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Exogenic as well as endogenic processes constantly shape the landscape. Aproper understanding of the physiographic character of a region remainsincomplete if the eects of endogenic processes are ignored.

All natural earthquakes take place in lithosphere(crust and the uppermostpart of the mantle ,portion of depth up to 200 km from surface of earth.

eect of tsunami would occur only if the epicentre of the tremor is !elowoceanic waters and magnitude is su"ciently high. #sunamis are wa$esgenerated !y the tremors and not an earthquake in itself.

%&')E% &* +*&-A#+& A&'# #/E +#E+&

irect %ources1rilling $olcanic eruptions

+ndirect %ources1-eteors,gra$itation,magnetic 3eld,seismic acti$ity

temp,pressure density increases as we go inside the earth

deepest drill at 4ola, in Arctic &cean, has eached depth of 52 km.

g is not the same at dierent latitudes, greater near the poles and less atequator !cos distance from centre at equator is greater than that at poles. galso dier according to mass of material, une$en distri!ution of mass withinearth in6uences this $alue. 7ra$ity anomalies gi$e us information a!out thedistri!ution of mass of the material in the crust of earth.

Earthquake

caused due to release of energy, which generates wa$es that tra$el in all

directions.

8hy does the earth shake9

release of energy occurs along a fault(sharp !reak in crustal rocks.

ocks along a fault tend to mo$e in opposite directions. As the o$erlying rockstrata press them, the friction locks them together.

ut, their tendency to mo$e apart at some point of time o$ercomes thefriction. As a result, the !locks get deformed and e$entually, they slide pastone another a!ruptly causing a release of energy, and the energy wa$estra$el in all directions. point where energy released is called

focus:hypocentre.

energy wa$es tra$elling in dierent directions reach the surface. point on thesurface, nearest to the focus, is called epicentre(3rst one to experiencewa$es is directly a!o$e focus.

Earthquake Waves

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two types

5.ody wa$es1generated due to release of energy at focus and mo$e in alldirections tra$elling through the !ody of earth.2 types1

a;1wa$es:primary wa$es1mo$e faster and are 3rst to arri$e at surface.

similar to sound wa$es. tra$el through gaseous, liquid and solid materials.

!%1wa$es:secondary wa$es1arri$e at surface with some time lag. can tra$elonly through solid materials. #his characteristic has helped scientists tounderstand the structure of the interior of the earth.

2.%urface wa$es1ody wa$es interact with the surface rocks and generatenew set of wa$es called surface wa$es. they mo$e along the surface. #hesurface wa$es are the last to report on seismograph. #hese wa$es are moredestructi$e. #hey cause displacement of rocks, and hence, the collapse ofstructures occurs.

#he $elocity of wa$es changes as they tra$el through materials with dierent

densities. #he denser the material, the higher is the $elocity. #heir directionalso changes as they re6ect or refract when coming across materials withdierent densities.

Propagation of Earthquake Waves(p parallel others perpendicular

;1wa$es $i!rate parallel to the direction of wa$e exerting pressure on thematerial in the direction of propagation createing density dierences inmaterial leading to stretching and squeerecorded arri$al of none of the wa$es those located !eyond 5@>? fromepicentre, record the arri$al of ;1wa$es only not %1wa$es.#hus, a ? and 5@>? from epicentre was identi3ed as the shadow ? away from the epicentre.entire ? does not recei$e%1wa$es. shadow

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(i tectonic earthquakes(most common ones1 generated due to sliding ofrocks along a fault plane.

(ii $olcanic earthquake(spl. class of #E1 con3ned to areas of acti$e$olcanoes.

(iii collapse earthquakes1when roofs of underground mines collapse causingminor tremors.

(i$ explosion earthquakes1 due to the explosion of chemical or nuclearde$ices.

($ reser$oir induced earthquakes.

