Journey into Earth's Interior: Seismology Insights & Layers Revealed

INTERIOR OF THE EARTH By Debjit Ghosh

The radius of the Earth is almost 6370 km and average density is 5.5 gm/cc. So no one can reach to the centre of the Earth and make observations or collect samples of the materials. So most of our knowledge about the interior of earth is largely based on estimations. Seismology has helped to get knowledge about the Earth sinterior.

Seismology is the science which studies various aspects of seismic waves generated during Earthquakes. Seismic waves are recorded with the help of Seismograph. Seismic waves are three types- 1) Primary Wave 2) Secondary Wave 3) Surface Wave

PRIMARY WAVE Primary waves are also called P waves and compressional or longitudinal waves. The rocks moves in the direction of the waves. It moves in high velocity( average 6 km/h). It passes through solid, liquid and gaseous materials of the interior of earth.

SECONDARY WAVE Secondary waves are also known as S waves or Shear waves or Transverse wave. The rocks move in vertical direction of the direction of the wave. Its velocity is lower than p waves, so these are recorded by the seismograph after p waves. It passes only through the solid materials of the interior of the Earth.

SURFACE WAVE The surface waves also called L waves. The L waves move very slowly and is recorded by the seismograph lastly. The destroying power is maximum. It only passes through the crust.

Thus by the study the different aspects of P and S waves we get knowledge about the interior of Earth. From the different observations Earth s interior is divided into three layers. These are- 1) Crust 2) Mantle 3) Core.

EARTH CRUST OceanicCrust It is the part of oceans The average depth of oceaniccrustis 5 km-7 km. The average density is 3.0 gm/cc. It mainlymade by basalts. It is only 18 crore yearsold. This mainly composedwith Silica(Si) and magnesium(Ma) so it generally known as SIMA. Continental Crust It is the part of continents The average depth of it 30 km-45 km and maximum depth is 100 km. Its average density is 2.7 gm/cc. It is mainly made by granites. It is almost380 crore years old. This mainly composed with silica(Si) and Aluminium(Al) so it generally knownas SIAL.

The discontinuity between SIAL and SIMA is known as Conrad Discontinuity . The discontinuity between crust and mantle is known as Mohorovicic discontinuity or Moho discontinuity.

MANTLE This layer is extended from moho discontinuity to Gutenberg discontunity. This area covers 82% of Earth s interior. This is divided into two parts outer mantle and inner mantle. Repti discontinuity found between outer and inner mantle. In the upper mantle between 50 km to 250 km moves very slowly because the density of materials are very low and viscous in nature. This is known as Asthenosphere.

The outer mantle is composed with Chromium(Cr), iron(Fe), Silica(Si) and magnesium(Mg) so it generally known as CRFESIMA. The inner mantle is composed with nickel(Ni), iron(Fe), silica(Si) and magnesium(Mg) so it known as NIFESIMA. The discontinuity between mantle and core is known as Gutenberg Discontinuity.

CORE The core is extended from Gutenberg discontinuity to centre of the Earth. This covers 17% of Earth s interior. The core is divided in to inner and outer core on the basis of Leman discontinuity . At the Gutenberg discontinuity the velocity of P waves decrease rapidly and S waves are disappeared, by this observations scientists conclude that inner core is liquid in nature.

After Leman Discontinuity the S waves are again appeared so from this observations scientists conclude that inner core is solid in nature and it is because of very high temperature, pressure and density. The average density of inner core is 12 gm/cc to 13.6 gm/cc. The core is mainly composed with nickel(Ni) iron(Fe) , so it called as NIFE.

QUESTIONS 1. What is the average density of Earth?(1) 2. How Seismology helps to identify different layers of Earth s interior?(5) 3. Whatare the difference between SIAL and SIMA?(3) 4. What is Conrad Discontinuity?(1) 5. What is GutenbergDiscontinuity?(1) 6. What is Moho Discontinuity?(1) 7. Why inner core is solid in nature?(3) 8. What is Asthenosphere?(1/3) 9. What do you mean by CRFESIMA and NIFESIMA ?(3) 10.What do you mean by NIFE ?(1) 11.Write the Characteristicsof Mantle.(3) 12. Briefly discuss about the layers of Earth s interior.(5)

Rocks & Minerals Minerals: A homogenous, naturally occurring substances with a definite internal structure is called minerals. Rocks: Rocks are combination of different minerals.

