when do volcanoes become dangerous?

Volcanoes have a fascinating history that can teach us a lot about how to understand potential volcanic hazards. It is important to know the history of past eruptions so we can be better prepared for future eruptions. In the 1800s, vulcanologist Giuseppe Mercalli studied the eruptive history of Vesuvius, and in doing so created what is now known as an eruption column. He was able to predict which areas would be hit first and last based on how high the plume reached from deposits below. These plume height lines allowed residents of nearby towns to evacuate or take precautions.

Deep inside the Earth it is very hot and some rocks slowly starts melting to form a thick flowing substance called magma. Magma is lighter than the rock around it, so it rises and collects in the rock crevices.When the magma find cracks on the rocks it pushes through those vents and cracks and rises to the top forcing a volcanic eruption.

Volcanic lava travels a long way around 20 miles radius from the area of activity and some can travel up to 100 miles radius.So the human population,animals and property are all at risk if they are in the vicinity of these danger zone.