This article is about volcanoes, their types and their distribution in the world. This topic is very important for exams conducted by UPSC, SSC and other state level commissions and boards. We urge you to read this topic very thoroughly to perform better in your exams.
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Volcano
A volcano is a vent or opening in the earth’s crust through which molten rock materials, rock fragments, ash, steam, and other hot gases are emitted slowly or forcefully in the course of an eruption.
Volcanoes are evidence of the presence of the intense heat and pressure existing within the Earth.
Constituents of volcanic eruption
The constituents of the volcanic eruption are water vapor, gases, lava and other pyroclastic materials. These are discussed below;
1. Water vapor and gases
They account for around 60% of the materials thrown out during volcanic eruption.
The water vapor comes out as superheated steam and the gases that come out include Sulfur dioxide, oxides of carbon and Nitrogen and Methane.
2. Lava
Hot molten rock materials beneath the solid outer crust is called magma. When this magma is thrown out from the magma chamber to the earth’s surface, it’s known as lava.
Magma is of two types namely Acidic magma and basic magma. The differences between the two are tabulated below;
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| Characteristics | Acidic Magma | Basic Magma |
|---|---|---|
| Composition | Higher concentration of silica and less iron | Higher iron and less silica |
| Color | Lighter in color | Darker in color |
| Temperature | – | Hottest of all the magma/lava |
| Nature | Highly thick/viscous: flows only for shorter distances before solidifying | Fluidic in nature: flows quickly and covers longer distances before solidifying |
| Melting Point | Materials have very high melting point | Materials have low melting point |
| Eruption Association | Generally associated with violent eruptions and elevated landforms such as mountains | Generally associated with fissure eruptions and forms volcanic plateaus/plains |
| Example | Granite | Basalt |
3. Pyroclastic Material
It consists of broken rock fragments, volcanic ash, volcanic dust and large stones.
Important Terms
1. Crater:
- Volcanic materials accumulate around the opening or hole, taking the form of a cone. The top of the cone has a funnel-shaped depression, which is called its crater.
2. Caldera
- Craters are formed by the outward explosion of rocks and other materials from a volcano.
- When a magma chamber beneath the volcano empties quickly and fails to support the weight of the land above it, the volcano collapses inwards and forms a caldera.
Types of Volcanoes:
On the basis of frequency of eruption: 3 types
(1) Active:
- Volcanoes which erupt frequently, or have erupted recently, or are in action currently.
- Examples: Stromboli in Mediterranean, Krakatoa in Indonesia, Mayon in Philippines, Mauna Loa in Hawaiian islands, & Barren Island in India.
(2) Dormant:
- Volcanoes which have not erupted in recent times.
- Examples: Vesuvius of Italy, Cotopaxi in South America.
(3) Extinct:
- Volcanoes which have not erupted in historical times.
- Examples: Mt. Popa of Myanmar (Burma) & Kilimanjaro of Tanzania.
On the basis of mode of eruption:
Volcanoes are of 2 Types:
(1) Central Type of Volcanoes:
- When the eruption in a volcano takes place from a vent or a hole, it’s called the central type of volcano.
- Majority of volcanic eruptions in the world are of this type, marked by violent explosions due to sudden escape of gases & molten rocks through the hole.
- Examples: Vesuvius & Fuji-Yama.
(2) Fissure Type Volcanoes:
- Sometimes, deep elongated cracks develop due to EQ (Earthquake) or faulting. The magma starts flowing through them quietly.
- This eruption helps in the formation of thick horizontal sheets of lava or a low dome-shaped volcano with a broad base.
- Examples: Deccan Traps of India.
On the basis of the fluidity of lava
On the basis of fluidity of lava, volcanoes are of two types;
(1) Basic/Basaltic/Shield Lava:
- Since the basic lava is rich in metallic minerals and has a low melting point, it has greater fluidity.
- Lava flows rapidly far and wide quietly with greater speed and spreads out in thin sheets over a large area. Thus, it leads to the formation of shields and lava domes.
- Examples: The shield volcanoes of Hawaiian Island in the Pacific Ocean.
- This has high Fe (Iron) and Mg (Magnesium) content but a low silica content.
(2) Acidic/Composite/Strato Volcano Lava:
- Contrary to basic lava, acidic lava is rich in silica and has a relatively high melting point.
- It is highly viscous and solidifies quickly.
- Hence, the acid lava volcanoes cause the formation of usually higher land features with steeper slopes.
Volcanic Landforms
The landforms that arise due to the volcanic eruption are called volcanic landforms. They are of two types;
- Extrusive
- Intrusive
1. Extrusive Volcanic Landforms:
These are the landforms formed by the material thrown out of the volcanoes. They can be divided into elevated landforms and depressed landforms.
