Theory:

A volcano is a vent from which a combination of melted rock, solid rock debris, and gas is erupted. It has a reservoir of molten material below the surface (magma chamber) is called magma, when magma rises to the surface it is called lava.
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Fig. 9 – Structure of the Volcano
 
It is well known that the Earth’s interior is very hot and as the depth increases from the surface the temperature too increases and heat rises, ultimately the heated rock (magma) begins rising slowly toward the surface. The escaping gases provide the driving force for volcanic eruptions and these volcanic gases are known as steam. When this gas-filled magma rises and collects in large pools called magma chambers. As the magma becomes lighter in weight it rises through channels. When it reaches the surface, it discharges out of vents, or openings, at the tops of the channels. Thus, when the Magma (molten rock) discharges out of vents is known as lava. A caldera is a volcanic feature formed by the collapse of a volcano into itself, making it a large, special form of volcanic crater (Fig. 9).
Causes for Volcanic Eruptions
The main cause for a volcanic eruption is the change in pressure within the volcano, forcing the magma to overflow the chamber it is holding. The eruption caused by the movement of tectonic plates is the most common type. In this kind of eruption the nature of lava will be sticky and thick with temperatures ranges from 800 to 1,000º C.

The other kind of eruption is caused when the tectonic plates move away from each other allowing magma to rise and fill the gap. Here the lava will be thin with gentle explosion and the temperature ranges between 800 and 1,200º C.
 
Finally, a decrease in external pressure can trigger an eruption as it minimizes the holding limit of the volcano by increasing the pressures inside the magma chamber.
Nature of Volcanic Eruptions
Volcanic eruptions are classified into two types:
  1. Fissure Eruption: Slow rises of magma and spread over a vast area on the surface of the earth. e.g. Deccan Plateau in India  and Columbian Plateau in North America.
  2. Explosive Eruption: it is characterized by lava that is thrown very high into the atmosphere due to the rapid rise of magma. E.g. eruption of Krakatoa on 27th August 1883 in Indonesia.
The lava flow of a volcanic eruption is affected by the viscosity. When the lava is rich in Silica and little water it is highly viscous and flows slowly. If it contains less silica and more water it is less viscous and flows swiftly.
Types of Volcanoes
Three main basic types of volcanoes are:
  1. Shield Volcano: Several flows of basaltic lava occur in a given region, and they can eventually pile up into the shape of a large mountain, called a shield volcano.
  2. Cinder Cone: This is the smallest type of volcano with a height of a few hundred meters, is known as a cinder cone. Cinder cones generally consist largely of gravel-sized pyroclastics.
  3. Composite Cone (Stratovolcano): The third kind of volcano is a composite cone, here the eruptions are sometimes effusive and sometimes explosive. Composite cones are therefore composed of lava flows and pyroclastic materials.
  4. Plug Dome: This type is viscous silica-rich magma, pushed up into the vent of a volcanic cone without flowing beyond it, it forms a plug dome.
 
On the basis of periodicity of eruption, volcanoes are divided into three types:
  1. Active Volcanoes: Constantly ejects lava, gases, ashes and fragmental materials. These types of volcanoes are found along the mid oceanic ridges representing divergent plate margins and convergent plate margins.
  2. Dormant Volcanoes: These types of volcanoes become quiet for some period after their eruptions and suddenly erupt again with very violence and cause enormous damage to human health and wealth.
  3. Extinct Volcanoes: Volcanoes without the indications of eruption are considered as extinct. Here, the crater is filled up with water and lakes are found.
Distribution of Volcanoes
Generally, there is a direct relationship between volcanic regions, earthquake zones and plate boundaries. It is observed that around 80% of volcanic activity is found along subduction boundaries. Following this, around 15% of volcanic activity is found in the Mid-ocean spreading areas and continental rifts. Rest is intra-plate volcanism.
 
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Fig. 10 – Distribution of Volcanoes
 
Circum-Pacific ring of fire: Most of the high volcanic cones and volcanic mountains are found in this belt, where there is active subduction of the Pacific, Nazca, Cocos, and juan de fuca plates (Volcanoes of Sumatra and Java, which lies over the subduction zone between the Australian plate and the Eurasian plate).
 
Mid-Atlantic belt: Volcanoes of fissure eruption type (basaltic type) occur along the mid-oceanic ridge, where seafloor spreading is in progress.
 
Mid-Continental belt: This belt includes volcanoes of the alpine mountain chains, Mediterranean Sea and those fault zones of Eastern Africa (volcanoes are caused due to collision of African, Eurasian and Indian plates).
 
Intra-plate volcanoes: The inner parts of plates away from the margins. These parts are also called  hot spot volcanoes as they occur in the middle of plate boundaries (Volcanoes of Hawaiian Islands).