Pyroclastic flows are mixtures of hot gas, ash and other volcanic rocks travelling very
quickly down the slopes of volcanoes. They are one of the most dangerous hazards posed by volcanoes.
Pyroclastic flows are so hot and choking that if one is caught in one the person will certainly be killed. Because these flows are very fast they cannot be out-runned. If a volcano that is known for producing pyroclastic flows is looking like it may erupt soon, the best thing is to evacuate all the people living near the volcano.
Nuées ardentes and ignimbrites are two types of pyroclastic flows. Ignimbrites contain mostly light material whereas a nuée ardentecontains denser material. Nuée ardente means glowing avalanche and was named for the pyroclastic flows seen at Mount Pelée in 1902.
(Fig. 2.35) Pyroclastic Surge
Credit: W.I. Rose
Pyroclastic surges are low density flows of pyroclastic material. They are more dilute,turbulent, and widespread in their effects than pyroclastic flows. Surges are less concentrated and less topographically controlled than flows.
Pyroclastic surges are very dangerous. They contain a lot of dangerous (toxic) gases that
can kill people.