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1 NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | [email protected] Volcanism The volcano is a narrow opening in the earth’s crust through which the molten rock material, magma (lava), volcanic ashes are emitted outward through an eruption. Such types of openings (vents) are found in those parts of the crust where the rock strata are relatively weaker than the surrounding areas. Volcanism Volcanism refers to an exogenous activity which includes the formation of magma, its upward movement, ejection of magma (lava) on the earth’s surface, and its cooling and solidification. volcano diagram 1 – Structure of Volcano Magma refers to the molten rock material present inside the earth in the asthenosphere. When this molten material comes out to the Earth’s surface through an opening of a volcano, it is known as the Lava. The process through
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Volcanism
The volcano is a narrow opening in the earth’s crust through which the molten
rock material, magma (lava), volcanic ashes are emitted outward through an
eruption. Such types of openings (vents) are found in those parts of the crust
where the rock strata are relatively weaker than the surrounding areas.
Volcanism
Volcanism refers to an exogenous activity which includes the formation of
magma, its upward movement, ejection of magma (lava) on the earth’s surface,
and its cooling and solidification.
volcano diagram 1 – Structure of Volcano
Magma refers to the molten rock material present inside the earth in the
asthenosphere. When this molten material comes out to the Earth’s surface
through an opening of a volcano, it is known as the Lava. The process through
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which this molten material or magma comes out from asthenosphere to the
Earth’s surface is known as volcanism.
Types of volcanic eruptions
Ejection of Lava to the surface occurs through either Fissure eruption or through
Central eruption.
Fissure eruption
In Fissure volcanic eruptions, the Lava comes out to the surface through the
cracks of the rock strata and hence the fissure eruptions are not much explosive.
The fissure eruptions are smooth and the Lava spreads to a larger area, so they
form landscapes such as plateaus etc.
volcano diagram 2 – Lava flow
Central eruption
In the central volcanic eruptions, the lava comes out to the surface through
narrow pipes and thus causes an explosion, during the ejection of magma onto
the surface. The explosive nature of eruptions leads to the formation of
mountains which are known as volcanic mountains. The different volcanic Islands
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throughout the world are actually volcanic mountains formed through Central
eruptions.
Causes of volcanic eruptions
In the interior of the earth, the radioactive substances undergo chemical
reactions which generate a large amount of heat. Apart from this, some amount
of residual heat which was captured at the centre of the earth during its
formation is also present. This leads to a creation of large temperature
difference between the inner and Outer layers of earth.
This huge temperature difference leads to the formation of convection currents
in the outer Core and the Mantle. Due to this, the molten magma along with
the gaseous materials comes out to the earth’s surface at the first available
opportunity. This mainly occurs in the weak zones of earth surface such as
divergent plate boundaries, and convergent plate boundaries etc.
Sometimes, the earthquakes may expose the fault zones in the rock strata
through which the Magma can escape to the earth’s surface leading to volcanic
eruptions.
Types of Lava in volcanism
Acidic or Andesitic or composite Lava
The acidic or composite Lava is highly viscous and has a high melting point. It
has a high percentage of silica content, low density and light colour.
The acidic lava flows slowly and they rarely travel far before solidification. This
leads to the formation of the cone-like structure having steep sides.
Due to the rapid solidification of this acidic Lava, the openings obstruct the flow
of new Lava, which results in loud explosions and pyroclasts (volcanic bombs).
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volcano diagram 3 – Cone-like Structure
Basic or Shield or Basaltic Lava
The basaltic or basic lava is highly fluid, and their temperature is about 1,000°C.
Basaltic lava is poor in silica, but are rich in Iron and manganese.
They have a dark colour and high fluidity. Due to their high fluidity, the basaltic
Lava is not very explosive, and they spread over great distances as thin sheets of
Lava.
The volcano formed by Basic Lava is gently sloping and they form a flattened
shield or dome with a wide diameter.
Active, dormant and Extinct volcanoes
The volcanoes erupting fairly frequently are known as active volcanoes. Kilauea
volcano of Hawaii, Grímsvötn volcano of Iceland and Etna volcano in Italy etc
are examples of active volcanoes which have been volcanoes erupting in the
recent past.
