Pyroclastic flows

Graphic showing dome collapse

Graphic showing fountain collapse

What are they and how do they form?

  • Mixtures of hot volcanic ash, boulders and gas
  • Also called 'nuées ardentes', meaning 'glowing clouds' in French
  • Temperatures can be from 100 to 600 degrees C
  • Typically travel at 70 mph or faster down the volcano

Volcanoes which erupt very viscous and sticky lavas to form domes can also produce pyroclastic flows, when the dome becomes unstable. As the dome grows, blocks are pushed out of the dome to make room for more lava. These blocks tumble down the dome face and disintegrate. This is commonly referred to as rockfall activity. Pyroclastic flows are produced when large portions of the dome collapse and disintegrate.

Eruption columns from explosive magmatic eruptions may collapse to form pyroclastic flows. The jet of ash and gas erupting from the vent heats cold air in the atmosphere. If enough air is heated, the mixture may become buoyant and form a vigorous rising plume. If the interior of the plume does not heat sufficient air, it will soon run out of momentum. This material is dense and undiluted by air, so collapses back on to the flanks of the volcano.


Photo showing rockfall activity
Photo of fountain collapse

Photo of pyroclastic flows
Photo of pyroclastic flow to the sea

What do they leave?

Pyroclastic flows produce deposits of hot ash and rocks around the flanks of the volcano. Temperatures may exceed 400 degrees centigrade in material several months old. These pictures show typical deposits from dome collapse and fountain collapse pyroclastic flows.

Photo showing deposits over Bethel
Photo of coastal deposits