Volcano

1. VOLCANO
Volcanoes destroy and volcanoes create. Yet, over a time span longer than human
memory and record, volcanoes have played a key role in forming and modifying the
planet upon which we live. More than 80 percent of the Earth's surface--above and below
sea level--is of volcanic origin.
The word volcano comes from the little island "Vulcano" in the Mediterranean Sea off
Sicily. Centuries ago, the people living in this area believed that Vulcano was the
chimney of the forge of Vulcan--the blacksmith of the Roman gods. The term volcano is
derived from a Roman deity of fire, Vulcan.
A volcano is most commonly a conical hill or mountain built around a vent that connects
with reservoirs of molten rock below the surface of the Earth. The term volcano also
refers to the opening or vent through which the molten rock and associated gases are
expelled. A volcano is essentially a fissure or vent, communicating with the interior, from
which flows of lava, explosive bursts of gases and volcanic ashes are erupted at the
surface. The fragmental materials produced during volcanic eruptions are known as
pyroclasts.
The parental material of the hot volcano products is called as magma, a Greek term
meaning any kneaded mixture. Magma in geological sense means any hot mobile
material within the earth, which is capable of penetrating into or through rocks. Magma
however, is not merely a molten rock, but when confined under pressure it is commonly
associated with gasses and vapours. Magma contains predominantly silicate melts
saturated with gasses dissolved in them.
The formation of magmatic sources is a continuous process. They are accumulated in the
upper part of the mantle. They are then ascended to the upper horizons of the earth's
crust. The movement of magma to the surface is due to hydrostatic pressure and also due
to the considerable increases in the volume as the solid rock melts. When the magma
penetrates and breaks the whole mass of the earth’s crust it is called as effusive
magamatism or volcanism.
However, if the magma solidifies within the earth at a depth it is called as intrusive
magamatism. The magma when it emerges out of earth’s crust it is called as lava and the
process is called volcanic eruption.
Magma is of two types acidic (Granitic) and basaltic (basic). The igneous rocks of the
Globe are formed chiefly by these two types. Rocks of other composition are mixtures of
granitic and basaltic magma.
Typical Volcano
A typical volcano has a vertical pipe. A
communication between surface of the earth and
the interior mass of magma or lava. It is
generally conical in shape.
It is the passage for lava hot gasses and
fragments of rocks when the volcano erupts. The
mouth of the volcano is called as "crater". It is
funnel shaped. When the crater is wide it is
called "Caldera", and it is formed due to violent
explosion. Resulting in the blowing up of the
summit of the volcanic cone or due to

2. subsidence or collapse. The mountain shape of a volcano is due to the accumulation of
lava and other material that come out of it. Lava on its way up through the fissures
sometimes cut across the country rocks as dykes instead of taking the usual central path.
These are also called discordant intrusions.
Sometimes these dykes penetrate through earth and eruption takes place forming
volcanic cones. These are called as secondary craters or volcanoes. Lava sometimes
spread laterally into the country rocks in the form of sills. Especially in sedimentary
rocks along the bedding planes. These are also called as concordant intrusions.
Products of Volcanoes:
The material which are thrown out or erupted by a volcano consist generally of all the
three phases of the matter i.e., solid, liquid and gases.
 Solid materials (Pyroclasts): These are solid materials thrown out by the pressure of
the rising lava generally of country rocks. Pyroclasts consists of fragments of
different sizes. The biggest fragments formed as "Volcanic blocks" (>32mm)
accumulates near the crater. Where as those with a size between walnut and pea
known as cinders or lapilli’s are thrown further away. The finest particles are called
as volcanic dust, is blown away still greater distances, which after consolidation
forms volcanic tuff. Solidified or semi solidified clots of lava, which are also thrown
along with the other materials, are called as volcanic bombs.
 Liquid materials (Lava): The bulk of the material erupted from volcanoes in the
form of an excessively hot and mobile liquid is called as lava. This lava is charged
with gasses, which escapes when it erupts on the earth surface and produce cavities in
the rock formed after cooling and consolidation.
 Gases: Gases are generally the first to reach the surface and acquire great heights
over a volcano. The most dominant gas is steam; carbon dioxide, sulphur dioxide and
carbon monoxide; nitrogen and hydrogen are other chief gases. Gasses play very
important role in the volcanic eruptions in that they impart to the magma sufficient
mobility and also supply pressure necessary for the ascent in the volcanic column.
Classification:
The classification of volcanoes is mainly based on the character of their eruption and on
the structure of volcanic systems. There are three types.
1. The effusive process: Which is characterised by flowing out of lava and it’s
spreading over the earth surface. In the effusive process, effusion is the dominant
process or the flowing out of lava on the surface in the form of streams along the
slopes eg., Hawaiian type.
2. The explosive process: Which finds its eruption in a blast and an injection of a large
amount of pyroclastic material (solid products of eruption).
3. The extrusive process: Characterised by the squeezing out or the extrusion of the
magmatic substances, to the surface in a liquid or solid state.
Some times these processes are mixed e.g., explosive, effusive, extrusive- depending
upon its character of eruption, volcanic structures and forms are found.

