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Structure of the Earth

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Introduction

Earth is the only living planet in our solar system. It is the place where we live. We perform various activities on earth like construct buildings, houses, roads, canals, drainage etc on the earth's surface. The earth's surface is composed of various things like soil, water, rocks, etc. Most parts of the earth are covered by soil but the inner surface of the earth contains the layers of rocks. The rocks are the hard and solid parts that are found on the surface of the earth. We use rocks for various purposes. Rocks are used for constructing buildings, houses, roads, canals, drainage etc. It is also used for making tools and getting metals and minerals.

Source: ww2.kqed.org Fig: Rocks
Source: ww2.kqed.org
Fig: Rocks

Rock

A rock is a naturally occurring solid material that is found on the surface of the earth. Major part the earth's crust is made of rocks. Rocks are found everywhere around us. They are found at the bottom of the sea. They are found on the bank of river, streams, etc. The mountains and hills are made up of rocks. The soil too is formed by the weathering of rocks. The rocks can be defined as the natural solid substances having one or more minerals or mineraloid. The outer layer of the earth lithosphere is made of rocks.

Physical Properties of Rocks

Structure: The rocks have different shapes and size. Some rocks are very small whereas some rocks are very big. The rocks are made up of particles of minerals. The rocks that are made of big particles are rough and the rocks that are made of small particles are smooth. The rocks contain different types of minerals which are held together by chemical bonds.

Hardness: Most of the rocks are hard in nature. These types of rocks are used in constructions of buildings, houses, dams etc. Soft rocks are also found in different parts of the earth. These types of rocks are not used in construction activities.

Colour: Different rocks have different colours. The colour of the rock is determined by the minerals present in the rock. Rocks are found in different colours like white, black, green, red, yellow, grey etc.

Types of Rocks

There are three main classes of rocks. They are classified on the basis of their origin and the method of formation. They are igneous rocks, sedimentary rocks and metamorphic rocks.

Igneous Rocks
Source: cmapspublic.ihmc.us Fig: Igneous Rocks
Source: cmapspublic.ihmc.us
Fig: Igneous Rocks

Igneous rocks are those rocks which are formed when magma cools and hardens. Sometimes the magma cools inside the earth and solidify in the crust in the colder region but sometimes it comes out in the earth surface in the form of a volcano due to high pressure and temperature. The lava cools down to form rocks. When the magma cools slowly, crystals grow from the molten liquid, and a coarse-grained rock forms. When the magma cools fast, small crystals and fine-grained rocks are formed. The igneous rocks may be plutonic or volcanic.

Volcanic rocks: Volcanic rocks are those igneous rocks that are formed on the surface of the earth by cooling down of magma or lava. Some of the common examples of volcanic rocks are basalt, pumice, obsidian etc. The magma comes out on the surface of the earth in the form of a volcano when it gets weak spot or cracks. The magma is then called lava. The lava cools down slowly and forms rocks. These rocks are known as volcanic rocks. They have small crystals as they cool down rapidly on the surface of the earth. Volcanic rocks are also known as extrusive rocks.

Plutonic rocks: Plutonic rocks are those igneous rocks which are formed before the magma reaches the earth surface. The plutonic rocks are formed when the magma cools down inside the earth's surface. Some of the common examples of plutonic rocks are gabbro, granite, peridiotite, dunite, etc. These types of rocks are formed over a long period of time. They are very hard in nature. These rocks contain big crystals as they cool down very slowly inside the surface of the earth.

Some Important Igneous Rocks

Granite: Granite is a best known igneous rock. It is mainly composed of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. It is very hard and strong. It is the most common igneous rock. It is a coarse- grained plutonic rock. Granite is used in the construction of houses, pavement, road, etc.

Pumice: Pumice is a light coloured porous igneous rock. It is formed during an explosive volcano eruption. It is formed by the rapid cooling of lava. So, it may or may not contain crystals. Pumice is an amorphous volcanic glass known as a "mineraloid".It is used for making light- weight concrete. It is also used in polishes, pencil erasers, cosmetic items, toothpaste etc.

