The Earth:

The earth, our homeland, is a dynamic Planet. The earth’s surface has lofty mountains, High plateaus, large plains and deep valleys etc. The earth’s surface is constantly undergoing Changes inside and outside. Have you ever Wondered what lies in the interior of the earth? What is the earth made up of? Let us learn about it

Earth is called as blue Planet. 71% of the earth is covered by Water

Spheres of the Earth:

Earth’s surface is a vast area of 510 million sq.km, where four spheres of the Earth interact. The abiotic spheres are the Lithosphere, atmosphere and hydrosphere. The biotic sphere is the biosphere. Together, These spheres constitute the planet, Earth.

The lithosphere is the solid outer Part of the Earth.

The atmosphere is a thin layer of Gases that surrounds the Earth.

The hydrosphere is the wateryPart of the Earth’s surface including oceans, Rivers, lakes and water vapour.

The biosphere is the layer of Earth Where life exists.

The terms ‘lithosphere’ and ‘crust’ are not the same. The lithosphere includes The crust and the uppermost Part of the mantle. All terrestrial planets have Lithosphere. The lithospheres of Mercury, Venus, and Mars are much thicker and more rigid than that of the Earth.

Interior of the Earth:

The structure of the earth may be compared to that of an apple.On the basis of The study of earthquake waves the spherical Earth is found to be three concentric layers. They are:

1. The crust,
2. The mantle and
3. The core.

The Crust:

The crust is the outermost layer of the Earth. Its thickness varies from 5 to 30 km. It Is about 35 km on the continental masses and Only 5 km on the ocean floors. Despite greater Thickness, the continental crust is less dense Than the oceanic crust because it is made of both Light and dense rock types. The oceanic crust is Composed mostly of dense rocks such as basalt.

The crust comprises two of distinct parts. The upper part consists of granite rocks and Forms the continents. It has the main mineral Constituents of silica and alumina. So it is Referred to as Sial. It has an average density of 2.7g/cm3.The lower part is a continuous zone of Denser basaltic rocks forming the ocean floors, Comprising mainly of silica and magnesium. It is Therefore called Sima. It has an average density Of 3.0g/cm3. The sial and the sima together form The earth’s crust. Since the sial is lighter than the Sima, the continents can be said to be ‘floating’ On a sea of denser sima.

The Mantle:

The next layer beneath the crust is called The mantle. It is separated from the crust by a Boundary called Mohorovicic discontinuity. The mantle is about 2,900 km thick. It is divided Into two parts. (i) The upper mantle with a Density of 3.4 – 4.4g/cm3. Extends down to 700 Km. (ii) The lower mantle having a density of 4.4 – 5.5g/cm3 extends from 700 to 2,900 km.

The Core:

The innermost layer of the earth is called The core. It is also known as barysphere. It is separated from the mantle by a boundary Called Weichart-Gutenberg discontinuity. The Core is also divided into two parts

1. The outer core, which is rich in iron, is in Liquid state. It extends between 2,900 – 5,150 km
2. The inner core, composed of Nickel And Ferrous (Nife), is solid in state. The central Core has very high temperature and pressure. It Extends from 5,150 km to 6,370 km. The average Density of core is 13.0 g/cm3

The Earth Movements:

The lithosphere is broken into a number Of plates known as the Lithospheric plates.Each plate, oceanic or continental moves Independently over the asthenosphere. The Movement of the Earth’s lithospheric plates Is termed as tectonic movements. The energy Required to move these plates is produced by the Internal heat of the earth. These plates move in Different directions at different speed.

At places, these plates move away from Each other creating wide rifts on the earth’s Surface. At some places, these plates come Closer and collide. When an oceanic plate Collides with a continental plate, the denser Oceanic plate is forced below the continental Plate. As a result of the pressure from above The rocks heats up and melts. The molten rocks Rise again forming volcanic mountains alongthe continental edge. Alternatively, a trench May be formed between two plates In some cases when two continental plates Converge, neither plate can be forced under The other. Instead, folds may be created. Great Mountain ranges like the Himalayas have been Formed in this way.The movement of these plates causes Changes on the surface of the earth. The earth Movements are divided on the basis of the forces Which cause them. The forces which act in the Interior of the earth are called as Endogenic Forces and the forces that work on the surface of The earth are called as Exogenic forces

Endogenic forces produce sudden Movements and Exogenic forces produce slow Movements. Endogenic movements produce Earthquakes and volcanoes that cause mass Destruction over the surface of the earth.

