Lithosphere (1)
- 1. TOPIC NO. 2 LITHOSPHERE • INTERIOR OF THE EARTH • WEGNER’S CONTINENTAL DRIFT THEORY • DAVIS CONCEPT OF CYCLE OF EROSION
- 2. INTERIOR OF THE EARTH What should you understand about the interior of the earth? • It is not possible to know about the earth’s interior by direct observations because of the huge size and the changing nature of its interior composition. • It is an almost impossible distance for the humans to reach till the center of the earth (The earth’s radius is 6,371 km). • Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of few kilometers. • The rapid increase in temperature below the earth’s surface is mainly responsible for setting a limit to direct observations inside the earth. • But still, through some direct and indirect sources, the scientists have a fair idea about how the earth’s interior look like.
- 3. Sources of Information about the interior of the earth Direct Sources: • Rocks from mining area • Volcanic eruptions Indirect Sources • By analyzing the rate of change of temperature and pressure from the surface towards the interior. • Meteors, as they belong to the same type of materials earth is made of. • Gravitation, which is greater near poles and less at the equator. • Gravity anomaly, which is the change in gravity value according to the mass of material, gives us information about the materials in the earth’s interior. • Magnetic sources. • Seismic Waves: the shadow zones of body waves (Primary and secondary waves) give us information about the state of materials in the interior.
- 5. • 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. • Direct Sources • 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.
- 6. • Indirect Sources • 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.
- 7. • 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.
- 8. • Earthquake • 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.
- 10. • Earthquake Waves • 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.
- 12. • 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.
- 13. • 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.
- 14. Emergence of Shadow Zone
- 15. • 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.
- 16. • Measuring 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.
- 17. • Structure of the earth’s interior Structure of earth’s interior is fundamentally divided into three layers – crust, mantle and core.
- 25. • SO HOW DO WE KNOW WHAT IS INSIDE PLANET EARTH? Earth’s Interior – about 6400 km from surface to core The deepest mine in the world – 3.8 km down (South Africa) *We have only drilled about 12 km into Earth Scientists have used two main ways to study Earth’s interior Direct evidence from rock samples • Indirect evidence from seismic waves Rock samples – direct evidence From drilling (up to 12 km) • Shows how Earth’s crust has changed
- 26. • Seismic waves – indirect evidence Geologists study earthquake waves – seismic waves • P-waves – fastest • go faster through more dense material • S-waves – Slower • Do not go through liquids • This is how we learned of the liquid outer core • So what did we learn? · Temperature – increases with depth · Pressure – increases with depth · The outer core is liquid – made a shadow zone where no S-waves were detected
- 28. • Crust • It is the outermost solid part of the earth, normally about 8-40 kms thick. • It is brittle in nature. • Nearly 1% of the earth’s volume and 0.5% of earth’s mass are made of the crust. • The thickness of the crust under the oceanic and continental areas are different. Oceanic crust is thinner (about 5kms) as compared to the continental crust (about 30kms). • Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL (Sometimes SIAL is used to refer Lithosphere, which is the region comprising the crust and uppermost solid mantle, also). • The mean density of the materials in the crust is 3g/cm3. • The discontinuity between the hydrosphere and crust is termed as the Conrad Discontinuity.
- 30. • Mantle • The portion of the interior beyond the crust is called as the mantle. • The discontinuity between the crust and mantle is called as the Mohorovich Discontinuity or Moho discontinuity. • The mantle is about 2900kms in thickness. • Nearly 84% of the earth’s volume and 67% of the earth’s mass is occupied by the mantle. • The major constituent elements of the mantle are Silicon and Magnesium and hence it is also termed as SIMA. • The density of the layer is higher than the crust and varies from 3.3 – 5.4g/cm3. • The uppermost solid part of the mantle and the entire crust constitute the Lithosphere. • The asthenosphere (in between 80-200km) is a highly viscous, mechanically weak and ductile, deforming region of the upper mantle which lies just below the lithosphere. • The asthenosphere is the main source of magma and it is the layer over which the lithospheric plates/ continental plates move (plate tectonics).
- 31. The discontinuity between the upper mantle and the lower mantle is known as Repetti Discontinuity. The portion of the mantle which is just below the lithosphere and asthenosphere, but above the core is called as Mesosphere.
- 32. • Core • It is the innermost layer surrounding the earth’s centre. • The core is separated from the mantle by Guttenberg’s Discontinuity. • It is composed mainly of iron (Fe) and nickel (Ni) and hence it is also called as NIFE. • The core constitutes nearly 15% of earth’s volume and 32.5% of earth’s mass. • The core is the densest layer of the earth with its density ranges between 9.5-14.5g/cm3. • The Core consists of two sub-layers: the inner core and the outer core. • The inner core is in solid state and the outer core is in the liquid state (or semi-liquid). • The discontinuity between the upper core and the lower core is called as Lehmann Discontinuity. • Barysphere is sometimes used to refer the core of the earth or sometimes the whole interior.
- 33. • Temperature, Pressure and Density of the Earth’s Interior • Temperature • A rise in temperature with increase in depth is observed in mines and deep wells. • These evidence along with molten lava erupted from the earth’s interior supports that the temperature increases towards the centre of the earth. • The different observations show that the rate of increase of temperature is not uniform from the surface towards the earth’s centre. It is faster at some places and slower at other places. • In the beginning, this rate of increase of temperature is at an average rate of 10C for every 32m increase in depth.
- 34. • While in the upper 100kms, the increase in temperature is at the rate of 120C per km and in the next 300kms, it is 200C per km. But going further deep, this rate reduces to mere 100C per km. • Thus, it is assumed that the rate of increase of temperature beneath the surface is decreasing towards the centre (do not confuse rate of increase of temperature with increase of temperature. Temperature is always increasing from the earth’s surface towards the centre). • The temperature at the centre is estimated to lie somewhere between 30000C and 50000C, may be that much higher due to the chemical reactions under high- pressure conditions. • Even in such a high temperature also, the materials at the centre of the earth are in solid state because of the heavy pressure of the overlying materials.
- 35. • Pressure • Just like the temperature, the pressure is also increasing from the surface towards the centre of the earth. • It is due to the huge weight of the overlying materials like rocks. • It is estimated that in the deeper portions, the pressure is tremendously high which will be nearly 3 to 4 million times more than the pressure of the atmosphere at sea level. • At high temperature, the materials beneath will melt towards the centre part of the earth but due to heavy pressure, these molten materials acquire the properties of a solid and are probably in a plastic state.
- 36. • Density • Due to increase in pressure and presence of heavier materials like Nickel and Iron towards the centre, the density of earth’s layers also gets on increasing towards the centre. • The average density of the layers gets on increasing from crust to core and it is nearly 14.5g/cm3 at the very centre.
- 37. • 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
- 38. • Important Facts • 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