There are about 800 active volcanoes on the earth’s surface. Besides these these, there are many thousands of extinct volcanic cones, most of which are under the seafloor. A great majority of volcanoes are found to be concentrated along the following belts.
- The Circum-Pacific Belt
- The Alpine-Indonesian Belt
- The Mid-Oceanic Ridges
- African Rift Valley
(i). Circum-Pacific Belt: The belt around the Pacific ocean contains the largest number of active volcanoes and therefore, it is called the ”ring of fire” It seems that the volcanoes are found associated with the boundaries of crustal plates. For example, the Circum-Pacific and the Alpine-Indonesian belts are the regions of recent crustal deformation, which lie along the collision edges of the crustal plates.
(ii). Mid-Oceanic Ridges: Similarly Mid-oceanic ridges represents big tensional cracks on the earth’s surface. A few volcanoes that exist on land away from the plate boundaries are perhaps due to localized heating by radioactivity.
(iii). African Rift Valley: African Rift Valley also known as the Great Rift Valley is a series of contiguous geographic trenches, approximately 7,000 kilometres (4,300 mi) in total length, that runs from the Beqaa Valley in Lebanon which is in Asia to Mozambique in Southeast Africa. While the name continues in some usages, it is rarely used in geology as it is considered an imprecise merging of separate though related rift and fault systems.
Today, the term is most often used to refer to the valley of the East African Rift, the divergent plate boundary which extends from the Afar Triple Junction southward across eastern Africa, and is in the process of splitting the African Plate into two new separate plates. Geologists generally refer to these incipient plates as the Nubian Plate and the Somali Plate. Like mid-oceanic ridges, the African rift valley also represent a very large series of volcanic eruptions.