Volcanoes are the openings on the surface of the earth through which the molten rock, mixed with various gases, ash and steam, from the core of the earth explodes. This molten rock has a temperature ranging from 700 to 1000 degree Celsius and it is known as magma when it is underground and the moment it is erupted out of the volcano, it is called lava. It is the lava and the ashes that get accumulated forming a cone shaped structure of the volcano. The lava solidifies and piles up after every eruption and therefore over the years some of the Volcanoes are mountain-like. Based on its frequency of eruption, there are active, dormant and extinct Volcanoes. Active Volcanoes, such as Mount Vesuvius of Italy and Suribachi of Japan, are currently erupting with intervals, dormant Volcanoes have been silent for a long time but could explode again in future and the extinct Volcanoes have remained dormant for more than 10,000 years. Mount Fuji of Japan and Mount Kea of Hawaiian Islands are dormant Volcanoes while Mount Kilimanjaro of Tanzania and Mount Warning of Australia are two of the famous extinct Volcanoes.

Volcanoes are majorly divided into three categories depending on its shapes which play an important role in the size, style and frequency of volcanic eruptions. They are:

• Shield volcano: It has a broad and flattened dome-like shape. It is formed after a gentle eruption caused by the very hot and runny magma. Thus, due to the gentle eruption, the lava flows slowly over the surfaces and cools down. It gradually runs down the slope reaching great distances.

• Composite volcano: It is also known as Strato Volcanoes and the Volcanoes have steep sided cones. The Volcanoes got this shape because of the slightly cooler, thick and sticky magma that made it hard for the gas bubbles to expand and escape. Thus, it generated more pressure resulting in a violent explosion throwing the lava high up in the air.

• Caldera volcano: It is the most dangerous category of Volcanoes. They have the coolest lava with temperature ranging from 650 to 800 degree Celsius. It also has the most powerful explosion that it empties the entire magma from the magma chamber. The depression caused by the eruption then is basin-shaped, circular and usually several kilometres in diameter.

The volcanic eruptions are caused because of the magma and the gases trapped in it. Since the magmas are the molten rocks inside the Earth, it has the same mass with the surrounding solid rocks but its volume increases. Thus the density of the magmas are lesser than the rocks (density=mass/volume). Because of the lesser density and the buoyancy, the magmas are moved towards the surface of the earth to erupt. Another reason behind its eruption is also the substances contained in the magma. The core of the Earth has water, sulphur dioxide and carbon dioxide dissolved in it. But as the magmas move upward towards the surface of the earth, it faces a decrease in the pressure and the solubility of the water and the gases decreases forming bubbles. The closer it gets to the surface of the earth, more bubbles are formed increasing its volume. When the volume of the bubbles reach 75 percent, the magmas that contain both molten and solid substances explode ferociously due to the increasing pressure of the bubbles. Another cause of the eruption is also the flowing in of new molten magmas. When magmas, whether of same or different compositions, flow into the chamber where there are magmas already in it, it moves the magmas to the surface and thus it erupts.

There are five different types of eruptions in which the Volcanoes all over the world erupt. They are as follows:

• Hawaiian eruption: Here, the lava erupts in the air from various vents in jets. The eruption follows a phenomenon known as fire fountaining where the magma from the vents erupt for hours or even days forming a fountain. It got its name because of the Kilauea volcano on the Big Island of Hawaii which is famous for its fire fountaining.

• Strombolian eruption: In this type of eruption, the lava bursts out from the crater every few minutes at various intervals. The explosions make lava reach heights of hundreds of meters which are mainly due to the bursting of large gas bubbles. It was named after the volcano of the Italian island of Stromboli.

• Vulcanian eruption: It is a very powerful eruption where the substances travel for more than 350 meters per second and go up in the air for several kilometres. Though it is violent, it is a short and relatively small explosion. It produces the rock fragments known as tephra and ash clouds. The eruption goes on for days, months or even years. It was named after the Italian island of Vulcano.

