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 boundaries. According to A.N. Strahler, a volcano is a conical or dome-shaped initial land form built by the emission of lava and its contained gases from a constricted vent in the Earth's surface.
According to Monk house, "a volcano consists of a vent or opening on the surface of the crust, through which material is forced in an eruption".
The material thus ejected may accumulate around the vent to form a hill, more or less conical in shape, or it may flow widely over the country rock to form an extensive level sheet. However, the landforms thus created depend to a large extent on the nature and type of the materials ejected.
The materials, no doubt, are not the same in all the cases. They also show a great variation during different stages in an individual eruption. The character of volcanic eruptions is governed primarily by the composition of the magma involved.
Materials emitted during an eruption are put into the following categories:-
1. Gaseous Materials:
Various gaseous compounds like those of sulphur, hydrogen, carbon dioxide and carbon mono oxide are emitted during the course of an eruption, most of which are dissipated directly into the atmosphere.
These gases present in the molten rock-material produce great heat through their inter-reactions within the lava being poured out on the surface.
It may be pointed out that gases are the main driving force in volcanic eruptions. Steam constitutes 80 to 95 per cent of the volcanic gases. Hydrogen, sulphur, carbon dioxide and different kinds of hydrocarbons are the constituents of magma.
A large part of the steam is derived from ground-water which percolates into volcanic vents or down to the heated rocks several hundred meters beneath the earth's surface. In addition, sea water also contributes to the supply of steam to the volcanoes located on the periphery of the ocean basins.
The expansive force of this steam and other constituent gases contribute to the mechanism of a volcanic eruption, particularly in case of explosive eruption. It is the steam which condenses to form clouds which yield torrential rains.
2. Solid matter or pyroclasts:
When a volcanic eruption takes place with a series of explosions, solid fragmental materials are ejected which are known as Pyroclasts. The ejected fragments vary in size from gravel and sand down to fine silt size. These fragments are called tephra.
Pyroclasts consist of materials from three different sources:
(i) 'Live Lava' that at the time of the formation of pyroclasts was molten or partly consolidated. The fragments vary in size from fine particles to pumice or scoriae and volcanic bombs.
(ii) 'Dead' Lava and pyroclasts detached from the walls of volcanic vent.
(iii) Pre-existing crustal rocks from beneath the volcanic cone.
It may be pointed out that the term volcanic bomb is used for the fragments from the live lava. During eruption these fragments are blown out. The solid materials from pre-existing crystal rocks or from the crystal foundations are known as ejected blocks.
The finer particles which fall from volcanic clouds are called volcanic ash. Deposits of these fine particles when solidified are known as volcanic tuff. The coarser materials form deposits of volcanic breccias with many angular fragments.
3. Magmatic materials:
Remember that so long as the molten rock materials lie beneath the earth's surface, they are known as magma, but after they escape on the earth's surface the term 'Lava' is used for them.
Depending on the percentage of silica present in the magma or lava, it is classified as under: (a) Acidic magma, which has high percentage of silica (more than 70 percent), (b) Basic lava, which has low percentage of silica (less than 50 percent)
It may be pointed out that the form of a volcanic cone is determined largely by the nature of lava. The type of lava also has much influence on the nature of eruption.
Acid lavas or magmas have a high melting point and, therefore, they are very viscous. Such lavas solidify rapidly, and so do not flow far. Lavas with high percentage of silica (felsic lava) build high, steep-sided cones.
They may also solidify in the vent and thus cause recurrent explosive eruptions. On the contrary, the basic lava, which has low percentage of silica and higher percentage of iron and magnesium (mafia lavas), is characterized by lower melting point, and so it flows rapidly to a great distance.
Such types of lavas produce a much flatter come of great diameter. In this case the eruption is generally quiet, without much explosive activity.
On the basis of newly consolidating flows of basic lava, they are classified into two contrasted types: block lavas and ropy lavas. However, internationally they are known by their Hawaiian names, aa lava and pahoehoe.
An 'aa lava' contains little gas and moves slowly. After cooling its surface forms a crust which is broken into angular blocks. In its flow bubbles are formed by the escaping gases. Pahoehoe lavas, on the other hand, are more fluid since they contain more gases.
They flow faster. As they move, there develops a thin, glassy crust which is molded into billow forms or coils of rope, so that they are also known as 'ropy lava'.
Pahoehoe lavas under some circumstances consolidate as a pile of tightly packed rounded masses, which are called pillow lavas.
As regards the mode of their formation, when ropy type of lavas flows over the sea floor, they consolidate and appear as if there was a heap of cushions. Examples of pillow lavas of the pre-Cambrian age can be seen in certain parts of Karnataka.