Chapter 4: NATURAL CATASTROPHIC EVENTS
types of natural calamities are
causes of volcanism aren’t random. The areas of high volcanic
activity are of ocean - ocean convergence and ocean continent
the regions is the Pacific ring of fire located in the
pacific ocean and consists of border areas of China,
Philippines, Japan, rocky mountains, Mexico , Andes mountains,
Indonesia, new Zealand, Tanzania.
reason for volcanoes is the ocean - ocean divergence
which creates peaceful basaltic eruptions as mid ocean
ridges are formed.
reason for volcanic eruptions is breaking up of plates
into blocks. The andesitic eruptions are seen in this
Intrusive land - forms:
magma which couldn’t come out
intrusive granite rock
shaped like waves
Fig 1: intrusive
cracks in the continental crust leads to basaltic flows which
spread across land creating lava plateau. E.g. Deccan plateau.
Dome volcano: The magma reservoir
creates volcanic islands. These create peaceful basaltic flow
with gentle flow. E.g.: Hawaii.
ocean continent plates collide and then viscous lava explodes
eruptions led to new layers of ash or lava.
Fig 2: Volcano types
hot water. Ground water heated by shallow source of magma. They
have chambers in interior and water comes out like fountains due
to pressure. Silica is dissolved in water.
Springs: The groundwater stored in rocks comes out of outlets called springs where the water table reaches the surface. The water may seep out gradually out of rocks or gush out as a fountain. Wells are man made for obtaining ground water by digging a hole in the ground till the water table is reached.
Water reaches interior and there
it is heated. Water is heated by magma or hot rocks. It is
common and is present in many places of the world. Cyano
bacteria are present in this water.
Fig 3: Hot springs vs geysers
Drought is a continuous and lengthy period during which no significant rainfall is recorded. In
India, the Meteorological Department of India defined drought as a period of at least 22 consecutive
days on none of which is there more than 0.25 mm (0.01 inch) of rainfall. However this definition doesn't apply to the whole country.
In general, the areas recording less than 60 cm of rainfall annually
and in which the variability of rainfall is more than 20 per cent are the drought prone areas in
Areas where the variability of rainfall varies between 20 to 60 per cent (like Rajasthan, west
of Aravallis, and east of the Western Ghats) are the chronic drought prone areas.
Ministry of Agriculture and the Ministry of Environment, a drought prone
area is defined as one in which the probability of a drought year is greater than 20 per cent. A
chronic drought prone area is one in which the probability of a drought year is greater than 40 per
cent. A drought year occurs when less than 75 per cent of the average annual rainfall is recorded.
prone areas of India are:
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