The molten material coming out from inside
the earth through a pipe or vent and
accumulating around the pipe in the form of
a mound, is called a volcano. All volcanoes
result from magma, molten material below the
earth�s surface, which is brought to the
surface as lava or volcanic fragments.
Volcanoes show considerable variation in size,
shape and the kind of material ejected. Most
volcanoes produce lava. Lava is at very high
temperatures, between 8000 and 1,2000
celsius and it contains steam and many other
gases.
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Let us find out how does magma,
originating inside the earth comes to the
surface as lava.
Towards the base of the earth�s crust, hot
rock maintains its solid state because of the
pressure from rock above it. When pressure is
reduced by a crack in the earth or rock that
lies above, the hot rock can change to a liquid
in small chambers known as magma
chambers. Zones of weakness, regions of
lessened pressure or already existing fissures
are perfect paths for the passage of magma.
Gas pressure in pockets in the earth forces the
magma to move. Sometimes as magma moves,
it melts overlying rocks or forces them aside.
Volcanic Features
Extrusion of lava takes place from an opening
called volcanic vent. A saucer shaped
depression is created around the vent, which
is known as crater. On occasion, the explosive
eruption of a volcano blows out an enormous
mass of magma and previously solidified lava
from a considerable depth. Such explosion is
associated with the collapse or subsidence of
the central part of the crater. A steep sided
crater is formed by the destruction of the
crater formed by the gradual construction of
volcanic matter.
This steep-sided
crater is known as caldera. Besides the lava
that flows out from a volcano, rock and
mineral fragments are also blown out from a
volcano known as pyroclastic material. The
pyroclastic material range in size from solid
blocks to fine dust. Spindle or spherical
shaped masses are known as volcanic bombs.
Sometimes fluid or plastic lava falls close to the
vent after being thrown in the air to form
spatter cone. Smaller particles of lava, ranging
from 4 to 25 mm, are called volcanic lapilli
whereas particles under 4 mm constitute
volcanic dust or ash. Volcanic ash, when
thrown up in air, remains in suspension for a
long period of time. It is either drifted away by
wind or falls down with rainfall and is
transported by running water to be deposited
as sedimentary layers called tuff.
Eruptive Styles
Magma may cool enough to solidify before it
reaches the surface of the earth, or it may cool
after it flows down the side of a volcano. If
eruption is violent, lava is hurled high into the
air, with solid rocks. Volcanoes may remain
dormant for thousands of years and then
suddenly erupt when the pressure has built up
beneath the ground. Eruption takes place in two
ways � fissure eruption and central eruption.
Fissure Eruption
The lava or pyroclastic material emanates from
long, narrow fissures or a group of such
fissures. The fluid lava flows away from the
fissures and does not build cones. Deccan
Plateau in India is an example of the fissure lava
eruption that took place about 60-62 million
years ago.
Central Eruption
The central eruptions are much like point
source eruptions, unlike the linear sources of
fissure eruption. The lava or pyroclastic
material erupts from a central vent or pipe and
gives rise to a cone. Viscous lava can barely
flow and produce volcanic domes.
Types of Volcanoes
On the basis of frequency of eruption, there are
three types of volcanoes: active, dormant and
extinct volcanoes. The volcanoes that erupt
frequently as compared to others are called
active volcanoes. The Barren Island of
Andamans in India is an example of active
volcano. The volcanoes that erupt
intermittently are known as dormant or
sleeping volcanoes. The volcanoes that have
not erupted for a very long time and have no
record of eruption in historic times are known
as extinct or dead volcanoes. The dormant
volcanoes may get activated suddenly and are
therefore dangerous. Krakatao in Indonesia
erupted in 1883 (the world�s greatest recorded
explosion).
Global Pattern of Volcanism
Chains of volcanoes are always associated
with places where the crust of the earth is
actively changing. The longest chain
surrounds the Pacific Ocean and is called the
�Ring of Fire�. Another group exists in the area
of the Mediterranean and in the Rift Valley in
eastern Africa. Whole groups and chains of
islands are sometimes formed in the ocean by
volcanoes that erupt time and again. This is
how the Hawaiian and Aleutian Islands were
created. The effects of volcanic activity are not
all bad. The ejected material that comes from
inside the volcano is unbelievably fertile.
Another important product of volcanic activity
is geothermal heat, a source of alternate
power.
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AKHAND PRATAP SINGH