Measuring Earthquakes

magnitude1 magnitude scale is known as ichter scale. #he magnituderelates to the energy released during the quake. magnitude is expressed ina!solute num!ers, 0150.

intensity of the shock1intensity scale is named after -ercalli, an +talianseismologist. +t takes into account the $isi!le damage caused !y the e$ent.+ts range is from 5152.

E**E)#% &* EA#/'A4E (i 7round %haking (ii ierential groundsettlement (iii Band and mud slides (i$ %oil liquefaction ($ 7round lurching($i A$alanches ($ii 7round displacement ($iii *loods from dam and le$eefailures (ix *ires (x %tructural collapse (xi *alling o!Cects (xii #sunami

3rst six ha$e some !earings upon landforms, while others may !e consideredeects causing immediate concern to life and properties of people.

STRUTURE !" T#E E$RT#

)rust1outermost solid part of earth. !rittle in nature.thickness $aries underoceanic and continental areas. &ceanic(mean thickness >km !eing thinneras compared to continental(D0 km. made up of hea$ier rocks ha$ing densityof D g:cmD. #his type of rock found in the oceanic crust is !asalt. #he meandensity of material in oceanic crust is 2. g:cmD.

-antle1portion of interior !eyond crust. extends from -ohoFs discontinuity toa depth of 2,G00 km.thickness ranges from 501200 km.density higher thancrustFs (D.@ g:cmD.upper portion of mantle is called asthenosphere(weak,@00km,main source of magma.lower mantle extends !eyond the asthenosphere

is in solid state.

)ore1coremantle !oundary is located at the depth of 2,G00 km.density atmantle core !oundary is around > g:cmD and at the centre of theearth(HD00km,is around 5Dg:cmD.outer core is in liquid state while inner coreis in solid state. made up of $ery hea$y material like nickel and iron aka nifelayer.

%!&$'!ES $'( %!&$') &$'("!RMS

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$olcano is a place where gases, ashes and:or molten rock material I la$a Iescape to the ground.

#he layer !elow the solid crust is mantle. +t has higher density than that ofthe crust. #he mantle contains a weaker

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continental mass, a mega ocean surrounded by the same. Thesuper continent was named PANGAEA, which meant all earth.

The mega-ocean was called PANTHALASSA, meaning all water.He argued that, around 200 million years ago, the supercontinent, Pangaea, began to split. Pangaea first broke into twolarge continental masses as Laurasia and Gondwanalandforming the northern and southern components respectively.Subsequently, Laurasia and Gondwanaland continued to

break into various smaller continents that exist today.

Evidence in Support of the Continental Drift

The Matching of Continents (Jig-Saw-Fit)

The shorelines of Africa and South America facing each otherhave a remarkable and unmistakable match. It may be notedthat a map produced using a computer programme to find the

best fit of the Atlantic margin was presented by Bullard in

1964. It proved to be quite perfect. The match was tried at1,000- fathom line instead of the present shoreline.

Rocks of Same Age Across the Oceans

The radiometric dating methods developed in the recent periodhave facilitated correlating the rock formation from differentcontinents across

the vast ocean. The belt of ancient rocks of2,000 million years from Brazil coast matches withthose from western Africa. The earliest marine deposits alongthe coastline of South America and Africa are of the Jurassicage. This suggests that the ocean did not exist prior to thattime.

Tillite

It is the sedimentary rock formed out of deposits of glaciers.The Gondawana system of sediments from India is known tohave its counter parts in six different landmasses of theSouthern Hemisphere. At the base the system has thick tilliteindicating extensive and prolonged glaciation. Counter parts ofthis succession are found in Africa, Falkland Island,

Madagascar, Antarctica and Australia besides India. Overallresemblance of the Gondawana type sediments clearlydemonstrates that these landmasses had remarkably similarhistories. The glacial tillite provides unambiguous evidence ofpalaeoclimates and also of drifting of continents.

