A volcano is a vent or an opening in the earth’s crust through which molten rock material, rock fragments, ash, steam and other hot gases are emitted slowly or forcefully in the course of an eruption.
These materials are thrown out from the hot interior of the earth to its surface.
Such vents or openings occur in those parts of the earth’s crust where rock strata are relatively weak.
Actually, volcanoes are evidence of the presence of the intense heat and pressure existing within the earth.
Hot molten rock material beneath the solid outer crust is known as magma.
When this magma is thrown out from the magma chamber 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.
The 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.
Types of Volcanoes
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.
The volcanoes which erupt frequently or have erupted recently or are in action currently are called active volcanoes.
Important among these include Stromboli in the Mediterranean, Krakatoa in Indonesia, Mayon in Philippines, Mauna Loa in Hawai Islands and Barren Island in India.
The volcanoes which have not erupted in recent times are known as dormant volcano.
They are as such the ‘sleeping volcanoes’.
Important among these are Vesuvius of Italy, Cotopaxi in South America.
Contrary to these two, there are volcanoes which have not erupted in historical times.
These are called extinct volcanoes.
Mount Popa of Myanmar (Burma) and Kilimanjaro of Tanzania are important extinct volcanoes.
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.
On the basis of mode of eruption, volcanoes are divided into two types:
 Central type of volcanoes
When the eruption in a volcano takes place from a vent or a hole, it is called a central type of volcano.
Different types of domes or conical hills are formed by this type of eruption depending on the nature of erupted materials.
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.
Visuvious and Fuji-yama belong to this group of volcanoes.
 Fissure type volcanoes
Sometimes, deep elongated cracks develop due to earthquakes or faulting.
The magma starts flowing through them quietly.
This mode of eruption is called 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.
On the basis of the fluidity of lava there are two types of volcanoes:
 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 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.
The shield volcano of Hawaiian Island in Pacific ocean is one of these volcanoes.
 Volcanoes of acid lava
Contrary to basic lava, acid lava is rich in silica and has a relatively high melting point.
Therefore it is highly viscous and solidifies quickly.
Hence, the acid lava volcanoes cause the formation of usually higher land features with steeper slopes.
Acid lava cones are of steeper slopes than basic lava shields.