3. Types of Volcanoes:
Geologists generally group volcanoes into four main kinds--cinder cones, composite
volcanoes, shield volcanoes, and lava domes.
i. Cinder cone: Cinder cones are the simplest type of volcano. They are built
from particles and blobs of lava ejected from a single vent. As the gas-charged
lava is blown violently into the air, it breaks into small fragments that solidify
and fall as cinders around the vent to form a circular or oval cone. Most cinder
cones have a bowl-shaped crater at the summit and rarely rise more than a
1000 feet or so above their surroundings.
ii. The composite cones or stratovolcanoes: Some of the Earth's grandest
mountains are composite volcanoes- sometimes called stratovolcanoes. They
are typically steep-sided, symmetrical cones of large dimension built of
alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and
may rise as much as 8,000 feet above their bases.
The essential feature of a composite volcano is a conduit system through
which magma from a reservoir deep in the Earth's crust rises to the surface.
The volcano is built up by the accumulation of material erupted through the
conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.
Most composite volcanoes have a crater at the summit, which contains a
central vent or a clustered group of vents. Lavas either flow through breaks in
the crater wall or issue from fissures on the flanks of the cone. Lava, solidified
within the fissures, forms dikes that act as ribs which greatly strengthen the
cone.
iii. Shield volcanoes: Shield volcanoes, are built almost entirely of fluid lava
flows. Flow after flow pours out in all directions from a central summit vent,
or group of vents, building a broad, gently sloping cone of flat, domical shape,
with a profile much like that of a warrior's shield. They are built up slowly by
the accretion of thousands of highly fluid lava flows called basalt lava that
spread widely over great distances, and then cool as thin, gently dipping
sheets. Lavas also commonly erupt from vents along fractures that develop on
the flanks of the cone.
iv. Lava Domes: Volcanic or lava domes are formed by relatively small, bulbous
masses of lava too viscous to flow any great distance; consequently, on
extrusion, the lava piles over and around its vent. A dome grows largely by
expansion from within. As it grows its outer surface cools and hardens, then
shatters, spilling loose fragments down its sides. Some domes form craggy
knobs or spines over the volcanic vent, whereas others form short, steep-sided
lava flows known as "coulees." Volcanic domes commonly occur within the
craters or on the flanks of large composite volcanoes.
Types of eruptions:
Fissure eruption: These are eruptions through a number
of weak zones or linear fissures distributed around the
volcanic region and having no cones or craters like central
eruptions. Deccan plateau of India is regarded as due to
fissure eruption.
Fumaroles: These are cracks or fissures in the earth’s crust through which only hot gases
come out regularly or intermittently. Generally, such gaseous eruptions precede to the
volcanic eruptions.
Magma
chamber

