Download GEOGRAPHY Chap – 7 VOLCANOES STD. 8 Q1. What is a volcano

GEOGRAPHY
Chap – 7 VOLCANOES
STD. 8
Q1. What is a volcano?
Ans. A volcano is a vent or a narrow opening in the earth’s crust connected by a conduit to
an underlying magma chamber. Through this lava, magma, rock fragments, ash, steam gases
etc. come out.
Eg. Mt. Etna (Italy)
Q2. Classify volcanoes on the basis of the frequency and intensity of the eruption.
Ans. On the basis of frequency Active
These are the volcanoes that
erupted lava, ash etc. in the
recent historic period and
are likely to erupt later.
Dormant
Extinct
These volcanoes have
These were active in the
erupted in the past and are
remote geological period and
likely to erupt again after
are not likely to erupt again.
remaining inactive for a fairly
long period.
Dormant means sleeping.
Eg. Mt. Etna, Mt. Stromboli
(Italy), Mauna Loa (Hawaii)
Eg. Mt. Vesuvius (Italy)
Eg. Mt. Kilimanjaro (Africa),
Mt. Aconcagua (South
America).
On the basis of intensity Conical Volcano
Magma and other materials get erupted
through a narrow conduit with great force.
They get accumulated around the point of
eruption.
Shield Volcano
Eruption takes place around along narrow
fissure or crack in the crust.
Large quantities of magma thus erupted
flows over a large area.
They lead to the formation of conical hills as
this kind of lava cannot flow over large areas
They form thick sheets of lava and extensive
lava plateau. This is because lava can spread
over a large area.
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Composite Volcanic Cone
Q3. Write in brief about the two different types of lava.
Ans. Lava can be of two different types –
a) Acid
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Rich in silica and poor in iron & magnesium.
Highly viscous and flow over a short distance.
Erupts with great explosion.
Forms high steep sided cones.
Vertical plug or spine can be formed from solidification of magma in
the pipe or conduit.
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Rich in iron and magnesium and poor in silica.
Lava is very hot and fluid by nature.
Eruptions are quiet
Forms cone or plateau with gentle slopes and broad summits.
b) Basic
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Q4. Classify the landforms of the volcanic regions.
Ans.
Extrusive Landforms
Formed by solidification of lava on the
surface of the land.
Intrusive Landforms
Formed by the solidification of magma
below the ground. They can be exposed to
the surface by the denudation of the
overlying rocks.
The vertical intrusions of magma within the
layers and crevices of rocks are called Dykes.
The horizontal intrusions of magma within
the layers and crevices of rocks are called
Sills.
Typical extrusive landform found is the cone
shaped hill called volcanic cone.
Most volcanoes start as cinder cones and
grow into large volcanic hills with alternating
layers of lava and ash. They are called
composite cones.
Q4. How is a crater formed?
Ans. When there is an explosive volcanic eruption, a portion of the summit gets blown up to
form a depression. This is called a crater. Sometimes lakes are formed on those depressions
called Caldera lakes.
Q5. What is a geyser? How are they different from hot springs?
Ans. Geysers are fountains of superheated steam and hot water that is usually emitted with
an explosion, at intervals triggered off by gases escaping from below.
It is associated with volcanic or thermal regions in which the underground water is heated
beyond boiling point. Eg. Geysers of New Zealand, Iceland and Yellowstone National park of
USA.
Hot springs are also called thermal springs. They are associated with areas where ground
water has sunk and is heated underground by internal forces. Such water usually rises
without any explosion. It is of medical value as certain minerals are dissolved in it.
Eg. Manikaran in the Kulu valley of the Himalayas.
Both geysers and hot springs are of great tourist attraction.
Q6. Name the two belts of the volcanoes of the world.
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Ans. The volcanoes are found mostly in the unstable portions of the Earth’s crust
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