Interior of earth
INTERIOR OF THE EARTH
The configuration of the surface of the earth is largely a product of the processes operating in the interior of the earth. Exogenic as well as endogenic processes are constantly shaping the landscape. A proper understanding of the physiographic character of a region remains incomplete if the effects of endogenic processes are ignored. Human life is largely influenced by the physiography of the region. Therefore, it is necessary that one gets acquainted with the forces that influence landscape development. To understand why the earth shakes or how a tsunami wave is generated, it is necessary that we know certain details of the interior of the earth. The earth-forming materials have been distributed in the form of layers from the crust to the core.
SOURCES OF INFORMATION ABOUT THE INTERIOR
The earth’s radius is 6,370 km. No one can reach the centre of the earth and make observations or collect samples of the material. Most of our knowledge about the interior of the earth is largely based on estimates and inferences. Yet, a part of the information is obtained through direct observations and analysis of materials.
The most easily available solid earth material is surface rock or the rocks we get from mining areas. Gold mines in South Africa are as deep as 3 – 4 km. Going beyond this depth is not possible as it is very hot at this depth. Besides mining, scientists have taken up a number of drilling projects to penetrate deeper depths to explore the conditions in the crustal portions. Scientists world over are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”. The deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km. This and many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.
Volcanic eruption forms another source of obtaining direct information. As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis. However, it is difficult to ascertain the depth of the source of such magma.
INDIRECT SOURCES to know Interior of earth
Analysis of properties of matter indirectly provides information about the interior. We know through the mining activity that temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths. Moreover, it is also known that the density of the material also increases with depth. It is possible to find the rate of change of these characteristics. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.
Another source of information are the meteors that at times reach the earth. The material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet. Hence, this becomes yet another source of information about the interior of the earth.
The other indirect sources include gravitation, magnetic field, and seismic activity. The gravitation force (g) is not the same at different latitudes on the surface. It
is greater near the poles and less at the equator. This is because of the distance from
the centre at the equator being greater than that at the poles. The gravity values also differ according to the mass of material. The uneven distribution of mass of material within the earth influences this value. The reading of the gravity at different places is influenced by many other factors. These readings differ from the expected values. Such a difference is called gravity anomaly. Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth. Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
Seismic activity is one of the most important sources of information about the interior of the earth. The study of seismic waves provides a complete picture of the layered interior. Seismology is the science which studies various aspects of seismic waves generated during the occurrence of earthquakes. An earthquake in simple words is shaking of the earth. It is a natural event. It is caused due to release of energy, which generates waves that travel in all directions.
SEISMOLOGICAL EVIDENCES for Interior of earth
The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together. However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions. The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre. It is always inside the earth. The energy waves travelling in different directions reach the surface. The deepest focus has been measured 700km from the earth’s surface. The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves. It is a point directly above the focus.
All natural earthquakes take place in the lithosphere. It is sufficient to note here that the lithosphere refers to the portion of depth up to 200 km from the surface of the earth. An instrument called ‘seismograph’ records the waves reaching the surface. A curve of earthquake waves recorded on the seismograph is given in following figure. Earthquake waves are basically of two types — body waves and surface waves.
- Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves. There are two types of body waves. They are called P and S-waves.
- P-waves move faster and are the first to arrive at the surface. These are also called `primary waves’. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials.
- S-waves arrive at the surface with some time lag as their velocity is slower than P-waves. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials, but not through the liquid materials.
(2) Surface waves move along the surface. Also called Long Period or L waves, these waves cover long distance on the surface of the earth. They move slower than P and S waves. So surface waves are the last to report on seismograph. Despite slow speed these waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.
Surface waves are of two types: Rayleigh waves which travel in the direction of wave motion, while Love waves travel at right angles to the wave motion.
The characteristics of all these waves are quite important. They have helped scientists to understand the structure of the interior of the earth. The variations in the direction and velocity of waves are inferred with the help of their record on seismograph. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Reflection causes waves to rebound whereas refraction makes waves move in different directions. Their direction changes as they reflect or refract when coming across materials with different densities. A regular change of density inside the earth causes a curved path to be followed by the seismic waves. Thus, the seismic waves become concave towards the earth’s surface.
As stated earlier S waves cannot pass through liquid. After in depth study of seismic waves, it was observed that S waves disappear at the angular distance of 120degrees from the epicentre and P waves are weakened. It can be seen that S waves are totally absent in the core of the earth. Thus it can be said that core of the earth is in liquid state.
STRUCTURE OF THE EARTH
Based on the chemical composition of the earth’s interior, Edward Suess has identified three zones of different matter below the outer thin sedimentary layer.
- Sial: It is located just below the outer sedimentary cover. It is composed of granites. This layer is dominated by Silica and Aluminium (SIAL). The average density is 2.9 whereas thickness ranges between 50-300 km. Continents have been formed by sialic layer.
- Sima: It is located just below the sialic layer. It is mainly composed of basalt and is the source of magma and lava during volcanic eruptions. Silica and MAgnesium (SIMA) are main constituents. Average density ranges between 2.9 to 4.7 whereas thickness varies from 1000-2000km.
- c) Nife: It is located just below the sima layer. It is composed of Nickel (Ni) and Ferrium (Fe). Thus this layer is made up of heavy metals which are responsible for very high density (11) of this layer. The diameter of this zone is 6880 km. The presence of iron imparts magnetic property to earth’s interior.
Based on seismic studies, the earth’s interior has been divided into three layers — crust, mantle and core.
It is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continental crust is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region. It is made up of heavier rocks having density of 3 g/cm3. This type of rock found in the oceanic crust is basalt. The mean density of material in oceanic crust is 2.7 g/cm3.
The portion of the interior below the crust is called the mantle. There is sudden increase in the velocity of seismic waves from 6km/s to 7 km/s at the base the lower crust due to sudden increase in the density of the material therein. This surface of sudden increase in wave velocity which separates the crust above from the mantle below is called as Mohorovicic or Moho discontinuity, named after A. Mohorovicic of Yugoslavia. The mantle having mean density of 4.6gm/cm3 extends for a depth of 2900km inside the earth. The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The upper portion of the mantle is called asthenosphere. The word astheno means weak. It is considered to be extending upto 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust’s (3.4 g/cm3). The lower mantle extends beyond the asthenosphere. The mantle is in solid state. Mantle is believed to be made up of silicate minerals rich in iron and magnesium.
As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth. The core-mantle boundary is located at the depth of 2,900 km. At this boundary called as Weichert-Gutenberg discontinuity, there is sudden change of density from 5.5gm/cm3 of mantle to 10gm/cm3 indicated by sudden increase in velocity of P waves. The outer core is in liquid state while the inner core is in solid state. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.