aresome volcanic cones in Madagascar, but active eruption has notbeen known so far. The West Indian islands have experiencedsome violent ex¬ plosions in recent times, e.g. Mt. Pelee inMartinique, and in St. Vincent further south. The LesserAntilles are made up mainly of volcanic islands and some ofthem still bear signs of volcanic liveliness. Elsewhere in theinteriors of continents— Asia, North America, Europe andAustralia, active volcanoes are rare.

Earthquakes

An earthquake in simple words is shaking of the earth. It is anatural event. It is caused due to the release of energy,which generates waves that travel in all directions. Anearthquake is measured with a machine called a seismograph.The magnitude of the earthquake is measured on the Richterscale. An earthquake of 2.0 or less can be felt only a little.An earthquake over 5.0 can cause damage from things falling. A6.0 or higher magnitude is considered very strong and 7.0 isclassified as a major earthquake.

The earth is never free from earthquakes for long and morethan 50,000 of them are recorded annually. Minor earth tremorscaused by gentle waves of vibration within the earth’s crustoccur every few minutes. Major earthquakes, usually caused bymovement along faults. can be very disastrous particularly indensely populated areas. Earthquakes themselves may cause onlyrestricted damage in the regions of occurrence, but their

after-effects can be very catastrophic. They produce gigantictidal waves, called tsunamis by the Japanese, which floodtowns and drown thousands of people. Fires break out beyondcontrol as gas mains are shattered and buildings collapse. Insevere earthquakes, fissures gape open, and the ground writhesand undulates in the passage of the ‘surface waves’. A waveheight of a quarter of an inch in the upheaval is sufficientto bring down most ordinary buildings. Roads, rail ¬ ways andbridges are buckled and twisted; telecommunications are cutwhen the cables are snapped. Hills are so shaken thatlandslides are widespread. As the vibration thins out at theedges, like the series of waves set up by a stone thrown intothe water, the damage is greatly reduced. Only the highlysensitive seismograph can record the movements of earthquakewaves.

Why does the earth shake?

The release of energy occurs along a fault. A fault is a sharpbreak in the crustal rocks. Rocks along a fault tend to movein opposite directions. As the overlying rock strata pressthem, the friction locks them together. However, theirtendency to move apart at some point in time overcomes thefriction. As a result, the blocks get deformed and eventually,they slide past one another abruptly. This causes the releaseof energy, and the energy waves travel in all directions. Thepoint where the energy is released is called the focus of anearthquake, alternatively, it is called the hypocentre. Theenergy waves travelling in different directions reach thesurface. The point on the surface, nearest to the focus, iscalled the epicentre. It is the first one to experience thewaves. It is a point directly above the focus.

Earthquake Waves

All-natural earthquakes take place in the lithosphere. Youwill learn about the different layers of the earth later inthis chapter. It is sufficient to note here that thelithosphere refers to the portion of depth up to 200 km fromthe surface of the earth. An instrument called ‘seismograph’records the waves reaching the surface. A curve of earthquakewaves recorded on the seismograph is given in Figure

that the curve shows three distinct sections each representingdifferent types of wave patterns. Earthquake waves arebasically of two types — body waves and surface waves. Bodywaves are generated due to the release of energy at the focusand move in all directions travelling through the body of theearth. Hence, the name body waves. The body waves interactwith the surface rocks and generate a new set of waves calledsurface waves. These waves move along the surface. Thevelocity of waves changes as they travel through materialswith different densities. The denser the material, the higheris the velocity. Their direction also changes as they reflector refract when coming across materials with differentdensities.

There are two types of body waves. They are called P and S-waves. P-waves move faster and are the first to arrive at thesurface. These are also called ‘primary waves’. The P-wavesare similar to sound waves. They travel through gaseous,liquid and solid materials. S-waves arrive at the surface withsome time lag. These are called secondary waves. An importantfact about S-waves is that they can travel only through solidmaterials. This characteristic of the S-waves is quiteimportant. It has helped scientists to understand thestructure of the interior of the earth. Reflection causeswaves to rebound whereas refraction makes waves move indifferent directions. The variations in the direction of wavesare inferred with the help of their record on a seismograph.The surface waves are the last to report on a seismograph.These waves are more destructive. They cause displacement ofrocks, and hence, the collapse of structures occurs.

