Volcanism includes all the phenomenon associated with movement of molten material from the interior of the Earth to the surface.

 A volcano is an opening in the earth’s crust through which molten materials comes out.  It is also the place where gases, ashes and molten rock material escape to the ground.

 The mantle contains a weaker zone called asthenosphere. It is from asthenosphere; molten rock material comes out to the surface.

 The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava.

 The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon

 Crater of a volcano is a cylindrical opening through which Magma or Lava comes out to the surface of the earth.

 A magma chamber is a large underground pool of molten rock under pressure sitting underneath the Earth’s crust. Generally, magma chambers are located close to the Earth’s surface, usually between 1 km and 10 km deep.

 A vent is the weak point in the Earth’s crust where hot magma has been able to rise from the magma chamber and reach the surface.

Types of volcano on the basis of eruption

Hawaiian Type

  In this type of volcano, extremely fluid lava comes out on the surface of the Earth.

  It is mostly basaltic and spreads over large areas.

Strombolian Type

  Strombolian type is the most typical volcano which are less explosive.

  There are several small explosive activities, where lava is more viscous.

  It has a power to explode the crust. e.g., Stromboli volcano, Lipari island (North Sicily), Mauna Loa volcano (Hawaiian island).

Volcanian Type

  In this type, lava is very viscous and the eruptions take place at longer intervals.

  Large quantities of pyroclastic materials are erupted from them. e.g., volcanoes in Lipari island group of Mediterranean Sea.

Vesuvian Type or Plinian Type

  In this type, long intervals take place between two eruptions.

  These are extremely violent and their lava is highly viscous. e.g., volcano that once existed near Naples in Italy.

Pelean Type

 In this type, lava is viscous and highly destructive and fast-moving mass gets erupted. e.g. Mount Pelee on the Martinique island in West Indies.

Types of volcano on the basis of activity

Active Volcano

 A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past.  These volcanoes constantly eject volcanic lava, gas and ashes.  e.g., Cotopaxi, Mt. Erebus, Mt. Etna, Strombolian, etc.

 Due to its recent activity and nearby population, Mount Etna has been designated a Decade Volcano by the United Nations

Dormant Volcano

 These are volcanoes, which were active in past, but for several years have not shown any activity.

 They can erupt very violently and cause huge damage. e.g., Mt. Krakatoa, Vesuvius (Italy), Fujiyama (Japan), etc.

Extinct Volcano

 A volcano which erupted in the distant geological past and there is no longer any active volcanicity. They show no indications of future eruptions. e.g. Popa (Myanmar), Devmand and Kohsultan (Iran).

Types of volcano on the basis of nature of eruption

Shield Volcanoes

 Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.

 The Hawaiian volcanoes are the most famous examples.

 Since the volcanoes are mostly made up of very fluid basalt, this reason, these volcanoes are not steep. They are characterised by low-explosivity. When water gets into the vent, they become explosive.

 The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone.

Composite Volcanoes

 Composite Volcanoes are characterised by the eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions.

 Along with lava, large quantities of pyroclastic material and ashes find their way to the ground.

 This material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.

Caldera

 These are the most explosive of the earth’s volcanoes. When they erupt, they tend to collapse on themselves rather than building any tall structure.

 The collapsed depressions are called calderas.

 Their magma chamber supplying the lava is not only huge but is also in close vicinity.

Flood Basalt Provinces

 These volcanoes outpour highly fluid lava that flows for long distances.

 There can be a series of flows with some flows attaining thickness of more than 50 m.

 The Deccan Traps from India, are a much larger flood basalt province. It is believed that initially the trap formations covered a much larger area than the present.

Mid-Ocean Ridge Volcanoes

 These volcanoes occur in the oceanic areas.

 There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins.

 The central portion of this ridge experiences frequent eruptions.