Volcanoes
In the simplest of terms, a volcano can be thought of as a mountain with an opening that goes downward below the surface of the earth. Deep below the surface, earth is extremely hot. This heat melts some of the rocks which become a thick flowing substance called magma. This magma, being lighter than surrounding rocks rises up through the opening and gets collected in magma chambers that are a part of the mountain visible to all. Sometimes, this magma comes out of the structure through fissures and cracks, and this is when we say that the volcano has erupted. The hot, flowing liquid that comes out from the volcano is called lava which is nothing but magma that is formed inside the volcano.
The lava, when it is thin and fast moving, causes more destruction than when it is thick and slow moving. More gases erupt out of thin lava than when it is thick. Destruction caused by lava is enormous, but it rarely kills people as people can get away from the site in time easily. It is when explosions accompany volcanic eruptions that they become more dangerous because of the presence of deadly ash that can suffocate plants, animals and human beings. Mudflows from volcanoes have sometimes buried entire villages and cities existing around them.
Volcanoes remain silent for thousands of years and then suddenly become active which is why people around them are not aware of the dangers.