Lithosphere

Welcome dear students! Today we are going to learn about Lithosphere from Class 8 Social_Science. After studying this chapter you will learn the meaning of lithosphere and its importance, the structure of the earth and its composition, structure of rocks, about the internal forces like volcanoes and earthquakes and their effects on life on the earth, the external forces like temperature, wind, rain and rivers, and the meaning of underground water and its importance.

The outer-most solid layer of the earth is known as the Lithosphere. Litho means rock. This layer is very thick in the continents and quite thin under the sea floor. The lithosphere consists of rocks, minerals, and soils. Life exists on this layer with the help of the atmosphere and the hydrosphere. Continents are parts of the lithosphere where different land forms like mountains, plateaus, and plains are found.

Do you know what is there inside the Earth? Our planet Earth is more than 4.6 billion years old and still in the process of changing. Humans are in quest of understanding this change and eager to know more about the inside of the earth. What is inside the earth is still a mystery. With years of study and research, human beings have been able to get information about the materials inside the earth up to 10 to 12 kilometers. Going beyond this depth is very difficult due to the increase of temperature, which rises by 10 degrees Celsius for every 32 meters. To understand more about the earth’s interior, humans are dependent on indirect evidences such as seismic waves and volcanic materials. The earth’s interior comprises various materials in different forms. On the basis of density of material, chemical composition, and physical state of matter, the earth’s interior is classified into three main layers. They are the Crust, the Mantle, and the Core.

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Let us look at the Crust. The crust is the uppermost layer of the earth, rich in Silica, Aluminium, and Magnesium. This is generally known as the Lithosphere. The depth of this layer is around 60 kilometers from the surface. In the upper part of the crust only lighter materials are found. It is called SIAL, which stands for Silica and Aluminium, or continental crust. The lower part of the crust is rich in Silica and Magnesium and it is called SIMA or oceanic crust.

Next is the Mantle. The Mantle is the second and the middle layer of the earth. The depth of this layer is up to 2900 kilometers from the surface. The materials are in a semi liquid or partially molten state which is called magma. The mantle is composed of dense and rigid rocks which have a predominance of minerals like magnesium and iron. The mantle has two parts. The first is the Upper mantle or the Asthenosphere, which is partially in a molten condition. The second is the Lower mantle or the Mesosphere, which is in a solid condition. The contact zone of the crust and the mantle is called Mohorovicic Discontinuity or Moho. The boundary that separates the Mantle from the Core is called Gutenberg Discontinuity. Here the rocks are different in chemical composition from those below and above.

Now we come to the Core. This is the innermost layer of the earth. The depth of this layer is up to 6371 kilometers from the surface. The most important materials of the core are Nickel and Ferrous, which means Iron. So it is known as NIFE. The core is divided into two sub layers. The outer core is known as the molten core, where the materials are in liquid and molten form. The inner core is known as the solid core.

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Let us look at a facts file about some of the deepest land mines in the world. First is the Kola deep Borehole. This borehole was drilled as a part of a scientific research deep drilling project near Murmansk, Kola Peninsula, Russia. It is the deepest hole artificially drilled into the earth, reaching 12.262 kilometers. Second is the Tau Tona Mine in South Africa, a gold mine currently the world's largest gold mine, reaching 3.9 kilometers. Third is The Champion Reef Gold mine at KGF, Karnataka in India, which is around 3.2 kilometers deep, making it the deepest mine in India.

Now let us move on to Rocks. Rocks are the solid inorganic substances that are found in the crust of the earth. They are aggregates of minerals. Rocks are formed due to various natural processes. On the basis of mode of formation, rocks are classified into three types. They are Igneous rocks, Sedimentary rocks, and Metamorphic rocks.

First, Igneous Rocks. The word Igneous means fire, derived from the Latin word Ignis or Sanskrit word Agni. Igneous rocks are those which have been formed by the cooling of molten matter of the earth. Igneous rocks were the first to be formed, therefore they are also called Primary rocks. The two important types of igneous rocks are Intrusive igneous rocks and Extrusive igneous rocks. Intrusive igneous rocks form when the molten materials, or magma, of the Earth’s interior do not reach the earth’s surface. They cool and solidify quite slowly below the surface and are called intrusive igneous rocks. These rocks are made of large crystals and are found at great depth inside the Earth. Examples include Granite, Diorite, and Gabbro. Extrusive igneous rocks are formed by solidification of magma above the earth’s surface. These rocks are generally fine grained or glassy because lava after reaching the surface of the earth cools and solidifies quickly. Examples include Basalt and Andesite. The rock materials in the liquid or molten state is called magma, and when it comes out from the earth it is called lava.

