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Offline Zahida Raees Raji

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« on: March 26, 2008, 03:37:02 PM »



 Mahmood Siddiqui & Uzma Mahmood

         In the previous episode the seismic waves commenced a journey to the centre of the Earth. The waves started to travel from the Crust, which is the outer most veneer of the globe. They narrated that crust is on an average 30 km thick and exists everywhere on the Earth?s surface; on land, under seas and the ocean floors and is of two types, viz. continental and oceanic. Penetrating deeper, the traveller waves have now reached the first station; the base of crust. Let?s see what they have to tell us as they descend further.

Station №1: Crust-Mantle-Junction
         Advancing deeper into the Earth?s interior, the traveller waves encounter a 0.5 km thick sheet or layer of a dense rock at the crust-mantle junction. The sheet runs along bottom of the continental and oceanic crusts, and separates the comparatively light-weighted crust from the dense and heavier underlying mantle. Andrija Mohorovičić (1857?1936), a Croatian seismologist, using seismic waves as tool discovered this high-density transitional layer in the year 1909 and in his honour it was named Mohorovičić Discontinuity (Moho or M? for short). As the seismic waves reach this layer, a discontinuity appears in terms of an abrupt increase in velocity of these waves. The sheet is interpreted to be composed of a metamorphic rock ?peridotite?, which is comprised of heavy minerals like olivine38, pyroxene39, calcium and magnesium garnet40, and a few other minerals that form under high-pressure conditions.


Floating Platform
         Below Moho starts Mantle; the next zone of the Earth?s interior. The upper 100 to over 200 km thick stratum of this zone, according to the seismic waves, is a solidified rigid and hard slab of peridotite that floats on underlying squishy and partly molten upper mantle. In view of composition and high density it is included in mantle but considering its toughness and rigidity it is counted with the lithosphere (discussed in next paragraph). No particular name appears to be previously given to this layer but in view of its role as a floating rocky platform on which the continental and oceanic crusts rest upon, it deserves a specific name and here we call it solidified peridotite layer (Figure 2-3).


Rafts Cruising on an Ocean of Fire
         Earth?s outer shell is known as Lithosphere. It is not a separate layer per se, but is a sum of all the layers the seismic waves have penetrated so far. In other words it is a unit made up of overlying continental or oceanic crusts plus the underlying solidified peridotite layer, with Moho sandwiched in between the two. Like the leather cover of a football, Earth?s outermost sheath the lithosphere, is built of a patch-work of over a dozen large and small segments called Lithospheric Plates (Greek lithos ?rock?) or Tectonic Plates (so well known in context of theory of plate tectonics). In area, these plates spread over hundreds of thousands of square kilometres but vertically they are less than 275 km thick. The plate thus symbolise a laden raft-like assembly where solidified peridotite layer represents the raft and the continental or oceanic crusts; the payload. With this setup the assembly cruises like a gigantic raft on heat-softened and partly molten 100?200 km thick heavy mobile rock material that exists in the upper part of mantle and is known as Asthenosphere (Greek aesthenos ?weak?). It is a weak solid as it is hot, almost at the melting temperature of peridotite and virtually, therefore, it is an ocean of fire.


         Lithosphere and Asthenosphere are of specific interest and relevance to the geoscientists, particularly to the economic geologists. For, these are the zones where magma chamber41 develops and ore materials originate from to form ore deposits at or close to the surface. At the same time, however, lithosphere and the upper mantle are the zones where destructive earthquakes initiate and cause heavy loss to life and property.