• Plinian eruption: It is the largest and most violent type of eruption. It releases columns of gas and ash up to a height of 50 kilometres at a speed of hundreds of meters per second. The ashes drift away from the column and travel for hundreds or thousands of kilometres thus forming a mushroom shape or resembling an Italian pine tree. It was named after a Roman historian, Pliny the Younger who discovered it after seeing the 79 AD Mount Vesuvius eruption.

• Surtseyan eruption: It usually happens to the Volcanoes that grew from underneath a sea. It thus has a hydromagnetic eruption where the lave mixes with the water. It forms ash, steam and scoria due to water’s contact with lava. The name was taken from the volcanic island of Surtsey which displayed similar form of eruption in the south coast of Iceland between 1963 and 1965.

A volcano also has its own anatomy that its eruptions follow. At the bottom of it, way down underneath the surface of the earth, there is the ‘magma chamber’ which is like a pool where magmas are stored or remain accumulated. It is, rather, a reservoir of magma. When the Volcanoes erupt, the magma travels to the surface of the earth through a canal known as ‘vent or pipe’. The vent then opens up at a bowl shaped opening known as ‘crater’ from where the magma explodes and turns into lava. The lava then flows down the sides of the crater and forms ‘volcanic cone’, which is the outer part of volcano whereas the solidified fragments in the lava explode as clouds of ash. The steams, gases and magma also divert its flow from the main vent and form ‘side vents’. The secondary (side) vents through which steam and gases explode are known as ‘fumeroles’ and in some of the side vents the magma gets solidified and forms dykes, sill and laccoliths. Dykes follow the straight path of the side vent, vertical to the layer of the rocks, sills are parallel to the existing rock-layers and laccoliths are the solidified magma that have been injected between two rock-layers and is connected to a dyke.

There are more than 10,000 identified Volcanoes out of which nearly 500 are active. Some of the most famous Volcanoes are:\

• Muana Loa of Hawaii: It is not only the highest volcano but also the highest mountain in the world. It has its roots on the seabed and it stands for 10,200 meters.

• Cotopaxi of Ecuador: It is the highest active volcano with a height of 5,911 meters above the sea level. Since 1738, it has had 50 eruptions. Its last eruption happened in 1904.

• Kilauea of Hawaii: It is the most active volcano and it has been continuously erupting since 1983 at a rate of 5 cubic meters per second.

• Stromboli of Italy: It is known as ‘Lighthouse of the Mediterranean’ for its continuous mild eruptions ever since the Roman times. Thus, it is the most active volcano in the history.

• Tambora of Indonesia: The eruption of Tambora in 1815 had killed the maximum number of people ever killed by any volcanic eruption. Ten thousand people were directly killed by the eruption and another 82,000 died of starvation and diseases that followed.

• Krakatau of Indonesia: The Krakatau eruption of 1883 has been recorded as the greatest explosion for it destroyed most of the island of Sundra Strait and also created a tsunami killing 36,000 people on the neighbouring islands.

There are some of the major volcanic eruptions that not only killed people but also destroyed towns, cities and even civilisations. Some of them are:

• The volcanic explosion of the Greek island, Santorini in 1626 B.C. destroyed the Minoan civilisation of Crete. The eruption had ash cloud that reached for 37 kilometres creating a tsunami of 50 meters.

• The Vesuvius eruption of 79 A.D. had the city of Pompeii in Italy buried 3 meters under the ashes from the eruption. Poisonous sulphur from the eruption had also killed nearly 2000 people.

• The towns of San Juan Parangaricutiro and Paricutin were devastated as the Paricurin eruption in Mexico in 1943-1944 buried the towns under the lava.

• The Nyirangongo eruption in January 2002 damaged 14 villages of D.R. Congo. Twelve thousand families were homeless and around 500,000 people had to evacuate their houses in the city of Goma.