INTERNAL STRUCTUTRE OF THE EARTH

INTRODUCTION

• The real interior of the earth is nowhere exposed to our direct observation . We can penetrate up to a few km below the earth surface.
• Indirect geo-graphical method are used to determine the properties of the material inside the earth.
• The seismic wave is the only way by which we can explore about the internal materials or interior of the earth.

• Seismological Evidence

(a)Basic facts

• Seismology is a branch that deals with the study of elastic or seismic wave generated during earthquake.
• Shock wave developed during big explosions are similar an all details to seismic waves.
• Important facts about seismic waves are relevant to the internal structure of the earth…

• In every earthquake there are 3 type of wave generated from the focus of the earthquake and up to surface of the earth.
• Name of the 3 waves are..
• P-waves
• S-waves
• L-waves

p and s waves travel through the body of the earth and hence distinguish as body waves. The L waves are confined mostly near to the surface and hence known as surface waves.

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• These waves are recorded during each earthquake in definite sequence at various seismographic stations and their records are known as seismogram.
• P-S waves are recorded in surface after passing through material deep in the earth.
• P-S waves travel with characteristic velocities through different media, so that form their arrival times many important conclusions can be made regarding the nature of the materials lying in their paths.

1. From the study of travel-time curves of P-s waves, it is possible to calculate the velocity of any one type of these waves at any depth within the earth. This gives velocity depth curves.
2. A through analysis o f the velocity depth curves, especially of sharp and prominent changes in them observed respectively from different record taken as indicator of major variation in the nature of the medium at these respective depth below the surface the analysis when started from the surface to the centre of the earth enables us obtain a generalized picture of the internal of earth.

Interpretation

• The interpretation of the internal structure of the earth from the study of seismic waves is based on detection of abrupt changes in the velocity of p-s waves during their travel from the focus to various seismographic stations on the earth.
• If the earth were of a uniform nature from its surface to the center, seismic waves travelling through it would be recorded on the opposite end without going any change in their velocity.

• Conversely, a major change in the velocity of seismic waves at some specific depths below the surface in numerous records can be taken to mean there is a change in the nature of medium (material) at that particular depth.
• Such a major change in the velocity is called seismic discontinuity.
• A number of seismic discontinuity is observed from the earthquake records.
• The depths calculated from these discontinuities show remarkable agreement and demarcating zones of different material composition within the earth.
• Two most significant seismic discontinuities are Mohorovicic discontinuity and the Mantle-Core discontinuity.

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• These basic discontinuities demarcate 3 major internal zones in the construction of earth: the crust, the mantle and the core.
• The mohorovicic discontinuity
• This is 1^st major discontinuity in seismic records for the earthquake and named based on the discover A. mohorovicic .
• It occurs in the seismic records at depth of 30-40 km below the continents, 5-6 km below the oceans and 60-70 km below the mountains.
• It is observed that both p-s waves reaching on these depth undergo sharp increase in their velocity.
• P wave attains velocity from 5.4 km/sec to 7.5 km/sec like this s wave attains velocity from 3.35 km/sec to 4.35 km/sec.

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• Mohorovicic discontinuity makes the lower limit of the skin of the earth known as crust.
• Crust is nearly 35 km thick below continental surface and about 5-6 km thick under ocean.
• The crust is to the earth what skin in the old apple very thin hard and wrinkled sheet of rock covering.

The mantle- core discontinuity

• the seismic wave which passes the Mohorovicic discontinuity continue to travel downwards with almost a uniform increase in velocity.

• The velocity of the waves increases with depth as the density of earth is creases with depth.
• At the depth of 2900 km there is one another discontinuity is suddenly observed in the record of seismic waves.
• At this speed p-waves become very sluggish and suffer a decrease in their velocities from 13.64 km/sec to 8.1 km/sec.
• S waves are practically stopped from going deeper in to the earth at this depth.

• The zone lying between these two discontinuity, M-discontinuity and Mantle-core discontinuity is known as MENTLE.
• The second discontinuity which is recorded at depth of 2900 km forming the end of mantle and begging of the third major zone of the earth is named as CORE.
• In every major earthquake, p-s waves are at all station lying between the epicenter and 142° arc distance(11000 km); further between 105° and 142° arc distances (11000-16000 km), only p-waves reappear.
• There is a shadow zone free from p and s waves in the record of each deep-seated earthquake. This zone indicate a different material as compare to upper zones.