Obsidian rock: It is a dark coloured igneous rock that is formed during the volcano eruption. It does not contain crystals as it is formed by rapid cooling of lava. Obsidian rocks are usually black in colour. It is used in making weapons and cutting tools, jewellery, etc. It also used in modern surgery.

Source: slideplayer.com Fig: Sedimentary Rock
Source: slideplayer.com
Fig: Sedimentary Rock
Sedimentary rock

Sedimentary rocksare formed by the accumulation of sediments. Whenparticles of sand, shells, pebbles, organic and inorganic substance and other fragments of material are deposited in one place by different agents like air, water, glaciers etc, due to high pressure and temperature, the deposited materials changes into rocks after very long period of time. These types of rocks are known as sedimentary rocks. In simple words, sedimentary rocks are the rocks that are formed from the deposited and sedimented substances under high pressure and temperature after a long period of time. They are very soft in nature. Sedimentary rocks are used to study about living things and their evolution as they contain fossils. Some of the common examples of sedimentary rocks are sandstone, shale, coal, limestone, dolostone etc.

Some Important Sedimentary Rocks

Conglomerate: It is a type of sedimentary rock that contains large rounded granules. It is composed of small rock particles or pebbles, granules, sand and quartz or silica. The conglomerate rock having silica is very strong. It is used in making concrete.

Sandstone: It is one of the most common type of sedimentary rock that is found in sedimentary basins throughout the world. It is composed of sand-size grains of mineral, rock, or organic material. They are also composed of quartz or feldspar or both. Sandstone may be of a different colour like grey, brown, yellow, red, white etc. They may be soft or hard. It is used as a constructional material.

Shale: It is a fine- grained sedimentary rock. It is black grey in colour. It is composed of silt and clay-size mineral particles. The granule that is smaller than sand is called silt and the collection of the granules that are smaller than sand is called silt and the collection of the granules that are smaller than silt is called clay. It is used to produce clay and cement. It is also used to make various useful objects.

Limestone: Limestone is a sedimentary rock composed of calcium carbonate that is found in the form of mineral calcite. It is the organic sedimentary rock that is made of skeletal fragments of marine organisms. It is also formed by the precipitation of calcium carbonate from lake or ocean water. It may be of a different colour like red, grey, black, green etc. It is mostly used in architecture.

Metamorphic rock

They are formed on the surface of the earth from the metamorphosis (change) of different types of igneous rocks and sedimentary rocks that occur due to high pressure, high temperature and chemical processes. Metamorphic rocks can be defined as the rocks which are formed from other types of rocks due to high temperature and pressure over a long period of time. Some of the examples of metamorphic rocks are given below in table,

S.No. Metamorphic rocks Formed from
1. Phyllite Shale or slate
2. Gneiss Granite or diorite
3. Quartzite Sandstone
4. Marble Limestone or dolestone
5. Schist Shale or slate
6. Slate Shale

Uses of Rocks

Some of the uses of rocks are described below,

  • Rocks are used in the construction of houses, buildings, monuments, bridges, roads, palaces, temples etc.
  • Various types of rocks are used for the decorative purposes. Rocks like marble, slate, limestone etc are used in the decoration of tables, houses, toilets, house floors, etc. Limestone is also used in colouring old and traditional houses.
  • As we know that, rocks are composed of different types of minerals like copper, iron, gold etc. So, various types of minerals are extracted from the rocks and are used for various purposes.
    Source: vibrantvictoria.ca Fig: Rocks used for making walls
    Source: vibrantvictoria.ca
    Fig: Rocks used for making walls
  • Rocks help to increase the tourisms industry as people from different areas visit the country for rock. The minerals extracted from rocks can be sold inside and outside the country. So, it helps in strengthening the national economy.
  • Various types of rocks are used in research projects. Sedimentary rocks help in the study of the evolution of living beings.