Propagation of Earthquake Waves

Different types of earthquake waves travel in differentmanners. As they move or propagate, they cause vibration inthe body of the rocks through which they pass. P-waves vibrateparallel to the direction of the wave. This exerts pressure onthe material in the direction of the propagation. As a result,it creates density differences in the material leading tostretching and squeezing of the material. Other three wavesvibrate perpendicular to the direction of propagation. Thedirection of vibrations of S-waves is perpendicular to thewave direction in the vertical plane. Hence, they createtroughs and crests in the material through which they pass.Surface waves are considered to be the most damaging waves.

The emergence of Shadow Zone

Earthquake waves get recorded in seismographs located at faroff locations. However, there exist some specific areas wherethe waves are not reported. Such a zone is called the ‘shadowzone’. The study of different events reveals that for eachearthquake, there exists an altogether different shadow zone.The figure shows the shadow zones of P and S-waves. It wasobserved that seismographs located at any distance within 105°from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from theepicentre, record the arrival of P-waves, but not that of S-

waves. Thus, a zone between 105° and 145° from epicentre wasidentified as the shadow zone for both the types of waves. Theentire zone beyond 105° does not receive S-waves. The shadowzone of S-wave is much larger than that of the P-waves. Theshadow zone of P-waves appears as a band around the earthbetween 105° and 145° away from the epicentre. The shadow zoneof S-waves is not only larger in extent but it is also alittle over 40 per cent of the earth surface. You can draw theshadow zone for any earthquake provided you know the locationof the epicentre

Types of Earthquakes

(i) The most common ones are the tectonic earthquakes. Theseare generated due to the sliding of rocks along a fault plane.

(ii) A special class of tectonic earthquake is sometimesrecognised as volcanic earthquake. However, these are confinedto areas of active volcanoes.

(iii) In the areas of intense mining activity, sometimes theroofs of underground mines collapse causing minor tremors.These are called collapse earthquakes.

(iv) Ground shaking may also occur due to the explosion ofchemical or nuclear devices. Such tremors are called explosionearthquakes.

(v) The earthquakes that occur in the areas of largereservoirs are referred to as reservoir induced earthquakes.

Measuring Earthquakes

The earthquake events are scaled either according to themagnitude or intensity of the shock. The magnitude scale isknown as the Richter scale. The magnitude relates to theenergy released during the quake. The magnitude is expressedin numbers, 0-10. The intensity scale is named after Mercalli,an Italian seismologist. The intensity scale takes intoaccount the visible damage caused by the event. The range ofintensity scale is from 1-12.

Frequency of Earthquake Occurrences

The earthquake is a natural hazard. If a tremor of highmagnitude takes place, it can cause heavy damage to the lifeand property of people. However, not all the parts of theglobe necessarily experience major shocks

Note that the quakes of high magnitude, i.e. 8+ are quiterare; they occur once in 1-2 years whereas those of ‘tiny’types occur almost every minute.

The Distribution of Earthquakes

The world’s distribution of earthquakes coincides very closelywith that of volcanoes. Regions of greatest seismicity areCircum -Pacific areas, with the epicentres and the mostfrequent occurrences along the ‘ Pacific Ring of Fire’. It issaid that as many as 70 per cents of earthquakes occur in theCircum-Pacific belt. Another 20 per cent of earth¬ quakes take

place in the Mediterranean-Himalayan belt including AsiaMinor, the Himalayas and parts of north-west China. Elsewhere,the earth’s crust is relatively stable and is less prone toearthquakes, though nowhere can be said to be immune to earthtremors

Effects of Earthquakes

Earthquake is a natural hazard. The following are theimmediate hazardous effects of earthquake:

(i) Ground Shaking

(ii) Differential ground settlement

(iii) Land and mud slides

(iv) Soil liquefaction

(v) Ground lurching

(vi) Avalanches

(vii) Ground displacement

(viii) Floods from dam and levee failures

(ix) Fires

(x) Structural collapse

(xi) Falling objects

(xii) Tsunami

The first six listed above have some bearings upon landforms,while others may be considered the effects causing immediateconcern to the life and properties of people in the region.The effect of the tsunami would occur only if the epicentre ofthe tremor is below oceanic waters and the magnitude issufficiently high. Tsunamis are waves generated by the tremorsand not an earthquake in itself. Though the actual quakeactivity lasts for a few seconds, its effects are devastatingprovided the magnitude of the quake is more than 5 on theRichter scale

Earthquake Preparedness

Where to take shelter during an earthquake —

Safe Spot – Under a kitchen counter, table or desk, against aninside corner or wall.