Placer Deposits

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The occurrence of rich placer deposits of gold in the Ghanacoast and the absolute absence of source rock in the region isan amazing fact. The gold bearing veins are in Brazil and it isobvious that the gold deposits of the Ghana are derived fromthe Brazil plateau when the two continents lay side by side.

Distribution of fossilWhen identical species of plants and animals adapted to livingon land or in fresh water are found on either side of themarine barriers, a problem arises regarding accounting forsuch distribution. The observations that Lemurs occur inIndia, Madagascar and Africa led some to consider acontiguous landmass Lemuria linking these threelandmasses. Mesosaurus was a small reptile adapted toshallow brackish water. The skeletons of these are found onlyin two localities : the Southern Cape province of South Africaand Iraver formations of Brazil. The two localities presently are

4,800 km apart with an ocean in between them. 31Force for Drifting

Wegener suggested that the movement responsible for thedrifting of the continents was caused by pole-fleeing force andtidal force. The polar-fleeing force relates to the rotation of theearth. You are aware of the fact that the earth is not a perfectsphere; it has a bulge at the equator. This bulge is due to therotation of the earth. The second force that was suggested by

Wegenerthe tidal forceis due to the attraction of the moonand the sun that develops tides in oceanic waters. Wegener

believed that these forces would become effective when appliedover many million years. However, most of scholars consideredthese forces to be totally inadequate.Post-Drift StudiesIt is interesting to note that for continental drift, most of theevidence was collected from the continental areas in the formof distribution of flora and fauna or deposits like tillite. Anumber of discoveries during the post-war period added newinformation to geological literature. Particularly, theinformation collected from the ocean floor mapping providednew dimensions for the study of distribution of oceans andcontinents.

Convectional Current TheoryArthur Holmes in 1930s discussed the possibility ofconvection currents operating in the mantle portion. Thesecurrents are generated due to radioactive elements causingthermal differences in the mantle portion. Holmes argued thatthere exists a system of such currents in the entire mantleportion. This was an attempt to provide an explanation to theissue of force, on the basis of which contemporary scientistsdiscarded the continental drift theory.

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Mapping of the Ocean Floor

Detailed research of the ocean configuration revealed that theocean floor is not just a vast plain but it is full of relief.Expeditions to map the oceanic floor in the post-war periodprovided a detailed picture of the ocean relief and indicatedthe existence of submerged

32FUNDAMENTALS OF PHYSICAL GEOGRAPHYmountain ranges as well as deep trenches, mostly locatedcloser to the continent margins. The mid-oceanic ridges werefound to be most active in terms of volcanic eruptions. Thedating of the rocks from the oceanic crust revealed the factthat the latter is much younger than the continental areas.Rocks on either side of the crest of oceanic ridges and havingequidistant locations from the crest were found to haveremarkable similarities both in terms of their constituents andtheir age.

Ocean Floor ConfigurationIn this section we shall note a few things related to the oceanfloor configuration that help us in the understanding of thedistribution of continents and oceans. You will be studying thedetails of ocean floor relief in Chapter13. The ocean floor may be segmented into three majordivisions based on the depth as well as the forms of relief.

These divisions are continental margins, deep-sea basins andmid-ocean ridges.

Figure 4.1 : Ocean FloorContinental Margins

These form the transition between continental shores anddeep-sea basins. They include continental shelf, continentalslope, continental rise and deep-oceanic trenches. Of these,the deep-sea trenches are the areas which are of considerableinterest in so far as the distribution of oceans and continentsis concerned.Abyssal Plains

These are extensive plains that lie between the continentalmargins and mid-oceanic ridges. The abyssal plains are theareas where the continental sediments that move beyond themargins get deposited.

Mid-Oceanic RidgesThis forms an interconnected chain of mountain system withinthe ocean. It is the longest mountain-chain on the surface ofthe earth though submerged under the oceanic waters. It ischaracterised by a central rift system at the crest, afractionated plateau and flank zone all along its length. Therift system at the crest is the zone of intense volcanic activity.In the previous chapter, you have been introduced to this type

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of volcanoes as mid- oceanic volcanoes.

