thumb|Pyroclastic flows sweep down the flanks of [[Mayon|Mayon Volcano, Philippines, in 2018]]

A pyroclastic flow, more broadly known as a pyroclastic density current, is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano. Pyroclastic currents travel at extremely high speeds and have extremely high temperatures.

Pyroclastic flows are the deadliest of all volcanic hazards and are produced as a result of certain explosive eruptions. They normally touch the ground and hurtle downhill or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.

Upon the eruption of volcanic matter, much of the material sinks to the ground and flows along it, forming a pyroclastic current. Currents are composed of a continuum of materials, spanning from denser, comparatively cooler rocks to warmer, lighter ashes and gasses. Denser rocks are more likely to be influenced by gravity, becoming separated from lighter ashes and gasses that continue flowing across the surrounding ground and air. The denser components of pyroclastic currents are what is specifically meant when referring to pyroclastic flow, whereas the lighter components of pyroclastic currents are known as pyroclastic surges. A name for pyroclastic flows that glow red in the dark is (French, "burning cloud"); this was notably used to describe the disastrous 1902 eruption of Mount Pelée on Martinique, a French island in the Caribbean.

Formation and movement of pyroclastic currents

Pyroclastic flows originate from the physical processes that occur prior to and during a volcanic eruption. Magma within the volcano degasses, promoting decompression, greater buoyancy, and rising motion of magma to the surface, which in turn allows for greater degassing. As more degassing occurs, the magma evolves from a gas-containing liquid to a mixture of condensed particles and gas with greater upward velocity. Upon eruption of this mixture to the surface and atmosphere, much of it sinks to the ground due to being denser than the atmosphere, forming the pyroclastic density current that then moves along the ground.

Upon entering the atmosphere, as a gravity current, pyroclastic currents are subject to gravitational pull toward the surface. Gravity is counterbalanced by the fluid forces associated with the turbulent processes within the current. This turbulence is courtesy of pyroclastic currents being a heterogeneous mixture. Made up of condensed volcanic matter and hot gas, pyroclastic flows are heterogeneous in their chemical composition. As a result, within a current, particle temperature, resistance to change in motion (inertia), velocity, and other properties vary. As a fluid, pyroclastic currents experience internal friction (viscosity). However, temperature, velocity, and inertial imbalances within the current overpower this viscosity, promoting the turbulent fluid flow that counteracts gravity. Reynold’s number measures the degree to which these property imbalances overpower viscosity. The higher Reynold’s number, the higher chances for turbulence within a fluid.

A key signature of turbulence are swirls known as eddies. Pyroclastic currents are multiphase, meaning their physical processes, including the formation of these eddies, occur at a wide variety of scales, from the molecular to the scale of the bulk matter of the pyroclastic current itself. As these eddies form through turbulence, they create paths of fluid flow. Meanwhile, gravity continues to push the bulk mixture downward. Within the heterogeneous mixture of the pyroclastic flow, heavier, more condensed material (e.g., rocks) will be more strongly impacted by gravity, sinking to the ground and flowing along it. By contrast, lighter, less dense material (e.g., ash), will be more strongly impacted by the fluid flow generated from turbulence, causing it to rise. The former component of pyroclastic currents is sometimes referred to as basal flow, whereas the latter is sometimes known as an ash plume.

Furthermore, two ways of quantifying this separation of pyroclastic material is through Froude’s number and Stokes’ number. Froude’s number is a measure of whether gravity or turbulent fluid force will more strongly dictate a fluid’s motion. Lower values of Froude’s number indicate that gravity is dominant while higher ones indicate that turbulent fluid motion is more dominant. Meanwhile, Stokes’ number is a measure of whether a particle or parcel of matter within the fluid will follow the flow path of the fluid or remain independent of it. Higher values of Stokes’ number suggest a particle will flow along the fluid path, while lower values suggest it will remain independent from the fluid path.

Examples

Eruptions throughout history reveal several specific mechanisms that can produce pyroclastic flow:

  • Fountain collapse of an eruption column from a Plinian eruption (e.g. Mount Vesuvius' destruction of Herculaneum and Pompeii in 79 AD). In such an eruption, the material forcefully 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. Pyroclastic flows travel at average speeds of 100 km/h (30 m/s, 60 mph), although they can travel up to 700 km/h (190 m/s, 430 mph). Temperatures of the material within a pyroclastic density current range from 200°C to 700°C (390-1300°F).
  • The 1902 eruption of Mount Pelée destroyed the Martinique town of St. Pierre. Despite signs of impending eruption, the government deemed St. Pierre safe due to hills and valleys between it and the volcano, but the pyroclastic flow charred almost the entirety of the city, killing all but three of its 30,000 residents.
  • On June 25, 1997, a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of Montserrat. A large, highly energized pyroclastic surge developed. This flow could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area (which was officially evacuated). Several others in the area suffered severe burns.

Interaction of pyroclastic currents with water

Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, shows that pyroclastic density currents can cross significant bodies of water. One flow reached the Sumatran coast as far as away.

Generally, pyroclastic currents can interact with water in many ways, with their properties dictating what exactly these interactions entail. With their heterogeneous assortment of chemical constituents, density varies greatly within pyroclastic currents. Components that are less dense than water are able to remain buoyant, whereas components denser than water will sink, isolating the pyroclastic current to its less dense components. Additionally, there may be currents whose density exceed that of water while still remaining buoyant. Such a phenomenon occurs by virtue of a bulk density lower than water brought about by dispersal of hot gas and solid ash within a pyroclastic current. The temperature difference between a water body and a pyroclastic current generates steam from the water body, propelling the current’s movement.

The angle at which a pyroclastic current enters a water body also influences the trajectory of its constituents. If the angle of entry is more parallel to the water body surface, the current is able to more seamlessly flow along the surface of the water more easily. By contrast, if the angle of entry is more perpendicular to the water body surface, the current will more deeply penetrate the water, forming mixing layers within it, before reemerging to the surface and flowing across it. Some volcanoes on Mars, such as Tyrrhenus Mons and Hadriacus Mons, have produced layered deposits that appear to be more easily eroded than lava flows, suggesting that they were emplaced by pyroclastic flows.

See also

  • Pyroclastic fall
  • Pyroclastic rock
  • Welded tuff

References

  • Sigurdson, Haraldur: Encyclopedia of volcanoes. Academic Press, 546–548. .

Notes

  • Pyroclastic Flows video