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Chapter 4
1. List three differences between volcanoes found at convergent plate boundaries and
those found at divergent plate boundaries.
Source of
magma
Intensity of
erution
Type of magma
Volcano at Divergent
Boundary
Volcano at Convergent
Boundary
Mantle
Rubbing of plates
Quiet
Violent
Basaltic
Granitic
Why are volcanoes not found in the Himalayas, in spite of the fact that they are
located at a convergent plate boundary?
2.
The Himalayas are located at a continent-continent convergent boundary. In such a case
the magma produced due to the rubbing of plates remains buried deep in the crust and
does not erupt as a continental plate is thicker.
3.
What are hot spots?
A hot spot is an area of great heat in the mantle. Such unusual conditions of heat are
thought to have been created due to concentration of radioactive elements.
4.
In what way do volcanoes over hot spots support the plate tectonic theory?
If a plate happens to pass over a hot spot, a continuous chain of volcanoes can occur on
the Earth’s surface. The leading volcano is the quietest and coolest and the most active
volcano is last in the chain. in this manner, the direction of plate movement can be
determined.
5.
Why are some volcanoes conical while some in the shape of a shield?
Conical volcanoes : If the lava erupting from a volcano is thick and viscous, it is
generally confined near the vent and does not travel far. The resultant shape is a conical
volcano. E.g. Mt. Fuji in Japan.
Shield volcano : Conversely, if the lava erupting from a volcano is thin and fluid, it is not
confined near the vent travels long distances. As a result, the volcano does not grow in
height, but grows in lateral extent. This is how a shield volcano is formed. E. g. Mauna
Loa, Hawaii.
6.
Explain the formation of the Deccan plateau.
When the Afro-Indian plate passed over the Marion and the Reunion hot spots about 100
mllion years ago, there was a colossal volcanic eruption over the Indian plate which
lasted for about 10 million years. The lava which spread over 5 lakh square kilometres
eventually cooled down and formed the Deccan Plateau.
7.
Distinguish between active, dormant and extinct volcanoes using examples.
Active volcanoes: Active volcanoes are those volcanoes which are either in a constant
state of eruption or which erupt at least once in every ten years. E. g. Krafla, Loki, Hekla
and Katla volcanoes in Iceland
Dormant volcanoes: Volcanoes which have not erupted in recent times or in history but
which are capable of eruption are classified as dormant or sleeping volcanoes. E.g. the
Karkatoa Volcano in Indonesia, Mt. St. Helens in USA.
Extinct volcanoes: As the name suggests, these are volcanoes which have not erupted in
historical times, and will never erupt again. Capulin Volcano, USA, Mt. Girnar, India
8.
Briefly describe the scale used to measure volcanic eruptions.
The volcanic explosivity index (VEI) is a scale used to measure the intensity of volcano
eruptions. It has divisions form 1 to 8. It is similar to the Richter and Mercalli scales,
which measure the intensity of earthquakes. The VEI measures explosivity, volume of
ash and the height ash reaches into the atmosphere. Additionally, it gives a description of
the eruption, a name for the eruption, and the total number of those eruptions in human
history.
9.
How does a geyser erupt?
The water deep down in fractures of resistant rocks is under higher pressure, so it doesn’t
boil. Instead, the heat is transferred vertically upwards. Finally, water in the upper part of
the fracture system boils, causing a little spurt of water. That loss of water reduces the
pressure and the whole thing boils and erupts as steam causing a full-scale geyser
eruption.
10.
In what way are bubbling mud pools different from hot springs?
In bubbling mud pools, the water which comes from underground is highly contaminated
with rock matter, mud and gases. This is because the hot water dissolves and eats away
into the relatively softer rock holding water. In a hot spring, the water is relatively fresher
since it is held in cracks of resistant rocks.
11.
List the constructive effects of volcanic eruptions.


with lava


Fertile Soil formed as a result of weathering of volcanic rocks
Precious stones such as diamonds are brought to the earth’s surface along
Hot springs are of medicinal and therapeutic value
Geysers can be used to avail geo thermal energy
Extra questions
5. Give an example of a volcano formed at a divergent plate boundary, convergent plate
boundary and over a hot spot.
Divergent plate boundary: Krafla and Loki volcanoes in Iceland
Convergent boundary: Mt. Fuji, Krakatoa
Over a hot spot: Mauna Loa in Hawaii
6. What is a hot spot? What is the belief behind its formation?
A hot spot is an area of great heat in the mantle. Such unusual conditions of heat
are thought to have been created due to concentration of radioactive elements in
the centre of eddy currents or whirlpools.
9. List the locations and examples where you findi) Active volcanoes ii) Dormant
volcanoes iii) Extinct Volcanoes. Which of the three is most dangerous? Why?
Active volcanoes :
- Divergent boundaries. E. g. Krafla, Loki, Hekla and Katla volcanoes in Iceland.
- Over Hot Spots. E. g. Mauna Loa, Mauna Kea and Kohala volcanoes in Hawaii.
- Convergent boundaries. E. g. Mt. St. Helens in California, Stromboli in Sicily.
Dormant volcanoes : Most of the world’s dormant volcanoes are found at
convergent plate boundaries. E.g. the Karkatoa Volcano in Indonesia, Mt. Fuji in
Japan.
Extinct volcanoes : Such volcanoes are usually those which were created when
the plate passed over an active hot spot. Examples of such volcanoes are Mt.
Kilimanjaro in Tanzania, The Deccan plateau and the Girnar hill in India.
Dormant volcanoes are most dangerous because they erupt because of the
pressure release associated with the sudden shifting of plates after hundreds of
years. As a result, they erupt violently without warning causing widespread
destruction.
11. How is a geyser formed? Which other features are associated with volcanic regions?
Water in rock fractures gradually heats up due to the heat of the magma. The
water down deep is under higher pressure, so it gets hotter without boiling. The
heat is transferred vertically upwards. When water in the upper part of the
fracture system boils, it pushes the water column above it. That loss of water
reduces the pressure on the water in the rest of the fracture system, and the
whole thing boils and causes a full-scale geyser eruption in the form of steam.
Other features associated with volcanoes are bubbling mud pools and hot
springs.
12. List the constructive and destructive effects of volcanoes on man and the
environment.
Destructive : • Loss of life
• Damage to property
Constructive: • Fertile Soil
• Source of precious stones
• Hot springs • Geo thermal energy