Pyroclastic Flow - Causes

Causes

There are several scenarios which can produce a pyroclastic flow:

• Fountain collapse of an eruption column from a Plinian eruption (e.g., Mount Vesuvius's destruction of Pompeii, see Pliny the Younger). In such an eruption, the material ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometres. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing down the flanks of the volcano.
• Fountain collapse of an eruption column associated with a vulcanian eruption (e.g., Montserrat's Soufrière Hills volcano has generated many of these deadly pyroclastic flows and surges.) In this style of eruption, the gas and projectiles create a cloud which is denser than the surrounding air and becomes a pyroclastic flow.
• Frothing at the mouth of the vent during degassing of the erupted lava. This can lead to the production of a rock called ignimbrite. This occurred during the eruption of Novarupta in 1912 which produced the largest flows to be generated during recorded history.
• Gravitational collapse of a lava dome or spine, with subsequent avalanches and flow down a steep slope e.g., Montserrat's Soufrière Hills volcano which caused nineteen deaths in 1997.
• The directional blast (or jet) when part of a volcano explodes or collapses (e.g., the May 18, 1980, eruption of Mount St. Helens) As distance from the volcano increases, this rapidly transforms into a gravity-driven current.