The Earth’s interior comprises different layers with varying physical properties and compositions. The layered structure of the Earth has been interpreted based on the studies of seismic waves, magnetic fields, meteorites, and other physical properties.
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Internal Structure Based on Chemical Composition
Base on the chemical composition, the internal structure of the earth is divided majorly into crust, mantle and core. The core is further sub-divided into outer core and inner core.
1. The Crust
The crust is the outermost layer of the Earth above the Mohorovicic discontinuity. It is defined based on composition. Thickness of the crust ranges from about 3 km at some oceanic ridges to about 70 km in collisional orogens. It consists of the sial or both the sial and the sima. The crust represents less than 0.1% of the Earth’s total volume.
Sial is a term used in petrology to refer to the upper layer of the Earth’s crust. This term is based on silica and alumina, the principal components of the rocks in this layer. Sial is known for being the primary source of granitic magma and distinguishes the upper continental crust.
Sima (an acronym for silica + magnesia) is a petrologic term for the lower layer of the Earth’s crust. It consists of rocks that are abundant in silica and magnesia. Sima is compositionally similar to the oceanic crust and the lower section of the continental crust beneath the sial.
Further, scientists divide the Crust into two types: continental crust (beneath land masses) and oceanic crust (beneath oceans).
Continental Crust
Continental crust is a type of the Earth’s crust which underlies the continents and the continental shelves. It ranges in thickness from about 25 km to as much as 70 km under mountain ranges, averaging ~40 km. The density of the continental crust averages ~2.7 g/cm3. It has the planet’s oldest rocks (billions of years old). The velocities of compressional seismic waves through its average ~6.5 km/s and are less than ~7.0 km/sec.
Oceanic Crust
The oceanic crust is a type of Earth’s crust that lies beneath the ocean basins. It is generally 5-10 km thick and denser than the continental crust, with a density of 2.9 g/cm3. Compressional seismic-wave velocities travels through it at 4-7.5 km/sec. Oceanic crusts are geologically young, with an age of 200 million years or less.
2. The Mantle
The mantle is a layer beneath the Earth’s crust and above the core. It extends from about 70-km to 2,900-km in depth. It contains about 80% of the Earth’s volume. The boundary between the crust and mantle shows a change in chemical composition. The mantle comprises more magnesium and iron than the minerals found in either the continental or oceanic crust. It can be divided into divided into the upper and lower mantle.
Upper Mantle
Upper mantle is part of the mantle which lies between the Moho to 660 km depth. It includes the lower part of the Iithosphere and the upper part of the asthenosphere. The density of upper mantle ranges from 3.4 g/cm3 to 4.4 g/cm3 with increasing depth. Likewise, P-wave velocity increases from about 8 to 11 km/sec with depth and S-wave velocity increases from about 4.5 to 6 km/sec with depth. Composition of upper mantle is presumed to be peridotitic in composition.
The region from the 410-km to the 660-km discontinuity within upper mantle is known as the transition zone. These two discontinuities are result of two important solid-state transformations: from olivine to wadsleyite at 410 km and from spinel to perovskite + magnesiowustite at 660 km.
Lower Mantle
Lower mantle is lower part of the mantle that lies below a depth of about 660 km to 2900 km at the core-mantle boundary. It is characterized by rather constant increases in velocity and density in response to increasing hydrostatic compression. The density of upper mantle ranges from ~4.4 g/cm3 to ~5.6 g/cm3 with increasing depth. The velocity of compressional seismic waves increases from ~10.7 km/s to ~13.7 km/s.
3. The Core
Core is the central zone or nucleus of the Earth’s interior, below the Gutenberg discontinuity at a depth of 2900 km. The core comprises about 16% of the Earth’s volume and 32% by mass. It is divided into an inner core and an outer core, with a transition zone between. Since only compressional waves propagate in the outer core, it is a fluid. The inner core, having a radius of approximately 1300 km, is solid, as shear waves have been observed to propagate through it. The magnetic field originates within the core.
3.1 Outer Core
The Outer Core is upper zone of the Earth’s core. It is extending from a depth of 2900 km to 5200 km. It is presumed to be liquid because it sharply reduces compressional-wave velocities and does not transmit shear waves. The density of outer core ranges from 9.9 to 12.2 g/cm3. The outer core is the source of the principal geomagnetic field.
3.2 Inner Core
The Inner Core is the central part of the Earth’s core. It is extending from a depth of about 5200 km to the center (6371 km) of the Earth. Its radius is about one third of the whole core. The inner core is considered solid, as evidenced by the observation of S waves that are propagated in it, and because compressional waves travel noticeably faster through it than through the outer core. The density of inner core ranges from 12.8 to 13.1 g/cm3.
Internal Structure Based on Physical Properties
The physical (or mechanical) properties of a material tell us how it responds to force, how weak or strong it is, and whether it is a liquid or a solid. Based on physical properties internal structure of the Earth is divided into – (1) Lithosphere, (2) Asthenosphere, (3) Mesosphere, (4) Outer Core and (5) Inner Core.