Stay Away from – Fire places, areas around chimneys, windowsthat shatter including mirrors and picture frames.

Be Prepared – Spread awareness amongst your friends and familymembers and face any disaster confidently.

Plate Tectonic

Since the advent of the concept of sea floor spreading, theinterest in the problem of distribution of oceans andcontinents was revived. It was in 1967, McKenzie and Parkerand also Morgan, independently collected the available ideasand came out with another concept termed Plate Tectonics.

A tectonic plate (also called lithospheric plate) is amassive, irregularly-shaped slab of solid rock, generallycomposed of both continental and oceanic lithosphere. Platesmove horizontally over the asthenosphere as rigid units. Thelithosphere includes the crust and top mantle with itsthickness range varying between 5 and100 km in oceanic partsand about 200 km in the continental areas. A plate may bereferred to as the continental plate or oceanic platedepending on which of the two occupy a larger portion of theplate. Pacific plate is largely an oceanic plate whereas theEurasian plate may be called a continental plate. The theoryof plate tectonics proposes that the earth’s lithosphere isdivided into seven major and some minor plates. Young FoldMountain ridges, trenches, and/or faults surround these majorplates . The major plates are as follows :

I Antarctica and the surrounding oceanic plate

II North American (with western Atlantic floor separated fromthe South American plate along the Caribbean islands) plate

III South American (with western Atlantic floor separated fromthe North American plate along the Caribbean islands) plate

IV Pacific plate

V India-Australia-New Zealand plate

VI Africa with the eastern Atlantic floor plate

VII Eurasia and the adjacent oceanic plate.

Some important minor plates are listed below:

(i) Cocos plate : Between Central America and Pacific plate

(ii) Nazca plate : Between South America and Pacific plate

(iii) Arabian plate : Mostly the Saudi Arabian landmass

(iv) Philippine plate : Between the Asiatic and Pacific plate

v) Caroline plate : Between the Philippine and Indian plate(North of New Guinea)

(vi) Fuji plate : North-east of Australia.

These plates have been constantly moving over the globethroughout the history of the earth. It is not the continentthat moves as believed by Wegener. Continents are part of aplate and what moves is the plate. Moreover, it may be notedthat all the plates, without exception, have moved in thegeological past, and shall continue to move in the future aswell. Wegener had thought of all the continents to haveinitially existed as a supercontinent in the form of Pangaea.However, later discoveries reveal that the continental masses,resting on the plates, have been wandering all through thegeological period, and Pangaea was a result of converging ofdifferent continental masses that were parts of one or theother plates. Scientists using the palaeomagnetic data havedetermined the positions held by each of the presentcontinental landmasses in different geological periods.Position of the Indian subcontinent (mostly Peninsular India)is traced with the help of the rocks analysed from the Nagpurarea.

Types of Plate Tectonic

There are three types of plate boundaries:

Divergent Boundaries:

Where the new crust is generated as the plates pull away fromeach other. The sites where the plates move away from eachother are called spreading sites. The best-known example ofdivergent boundaries is the Mid-Atlantic Ridge. At this, theAmerican Plate(s) is/are separated from the Eurasian andAfrican Plates

.

Convergent Boundaries:

Where the crust is destroyed as one plate dived under another.The location where sinking of a plate occurs is called asubduction zone. There are three ways in which convergence canoccur. These are: (i) between an oceanic and continentalplate; (ii) between two oceanic plates; and (iii) between twocontinental plates.

Transform Boundaries:

Where the crust is neither produced nor destroyed as theplates slide horizontally past each other. Transform faults

are the planes of separation generally perpendicular to themidoceanic ridges. As the eruptions do not take all along theentire crest at the same time, there is a differentialmovement of a portion of the plate away from the axis of theearth. Also, the rotation of the earth has its effect on theseparated blocks of the plate portions.

