The term volcano has been derived from Vulcan-The Roman God of Fire. A volcano was known as the mountain of fire & people used to regard it with faith.
A volcano is not a mountain.It is a vent or an opening, usually circular or nearly circular in form, through which heated materials consisting of water, gases & liquid lava & rock fragments are erupted from the highly interior to the surface of the earth. Volcanology is the term given to the study of volcanoes & the scientists who study them are called Volcanologists. Volcano has two components, one of them operates below the earth’s crust & the other above the crust i.e. the endogenetic mechanism & the exogenetic mechanism.
The endogenetic mechanism includes the creation of hot & liquid magma & the gases in the mantle & the crust,their expansion & upward ascent , their intrusion & cooling & solidification in various forms below the crustal surface. They form batholiths, laccoliths, sills, dykes, phacoliths etc. The exogenetic mechanism includes the process of appearance of magma as lava, volcanic dust & ashes, fragmental materials, mud, smoke etc in different forms such as fissure flows or violent volcanic eruptions, hot springs, geysers, fumaroles etc.
The volcano of explosive type has a volcanic cone, which is formed when the erupted material accumulates around the vent. The vent is an opening of circular or nearly circular shape at the centre of the cone. The vent is connected to the interior of the earth by a narrow pipe. The volcanic materials erupt through this pipe. A funnel-shaped hollow at the top of the cone is called the crater.
There is a wide variation in the mode of volcanic eruptions & their periodicity. Accordingly, the volcanoes can be classified into 3 types:
1) Active volcanoes erupt frequently & constantly & eject volcanic lava, gases, ashes & fragmentsl materials. There are more than 500 active volcanoes in the world. Etna & Stromboli are examples. Most of the active volcanoes are found on the eastern & western margins of the pacific ocean & along the mid-oceanic ridges.
2) Dormant volcanoes have become quiet after eruption. They do not show any indication of future eruption. These are also called sleeping volcanoes.Mt vesuvius is the example.
3) Extinct volcanoes have not erupted for thousand of years. It may be pointed out that no volcano can be declared permanently dead,as we do not know what is exactly happening below the ground surface.
Causes of volcanism :
1) A gradual increase of temperature with increasing depth at the rate of 1 degree Celsius every 32 meters.
2) Magma is formed due to lowering of melting point, which in turn is caused by the reduction in pressure of the overlying material.
3) Gases & vapour are formed due to heating of water, which reaches underground through percolation.
4) The ascent of magma forced by vast volume of gases & water vapour.