Sailing Continents
         According to the theory of plate tectonics42, continents and ocean floors, riding the peridotite raft, move around on the face of the Earth. Tectonic plates may move away from each other (divergent movement), or come closer and collide with each other (convergent movement), or may slide-past each other (lateral movement). Convection currents43 in asthenosphere are believed to be responsible to drive the lithospheric plates and cause drifting of the continents and ocean floors. Through these movements, large land masses and ocean floors have travelled long distances. For instance, about 170 million years ago India, together with eastern half of Pakistan, was located in the vicinity of South Pole and has sailed thousands of kilometres to reach to the present position about 20 million years ago. It would be astounding to note that the piece of land that now forms eastern Balochistan (east of Chaman, Nushki Kalat, Khuzdar and Bela) was not always at its present position. It has rather drifted from a faraway position, somewhere in present Arctic Circle, to annex to the indigenous land of the western Balochistan. Of course, the voyage was made aboard the raft of solidified peridotite layer that sails on the surface of asthenosphere over 200 km below the Earth?s surface. At the same time, original crust of eastern Balochistan was pushed northward to Chitral and beyond to give rise to the high mountain ranges of the Himalayas and Karakoram (to be discussed in details in next chapter).


         The lithospheric plates are well buoyant and move around like ice bergs in ocean. To maintain the isostatic balance or buoyancy (i.e.,equilibrium between the hard floating lithospheric plate and soft asthenosphere) the continental crusts being light-weighted rise high and constitute mountain ranges with some peaks towering over 8 km in the air. Under high mountains and large plateaux, the continental crust often gets twice as thick as under plains (Figure 2-3). Due to the additional load of the mountains, the solidified peridotite layer bends downwards. This happens as rocks of the crust and the rocky material of the mantle are solid and strong over short period of time (hundreds of years); over long terms (thousands to millions of years), however, they are weak. Under prolonged high pressure, the lithosphere flows slowly like viscous fluid. Large mountains, accordingly, sink under their own weight as the solidified peridotite layer below the crust bends and bulges downward into the soft upper mantle. A downward projection or ?root? of continental lithosphere is thus formed as is shown in Figure 2-3.

(solidified top of Asthenosphere) on which the crust rests upon

(Source: Press and Siever, 2002).


         This provides additional buoyancy to the lithosphere, which is protected from side-wise tilting owing to heavy weight of the high mountains. This arrangement is in perfect agreement with several Quranic verses about the underground shape and function of mountains ? And We fixed mountains on the Earth so that it may not sway . . .? (Al-Ambia 21:31 and Luqman 31:10)  and  ?Isn?t it so that We made the Earth a floor, and nailed mountains into it like pegs.? (Al-Naba 78:6&7). Now, here is the explanation of modern science. ?. . . Relatively light-weighted continental crust projecting into the denser mantle serves as buoyant root providing ?flotation? for the continent. The root is deeper under mountains, where flotation is required to support the heavier load, in accordance with the principle of isostasy44? (Press & Siever; 2002).

Ocean Above and Ocean Below
         Oceanic lithosphere comprises about 6 km thick sheet of oceanic crust that rides over some 100 km thick solidified peridotite layer. Unlike light-weighted continental lithosphere the oceanic lithosphere, due to high density of the riding crust, stays submerged at about 4 to 5 km below the sea level to drifts on the underlying asthenosphere. Carrying an ocean of water on its back, the oceanic lithosphere floats on an ocean of molten rocky material. Unlike continental lithosphere, the oceanic lithosphere is of uniform thickness and no bending or down bulging of either the oceanic crust or the underlying solidified peridotite layer takes place.

Station № 3: The Mantle
         Having crossed the wonderland of Curst-Mantle junction, the seismic finally step into mantle itself. The word mantle comes from Latin word mantellium, meaning ?cloak? or ?cover?, mantle thus cloaks the core. Extending from base of the crust (~30 km) to the surface of core (~2900 km), mantle is the thickest zone of the Earth?s interior. According to the seismic weaves, it is composed of dense, iron-magnesium silicate minerals and is divided into concentric layers by phase changes that are caused by increase in pressure with depth. It is this zone that is attributed to supply metals to the crust, where they form ore deposits. Mantle is divided into two zones; upper and lower mantle. Upper Mantle, which begins below Moho and extends to the depths of about 700 km, is composed mostly of peridotite. The topmost 100?200 km thick layer is solid, as has been mentioned earlier in the text as solidified peridotite layer. The next 200 km thick portion called Asthenosphere is partly molten (up to 10 %). Density of upper mantle just below Moho is 3.3g/cm3. The Lower Mantle occurs between 700 and 2900 km depth. This 2200 km thick zone comprises solid, strong and dense peridotite. Composition of the material remaining same as that of the upper mantle, density of lower mantle rises to 5.5 g/cm3 near the base at the mantle-core boundary. This increase in density is attributed to readjustment of the silicate atoms of olivine and pyroxene to more compact arrangement due to the very high load of the overlying rocks. Mantle occupies about 80% of the Earth?s volume.

