A volcano is a place where gases, ashes and/or molten rock material — lava — escape to the ground.
The mantle contains a weaker zone called asthenosphere.
The material in the upper mantle(asthenosphere ) portion is called magma.
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 ninogen compounds, sulphur compounds and minor amounts of chlorene, hydrogen and argon.
Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Major types of volcanoes are as follows:
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.
- They become explosive if somehow water gets into the vent; otherwise, they are characterised by low-explosivity.
Composite Volcanoes
- These yolcanoes are characterised by eruptions of cooler and more viscous lavas than basalt.
- These volcanoes often result in explosive eruptions.
- This material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite vo1canoes.more than 50 m. Individual flows may extend for hundreds of km.
- The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.
Mid-Ocean Ridee Volcanoes
- These volcanoes occur in the oceanic areas.
- There is a system of mid-ocean ridges more than 70,000 km long that sketches through all the ocean basins.
- The central portion of this ridge experiences frequent eruptions.
