Volcanic refers to the transference of material from the earth’s interior towards the surface. Such transference is evidenced by igneous masses, which may be abyssal, hypabyssal and extrusive. Such transference has been responsible for the formation of igneous rocks of the crust. It has also been responsible for the extrusion of juvenile (original water from earth’s interior) water on the surface and the various gases, which make up the atmosphere and thus volcanic has been responsible for the appearance of life on the earth. Most of the oceanic crust and part of the continental crust is made of rocks produced by volcanic.
Relatively new volcanic products, however, occupy relatively small areas of the land surface of the earth. When volcanic matter comes on the surface, it forms new ground for geomorphic evolution. Volcanism produces what was regarded by W.M. Davis as geomorphic ‘accident’ because it interrupts the normal progress of the cycle by burying the pre-existing surface and presenting new ground, new altitude and rocks for the geomorphic processes.
The volcanic matter takes two major types of forms on the surface of the earth. One type, which comes out through fissures, consists of basic lava and spreads out over relatively extensive area as plateaus or shields. The central type generally consists of acid matter and forms cones. Let us now consider the fissure type and resulting forms.