Introduction
At the beginning of the creation of the solar system, the earth was a hot gas ball. It is located at the third position from the sun. The surface of the earth then gradually cooled as a result of billions of years of heat radiation and changed its gaseous state to a liquid state. This results in the accumulation of heavy metals in the center and light matter on the surface of the earth. This creates differences in the inner and outer structure of the earth in which light and heavy elements are arranged according to the variation of mass to form different layers of the earth. Therefore What are the three layers of earth forms after the settlement of the atmosphere is the main topic of discussion.
Pressure and heat gradually increase from the surface of the earth to the interior of the earth. Thus the temperature of the earth’s interior is very high hence the elements here are in a liquid or gaseous state. However scientists have noticed the dynamics of different seismic waves and assumed that elements from the Earth’s center to the Earth’s surface are arranged in layers. There are mainly three layers of earth (2) & (5).
What are the three layers of earth?
This planet in the solar system is slightly flattened. The radius of the earth is about 6400 km. Just like the onion peel, the earth also has different density and characteristics layers.
1. The core
2. Mantel
3. The crust.
The sources from which the concept of the earth’s layer is derived are mainly divided into two parts, such as direct sources and indirect sources. Meteors or cosmic objects, the earth’s gravitational force, the earth’s magnetism, seismic waves, etc. are indirect sources. Surface rock, mineral extraction, deep ocean excavation, and volcanic eruptions, etc. are direct sources.
Geologists have been able to make some analyses of the earth’s layers from mineral extraction and deep ocean excavations among direct sources. For example, the deepest mining area in the world extends to a depth of only 5 km. ocean drilling has been able to dig up to 12 km at most (4).
Earth’s layers description
Based on the movement of seismic waves, the interior of the earth is divided into three main layers.
1. The crust
The rocky, solid thin outer part of the earth’s surface is called the crust. It covers the interior of the earth like the shell of an egg. This layer is also called the lithosphere. The word “lithos”, means rocks as it is made up of different rocks so it is called lithosphere (6) & (1).
Properties of the Crust
1. The crust is usually brittle and occupies only 1% of the total mass of the earth.
2. Its average depth is about 60 km below the continent and about 5 km below the ocean.
3. This layer is composed mainly of solid rocks like granite and basalt. As the depth of the crust increases, so does the density of the rock.
4. The density of the crust is 2.9 gm/cm³.
5. It has two parts namely continental crust and oceanic crust.
6. All the continents of the world are formed by continental crust. This layer is about 40 km thick on average. But in the mountain regions, it is 60 to 70 km thick.
7. The oceanic crust is located at the bottom of the world’s oceans. It is only 6 to 8 km thick (3) & (7).
Classification
According to its composition and density, it is divided into two parts.
1. Continental crust
The continental crust is usually composed of acidic rocks like granite. It is about 30 km thick and has a density of 2.7 gm/cm³. Its main components are silicon (Si) and aluminum (Al). That is why this layer is called Sial.
2. Oceanic crust
It is made up of alkaline rocks like gabbro and basalt. The thickness of this layer is about 5 to 8 km. Silicon (Si) and magnesium Mg) are the main components of the oceanic crust. So this layer is also called Sima. The density of this layer is 2.9 gm/cm³ (2).
The Conrad discontinuity is a line that separates the continental crust or Sial and the oceanic crust or Sima.
Elements of the earth crust
According to Clark & Washington, the elements of the earth’s crust are
| Elements name | Amount of elements (%) |
| 1. Oxygen | 46.7 |
| 2. Silicon | 27.7 |
| 3. Aluminum | 8.1 |
| 4. Iron | 5.2 |
| 5. Calcium | 3.6 |
| 6. Sodium | 2.8 |
| 7. Potassium | 2.6 |
| 8. Magnesium | 2.1 |
Boundary lines or discontinuities
The boundary line between the earth’s crust and the mantle is called Moho or Mohorovicic discontinuity. In 1909 Croatian seismologist Andrija Mohorovicic first discovered this boundary line between the earth’s crust and the mantle (2).
Importance of Earth’s crust
The crust is the highest surface of the earth. All living things live on the surface of the crust. Different types of geological activities such as earthquakes, the origin of different types of rocks, creation of landforms such as mountains, plateaus, plains, etc. can be observed on the crust. This crust is a source of mineral resources and a source of groundwater, therefore, having immense importance (2).
2. The mantle
The lower part of the crust is the mantle having a thickness of 2900 km of the earth’s interior. Temperature and density at the bottom are much higher than at the top of the mantle. So there is a natural change between the substances of the upper and the lower part. It occupies 80% of the total area of the earth. Most of the mantle is composed mainly of solid rocks (6).
