A volcano is a vent or opening usually circular in form through which heated materials consisting of gases, water, liquid lava and fragments of rocks are ejected from the highly heated interiors to he surface of the earth.
Volcanic eruptions are closely associated with several interconnected processes such as
- The gradual increase in temperature with increasing depth at a rate of 1°c per 32 m due to heat generated by degeneration of radioactive elements inside the earth
- Origin of magma because of lowering of melting point caused by reduction in pressure of overlying rocks due to fractures caused by splitting of plates
- Origin of gases and water – vapour due to heating of water
- Ascent of magma due to pressure from gases and vapour
- Occurrence of volcanic eruptions. These eruptions are closely associated with plate boundaries.
Classification of Volcanoes
Volcanoes are classified under different schemes:
- Active Volcano: Volcano which erupt periodically. E.g. Maona Loa in Hawaii, Etna in Sicily, Vesuvius in Italy, Stromboli in Mediterranean Sea, etc.
- Dormant Volcano: Volcano which has been quiescent for a long time but in which there is a possibility of eruption. E.g. Fujiyama in Japan, Krakatoa in Indonesia, Barren island Volcano in Andamans, etc.
2. Classification on the basis of Mode of Eruption
- Central Eruption Type or Explosive Type: E.g. Hawaiian type, Strombolian type, Volcanian type, Pelean type, Vesuvius type, etc.
- Fissure Eruption or Quiet Eruption Type: Large quantities of lava quietly flow up from fissures and spread out over the surrounding areas. Successive lava flow results in the growth of a lava plateau. E.g. Deccan Plateau, etc.
Distribution of Volcanoes in the World
- About 15% of world’s active volcanoes are found along the “Constructive or Divergent” plate margins, whereas 80% volcanoes are associated with the “Destructive or Convergent” plate boundaries.
- The Circum – Pacific belt or the ‘Ring of Fire’. It extends across the Kamchatka Peninsula, Kurile Islands, the Islands of Japan, Philippines, New Guinea, New Zealand and the Soloman Islands. It also passes through the Antarctica and the western coast of America.
- The Mid – Continent belt includes volcanoes of Alpine mountain chain, the Mediterranean Sea and the fault zone of eastern Africa. E.g. Stromboli, Vesuvius, Etna, Kilimanjaro, etc.
- The Mid – Atlantic belt in which the volcanoes are fissure eruption type. E.g. Iceland, Canary Islands, Cape Verde, Azores, etc.