*    Introduction
    The lithosphere is broken into a number of plates known as the Lithospheric plates. 
    These plates move around very slowly – just a few millimetres each year. 
    This is because of the movement of the molten magma inside the earth. The molten magma inside the earth moves in a circular manner.
    The movement of these plates causes changes on the surface of the earth. 

    Earth movements : 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.

*    Earthquakes  
    The sudden mild or violent shaking of a part of the earth is called an earthquake. It is generally accompanied by a rumbling sound and tremors. 
  One of the causes of earthquakes is the movement of the molten rock inside the earth’s crust. Sometimes this molten rock is thrown out and volcanoes are caused. This eruption also causes earthquakes.
    
    Earthquakes occur in places where the earth’s crust is weak. 
    Violent earthquakes are frequent in areas where there are volcanoes, e.g., the belt surrounding the Pacific Ocean.
    The tectonic or horizontal movements of the earth bring about disturbances in  its interior portions. 
    The point where these vibrations originate is called the focus of the earthquake. The point above the focus on the earth’s surface is called the epicentre of the earthquake.

 

An earthquake lasts for a few seconds, but it causes a huge loss of life and property. In India, earthquakes occur frequently in the areas such as foot-hills of the Himalayas and Ganga-Brahmaputra valley. 
The vibration of the earthquakes are recorded by an instrument called Seismograph. 
The magnitude of earthquake is measured by Richter Scale.
The number of this scale range from 0 to 9.
 Earthquake of magnitude 7 or above are very destructive and cause heavy damage to life and property.
 Earthquakes bring about changes on the surface of the earth in a number of ways. They may cause landslides in the hilly areas and cracks in the earth’s crust. In the coastal areas the earthquakes sometimes submerge land or old islands and sometimes   they give birth to new ones. Sometimes hidden minerals buried under deep earth reveal themselves on the surface of the earth.

 

*   EARTHQUAKE PREPAREDNESS
    Where to take shelter during an earthquake —
    Safe Spot – Under a kitchen counter, table or desk, against an inside corner or wall.
    Stay Away from – Fire places, areas around chimneys, windows that shatter including mirrors and picture frames.
    Be Prepared – Spread awareness amongst your friends and family members and face any disaster confidently.

*   Volcanoes      
    Volcanoes are openings in the earth’s crust through which materials are thrown out from the interior of the earth. 

    The materials thrown out include hot molten rock or lava along with ash, some solid rock particles, steam and gas. 

    The opening itself is called a vent around which a conical mountain may be formed. 
    The funnel-shaped basin surrounding the vent is called a crater. 

    The  liquid  of the volcano, so long as it remains within the surface of the earth, is called magma. But when this magma rises slowly to the surface of the earth, it is called  lava.      

    

  Volcanoes are classified into three types on the basis of their composition and shape. They are :
   According to the nature and frequency of eruptions, volcanoes around the world have been divided into three categories:
    1.   Active volcanoes        2.   Dormant volcanoes        3.   Extinct volcanoes           

Active volcanoes
These erupt quite frequently and are usually found where folding and faulting is active. The crust of the earth in such regions is under tremendous pressure and, therefore, tends to crack. We find a belt of such active volcanoes around the Pacific Ocean, which is called the Ring of Fire. Examples of active volcanoes include Mt Etna in Italy and Barren Island in the Andaman Sea in India. There are about 500 active volcanoes in the world.  

Dormant volcanoes
These volcanoes have not erupted for a long time. They are also called sleeping volcanoes as they may erupt though they have not done so in a long time. Mt Vesuvius in Italy is a good example of a dormant volcano. When they come to life, dormant volcanoes can be really destructive.

Extinct volcanoes
These volcanoes are dead and inactive. There is virtually no movement inside such structures. Mt Kilimanjaro in East Africa is one such volcano. The peak of this mountain is now covered with ice. Crater Lake, in Oregon, USA, is another example where a lake has formed in the crater over the years. Such lakes are called Crater Lakes.

*   MAJOR LAND FORMS Do you know? 
    The landscape is being continuously worn away by two processes – weathering and erosion. 
    Weathering is the breaking up of the rocks on the earth’s surface.
    Erosion is the wearing away of the landscape by different agents like water, wind and ice. 
    The eroded material is carried away or transported by water, wind, etc. and eventually deposited. 
    This process of erosion and deposition create different landforms on the surface of the earth.

