Interior of the Earth

Direct Sources: Mining and deep drilling projects have provided large volume of information through the analysis of materials collected at different depths. Also when  the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis. •Indirect Sources  Analysis of properties of matter indirectly provides information about the interior. Through the  mining activity, it  is  known that  temperature and  pressure increase  with the  increasing distance from the surface towards the interior in deeper depths. •Also, the density of the material  also  increases  with depth. 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. Earthquake •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. •An earthquake is  a  natural event 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. Earthquake Waves •All natural earthquakes take place in the lithosphere. 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. 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. •There are two types of body waves. They are called P and S-waves. 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. •Other three waves vibrate perpendicular to the direction of propagation. 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. •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. Types of Earthquakes •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. (i)    The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane. (ii)  A special class of tectonic earthquake is sometimes recognised as volcanic earthquake, confined to areas of active volcanoes. (iii) In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors called as 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. 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(0-10). •The magnitude relates to the energy released during the quake. The intensity scale is named after Mercalli, an Italian seismologist. •The intensity scale(1-12) takes into account the visible damage caused by the event.

Structure of The Earth

The Crust:  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 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 Mantle:  The portion of the interior beyond the crust is called the mantle. The mantle  extends from Moho’s discontinuity  to  a  depth  of 2,900  km. The upper portion  of the mantle  is  called  asthenosphere. 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 crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid state. •The Core:  The core  mantle boundary  is located at the depth of 2,900 km. The outer  core  is in liquid state while the inner  core  is in solid state. The density of material at the mantle core  boundary is around  5 g/cm3 and at the centre  of the earth at 6,300 km, the density value is around 13 g/cm3. The core  is made  up of very heavy material mostly  constituted  by nickel and iron. It is  sometimes  referred to as  the nife layer. Volcanoes and Volcanic Landforms •A volcano is a place where  gases, ashes and/or  molten rock  material – lava – escape to the ground.  A volcano is called an active volcano if the materials  mentioned are being released  or have been released out in the recent past. The layer below the solid crust is mantle with higher  density than that of the crust. •The mantle contains a weaker zone called asthenosphere.  It is from this that the molten rock materials find their way  to the surface.  The  material in the upper  mantle portion is called magma. Once  it starts moving towards the crust or it reaches the surface, it is referred to as lava. •The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon. Volcanoes  are  classified  on  the  basis  of  nature  of  eruption  and  the  form  developed  at  the  surface.  Major  types  of volcanoes are as follows: 

Shield Volcanoes, barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth. The Hawaiian  volcanoes are the most  famous  examples.  These volcanoes are mostly  made  up of basalt, a  type of lava  that  is  very fluid  when erupted. For this  reason, these  volcanoes are not steep. They  become explosive if somehow water gets into the vent; otherwise, they are characterised by lowexplosivity. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone. •Composite Volcanoes: These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt.  These volcanoes often result in explosive eruptions. Along with lava, large quantities  of pyroclastic material  and ashes find their way to the ground. This material  accumulates in the vicinity of the vent openings leading to formation of layers, and this makes  the mounts appear as composite volcanoes.  •Caldera: These are the most explosive of the Earth’s volcanoes. They  are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity. •Flood Basalt  Provinces:  These volcanoes outpour highly fluid lava that flows for long distances. Some parts  of the world are covered by thousands  of sq.  km of thick basalt  lava flows. There can be a  series  of flows with some flows attaining thickness of more than 50 m. Individual flows may extend for hundreds of km. The Deccan  Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province. •Mid-Ocean Ridge Volcanoes:  These volcanoes occur in the oceanic areas. There is  a system  of midocean ridges more than 70,000 km long  that stretches through  all the ocean basins. The central portion of this ridge experiences frequent eruptions. 

Volcanic Landforms

Intrusive Forms: The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion. Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust). •The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms.

Batholiths:  A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. They appear on the surface only after the denudational processes remove the overlying materials. •They cover large areas, and at times, assume depth that may be several km. These are granitic bodies. Batholiths are the cooled portion of magma chambers. •Lacoliths:  These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths. It can be regarded as the localised source of lava that finds its way to the surface. The Karnataka plateau is spotted with domal hills of granite rocks. Most of these, now exfoliated, are examples of lacoliths or batholiths. •Lapolith,  Phacolith  and Sills:  As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. •In case it develops into a saucer shape, concave to the sky body, it is called lapolith. •A wavy mass  of intrusive rocks, at  times,  is  found at  the base  of synclines or at  the top of anticline  in folded igneous country. Such wavy materials  have a definite conduit to  source beneath  in the form of magma chambers (subsequently developed as batholiths). These are called the phacoliths. •The near horizontal  bodies of the intrusive  igneous  rocks are called  sill  or  sheet,  depending on  the  thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills. •Dykes: When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost  perpendicular to  the ground. It gets cooled in the same  position  to develop a  wall-like  structure. Such structures are called dykes. These are the most  commonly found intrusive forms in the western Maharashtra  area.  These  are considered  the feeders  for the  eruptions  that  led  to  the  development  of  the Deccan traps.