Earth’s layers

Earth’s Layers

There is more to earth than its surface. The interior of the earth is made up of several concentric layers, including their unique chemical and physical properties, and can impact life on Earth’s surface. The structure of the earth is divided into four layers, namely the crust, the mantle, the outer core, and the inner core. However, the earth’s interior remains mysterious to us. Recently the advancement in seismology has allowed us to learn more about the layers of Earth. In this topic, we will further discuss in detail the different layers of earth, their constitution, its physical and chemical properties.

Vibration of waves

Now let us first understand two types of waves, one is transverse, and another is longitudinal, transverse waves travel perpendicular to the direction of wave propagation, whereas in the case of longitudinal waves the waves travel to the direction parallel to the wave propagation. Longitudinal waves travel through solids, liquids, and gases, whereas transverse waves travel only through solids. This is because the primary wave is a longitudinal wave, being compressional, it also depends on the shear strength of a material when it comes to its motion. In the case of transverse waves or secondary waves, their shear strength completely depends on the rock material, and when these waves travel from solid to liquid medium, then this wave suddenly gets lost due to the absence of sudden waves in water (shear strength), it gets lost in liquid. The vibration will be horizontal when it comes to transverse waves. Whereas in the case of longitudinal, the particle vibrates perpendicular to the horizon, there they don’t get lost when they enter a liquid medium, but the intensity decreases with a decrease in velocity.

Shadow Zone of S & P waves

Imagine earth is a homogenous medium throughout, then it would not have any layers and if there was an earthquake then it would have reached every part of the earth. Although, due to the presence of transverse waves, shock waves (earthquakes) are not able to reach 40% of the earth. Shadow waves are established due to the presence of liquid at the centre, and we know that secondary waves can’t travel in liquid. 

Layers of Earth

Scientists came up with different concentric layers of earth. Now one by one we will discuss their physical properties.

Crust

• The crust is about 15 km to 20 km thick
• In the continental region, it is thick, whereas in the oceanic region its thickness completely decreases
• Below the crust, there is a sudden change in the medium, which is called the asthenosphere. The asthenosphere is a part of the upper mantle, it is important to study plate tectonics
• It is a semi-solid or ductile medium, in a form of molten ion (not rigid but cannot flow easily)
• The asthenosphere, being ductile, is suited beneath the lithosphere.

Lithosphere

• The lithosphere is the rigid outer part of the earth, with thickness varying between 10-200 km
• It includes 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 large-scale changes in the earth’s geological structure (folding, faulting).

Convectional cycles

• The core being very hot and the upper surfaces being very cold, there is a movement of molten material which moves to top most layers
• The hot materials travel up to the top and then move back to the core, this movement of molten matter forces the plates to move. Therefore, convectional cycles are the major driving force behind plate tectonics, and the Asthenosphere facilitates tectonic movement.

Mantle

• The mantle is in a semi-molten or semi-solid state, followed by the inner liquid core
• The lower mantle extends beyond the asthenosphere. It is in a solid state
• The density ranges in the lower mantle from 3.3 g/cm3 to 5.7 g/cm3
• Due to the temperature difference, there is a convective material circulation in the mantle
• High-pressure conditions should inhibit seismicity in the mantle.

Asthenosphere

• The top portion of the mantle
• It lies just beneath the lithosphere, extending up to 80-200 km
• Asthenosphere is highly viscous, mechanically weak, and ductile, and its density is higher than that of the crust.

The Outer Core

• The outer core lies between 2900 km and, 5100 km below the earth’s surface
• The outer core is composed of iron mixed with nickel and trace amounts of lighter elements
• The density ranges from 9.9 g/cm3 to 12.2 g/cm3
• The temperature ranges from 4400 °C in the outer regions to 6000 °C near the inner core
• Dynamo theory suggests that convection in the outer core along with the Coriolis effect gives rise to Earth’s magnetic field.

The Inner Core

• The inner core extends from the centre of the earth to 5100 km towards the earth’s surface
• The inner core is generally believed to be composed primarily of iron (80%) and some nickel. Since this layer can transmit shear waves, therefore, it is solid
• Earth’s inner core rotates slightly faster relative to the rotation of the earth’s surface
• The density ranges from 12.6 g/cm3 to 13 g/cm3
• The temperature is around 6000֯ C.

Conclusion

The total Earth Radius measures 6370 km. Earth’s diameter is about 12756 km at the equator and 12715 km at the poles. Crust measures around 0.5% volume of the earth. Mantle measures around 83% of the volume of earth. Earth’s core is about 16% of the volume of earth. Temperature, Pressure, and Density increase with increasing distance from the surface of the earth. Gravitation force is greater near the poles and lesser near the equator. A seismograph is an instrument used to measure the waves reaching the earth’s surface.