The word volcano has been derived from the island of Vulcano, which lies off the north-east coast of Sicily. A volcano is essentially a conical or dome shaped hill or mountain, formed by the extrusion of lava or any other pyroclastic materials from a vent.
Volcano is considered to be the most significant landform created by volcanism. Volcanism is the general term used to cover all the phenomena related to the eruption of magma to the surface of the earth. It is one of the most important evidences of the dynamic nature of the earth and arises from the forces which are endogenous in nature.
Causes & Formation of Volcanoes
As it has already been indicated, a volcano is the result of the process of volcanism in which lava is extruded on the surface of the earth. This process is also known as effusive magmatism.
Volcanism is considered to be the outcome of the release of high pressures which build up within the magma-chambers below toe ground surface. The origin of volcanism can be dealt with the explanation for-
(a) the origin of magma with its high temperature;
(b) the origin of volcanic gases; and
(c) the extrusion of magmas.
Magma may be produced because of the factors like-
(i) Geothermal gradient i.e. the increase of temperature
(ii) Accumulation of radioactively generated heat;
(iii) Relaxation of pressure locally etc. As we know, because of the high pressure at depth, the subcrustal region is in a viscous state. Any release of pressure due to some dia- strophic movements melt the rock below and thereby producing pockets of magma.
It is also believed that water percolating through the crust gets converted in to steam due to increase of temperature with depth. Besides, a number of gases may be produced due to the effect of magma on the surrounding rocks.
The steam and other gases may force the magma upwards causing eruption. There are instances when the eruptions are accompanied by powerful explosion of gases.
The magma once produced will find its way upwards by the pressure of the overlying rock through fissures, joints, cracks etc, the development of which is mostly associated with crustal deformation.
Reaching the surface, igneous material may pour out in tongue-like lava-flows or may be ejected as tephra (pyroclas- tic debris) under pressure of confined gases. Whenever, the magmatic material is ejected from an opening it spreads around the outlet and gradually cools and consolidates.
The process of repeated eruption, cooling and consolidation over a period of time gives rise to a conical structure, commonly known as a volcano.
Every well-developed volcanic cone has near its top, a funnel-shaped depression that acts as the avenue for the magmatic materials to rise. This depression at the top of a volcanic-cone is known as crater.
The crater is connected with the magma reservoir at the bottom through a pipe-like conduit, called volcanic pipe. Sometimes lava may also issue from the sides of the volcano, giving rise to secondary, satellitic or parasitic cones on the flanks of the main structure.