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Second, Sedimentary Rocks. The word Sedimentary is derived from a Latin word Sedimentum, which means settling down. Sedimentary rocks are formed by the agency of water, wind, and ice. These agents break and erode the igneous rocks, transport those broken fragments, and deposit them at certain places. The deposit of these materials often occurs in the form of layers or strata. Therefore sedimentary rocks are called stratified rocks. The sedimentary rocks are formed after the disintegration of igneous rocks. Therefore they are called secondary rocks. These rocks are also called aqueous rocks because they are formed in water bodies like lakes, seas, and ocean beds. There are three types of sedimentary rocks. Mechanically formed sedimentary rocks are rocks built up by fragments of pre-existing rocks which have been produced by the processes of weathering and erosion. Examples are Sandstone, also known as Arenaceous rocks, and Shale, also known as Argillaceous rocks. Chemically formed sedimentary rocks are commonly formed by the process of evaporation of water containing salts in solution. Examples are Rock salt and Gypsum. Organically formed sedimentary rocks are those derived by the accumulation of remains of organisms, such as shells of marine organisms, and remains of plants and animals. Examples are Limestone, also known as Calcareous rocks, and Coal, also known as Carbonaceous rocks.

Third, Metamorphic Rocks. Metamorphic rocks are those which are formed by the process of metamorphism or alteration of pre-existing rocks. Metamorphism means change of form which may be physical or chemical or both by the influence of heat and pressure. Let us look at the examples of rock transformations. Granite changes into Gneiss. Basalt changes into Schist. Limestone changes into Marble. Sandstone changes into Quartzite. Coal changes into Graphite. Graphite changes into Diamond. Metamorphic rocks are the hardest rocks on the earth. These rocks supply precious stones. Examples include Sapphire, Ruby, Emerald, and Diamonds.

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Now we will study the Forces of the Earth’s Crust. The Earth has two important forces that change the face of the earth. They are Indogenic forces and Exogenic forces. Indogenic forces are the forces which originate inside the crust and influence the surface features of the earth. Examples include Volcanoes and Earthquakes. The internal forces are also called Endogenic forces.

Let us discuss Volcanoes. A volcano is a vent or narrow opening in the Earth’s crust connected by a pipe to an underlying magma chamber, through which magma, rock fragments, lava, ash, steam, flames, and other gases are emitted from the interior of the Earth. A passage in the earth’s crust through which magma and other volcanic materials are ejected is called a Vent. The funnel-shaped hollow at the top of the cone of a volcano is called a Crater. A large basin-shaped crater bound by steep sides is known as a Caldera.

On the basis of the periodicity or frequency of eruption, volcanoes are classified into three types. Active Volcanoes are those which constantly eject lava, gases, and ashes. There are about 1350 active volcanoes in the world. Examples are Mount Stromboli and Mount Etna in Italy, Saint Helens in the United States of America, Mauna Loa in the Hawaiian islands, and Pinatubo in the Philippines. Dormant Volcanoes are those which have erupted in the past and are likely to erupt again but have remained inactive for fairly long periods. Examples are Mount Vesuvius in Italy, Mount Fujiyama in Japan, Mount Kilimanjaro in Tanzania, and Mount Krakatoa in Indonesia. Extinct Volcanoes are those which were active in the remote geological periods. These are not likely to be active once again. Examples are Aconcagua in Argentina, Gorongoro in Tanzania, and Arthur’s Seat in Scotland.

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The materials ejecting out from the volcanoes are of three types. Solid materials include Volcanic bombs, cinders, scoria, pumice, dust, and ash. Liquid material is Lava. Gases include Sulphur, Hydrogen, and Carbon dioxide.

Volcanoes occur in many regions of the world, including islands, young mountain ranges, and plateaus of the continents. The most important regions are the Circum-Pacific Belt or Pacific Ring of Fire, which includes coastal margins of the Pacific Ocean consisting of the Philippines, Japan, the United States of America, Central America, and South America, where the world's largest volcanoes are found. The second is the Mid-Atlantic Belt, which includes Iceland and West Indies islands. The third is the Mid-Continental Belt, which includes Italy, Spain, France, Greece, and Turkey. Apart from the above major volcanic zones, volcanoes also occur in islands such as Hawaii and Indonesia.