4. Kinds of Volcano:
Depending upon the time interval between two volcanic eruptions the volcanoes are
divided in the following 3 kinds-
1. Active Volcano: is one, from which a volcanic eruption can be erupted at any
time. The eruption may take place after a few years or tens of years.
2. Dormant Volcano: is one, which erupts with a period of rest that could be a few
years, or several years.
3. Extinct Volcano: is one, where the activity of the volcano seems to have ceased
altogether.
Types of volcanic eruptions:
During an episode of volcanic activity, a volcano commonly displays a distinctive pattern
of behavior. Some mild eruptions merely discharge steam and other gases, whereas other
eruptions quietly extrude quantities of lava. The most spectacular eruptions consist of
violent explosions that blast great clouds of gas-laden debris into the atmosphere.
The type of volcanic eruption is often labeled with the name of a well-known volcano
where characteristic behavior is similar- hence the use of such terms as "Strombolian,"
"Vulcanian," "Vesuvian," "Pelean," "Hawaiian," etc. Some volcanoes may exhibit only
one characteristic type of eruption during an interval of activity--others may display an
entire sequence of types.
 Hawaiian Type: The lava in this type is basaltic (basic). It is easily mobile and
has a high temperature (1200 C). In the crater the lava is all the time in a state of
boiling when it rises its mobility increases and spreads all over the crater forming
large boiling lava lake. The gasses are expelled without bursts.
 Strombolian Type: In a Strombolian-type eruption, huge clots of molten lava
burst from the summit crater to form luminous arcs through the sky. Collecting on
the flanks of the cone, lava clots combined to stream down the slopes in fiery
rivulets. It is similar to Hawaiian except that the somewhat more viscous lava is
ejected upward as fountains from a lava lake in the crater at regular, rhythmic
intervals of about several minutes to hours. The continued source of heat for
maintaining the liquid form of lava seems to be bubbles of burning gas that rise
freely through the melt from several vents.
 Volcanic type: in this type of eruption, a dense cloud of ash-laden gas explodes
from the crater and rises high above the peak. Steaming ash forms a whitish cloud
near the upper level of the cone.
 Vesuvian Type: this is similar to volcanic type but it is typified by the eruption of
Mount Vesuvius in Italy. Here, great quantities of ash-laden gas are violently
discharged to form cauliflower-shaped cloud high above the volcano.
 Explosive type: This is the category, which comprises a large number of
volcanoes in which the greatest significance is with the large gaseous - explosive
processes involving mass ejecting of the solid products of eruption and practically
no out flow of lava.
 The Pelean type: In a "Peléan" or "Nuée Ardente (glowing cloud) eruption, a
large quantity of gas, dust, ash, and incandescent lava fragments are blown out of
a central crater, fall back, and form tongue-like, glowing avalanches that move
downslope at velocities as great as 100 miles per hour. Such eruptive activity can
cause great destruction and loss of life if it occurs in populated areas. The activity

5. is violently explosive, as a result of very viscous gas rich acidic lava plugging the
vent violently over the crater rim or breaking out laterally.
 Phreatic Type (or steam-blast): Here eruptions are driven by explosive
expanding steam resulting from cold ground or surface water coming into contact
with hot rock or magma. The distinguishing feature of phreatic explosions is that
they only blast out fragments of preexisting solid rock from the volcanic conduit;
no new magma is erupted. Phreatic activity is generally weak, but can be quite
violent in some cases.
 Plinian Type: The most powerful eruptions are called "plinian" and involve the
explosive ejection of relatively viscous lava. Large plinian eruptions can send ash
and volcanic gas tens of miles into the air. The resulting ash fallout can affect
large areas hundreds of miles downwind. Fast-moving deadly pyroclastic flows
("nuées ardentes") are also commonly associated with plinian eruptions.
Effects of Volcano:
Whenever the volcanic eruption takes place the places around or the area under influence
of volcano will be burnt and inflamed, leading to mass destruction of life and property.
Flooding of volcanic mud and deposition of ash around the volcano, which causes dust
pollution, contamination of lakes or water bodies, suffocation and introduction of harmful
gases in the atmosphere leading to death of people and loss of property. Some times
earthquakes are triggered due to volcanic eruptions and that can devastate the land
surface and human settlements.
Distribution:
A great majority of major and minor volcanoes of the world appear to be concentrated
along two main belts. The Circum Pacific Belt and Alpine - Himalayan Belt. The first
belt is well defined and covers many parts of America, Japan, East Indies, and New
Zealand etc., the belts are not continuous.