Volcanoes usually produce three types of materials viz. solid, liquid and gaseous.
(a) Solid products
Enormous quantities of solid materials are thrown out by volcanoes during an eruption. They consist of fragments of rocks or pieces of already cooled lava.
The ejection of the solid materials are usually accompanied by violent explosions. The solid materials, during the initial stages of volcanism, mostly contain the fragments of the crustal rocks through which the pipe of the volcano passes; but at later stages they consist mostly the fragments of solidified lava, resulted from the partial solidification in the molten reservoir beneath the surface as well as the solidified lava of earlier eruptions.
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The rock fragments ejected during volcanic-eruptions are called pyroclasts or tephra. Generally, larger fragments fall at the edge of the crater and slide down its inner and outer slopes, while smaller ones are thrown into the surrounding plains or pile up at the foot of the cone.
According to their size and shape the pyroclastic materials are classified as follows:
(i) Volcanic blocks
These are the largest masses of rock blown out. These are either the masses of the solidified lava of earlier eruptions or those of the pre-existing rocks. They are usually angular and the diameter of the fragments is always above 32 milimeter. Thus they are the huge solid fragments ejected during a volcanic activity.
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(ii) Volcanic bombs
These are rounded or spindle-shaped masses of hardened lava, which may develop when clots of lava are blown into the air and get solidified before reaching the ground.
Their ends are twisted, indicating rapid rotation in the air while the material was plastic. Because of their somewhat rounded appearance, they are known as volcanic bombs. The diameter of these fragments are always above 32 milimeter.
Bread-crust bombs are those volcanic bombs which present a cracked surface, may be due to the approximately solid state of the material from which they have been formed, which gives the appearance of the crust of a bread.
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(iii) Cinders or lapilli
The size of the fragments is between 4 mm to 32 mm, and are shaped very much like bombs. The term ‘lapilli’ is used when the fragments are not conspicuously vesicular; and in case of vesicular fragments they are known as cinders. Still smaller fragments are called volcanic-sand.
(iv) Ash
These particles range in size from 0.25 mm to 4mm and as such, are the fine particles of lava.
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(v) Fine-ash or volcanic dust
These are the minute pyroclas- tic materials, and their diameter is always less than 0.25 mm. In many instances volcanic dust was carried by wind to enormous distances and scattered over a vast territory forming volcanic dust layers.
Pyroclastic materials accumulating on the slopes and adjoining areas of a volcano with gradual compaction and cementation gives rise to rocks called Volcanic-tuffs.
These tuffs when consist of angular fragmental materials, they are known as Volcanic- breccia; when volcanic bombs are predominant in the tuffs, they are referred to as Volcanic-agglomerates.
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The diameter of these fragments are always larger than 20 mm. The welded tuffs are commonly known as Ignimbrites. In certain instances, a great cloud of superheated vapours and incandescent rock material and volcanic ash are violently emitted during the eruption. These are called Nuees ardentes and are sometimes referred to as glowing avalanches.
(b) Liquid products
Lavas are the major and the most important liquid product of a volcano. As we know, the magma that has flowed out on to the surface is called lava. All lavas contain gases, but because of the high pressure that prevails in the interior of the earth the content of gases and vapours in the magma is more.
According to the composition and the gas content, the temperature of lavas during eruptions usually ranges between 900°C to 1200°C. Like magma, lava is also divided in to three types viz. acidic, medium and basic, depending on the silica content
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Acid lavas contain a high proportion of silica, have a high melting point and are usually very viscous and therefore their mobility is low. They cool very slowly and contain many gases in a dissolved state.
They congeal at relatively short distances from the crater. Rhyolites, composed of orthoclase feldspar and quartz are the examples of acid lavas.
The lavas of intermediate or medium composition have the silica content between 55 to 60%. Andesite lavas are the best examples of the lavas of intermediate nature and they mostly characterize extrusions around the margins of the Pacific.
The basic lavas contain low percentage of silica, which is usually 50% or less. These lavas melt at lower temperature, and have a high density as well as liquid consistency. They cool quickly and contain little gas.
These lavas are highly mobile and spread over large distances, forming flows or sheets. Basalts are the best examples of the basic lava.
Since the lava behave differently depending on their chemical composition they give rise to different configurations when consolidated, as described below:
(i) Lava tunnels
Sometimes the outer surface of the lava flows; cools and solidifies first forming a crust while the lava is still in a liquid state inside. This enclosed liquid may drain out through some weak spots of the solidified flow forming a tunnel called a lava-tunnl.
(ii) Block lava
It is also known as aa-lava. In this case, the gases escape explosively from the partly crystallized flows thus break the congealing crust in to an assemblage of rough and uneven blocks.
The escape of gases increase the viscosity of the lava and helps in rapid cooling, giving rise to a solidified lava flow with spiny, rubbly surface. It is therefore the Hawaiian name, aa (pronounced ah-ah meaning rough or spiny) is applied to this type of lavas.
(iii) Ropy-lava
Lavas with low-viscosity remain mobile for a longer period. These lavas usually contain much entrapped gas and cool very slowly.
The lava spreads out in thin sheets and congeals with a smooth surface which wrinkles or twisted into ropy form like that of a stream of flowing pitch. It is also called Pahoehoe-structure.
(iv) Pillow lava
Lava erupted under water-logged sediments in sea-water, beneath ice-sheets, or in to rain soaked air, characteristically emerges as a pile of rounded bulbous blobs or pillows. Basic lava of spilitic type often presents pillow structure.
(v) Vesicular or Scoriaceous structure
When lavas heavily charged with gases and other volatiles are erupted on the surface, the gaseous constituents escape from the lava, due to the decrease of pressure, giving rise to a large number of empty cavities of variable dimensions on the surface of the lava-flows.
Due to the presence of vesicles or cavities, the resulting structure is known as vesicular- structure. These cavities when filled up subsequently with secondary minerals, the structure is called amyg- daloidal structure and the infillings asamygdales.
A highly vesicular rock, which contains more gas space than rock, is known as ‘Scoria’. In more viscous lavas, when the gases cannot escape easily and the lava quickly congeals, it forms Pumice or ‘Rock -froth’, which contains so much void space that it can float in water.
(vi) Jointing
As a consequence of contraction due to cooling joints are developed in the lava flows, which may be manifested in the form of sheet, platy or columnar structures, (c) Gaseous Products Volcanic activity is invariably associated with emanation of steam and various gases from the volcanoes.
Water vapour constitutes about 60 to 90% of the total content of the volcanic gases. Second in abundance to steam among volcanic gases is carbon-di-oxide.
Amongst other gases which have been detected in considerable quantities, hydrochloric acid, sulphuretted hydrogen, sulphur-dioxide, hydrogen, nitrogen, boric-acid vapours, phosphorous, arsenic vapour, argon, hydrofluoric acid etc. are the most important
The vents emitting sulphurous vapours are called Solfataras’ when carbon-dioxides are emitted they are called ‘Mofettes’ and in the case of emission of boric-acid vapours, they are known as Saffioni.