Volcanoes are classified in to various types on the following bases
(a) Continuity of eruption;
(b) Nature of eruption; and
(c) Mode of eruption;
(a) Continuity of eruption
On this basis volcanoes are classified as-Active, Dormant and Extinct The active volcanoes are those which still erupt either, intermittently or continuously.
The volcano which has not erupted for a longtime but are expected to be active at any time is called a dormant volcano; whereas an extinct volcano is one which has stopped eruption over a long time. Killimanjaro in Tanzania (Africa) is considered to be a dormant volcano.
(b) Nature of eruption
Depending on factors like chemical composition of lavas, the amount of gas contained in them and their pressure, temperature etc. the volcanic eruptions may be quiet, intermediate or violent.
In quite-type of volcanoes, the lava erupts quietly without any explosion. In this case the lava is of basaltic composition, which is highly fluid and holds little gas.
Intermediate-type of volcanoes erupt intermittently with explosion in the beginning and gradually the explosive action dies down and lava is emitted quietly.
In the Violent-type of volcanoes, there are explosive eruptions. The lava in such cases are of acidic nature and have high degree of viscosity. These explosive volcanoes usually produce huge quantity of pyroclastic materials.
(c) Mode of eruption
On the basis of the mode of eruption volcanoes are classified as-Central-type and Fissure-type.
1. Central type
This type of Volcanoes are represented by a cone crowned by a bowl-like depression called the crater and a vent, connecting the crater with the magma-chamber, through which the eruption products reach the surface.
A number of central types of volcanoes have been recognized depending on the chemical composition of the lava, gaseous contents and the nature of the volcanic structure, as follows: –
(i) Hawaiian type
In such cases the lava begins to pour over slowly the edge of the crater and flow down the slope. Thus there is silent effusion of lava without any explosive activity.
Sometimes, the lava, foamed by gases, is sprayed into the air and solidified in the form of long glassy threads known as Pele’s Hair. The lava is basaltic in nature. The Hawaiian type of volcanic eruption is characteristic of Mauna Loa and Kilauea on the Hawaiian Islands.
In this type, the eruptions are rhythmic and they occur at intervals of 10 to 15 minutes. The lavas emitted are of basaltic composition which are less mobile and with more viscosity in comparison to those of Hawaiian types, because of more accumulation of gases.
Moderate explosions occur with the eruption, ejecting volcanic bombs, lapilli and slags.
The Volcano ‘Stromboli’ in the Mediterranean Sea shows this type of eruption. Since glow from the ejected masses are visible afar to men on ships with regularity, it is called the Light House of the Mediterranean.
In this case, eruption takes place at longer intervals and the lava is more viscous which quickly solidifies between consecutive eruptions, producing explosions.
Each new-explosion causes the shattering of the congealed cover. They emit much ash. This type is named after ‘Volcano’ in the Lipari Islands, north of Sicily, which shows this type of eruption.
These are characterized by extremely violent eruptions of lava which is highly charged with gases, possessing a relatively high degree of viscosity, during a long period of superficial quiescence.
The eruptions occur after long intervals, usually measured in lens of years, ejecting huge amount of volcanic products in the form of volcanic ashes, lapilli, bombs etc.
The lava flowing out of the crater runs down the slope of the cone. After the volcanic activity subsides, it remains at rest for an indefinite time.
Volcanoes like Vesuvius, Etna located in the Mediterranean shows this type of eruotion; hence of the name.
These are the most violent type of Vesuvian- eruption. In such cases, huge quantities of fragmenial products are ejected with little or no discharge of lava.
This is the most violent type of all the eruptions. Here the lava is of andesitic composition, and highly saturated with gases and possesses a high degree of viscosity.
Such lava congeals in the crater and because the vent gets plugged, the free exit of gases is prevented thus creating a tremendous pressure beneath the plug.
Here the lava, therefore, force its way out through side fissures and sweeps down the slopes as avalanche of molten rock-materials of self-explosive type and gases.
This combination of extremely hot, incandescent fine ash and coarse rock fragments permeated with hot gases is known as Nudes ardentes.
The town of St. Pierre at the foot of Mont Pele’e was destroyed by nue’es ardentes, during the eruption of Mont Pele’e in 1902. This type of eruption is exhibited by Mont Pele’e located in the Island of Martinique in West Indies.
2. Fissure type
Sometimes volcanic eruptions take place along a fissure or a group of parallel or closed fissures. Usually volcanic cones are not produced through fissure-eruptions. Lava, flowing out of fissures, spreads out over extensive areas forming lava sheets.
Fissure – eruptions are characterized by quiet welling out of molten lava. The Deccan Traps in India are made up principally of basaltic lava-flows, which were erupted mostly through fissures and covered a major portion of the Deccan-plateau.