5 Types Of Mechanical Layers Of The Earth

Structure of the Earth

Introduction

Earth is the place where we live, and this is the third planet from the sun. 70% of Earth is covered by water. Earth is a unique planet. It supports life since many years. It is round in shape and is not a solid piece of rock. It is made up of various layers. Each layer of earth has different physical and chemical characteristics. It is also called water planet or blue planet due to the presence of water on it. The only planet that has water on its surface is Earth. There are many features on the Earth’s surface. These features are called landforms.  

Earth’s structure 

The interior of the Earth comprises of many circular layers of which the crust, the mantle, the outer core, and the inner core are important because of their distinctive physical and chemical characteristics. 

The crust is a solid silicate, the mantle is in the form of viscous molten rock, the outer core is in the form of a viscous liquid, and the inner core is in the form of a dense solid. 

Mechanically, the Earth’s layers are divided into lithosphere, asthenosphere, mesospheric mantle (it is part of the Earth’s mantle present below the lithosphere and the asthenosphere), outer core, and inner core. 

Chemically, Earth is divided into the crust, upper mantle, lower mantle, outer core, and inner core. 

Fig. No. 2: Structure of Earth 

Crust

• Crust is the thinnest outermost layer of the Earth. It is known as lithosphere. Crust makes up 0.5-1.0% of the Earth’s volume and less than 1% of Earth’s mass. 
• The average density of the Earth’s crust is about 2.7 g/cm³ (average density of the Earth is 5.51 g/cm³). Density of Earth’s crust increases with depth. 
• The thickness of the Earth’s crust is in the range of 5-30 km in case of the oceanic crust and 50-70 km in case of the continental crust. 
• The oceanic crust is 5 km thick and  consists of silica and alumina, and the continental crust consists of rocks. 
• Continental crust is land where we live and it is 70 km thick in the areas of mountain systems, whereas it is in the range of 70-100 km thick in the Himalayan region. 
• The temperature of the Earth’s crust ranges from about 200°C to 400°C at the boundary with the underlying mantle. The temperature of the Earth’s crust increases with the depth. 
• In the upper part of the crust, the temperature increases by as much as 30°C for every kilometer. 
• The outer covering of the Earth’s crust is of sedimentary material. Crystalline, igneous, and metamorphic rocks which are acidic in nature are present below the sedimentary material. 
• The lower layer of the Earth’s crust comprises of basaltic and ultra-basic rocks. 
• The continents comprises of lighter silicates called sial [it is a combination of silica + aluminium] while the oceans comprises of heavier silicates called sima [it is a combination of silica + magnesium]. 
• The continental crust is comprises of lighter (felsic) sodium, potassium, aluminium, silicate rocks, like granite. 
• On the other hand, the oceanic crust comprises of dense (mafic) iron, magnesium, silicate igneous rocks, like basalt. 

The Mohorovicic (Moho) discontinuity 

Mohorovicic (Moho) discontinuity creates the boundary between the crust and the upper area of the mantle (asthenosphere) where there is a discontinuity in the seismic velocity. 

It occurs at an average depth of about 8 kilometers under ocean basins and 30 kilometers underneath continental surfaces. 

The basis of the Mohorovicic discontinuity (Moho) is thought to be a change in the chemical composition of rocks containing feldspar (above), to rocks that do not contain feldspars (below). 

Lithosphere 

• The lithosphere is the rigid outer part of the Earth. Its thickness varies between 10-200 km. 
• It comprises the crust and the upper part of the mantle. 
• The lithosphere is broken into tectonic plates (lithospheric plates), and the movement of these tectonic plates causes significant changes in the Earth’s geological structure, such as folding, faulting. 
• The source of heat that pushes plate tectonics is the elemental heat left over from the formation of the planets and the radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle. 

Mantle 

Mantle is made up of rock; it is hot and is present below the crust. 

• It expands up to a depth of 2900 km below the crust. The mantle is divided into the upper and lower mantle. 
• Mantle mainly comprises of silicate rocks that are rich in iron and magnesium. The mantle is made up of constituent elements – 45% oxygen, 21% silicon, and 23% magnesium (OSM). 
• In the mantle, temperatures vary around 200°C at the upper boundary with the crust to about 4,000°C at the core-mantle boundary. 
• Because of the difference in temperature, there is a circulation of convective material in the mantle (through solid, the high temperatures in the interior of the mantle cause the silicate material to be adequately ductile). 

