Layers of earth

Internal structure & composition of the earth

1. Layers / Divisions of the earth

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4. Layers / Divisions of the earth The Core, Mantle, and Crust constitute the three main layers of earth (Fig. 5.1). Compositionally (chemically), the outer thin crust is mostly silicate (SiO 2 -based) and mantle, the layer below is accompanied by metal oxides (such as MgO , FeO , Al2O 3 , CaO , and Na 2 O) in mineral composition (Taylor and McLennan 1985) (see also Fig. 4.3 ). Mantle is the largest by volume, making up almost 87 % of the earth. The base of the mantle to the center of the earth, is made up of iron (90 %), nickel (5 %) with a possible admixture of carbon, silicon, oxygen, sulfur, and hydrogen (comprising around 5 % by mass). The core makes up 35 % of the total mass of the earth and is probably made of almost pure iron, perhaps even in a single crystal form. 4

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8. Continental Crust (0–75 km) This is the outer most layers and forms the surface of the earth. It is primarily composed of crystalline rocks with low-density buoyant minerals that are largely dominated by silicates (Quartz; SiO 2 ) and feldspars (metal-poor silicates). As cold rock deforms slowly, this rigid and brittle outer layer is also called the Lithosphere ( lithos meaning rocky or strong layer) 8

9. Oceanic Crust (0–10 km) The majority of the earth’s crust was made through volcanic activity. The oceanic ridge system, a 40,000 km long network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, and covers the ocean floor with Basalt, an igneous rock. Hawaii and Iceland are two classic examples of such accumulations. 9

10. Upper Mantle (10–400 km) Solid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions. These include such minerals as Olivine [(Mg, Fe) 2 SiO 4 ], Pyroxene [(Mg, Fe)SiO 3 ], and others that crystallize at high temperatures. The asthenosphere, part of the upper mantle (Fig. 5.1), might well be partially molten. 10

11. Transition Region (400–650 km) The transition region or Mesosphere (for middle mantle) is sometimes also called the Fertile layer (Fig. 5.1). It is the source of basaltic magma and complex aluminum- bearing silicate minerals containing calcium, aluminum, and garnet. When cold, this layer is dense due to the presence of garnet. It is buoyant when hot as these minerals melt easily to form basalt which rises through the upper layers as Magma. 11

12. Lower Mantle (650–2890 km) The lower mantle (Fig. 5.1) is probably composed of silicon, magnesium, and oxygen with some amounts of iron, calcium, and aluminum. D”” Layer (2700–2890 km) It is also called the D prime (D””) and is 200–300 km thick. Although it is often identified as part of the lower mantle (Fig. 5.1), seismic data suggest that this layer might differ chemically from the lower mantle. 12

13. Outer Core (2890–5150 km) The outer core is hot and composed of electrically conducting liquid made mainly of iron and nickel. This conductive layer (Fig. 5.1) combines with earth’s rotation to create a dynamo effect that maintains a system of electrical currents, thereby, creating the earth’s magnetic field. This layer is not as dense as pure molten iron, thus, suggesting the presence of lighter elements also. It is suspected that about 10 % of the layer is composed of sulfur and oxygen as these elements are abundant in the cosmos and also dissolve readily in molten iron. 13

14. Inner Core (5150–6378 km) The inner core is made of solid iron and nickel and is suspended in the molten outer core, unattached to the mantle (Fig. 5.1). It is believed to have solidified as a result of pressure-freezing which occurs to most liquids under extreme pressure. 14

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22. Geodynamics is generally concerned with processes that move materials throughout the Earth. In the Earth’s interior, movement happens when rocks melt or deform and flow in response to a stress field. This deformation may be brittle, elastic, or plastic, depending on the magnitude of the stress and the material’s physical properties, especially the stress relaxation time scale. Rocks are structurally and compositionally heterogeneous and are subjected to variable stresses, so it is common to see different types of deformation in close spatial and temporal proximity. When working with geological timescales and lengths, it is convenient to use the continuous medium approximation and equilibrium stress fields to consider the average response to average stress. 22

23. The interior structure of the Earth is layered in spherical shells. These layers can be defined by their chemical and their rheological properties.  Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.  23

24. The crust ranges from 5–70 kilometers (3.1–43.5 mi) in depth and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense ( mafic ) iron magnesium silicate igneous rocks, like basalt. The thicker crust is continental crust, which is less dense and composed of ( felsic ) sodium potassium  aluminium  silicate rocks, like granite. The rocks of the crust fall into two major categories – sial and sima (Suess,1831–1914). It is estimated that sima starts about 11 km below the Conrad discontinuity. The uppermost mantle together with the crust constitutes the lithosphere. The crust-mantle boundary occurs as two physically different events. 24

25. Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. The mantle is divided into upper and lower mantle. The upper and lower mantle are separated by the transition zone. The lowest part of the mantle next to the core-mantle boundary is known as the D″ (pronounced  dee -double-prime) layer. 25

26. The pressure at the bottom of the mantle is ≈140 GPa  . The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales.  Convection of the mantle is expressed at the surface through the motions of tectonic plates.  26

