Structure of the Earth’s Systems

In this article, we’ll learn about the structure of the Earth’s Systems. Let’s begin with the introduction.

Introduction

Earth is 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 has supported life for many years. It is round 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 a 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.

Structure of the Earth’s Systems

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

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

Crust

• The crust is the thinnest outermost layer of the Earth. It is known as the lithosphere. The crust makes up 0.5-1.0% of the Earth’s volume and less than 1% of the Earth’s mass.
• The average density of the Earth’s crust is about 2.7 g/cm³ (the average density of the Earth is 5.51 g/cm³). The density of Earth’s crust increases with depth.
• The thickness of the Earth’s crust is in the range of 5-30 km in the oceanic crust and 50-70 km in the case of the continental crust.
• The oceanic crust is 5 km thick and consists of silica and alumina; the continental crust consists of rocks.
• The continental crust island where we live is 70 km thick in mountain systems and 70-100 km wide 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, are present below the sedimentary material.
• The lower layer of the Earth’s crust comprises basaltic and ultra-basic rocks.
• The continents comprise lighter silicates called sial [a combination of silica + aluminum], while the oceans contain heavier silicates called sima [a combination of silica + magnesium].
• The continental crust comprises lighter (felsic) sodium, potassium, aluminum, and silicate rocks, like granite.
• On the other hand, the oceanic crust comprises dense (mafic) iron, magnesium, and 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 have 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 and 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

The 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 silicate rocks that are rich in iron and magnesium. The mantle consists of constituent elements – 45% oxygen, 21% silicon, and 23% magnesium (OSM).
• In the mantle, temperatures vary from around 200°C at the upper boundary with the crust to about 4,000°C at the core-mantle boundary.
• Because of the temperature difference, 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 continuously move up and down due to internal heat from the core area, forming convective currents.
• The movement of tectonic plates shows the mantle’s convection at the surface.
• These currents cause rock plates to move and collide, resulting in
  earthquakes.
• The combination of the upper mantle and crust forms tectonic plates. These plates move very slowly. The point where plates touch each other is called a fault.

 

Asthenosphere

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

The density of the 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 primary source of magma that reaches the surface during volcanic eruptions.

Outer core

• This layer surrounds 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 the inner core. It is not present under sufficient pressure to remain in a solid state.
• The outer core comprises nickel, iron, and small trace elements. These two metals are liquid 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 extreme 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 its constant circulatory motion.
• 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 essential 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 more desirable 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 a 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 boiling 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., the inner and 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 is 6000֯ C, i.e.,1000֯C hotter than previously thought.
• At 6000°C, this inner core is as hot as the sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid.

Seismic Discontinuities

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