Pyroclastic flows and lahars are the greatest volcanic hazards. More people have died due to these hazards than any other volcanic hazard.
Pyroclastic flows are fluidized masses of rock fragments and gases that move rapidly in response to gravity. They can form when an eruption column collapses, or as the result of gravitational collapse or explosion on a lava dome or lava flow. Pyroclastic flows are often hot and can incinerate, burn, and asphyxiate people. Gases within a pyroclastic flow can explode and cause ash to rain down on nearby areas. Pyroclastic flows can transform into lahars, enabling travel over even further distances.
Lahars are similar to pyroclastic flows but contain more water due to the presence of snow and ice, or water in a river, dam or lake. Lahars usually travel down valleys. They have a wide range of velocities varying from 1 m/s to 40 m/s. Lahars can travel long distances, some hundreds of kilometers from their source, often transporting very large boulders.
Pyroclastic flows and lahars are extremely dangerous especially to those living on the slopes and in valley areas near a volcano. Lahars can undercut banks and cause houses on those banks to be destroyed. Lahars can bury and destroy manmade structures including roads and bridges. At Nevado del Ruiz, lahars destroyed an entire city; filling the first floor of a hospital with mud, breaking windows, floating cars, and leaving debris in the tops of trees.