Chapter 4 : Volcanism
After having learnt this chapter, you should be able to
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Apply the knowledge of the Plate Tectonic Theory to understand volcanism
Cite the locations at which volcanoes are found along with exceptions
List examples of volcanoes found at each location
Explain hot spots
State the shapes in which volcanoes may be found
Give reasons for the different shapes in which volcanoes exist
Describe the formation of the Deccan Plateau
Distinguish between active, dormant and extinct volcanoes
List the locations of active, dormant and extinct volcanoes along with examples
Describe ‘Volcanic Explosivity Index’
Explain how volcanism causes hot springs, geysers and bubbling mud pools
List the constructive and destructive effects of volcanoes
Mark the location of important volcanoes on the tectonic map of the world
Introduction
The process by which magma is produced and erupts on the earth's surface is known as volcanism. It
would be easier to understand volcanism if we examine the locations where volcanoes are found on
the earth's surface. Volcanoes can mainly be found at three locations on the earth.
1. Divergent boundaries: Volcanic activity at the divergent boundaries like the mid-oceanic
ridges is pronounced because the oceanic
crust is thinner so the magma from the
earth’s interior can easily erupt to the
surface. Such volcanoes are usually active
but quiet. The lava erupting out of these
volcanoes is basaltic in nature. The Krafla,
Loki, Hekla and Katla volcanoes in Iceland
are examples of such volcanoes.
2. Convergent boundaries: At convergent
boundaries, friction, combined with the
energy of the earth’s interior melts some of
the rock in the descending plate, forming
magma. The magma, hotter and lighter that
the surrounding rock erupts as volcanoes.
Most volcanoes found at these locations are
violent and erupt without warning. The lava
erupting out of these volcanoes is granitic in
nature. Volcanoes are likely to be found at
convergent boundaries where the overriding
plate is a thinner oceanic plate. E. g. Mt. Fuji in Japan and Karkatoa in Indonesia and Mt. St.
Helens in Washington state, USA. If both the plates are continental (like in the Himalayas),
magma is likely to remain buried deep in the crust, and does not erupt.
S N Kansagra School/Std-9/Dept. of Geography/2009-10
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3. Over 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. If a plate happens to pass
over a hot spot, a continuous chain
of volcanoes can occur on the
Earth’s surface. E.g. the Hawaiian
chain of islands. viz., Mauna Loa,
Mauna Kea, etc.
Check your progress - 1
1. List three differences between volcanoes found at convergent plate boundaries and
those found at divergent plate boundaries.
2. Why are volcanoes not found in the Himalayas, in spite of the fact that they are
located at a convergent plate boundary?
3. What are hot spots?
4. In what way do volcanoes over hot spots support the plate tectonic theory? Support
your answer with an example.
Volcanoes can be classified on two bases - on the basis of their shape, and on the basis of their
activity.
On the basis of their shape, volcanoes can be classified as follows.
1. Conical or Dome 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.
2. 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.
3. Fissure volcano: In this case, lava erupts out of cracks in the earth’s crust which may range
from a few metres in length to a few hundred kilometres in length. The lava is distributed
over an area that may be several thousand square kilometres in extent. Rather than building a
cone, the lava forms flat topped hills in a series of steps.
The Deccan plateau is an example of fissure volcano eruption that happened about 100 million years
ago. It is believed that at that time, India, Africa and Madagascar were close together and that the
Deccan eruption forced them apart. The eruption is believed to have happened because the plates
passed over the Marion and Reunion hot spots. Much of the eruption at that time happened over
India and lasted for about 8-9 million years. The area covered comprises over 200,000 sq mi
(518,000 sq km). At some places, the solidified lava is about 3 kms thick. The Karroo and Ferrar
basalts in Africa and the Columbia River basalts are believed to have been formed at the same time.
Check your progress – 2
5. Why are some volcanoes conical while some in the shape of a shield?
6. Explain the formation of the Deccan plateau.
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Volcanoes can also be classified according to their activity.
1. 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. Active volcanoes are likely to be
found in the following places (according to the order of appearance).
- 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
2. 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. They may
erupt any time and such an eruption is usually very violent. Most of the world’s dormant
volcanoes are found at convergent plate boundaries. At these locations, the sudden shifting of
a plate may cause a dormant volcano to erupt. E.g. the Karkatoa Volcano in Indonesia, Mt.
Fuji in Japan.
3. Extinct volcanoes: As the name suggests, these are volcanoes which have not erupted in
historical times, and will never erupt again. Such volcanoes are usually those which were
created when the plate passed over an active hot spot. When the plate is no longer over the
hot spot, the eruption dies down, the lava cools down, and the volcano does not erupt again.
Whether or not the plate is over the hot spot at present can be determined using heat flow
maps. Examples of such volcanoes are Mt. Kilimanjaro in Tanzania, The Deccan plateau and
the Girnar hill in India.
Volcanic explosivity index
The volcanic explosivity index (VEI) is a scale used to measure the intensity of volcano
eruptions. 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.
Note that in human history, there have been no VEI 8 eruptions. This does not mean that a
VEI 8 eruption has not occurred sometime in the geologic past.
The 1815 eruption of Tambora in Indonesia is the largest volcanic eruption (VEI=7) in
recorded history. It caused massive loss of life (92,000), in which 10,000 people died as a direct
result of the eruption, and an estimated 82,000 died from starvation. In all about 150 km³ were
erupted from the volcano.
Check your progress – 3
7. Distinguish between active, dormant and extinct volcanoes using examples.
8. Briefly describe the scale used to measure volcanic eruption.
Features associated with volcanic eruptions
When water in the cracks in rocks comes in contact with hot rocks near the magma chamber,
it may result in the formation of bubbling mud pools if it eats away the surrounding rock because of
the heat and dissolved gases.
If the surrounding rock is more resistant, hot springs or geysers may result. Water in rock
fractures gradually heats up to near the boiling point. The water down deep is under higher pressure,
so it gets hotter without boiling. The heat is transferred vertically upwards. Finally, water in the
upper part of the fracture system boils, causing the geyser to give a little spurt. That loss of water
reduces the pressure on the water in the rest of the fracture system, and a few more points begin to
boil, causing more spurts. This reduces the pressure further and the whole thing boils and causes a
full-scale geyser eruption.
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A hot spring may result if there is a greater water column above the point at which water
begins to boil. The water at the surface then is not pushed out but being hot (not boiling), flows as a
hot spring. That means if either the rocks aren’t so hot or the magma chamber is deep, hot springs
may result.
Influence of volcanic eruptions on man and environment.
Destructive
• Loss of life
• Damage to property
Constructive
• Fertile Soil formed as a result of weathering of volcanic rocks
• Source of precious stones
• Hot springs are of medicinal and therapeutic value
• Geo thermal energy
Check your progress – 4
9. How does a geyser erupt?
10. In what way are bubbling mud pools different from hot springs?
11. List the constructive effects of volcanic eruptions.
S N Kansagra School/Std-9/Dept. of Geography/2009-10
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