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IAS Geography Notes: Volcanoes

Last Updated - March 07, 2017

Volcanoes are one of the important topics in Geography of the year 2016-17. As complete one chapter is dedicated to Volcanoes, types and distribution of volcanoes, volcanic landforms etc. So, we will try to give you a conceptual view of Volcanoes throughout this article. It will be helpful for the aspiring candidates preparing for IAS 2017.

IAS is a national level entrance examination organized by UPSC. IAS Exam selection process comprises preliminary, mains and personal interview round.

A Volcano is an opening on the surface of a planet that allows material warmer than its surroundings to escape from its interior. Volcanoes occur when material significantly warmer than its surroundings to escape from its interior. When a volcano erupts, it expels lava, gases and rocks with tremendous force. An eruption can be explosive, sending material high into the sky or it can be calmer, with gentle flows of material.

These volcanic areas usually form mountains built from the many layers of rock or other material that collect around them. On earth, the erupted material can be liquid rock such as Lava and Magma. There are three reasons why Magma might rise and cause eruptions onto Earth’s surface.

  • Magma can rise when pieces of Earth’s crust called tectonic plates slowly move away from each other.
  • It rises up to fill in space. When this happens, underwater volcanoes can form.
  • Magma also rises when these tectonic plates move toward each other. When this happens, part of earth’s crust can be forced deep into its interior.

Read About IAS Mains Geography Syllabus


Difference between Magma & Lava

Magma is used to denote the molten rocks and related materials seen inside the earth. The Asthenosphere is the weaker zone of the mantle, usually is the source of magma. When the magma came out to the earth surface through the vent of volcano, it is called Lava. Therefore, Lava is the magma on earth surface. Volcanism is the process by which solid, liquid and gaseous material escape from the earth’s interior to the surface of the earth.


Types of Volcanoes

Volcanoes are categorized on the basis of nature of eruption and the layer developed at the surface. It can also be classified based on the frequency of eruption, mode of eruption and characteristic of Lava.

Shield Volcanoes

Shield Volcanoes are the largest volcanoes in the world as the Lava flows to a far distance. They are not very steep but are far and wider. They extend to a great height as well as distance. They have low slopes and consist almost entirely of frozen Lava. These volcanoes are mostly made up of basalt, a type of Lava that is very fluid when erupted. They are of low explosive in general, but if water gets into the vent they may turn explosive. The upcoming lava moves in the form of a fountain and throws out the cone at the top.

Cinder Cone Volcanoes

These are extrusive igneous rocks and small in size. It is also known as Scoria. It consists of loose, grainy cinders and almost no lava. They have a small crater on top and have very steep sides.

Composite Volcanoes

Composite Volcanoes are cone in shape with moderately steep sides. Sometimes, they have small craters in their summits. They are also called as Strato because they consist of layers of solid lava flows mixed with layers of sand. Composite volcanoes are characterized by the eruption of a cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions. Large quantities of pyroclastic materials and ashes find their way to the ground along with lava. This material accumulates in the vicinity of the vent openings and leading to the formation of layers and this makes the mount appears as composite volcanoes.

Caldera

Caldera is the most explosive of the volcanoes on the earth. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that its magma chamber is large and in close vicinity. A caldera differs from a cater in such a way that a caldera is a huge depression caused by a collapse after a large- scale eruption and a crater is a small, steep wide, volcanic depression bored out by an eruptive plume.

Flood Basalt Provinces

These volcanoes outflow highly fluid lava that flows for long distances. The Deccan traps are much larger flood basalt province. They are presently covering most of the Maharashtra plateau.

Mid-Ocean Ridge Volcanoes

There is a system of mid-ocean ridges more than 70,000 KM long that stretches through all the ocean basins. These volcanoes occur in the oceanic areas. The central portion of this ridge experiences frequent eruptions.

Distribution of Volcanoes

The distribution of the volcanoes in the world are found in three well defined belts as mentioned below:

  • The Circum Pacific Belt
  • The Mid-World Mountain Belt
  • 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 of Oceans. Interior parts of the continent are generally free from their activity. Most of the active volcanoes are found in the pacific region.


Volcanic Landforms

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 from the inside itself. Depending on the location of cooling of lava, igneous rocks are classified as:

Volcanic Igneous rocks – These are also known as Extrusive igneous rocks. Cooling of the rock occurs  at the surface of the earth. E.g; Basalt, Andhesite etc.

Plutonic Igneous rocks – These rocks are also known as Intrusive Igneous rocks. Cooling of the rocks takes place in the crust and not over the surface. E.g; Granite, Gabbro, Diorite etc. Plutonic Igneous rocks are divided into the following types according to their forms.

  1. Batholoths: A large body of magnetic material that cools in the deeper depth in the form of a large dome. These are granitic bodies. They sometimes appear on the earth surface when the denudation processes remove the overlying materials.
  2. Laccoliths: Large dome shaped intrusive bodies with a level base and pipe-like conduit from below. Resembles a composite volcano structure, but beneath the earth.
  3. Lapoliths: They are saucer shaped, concave to the sky.
  4. Phacoliths: Wavy materials which have a definite conduit to source beneath.
  5. Sheets: They are near the horizontals bodies of intrusive igneous rocks. Thinner ones are called as sheets and while thick horizontal deposits are called sills.
  6. Dykes: When the lava comes out through cracks, they solidify almost perpendicular to the ground to form wall like structures called Dykes.

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