While dealing with Volcanoes we come across certain terms like Vulcanicity or Vulcanism. Let us first consider the difference between the terms Vulcanicity and Volcanoes.

Even the terms like volcanism or vulcanism are interchangeably used to illustrate the volcanic activity or phenomena either on the earth's surface or beneath it.

According to Monkhouse, the term 'Vulcanicity' is very comprehensive and broad. It includes all the processes by which solid, liquid or gaseous materials present beneath the earth's surface are forced into the earth's crust or escape on to the surface.

The phenomenon of Vulcanicity is associated both with periods of earth movement and also with the main areas of crustal instability.

In the opinion of Woodridge and Morgan, the implication of the term vulcanicity is very wide. It covers all those processes in which molten rock material or magma rises into the crust or is poured out on its surface, there to solidify as a crystalline or semi-crystalline rock.

In either case, this activity (vulcanicity) may have a profound effect on surface features. It may be pointed out that there are two forms of vulcanicity: intrusive and extrusive.

A volcano, on the other hand, is the resultant effect of the process of vulcanicity. A. Holmes has defined a volcano as under: "A volcano is essentially a fissure or vent, communicating with the interior, from which flows of lava, fountains of incandescent (red-hot) spray, or explosive bursts of gases and volcanic ashes are erupted at the surface".

In other words, any opening in the crust of the earth that allows molten rock-forming materials to reach the surface is properly called a volcano. The deposits of Lava surrounding the vent area also considered to be part of the volcano.

It may be noted that lavas are poured onto the surface along both the divergent and convergent plate