Some Important Rocks Found In Nepal

In Nepal, various types of rocks are found. Some important rocks found in Nepal are discussed below,

  • Conglomerate: It is a sedimentary rock and is found on the banks of rivers and streams of Nepal.
  • Sandstone:It is a classic sedimentary rock with sand- sized granules. It is also found on the banks of river and streams of Nepal.
  • Silica: It is formed by the combination of silica and oxygen and is found on the banks of rivers and streams.
  • Limestone: It is a sedimentary rock composed of calcium carbonate. It is used in making lime and cement. It is found in Godawari, Bhaise, Chovar, Udaypur, Jogimara and other parts of the country.
  • Marble: It is produced from the metamorphism of limestone or dolostone. It is also composed of calcium carbonate. It is used in the construction of buildings, houses, temples, pavements etc.It is found in Godawari, Lalitpur.
  • Slate: It is formed from the metamorphism of schist. It is used for making roofing of houses and making writing boards. In Nepal, it is found in Bandipur, Tanahun district.



  • Rocks are the naturally occurring solid substance found on the surface of the earth.
  • The rocks that are formed by big grains are rough and the rocks made of small grains are smooth.
  • Plutonic rocks are those igneous rocks which are formed by cooling down of magma inside the earth's surface.
  • Volcanic rocks are those rocks that are formed by cooling down of magma or lava on the earth's surface after the volcanic eruption.
  • Sedimentary rocks are formed from the deposited and sedimented substances under the high pressure and temperature after a long period of time.
  • Metamorphic rocks are those which are formed from other types of rocks under the action of high temperature and pressure after a long period of time.
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Very Short Questions

The three types of rocks with one example of each are given below,

  • Igneous rocks: Granite
  • Sedimentary rocks: Shale
  • Metamorphic rocks: Gneiss

 Metamorphic rocks can be defined as the rocks which are formed from other types of rocks due to high temperature and pressure over a long period of time. Any two examples of metamorphic rocks are given below,

  • Marble: formed from limestone or dolostone.
  • Gneiss: formed from granite or diorite

Igneous rocks are those rocks which are formed when magma cools and hardens. The two types of igneous rocks with two examples of each are given below,

  • Plutonic rocks: granite and diorite
  • Volcanic rocks: Basalt and Pumice

Any three uses of rocks are given below,

  • Rocks are used as gravel for the construction of roads and bridges.
  • Rocks are used to build monuments.
  • Hydroelectric dams are build by using rocks.

Any three differences between mineral and rocks are given below in table,

Mineral Rock
It is a pure substance. It is a mixture of minerals.
It is inorganic. It may be organic.
It is solid and crytstalline. It may or may not be crystalline.

 

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  • How many types of rocks are there?

    6
    4
    3
    5
  • Which one of the following is an example of plutonic rocks?

    Sandstone
    Obsidian
    Basalt
    Granite
  • Which one of the following is an example of volcanic rocks?

    Obsidian
    Shale
    Marble
    Sandstone
  • Which one of the following is an example of sedimentary rocks?

    Basalt
    Conglomerate
    Obsidian
    Marble
  • Which one of the following is an example of metamorphic rocks?

    Obsidian
    Basalt
    Sandstone
    Marble
  • The rocks formed by the sediment of the particles of other rocks, when they remain at pressure for a long time, is called

    Sedimentary rocks
    Volcanic rocks
    Igneous rocks
    Metamorphic rocks
  • When the magma gets cooled inside the earth, it forms

    Volcanic rocks
    Plutonic rock
    Metamorphic rocks
    Sedimentray rocks
  • Gneiss is metamorphic of

    Granite
    Shale
    Slate
    Sandstone
  • Sandstone changes into

    Quartzite
    Shale
    Sandstone
    Slate
  • Marble is formed from

    Sandstone
    Limestone or dolostone
    Shale or slate
    Shale
  • Phyllite is formed from

    granite or diorite
    limestone or dolostone
    Sandstone
    Shale or slate
  • Quartzite is formed from

    limestone or dolostone
    Shale
    granite or diorite
    Sandstone
  • When a volcano erupts and the magma of the earth comes out on the earth's surface, it forms rock by cooling called

    Metamorphic rocks
    Plutonic rocks
    Volcanic rocks
    Sedimentaryrocks
  • In which one of the place of Nepal, is slate found?

    Parsa
    Chovar
    Godawari
    Bandipur
  • Gneiss is an example of

    Metamorphic rock
    Volcanic rocks
    Sedimentary rock
    Plutonic rocks
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