The term lithosphere refers to the solid layer of the earth which consists of rocks of various types. It rests on the top of the mantle and contains sial and sima. All the landforms are formed on the solid surface of the earth.
Interior Structure of the Earth
Our knowledge about the interior of the earth is very limited because we do not have direct access to conditions prevailing within the earth. However, majority of the findings are of the opinion that the earth’s internal structure comprises of more or less concentric layers lying one above the other. The outermost layer is known as crust. Its average thickness is about 33 km. It is the solid layer of the earth which is composed of different types of rocks. Immediately below the c rust is the mantle which extends upto a depth of 2900 km below the surface. In between the crust and the mantle there is Mohorovicic discontinuity. Mantle is divided into two broad layers namely, the upper mantle and the lower mantle. Part of the upper mantle upto a depth of 400 km is known as asthenosphere and the part between 400 and 650 km is called transitional zone. Below the mantle i.e. beyond the depth of 2900 km, there is core of the earth. At the depth of 2900 km, there is another discontinuity which is known as Gutenberg discontinuity. The core is divide d into outer core (liquid) and inner core (solid).
These two layers of the core are separated from each other by a transition zone (liquid) which extends from depths of 4600 to 5151 km. Density, temperature, pressure and earthquake, waves are some of the sources to know about the earth’s interior.
According to Newton’s laws of gravity, earth’s density as a whole is 5.5. The sedimentary rocks on the surface of the earth have an average density of 2.7. Beneath the sedimentary rocks, there are igneous rocks with average density varying from 3.0 to 3.5. This means that the density inside the earth must be much higher. It is estimated that the ‘Core’ of the earth has a density of 11-12. It may rise to 13-14 at the centre of the earth.
It is estimated that temperature within the earth increases at the rate of 12°C per km at a depth of 100 km. It is 2°C per km in the next 300 km and 1°C per km below it. By this calculation, the temperature should be nearly 2900°C at the core of the earth. The actual temperature inside the earth is lower than the melting point upto a depth of 2900 km.
Like density and temperature, pressure also increases with the increase in depth inside the earth. This may be due to the overlying layers or the presence of heavier materials at the core of the earth. It is estimated that the pressure at the centre of the earth is about 3500 kilobars. It is due to high pressure inside the earth that volcanic eruptions take place and tectonic forces are at work.
The science of earthquakes is known as seismology and the evidences based on earthquake waves are known as seismological evidences. Earthquake waves have proved to be the most important source of information in the recent past. When an earthquake occurs at a particular place, three types of waves start travelling at different velocities in different directions. P. waves or primary waves are the fastest waves which travel with an average velocity of 8 km per second. They are longitudinal waves which can travel through all the mediums including solids, liquids and gases. S. waves are also known as secondary. These are transverse waves which can travel through solids and liquids and are lost in gases. They travel at an average velocity of 4 km per second. L. waves are also known as Long waves. They travel at the average velocity of 3 km per second. They affect only the surface of the earth and die out at smaller depth.
Behaviour of the Earthquake Waves
It shows the behaviour of the earthquake waves. There is a change in the course and velocity of the waves on crossing the boundaries of different zones inside the earth. If the ground through which the waves travel is solid, they behave in one way. If it is liquid, the waves behave in a different way. Their velocity in both the cases differ. The earthquake waves suffer reaction and refraction when they travel through different media.
All the three P, S and L-waves are recorded near the focus of the earthquake. They follow a curved path in the interior of the earth which proves that the density increases inside the earth. P and S-waves are recorded along the surface upto a distance of 11,000 km from the focus of the earthquake. Their velocity also increases with the increase in depth. But this process continues only upto a depth of 2,900 km. Beyond this depth, S-waves disappear and P-waves travel with reduced velocity. This process shows that the core of the earth behaves like solids while mantle upto a depth of 2,900 km behaves like liquids. On reaching the core, S-waves disappear and P-waves are refracted as a result of which there are no waves for a distance of 5,000km beyond the 11,000 km mark. This area is known as Shadow Zone. Normally, shadow zone is found between 103° and 143° distance from the focus of the earthquake.
The presence of shadow zone on the surface of the earth shows that earth’s core is composed of heavy materials like nickel and iron whose density is 11-12.
Three Main Layers of Earth
Famous Austrian Geological Suess has divided the earth into three main layers:
The uppermost layer is mainly composed of Silicon and Aluminium (Ai) and is known as Sial (Si + al). Continents are mostly composed of lighter silica having average density of 2.75 to 2.90. Its maximum depth is 100 km.
The second layer is mainly made up of Silica (Si) and Magnesium (Ma) and is known as Sima (Si + ma). This layer has average density of 2.90 to 4.75 and extends upto a depth of 2,900 km.
This extends from a depth of 2,900 km. to the centre of the earth. It is mainly composed to Nickel (Ni) and Ferrous (Fe) and is named Nife (Ni + fe). Its density is 11-12.