Volcanoes can cause great damage and loss of life.
One famous example is the eruption of Vesuvius in 79 AD, which buried the Roman towns of Pompeii and Herculaneum.
Many of the people killed in that eruption were killed by pyroclastic flows, which are hot clouds of gas full of tiny particles of volcanic rocks.
These clouds rush down at high speed.
The towns were buried in volcanic ash, and excavation has revealed buildings and streets.
The 79 AD eruption of Vesuvius, and the buried towns, are famous in the history of geology and in archaeology.
This diagram gives examples of some types of volcanic activity.