Broadly speaking, there are two types of volcanoes: (i) Volcanoes of the central type of eruption, and (ii) Volcanoes of fissure type eruption. The central type of volcanoes has certain characteristic features.
In this type the volcanic activity is centralized about a pipe-like conduit. Lavas of a central volcano do not always issue from the crater. Such volcanoes are characterized by cone and crater structure.
Sometimes the lavas blow off the whole top of the hill by explosion and a new channel is opened on the sides of the volcanic cone. Very large craters, called Calderas, are formed by the violent explosions or by subsidence of the mountain tops.
Contrary to the central type, sometimes a volcano erupts from a fissure – a linear crack, fault or other structural weakness in the crust. The outpourings of basic and very fluid lava occurring through the fissures lead to the formation of thick lava plateaus.
The Lake fissure eruption is Iceland in 1783 poured out huge volumes of lava from a 28 km long fissure. Usually there is no explosive activity, though there may be a few exceptions.
In 1886, a fissure eruption occurred in New Zealand at Tarawera accompanied by explosions and the ejection of ash. As a result of fissure eruption a lava plateau of great thickness is built up by a series of individual flows.
Deccan Plateau of India (260,000 km 3 ) is a typical example of volcanic plateau formed as a result of several fissure eruptions in the pre-historic times (Fig. 5.2a). Such plateaus are also found in other countries of the world (Fig. 5.2b). There are various types of eruption in the central type of volcanoes.
Before discussing the principal types of volcanic eruptions, it is necessary to know the primary factors which determine the nature and type of eruption.
They are: (i) Viscosity of magma. Viscosity implies magma’s resistance to flow, its thickness, or the degree of fluidity, (ii) The chemical constituents of magma i.e. whether it is acidic or basic.
Even though there are a number of classifications of volcanoes on the basis of the type of eruption, the most popular classification is that of Lacroix.
According to him, there are six principal types of volcanic eruptions: (i) the Hawaiian type, (ii) Vulcanian type, (iii) Strombolian type, (iv) Pelean type, (v) Vesuvian type, and (vi) Plinian type.
(i) Hawaiian Type:
In this type of eruption, huge quantity of extremely fluid magma is expelled without the explosive liberation of gases or ejection of the pyroclasic materials.
In this type of eruption lava flows out from a fissure or a central vent to form a shield volcano. The best known examples of this type of eruption are the Hawaiian Island volcanoes. Other examples are found in the Samoa group and in Iceland.
Hawaii has been built up from the sea floor by the combination of a number of shield volcanoes. Such volcanoes are highly active in Hawaii Islands. Kilauea in Hawaii is the world’s largest active volcano.
It has the appearance of a broad-domed shield. That is why the Hawaiian type is also known as Shield Volcano. The great basalt plateaus of the Columbia Plateau and Iceland are fine examples of the Hawaiian eruption.
(ii) Vulcanian Type:
In the Vulcanian type eruption the lava is relatively more viscous and pasty. The result is that between successive eruptions, the lava gets solidified. Hard crusts are formed in the volcanic vents creating obstruction for the subsequent eruptions.
The crust so formed allows the accumulation of gases which gather strength. Due to these gases, the subsequent eruptions are more violent so that the viscous lava and its crust are shattered to angular fragments of all sizes.
The resulting clouds being ash-laden are dark or rather black. As these clouds rise up in the atmosphere, they assume a cauliflower shape.
It may be pointed out that Vulcanian eruption is characterized by the brilliant lightning flashes in the cloud or between the cloud and the ground.
The friction of tephra particles generates electrical discharges. Thunderstorms are gene-rated by the heat and torrential muddy rainfall follows.
Remember that Vesuvius is a better example of Vulcanian eruption than Vulcano itself.
Vulcano, an island north of Sicily was named after the Roman god of fire and its name was given to this type of volcanic activity.
(iii) Strombolian Type:
This type is named for the famous volcano Stromboli situated in the Mediterranean Sea north of the island of Sicily. Lava in this type is basaltic and less fluid than in the Hawaiian type.
Gases escape with moderate explosions which may be rhythmic or nearly continuous. Volcanic bombs and lumps of Scoria which are red-hot are blown out. Eruptions normally take place at short intervals.
However, sometimes violent explosions occur and fragmental materials such as dust, volcanic bombs, scoria and pumice etc. are hurled into air. These materials sometimes fall back into the fiery crater.
It may be pointed out that Stromboli is in constant eruption. However, other volcanoes of this type do not erupt constantly. The persistent lava foundation activity of Stromboli has caused it to be known as “lighthouse of the Mediterranean”.
(iv) Pelean Type:
This type of eruption is characterised by being the most explosive of all types. The lava is extremely viscous. Due to the growth of an obstructive dome above the conduit, the upward escape of lavas is resisted, so that each successive eruption has to clear away these domes.
When eruption of Pelean type occurs, the explosions are extremely violent and the cones and craters are completely demolished.
The Mt. Pelee which erupted on May 8, 1902, a large number of huge blocks of lava, more than 100 tons in weight, were thrown away at great distance from the crater.
In the Krakatoa eruption of 1883, a large part of the old cone was blown off. In 1911, Mt. Taal in the Philippine Islands erupted so violently that the old crater walls were partly blown away and a new crater was created.
In 1902, in Mount Pelee’s disastrous eruption there was a sudden outburst of a dark and heavy cloud, consisting of hot gases and incandescent dust, which rolled down the mountain side with great rapidity.
Everything in its path was burnt and destroyed. “These downward-rolling explosive blasts, one of which demolished St. Pierre, are referred to as nuee ardente.
The nuees ardentes are referred to as ‘glowing avalanches’. They consist of self-expansive particles and are more mobile than any lava flow could be” – A. Holmes.
In addition, there are other types of eruptions as under:
(v) Vesuvian Type:
This type of eruption is characterised by the extremely violent expulsion of magma which is highly charged with gases that are highly explosive.
Because this type of eruption is violent, the ejected materials are blown off to greater heights in the atmosphere.
Another characteristic feature of this type of eruption is that due to concentration of explosive gases through the conduit, lava flows escape from fissures and vents on the flanks.
Thus, the main channel (conduit) is made empty down to a considerable depth. Because of the reduction of overhead pressure, the underlying magma expels itself through the crater as vast clouds of volcanic gases which assume the shape of a cauliflower.
They are luminous even at night. From these clouds which attain great heights, there are showers of volcanic ashes.
(vi) Plinian Type:
This is the most destructive and explosive type of eruption. As a matter of fact, it is just the most violent type of the Visuvian eruption.
In this type of eruption there is a great blast of up-rushing gas which rises to a height of many kilometers. At great heights the emitted gas spreads out into an expanding cloud of globular masses of gas and vapour.
The proportion of volcanic ash is low. Since this type of eruption was first observed and recorded by Pliny during the cataclysmic eruption of Visuvius in A.D.79, its name was given after him.