Internal Processes:

The internal processes Generate heat and eject Materials from deep below The Earth’s crust. Internal Radioactivity is the Principal source of power For this process.

Plate Tectonics:

The lithosphere is divided into a number Of huge slabs of rocks called ‘Tectonic Plates.’ These tectonic plates are divided into Major and minor plates. These plates float independently over the mantle. Collisions Of these plates produce mountain ranges And other irregular surface features, both on Land and the ocean floor. This phenomenon Is called ‘plate tectonics’. The movement Of tectonic plates is due to thermal energy From the mantle. Now we have a better Understanding about the plate movements And its relation to Earthquake and volcanic Activities.

Types of Plate Boundaries:

Convergent Boundary – Here the plate moves Toward each other and sometimes, a plate Sinks under another. The location where the Sinking of a plate occurs is called a subduction Zone (eg) Fold Mountain-Himalayas.

Divergent Boundary – Here the plates pull Away from each other as magma pushes up From the mantle (eg) Mid Atlantic Ridge

Conservative/Transform Boundary – Here The plates slide horizontally past each other.(eg) San Andres Faul

Movements of Continental Plates:

Due to lateral compressional forces, The plates are forced to move upwards And downwards. This is called ‘Folding’

Mountains formed by folding are called fold Mountains. The process of folding createsLofty mountain ranges such as the Himalayas And the Alps According to plate tectonics, the plates Are in constant motion with an average rate Of few centimetres per year. The movement Might seem slow, but over millions of years, The plates and the continents riding on them Move a long way. For example, about 250 Million years ago, the Indian Plate was a part Of the Gondwana land, which comprised of Modern Africa, Australia, Antarctica, and South America.Approximately 140 million years Ago, the Indian plate broke away from the Ancient super continent ‘Gondwana’ and Began moving north and collided with Asia. The collision with the Eurasian Plate along The boundary between India and Nepal Formed the Orogenic belt that created the Tibetan Plateau and the mighty Himalayan Mountains.

Earthquake:

Earthquakes are generally caused by The sudden vibrations in the Earth’s crust, Which spreads outward in all directions as Waves from the source of disturbance. The Point of origin of an Earthquake is called ‘Focus’ (Hypocenter) which generates a Series of elastic waves. ‘Epicentre’ is a point On the Earth’s surface that lies directly above The focus. The impact of the Earthquake is felt The most at the epicentre.

Seismic Waves:

Earthquakes generate seismic waves. The Nature, force and speed of these seismic waves Depend on the nature of the medium through Which it passes. Accordingly, there are three Major types of waves.

Primary or P-waves:

It is  the fastest of all The Earthquake waves and the first to reach The epicentre. These waves pass through solids, Liquids and gases, either through push or pull With an average velocity of 5.3km per second to 10.6 km per second.

Secondary or S-waves:

It travel only Through solids. These transverse waves shake The ground perpendicular to the direction in Which they propagate. The average velocity of These waves is 1Km per second to 8 km per Second.

Surface Waves (or) L-waves are similar:

To P-waves but they travel primarily along The ground surface. These waves travel Comparatively slower and are the most Destructive waves. The average velocity of These waves are 1 km per second to 5 km per Second.

The earthquake waves are recorded by An instrument known as seismograph. The Magnitude of an earthquake is measured by the Richter scale. The numbers on this scale range From 0 to 9.

Causes of Earthquake:

The chief cause of earthquake is the sudden Slipping of the portion of the earth’s crust along Fractures or faults. The movement of the molten Rocks underneath the surface produce strains Which break the rocks apart. The sudden shifting Of landmass causes upheavals in the crust of The earth sending vibrations or waves into the Surrounding portions of the earth. Sometimes The surface of the earth itself cracks.

Effects of Earthquakes:

Earthquakes may cause changes in the Earth’s surface. Vibrations often set landslides In mountainous regions. A greater danger in An earthquake is the falling of buildings. Most Of the houses which collapsed were made of Mud and bricks and proved to be death traps. Underground water system is naturally disturbed By such movements. Fire is another great danger.

