On the basis of frequency of eruption, there are active, dormant and extinct or ancient volcanoes. The volcanoes which erupt fairly frequently as compared to others are active. Only a few volcanoes remain more or less continually in eruption for a long period, but intermittent activity is more common. The dormant (from Latin word dormer, meaning, ‘to sleep’) volcanoes are those in which eruption has not occurred regularly recently.
These volcanoes undergo long intervals of repose during which all external signs of activity cease. Those volcanoes in which no eruption has been recorded in historic times are said to be extinct. Before a volcano becomes extinct, it passes through a waning stage during which steam and other hot gases and vapors are exhaled. These are known as fumaroles or solfataras.
Sometimes, a volcano thought to have become dormant suddenly becomes active. The Barren Island in an Andaman and Nicobar Islands of India is an example.
There are no volcanoes in the Himalayan region or in the Indian peninsula. Barren island, lying 135 km north-east of Port Blair, was thought to be dormant since it last erupted in early nineteenth century. It suddenly became active again in March 1991.
A second phase of eruption started in January 1995. The island has its base 2,000 m below sea level and its crater is about 350 m above the level of the sea. After its activity in the nineteenth century, it passed though a mild solfataric stage as evidenced by the sublimations of sulphur on the walls of the crater. The other volcanic island in Indian Territory is Narcondam, about 150 km north-east of barren island; it is probably extinct. Its crater wall has been completely destroyed.