The study of the propagation of seismic waves in the interior enabled scientists to theorize about its structure. Based on the abrupt changes in the paths of seismic waves, the earth’s structure has been demarcated into three zones.
They are the Crust, the outer and a very thin layer of the earth; the Mantle, an intermediary thick layer with a large volume of rocks below the crust and the Core, the innermost layer, which is spread all around the centre of the earth.
Crust
The earth’s crust is the outermost and thinnest layer, with an average depth of 5km below oceans and 40 km below the continents. Its depth reaches about 70 km below the mountains. Apart from a very thin sedimentary layer on the continental crust and adjoining ocean floors, it is primarily composed of igneous and metamorphic rocks.
Its lower limit is very clearly marked as both P and S earthquake waves increase due to abrupt changes in density. At the lower limit of the crust, the velocity of P waves is 7 km/second, which increases to 8 km/second immediately entering the mantle lying below. The same thing happens with S waves which are 3.7 km/second in the crust but increase to 4.5 km/second after their boundary.
The density of the crust at the surface is 2.7 g/cm3, and at the bottom limit, it is 2.9 g/cm3. The demarcating limit is known as Mohorovicic or Moho discontinuity from where the mantle starts.
Mantle
Just after the upper layer, the density of the mantle at the boundary increases to 3.0 g/cm3, from where the velocity of both P and S waves increase very significantly. As mentioned in the above paragraph, the increase in velocity of seismic waves is due to increased density existing there.
It extends from the base of the crust to about 2900 km below the surface (Figure 12). It occupies over 80% of the earth’s volume and 65% of the total mass. The mantle is composed of ultramafic rocks. It is igneous in nature and very rich in minerals. It is composed of magnesium and iron with very low silica content. The chemical composition of the mantle is almost similar throughout.
But there is a change in temperature and pressure inside. With increasing depth, the physical properties change, and therefore, the behaviour of the rocks also changes accordingly. Roughly up to a depth of 100 to 250 km from the surface, rocks are firm, solid and rigid. Below this depth, the state of the matter is partially molten and plastic in behaviour. This plastic and semi-solid belt extends about a depth of 700 km.
Lithosphere
It is the top upper solid and rigid layer of the earth. Its depths vary from 100 km to 250 km, below which the matter is in a semi-solid state. Therefore, the lithosphere consists of the mantle’s upper solid part and the entire crust. The complete solid upper part of the earth is divided into several plates of which the surface is made up, including continental and oceanic.
Asthenosphere
The term asthenosphere is derived from the Greek word ‘asthenos’, meaning ‘weak’. Therefore, the literal meaning is to denote the weak layer in terms of fluidity.
Geologists believe that the asthenosphere is semi-solid. It is composed of silicates, both iron and magnesium. The overall consistency is just like hot tar, the material used to make blacktop roads. Its depth is from 100 km to 700 km. Both waves, P and S, slow down while propagating through this layer.
This clearly suggests that this layer is not fully melted, as S waves are not stopped but pass with slower velocity. Both asthenospheres and the solid upper portion of the mantle together are known as the upper mantle.
Lower Mantle
The concentric layer is solid from 700 km to about 2900 km. In this zone, the velocity of both waves, P and S, increases abruptly until it reaches a depth of 2900 km. Beyond this depth, the velocity of P waves decreases very drastically from 13.7 km/second to 8.4 km/second, and S waves disappear completely.
It clearly suggests that at this depth, the state of matter is liquid or molten form as S waves are not entering. This distinct characteristic is defined by a boundary between two dissimilar sections of the earth. It is known as Gutenberg discontinuity. It is referred to as the core-mantle boundary (CMB).
Core
It is the innermost layer of the earth, starting from the Gutenberg discontinuity to the centre of the earth. It is completely spherical in shape. It has a volume of only 17% of the Earth, but it contains 34% of the mass. It is because of the very high density of the material existing there. The density of the core at the Gutenberg discontinuity is 5.5 g/cm3, whereas the density of the core near the boundary is about 10 g/cm3.
The core is made up of iron and nickel. Both of them have lower melting temperatures than the material lying above. As mentioned in the above paragraphs, the disappearance of S waves and abrupt reduction in the P wave’s propagation leads to conclude that the material is in a liquid state. The liquid state of matter is recorded till a depth of 5150 km.
After this depth, it is in a solid state. Here, the velocity of P waves increases. It is solid because the pressure is excessive and the melting temperature is increased but the temperature needed to melt is lower than the required at that pressure (Figure). Therefore, the upper liquid part is known as the outer core, and the inner solid is the inner core.
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