We know that the Earth’s interior contains molten magma and that it is under great pressure. This makes it come out through any opening or fault line to the surface of the Earth.The opening through which the magma, gas and steam come out is called a vent. The molten magma, which comes to the surface through a vent, may flow out silently burning everything in its way, or it may burst out accompanied by gases, ash and steam.The top of a volcanic cone has a circular depression with high walls. This depression is called a crater. Often when there is no volcanic activity for a very long time, the lava solidifies in the vent forming a volcanic plug. Volcanoes are generally classified according to their activity.An active volcano is one, which has erupted recently and may erupt again. There are some like Mt. Etna in Sicily, which erupt quite often.A dormant volcano is one, which is known to have erupted a long time ago and has every possibility of erupting again. Mt. Fuji is a dormant or sleeping volcano. Extinct volcanoes are the ones, which have not erupted for thousands of years. There is no evidence of volcanic activity in recent times. Mt. Kilimanjaro in Taanzania is one such extinct volcano.