INTERNAL_structure_OF_THE_EARTH_IN_REFERENCE_TO_SEISMOLOGY.pdf
- 1. INTERNAL STRUCTURE OF THE EARTH IN REFERENCE TO SEISMOLOGY
- 2. SEISMILOGY • Disturbance within Earth's interior, which is in a constant state of movement, result in the release of energy in packets known as seismic waves. An area of geophysics known as seismology is the study of these waves and their effects, which often can be devastating when experienced in the form of earthquakes.The latter do not only take lives and destroy buildings, but they also produce secondary effects, most often in the form of a tsunami, or tidal wave. Using seismographs and seismometers, seismologists study earthquakes and other seismic phenomena, including volcanoes and even explosions resulting from nuclear testing. They measure earthquakes according to their magnitude or energy as well as their intensity or human impact. Seismology also is used to study Earth's interior, about which it has revealed a great deal.
- 3. SEISMIC WAVES • When an earthquake occurs the seismic waves (P and S waves) spread out in all directions through the Earth's interior. Seismic stations located at increasing distances from the earthquake epicenter will record seismic waves that have traveled through increasing depths in the Earth. • Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass. Seismic waves travel more quickly through denser materials and therefore generally travel more quickly with depth. Anomalously hot areas slow down seismic waves. Seismic waves move more slowly through a liquid than a solid. Molten areas within the Earth slow down P waves and stop S waves because their shearing motion cannot be transmitted through a liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves. • When seismic waves pass between geologic layers with contrasting seismic velocities (when any wave passes through media with distinctly differing velocities) reflections, refraction (bending), and the production of new wave phases (e.g., an S wave produced from a P wave) often result. Sudden jumps in seismic velocities across a boundary are known as seismic discontinuities.
- 6. SHADOW ZONE • The shadow zone is the area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves.The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.
- 7. LOW VELOCITY ZONE • Low-velocity zone (LVZ) The zone within the upper mantle beneath the oceans within which seismic P- waves are slowed and S-waves are slowed and partially absorbed.The top of the zone is some 40–60 km deep near the oceanic spreading ridges, and this depth increases to 120–160 km beneath the older oceanic crust.The bottom of the zone is poorly defined, but in the region of 250–300 km in depth. Beneath the continents, a restricted low-velocity zone occurs beneath crust areas subjected to orogenesis during the last 600 million years or so, but is not found beneath cratonic areas. It is attributed to the presence of a 0.1% fluid phase and commonly ascribed to the partial melting of mantle rocks at these depths. It is often considered coincident with the asthenosphere, but probably this is valid only for oceanic areas.
- 8. EVIDENCE FOR INTERIOR EARTH AND COMPOSITION • The structure of Earth's deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the depths of layers of molten and semi-molten material within Earth. Because different types of earthquake waves behave differently when they encounter material in different states (for example, molten, semi-molten, solid), seismic stations established around Earth detect and record the strengths of the different types of waves and the directions from which they came. Geologists use these records to establish the structure of Earth's interior.
- 9. 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.
- 10. 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).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.
- 12. 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.
- 14. ACCORDING TO SUESS NAME THICKNESS MATERIALS DENSITY REMARKS SIAL 50 – 300 KM SILICATE, ALUMINIUM, POTASSIUM AND SODIUM 2.9 CONTINENTS ARE FORMED BY THIS LAYER SIMA 1000 – 2000 KM SILICA, MAGNESIUM,IRON MAINLY BASIC MATERIALS 2.9 – 4.7 OCEANIC CRUST ARE FORMED LAND SOURCE OF LAVA AND MAGMA NIFE 6880 KM ( DIAMETER ) NICKEL, FERRIUM, HEAVY METAL WITH VERY HIGH DENSITY 11 THIS LAYER INDICATES THE MAGNETIC PROPERTY AND RIGIDITY OF EARTH
- 15. ACCORDING TO DALY NAME THICKNESS MATERIALS DENSITY OUTER ZONE 1600 KM MAINLY SILICATES MATERIAL 3.0 INTERMEDIATE ZONE 1280 KM MIXTURE OF IRON AND SILICATES 4.5 - 9 CENTRAL ZONE 7040 KM IRON AND IN SOLID STATE 11.6
- 16. ACCORDING TO VAN DER GRACHT LAYER THICKNESS DENSITY OUTER SIALIC CRUST 60 KM UNDER CONTINENTS, 20 KM UNDER ATLANTIC OCEAN AND ABSENT UNDER PACIFIC OCEAN 2.75 – 2.9 INNER SILICATE MANTLE 60 – 1140 KM 3.1 – 4 .75 ZONE OF MIXED METALS AND SILICATES 1140 – 2900 KM 4.75 – 5.0 METALIC NUCLEUS 2900 – 6371 KM 11.0
- 17. ACCORDING TO HAROLD JEFFREYS 1. Outer layer of sedimentary rocks 2. Second layer of granites 3. Third layer of diorite 4. Fourth layer of dunite
- 18. GENERALISED CLASSIFICATION • Lithosphere: 100Km thickness, composed of granites, silica and aluminium are dominant elements, average density is 3.5. • Pyrosphere: 2780 Km thickness, dominant rock is basalt and average density is 5.6. • Barysphere: 2800 to nucleus of the core, mainly composed of iron and nickel and average density 8-11.