Rates of Plate Movement:

The strips of the normal and reverse magnetic field thatparallel the mid-oceanic ridges help scientists determine therates of plate movement. These rates vary considerably. TheArctic Ridge has the slowest rate (less than 2.5 cm/yr), andthe East Pacific Rise near Easter Island, in the South Pacificabout 3,400 km west of Chile, has the fastest rate (more than15 cm/yr).

Force for the Plate Movement:

At the time that Wegener proposed his theory of continentaldrift, most scientists believed that the earth was a solid,motionless body. However, concepts of seafloor spreading andthe unified theory of plate tectonics have emphasised thatboth the surface of the earth and the interior are not staticand motionless but are dynamic. The fact that the plates moveis now a well-accepted fact. The mobile rock beneath the rigidplates is believed to be moving in a circular manner. Theheated material rises to the surface, spreads and begins tocool, and then sinks back into deeper depths. This cycle isrepeated over and over to generate what scientists call aconvection cell or convective flow. Heat within the earthcomes from two main sources: radioactive decay and residualheat. Arthur Holmes first considered this idea in the 1930s,

which later influenced Harry Hess’ thinking about seafloorspreading. The slow movement of the hot, softened mantle thatlies below the rigid plates is the driving force behind theplate movement

Movement of Indian Plate

The Indian plate includes Peninsular India and the Australian

continental portions. The subduction zone along the Himalayasforms the northern plate boundary in the form of continent-continent convergence. In the east, it extends throughRakinyoma Mountains of Myanmar towards the island arc alongthe Java Trench. The eastern margin is a spreading site lyingto the east of Australia in the form of an oceanic ridge in SWPacific. The Western margin follows Kirthar Mountain ofPakistan. It further extends along the Makrana coast and joinsthe spreading site from the Red Sea rift southeastward alongthe Chagos Archipelago. The boundary between India and theAntarctic plate is also marked by an oceanic ridge (divergentboundary) running in roughly W-E direction and merging intothe spreading site, a little south of New Zealand. India was alarge island situated off the Australian coast, in a vastocean. The Tethys Sea separated it from the Asian continenttill about 225 million years ago. India is supposed to havestarted her northward journey about 200 million years ago atthe time when Pangaea broke. India collided with Asia about40-50 million years ago causing rapid uplift of the Himalayas.The positions of India from about 71 million years till thepresent are shown in Figure It also shows the position of theIndian subcontinent and the Eurasian plate. About 140 millionyears before the present, the subcontinent was located assouth as 50oS. latitude. The two major plates were separatedby the Tethys Sea and the Tibetan block was closer to theAsiatic landmass. During the movement of the Indian platetowards the Eurasian plate, a major event that occurred wasthe outpouring of lava and formation of the Deccan Traps. Thisstarted somewhere around 60 million years ago and continuedfor a long period of time. Note that the subcontinent wasstill close to the equator. From 40 million years ago andthereafter, the event of formation of the Himalayas tookplace. Scientists believe that the process is still continuingand the height of the Himalayas is rising even to this date.

Hot Springs and Geysers

Geysers are fountains of hot water and superheated steam thatmay spout up to a height of 150 feet from the earth beneath.The phenomena are associated with a thermal or volcanic regionin which the water below is being heated beyond boiling-point( 100°C. or 212°F.). The jet of water is usually emitted withan explosion and is often triggered off by gases seeping outof the heated rocks (Fig. 33). Almost all the world’s geysersare confined to three major areas: Iceland, the Rotoruadistrict to North Island. New’ Zealand and Yellowstone Park ofU.S.A. The worlds best-known geyser is perhaps “ Old Faithful’in Yellowstone National Park. Wyo¬ming which erupts at regularintervals — every 63 minutes on the average.

Hot Spring or thermal springs are more common and may be foundin any part of the earth where water sinks deep enough beneaththe surface to be heated by the interior forces. The waterrises to the surface without any explosion. Such springscontain dissolved minerals which may be of some medical value.Iceland has thousands of hot springs. Some of them have beenharnessed to heat houses, swimming pools and other domesticpurposes. Hot springs and geysers have become touristattractions e.g. in Japan and Hawaii.

REFERENCES: NCERT and G.C LEONG