What causes a Pyroclastic Flow

A pyroclastic flow is a movement of material from an erupting volcano. Volcanoes come in many types, relating in part to the acidity of the lava, the level of ash produced and whether they erupt gradually or violently. The word pyroclastic has its origin in its two parts: pyro – referring to fireworks, and clastic – a word used for sediments made from transported rock material.

Volcanoes occur mainly along the boundaries between tectonic plates. If two plates are either moving together – as in central America and the West Indies, or moving apart – as in Iceland, molten material from below the Earth’s crust is able to find its way to the surface and will sometimes erupt through the surface as a volcano.  

Material erupting from a volcano can be in the form of lava (molten rock), rocks, ash, cinder or dust, or a combination of any of the five. Normally the material in a pyroclastic flow is a mixture of solid to semi-solid material which has become liquified or fluidized together with extremely hot gases which constantly expand. These flows have an emulsified paint-like consistency which move down the side of a volcano under gravity, much like an avalanche, at amazing speeds. Because of this speed, their temperature and the deadly materials they contain, pyroclastic flows are the most lethal of all volcanic phenomena.

Their speed of advance can be equated to wind speeds around a hurricane’s centre, with 100km per hour having been recorded. This speed is generated due to the lack of friction between the particles in the flow, and this, in turn, is due to the amount of fluidization in the material. The larger the flow, the greater the speed, but even relatively slow ones can advance much faster than a human can escape.

Temperatures within a flow are almost beyond belief for something on Earth, and 1,075 degrees C has been recorded in flows from Mt Pelee on Martinique in the West Indies.

Vulcanologists describe two types of pyroclastic flows, ignimbrites and nuees ardentes. An ignimbrite is a flow of pumice, a froth of lava containing air spaces much like a sponge. This type of rock is formed immediately as dissolved gases come out of solution in the atmosphere as the rock solidifies. Often they are produced when the cone or column of a volcano collapses on itself.

Nuee ardente means glowing cloud, a name which again refers to Mount Pelee. These are flows of much more dense material, often accompanied by clouds of ash. It is this ash which can glow at night as it drifts away from the volcano. Within the ash are often huge blocks of rock or volcanic bombs which have been hurled from the volcano’s crater during an eruption.

Pyroclastic flows can cover an area incredibly quickly before cooling into solid rock. In some valleys near Mount Pinatubo in the Philippines, flows have been recorded up to 220 metres thick.

The cause of the flows, therefore, is volcanic eruption, but not all volcanoes will produce the right mix of materials necessary for a pyroclastic flow.