Here is a facts file on super volcanic eruptions of the world. These include Mount Vesuvius in Italy, Mount Krakatoa in Indonesia, Mount Pelee in the West Indies, and Mount Fujiyama, a volcano worshipped by the people of Japan.

Now let us move on to Earthquakes. An earthquake is a shock or series of shocks or tremors, due to a sudden movement of crustal rocks generated within the crust or mantle. The point of origin of the earthquake in the earth’s crust is called the Seismic focus or Hypocentre. The point on the earth’s surface vertically above the seismic focus is called the Epicentre. The scientific study of earthquakes is called Seismology. The origin, time, velocity, and direction of seismic waves are recorded by an instrument known as a Seismograph.

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Causes of Earthquakes include Plate Tectonics. The earth’s crust consists of many major and minor plates. These plates are not stationary. The plate boundaries are dynamic places and are the primary location of earthquake activity, such as the Circum-Pacific Belt. Volcanic Eruptions cause volcanic earthquakes by gas explosions, like the Krakatoa volcano of Indonesia. Faulting occurs when a fracture in a rock along with a great deal of displacement takes place. Earthquakes occur when movement of plates takes place along a line of fracture, like the San Andreas fault of California in the United States of America. Man made factors also cause earthquakes. Over interaction of man with nature is one of the main causes. The extraction of minerals, deep underground mining, huge dams and reservoirs, and nuclear tests cause earthquakes. Examples include the Koyna dam in Maharashtra and the Hoover dam in the United States of America.

Earthquake or seismic waves originate in the seismic focus and travel towards the epicentre in ripples or concentric circles. The three important earthquake waves are Primary Waves, Secondary Waves, and Surface Waves. Primary Waves are also known as longitudinal or compressional waves. These are the fastest earthquake waves and the first waves to reach the surface. They can pass through solid, liquid, and gaseous matters. Secondary Waves are also known as transverse or distortional waves. These waves cannot pass through liquids. They reach the epicentre after the primary waves. Surface Waves are also called long waves. They are the slowest earthquake waves but are responsible for the highest destruction on the surface of the earth. The magnitude and intensity of earthquakes is recorded by using the Richter scale. Earthquakes are very dangerous and destructive. They cause large-scale deaths, loss of property, landslides, flash floods, and damage to bridges, roads, and railway lines.

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Tsunami is a large sea wave occasionally experienced along the coasts of Japan and in other regions, caused by an underwater earthquake. In the Japanese language, Tsunami means harbour waves. On 26th December 2004, a Tsunami in the Indian ocean swept the coastal low lands of Indonesia, India, Sri Lanka, and Thailand. On 11th March 2011, a Tsunami in the Pacific ocean caused huge damage in the eastern coastal regions of Japan, including Fukushima, Sendai, and Miyagi. Some seismic recording centers of India are Gowribidanur, Kodaikanal, Pune, Hyderabad, and Dehradun.

The Distribution of earthquakes is as follows. First, the coastline around the Pacific ocean, which includes New Zealand, Philippines, United States of America, Japan, and Peru. Second, the coastal area around the Mediterranean sea. Third, the Himalayan Mountain region of India, specifically the Shivalik region. Earthquakes and Volcanic Regions overlap in three belts. The Circum-Pacific Belt covers regions around the Pacific ocean like New Zealand, Philippines, Japan, United States of America, and Peru. The Mediterranean Belt covers regions around the Mediterranean sea. The Himalayan Belt covers the Siwalik region of India.

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Now let us discuss Exogenic forces. Exogenic forces are the natural forces that modify the surface of the earth. The important forces are temperature, wind, rainfall, snowfall, river, and glacier. These forces act on the surface of the earth and constantly change its features.

Weathering is the wearing away or breaking down or gradual disintegration of rocks by agents like temperature, rainfall, and wind present in the atmosphere. The three types of weathering are Mechanical weathering, Chemical weathering, and Biological weathering. Mechanical weathering occurs when the rock is broken and disintegrated without any chemical alteration. The process is called Physical weathering or Mechanical weathering. The important processes of mechanical weathering are Granular disintegration, Block disintegration, and Exfoliation. The agents of mechanical weathering are temperature, wind, and frost. Chemical weathering is mainly brought about by the action of substances dissolved in rainwater. This type of weathering results in changing the composition of minerals present in the rocks. There are four types of chemical weathering. Oxidation occurs when oxygen dissolved in water reacts with certain minerals, especially iron, to form oxides. Carbonation occurs when rainwater falls on limestone rocks, the calcium carbonate present in the rocks absorbs carbon dioxide from rainwater and becomes calcium bicarbonate. Hydration is the process by which some minerals in crystalline form absorb water and become a powdery mass. Feldspar is a common rock forming crystalline mineral. Solution occurs when rain falls on the surface of the land, and rainwater dissolves soluble minerals present in the rocks. Biological weathering involves living organisms like plants, animals, and human beings playing a role in the weathering of rocks. This type of weathering includes both physical and chemical weathering. Plants cause weathering through growth and expansion of roots. Animals like burrowing animals, earthworms, rabbits, and rats cause weathering. Human beings cause weathering through mining, quarrying, and construction works.

