Volcanoes

 

(Volcanoes)

                                                                          

A volcano is a vent or fissure in the earth’s through which hot lava and volcanic gases are thrown out. Volcanic eruptions may be either explosive or quiet.

      The earth’s upper mantle (asthenosphere), under the crust, is nearly molten.  Magma originates at this depth. They migrate upwards, often along faults .in volcano the magma rises through a chimney-like opening called  “vent” and reach the surface of as lava. At the surface pressure in the rising magma falls as result dissolved gases are separated out such a magma is called “ lava’’.

VOLCANO

           In addition to emission of gases and molten lava, vast quantities of fragmental materials (known as pyroclasts)also produced during volcanic eruptions. TThis material accumulation around the vents. Conical hill like masses formed in this way is called volcanic cones.   volcanoes often have side vents as well. the smaller cones formed around these side vents are called “parasitic cones”.

 Crater -  A circular depression found at the top of-of volcanic cones is called crater.

Caldera – when a crater greatly enlarged is called caldera. A caldera is a gigantic depression the diameter of which may be 16 km or more .M ost of the calderas may have resulted either due to blowing of the summit of the volcanic cone by explosive action or it may be due to subsidence.

 

CALDERA

Types of volcanoes – 

 

1--Action volcanoes  

2-- dormant volcanoes  

3-- extinct volcanoes.

Active Volcanoes --  an active volcano is one which erupts very often. they mostly occur at crustal plate boundaries. The volcanoes that occur along a great are around the Pacific ocean from Chile to the east indies, are an example of active volcanoes.

Dormant volcanoes  --  the volcanoes which show eruptions after long intervals of timed are called dormant volcanoes. during the dormant period, they appear quite inactive.

Extinct volcanoes --  an ancient volcano which has not shown any volcanic activity since a very long time in the geological history is called extinct volcano.

        On the basic of ma ode of eruption and on the basis of nature of eruption different types of volcanoes have been recognized.

      

Basing on the mode of eruption volcanoes are classified as.

       Central types --wheres the products escape through a single pipe or vent.

   Fissure types --where the ejection of lava takes place from a long fissure or a group of parallel or closed fissures.

      Basing on the nature of the ruption, volcanoes may be of two types as.

 Explosive types – In which case the lava is of acidic (felsic )in nature and because of their high degree of viscosity they produce explosive eruptions.

  Quiet types --  In this case, the lava is basaltic composition (mafic lava), which is highly fluid and holds little gas with the result that eruptions are quiet and lava can travel long distances to spread out in thin layers.

       Besides the above, a number of other types of volcanoes have been identified according to their degree of explosive activity and nature of ethe ruption. They are followed….

Hawaiian type – Silent effusion of lava without any explosive activity or in the Hawaiian types eruptions, the mobile lava of basaltic composition is discharged relatively quietly .the lava flows out from lava pools occupying craters or from fissures. sometimes fountains of lava are formed due to rapid emission of gases. This type of eruption is characteristic of Hawaiian volcanoes particularly Mauna Loa and Kilauea.

HAWAIIAN VOLCANO

Strombolian type – In the strombolian type eruptions the viscous lavas erupt with the moderate explosion. Clots of incandescent lava are thrown out to from bombs or lumps of scoria. Stromboli volcano in Sicily exhibits such eruptions.

STROMBOLIAN ERUPTION

Vulcanian types – This type of eruption is more violent than the strombolian type. The viscous lava solidifies quickly thereby causing severe explosions. This results in the ejection of solid rock fragments of all sizes including cinders, ash, and dust. Dark clouds of dust are usually seen. 

VULCANIAN ERUPTION

Vesuvian types --  In this case, the lava highly charged with gases erupts with extreme violence. A large amount of gases and dust are thrown up into the air which forms a cauliflower-like ca loud at great heights.

Plinian types – It is the most violent type of volcanic eruption in which a blast of uprushing gases rises to great heights.

Pelean type -- In this type of eruption the most viscous lava is thrown out. the blocked vent is cleared due to violent explosions. The hot gases and volcanic ash spread very widely through the air.