Active volcanoes are dangerous for a number of reasons, including their tendency to erupt and send poisonous gas and choking ash across wide areas. Even when they aren't erupting, volcanoes can potentially cause major landslides and sudden mudflows.
Volcanic hazards can be grouped into three categories: ejecta, flow hazards and water hazards. The best-known hazards of living near a volcano are the risks associated with ejecta. Most volcanoes vent dangerous gases, such as sulfur dioxide and hydrogen sulfide, that are toxic to animals. Volcanoes also eject ash and chunks of debris called tephra that can fall out over hundreds of square miles and bury everything in their path. Lava usually flows slowly, but it is glowing hot and burns through everything it comes into contact with.
Flow hazards around a volcano involve rockslides of various threat levels. Simple rockslides occur often on the slopes of active volcanoes as loose tephra from old eruptions suddenly gives way and falls down the grade. One special kind of landslide called a pyroclastic flow occurs when a dome collapses or the side of a volcano erupts. A pyroclastic flow resembles a gigantic shotgun blast of superheated rock that can level the surrounding countryside. A related, but more sinister, threat is the lahar. Lahars occur when water comes into contact with hot magma and begins flowing. Along the way, this hot flash flood picks up enough mud and rock to take on the consistency of wet concrete, and it can flow at speeds of up to 40 mph for up to 50 miles, devastating everything in its path.