Earthquakes : The tremors felt on the earth’s crust for a few seconds is called earthquake. In earthquakes the surface of the earth moves backward and forward or up and down.
Following are the causes of Earthquakes.
(i) The dislocated parts of the earth move up and down or to the sides and cause earthquakes.
(ii) The internal movements at the time of volcanic activity cause earth tremors.
(iii) The plates in the interior of the earth, move, shake and rub against each other causing the earthquakes.
The movement of interior plates causes most earthquakes in the world.
Seismic Waves : The focus of the earthquake beneath the earth is the source of the seismic waves. The seismic waves from focus travel in all directions. Seismic waves are felt on the surface, just above the focus. This is called epicentre. It is the point on the earth’s surface closed to the focus.
Seismic waves first reach to the epicentre. This area gets biggest shocks of greater intensity. The intensity goes on decreasing as we go away from the epicentre.
An instrument called seismograph is used to record seismic waves. These waves are of three types :
Primary waves : Primary waves reach the ground surface first. The particles in these waves move backward and forward in the direction of the waves.
Secondary waves : After the primary waves reach the secondary waves on the ground surface they are more destructive then the primary waves. The particles in these waves move perpendicular to the direction of the waves.
Surface or long waves : In earthquake waves resembling the ripples are created on ground surface. These are called surface or long waves. They originate at the epicentre and travel along the ground surface. These are the most demaging and destructive waves.
Earthquake Zones
Earthquakes and volcanic eruptions occur on both the coasts of the Pacific Ocean i.e. along the western coast of North and South America and along the eastern coast of Asia. Another zone is the Mediterranean belt. This belt stretches from the Mediterranean region across the Alps, the Himalayas and later merges with the Pacific belt near New Guinea.
Earthquakes are very common in Japan.
The earthquake zones in India : The movement of Indian plate is from south to north. So we feel earthquakes in the Himalayan region. Assam, Arunachal Pradesh, Nagaland, Tripura, Manipur, Mizoran, Andaman and Nicobar Islands, Jammu and Kashmir, the north-western region of Uttar Pradesh, the northern region of Bihar etc. are also prone to earthquakes.
During the last few years, there have been several earthquakes of varying intensities in Maharashtra. The Koyananagar and Killari earthquakes proved quite devastating. A couple of years ago, a devasted earthquake in Gujarat proved fatal for thousands of lives and property worth billions were damaged.
Results of the Earthquakes
1. Earthquakes give rise to cracks on the ground surface.
2. Some places are raised and some are lowered down by earthquakes.
3. Landslides occur.
4. River changes their courses or even disappear.
5. Wells dry up.
6. Buildings and bridges get collapsed.
7. Roads and railways get damaged.
8. People are killed in big earthquakes.
9. Earthquakes under the sea cause huge waves. These waves cause great damage especially in Japan and Western Coast of South America.
Precautions for Earthquake zones
1. One should be away from the high buidlings when earthquake tremors are felt.
2. If you are in a building, run away to the open ground as soon as possible.
3. Inside a building one should stand in a doorframe.
4. All electric lights, should be swtiched off.
5. Gas cylinders, water taps should be closed.
6. It is a natural calamity, victims should be given immediate help.
Volcanic Activity
Ground gets ruptured and magma and molten rocks beneath the ground come out. Sometimes, it cools down beneath the ground surface instead of pouring out. These activities are called volcanic activities.
Volcanic eruption : The eruption of the magma from the ground surface is called a volcanic eruption. After the eruption a loud rumbling noise is created and the surrounding area experience tremors. The ground temperature rises in the area. At the time of eruption, the magma, steam, fragments of rock, dust and gaseous substances come out with great force from beneath the ground through a cone like structure.
The opening of this pipe from the earth’s surface is known as the vent which forms a crater. The lava which goes quite high into the sky during an eruption, falls to the ground as solid fragments. Dark clouds are formed in the sky and it begins to rain heavily. The volcanic ash and dust mixes with the rain water giving rise to hot mud flows.
Types of Volcanic Eruptions : Volcanic activity is of two types depending on the manner of ejection of the magma—1. Central eruption, and 2. Fissure eruption.
1. Central eruption : This type of activity is very explosive, because lava, steam, gas, dust, smoke, stone fragments are ejected in the form of a narrow pipe from beneath the ground surface. The material ejected in this type of eruption give rise to conical or dome-shaped hills.
Some examples of volcanic mountains formed due to central eruption are Mt. Kilimanjaro in Africa, the Fujiyama in Japan and the Vesuvius and Mount Etna in Italy. In India a volcanic eruption took place in 1994, on the Andaman group of islands.
2. Fissure eruption : In this type of eruption a very long fissure develops in the ground surface. The molten rock, rock fragments, steam and gases come out slowly. These eruptions have a very slow speed and lava is more fluid, it spreads over longer distances. The lava cools down on the ground surface over a period of time, increasing the thickness of the ground surface in that area. This type of eruptions give birth to the formations of Basalt plateaus.
In olden times a huge crack developed in the earth’s crust parallel to the western coast of India. From this the lava poured out several times and spread over extensive stretches of land. Basalt rocks were formed from this lava and the Deccan Plateau came into existence. In Brazil such basalt plateaus are also found. South America and Saudi Arabia in West Asia also exist such plateaus.
In Maharashtra, the fertile black soil has been frormed from the basalt rocks.
Shopping Cart