Crater Lakes and Salines Lakes


Crater lakes are formed by the impounding of water in the craters of extince volcanoes. These lakes have a circular outline and are surrounded by deep walls of lava. Usually Crater Lakes are very deep and their depth is the greatest near the centre. The Crater Lake of Oregon and the Caldera of Roman Campagana are some of the most outstanding lakes of this type. Such lakes are also found in the Avergne District of France and the Eifle region of Germany.
Salines lakes are the salt water lakes of arid and desert regions. These are not so short-lived as the playas and are more permanent. They have a more constant water supply but are bitter in taste because of the lack of an outlet. But these may also disappear leaving behind a ‘salt-Pan’. The best case in point is the 4000 square miles saline Lake Eyre, which lies in the ‘Dead Heart’ of Australia.

Coulee lakes and Sink Lakes


Coulee lakes are also of volcanic origin but they are formed by the flow of lava across the valley of some river so that it blocks the river flow and gives rise to lakes. Lake Tana of Absyssinia is a characteristic example. Several lakes of this type are found in Iceland. Such lakes are usually rectangular in outline but may be irregular also. Sink lakes are characteristic of limestone regions, where the water descending through the shallow holes produces large underground caves. In case of roof of such a subterranean cavern collapses, a basin is formed. Such a basin resembles a sink and has also been formed by the sinking of the land. The lakes formed in these depressions are known as ‘Sink Lakes’. These are usually small in size.

What is Volcano?


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.