1. Lithosphere
Lithosphere (“rock sphere”) includes the crust and part of the upper mantle. It is solid, strong, and rigid outer layer of a planet. Earth’s lithosphere varies greatly in thickness, from as little as 10 km in some oceanic areas to as much as 300 km in some continental areas.
2. Asthenosphere
Asthenosphere (“weak sphere”) is the layer of the Earth below the lithosphere, which is weak and in which isostatic adjustments take place. It is extending from the base of the Iithosphere to the 660-km discontinuity, is by comparison a weak layer that readily deforms by creep. It is a part of the upper mantle where temperature and pressure are just right so that part of the material melts, or nearly melts. Under these conditions, rocks lose much of their strength and become soft and plastic and flow slowly. It is presumed that magmas may be generated in this zone. Here seismic waves are strongly attenuated.
The boundary between the asthenosphere and the overlying lithosphere is mechanically distinct but does not correspond to a fundamental change in chemical composition. The boundary is simply a major change in the rock’s mechanical properties.
3. Mesosphere
Mesosphere (“middle sphere”) is the region between the asthenosphere and the core. The rock below the asthenosphere is stronger and more rigid than in the asthenosphere. It is so because the high pressure at this depth offsets the effect of high temperature, forcing the rock to be stronger than the overlying asthenosphere.
Major Discontinuities within the Earth
1. Mohorovicic or Moho Discontinuity
Mohorovicic discontinuity is defined as the boundary or sharp seismic-velocity discontinuity that separates the Earth’s crust from the subjacent mantle. It marks the level in the Earth at which P-wave velocities change abruptly from 6.7-7.2 km/sec (in the lower crust) to 7.6-8.6 km/sec or average 8.1 km/sec (at the top of the upper mantle). The depth of Mohorovicic discontinuity ranges from about 5-10 km beneath the ocean floor to about 40 km below the continents. In some cases, it may reach up to 70 km under mountain ranges. Mohorovicic discontinuity probably represents a chemical change from basaltic or simatic materials above to peridotitic or dunitic materials below, rather than a phase change (basalt to eclogite). However, the discontinuity should be defined by seismic velocities alone. It is variously estimated to be between 0.2 and 3 km thick. It is named in honour of its discoverer, Andrija Mohorovicic (1857-1936).
2. Gutenberg Discontinuity
Gutenberg discontinuity is the seismic-velocity discontinuity at 2900 km depth. It marks the mantle-core boundary, at which the velocities of P waves are reduced and S waves disappear. It probably reflects the change from a solid to a liquid phase as well as a change in composition. Gutenberg discontinuity is named after Beno Gutenberg, a seismologist.
3. The low-velocity zone (LVZ)
The low-velocity zone (LVZ) in the upper mantle is characterized by low seismic-wave velocities, high seismic energy attenuation, and high electrical conductivity. The bottom of the LVZ, sometimes referred to as the Lehmann discontinuity, has been identified from the study of surface wave and S-wave data in some continental areas (Figure 4.1) (Gaherty and Jordan, 1995). This discontinuity, which occurs at depths of 180-220 km, appears to be thermally controlled and at least in part reflects a change from an anisotropic lithosphere to a more isotropic asthenosphere. Because of the dramatic drop in S-wave velocity and increase in attenuation of seismic energy, it would appear that partial melting must contribute to producing the LVZ.
4. The Transition Zone
The zone from the 4l0-km to the 660-km discontinuity is known as the transition zone.
4.1 The 410-km Discontinuity
The transition zone is the part of the upper mantle where two major seismic discontinuities – one at 410 km and the other at 660 km occur. High-pressure experimental results indicate that at about 14 Gpa equivalent to 410 km burial depth in the Earth, the Mg-rich olivine breaks to a high-pressure phase known as wadsleyite (beta phase). No chemical composition change occurs during this phase change or other phase changes. Mantle olivine (F090) completely transforms to wadsleyite over a < 300 Mpa pressure range at appropriate temperatures for the 410-km discontinuity (~ 1000 °C) (Ita andStixrude, 1992).
4.2 The 660-km Discontinuity
Similar to the 410-km discontinuity, it appears that a phase change in Mg2Si04 is responsible for the 660-km discontinuity (Christensen, 1995). High-pressure experimental studies suggest that spinel transforms to a mixture of perovskite and magnesiowustite at a pressure of about 23 Gpa, and can explain both the changes in seismic velocity and density at this boundary if the rock contains 50-60 per cent spinel. Magnesiowustite and Mg-perovskite are high-density and appear to comprise most of the lower mantle.
Did you know that the lower mantle is mostly made up of two extremely dense minerals, Mg perovskite and magnesiowustite? It’s fascinating how these minerals can withstand such intense pressure and heat deep within the Earth!
5. The D” layer
The D” layer is a region of the mantle exists within a few hundred kilometres of the core where seismic velocity gradients are anomalously low (Young and Lay, 1987; Loper and Lay, 1995). A small temperature gradient (1-3 °C/km) can conduct heat from the core into the D” layer. Seismic wave diffraction by the core causes poor resolution in this layer, leading to limited knowledge of its structure details. Estimates of the thickness of the D” layer suggest that it ranges from 100 to about 500 km.
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