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Old Friday, November 18, 2011
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Roshan wadhwani Roshan wadhwani is offline
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Earthquake:


Earthquakes are those movements of the earth crust which make the ground vibrated and shake backwards and forwards or in simple words an earthquake is trembling in the earth.

The shocks waves are generated at a point within the crust called the focus, and the point on the earth’s surface vertically above the focus is called the epicentre of the earthquake. The shock waves travel in all directions from the focus. On the earth’s surface, the shaking is the strongest near the epicentre. These waves are detected by seismograph.

Occurrence Of Earthquake:
i) Earthquakes occur when rocks subjected to great stress suddenly break, releasing the accumulated energy, which shakes the ground. Vibrations spread out from the epicentre like ripples in water.
ii) It may also be caused by movements of the plates, resulting from convection currents in the hot mantle of the earth.
iii) Earthquakes are also associated with volcanic activity-eruption of magma. Collapse of mines can also produce small earthquakes.

Volcanoes:

An opening in the earth's crust through which molten lava, ash, and gases are ejected.

Occurrence Of Volcanoes:
i) Rocks below the Earth have a very high temperature.
ii) The great pressure upon these keeps them in a semi-solid state.
iii) If the pressure weakens, then some of rocks become liquid.
iv) This liquid is called magma.
v) The magma forces its way into cracks of the crust and may either reach the surface of the earth where it forms lava and flow out.

Types Of Volcanoes On The Basis Of Activity:
There are three types of volcanoes on the basis of volcanic activity, which are as under.

i) Active Volcanoes:
Volcanoes are said to be active when they frequently erupt or at least when they have erupted within recent time.

ii) Dormant Volcanoes:
The volcanoes that have been known to erupt and show signs of possible eruption in the future are describes as dormant volcanoes.

iii) Extinct Volcanoes:
The volcanoes that have not erupted at all in historic times but retain the features of volcanoes are termed as extinct volcanoes.

Causes of The volcanoes Eruptions:
i) Seafloor spreading
ii) Convergence of lithospheric plates
iii) Percolation of cold water
iv) Orogenic Movements
v) High temperature in the interior of the Earth
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