A volcano is an opening in the crust of the earth, connected by a conduit to an underlying magma chamber, from which molten lava, volcanic gases, steam, and pyroclastic materials are ejected.

Volcansim (Volcanicity) 

Volcansim includes all the processes and mechanisms related to the origin of magma, gaseous vapour., etc their ascent and appearance on the earth's surface in various forms. 

Causes of Volcanism 

The main causes of volcanism are as follows: 

  • With increasing depth in the earth's interior, the temperature increases gradually at the rate of 1°C per 32 metres (the rate varies at greater depths). This is caused mainly by the disintegration of radioactive elements inside the earth. 
  • Molten magma forms because of lowering of melting point in the earth's interior caused by reduction in the pressure due to splitting of plates and their movement in opposite directions. 
  • Volcanic eruptions are closely associated with the movement of plates. Most of the active fissure volcanoes are found along the oceanic ridges representing the divergent boundary, while explosive types of volcanoes are found in the zone of convergent plates. 

Lava and Magma 

  • Molten rock materials arc called magma when they are below the earth's surface, whereas when they emerge on the earth's surface they are called lava.

Types of Volcanoes 

Volcanoes are of three types: 

Active Volcanoes 

  • It continuously ejects lava, gases, and fragmental materials. 
  • About 100 per cent of world's active volcanoes are situated along the perimeter of the Pacific Ocean.
  • Noted examples are: Mona Loa in Hawaii Island (USA), Etna and Strain-boll in the Mediterranean Sea, Pinatubo volcano of Philippines, etc. 
  • Stransboli is also known as the Light House of the Mediterranean. 

Dormant Volcanoes 

  • Those volcanoes which have been quiescent for a long time but in which there is a possibility of further eruption. 
  • Mt Vesuvius mountain of Italy, Mt Fijiyansa ofJapan and Mt Krakatoa of Indonesia are famous examples of such volcanoes. 

Extinct Volcanoes 

  • A volcano in which the eruption has completely stopped and is not likely to recur is referred to as an Extinct Volcano.
  • Mt Popa of Myanmar, Mt Kilimanjaro of Africa, Mt. Demvand and Koh-Sultan of Iran are examples of Extinct Volcanoes. 

Spatial Distribution of Volcanoes 

Circum-Pacific Belt (Ring of Fire) 

  • This is the volcanic zone of the convergent oceanic plate boundary and includes the volcanoes of the eastern and western coastal areas. 
  • The zone begins from the Erebus mountain of Antarctica and runs north-wards through the Andes and the Rockies, and then to volcanoes of the island arcs (e.g. Sakhalin, Kamchatka, Japan, Philippines etc). 
  • Ojas del Salado is the highest active volcanic mountain of the world. along with Mt Cotopaxi. It is also situated in the circum-pacific zone. This zone includes more than 60 per cent of world's volcanoes. 

Mid-Atlantic Zone 

  • This zone represents the divergent boundary or splitting zone of plates located along the mid-Atlantic ridges. 
  • Iceland located on the mid-Atlantic ridge, is the most active volcanic area 
  • Other examples are: St. Helena, Azores Islands etc.

Mid-Continental Zone 

  • This is the volcanic zone of convergent continental plate boundary that includes the volcanoes of Alpine mountain chains, the Mediterranean Sea and the fault zone of Eastern Africa. 
  • The Himalayas form the most striking exception because none of the mountains are volcanic in the entire range.. 

Intra-Plate Volcanoes 

  • Some volcanoes are also found in the inner parts of continental or oceanic plates, contrary to the general trend e.g. Hawaiian Volcanic Chain, Re-Union island. Rhine Graben, etc, which lie beyond convergent or divergent boundary. 
  • This volcanism is the result of the activity of Hot Spots in the Mantle. Hot Spots are the mantle plumes of hot material whose location is stationary. 

Volcanic Topography 

  • When magma solidifies below the earth's surface then intrusive volcanic topography such as Batholith, Phacolith, Still, Dyke, etc. are formed. 
    Crater & Caldera: A funnel-shaped depression formed at the mouth of a volcanic vent is called a crater. Enlarged form of a crater is generally called Caldera. 
    Geysers and Hot Spring: Geysers are a special type of hot springs from which a column of hot water and steam is explosively discharged at intervals.
  • The difference between hot springs and geyser is that there is continuous spouting of hot water from the former, while there is intermittent spouting of water from the latter. 
  • Yellow Stone National Park of USA is famous for its geyser.
    Fumaroles: These are vents through which there is intermittent emission of gases, smoke, and water vapour. 
  • Fumaroles are found in groups near the Katmai Volcano of Alaksa, which is known as the Valley of Ten Thousand Smokes.