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Those volcanoes in which the eruption has not taken place regularly in the
recent past are known as dormant volcanoes. The volcanoes erupting after
undergoing long intervals of repose are the dormant volcanoes. The Fujiyama
volcano of Japan, Krakatoa volcano of Indonesia and the Narcondam island
volcano of Andaman and Nicobar islands are the examples of dormant
volcanoes.
Extinct volcanoes are those where the volcanic eruption had taken place in
historic times but they are not active today. Mount Chimborazo in Ecuador,
Mount Kenya in Eastern Africa, and Popa in Myanmar etc. are the examples of
extinct volcanoes.
Negative effects of volcanic eruptions
Volcanic eruptions are a highly damaging natural disaster and are highly
destructive in nature. volcanic eruptions have been responsible for the
destruction of whole cities and towns by the advancing lava.
Violent earthquakes are associated with volcanic eruptions which have often
caused damage to life and property. The mudflows of volcanic ashes which get
saturated by rainfall can bury the nearby areas.
The earthquakes are associated with volcanism and in coastal areas, they can
cause tsunamis which have often caused the large destruction of life and
property.
Different gases released from volcanic eruptions such as carbon dioxide,
hydrogen fluoride, Sulphur Dioxide etc are hazardous to human life and
environment. The volcanic gases such as Sulphur Dioxide etc have also been
responsible for causing acid rain.
Large volcanic eruptions inject a large number of Sulphur aerosols in the
stratosphere which can lead to the lowering of surface temperature and
increase in the depletion of Ozone layer. The release of SO2 from volcanic
eruptions has been responsible for lowering of earth’s temperature leading to
crop failures and famines.
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Positive impacts of the volcanic eruption
Volcanic eruptions are responsible for the formation of new landforms such as
islands, plateaus, Volcanic Islands, and mountains etc. The volcanic lava, ash
and dust are very fertile for the cultivation of different plants. The weathering
of Volcanic rocks leads to the formation of fertile soil.
volcanic eruptions are also the source of mineral resources. They bring useful
and important minerals resources to the surface of Earth. For example, the
diamond mines of the kimberlite rocks of South Africa, are actually the part of
an ancient volcano.
The areas surrounded by the active volcanoes give rise to the formation of
springs and geysers. These springs and geysers can even be used for the
generation of geothermal electricity. The Yellowstone National Park of USA
generates electricity from the geothermal electricity. The Puga Valley of Ladakh
in India is also a promising spot for geothermal electricity.
The landforms formed by the volcanic eruptions are also great tourist spots and
have a great natural beauty. For example, the Yellowstone National Park of USA
is a great tourist spot.
Apart from these, Volcanic rocks are also used as raw materials for various
building and Engineering purposes etc.
Hot springs and Geysers
When the underground water which percolates down through the porous rocks
is subjected to the heat of the underlying Rock Strata which is in the contact of
hot magma it gives rise to geysers and hot springs.
When the water comes in contact with the intense heat of these rocks, it gets
heated and rises in the form of capillaries and narrow roots through the porous
rocks. When this heated water comes to the surface it undergoes expansion
and gets converted into steam leading to the formation of geysers and Springs.
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Diagram- Geyser
Geysers: when the heated water at high pressure comes out of the surface and
bursts into steam, it is known as Geysers. In most of the cases, a carter like
structure is formed at its mouth.
Springs: When the hot water comes out to the surface in a smooth manner it is
known as a spring.
Most of the world’s geysers are found in the areas of Iceland, New Zealand and
the Yellowstone National Park of USA. The hot springs and geysers of Japan and
Hawaii are great tourist attractions.
Geysers are found in very few regions, while the hot water springs are found all
over the world.
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volcano diagram – Volcanic_hot_spring
Distribution of the volcanoes around the world
Till now, around 480 major active volcanoes have been found out of which
around 400 are found in the areas around the Pacific ocean. While the others
are in the Alpine Himalayan belt, Atlantic Ocean, Indian ocean etc. The
Himalayas do not have an active volcano.
The converging plate margins and the mid-oceanic ridges are the areas of high
volcanic activity and earthquakes. The volcanic zones and earthquake zones are
more prominent around the converging plate boundaries.
