A volcano is a sudden and abrupt explosion in the crust of the earth through which magma, gas, dust, smoke and solid material is ejected. The hot molten magma below the earth’s surface is brought to the surface as lava. Lava is ejected at great pressure through a pipe which is known as its ‘Vent’ or ‘Neck’. The lava forms a cone-like feature on the surface of the earth which is known as ‘Volcanic Cone’. The top of the cone has funnel-shaped or saucer-shaped depression which is known as the crater. The diameter of the crater may vary from a few metre s to a few kilometres. A Crater Lake is formed when rain water gets accumulated in the crater. Sometimes lava comes to the surface through joints on the sides of the main cone and forms secondary cones.
Three Types of Volcanoes
Depending upon the frequency of their eruption, three types of volcanoes can be recognised.
Active Volcanoes keep on ejecting volcanic material at frequent intervals. Etna of Italy and Stromboli of Sicily are outstanding examples of active volcanoes.
Dormant Volcanoes are those in which eruption has not occurred regularly for a long time but can occur at any time. Vesuvius of Italy and Barren Island to the east of Andaman’s in India are examples.
Extinct Volcanoes are those volcanoes which have not recorded any eruption in historic times and the possibility of future eruption is remote. Popa of Myanmar is its outstanding example.
Volcanic features may be classified as extrusive or intrusive depending upon whether the volcanic material is deposited above the earth’s surface or below it.
Extrusive Volcanic features
Lava cones or lava domes are the most common extrusive features created by volcanic eruptions. Ash or Cinder Cone is formed when the volcanic material thrown out by volcano cools down quickly in small solid pieces known as cinders. The solid particles are rained down and form a circular cone around the crater. This cone is called ‘cinder cone’. Acid lava cone or dome is formed by the viscous acid lava. This lava deposits itself near the neck immediately after explosion and forms a dome after solidification. Basic lava cone or lava shield is formed by lava having low silica content which was out quickly and forms a shield. This shield has wide base and low slope.
Composite cone is the largest and the highest volcanic cone. Such cones are formed by lava, ash, etc. which are deposited one after the other in almost parallel layers. Sometimes the main cone is covered by a number of smaller cones which are called parasite cones.
Caldera is a large roughly circular volcanic depression open several hundred square kilometres in area. Caldera usually has a number of smaller vents and can also contain a large crater lake.
Volcanic plug is formed by the solidification of lava in the vent of the volcano after eruption is over. The plug becomes clearly visible after the erosion of the upper rock material. When more fluid with low silica content comes out of a volcano, it spreads to great distance and covers a vast area with thick sheet of lava. Thus, lava plateaus are formed. The Deccan plateau of India was formed in this way.
Intrusive Volcanic Features
These features are formed when lava is not able to r each the surface and gets solidified within the earth’s interior. Batholith is the largest intrusive form which is usually hundreds of kilometres long and 50–80 km wide. Batholith of smaller size is called stock. Lacolith is formed when the magma pushes the upper layer with great force and assumes dome slope. Lapolith is formed when magma solidifies in saucer shape. When magma is deposited in wave like form, phacolith is formed. Sill is formed when magma solidifies in parallel layers. Its thickness varies from one metre to several hundred metres. When the lava makes its way through cracks and the ssures developed in the land, it solidies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.
World Distribution of Volcanoes
486 active volcanoes have been found all over the world since 1500 A.D. The total comes to 522 if the more ancient known eruptions are taken into account. Following are the main volcanic belts of the world. Borders of the Pacific Ocean. This is the most outstanding volcanic belt of the world in which 403 active volcanoes are found. This is known as the ring of fire. On the east coast of the
Pacific Ocean, this belt extends from Cape Horn of South America to Alaska in North America. Along the western coast of the Pacific this belt extends from Ross islands of the Antartic region in the south to Kurile islands in the north.
This is also known as Alpine-Himalayan belt. It starts from Madeira and Canary Islands and crosses the Mediterranean Sea beyond which it runs along the Alps and Himalayan ranges. After crossing the Himalayas, this belt crosses Yunan, Myanmar, Andaman and ends in Indonesian Islands.
The African Rift Valley. It stretches from Lake region to Red Sea in the eastern part of Africa and further goes upto Palestine in the north. Mt. Kiliamanjaro of Tanzania is an extinct volcano and lies out of this rift valley. Some volcanoes are situated in the Mid-Atlantic ridge where Cape Verde is very famous. Azores, St. Helena, and Ascencion are volcanic islands but they are mostly inactive. Iceland has about 20 very active volcanoes. Comoro, Mauritius and Reunion are important volcanoes near Madagascar in the Indian Ocean. Some active volcanoes such as Arelus and Terror are found close to Antarctica. Hawaiian Islands have many volcanoes.