*    Work of the River as an Agent of Erosion and Deposition
    In view of the change in the water supply and slope of land along the path of the river, the river course is- usually divided into three stages which are known as :
    (1) The Upper Course or the Mountain Stage.
    (2) The Middle Course or the Plain Stage.
    (3) The Lower Course or the Delta Stage.

(1)    The Upper Course or the Mountain Stage— In this stage, as the slope of the land is generally very steep, the water of the river rushes down with a great speed. Consequently the river not only erodes its sides and bed but also carries stones and                fragments of rocks with it. In this way the river is busy, in its upper course, in erosion and transportation. By eroding its sides and bed it develops gorges, V-shaped valleys, waterfalls and rapids, etc.

(2)    The Middle Course or the Plain Stage — In this stage the slope is not so steep. It becomes quite gradual, so the river slows down and becomes fit both for navigation and irrigation. The river, when it overflows its bank, deposits the alluvial sediments,           which it brings from the mountains, on both the sides. Thus are formed the flood plains which are often level and very fertile. When the course of a river lies in the flat plain, irregularities of the ground force the river to swing from side to side, thereby                making big loops. Such loops formed by the river in the flat plain are called meanders. Due to continuous erosion and deposition along the sides of the Meanders the ends of the meander loop come closer. In due course of time the meander loop cuts             off from the river and forms a lake called Ox-bow lake.
      Levees: The raised banks of flood plain are called levees.


(3)    The Lower Course or the Delta Stage —
        This is the last stage of the river before it merges into the sea. Its speed now slows down and it becomes quite difficult for it to carry the sediments it had brought with it. So the deposition process begins with a rapid speed. The level of its bed rises and            the river gets divided into many channels called distributaries before it falls into the sea. Thus are formed the deltas of the river which contain deposits of alluvium (or soil) and hence they are very fertile.

 

*   Work of Sea Waves
    The erosion and deposition of the sea waves gives rise to coastal landforms.
    Sea waves continuously strikes at the rocks. Cracks develop overtime they become larger and wider.
    Thus, hollow like caves are formed on the rocks. They are called Sea caves.
    As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches.
    Further, erosion breaks the roof and only walls are left. These walls like features are called stacks.
    The sea rocky coast rising almost vertically above sea water is called sea cliff.
    The sea waves deposit sediments along the shores forming beaches.

 


  

*   Work of Ice
    Glaciers are “rivers” of ice which too erode the landscape by bulldozing soil and stones to expose the solid rock below. 
    Glaciers carve out deep hollows. 
    As the ice melts they get filled up with water and become beautiful lakes in the mountains. 
    The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.

*    Action of Wind
    Wind, too, like ice and water, erodes transports and deposits rock materials. 
    While blowing over land, it lifts and carries fine soil, sand, etc. When the wind slows down, it can no longer carry its entire load, and deposits some of the material it carries. 

 

The action of wind is more prominent in arid and semi-arid regions. This is due to several reasons.

•     In arid regions, the wind is dry and light, and hence blows at a greater speed. This enables it to erode and carry away more material.

•      The lack of trees in these regions means that there are lesser barriers to the flow of air than in areas with trees. This allows winds to flow freely.

•      Dry soil is loose, and in the absence of vegetation to hold it down it is easily carried away by the wind.
      Wind-blown depressions The blowing away of soil often creates depressions in arid and semi-arid regions. When such a depression becomes deep enough to reach the surface of groundwater, an oasis is formed. The Qattara Depression in the Egyptian        desert is an example of an oasis.
     Mushroom rocks Winds carry heavier rock fragments at a lower level. So, when wind hits a mass of rock, it tends to erode the lower part of the rock more than the upper part. This gives the rock a shape resembling a mushroom. Such rocks are called             mushroom rocks.

    Sand dunes When wind laden with sand comes up against an obstacle or its speed drops, it deposits some of the sand it carries. As deposition continues, a small hill of sand, known as a sand dune, is formed. 
    Sand dunes are generally mobile. They move in the direction of the wind.
    When such sand is deposited in large areas, it is called loess.
    Large deposits of loess is found in China.

                                                              “Do you know”
    There are thousands of small waterfalls in the world. The highest waterfall is Angel Falls of Venezuela in South America.

   The other waterfalls are Niagara Falls located on the border between Canada and USA in North America and Victoria falls on the border of Zambia and Zimbabwe in Africa. 

 

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