• In the mantle, rocks move continuously up and down due to internal heat from the core area and form convective currents. 
• Convection of the mantle is shown at the surface by the movement of tectonic plates. 
• These currents cause rock plates to move and collide with each other that results in 
  earthquakes. 
• Tectonic plates are formed by the combination of the upper mantle and crust. These plates move very slowly. The point where plates touch each other is called a fault. 

Asthenosphere 

Asthenosphere (astheno means weak) is the upper portion of the mantle. It is present just below the lithosphere ranging up to 80-200 km. 

Density of asthenosphere is higher than that of the crust. It is ductile, and mechanically weak. These characteristics of the asthenosphere help in the movement of plate tectonic and isostatic modifications (the elevated part at one part of the crust area is balanced by a depressed part at another crust area).  

Asthenosphere is the main source of magma that reaches to the surface during volcanic eruptions. 

Outer core 

• This layer is surrounding the inner core. It is situated between 2900 km and 5100 km below the surface of the Earth. 
• It is present in the liquid state, though it has the same composition as that of the inner core. It is not present under sufficient pressure to remain in a solid state. 
• Outer core comprises of nickel, iron, and small amount of trace elements. These two metals are in liquid state due to tremendous heat in the outer core. 
• The inner core is in a solid state even though its temperature is higher than the outer core. Here, huge pressure, produced by the weight of the rocks spreading over the surface is very strong to bring together the atoms tightly and avoid the liquid state. 
• The density of the outer core varies from 9.9 g/cm3 to 12.2 g/cm³. 
• The temperature of the outer core varies from 4400°C in the outer core regions to 6000°C near the inner core region. 
• The outer core creates a magnetic field around the Earth due to constant circulatory motion of it. 
• The benefit of this magnetic field is that it protects the Earth from the sun’s damaging solar wind. 

• This layer (the outer core) is very important because without this layer, Earth will not have a magnetic field and without a magnetic field, Earth will not have life, ocean and atmosphere on it. 

Inner core 

This layer is the hottest layer on Earth with a temperature of 7000°C. It is hotter than the sun’s surface. The inner core spreads from the center of the Earth to 5100 km below the Earth’s surface. 

• It is below the outer core and comprises iron and nickel. 
• Though it is the innermost layer, it is present in the solid state because it is under high pressure from the weight of layers present above it. 
• Since the inner core layer can transmit shear waves (transverse seismic waves), it is in the solid state. (When P-waves hit the outer core, i.e., the inner core boundary, they give rise to S-waves) 
• The inner core of Earth rotates slightly faster as compared to the rotation of the surface. 
• The solid inner core is very hot to keep a permanent magnetic field. 
• The density of the inner core varies from 12.6 g/cm³ to 13 g/cm³. 
• The core, i.e.,inner core, and the outer core accounts for only around 16% of the Earth’s volume but 33% of Earth’s mass. 
• Scientists have shown that the temperature near Earth’s center to be 6000֯ C, i.e,1000֯C hotter than previously thought. 
• At 6000°C, this inner core is as hot as the surface of the Sun, but the effect of crushing pressure caused by gravity avoids it from becoming liquid. 

Seismic Discontinuities 

• Seismic discontinuities are the areas on Earth where seismic waves act very differently as compared to the surrounding regions due to a noticeable change in physical or chemical properties. 
• Mohorovicic Discontinuity (Moho): It divides the crust from the mantle. 
• Asthenosphere: It is a highly viscous, mechanically weak, and ductile part of the mantle. 
• Gutenberg Discontinuity: It lies between the mantle and the outer core. 

Summary

• Earth is made up of various layers – crust, mantle, outer core, and inner core.
• Earth is also called a water planet or blue planet due to the presence of water on it.
• Crust is the thinnest outermost layer of the earth.
• Mantle is mainly made up of silicate rocks that are rich in iron and magnesium. In the mantle, rocks move continuously up and down due to internal heat from the core area and form convective currents.
• Outer core is present in liquid state at the 5000ﹾC temperature.
• The inner core is the hottest layer on Earth with a temperature of 7000ﹾC.
• Seismic discontinuities are the areas on Earth where seismic waves act very differently as compared to the surrounding regions due to a noticeable change in physical or chemical properties.
• Asthenosphere (astheno means weak) is the upper portion of the mantle. It is present just below the lithosphere ranging up to 80-200 km.