Distribution of Earthquakes and Volcanoes

Study the maps showing the distribution of seismic activityand volcanoes given in Figure 4.2. You will notice a line of dotsin the central parts of the Atlantic Ocean almost parallel to thecoastlines. It further extends into the Indian Ocean. It

bifurcates a little south of the Indian subcontinent with onebranch moving into East Africa and the other meeting a similarline from Myanmar to New Guiana. You will notice that thisline of dots coincides with the mid- oceanic ridges. The shaded

belt showing another area of concentration coincides with theAlpine-Himalayan system and the rim of the Pacific Ocean. Ingeneral, the foci of the earthquake in the areas of mid-oceanicridges are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, theearthquakes are deep-seated ones. The map of volcanoes alsoshows a similar pattern. The rim of the Pacific is also calledrim of fire due to the existence of active volcanoes in this area.

CONCEPTOFSEAFLOORSPREADINGAs mentioned above, the post-drift studies providedconsiderable information that was notDISTRIBUTION OF OCEANS AND CONTINENTS33

Figure 4. 2 : Distribution of earthquakes and volcanoesavailable at the time Wegener put forth his concept ofcontinental drift. Particularly, the mapping of the ocean floorand palaeomagnetic studies of rocks from oceanic regionsrevealed the following facts :(i) It was realised that all along the mid- oceanicridges, volcanic eruptions are common and they bring hugeamounts of lava to the surface in this area.

(ii) The rocks equidistant on either sides of the crest ofmid-oceanic ridges show remarkable similarities in terms ofperiod of formation, chemical compositions and magneticproperties. Rocks closer to the mid-oceanic ridges arenormal polarity and are the youngest. The age of the rocksincreases as one moves away from the crest.

(iii) The ocean crust rocks are much younger thanthe continental rocks. The age of rocks in the oceanic crustis nowhere more than 200 million years old. Some of the

continental rock formations are as old as 3,200 millionyears.(iv) The sediments on the ocean floor are unexpectedlyvery thin. Scientists were expecting, if the ocean floors wereas old as the continent, to have a complete sequence ofsediments for a period of much longer duration. However,nowhere was the sediment column found to be older than200 million years.

(v) The deep trenches have deep-seated earthquake

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occurrences while in the mid- oceanic ridge areas, thequake foci have shallow depths.

These facts and a detailed analysis of magnetic properties ofthe rocks on either sides of the mid-oceanic ridge led Hess(1961) to propose his hypothesis, known as the sea floorspreading. Hess argued that constant eruptions at the crest of

oceanic ridges cause the rupture of the oceanic crust and thenew lava wedges into it, pushing the oceanic crust on eitherside. The ocean floor, thus spreads. The younger age of theoceanic crust as well as the fact that the spreading of oneocean does not cause the shrinking of the other, made Hess34FUNDAMENTALS OF PHYSICAL GEOGRAPHYUauridary

TVnikrfhrmn nivrrjjfmMiilc Buundai}-| hv,1111-L.ir\-CwumjJnil+N.Blr llnuml.Br$( fiilrM iii hl IfP &ir IVOl! IIa. |sl>iic555 lii iO 51.lr>. lFigure 4. 3 : Sea floor spreadingPLATETECTONICSmmmm;v, S:*think about the consumption of the oceaniccrust. He further maintained that the ocean

Since the advent of the concept of sea floorfloor that gets pushed due to volcanic spreading, the interest inthe problem oferuptions at the crest, sinks down at the distribution ofoceans and continents wasoceanic trenches and gets consumed.

revived. It was in 1967, McKenzie and ParkerThe basic concept of sea floor spreading hasand also Morgan,independentiycollectedthe

been depicted in Figure 4.3.available ideas and came out with another

unions years ago: __________________,,

S40(

\XXE \