Halts of the Mantle

At least three notable transitional layers and discontinuities occur in mantle at the depths of 390, 660 and 2700 km. These are Halts from where the seismic waves pass-by just casting a brief glance. These layers are believed to be formed as a result of the phase changes. At the first discontinuity layer at the depth of 390 km, the normally stable crystal lattice of olivine, presumably due to excessive temperature and pressure, is transforms into higher density configurations. Likewise, discontinuity layer at the depth of 660 km also is assumed to be formed as a result of the olivine and pyroxene undergoing transformation to denser crystalline structures. As a result, a silicate mineral ?perovskite? out of the two (olivine and pyroxene) is formed. This transformation results in 10 percent increase in density. The third discontinuity, also called ?D? layer, occurs close to the bottom of lower mantle at the depth 2700 km. This is a 200-300 km thick seismic discontinuity, which suggests that the layer is chemically different from rest of the lower mantle. The material of which the layer is composed possibly sank through the mantle but owing to its low density could not sink deeper into the denser core.

Station № 4: The Core
         As seismic waves descend further below the mantle they arrive at the Earth?s core, a spherical body 6940 km in diameter, i.e. larger in size than the planet Mars. This huge orb is readily divisible into two distinct zones, viz., the Outer Core and the Inner Core.

A Ball of Molten Iron

The Outer Core occurs below mantle at the depth of 2900 km from the Earth?s surface. Having a thickness of 2200 km, it extends downward to a depth of 5100 km. The core-mantle boundary, also known as Wiechert-Gutenberg Discontinuity, is the location of marked changes in density of the composing material. Density, which was 5.5 g/cm3 at the base of mantle increases to 9.9 g/cm3 in the outer core. In view of the high density it may be inferred that the outer core is comprised of iron and nickel with small amount of lighter elements possibly sulphur and oxygen. One of our travellers (the S waves) does not know how to swim and stops at the mantle-core boundary. This behaviour of the S waves suggests that the outer core occurs in molten form. Outer core, therefore, is an enormous ball of molten iron that engulfs the inner core, just like pulp of apricot envelops its stone (seed). Convection current occurs in this molten mass which is electrically conductive. This, together with the Earth?s rotation, creates a dynamo effect that possibly causes a system of electrical current resulting in creation of the Earth?s magnetic field. The outer core accommodates 30.8 percent of the Earth?s mass.

Final Destination: Centre of the Earth
         The seismic waves finally reach the endpoint of their journey; the Inner Core. With a radius of 1270 km, the inner core is a spherical body slightly larger in size than the Moon. Like outer core, the inner core also is believed to be built of an alloy of iron and nickel together with oxygen and sulphur. Contrary to the outer core, however, the inner core is a solid ball at the heart of which lies the center of the Earth at an average depth of 6371 km from the surface.
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Offline Zahida Raees Raji

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« Reply #1 on: March 26, 2008, 03:40:56 PM »



Word ?seismic? comes from Greek word seismos, for ?earthquake?. These waves travel from crust to core and thus are used as a tool to probe the Earth?s interior. Four types of seismic waves are recognised: (i) Primary or (P) waves (ii) Secondary or Shear (S) waves (iii) Surface, Long, or Love-waves and (iv) Rayleigh waves. The P and S waves travel through the Earth?s body and are suitable in study of the Earth?s interior (Figure 2-4). Other waves, i.e., Surface, and Rayleigh waves, travel only at the Earth?s surface and do not penetrate deep into the Earth and therefore, are of little use to study the Earth?s interior.

