If we look at the map showing the distribution of volcanoes in the world, it will immediately be clear that volcanoes are not randomly distributed on the surface of the earth, rather they are found in certain well-defined belts.
These belts coincide with major lines of weakness is the earth’s crust and these happen to be the earthquake zones also. These weaker zones are also characterised by the young folded mountains.
Besides, volcanoes are also associated with the plate boundaries. Volcanoes are also found in the meeting zones of the continents and oceans.
However, there are many scattered volcanoes which have no relation to the belts of folding, and which show no linear arrangement.
There are three volcanic belts, besides many volcanoes which are outside these belts. The three volcanic belts are as under:
1. The Circum-Pacific belt
2. The Mid-Continental belt
3. The Mid-Atlantic belt.
1. The Circum-Pacific belt:
This is the most important belt of volcanoes. This is the so called Ring of Fire. The belt extends through the Andes of South America, Central America, Mexico, the Cascade Mountains of Western United States, the Aleutain Islands, Kamchatka, the Kuril Isles, Japan, the Philippines, Celebes, New Guinea, the Solomon Islands, New Caledonia and New Zealand.
This belt has 80 active volcanoes. The Circum-Pacific belt meets the mid- continental belt in the East Indies. This belt is characterised by high volcanic cones and volcanic mountains. The volcanoes of the Aleutian Island, Hawaii Island and Japan are found in Chains.
Cotapaxi is the highest volcanic mountain (6035m) in the world. Other important volcanoes found in this belt are Fuziyama, Shasta, Rainer and Hood.
In Alaska there is a Valley of Ten Thousand Smokes. It may be pointed out that in this belt volcanic eruptions occur because of the subduction of the Pacific plate below the Asiatic plate.
In Equador, South America, there are about 22 volcanoes out of which 15 are more than 4450 metres above the sea level. Besides, other high volcanic mountains are St. Helens (Washington, U.S.A.), Kilavea (Hawaii Island, U.S.A.), Mt. Taal, Pinatubo and Mayon (Philippines). It may be mentioned that the volcanoes of Hawaii Island are situated in the intra- plate region.
2. The Mid–Continental belt:
This belt has various volcanoes of the Alpine mountain chain, Mediterranea Sea (Stromboli, Vesuvius, Etna etc.), Volcanoes of the Aegean Sea. Mt. Ararat, Elburz and Hindukush are also included in this belt.
It is interesting to not that there are several volcanic free zones found along the Alps and the Himalayas. The Rift Valleys of Africa have volcanoes such as Kilimanjaro, Elgon, Birunga and Rungwe etc.
In the region where the boundaries of Persia, Afghanistan, and Baluchistan meet, there are several volcanic cones of large size, and one or two of them emit steam and other gases. This region has also a few extinct volcanoes.
3. The Mid- Atlantic belt:
As the name indicates, this belt includes the volcanoes of the Mid-Atlantic Ridge. The volcanoes associated with the Atlantic Ocean are located either on swells or ridges rising from the sea floor, or on or near the edge of the continent where it slopes abruptly into the deep oceanic basins. However, in each case, the volcanoes are associated with zones of crystal movement.
The volcanoes formed along the Mid-Atlantic Ridge actually represent the splitting zone of the American plate moving towards west and the Eurasian plate moving towards east.
In the splitting zone stated above there is constant upwelling of magmas. Thus, it is a zone of crustal weakness. Volcanoes in this belt are generally of fissure-eruption type. Volcanoes of Lesser Antilles, Azores, St. Helens etc. are included in this belt.