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Denudation is the action of changing landscape or changing the surface of the earth by various natural agents such as Rivers, Glaciers, Underground water, Wind, and Sea waves. The work of these natural agents are Erosion, Transportation, and Deposition. These processes produce distinct relief features.

A mass of fresh water flowing from its source to mouth along a definite course is called a River. The river is a most prominent agent of denudation. The place where a river takes its birth is called Source, and Mouth is the point where it meets the sea or ocean. Tributaries are the feeders or small streams which supply water to the river along its course. The point where a tributary joins the main river is called Confluence. The course of the river from its source to mouth is divided into three stages: Upper course, Middle course, and Lower course. In the Upper Course, vertical erosion is most common due to steep slopes leading to deepening of valleys, resulting in the formation of gorges, canyons, V shaped valleys, waterfalls, and rapids. Examples include Angel falls, Niagara falls, and Jog falls. In the Middle Course, the river path has a moderate slope and velocity is less than that in the upper course. The volume of water increases when many tributaries join the main river. The main work of the river in this stage is transportation of sediments and little deposition. The important landforms in this stage are Alluvial fans and Meanders. In the Lower Course, the slope of the river course is very minimum. The volume of river water is more and deposition is the main work of the river. The important landforms in this course are Flood plains, Natural levees, Ox-bow lakes, and Deltas. Examples include Sundarban delta and Nile delta. The work of river is called Fluvial Cycle. An Estuary is the tidal mouth of a river broadening into the sea or ocean. A Delta is a fan shaped, low lying area of deposits at a river mouth.

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Glaciers are slowly moving, compacted masses of ice and snow found in the high mountains and polar regions. The mass of ice or snow is pulled down by its weight and due to the force of gravity. Glaciers are divided into two types: Continental glaciers and Mountain glaciers. Continental glaciers are extensive ice sheets found in polar regions. Examples are Greenland and Antarctica. Mountain or Alpine or Valley glaciers are found in high mountainous regions.

Glacier as an agent of denudation performs the work of erosion, transportation, and deposition. The erosional work of glaciers is mainly side cutting. The landforms resulting from this are Cirque, Horn, Arete, U shaped valleys, Hanging valleys, and Rock steps. The transportation and the deposition work of glaciers go almost together. The important depositional landforms produced by glaciers are Moraines, Drumlins, Esker, Kames, Outwash plains, and Till plains. Moraines are the most important landforms of the glaciated region. Moraines are of four types. Lateral moraines are rock debris deposited along the sides of the glacial valley. Medial moraines form when two lateral moraines meet. Ground moraines are rock materials found on the floor or at the bottom of the glacial valley. Terminal moraines are the glacial deposits found at the end of the glacier. The work of Glacier is called glacial cycle.

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Underground water is the subsoil water found on account of percolation or seepage of water into the ground. The underground water which seeps into the ground passes through various layers of rocks. The rocks which allow the water to percolate are called pervious or porous rocks, and the rocks which do not allow the water inside are called impervious or non-porous rocks. The porous rock beds which hold a large amount of underground water are called aquifers. The pervious rocks allow and hold water and form springs. A Spring is a place where the underground water comes out naturally. Types of spring include Perennial spring, which is a spring through which water comes out continuously. Intermittent springs are springs through which water comes out intermittently, meaning not continuously, and they are also called Periodic springs. Hot springs occur whenever warm or hot water comes out naturally. They are usually found near volcanic regions. Geysers throw a jet of hot water, like a fountain, and steam into the air at regular or irregular intervals. An example is Old Faithful in Yellowstone National Park of the United States of America. Artesian wells form when underground water is stored in a basin shaped layer between two non-porous rocks, so the water cannot come out naturally. If an artificial hole is made to the porous rock, the water comes out like a fountain. These are called Artesian wells. These wells are common in Australia.