Properties
1. The mantle is about 2900 km thick with a temperature of about 870°C to 2200°C.
2. This zone is made up of various types of rocks, and sands, as well as light metals like iron, nickel, magnesium, chromium, etc.
3. Due to low heat and pressure, the rocks in the upper layer of the mantle remain in a solid state.
4. The lower part of the mantle has too much heat and pressure. That is why the rocks in this zone are in a liquid state. These heated liquid rocks are called magma.
5. Components of this layer are about five times heavier than water.
6. Volcanoes, earthquakes, etc. are usually seen at this layer.
7. The density of this layer is 3.4 to 5.6 gm/cm³.
8. Here the velocity of the P waves is 6.9 km to 8.1 km per second (3) & (5).
Classification
The mantle is divided into three parts.
1. Upper mantle
The upper mantle extends from the lower part of the rock to 410 km. This part of the mantle is called Crfesima because it contains chromium (Cr), iron (Fe), silica (Si), and magnesium (Mg).
The presence of temperature in the lower part of the upper mantle helps to create convection currents in this part. This convection flow plays an important role in the movement of the plate.
The velocity of the P wave in this zone is 8 km/second. The upper mantle is made up of about 55% olivine, 35% pyroxene, and 5 to 10% calcium oxide and aluminum oxide.
2. Transition zone
The zone from 410 to 660 km below the upper mantle is known as the transition zone. Here the rocks are present in crystalline form.
3. Lower mantle
The lower mantle extends from 660 km to 2900 km, it indicates the end of the mantle. This zone is called Nifesima because it contains nickel (Ni), iron (Fe), silica (Si), and magnesium (Mg).
The combined zone of the lower mantle and core is known as the “D” layer. The mantle plume or superplume on which the earth’s hotspots are located, is formed from this “D” layer of the lower mantle.
This part has the highest temperature and density as compared to the other parts of the mantle. The substances in the lower mantle are in a liquid state due to excessive pressure (2) & (3).
The boundary line that separates the upper and lower mantle is the Repetti discontinuity.
Components of mantle
The mantle is made up of various elements.
Oxygen – 44.5%
Silicon – 21.5%
Magnesium – 22.5%.
Besides, there are also iron, aluminum, calcium, sodium, and potassium present in this layer (3).
Boundary lines or discontinuities
There is a boundary line between the mantle and the core. This line is called Gutenberg discontinuity. In 1912 seismologist Gutenberg first discovered this discontinuity. The P wave of the earthquake can enter beyond this separation line but the S wave cannot enter (3).
3. The core
The lowest layer of the earth is called the core. It is the next layer of the mantle. The area from 2900 km depth to 6371 km in the center of the earth is called the core. This layer is formed by the gravitational force of the earth concentrating heavy elements like iron, nickel, etc. around the center of the earth.
Properties
1. This layer is made up of heavy and dense composite elements like iron, nickel, mercury, etc. The total radius of the core is about 3485 km.
2. The density of this layer is 12.6 to 13.1 gm/cm³ with a temperature of the core is about 4000°C to 6000°C.
3. Here the velocity of the P waves is 11.2 km/second.
4. This zone is called Nife because of its high content of nickel (Ni) and iron (Fe).
5. It is 10-12 times thicker than water and twice as thick as other parts of the earth.
6. The core has a liquid outer layer about 2270 km thick and has a solid interior which is about 1216 km thick.
7. At extreme temperature and pressure, all substances at this layer are in a dense and elastic state (4) & (1).
Classification
The earth’s core is divided into two parts.
1. Outer core
The outer core is liquid. This zone extends from 2900 to 5150 km. The density of the outer core is 12.6 gm/cm³. This part of the core is mainly composed of iron and nickel. The S waves cannot enter this layer.
Seismic waves flow slowly through this part. The magnetic force is created in this outer core. It is extremely heated that the metals inside are all in a liquid state. The temperature of the outer core is 4500°C to 5000°C. The thickness of the outer core is about 2300 km (3).
2. Inner core
The lower part of the outer core is known as the inner core. This layer is formed by iron. The inner core extends from 5150 km to 6371 km. According to scientists, the inner core is in a solid state. This is because the melting point of elements in this layer is much higher than the temperature at this layer due to the presence of strong pressure. So this part is in a solid state despite the extreme heat.
The density of the inner core is between 9.5 to 12.5 gm/cm³. Here the temperature is very high. The temperature of the inner core is 5200°C. This layer is about 1220 km thick (2).
Boundary line or discontinuities
The boundary line that separates the outer and inner core is the Lehmann discontinuity. The velocity of the P wave and S wave suddenly increase in this boundary line (5) & (7).
Written By: Manisha Bharati
Reference
1. Geotectonics and Geomorphology (First Edition-2017)” by Dr. Sunil Garg. Publisher: Random Publications, Gideon Paxton edition (1 January 2017).
