Volcanoes, Types of Volcanoes, Distribution of Volcanoes Part – 3

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Volcanoes

A volcano is a vent or an opening in the earth’s crust through which molten rock material, rock fragments, ash, steam, and other hot gases are emitted slowly or forcefully in the course of an eruption. These materials are thrown out from the hot interior of the earth to its surface. Such vents or openings occur in those parts of the earth’s crust where rock strata are relatively weak.

Volcanoes are evidence of the presence of the intense heat and pressure existing within the earth. Hot molten rock materials beneath the solid outer crust is known as magma. When this magma is thrown out from the magma chamber to the earth’s surface it is known as lava. The magma and the gases stored within the earth’s surface keep trying to come out to the surface through a line of weakness anywhere in the crust. The tremendous force created by magma and its gases creates a hole in the crust and the lava spreads out on the surface along with ash and fragmented rock materials. The process by which solid liquid and gaseous materials escape from the earth’s interior to the surface of the earth is called vulcanism. The volcanic materials accumulate around the opening or hole taking the form of a cone. The top of the cone has a funnel shaped depression which is called its crater.

A. TYPES of VOLCANOES

Volcanoes are classified on the basis of the nature of vulcanism. On the basis of the frequency of eruption, volcanoes are divided into three types:

Active: The volcanoes which erupt frequently or have erupted recently or are in action currently are called active volcanoes. Important among these include Stromboli in Mediterranean, Krakatoa in Indonesia, Mayon in Philippines, Mauna Loa in Hawaii Islands, and Barren Island in India.

Dormant: The volcanoes which have not erupted in recent times are known as dormant volcano. They are as such the sleeping volcanoes. Important among these are Vesuvius of Italy, and Cotopaxi in South America.

Extinct: There are volcanoes which have not erupted in historical times. These are called extinct volcanoes. Mount Popa of Myanmar, and Kilimanjaro of Tanzania are important extinct volcanoes.

It is not, always very simple to categorise a volcano as dormant or extinct. For example, the Vesuvius and Krakatoa became suddenly active after lying dormant for hundreds of years.

On the basis of mode of eruption, volcanoes are divided into two types:

Central Type Volcanoes: When the eruption in a volcano takes place from a vent or a hole, it is called a central type of volcano. Different types of domes or conical hills are formed by this type of eruption depending on the nature of the erupted materials. Majority of volcanic eruptions in the world are of this type. It is marked by violent explosion due to sudden escape of gases and molten rocks through the hole. For example, Vesuvius and Fuji-Yama volcanoes.

Fissure Type Volcanoes: Sometimes, deep elongated cracks develop due to earthquakes or faulting. The magma starts flowing through them quietly. This mode of eruption is termed as fissure type of eruption. This eruption helps in the formation of thick horizontal sheets of lava or a low dome shaped volcano with broad base. It may also form what are identified as lava plateaus, and lava shields. For example, the Deccan Traps of India.

On the basis of the fluidity of lava, there are two types of volcanoes:

Volcanoes of Basic Lava: Since, basic lava is rich in metallic minerals and has a low melting point, it has greater fluidity. In this type of eruption, lava flows far and wide quietly with greater speed and spreads out in thin sheets over a large area. Thus, it leads to the formation of shields and lava domes. For example, the shield volcano of Hawaiian Island in Pacific.

Volcanoes of Acid Lava: Acid lava is rich in silica and has a relatively high melting point. Therefore, it is highly viscous and solidifies quickly. Hence, the acid lava volcanoes cause the formation of usually higher land features with steeper slopes. Acid lava cones are of steeper slopes than basic lava shields.

B. Distribution of Volcanoes

There are about 500 volcanoes in the world. Most of these volcanoes are found in three well defined belts, the Circum-Pacific belt, the Mid-World Mountain belt, and the African Rift Valley belt. Volcanoes are closely related to the regions of intense folding and faulting. They occur along coastal mountain ranges, on islands, and in the mid-oceans. The interior parts of the continents are generally free from volcanic activity. Most of the active volcanoes are found in the Pacific region. About 83 active volcanoes are located in the Mediterranean region. The Circum-Pacific region has the greatest concentration of volcanoes; thus, it is called the Pacific Ring of Fire.

This ring extends along the Andes mountains of south America to Alaska and from the Aleutian Islands to Japan, Philippines, Indonesia to New Zealand. The Mid-world mountain belt occupies the second position with regard to the numbers of volcanoes. It runs from the Alps in Europe to Asia Minor and crossing through the Himalayan region joins the Circum-Pacific belt. Most of the volcanoes are extinct in the African rift valley region. Mt. Cameroon is the only active volcano which is situated in Central West Africa.

World Distribution of Active Volcanoes

World Distribution of Active Volcanoes

World Distribution of Active Volcanoes

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