Underground water performs the work of erosion, transportation, and deposition. The work of underground water is predominant in limestone regions. The important landforms associated with the underground water are Lapies, Sinkholes, Limestone caves, Stalactites, Stalagmites, and Calcite pillars. Relief features formed by underground water in limestone regions is called Karst topography. Examples of limestone caves include Akalagavi, also known as Ulavi caves of Uttara Kannada in Karnataka, and Belum and Bohra caves of Andhra Pradesh.

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The denudation work of wind is common in the arid or desert regions. When the winds of high velocity blow over desert areas they erode, transport, and deposit materials to produce different landforms. The important landforms associated with the erosional work of the wind are Rock pedestals, Mushroom rock, and Inselberg. The depositional landforms are Sand dunes, which include Longitudinal sand dunes, Barchans, and Loess deposits. Barchans are semi-circular or crescent shaped sand deposits most common in the deserts. Loess is the sand particles found beyond the borders of deserts. An example is the Loess plain, also known as Yellow Soil in China. Work of wind is called Aeolian cycle.

Sea waves are also an important exogenic agent of denudation. Sea waves are the regular undulation of water on the sea or ocean. The work of sea waves is significant along the sea shore or coasts. The important landforms associated with sea waves are Cliff, Sea cave, Sea stack, Sea arch, Headland, Sand bars, Beaches, and Lagoons.

Now let us move to the exercises. I will answer each question completely.

Exercise One: Fill in the blanks with suitable words. One. The continental crust is also called SIAL or Silica and Aluminium crust. Two. Vast basin shaped volcanic mouth is Caldera. Three. The most destructive earthquake waves are Surface Waves or Long Waves. Four. Stalactites and Stalagmites are most common in Limestone caves or Karst topography regions. Five. The Beaches are formed by Sea Waves work.

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Exercise Two: Answer the following questions briefly. Six. Mention the three major layers of the interior of the Earth. The three major layers are the Crust, the Mantle, and the Core. Seven. Name the types of volcanoes on the basis of frequency of eruption. The types are Active Volcanoes, Dormant Volcanoes, and Extinct Volcanoes. Eight. Mention the important earthquake zones of the world. The important earthquake zones are the coastline around the Pacific Ocean including New Zealand, Philippines, United States of America, Japan, Peru, the coastal area around the Mediterranean sea, and the Himalayan Mountain region of India, specifically the Shivalik region. Nine. What is weathering? Name the three main types of weathering. Weathering is the wearing away or breaking down or gradual disintegration of rocks by agents like Temperature, rainfall, and wind present in the atmosphere. The three main types are Mechanical weathering, Chemical weathering, and Biological weathering. Ten. Name the landforms associated with the work of river. The landforms associated with the work of river include gorges, canyons, V shaped valleys, waterfalls, rapids, alluvial fans, meanders, flood plains, natural levees, ox-bow lakes, and deltas.

Exercise Three: Match the following. One. SIMA matches with Oceanic crust. Two. Sandstone matches with Sedimentary rock. Three. Epicentre matches with Earthquake. Four. Geyser matches with Underground water. Five. Loess matches with Yellow soil.

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Exercise Four: Define the following. Eleven. Aqueous rocks are sedimentary rocks formed in water bodies like lakes, seas, and ocean beds. Twelve. Pacific ring of fire is the Circum-Pacific Belt, a coastal margin of the Pacific Ocean consisting of Philippines, Japan, United States of America, Central America, South America, where the world's largest volcanoes are found. Thirteen. Mechanical weathering is the process where rock is broken and disintegrated without any chemical alteration, also known as physical weathering. Fourteen. Moraine is the most important landform of a glaciated region, consisting of rock debris deposited by glaciers. They are classified into lateral, medial, ground, and terminal moraines. Fifteen. Carbonaceous rocks are organically formed sedimentary rocks derived by the accumulation of remains of organisms, such as coal. Sixteen. Tsunami is a large sea wave occasionally experienced along coasts, caused by an underwater earthquake. In Japanese it means harbour waves. Seventeen. Continental glacier is an extensive ice sheet found in polar regions, like those in Greenland and Antarctica. Eighteen. Hot spring is a place where warm or hot water comes out naturally, usually found near volcanic regions.

Exercise Five: Terms to remember. The terms are NIFE, Arenaceous and Argillaceous rocks, Tectonic forces, Mountain glacier, Aeolian cycle, and Beaches.

Exercise Six: Activities. Activity One: Collect the different rocks from your environment. Activity Two: During a school excursion, observe how a waterfall is formed.

Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]