27. The average density of Earth is 5,515 kg/m 3 . Because the average density of surface material is only around 3,000 kg/m 3 , we must conclude that denser materials exist within Earth's core. Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ≈1,220 km and a liquid outer core extending beyond it to a radius of ≈3,400 km. The densities are between 9,900 and 12,200 kg/m 3  in the outer core and 12,600–13,000 kg/m 3  in the inner core. 27

28. The inner core was discovered in 1936 by  Inge Lehmann and is generally believed to be composed primarily of iron and some nickel. It is not necessarily a solid, but, because it is able to deflect seismic waves, it must behave as a solid in some fashion. Experimental evidence has at times been critical of crystal models of the core 28

29. 5 discontinuities Conrad – between outer & inner crust Mohorovicic - between crust & Mantle Repetiti – between outer and inner mantle wiechert gutenberg – Between mantle & core Lehmann – Between outer & inner core. 29

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35. Conrad discontinuity The Conrad discontinuity corresponds to the sub-horizontal boundary in continental crust at which the seismic wave velocity increases in a discontinuous way. This boundary is observed in various continental regions at a depth of 15 to 20 km, however it is not found in oceanic regions. The Conrad discontinuity (named after the seismologist Victor Conrad) is considered to be the border between the upper continental crust and the lower one. It is not as pronounced as the  Mohorovičić discontinuity, and absent in some continental regions. 35

36. Up to the middle 20th Century the upper crust in continental regions was seen to consist of  felsic rocks such as granite ( sial , for silica- aluminium ), and the lower one to consist of more magnesium-rich  mafic rocks like basalt ( sima , for silica-magnesium). Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct two layers, sial and sima 36

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38. Low-velocity zone The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity. It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves. A second low-velocity zone (not generally referred to as the LVZ, but as ULVZ) has been detected in a thin ≈50 km layer at the core-mantle boundary. These LVZs may have important implications for plate tectonics and the origin of the Earth's crust. 38

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40. Mohorovičić discontinuity The  Mohorovičić discontinuity usually referred to as the  Moho , is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist   Andrija Mohorovičić , the Moho separates both the oceanic crust and continental crust from underlying mantle. The Moho lies almost entirely within the lithosphere; only beneath mid-ocean ridges does it define the lithosphere– asthenosphere boundary. The Mohorovičić discontinuity was first identified in 1909 by Mohorovičić , when he observed that seismograms from shallow-focus earthquakes had two sets of P-waves and S-waves, one that followed a direct path near the Earth's surface and the other refracted by a high-velocity medium. 40

41. The Mohorovičić discontinuity is 5 to 10 kilometres (3–6 mi) below the ocean floor, and 20 to 90 kilometres (10–60 mi), with an average of 35 kilometres (22 mi), beneath typical continental crusts Below Indian peninsula 35 km Below western Ghats 50 km Below Himalayas 80 km 41

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43. Repetti discontinuity Repetti discontinuity is a postulated boundary layer between two layers of the lower mantle. It is defined by an increase in seismic velocities with depth. It was named after William C. Repetti , an American geophysicist who investigated this boundary layer with seismological methods as part of his doctoral thesis completed at St. Louis University in 1930. 43

44. This is not a zero-order boundary, but a gradual transition. However, seismic studies with short-period data suggest that the transition region has a relatively small thickness (≈ 10 km). The cause of the Repetti discontinuity is so far unclear and its occurrence has so far only been proved by relatively few observations. These studies were mostly related to subduction zones and showed a seismic discontinuity at very different depths (between 900 and 1080 km). 44

45. According to the average depth of its occurrence, it is sometimes referred to in the literature as a 920 km discontinuity. 45

46. Gutenberg Discontinuity The Gutenberg discontinuity occurs within Earth's interior at a depth of about 2,900 km (1,800 mi) below the surface, where there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, and cannot transmit through liquids, so it is believed that the unit above the discontinuity is solid, while the unit below is in a liquid, or molten, form. This distinct change marks the boundary between two sections of the earth's interior, known as the lower mantle (which is considered solid) and the underlying outer core (believed to be molten). 46

47. The molten section of the outer core is thought to be about 700°C (1,292°F) hotter than the overlying mantle. It is also denser, probably due to a greater percentage of iron. This distinct boundary between the core and the mantle, which was discovered by the change in seismic waves at this depth, is often referred to as the core-mantle boundary, or the CMB. It is a narrow, uneven zone, and contains undulations that may be up to 5-8 km (3-5 mi) wide. These undulations are affected by the heat-driven convection activity within the overlying mantle, which may be the driving force of plate tectonics-motion of sections of Earth's brittle exterior. These undulations in the core-mantle boundary are also affected by the underlying eddies and currents within the outer core's iron-rich fluids, which are ultimately responsible for Earth's magnetic field. 47

48. The boundary between the core and the mantle does not remain constant. As the heat of the earth's interior is constantly but slowly dissipated, the molten core within Earth gradually solidifies and shrinks, causing the core mantle boundary to slowly move deeper and deeper within Earth's core. The Gutenberg discontinuity was named after Beno Gutenberg (1889-1960) a seismologist who made several important contributions to the study and understanding of the Earth's interior. It has also been referred to as the Oldham-Gutenberg discontinuity, or the Weichhert -Gutenberg discontinuity 48

49. The core–mantle boundary of the Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2891 km (1796 mi) depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth. This discontinuity is due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic  perovskite  mineralogy of the deep mantle named post- perovskite . 49