28 January 2015

Our Changing Earth

Our Changing Earth

Earth Movements : Chapter Summary

Alfred Wegener’s theory of continental drift proved that millions of years ago, all the continents were joined in a super-continent, which he called Pangaea.

Due to the movements of the lithospheric plates, the super continent broke into large landmasses that drifted away. The big landmasses became continents and Water collected between these landmasses to form the oceans.

The lithosphere consists of a number of plates called the lithospheric plates or the tectonic plates. The circular movement of the molten magma inside the earth causes the tectonic plates to move slowly. The earth’s movements can be categorised into two types based on the forces that initiate these movements i.e. endogenic or exogenic.

Some movements are caused by forces acting in the interior of the earth called endogenic movements. Forces on the surface of the earth result in exogenic movements.

Endogenic forces can be sudden or diastrophic. Sudden endogenic movements may result in natural disasters, like earthquakes, eruptions of volcanoes and landslides. Diastrophic forces refer to forces generated by the movement of the solid material of the earth’s crust.

Exogenic forces can take the form of weathering, erosion and deposition. Weathering is the breaking of rocks on the earth’s surface by different agents like rivers, wind, sea waves and glaciers. Erosion is the carrying of broken rocks from one place to another by natural agents like wind, water and glacier. 

Must Read:  Our Changing Earth Solved Question Answers

Earthquakes : Chapter Summary

An earthquake is a sudden shaking and trembling of the earth. This shaking and trembling causes the movement of a series of shock waves on the earth’s surface.

Earthquakes are caused by the sudden breaking and movement of the tectonic plates of the earth's rocky outermost crust. Earthquakes typically occur on plate boundaries.

Every earthquake has a focus and an epicentre. The epicentre of an earthquake is the location on the earth’s surface where the damage from an earthquake is the maximum. The point of origin of an earthquake is its focus.

An earthquake results in a series of shock waves on the earth’s surface. These shock waves are called seismic waves. A seismograph records the vibrations of the seismic waves caused by an earthquake.

There are three types of earthquake waves or seismic waves.
  • P waves or longitudinal waves.
  • S waves or transverse waves
  • L waves or surface waves

The Richter scale is used to measure the intensity of an earthquake. 

Volcanoes : Chapter Summary

An opening in the earth’s crust that allows hot molten lava, ash and gases to escape from below the surface, is called a volcano.

Lava is molten rocks or magma coming out from the magma chamber through a vent, which is located in the middle of a volcanic mountain and connected to a pool of lava under the earth’s surface. With time, the lava and ash flow out of the magma chamber, cool down, solidify and accumulate in layers forming a Volcano.

The earth is composed of three layers: Crust, Mantle and Core. The mantle consists of magma, on which the lithospheric plates move. Raise in temperature and pressure force the magma to flow out through an opening in the earth’s surface, as lava in a volcanic eruption. The tectonic motion that causes earthquakes also causes volcanic eruptions.

On the basis of the frequency of eruption, volcanoes are categorized into three types, i.e. Active volcanoes, Dormant volcanoes and Extinct volcanoes.

Active volcanoes are those that erupt regularly i.e. a volcano that has erupted in the recent period of time. An active volcano can erupt at any time. In the basin of the Pacific Ocean lies a belt of active volcanoes. This area is called the Pacific Ring of Fire.

Mauna Loa in the Hawaii Islands is the world’s largest volcano. It is about six miles tall from the sea
floor to the summit.

Volcanoes which have not erupted for many years, but have the potential to erupt again, are called as dormant volcanoes. While the volcanoes which have not erupted in thousands of years and show no signs of doing so in the future too are called as Extinct Volcanoes.

Volcanoes further are categorized according to their shapes as well: Shield Volcanoes,   Cinder Cone Volcanoes and Composite Volcanoes.

A shield volcano is very broad with a large base and is common in oceanic areas. Mauna Loa is an example of a shield volcano.

