A Volcanoes are naturally occurring fissures or vents in the earth’s crust through which hot lava and volcanic gases comes out regularly or intermittently are termed as volcanoes. Volcanic eruption may be either explosive or quiet.

The upper mantle of the earth is nearly molten. Magmas present in this depth migrate upwards, often along fault or cracks. In a volcano the magma rise through chimney like opening called “vents” and reaches to the surface of the earth as lava. When the magma comes at the surface due to decrease in temperature and pressure the magma starts to segregate. When the magma reaches to the surface it is called “Lava“.

Structure of a volcano

Types of Volcanoes

Volcanoes commonly do not erupt continuously for long periods of time. On the basis of mode of eruption as well as on the basis of nature of eruption, volcanoes are divided into  three types:

  • Active Volcanoes – An active volcano is one which erupts very often. They mostly occur at crustal plate boundaries. The volcanoes that occur along at pacific ocean from Chile to East India are example of active volcanoes.
  • Dormant Volcanoes-  The volcanoes which show eruptions after a long intervals of time are called Dormant Volcanoes. During dormant period they appear quite inactive.
  • Extinct Volcanoes- An ancient volcano which has not shown any volcanic activity since very long time in geological history is called extinct volcanoes.

Volcanic Products

The material which are thrown out or erupted by a volcano consist generally of all three phases of matter ( Solid, Liquid and Gases), all of these materials are erupted at extensively high temperature and pressure.

(a) Liquid Products- The Lava

The bulk of the material erupted from a great majority of volcanoes has been in the form of an excessively hot, varyingly viscous and mobile liquid termed as Lava. The behavior of lava after its eruption and during its flowing depends upon factors like composition, temperature, viscosity, gaseous content and the pressure at which it has been erupted.

Most commonly, lava are basic in character, quite mobile in nature and poured out without much noise from the volcanoes. Acidic lava are however are highly viscous and are erupted with explosive action.

(b) Gaseous Products

The gaseous products are generally the first to reach the surface during a volcanic eruption. The most important constituent of volcanic gases is steam. It forms nearly 90% of the total gas content. The other chief gases, in the order of abundance are Carbon Dioxide, Nitrogen and Sulfur Dioxide and smaller amount of Hydrogen, Carbon Monoxide, Sulfur and Chlorine. The density of magma and molten lava is reduced by the presence of dissolved gases.

(c) Solid Products

Besides gases and liquid lavas, volcanoes eject solid rock fragments of various size. These rock  fragments are thrown out by the escaping gases during violent eruptions. When fragments of very viscous lava are blown off into the air, they solidify quickly and fall to the ground as pyroclasts. The solid rocks fragments produced during volcanic eruptions are called “pyroclasts“.


Features Associated with Volcanism

  1. Fumaroles-  They are fissures or vents through which volcanic gases are ejected. Fumaroles emitting sulphurous vapour are called ‘Salfataras‘ and those which emit carbon dioxide and boric acid vapours are known as ‘Mofettes‘ and ‘Saffoni‘ respectively.
  2. Hot Springs-  These are fissures through which hot water escapes. The water usually gets heated with increased temperature below, may be magmatic or radioactive heat.
  3. Geyser- Hot springs ejecting boiling water and steam at regular intervals are geysers. Siliceous deposits formed around geysers are known as “geyserite“.

Pseudo- Volcanic features- Mud- volcanoes, meteor craters are of non-volcanic origin and are example of pseudo-volcanic features.


Causes of Volcanism

  1. Release of high-pressure, which buildup within magma chamber below the ground surface.
  2. Accumulation of radioactive heat produces magma, of course other factors like friction heat and the increase of heat with depth causes the formation of magma and their eruption on the surface causes volcanism.
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