A volcano is a mountain having an opening on the surface of the earth from which fire, smoke and ashes come out continuously. Mountains of this type are created by upheavals inside the Earth. The formation of volcanoes can be understood as follows. Temperature inside the Earth goes on increasing as we go into the interior of the Earth. At a depth of approximately 30 kilometres, the temperature is so high that it can melt rocks. When rocks inside the Earth get melted, they start expanding. These molten rocks are known as ‘magma’. In some parts of the Earth this magma starts coming up through the openings in the Earth’s crust. When the pressure exerted by this magma is considerably high and the Earth’s crust at some places is weak, the crust erupts at these places and as a result, hot gases, liquid and solids material of the red molten rocks start coming out. This is called volcanic eruption. The ejected hot smoke, ashes and stone pieces constitute what we call lava. This lava then solidifies in the shape of a cone and, on cooling, it takes the form of a mountain on the surface the Earth. Fire and smoke keep on flowing out of the opening of the volcano until the molten material inside the Earth is exhausted. Such volcanoes, from which lava stops flowing out, are called ‘dead volcanoes’. There are more than 450 volcanoes in the world. The number of volcanoes in Indonesia is quite large. The highest dead volcano of the world is in Argentina, it is 6,960 metres high. One of the most violent volcanic eruptions was on the island of Krakatoa, near Sumatra, in 1883, which produced tidal waves in the oceans throughout the world.