Volcanism: The mystery behind Volcanic eruptions

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 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 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 throughout the world are actually volcanic mountains formed through Central eruptions.

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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).

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, Gr msv tn volcano of Iceland and Etna volcano in Italy etc are examples of active volcanoes which have been volcanoes erupting in the recent past.
  • 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.

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.


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.


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.
  • 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.
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.

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.
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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