By now you should be aware that in the bowels of the Earth all is not still. On the contrary, it is very active. One obvious example the the Earth’s internal working is the activity of volcanoes. There are many famous instances of a volcano erupting and spewing white hot, molten rock from it mouth; if you have been to Pompeii or just seen pictures of it you will realize how a whole city and most of its inhabitants can be engulfed in the fiery ash suddenly erupting from what was thought to have been dead volcano.
A more recent example was the eruption of Mount Saint Helens in the United States in 1980. This eruption had the force of 500 of the nuclear bombs dropped on Hiroshima in 1945. Perhaps the most famous of all volcanoes is Krakatoa. This volcano had remained dormant for 200 years, when on 27 August 1883, the volcano exploded with such force that the whole island was ripped apart. The noise was heard almost 5000 kilometers away in Australia, and as tidal wave of 36 meters high was sent across the sea to Java, killing 36,000 people. You can understand, then, that it is a very great force that makes volcanoes erupt.
Volcanoes were once thought to be burning mountains and it is not difficult to understand why. But nowadays, a volcano is usually defined as a direct feed from a magma chamber in the interior of the Earth to the Surface, Remember that a magma is any hot material within the Earth which can flow and penetrate into or through the surface rocks. A magma, then, is usually thought to consist of a kind of ‘porridge’ of solid rock material, liquids and semi plastic substances which are made to flow by the lubrication of hot gases under great pressures.
Two main causes are responsible for the birth of a volcano. Explosive may be caused by pressure resulting form the build up of gases within the Earth or a crack int the crust can be opened up forming a passage or vent. Volcanic lava can then flow through it. It is important to remember that the classic cone shaped body which you might immediately think of as a volcano, only forms as a result of volcanic activity. It is the pile of ash and cooled lava that has flowed or been thrown out of the original fissure or crack. A volcano may erupt only once, and it may not emit enough material to build up the well known cone. This cone usually occurs when there are successive eruptions from the same vent.