The physical properties of the Earth such as internal structure, density , Pressure and elasticity is provided by means of Seismic waves study. However the contribution of mineralogical and Petrological studies can not be undermined when it comes to revealing how the interior of the Earth may look like. I have tried to bring to you, through this post, how Our planet Earth is being subdivided. Keep reading!
There are discontinuities that separate Earth into layers based on rock properties or seismic velocity changes.
Moho is the term that is frequently used which means DEPTH
They are described as follows here down,
i/ Seismological Moho
It is the depth at which the P - wave velocity exceeds 7.6 kms-1
ii/ Petrological Moho
It is the depth where rock types change.
Based on above Moho the following internal discontinuities of the Earth are derived,
1: Gutenberg Discontinuity
This lies between the mantle and Core part of the Earth.
2: Mohorovic Discontinuity
It lies between the Crust and the Mantle.
3: Conrad Discontinuity
It is an interface that separates the continental crust into an Upper crustal layer and lower crustal Layer.
TRANSMISSION OF SEISMIC WAVES IN THE EARTH.
The Concept of Shadow Zones.
Shadow zone is defined as regions where no passes of seismic waves (P and S) in the Earth's interior. It is evident that within the core P - waves propagated with lower velocity than at the mantle.No S - waves arrive in the Shadow zone which indicates that the core must be fluid.
TYPES OF SHADOW ZONES
There are two (2) types of Shadow zones as explained below
i/ P - waves Shadow Zones
It develops between angular distance 103° and 142° due to abrupt inward bending of seismic rays, It results in no P - waves arrival at 103° and 142°.
ii/ S - waves Shadow Zones
It develops between angular distance 103°, where no S - waves arrive. It is evident that S - waves do not pass through the Outer core.
Shadow zones develop when the mantle velocity is higher than the Outer core Velocity.
The internal Structure of the Earth can be divided into three (3) Layers (regions) as described below
- Crust (Outermost)
- Mantle (Middle)
- Core (Innermost)
1: THE CRUST
It is the uppermost / Outermost part of the Earth that contributes about 1% by Volume of the Earth. It divided into two (2) subdivisions,
i/ Continental Crust
It has a thickness of 35 - 40 km under young mountain, with granitic rock composition.
However, its thickness increases with age of crustal rocks such as 50 - 70 km under older mountains.
ii/ Oceanic Crust
It is 5 - 10 km in thickness, with Basaltic rocks composition.
2: THE MANTLE
The middle part of the Earth lies between the Outer Core and Lower Crust. It contributes about 83% by Volume of the Earth.
It is subdivided into three (3) layers
i/ Upper Mantle
It is a Lid Layer or Low Velocity Layer. When Upper mantle meets the lower Crust it is termed as LITHOSPHERE. Lithosphere is brittle and rigid, characterized by an increase in P and S waves Velocities. It has a thickness of 80 - 120 km.
ASTHENOSPHERE: It is the region with reduced rigidity characterized with decrease of seismic velocities. It is a viscous layer that decouples the Lithosphere.
COMPOSITION OF UPPER MANTLE
It is composed of ultramafic rocks with peridotite and Olivine silicate minerals, such as [Mg,Fe]2SiO4
ii/ Transition Zone
It is the region characterized by phase change such as transformation of silicates minerals, Olivine type lattice to more closely packed Spinel type lattice due to increase of high pressure triggered by increases of depth. It lies between depths of 410 - 660 km.
iii/ Lower Mantle: It lies between depths of 660 - 2885 km.
3: THE CORE
It is the innermost region of the Earth that contributes about 16% by Volume of the Earth. It has a radius of about 3480 km. It has a density of 10 - 13g/cm3. It has divided into two (2) subdivision,
i/ Outer Core
This is Liquid in nature part of the Core. It is Liquid due to increased melting caused by high temperature associated with geothermal gradients. It is composed of Iron (Fe) and Nickel (Ni) with other lighter elements.
Because of its high temperature, the outer core exists in a low viscosity fluid-state that undergoes turbulent convection and rotates faster than the rest of the Earth. This causes eddy currents to form in the fluid core, which in turn creates a dynamo effect that is believed to influence Earth's magnetic field. The average magnetic field strength in Earth's outer core is estimated to be 25 Gauss (2.5 mT), which is 50 times the strength of the magnetic field measured on Earth's surface.
ii/ Inner Core
This is Solid in nature part of the Core. It is solid due to the rapid increase of compaction caused by high pressure.
Figure: Concentric Layers of the Earth with their associated depths and radius.
Every part of the internal region of the Earth has its own importance, however the outer core is believed by most geoscientists to contribute to geomagnetic fields, that if these fields were not present in our planet, even our life could be at risk, even if it does not exists.!.
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