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• This zone start from 2900 km and continues right up to the center of earth which defines the boundaries of the core within the earth.
• (c ) Final Picture
• The final picture of earth interior which is obtained from seismic records of the earthquake divides it into 3 zone or shell or spheres: the crust, the mantle and the core.
• These shell also further subdivided into different layer with definite characteristics.

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The crust

• It is the uppermost shell of the earth that extends to a variable depths below the mountain (75 km). Continents (35 km) and oceans (5 km).
• The mohorovicic discontinuity marks the lower boundary of the crust.
• The following is given about the thickness of the crust..
• Mountainous areas.-
• Under Himalaya crust thickness= 70-75 km

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• Under Hindukush mountain thickness = 60 km
• Under Andes = 75 km thick
• Continental areas= 30-40 km this has considerable variation in thickness.
• Oceanic Areas = it has variable value of thickness from maximum value of 19 km to low value of 5 km in deep oceans.
• Continental crust is further divided in to 3 parts..
• The A or the upper layer is between 2-10 km thick and is of low density (2.2 g/cc) and this is made up of sedimentary rocks. And in this p-waves have velocity from 1.8 to 5.0 km/sec.

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(2) The B or middle layer of the continental crust having relatively dense (2.4 to 2.6 g/cc) and seismic wave having velocity of 5 to 6.2 km/sec.

• This layer also called granite layer as its made up of granites , gneisses and other related igneous and metamorphic rocks.
• In some places it acquires thickness of 20 km or more in some places it exposed on surface because the overlying A layer has already been removed due to prolonged erosion by weathering agents.
• Some time this layer is referred as SIAL(Si=silica, AL= alumina).

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(C) The C layer is the lowermost layer of the continental crust and has density of 2.8 to 3.3 g/cc in which p waves attains velocity of 6 to 7.6 km/sec.

• this layer also known as basaltic layer of the crust and acquires a thickness 25-40 km under the continents.
• It is sometimes also called as SIMA(Si= silica, Ma=magnesium).

the Oceanic crust, it is generally the extension of C layer of the continental crust that makes the top layer of the oceans in most cases A and B layers being practically absent from there. The oceanic crust is estimated to have a volume of 2.54* 10 power 9 cc with an average density of 3 g/cc.

The mantle

• It is the second zone of the earth starting from the lower boundary of the crust continue up to a depth of 2900 km.
• The exact nature of the mantle yet incompletely understood. Even it has been sub-divided into upper and lower mantle.
• The boundary between this layer is of 900-1000 km below the earth and upper layer also sub-divided into 2 layer of 400 and 600 km thickness respectevily.

• The density rises from 3.3 g/cc to 5.7 g/cc.
• The part of the upper mantle from 100 km to 500 km depth, is in plastic rather than solid state. This zone is known as asthenosphere( greek “asthenes”-without strength).
• It is believed to be the source of much volcanic activity of the earth and many other processes.

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The core

• It is the innermost concentric shell and its boundry begins at depth of 2900 km from the surface and extends to the center of the earth.
• Core itself has 2 different zone.
• The outer core comprises the region from a depth of 2900 km to 4500 km below the earth surface and behaves like a liquid because S waves from the earthquake shocks reaching in this zone are n0t transmitted through this zone at all.
• The inner core with a thickness of around 1790km is believed to be a solid metallic body.

• The significant variation in the density of the material immediately outside and inside of the core .
• At the base of mantle density is referred as 5.7 g/cc that jumps to 9.9 g/cc at the top of the core. This value reaches a figure of 12.7 g/cc at the boundary of inner core and becomes 13 g/cc at the center of the earth.
• As regard the chemical composition of the inner core, the hypothesis that it is made up chiefly iron and nickel.
• Seismologically, velocities of p waves recorded in the core bear close resemblance tl those recorded for nickel iron alloys.

• The high density of earth as a planet. Mean density of earth is 5.517 g/cc.
• the density of the crust is put at 2.7 g/cc.
• The density of the material of mantle is 3.3 to 5.7 g/cc.
• Density for the material of core = 12 g/cc.
• The composition of meteorites. In fact 3 types of iron bearing meteorites have been studied…
• Hexahedrites- a nickel content- 5-6.5%.
• Octahedrites- nickel content-6.5 to 16%
• Ataxites- fine grained in nature and contain nickel content in still higher proportions.

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• It is argued by many that the iron meteorites are actually fragments from the core of a planet or planet like body of our solar system that suffered disintegration during the process of evolution of solar system.