What are the effects of volcanoes?

A volcano is actually an opening or a fissure, in the earth' crust , through which lava or molten rocks, ash and toxic gases present below the surface of Earth, are discharged by a sudden, violent eruption. Sometimes, it can be a mountain like structure with a bowl shaped depression at the top, through which these substances are expelled. The term volcano, is derived from the name of the Roman God of fire, Vulcan. Volcanic structures are usually formed at places where the tectonic plates are either converging or diverging. A stretching or thinning of the Earth's crust, can also lead to the formation of volcanoes. They are often classified into three types, on the basis of their frequency of eruptions, i.e. active, dormant and extinct. The active volcanoes are characterized by regular eruptions, while the dormant volcanoes are those that erupted in the past, but are silent now. On the other hand, an extinct volcano is the one that erupted in the remote past and is unlikely to erupt again.