A volcano is an opening where gas and rock come from inside the earth on to the surface. The rock is usually hot – in most cases, it is so hot that it is molten (melted).
Sometimes molten rock flows out quietly. At other times expanding gas causes explosions. They throw pieces of rock high into the air to shower down around the opening. An outburst of either kind is called a volcanic eruption. The opening where it takes place is a volcanic vent.Molten rock is called magma by geologists. It probably always contains gas. The gas may be dissolved in the magma, like the gas in soda pop before bottle is opened. Or it may be in the form of bubbles, like those that appear in soda pop after a bottle opened.
Magma also usually contains crystals of minerals that are forming ini it. When magma is very hot, the crystals are dissolved in it, just as sugar dissolves in boiling water. When magma reaches the earth’s surface, it cols and freezes into a solid rock. Magma that has come out on to the surfaceis called lava .
There are few kinds of volcanic eruptions. Some volcanic eruptions are gentle outpourings of lave. Some are violently explosive. One of the greatest explosive eruptions ever seen took place on 1883 in Krakatau, a volcanic island that is part of Indonesia. For three months the volcano had been mildly active. Then the eruption suddenly became violent. Tremendous explosions were heard as far as 6.400 kilometres away. fine ash was blown high into the upper atmosphere. This dust drifted slowly three times around the earth and caused briliant red sunsets as far away as England. The eruption destroyed most of the Krakatu Island and parts of nearby islands. The top of the mountains sank downl beneath the sea. The sudden sinking and great explosion caused tsunamis. These huge waves of water swept up on to the shores of Java and Sumatera, drowning more than 35.000 persons.
However, there are some benefits from volcanoes. Most volcanic energy is in the form of heat. This heat could be a very important source of power. But so far we have been able to use only a very small part of it. In several areas, volcanic stream is being used to make electricity. The most important result of a volcanic action is fertile soil. Volcanoes bring to the surface many of the minerals that plants need to grow well. In a way,this has led to trouble. People have clustered in the fertile volcanic regions. And as a result, eruptions have sometimes killed many people and done great damage to their properties.