A pyroclastic flow is so dangerous because it moves so quickly it does not give anyone the opportunity to escape its potential effects. Furthermore it can move uphill as it is driven both by the power of the eruption and by the gravity force of the material behind it.
Pyroclastic flows are mixtures of gas, ash, rock, lava and cinder which move down the sides of a volcanic crater during an eruption. There are two main types, nuées ardentes and ignimbrites and the difference lies in the density of the material contained in the flow. Nuées ardentes contain dense material whereas ignimbrites contain lighter material like pumice.
It is largely the speed at which pyroclastic flows can move which causes their danger. 450 miles per hour is an extraordinary speed for any natural phenomena, but flows up to this rate have been registered in pyroclastic flows. High temperatures within a flow also have an effect, and temperatures of up to 1,000 degrees C have been reported in some cases. The danger arising from flows which are so hot and so fast moving is most easily examined by considering each volcanic material in turn.
The lightest material, both within and given off from a flow, will be hot gases. These will be highly toxic and will therefore cause the immediate asphyxiation of any life form. When Mount Merapi erupted in Indonesia, the gases in the ensuing flow killed 122 people.
Ash at an extremely high temperature will petrify any living thing on contact. Most people have seen pictures of people living in Pompeii who were killed in seconds by the eruption of Mount Vesuvius. It is generally considered that death will have been caused by hot gas, as gas, being light, would have travelled more quickly, but the following hot ash petrified the bodies of the dead, leaving extraordinary statuesque remnants of the everyday life in the ancient Roman city.
Lava sets fire to anything it touches, so trees, buildings and anything in the path of a lava flow will be destroyed immediately.
A lahar is formed if a pyroclastic flow contains water. Such water can come from within the volcano itself, or from snow and ice on the upper sides of the crater. The mixture of ash and water forms mud which can flow downhill at the same sorts of speed as a drier pyroclastic flow, but will set as soon as it comes to rest.
There have been tragic examples in recent years of lahars which have swept down to fill valleys in which villages were situated. As the mud sets to virtual concrete as soon as it stops moving, there is no chance of survival for anyone in the path of a lahar. 2,500 people were killed when a lahar swept down from Casito volcano in Nicaragua in 1998, and over 200,000 cubic metres of mud engulfed the towns of El Porvenir and Rolando Rodriguez.
One question which is sometimes asked about volcanoes is why people are prepared to live so close to them, when there is such obvious danger. The answer is often related to the fertility of the soil which weathers down from volcanic rocks. It can be so productive, and therefore profitable, that people are prepared to take the risk.
The risk is undoubtedly considerable, and by the very nature of pyroclastic flows and lahars, their temperature and their speed, there is no opportunity to change one’s mind and escape at the last minute. The last minute will be literally that.