The interior of Earth can be observed through direct evidence such as rock samples from mining, deep ocean drilling project, volcanic eruptions and indirect evidence such as seismic waves, meteorite investigation, gravitation force, magnetic field etc.
- 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 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.
- 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. However, it may be noted that the material that becomes available for analysis from meteors, is not from the interior of 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 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 produced during an earthquake provides a complete picture of the layered interior.
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. 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. The energy waves travelling in different directions reach the 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. 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. 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. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials. This characteristic of the S-waves is quite important. It has helped scientists to understand the structure of the interior of the earth.
- The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.
The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities. Reflection causes waves to rebound whereas refraction makes waves move in different directions.
The variations in the direction of waves are inferred with the help of their record on seismograph. The surface waves are the last to report on seismograph. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.
Propagation of Earthquake Waves
Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass.
- P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material.
- The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.
- Surface waves are considered to be the most damaging waves.
Emergence of Shadow Zone
Earthquake waves get recorded in seismographs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone. Above figure show the shadow zones of P and S-waves. It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from epicentre, record the arrival of P-waves, but not that of S-waves. Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves. The entire zone beyond 105° does not receive S-waves. The shadow zone of S-wave is much larger than that of the P-waves. The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface. You can draw the shadow zone for any earthquake provided you know the location of the epicentre.
Types of Earthquakes
- The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
- A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
- In the areas of intense mining activity, sometimes the roofs of underground
mines collapse causing minor tremors. These are called collapse earthquakes.
- Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
- The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
The earthquake events are scaled either according to the magnitude or intensity of the shock.
- The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.
- The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.
Structure of the Earth
The crust is the outermost brittle solid part of Earth ranging from 5 – 70 kms. The crust is of two types:
- Continental Crust: Mean thickness is around 30 km, made of SIAL (Silica and Aluminium) and is thicker than oceanic crust. Its density is around at 2.7 g/cm3
- Oceanic Crust: Mean thickness is about 5 km made of SIMA (Silica and Magnesium). Oceanic crust is basaltic in origin and relatively of a younger age than the continental crust. The basaltic crust is denser at 3.0 g/cm3
- The mantle extends up to 2890 km.
- Asthenosphere: The upper portion of the mantle which extends up to around 400 km. It is the primary source of magma.
- The density of mantle is 3.4 g/cm3
- The lower mantle is in the solid state which extends up to the Core-Mantle boundary. This layer is called as the D″ (pronounced deedouble-prime) layer.
Note: The Crust and Upper part of mantle combined are called as Lithosphere.
The core extends to 2870 – 6370 km. It is divided into
- Liquid Outer Core
- Solid Inner Core: Made of NIFE – Nickel and Ferrous.
Note: Inner core rotates slightly faster than the rest of the planet. The density at the outer core is at 5.5 g/cm3, which increases to 13 g/cm3 in the inner core.
Dynamo theory: It suggests that convection in the out er core, combined with the Coriolis effect, gives rise to Earth’s magnetic field.
Boundaries in the Earth’s interior
- Conrad Discontinuity: Between Upper and Lower Continental Crust.
- Mohorovičić discontinuity, “Moho”: Crust-Mantle boundary
- Gutenberg discontinuity: Core-Mantle boundary
- Lehmann discontinuity: Boundary between Outer and Inner Core
- Earth’s radius: 6370 km
- Earth’s diameter: About 12756 km at equator & about 12715 km at the poles
- Crust: 0.5 % of the volume of the Earth
- Mantle: 83 % of the volume of the Earth
- Core: 16 % of the volume of the Earth
- Temperature, Pressure and Density increases with the increasing distance from the surface to the interior in deeper depths
- Gravitation force is higher near the poles and lesser near the equator
- Gravity anomaly is the difference in gravity value according to the mass of the material