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IAS Prelims : GS Physical Geography : Volcanoes

Feb 24, 2015 18:15 IST

    Volcano is a place on Earth’s surface where molten rock, gases and pyroclastic debris escape to the ground and erupt through the earth’s crust. When magma (molten rock) from within earth’s upper mantle (asthenosphere) finds its way to the surface, volcanoes are formed. Once magma starts moving towards the crust or it reaches the surface it is called lava.

    The process of volcanic eruptions has been described by plate tectonic theory. This theory holds that earth’s crust is divided into several major and minor rigid slabs called plates. These plates move horizontally over the underlying asthenosphere. These plate sometimes move towards each other, sometimes they move apart and other times one will sink while the other rises over it. When a tectonic plate sinks, it melts and then finds its way to the surface. Since such phenomenon occurs only along plate boundaries, volcanoes are mostly found along plate boundaries.

    Stages of volcanoes

    Scientists have categorized volcanoes into three main categories: active, dormant, and extinct. An active volcano is one which has recently erupted and there is a possibility that it may erupt soon. A dormant volcano is one which has not erupted in a long time but there is a possibility it can erupt in the future. An extinct volcano is one which has erupted thousands of years ago and there’s no possibility of eruption.

    Types of Volcanoes

    Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Major types of volcanoes are as follows:

    1.    Shield Volcanoes: They are shaped like a bowl or shield in the middle with long gentle slopes. The reason for their gentle slopes is that they are made up of basalt, a type of lava that is very fluid when erupted. They are characterized by low-explosivity.


    2.    Composite Volcanoes (Stratovolcanoes): These are characterized by eruptions of cooler and more viscous lavas than basalt. The erupted materials are accumulated near the vicinity of the vent openings leading to formation of layers. These layers make volcano appear as composite ones. They are characterized by high-explosivity.


    3.    Caldera: These are the most explosives volcanoes. Because of their high- explosivity they do not build any tall structure, rather they tend to collapse on themselves thus forming a steep bowl shaped depression called caldera.


    4.    Flood basalt provinces:  These are formed by outpouring or series of eruptions of highly fluid lava that flows for long distances. They are also called traps. Some parts of the world are covered by thousands of square kilometers of thick basalt lava flows.

    5.    Mid-Oceanic ridge volcanoes: The mid-ocean ridge is a continuous range of undersea volcanic mountains. The central portion of these mountains experience frequent volcanic eruptions.

    Volcanoes can have both advantageous and disastrous effects.

    Positive effects

    1.    The natural scenery created by volcanic eruptions becomes a tourist attraction.

    2.    It helps agriculture as the lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil.

    3.    The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy.

    Negative effects

    1.    Volcanic eruptions close to human settlements may destroy lives, property and infrastructure.

    2.    If the ash and mud from a volcanic eruption mix with rain water or melting snow, fast moving mudflows are created. These flows are called lahars. Lava flows and lahars can destroy settlements and affect agriculture and forests.

    3.    Ash discharged into the atmosphere affects bird migration. They also create problems for aircrafts due to low visibility.

    4.    CO2 and other gases released add to the green house effect. Toxic gases create health problems while other gases like Sulphur di oxide cause acid rain.

    The cooling of magma during or after volcanic eruptions results in various landforms such as batholiths, lacoliths, lapoliths, phacoliths, sills and dykes.

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