A cinder cone volcano has steeply sloped sides made of layers of ash and loose rock and a composite volcano is made of strata or layers. These layers are formed when cinders and ash pile on top of each other, and lava flows on top of the pile. It cools and hardens to form a layer and during next eruption, the process of layer formation begins again. 

Major Landforms - Work of Water : Chapter Summary

Get to know about Our Changing Earth (Ncert / Cbse Solutions & Revision Notes), Chapter Summary Earth Movements, Earthquakes, Volcanoes,Major Landforms - Work of Water Major Landforms - Work of Ice and Wind, CBSE / NCERT Revision Notes, CBSE NCERT Class VII (7th) | Social Studies | Geography, CBSE NCERT Solved Question Answer, CBSE NCERT Solution.The process involved in the creation of the various landforms on earth i.e. mountains, plains and
plateaus are endogenic and exogenic. The endogenic forces like earthquakes and volcanic eruptions bring about rapid change and even mass destruction.

The Exogenic forces involve weathering, erosion and deposition. Weathering and erosion are two processes, constantly working together, wearing away the earth’s surface and lowering it.

The process of weathering involves breakdown of rocks and soils on the earth’s surface through direct contact with atmospheric conditions, and agents like water, ice and wind while erosion involves the loosening and carrying away of rocks and debris formed by weathering. The eroded material is carried away by agents i.e. water, ice and wind and deposited at other locations.

This results in the rebuilding of the earth’s surface at the new location. Water is the most powerful agent of erosion. Waterfalls, are a result of erosion by rivers.

Waterfalls are formed when the river water tumbles at a steep angle over erosion-resistant rocks or the side of a steep valley.

Flowing down from the mountains, rivers enter plains. A river never flows straight but in twists and turns, forming large bends. These bends are called meanders. The water of a river continuously erodes and deposits sediments along the sides of these meanders. This way the ends of the meander loop come close with time.

At a point, the ends of the meander loop meet and it gets cut off from the river forming a lake. Such a lake formed from the erosion and deposition along the meanders of a river is called an ox-bow lake.

When a river overflows it floods the areas around it depositing layers of fine soil and other types of sediment along the banks of the river. The areas subject to recurring floods get deposits which gradually spread over to build flat, fertile plains, called flood plains. The raised banks of such flood plains are called levees.

The speed of the river decreases owing to the flat land and it starts breaking up into distributaries and depositing the sediments. At some point, the speed of the river water becomes so low that it is not able to the sediments further. The river hence, starts depositing the sediments.

The deposition of sediments forms a mouth at the end of each distributary of the river which collects to form a delta.

The sea water in the form of waves carries on the task of erosion and deposition. As the sea waves keep striking at the rocks, they develop cracks which with time increase in size and width, gradually forming sea caves.

As water keeps eroding these caves, the cavities increase in size until only the roof is left from the original rock, resulting in the formation of a sea arch. Further erosion results in the destruction of the roof also, leaving only the walls of the original cave called stacks.

Where the sea waves encounter erosion-resistant rocks parallel to the coast, sea cliffs are formed. Sea waves continuously transport and deposit sediments along the shores and with time, these deposits form beaches.

Major Landforms - Work of Ice and Wind : Chapter Summary

Glaciers are found in snow-capped mountain ranges at great heights and are basically rivers of ice. The glaciers pick up and carry soil and loose stones along with ice which exposes the solid rock below. The forced movement of these materials with the glacier cause deep scratches and large holes to be carved in the bedrock.

As the ice melts, these holes get filled with water, and form picturesque lakes in the mountainside. When the materials carried by the glacier get deposited, they accumulate and form glacial moraines. Wind blowing in a desert carries and deposited sand from one place to another, in the form of low hill-like structures, called sand dunes, mushroom rocks and loess.

Winds carrying heavy, thick grains grind against the lower section of the rock while the upper portions of these rocks are less exposed to erosion and remain wide. The erosion hence gives it the shape resembling a mushroom.

When the sand grains are very fine and light, the wind carries it over very long distances. When such sand is deposited in large areas, it is called loess.