Types of Rocks Sedimentary Rocks Metamorphic Rocks Igneous Rocks Extrusive Igneous Rocks Intrusive Igneous Rocks

IGNEOUS ROCKS Igneous rocks are defined as types of rocks that are formed when molten rock (rock liquefied by intense heat and pressure) cools to a solid state. Characteristics of the rocks: These rocks are formed by the molten lava. These are created first on the earth surface. So, these rocks are called Primary Rocks . They can be either glassy or coarse. The rocks have not any layers. The rocks does not contain any fossil deposits. They usually do not react with acids. They are hard in nature. The Igneous rocks are two types. 1. Intrusive Igneous Rocks 2. Extrusive Igneous Rocks

Intrusive Igneous Rock: Intrusive igneous rocks are rocks that crystallize below the earth s surface resulting in large crystals as the cooling takes place slowly. Diorite, granite, and pegmatite are examples of intrusive igneous rocks. Fig: Pegmatite Fig: Granite

Extrusive Igneous Rock: Extrusive igneous rocks are rocks that erupt onto the surface resulting in small crystals as the cooling takes place quickly. The cooling rate for a few rocks is so quick that they form an amorphous glass. Basalt, tuff, pumice are examples of extrusive igneous rock. Fig: Basalt Fig: Pumice

Sedimentary Rocks A sedimentary rock is a type of rock that is formed from sediment or pieces of pre-existing rocks or organisms. Characteristics of the rocks: 1. These rocks are formed by eroded rock materials. 2. They are soft in nature. 3. The rock contains different layers. 4. The rock contains fossil deposits in between two layers. 5. Sedimentary rocks are also known as secondary rocks as they are formed by the sediments of other rocks denuded and deposited by the agents of gradation. 6. These rocks are generally not crystalline in nature. 7. Most of the sedimentary rocks are permeable and porous.

Examples of Sedimentary Rocks Fig: Limestone Fig: Sandstone Fig: Gypsum Fig: Dolomite

Metamorphic Rocks Metamorphic refers to change to form or transform. In nature, various things go through metamorphism in order to transform. In Geology, metamorphic rock is the name given to those rocks that undergo a change or through the process of metamorphism. Metamorphism includes the change in mineralogy as well as the change in the fabric of the original or pre-existing rock. The metamorphic rocks may form by igneous, sedimentary or other metamorphic rocks undergoing the process of metamorphism or physical changes due to factors like heat, pressure or chemical reactions.

Characteristics of Metamorphic Rocks These rocks are formed by igneous, sedimentary or metamorphic rocks. These are hard in nature. The arrangements of minerals are different from previous rocks. Metamorphic rocks are highly resistant to erosion They have crystalline & have banded or foliated texture Examples of Metamorphic Rocks Fig: Slate Fig: Gneiss Fig: Marble

Table: Metamorphic Rocks Name of Rock Metamorphic Rock Granite Gneiss Limestone Marble Sandstone Quartzite Shale Slate

Rock Cycle A rock cycle is a fundamental concept that shows the transitions among the three basic rock types (metamorphic, igneous and sedimentary). In simple terms, it is a chain of processes that produce and transform the rock types in Earth s crust.

Uses of Rocks Igneous Rocks: Granite, which is used in the construction of buildings and statues. Pumice manufactures toothpaste due to its ability to clean any tough stains thoroughly. Basalt is used as gravels for construction. Sedimentary Rocks: Limestone is used to make cement. Limestone and sandstone are used for building stones Quartz is a type of sedimentary rock which is used to make glass. Rock gypsum is used to make plaster. Metamorphic Rocks: Slate and marble are used as flooring materials. Slate is used in old fashion chalkboard. Marble is used for making different sculptures.