LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 15 : SOME NATURAL PHENOMENA

EARTHQUAKES:

"An earthquake is a sudden shaking or trembling of the earth which lasts for a very short period."

"Earthquakes can cause immense damage to houses, other buildings, bridges, dams and people etc. A lot of people get killed when they get buried under the debris of collapsed houses and other buildings during an earthquake. Earthquakes can also cause floods, landslides and tsunamis."

Earthquake is a destructive natural phenomenon which can not be predicted in advance. 

STRUCTURE OF EARTH:

The inside of earth is made up of three main layers:  Core, Mantle, Crust. 

Core: The innermost part of the earth is called its core. The core of earth is made up mostly of iron. The core of earth is extremely hot. Most of the earth's core called outer core is liquid (molten iron) whereas the inner part of the core called inner core is under such high pressure that it is solid (solid iron).  

Mantle: The central region of earth (between the core and crust) is called mantle. Mantle is mostly made of dense, solid rocks. Some of the mantle is, however, a mixture of solid rocks and hot molten rocks (liquid rocks) like the lava from a volcano. Heat coming from the core of earth warms the mantle. This heating sets up huge convection currents in the mantle. The giant convection currents occuring in the mantle can make the mantle move very slowly.

Crust:

The outermost layer of earth is called crust. The crust of earth is made of comparatively lighter rocks than that of mantle. The crust of earth is thicker where there is land. The crust of Earth is not in one piece. It is divided into many plates and those are called continental plates and oceanic plates. The plates of earth on which continents exist are called continental plates, whereas those plates of earth on which oceans exist are called oceanic plates. The solid plates which make up the earth's crust are floating on the partially molten rocks of mantle beneath. Due to convection currents taking place in the mantle, the plates of earth's crust are moving around very, very slowly. The reason the earthquakes occur is that the earth's crust is made of a number of plates which are able to move. 

Why do Earthquakes occur:

The Earthquakes occur when the moving plates of the earth's crust:

(i) slide past one another, and

(ii) collide with one another. 

When the entangled rocks of the crust plates break open suddenly with a big jolt, the Earthquake occurs releasing a tremendous amount of energy. 

A tremendous amount of energy is released when two plates collide with eache other. This large amount of energy produces a wave throughout the earth, which is known as shock wave. It is also known as Seismic Waves. 

(iii) Tremors on the earth can also be caused due to the strike of large mateorites to the earth and it can be due to the  explosion of nuclear devices. 

SEISMIC ZONES ( FAULT ZONES )

The weak zones of earth 's crust which are prone to earthquake are called seismic zone.