Syllabus: General Studies Paper 1 (Indian Geography)
Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Elaborate. Also, discuss the volcanic land forms?
A volcano is a rupture on the crust from where gases, ashes and/or molten rock material – lava escape to the ground. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava. The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, Sulphur compounds and minor amounts of chlorine, hydrogen and argon.
A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past. In India, Barren Island (Andaman and Nicobar Islands) has the only active volcano of the country. Recently, population of a part of Hawaiian island was evacuated due to massive lava eruption.
Major types of Volcanoes:
Different types of Volcanoes can be described in terms of activity and can be:
- Still active and erupt frequently
- Dormant (temporarily inactive but not fully extinct)
- Extinct (never likely to erupt again)
Volcanoes can also be described by their shape or type
- Shield Volcanoes:
- Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth. The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes are not steep.
- Composite Volcanoes:
- These volcanoes are characterized by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions.
- These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure.
- Flood Basalt Provinces:
- These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows. The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.
- Mid-Ocean Ridge Volcanoes:
- These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins.
- Intrusive Forms:
- The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion.
- A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. They appear on the surface only after the denudation processes remove the overlying materials. They cover large areas, and at times, assume depth that may be several km.
- These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths.
- Lapolith, Phacolith and Sills:
- As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms.
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