(A) Elevated Land Forms
The elevated volcanic landforms include volcanic mountains and plateaus.
Volcanic Mountains
Depending upon the materials and the height, the volcanic mountains are classified into the following four types;
(1) Cinder Cone:
- It is produced due to the deposition of ash and broken raw material (cinder).
- These are weak cones and are made up of loose sediments and hence their height is very low.
(2) Composite cones or Strato Volcanoes
- These are made up of alternate layers of cinder and lava.
- When lava solidifies, it cements the underlying cinder and hence these mountains are stronger and highly resistant to erosion.
- They form the highest volcanic mountains of the world.
- Its examples are Mt. Acancagua in South America and Mt. Fijiyana in Japan.
(3) Parasitic Cone:
- In this cone, several secondary branches originate from the main feeder vent.
- It is characterised by the presence of various smaller cones or mountains on a main mountain.
- Its example is Mt. Shasta.
(4) Shield Cone or Basic Lava Cone:
- It is formed upon the solidification of basic (basaltic) lava.
- Since basaltic lava takes a longer time to solidify, hence such mountains are wider and of very small height.
Volcanic Plateaus
These are very large and extensive plateaus formed due to the basaltic lava.
- When this basaltic lava undergoes weathering, it leads to the formation of black soil, which is very important for crops like cotton.
- Its examples are Deccan plateau in India, Columbia snake plateau in USA, Siberian plateau in Russia and Drakensburg plateau in South Africa.
(B) Depressed Land Forms
The depressed volcanic landforms include craters and calderas
(1) Craters:
Depressions formed at the mouth of a volcanic vent is called a crater. A volcanic mouth is usually funnel-shaped. When a crater is filled with water, it becomes a crater lake.
Example: Lonar Lake in Maharashtra.
(2) Calderas: Enlarged forms of a crater. Formed due to collapsing of a volcano.
2. Intrusive Volcanic Landforms:
These landforms are formed when magma cools below the surface of earth.
(1) Batholiths:
- A large body of magmatic material (generally Granite) that cools in the deeper depths in the form of a large dome.
- It is very deep and gets exposed rarely.
(2) Laccoliths:
- Large dome-shaped intrusive bodies with a level base and pipe-like conduit from below.
(3) Lapoliths:
- They are saucer-shaped, concave to the sky.
- They are formed when magma finds some horizontal space to travel along. If it solidifies in the saucer shape, it is called lapolith.
(4) Sheets/Sills:
- They are the near-horizontal bodies of intrusive igneous rocks. Thinner ones are sheets, and thicker horizontal deposits are sills.
(5) Dykes:
- When the lava comes out through cracks and fissures, they solidify almost at the ground to form wall-like structures called dykes.
- it is almost perpendicular to the ground.
(6) Phacolith:
- It is a lens shaped mass found in pairs occupying a top of an anticline ot base of a syncline.
- It is always connected with a pipe like structure from below.
Hot Springs and Geysers
Apart from the above discussed landforms, volcanoes also give rise to hot springs and geysers around the world.
- When the ground water comes in touch with the underlying magma, it becomes hot. Due to this increase in temperature, the water expands and comes out in the form of hot springs or geysers.
- The fundamental difference between a geyser and a hot spring is that in geysers water comes out with great force.
- The regions of hot springs and geysers are famous for;
- medical tourism.
- geothermal energy.
Worldwide Distribution of Volcanoes
The world distribution of volcanoes can be explained with the help of Plate Tectonic theory. Volcanoes are associated with convergent and divergent plate boundaries. There are four major volcanic zones:
(1) Circum-Pacific Belt / Pacific Ring of Fire:
- It has 60% of the world’s volcanoes or volcanic eruptions.
- Represents a convergent plate boundary, where:
- The Pacific plate is subducting below the Indo-Australian plate, Eurasian plate, North American plate, and Antarctic plate.
- Starts from Antarctica and includes major volcanoes of:
- New Zealand, Philippines, Japan, Rockies, and Andes.
(2) Mid-Continental Belt:
- It is a convergent plate boundary, where:
- The African plate and Indian plate are subducting below the Asian plate.
- This region has 20% of the world’s volcanoes.
- Important mountains in this region are:
- Mt. Stromboli, Mt. Etna, etc.
- The mid-continental belt has one volcano-free zone and that is the Himalayan zone.
(3) Mid-Oceanic Belt:
- It has 15% of the world’s volcanoes, representing a divergent plate boundary.
- Volcanoes occur along the mid-oceanic ridges and are of the silent fissure type of eruption.
- Example: Volcanoes of Iceland.
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