On 12th March, 2011 Krishna saw the pictures of a tsunami in the newspaper. After reaching school, she asked her class teacher, “Sir, I read the news about tsunami in the newspaper. How does a tsunami occur?” Sir replied, “Natural disasters are of two types. Disasters like earthquake, tsunami and volcano occur due to the changes in the interiors of the Earth, whereas disasters like floods, cyclone, forest fires, drought, etc. occur due to the changes on the surface of the Earth.”


The weak upper crust of Earth suddenly experiences tremors either due to collision of plates or due to the tremendous pressure in the interior of the earth. Waves/tremors of earthquake originate from its hypocenter which is deep down below the surface of the earth. Such sudden shaking or trembling is known as an ‘earthquake’. Earthquake waves are created from the epicenter during the process of an earthquake. Earthquake occurs due to internal disturbance in the earth.

(I) Earthquake caused by volcanic eruption
(II) Earthquake caused by Strike-slip faults
(III) Earthquake caused by Subduction in an active subduction zone.


Volcano refers to an opening in the Earth’s crust from which motion lava, rock fragments, ashes, dust and gases are ejected form below the Earth surface.

1] Dormant volcano
2] Active volcano

Volcanic eruption occurs due to four main reasons: 

(1) High temperature in the interior of earth 
(2) Creation of magma 
(3) Creation of gases and vapour 
(4) Rising of magma


Due to volcanic eruption or earthquake on the sea floor, high oceanic waves are created. They are known as tsunami. They are high and have unimaginable length. Their length is approximately 700 to 1600 kms. These waves travel with a great speed and force. By the time they reach the coast, they attain great height and power which causes a great deal of destruction in the coastal regions. It was witnessed on 11th March, 2011 in Japan. Strong and whirling winds also create big and strong waves. Once the storm gets over,
these waves become tall with a high wavelength which can travel great distances. Such waves cause a great deal of destruction in the coastal regions. Hence they are known as the destructive waves. At times, rock falls, ice-falls or underwater 

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