Notes
Points to be Learn :
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Natural calamities :
- Various natural events occur on the earth from time to time e.g. floods, storms, snowfall extreme rainfall, etc.
- Similarly natural calamities like earthquakes and volcanic eruptions occur because of the movements below the earth’s surface. There is loss of life and property because of natural calamities.
The nature of endogenetic (internal) movements : Generally, these movements in the earth’s interior occur in the upper layer of the mantle.
- Tremendous energy is released by radioactive materials in the mantle.
- These energy waves travel from one place to another. Due to such a flow of energy, instability is caused in the mantle.
Classification of endogenetic (internal) movements :
The movements are classified on the basis of their velocity, direction and the landforms they produced.
(i) Endogenetic movements based on velocity : On the basis of velocity, endogenetic movements are classified into slow movements (occurring continuously) and sudden movements (occurring in the forms of events).
(ii) Endogenetic movements based on direction : On the basis of direction, endogenetic movements are classified into horizontal movements, upward movements and downward movements.
- Upward m0vement:The movement of energy waves from the lower level to upper level in the earth’s interior is called upward movement.
- Downward movement : The movement of energy waves from the upper level to the lower level in the earth’s interior is called downward movement.
(iii) Endogenetic movements based on landforms : On the basis of direction, endogenetic movements are classified into continent-building movements and mountain- building movements.
Slow movements : The effects of slow movements can be seen in the form of formation of mountains and continents on the earth's crust.
Mountain-building movements : Mountain-building movements include the movements that lead to formation of fold mountains, block mountains and rift valleys.
Fold mountains : Due to energy waves working towards each other and in horizontal way, the layers of the soft rocks form folds. As a result, the surface of the earth gets uplifted and fold mountains are formed.
Block mountains : Due to energy waves moving away from each other and in horizontal way, the tension on the layers of rocks increases and it leads to the formation of fractures (faults) in rocks. Similarly, waves coming towards each other in hard rocks, also form faults due to compression. When a part of the earth’s crust in between two Parallel faults is lifted, it looks like a block. Such a landform is known as a block mountain. For example, Black Forest mountains in Europe, the Meghalaya Plateau of India, etc.
Rift valleys : Sometimes, two fractures develop side-by-side in the earth’s crust‘ The land between the two fractures subsides. This subsided deep part is called rift valley. For example, the rift valley of river Narmada in India, The Great Rift Valley of Africa, the Rhine river rift valley of Europe.
Continent-building (Epeirogenic) movements : Due to some slow movements, a vast part of the earth’s crust is uplifted or gets subsided. When the part of the earth’s crust is uplifted above the mean sea level, the continents are formed. Extensive plateaus can also be formed because of such movements. If the originally continental portion of the crust subsides below the sea level, it forms a part of the seabed.
Sudden movements : The effects of sudden movements can be seen in the form of earthquakes and volcanoes.
Earthquake : Earth means the ground and the quake means the trembling.
- Because of the movements occurring in the interior of the earth, tremendous tension is created in the earth’s crust. When tension goes beyond limits, the energy is released in the form of waves.
- This leads to the trembling of the earth’s surface and it is known as the occurrence of earthquake.
- The magnitude of earthquake is measured by Richter scale.
Causes of earthquake :
Focus and epicenter : Because of the movements occurring below the earth’s surface, tension is created and it keeps on accumulating.
- High energy is released at the place where this tension mounts up. This is the centre of the earthquake. It is called the focus or hypocenter.
- The place on the earth’s surface where the energy waves reach first is called the epicenter of the earthquake. The epicenter is perpendicular to the focus.
Seismic waves : The seismic waves can be divided into primary, secondary and surface waves as follows :
Primary or ‘P’ waves : These Waves are the first one to reach the surface of the earth after the energy is emitted in the earth’s interior. Secondary or ‘S’ waves : The waves which reach the earth’s surface after the primary waves are called secondary waves. Surface waves or ‘L’ waves : These waves are generated after the main ‘P’ waves and ‘S’ waves reach the epicenter.
Seismogram :
Through seismogram, graphs showing movement of seismic waves can be generated. These graphs are known as seismographs and they helps in knowing the magnitude of the earthquake. With their help, even micro-seismic waves can also be measured.
Effects of earthquake :
Volcanoes : Hot solid, liquid and gaseous materials are thrown out from the mantle of the earth onto the surface of the earth. This process is called volcanic eruption. During this process ash, water vapour, various types of poisonous and inflammable gases, hot molten magma, etc. are thrown out. When the molten magma comes out on the surface it is called lava.
Types of volcanoes on the basis of types of eruption : The following are the types of volcanoes on the basis of types of eruption : Types of volcanoes on the basis of periodicity of eruption : The following are the types of volcanoes on the basis of periodicity of eruption :
Effects of volcanoes :
| Andaman and Nicobar Islands are located to the south-east of the mainland of India. On this archipelago lies the Barren Island, India’s only active volcano. This volcano was dormant for a long time. But, in February 2017, it started erupting.
Mainly dust, smoke and mud is coming out of this eruption along with some lava. |
Plates ; The crust (the outermost layer of the earth) is not continuous. It is separated into small and large pieces. These pieces are known as plates. Plates float over the mantle and also move independently. Indo-Australia, Africa, Eurasia, North America, South America, Pacific and Antarctica are seven major plates. The earth’s crust is made up of such plates. Oceans and continents are spread over these plates. Depending on the compression and tension formed in the interior of the earth, these plates move in various directions.
Relation between areas of earthquakes and volcanoes with plate boundaries :