On the basis of periodicity of eruption, volcanoes are classified into three types: (i) active volcanoes, (ii) dormant volcanoes, and (iii) extinct volcanoes.
(i) Active Volcanoes:
Volcanoes, from whose vents there is constant emission of volcanic materials such as, lava, ashes, lapilli, pumice and gases etc. are called active volcanaoes. Some of the active volcanoes are continually in eruption, while in other cases the eruptions are intermittent.
According to Worcester, there are about 500 active volcanoes in the world today. Besides, there are other active submarine volcanoes whose number it is difficult to determine.
Most of the active volcanoes are found in the Circum-Pacific Belt which is known as ‘the Ring of Fire’.
A few examples of active volcanoes are: Etna and Visuvius, Mount Pelee (Martinique), Mount Karmai (Alaska), Mount Saint Helens, Nevado Del Ru’z (Columbia), Mount Unzen (Japan), Mount Pinatubo (Philippines), Mount Redoubt (Alaska) and Mount Mayon (Philippines).
The Stromboli volcano emits so much fire and incandescent gases that it is known as ‘the Light House of the Mediterranean Sea’.
Besides active volcanoes are found in Alaska, Aleutian Islands, Kamchatka, the Kurile Islands, Japan, Taiwan, Philippines and the Moluccas group.
(ii) Dormant Volcanoes:
There are certain volcanoes which do not show any sign of volcanic activity for a long time. They are in a state of slumber. They appear to be extinct.
But all of a sudden there is explosive eruption with the result that there is unimaginable loss of life and property. Such volcanoes are said to be of the dormant type. The Vesuvius volcano is a typical example of a dormant volcano.
This volcano which erupted in A.D. 79 remained dormant for more than 1700 years. Then it was in the year 1631 that there was an explosive eruption.
Ever since, it has been intermittently active, though from 1500 to 1631, it was dormant for a period of 131 years. Even centuries of inactivity do not mean extinction.
It may be pointed out that when a dormant volcano becomes active, the eruption is apt to be exceptionally violent, for during the period of quiet, the lava in the channel has solidified and given rise to an obstacle which only a great and violent explosion can remove.
It is worth mentioning that violent eruptions of such dormant volcanoes are generally preceded and accompanied by earthquakes, some of which have been very destructive. Mount Kilimanjaro is also a fine example of dormant volcano.
(iii) Extinct Volcanoes:
This is an extreme case when a volcano permanently stops its volcanic activity. Even though the cone and its crater may still be present, it no longer emits steam.
Examples may be given of the volcanoes of the Eiffel district of Western Germany; of the Auvergne in central France; and of a host of volcanoes in Western United States.
In the northeast of Ireland, in Skye, Mull and other islands of the Hebrides, there are huge flows of basaltic lava of the Tertiary age. They belong to the volcanic area once spread as far as Greenland.
There are several extinct volcanoes which lie on the eastern side of the Rhine, and also on the inner border of the Carpathians.
It may be pointed out that it is rather difficult to classify definitely a volcano as active, dormant or extinct.
Some of the volcanoes that are today dormant may become active. Monte Somma was considered an extinct volcano by the inhabitants of Pompeii and Herculaneum in 79 A.D. because it had been inactive for 700 years, yet in that year (79 A.D.) there was a disastrous eruption.
How a volcano becomes extinct or inactive is associated with the movement of plates. As we know, a volcano can form over the hot spot, but because the plate is moving, the volcano is carried away from its source of magma after a few million years and becomes inactive. It is replaced at the hot spot by a new volcano a short distance away. A chain of volcanoes and volcanic islands results.