The P waves, which are compressional and dilatational (pushing and pulling) in nature, penetrate through the crust at speed of 5.8 to 6.4 km per second. In deeper parts of the Earth the velocity of P waves increases with increasing density and rigidity of the material. The waves spread out in alternate pushes (compressions) and pulls (relaxations) in the rocks. Like sound waves, the P waves can travel through solids, liquids, molten materials and gases. The S waves (shear waves), on the other hand, travel through the crust at about half the speed of P waves?about 3.5 km/second. These waves push the particles of material they pass through perpendicular to the path of their propagation (advancement). The S waves, however, are not capable of travelling through liquids, molten masses and gases and are stopped at the solid-liquid interface. With this much basic information about the P and S waves, the geoscientists undertook the grand task of unearthing the Earth?s interior?from crust to core. 

Study of these waves, as they pass through the Earth?s depths, provides data that can be readily translated in terms of information as if it were collected by direct observation. Seismic waves triggered either by an earthquake or generated by a manmade explosion, propagate in all directions from the focus of the earthquake (or the explosion site in case of manmade explosion). Due to varying chemical composition and density and physical state of material present in different zones the seismic waves do not travel uniformly thorough the Earth?s interior. Velocity of the waves, while passing through various internal zones and layers is accelerated or retarded. The waves travelling straight in one zone may get deflected, bounced back or altogether absorbed at the boundary with the other zone. Coming across certain zones with molten material, the S waves stop abruptly and entering denser materials both the P and S waves are deflected. Thus, waves travel with different velocities in two different zones lying side-by-side or one over the other and while doing so provide immensely useful information about the Earth?s inaccessible interior. Such attitudes of the waves are interpreted in terms of composition, rigidity, density and physical state of the material, thickness of the zones and layers these materials occupy. Also the depths at which these zones occur, can be precisely determined. For instance, P wave velocity is 6.8 km per second in the lower part of the crust but just below Moho its velocity increase to 8 km per second in the mantle. This high velocity of the P waves in mantle is not in agreement with any other rock but peridotite. Mantle, therefore, is inferred to be comprised of peridotite. Similarly, velocity of the P waves, which is about 13.7 km per second at the base of mantle, suddenly drops down to 8 km per second below the core-mantle boundary. And, the S wave velocity, which is 7.3 km per second at the base of the mantle, abruptly drops to zero at the outer core. The marked change of the P and S wave velocities suggest that the material constituting the outer core occurs in molten form and that the inner core occurs as solid.


Press, F. and Siever, R., 2002: Understanding Earth (3rd ed) W. H. Freeman and Company, New York. U.S.A.


38. Olivine: iron or magnesium silicate

39. Pyroxene: iron, magnesium or calcium silicate

40. Garnate: silicate of various metals e.g. calcium, magnesium, iron etc., used as heavy weighted, semi-     precious gemstone

41. Magma chamber: a magma-filled cavity within lithosphere.

42. Tectonic plate: (from Greek, tektonikos, for buildings) Rigid pieces of Earth?s lithosphere that move over asthenosphere.

43. Convection currents: Movement within liquefied mantle caused by heat transfer from Earth?s core. This movement is most likely responsible for motion of Earth?s tectonic plates. 

44. Principle of Isostasy: the mechanism whereby areas of crust rise until mass of their topography is buoyantly supported or compensated by thickness of the crust below, thereby forming crustal ?roots?. In simple words continents being lighter than the mantle float on it like ice cubes float in a glass of water. Bigger ice cubes that stand higher above the water surface have larger portions submersed under water to provide greater buoyancy.


Source: Monthly Sangat

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