An earthquake which originates below Or near the sea causes great disturbance in the Water. The floods and waves cause great loss Of life, sometimes more than the earthquake itself. Tsunami, a Japanese term, is the name Given to the huge waves caused in the sea by an Earthquake. Tsunamis are quite common along The coasts of Japan and other regions in the Pacific Ocean

Distribution of Earthquakes:

The world’s distribution of earthquakes Coincide very closely with that of volcanoes. Regions of greatest seismicity are circum-Pacific Areas, with the epicenters and the most frequent Occurrences along the Pacific Ring of Fire. It is Said that about 68 % of earthquakes occur in This belt. Remaining 31 % of earthquakes take Place in the Mediterranean-Himalayan belt Including Asia Minor, the Himalayas and parts Of north-west China. The remaining percent of Earthquakes occur in Northen Africa and Rift Valley areas of the Red sea and Dead sea.

In India, the Himalayan region and the Ganga-Brahamaputra valley are prone to Earthquakes. A number of earthquakes have Been experienced in this region. Some of them Were very severe and caused extensive damage, e.g., the earthquake of Uttar Kashi in 1991 and Chamoli in 1999. The Deccan Plateau, which Was supposed to be comparatively free from The dangers of the earthquakes, has experienced Two severe earthquakes in the past, the Koyna (Maharashtra) earthquake in 1967 and the Latur earthquake in 1993.

Tsunami:

The word ‘Tsunami’ is a Japanese term, Meaning harbour waves. It is adopted to describe large seismically generated sea waves caused

By Earthquakes, submarine explosions and Landslides. These waves travel at a great speed (more than 500 km per hour) and the length of The waves exceeds 600 km. These waves reach to A height of more than 15 m near the sea shore And are capable of causing destruction along the Coastal area.

 The 2004 Indian Ocean Earthquake that Caused tsunami is the sixth-deadliest natural Disaster which travelled at a speed of 600 Km per hour with an estimated death toll of 2,80,000. The Earthquake which occurred Near Indonesia at 00.58 hours took nearly 7 Hours to reach Chennai.

On 26 December 2004 a Tsunami occurred in the Indian Ocean. It was the resultOf the Indio-Australian Plate Subducting below the Eurasian Plate. It was caused by an Earthquake Measuring a magnitude of above 9 in the Richter scale. The Earthquake caused the Seafloor to uplift, displacing the seawater above.

Volcanoes:

A volcano is a vent or an opening on the Surface of the Earth crust, through which hot Solid, liquid and gaseous materials (Magma) Erupt out to the surface from the Earth’s Interior. Magma rises up and ejects on the Surface as Lava. Volcanoes are also formed When plates move apart. Volcanoes generally have the following Major components. They are:

1. Magma chamber – a large pool of liquid Rock found beneath the surface of the Earth
2. Vents – an opening serving as an outlet For air, smoke, fumes, magma etc

• Volcanic cone – a landform built by the Magma ejected from the vent in the Shape of a cone.

1. Crater – a bowl shaped depression found At the top of the volcano through which The magma flows out.

Causes of Volcanic Activity:

The temperature increases as the depth Increases at the rate of 1ºC for every 32 metres.

There is also great pressure. At a depth of about 15 km the pressure is about 5 tonnes per cm2Of Rock.

 Under these circumstances, the interior of The earth is in a semi-molten state called magma.The magma, under great pressure has the Capacity to dissolve great volume of gas; some Gases are also combustible. This makes volcanic Material burst forth through the weak spots in The earth’s crust.

Nature of volcanic eruptions:

Sometimes, magma rises slowly to the Surface and spreads over a vast area. This is known as fissure eruption. Some plateaus and plains have been formed in this way, e.g., Deccan Plateau in India and the Colombian Plateau in North America. If the magma rises quickly to the surface, lava is thrown high into the atmosphere. Besides lava, ash, steam, gases and pieces of rocks are also thrown out. This Type of eruption is known as explosive eruption. The terrible explosion on 27^th August 1883 in the island of Krakatoa, Indonesia is an example for explosive type of eruption.

The viscosity of lava is determined by the Amount of silica and water in magma. Highly Viscosity lava is rich in silica and has little water. Low viscosity lava has little silica, but a lot ofWater. It moves rapidly forming smooth flows.

