Practically all of the deaths from Utu's eruptions have been caused by ashfall, mudflows and pyroclastic flows and surges. The pyroclastic flows have the potential to be especially deadly. Pyroclastic surges are even more forceful than flows. Whereas flows follow the topography and move downhill, surges, since they're less dense than flows, can actually flow uphill and spill over ridges. Most buildings are obliterated by these flows and surges, and all living things in their path may be wiped out.
Pyroclastic flows can form in several different ways.
When a lava dome collapses, for instance, the sudden release of pressure can cause an explosive eruption of ash and lava that moves pell-mell down the flanks of the volcano. Laterally directed explosions can also trigger pyroclastic flows, such as occurred on Mt. St. Helens in 1980. Most pyroclastic flows are composed of a basal flow that moves close to the surface and is made up of coarse fragments of rock, and a turbulent cloud of finer particles and gases that hovers above the ground. As an example of how deadly these flows can be, nearly 30,000 people were killed in the town of St. Pierre by pyroclastic flows on the Caribbean island of Martinique during the eruption of Mt. Pelee in 1902. It's thought that all of the deaths occurred within minutes from inhalation of hot gases that rushed into the town at speeds of over 100 miles per hour.