volcanoes

Volcanoes: The volcanic phenomenon is a majestic natural phenomenon which we can neither prevent nor regulate.  But the ejection is not alike in all the cases.  On the basis of frequency of eruption, there are active, dormant and extinct volcanoes.  The volcanoes which erupt frequently as compared to other are active, best example is Mt. Stramboli.  The dormant or sleeping volcanoes are one which eruption has occurred in the past and every possible chance of eruption is there in the future, the best example is Mt. Vesuvius.  On the other hand Extinct is one where once the eruption had occurred and the possibility of eruption again is ruled out, example is Mt. Kilimanjaro.  Many a time a volcano thought to be extinct may suddenly become active. It happened in case of Vesuvius and Krakatao.

Volcanic types:  Volcanic eruptions are divided into two principal cases based on the modes of eruption

i.                     Central eruption

ii.                   Fissure eruption

CENTRAL ERUPTION:  Eruption is confined to a pipe-like vent and after the eruption, cone and crater structure is developed.  Example is Mt. Cotapoxi of Ecuador. In the central eruption the nature and intensity of the eruption show great variation according to the pressure of gases and viscosity of the lava.  When the lava is acidic meaning more silica content, therefore more viscous, the eruption is explosive.  On the contrary, when the lava is basic meaning, less content of silica, the eruption is peaceful and absence of explosion.

     Volcanoes of the central eruption may be sub divided into five types based on the nature and intensity

i.                     Hawaiian type

ii.                   Strambolian type

iii.                  Vulcanian type

iv.                 Vesuvian type

v.                   Pelean type

Hawaiian type : This type shows less explosion and eruption is peaceful.  Lava is thin basaltic variety.  Pit like crater or lateral cracks develops as lava rivers.  When the wind is strong the lava pieces are stretched into long shiny threads known as ‘Pele’s hair’ in the Hawaiian islands.  Ex. Hawaiian islands, Columbia plateau and Iceland.

Strambolian type : The basaltic lava is not quite as thin as in the Hawaiian type, the gases come either continuously or interruptedly with moderate explosive action.  The explosion is relatively mild, liquid lava fragmental materials are also ejected.  No cloud of black smoke can be seen either in Hawaiian or strambolian type.  This type is majorly found in the stramboli island in the Mediterranean sea.  The lava fountain activity of stramboli, reflected at night as a red glow on the down side of a towering steam plume has caused the volcano to be known as “ light house of the Mediterranean”.

Vulcanian type :  This type has been named after the volcano located in the Lipri islands, north of Sicily in the Mediterranean sea.  Vulcanian activity is explosive.  In this lava is so thick and viscous that it is unable to remain in liquid condition after coming in contact with air and solidifies and seals the mouth of the crater in between two eruptions.  This blocks the passage of the gases from inside, and after sometime when the gases have collected in adequate quantities and their pressure is intense, they force through the solidified vent with explosion.  Masses of black clouds filled with ash and dust rise to great heights and give the appearance of huge cauliflower from a distance.

Vesuvian type : In this type there is a violent explosion due to the intensity of the gases and the lava comes out with great force.  Lava comes out from the lateral cracks, the gases keep on accumulating in the main vent.  When the pressure has eased a little bit on the top on account of the ejection of lava from the lateral cracks, then under the influence of intense gases the lava comes out rapidly with a explosion.  The gases forming cauliflower like clouds.  The clouds rise to great heights and look very shining and bright, causing downpour of ash.

Pelean type : This type of Volcanic eruption is Violently explosively and the lava is highly viscous.  At the time of eruption the dense lava solidifies and closes the mouth of the crater and a dome is formed there.  After sometime, the powerful gases trapped inside either break through this obstacle or come out along the hill slope, and then exceedingly dense mass of hot, highly gas charged lava mixed with fragmental materials and ash flows down the slope like a avalanche.  These have been called “Nuees ardentes”.  This nuees ardentes is extremely dense but highly mobile on account of gases and moves rapidly down the hill slope almost without friction.  It is the most terrifying form of explosive volcanic activity.  It is however, soundless, as in spite of the fact that it has the velocity of winds in a hurricane.

FISSURE ERUPTION:  Lava of basic composition is less viscous and hence less explosive than central eruption.  There is a voluminous lava flow which spreads out over large areas like a thin sheet because of its low viscosity, covering the pre-existing topography.  Such outflows of basaltic lava builds large lava domes, shield volcanoes also lava plains and plateaus.  Basalt emerges from numerous fissures rather than a single pipe like vent, forming a huge plateau or the plain covering larger areas. Examples for this type are Deccan plateau of India, Columbia plateau of USA.