Barren Island is situated in the Andaman Sea, and lies about 138 Km northeast of the territory’s Capital. It is only in active volcano along the chain from Sumatra to Myanmar. Last Eruption occurred in 2017.

Based on the periodicity of eruptions, Volcanoes are classified into

• Active volcano,
• Dormant volcano,
• Extinct volcano.

Active Valcano:

Valcanoes that erupt frequently are called Active volcanoes. Most of the active volcanoes lie In the Pacific Ring of Fire belt which lies along The Pacific coast. There are about 600 active volcanoes in the world, such as Mt. Stromboli in Mediterranean Sea, St.Helens in USA, Pinatubo in Philippines. Mauna Loa in Hawaii is the World’s biggest active volcano.

Dormant Valcano:

These volcanoes have shown no sign of Activity for many years but they may become Active at any time. These are called Sleeping Volcanoes. Vesuvius mountain of Italy, Mt Fujiyama of Japan, Mt. Krakatoa of Indonesia Are famous examples of this type.

Extinct volcano:

A Volcano has not erupted in past 1000 Years is often listed as Extinct volcanoes. The top of extinct volcanic mountains have Been eroded.  Mt Popa of Myanmar and Mt. Kilimanjaro and Mt. Kenya of Africa are Examples of extinct volcanoes.

Volcanoes can also be classified based on their structure and composition as composite Volcano, shield volcano and dome volcano

Composite Volcano:

Composite volcano, also known as strata Volcano, is a conical volcano built by many Layers of hardened lava, pumice and volcanic Ash. These are commonly found in the Pacific Ocean Eg. Mt. Fujiyama, Japan

 Volcanic Dome:

A lava dome or volcanic dome is roughly A circular mound formed due to the slow Ejection of viscous lava from a volcano. As the

Lava is rich in silica with intense viscosity; it is Prevented from flowing far from its vent. E.g. Parícutin, Mexico

Shield Volcano:

Shield volcanoes are formed by intense Viscous lava.These are shallow Depositions with Gently sloping sides.Hence the lava flows Out in all directions To create a shield.E.g. Mauna Loa, Hawaii

Distribution of Volcanoes in the world:

Volcanoes are located in a clearly-defined Pattern around the world. They are closely Related to regions that have been intensely Folded or faulted. There are about 600 active Volcanoes and thousands of dormant and extinct ones. They occur along the coastal Mountain ranges, as off-shore islands and In the midst of oceans, but there are a few in the interior of continents. The volcanic belts are also the principal earthquake belts of the World. There are three major zones of volcanic Activities in the world. They are:

1. The Circum – Pacific belt
2. The Mid continental belt
3. The Mid Atlantic belt

Circum Pacific Belt:

This is the volcanic zone of the convergent Oceanic plate boundary. It includes the Volcanoes of the eastern and western coastal Areas of Pacific Ocean. This zone is popularly Termed as the Pacific Ring of Fire which has Been estimated to include two-thirds of the World’s volcanoes.

Mid continental belt:

This is the volcanic zone of convergent Continental plate boundaries that includes The volcanoes of Alpine mountain chains, the Mediterranean Sea and the fault zone of eastern Africa. The important volcanoes are Vesuvius, Stromboli, Etna, Kilimanjaro and Kenya. Surprisingly, the Himalayas have no active Volcanoes at all.

Mid Atlantic Belt:

This belt represents the divergent Boundary of plates located along the mid-Atlantic ridges. Volcanoes of this area are Mainly of fissure eruption type. Iceland is the most active volcanic area and is located on the mid-Atlantic ridge. St. Helena and AzoresIsland are other examples.

Effect of Volcanoes:

Constructive Effects:

Volcanic materials enrich the soil fertility that promotes agricultural activities. The Hot volcanic region helps in generating geothermal energy. Many dormant and active Volcanoes are the most attractive tourist spots of the world. Most of the volcanic materials are used as building materials.

Destructive Effects:

Volcanic eruption causes Earthquakes, flash Floods, mud slide and rock fall. Lava can travel Very far and burn, bury, or damage anything in its path. The large amount of dust and ash Makes breathing hard and irritable. Volcanic Eruptions can alter the weather conditions and disrupt transport (Iceland volcanic Eruption) in and around the volcanic region.