Pacific Ring of Fire
The circum Pacific region or Pacific Ring of Fire has the largest concentration of
active volcanoes. It has almost two-thirds of active volcanoes.
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The Aleutian islands of Kamchatka, Japan, the areas of Philippines, Indonesia,
Islands of Solomon, Tonga and North Island, New Zealand, the Andes to Central
America and up to Alaska are the part of Pacific rim of fire.
volcano diagram – volcanoes in pacific rim
volcanoes along the Atlantic coast
The Atlantic coast has a comparatively fewer number of active volcanoes. But it
has many dormant volcanoes such as Saint Helena, Cape Verde islands etc. The
volcanoes of Iceland and Azores along the Atlantic coast are active volcanoes.
Volcanoes in the Mediterranean region
The Alpine folds, such as Vesuvius, stromboli (also known as the Lighthouse of
Mediterranean) and the Aegean Islands are the areas of the Mediterranean
region where active volcanoes are found.
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Volcanoes in the great rift region
Mount Kilimanjaro and Mount Kenya of the East African Rift Valley have some
extinct volcanoes. Mount Cameroon is the only volcano active in West Africa.
Volcanoes in other parts of the world
Other regions such as West Indian Islands have experienced some volcanic activity
in the recent past. Mount Pelee of the Lesser Antilles is a volcanic Island where
the last eruption took place in 1929.
Volcanoes in India
The Barren Island of Andaman and Nicobar Islands which is in the northeast of
Port Blair is a volcanic island. The Barren Island volcano was the last active
recently in 2017 and in 1991 and 1995.
Narcondam which is in the north-east of Barren Island is another volcanic Island
in India. Narcondam volcano has not been active in the recent past. Other parts
of India do not have an active volcano.
Distribution of earthquakes
The distribution of earthquakes in the world coincides closely with that of
volcanoes. The Circum Pacific area, along with the Pacific Ring of Fire, is the
region of greatest systemic activity with most frequent occurrences of
earthquakes. These areas also have the most number of volcanic activity and
Volcanic Islands. Around 70% of world’s total earthquakes take place in the
Circum Pacific belt.
Around 20% of earthquakes take place in the Mediterranean Himalayan belt
which includes the Asia Minor, the Himalayas and the parts of northwestern
China. The earth’s crust is relatively stable in other parts of the world and they
are less prone to the threat of earthquakes. However, no place in the world can
be completely immune to the earthquakes.
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Some important volcanic eruptions
Mount Vesuvius in Italy
Mount Vesuvius is a composite volcano, which is around 4000 feet above the
bay of Naples. The volcano erupted in AD 79 and buried the city of Pompeii
located in the Southwest of this volcano. The volcanic ashes and the torrential
rainfall afterwards buried the city and killed its inhabitants.
Since the city was buried with the volcanic ashes, it was good infertility for the
cultivation of crops. This tempted farmers to begin a settled life on the slopes of
Mount Vesuvius. This volcano again erupted in December 1631 and destroyed
around 15 towns and killed their inhabitants.
Mount Krakatoa in the Sunda Strait
In August 1883, Mount Krakatoa located in the centre of the strait, between
Java and Sumatra, exploded violently; this was one of the greatest volcanic
explosion known to the mankind.
The Krakatau island was not inhabited, so nobody was killed due to the lava
flows and ashes. However, it set up the Tsunami waves of over hundred feet
high which submerged the coastal areas of Indonesia and drowned around
36000 people.
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Mount Tambora, Indonesia in 1815
volcano diagram -Mount_Tambora_Volcano
It is one of the deadliest volcanic eruption in the recent human history which
was responsible for the death of around 120,000 people. The volcano erupted
on 10 April 1815 and was the most powerful volcanic eruption in the last 500
years. It sent volcanic ashes and gases like SO2 in the sky. It also led to the
creation a series of Tsunami waves.
Due to the emission of large amount of SO2, the world experienced large
temperature drop which was responsible for the crop failures in various parts of
the world. Thousands of people died due to starvation in China, while the price
of Grains quadrupled in Switzerland after 2 years of the volcanic eruption.
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