What is a Volcano?
It is an opening in crust of earth through which rock fragments, steam, molten rock material, lava, and other hot gasses are gradually or violently ejected during an eruption.
These openings occur where rock strata are relatively weak.
Beneath the solid outer crust, there is hot molten rock material known as magma.
When this magma comes to the earth’s surface it is known as lava.
The process by which solid, liquid and gaseous materials escape from the earth’s interior to the surface of the earth is called volcanism.
Volcanoes are classified on the basis of the nature of volcanism.
On the basis of the frequency of eruption, volcanoes are of three types: Active, Dormant and Extinct.
 Active Volcano
Those volcanoes which erupt frequently or have erupted recently are termed as active Volcano.
Important among these include Stromboli in the Mediterranean, Krakatoa in Indonesia, Mayon in the Philippines, Mauna Loa in Hawai Islands and Barren Island in India.
 Dormant Volcano
The volcanoes which have not erupted in recent times are known as a dormant volcano.
They are as such the ‘sleeping volcanoes’.
Important among these are Vesuvius of Italy, Cotopaxi in South America.
 Extinct Volcano
Contrary to these two, there are volcanoes that have not erupted in historical times.
These are called extinct volcanoes.
Examples: Kilimanjaro of Tanzania and Mount Popa of Myanmar (Burma)
It is not, always very simple to categorize a volcano as dormant or extinct.
For example, the Vesuvius and Krakatoa became suddenly active after lying dormant for hundreds of years.
Volcanoes are divided into two types on the basis of mode of eruption.
 Central type of volcanoes
This type of volcano erupts from a vent or hole.
Different types of domes or conical hills are formed by this type of eruption depending on the nature of erupted materials.
The majority of volcanic eruptions in the world are of this type.
The other characteristic of this mode of eruption is that it is marked by a violent explosion due to a sudden escape of gases and molten rocks through the hole.
Visuviousand Fuji-yama belongs to this group of volcanoes.
 Fissure type volcanoes
Due to earthquakes or faulting, deep elongated cracks develop and he magma starts flowing through them quietly.
This mode of eruption is called a fissure type of eruption.
This eruption helps in the formation of thick horizontal sheets of lava or a low dome-shaped volcano with a broad base.
It may also form what is identified as lava plateaus, and lava shields, Deccan Traps of India is one example of fissure type of eruption.
There are two types of volcanoes based on the fluidity of lava:
 Volcanoes of Basic lava
Since the basic lava is rich in metallic minerals and has a low melting point, it has greater fluidity.
In this type of eruption, lava flows slowly far and wide with higher intensity, spreading over a vast area in thin sheets.
Thus, it contributes to shields and lava dome formation.
The shield volcano of Hawaiian Island in the Pacific Ocean is one of these volcanoes.
 Volcanoes of acid lava
Acid lava is rich in silica.
It has a relatively high melting point.
Therefore it is highly viscous and solidifies quickly.
Acid lava volcanoes produce higher land features with steeper slopes.
The cones of acid lava are steeper than the basic shields of lava.