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.
In certain cases, pyroclastic density currents can flow over water, with its denser constituents sinking into and mixing with it. Waters can even recede as a result of land production from the mass and volume flux of pyroclastic currents. Additionally, pyroclastic currents can be observed on other astronomical bodies, such as the moon.
Etymology
The word pyroclast is derived from the Greek (), meaning 'fire', and (), meaning 'broken in pieces'. From p. 68: "The word 'ash' is not a very good one to include all the mechanical accompaniments of a subaerial or subaqueous eruption, since ash seems to be restricted to a fine powder, the residuum of combustion. A word is wanting to express all such accompaniments, no matter what their size or condition may be, when they are accumulated in such mass as to form beds of 'rock.' We might call them perhaps 'pyroclastic materials,' [...] " A name for pyroclastic flows that glow red in the dark is (French for 'burning cloud'); this was notably used to describe the disastrous 1902 eruption of Mount Pelée on Martinique, a French island in the Caribbean.Lacroix, A. (1904) La Montagne Pelée et ses Eruptions, Paris, Masson (in French) From vol. 1, p. 38: After describing on p. 37 the eruption of a "dense, black cloud" (nuée noire), Lacroix coins the term : "" (Shortly after the eruption of what I will call henceforth the dense, glowing cloud [nuée ardente], an immense cloud of cinders covered the entire island, sprinkling it with a thin layer of volcanic debris.)Although the coining of the term in 1904 is attributed to the French geologist Antoine Lacroix, according to: the term was used in 1873 by Lacroix's father-in-law and former professor, French geologist Ferdinand André Fouqué in his description of the 1580 and 1808 eruptions of the volcano on the island of São Jorge in the Azores.
From p. 1199: "" (One of the strangest phenomena of this great eruption is the production of what contemporary witnesses called nuées ardentes.) From p. 1200: "" (The detonations cease on the day of the 17th, but then [there] appear burning clouds [nuées ardents] similar to those of the eruption of 1580.) Marjorie Hooker – (Hooker, 1965), p. 405 – records that Father João Inácio da Silveira (1767–1852) from the village of Santo Amaro on São Jorge island wrote an account of the 1808 eruption in which he described an ("burning cloud" in Portuguese) that flowed down the slopes of the volcano. Silveira's account was published in 1871 and republished in 1883.
From pp. 439–440: "" (On the seventeenth of the said month of May … suddenly there arose a typhoon of fire out of the volcano and [it] entered the farm lands, heaved up all those fields down to the vineyards, with all the trees and hedges, forming a fearsome and burning cloud [ardente nuvem] and running down to the church, burned more than thirty people in the church and in the fields…)
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.Myers and Brantley (1995). Volcano Hazards Fact Sheet: Hazardous Phenomena at Volcanoes, USGS Open File Report 95-231
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. Denser rocks within a pyroclastic material have a tendency to sink under the influence of gravity, falling to and traveling along the ground and departing from the fluid flow paths generated from turbulence. In turn, these components of pyroclastic current have lower values for Froude’s and Stokes’ numbers. Lighter ashes and gases within a pyroclastic current are more strongly influenced by turbulent fluid motion, traveling along the fluid path. Hence, they have higher values for Froude’s and Stokes’ numbers.
Terminology To fully clarify terminology, the denser components of a pyroclastic current which sink and become part of the ground-hugging basal flow are part of what is known as a pyroclastic flow. Meanwhile, the more buoyant components of a pyroclastic current which become more significantly influenced by fluid forces are part of what is known as a pyroclastic surge. Pyroclastic flows merely lie within the continuum of what occurs to the volcanic matter contained within a pyroclastic current. In other words, the lighter a material within the pyroclastic flow, the more likely it will become part of a pyroclastic surge, while the heavier the material, the more likely it will become part of a pyroclastic flow.
Pyroclastic surges can also be referred to as "fully dilute pyroclastic density currents." Their lower density sometimes allows surges to flow over higher topographic features or water such as ridges, hills, rivers, and seas. Surges may also contain steam, water, and rock at less than ; these are called "cold" compared with other flows, although the temperature is still lethally high. Cold pyroclastic surges can occur when the eruption is from a vent under a shallow lake or the sea. Fronts of some pyroclastic density currents are fully dilute; for example, during the eruption of Mount Pelée in 1902, a fully dilute current overwhelmed the city of Saint-Pierre and killed nearly 30,000 people.Arthur N. Strahler (1972), Planet Earth: its physical systems through geological time
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. 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). The gas and projectiles create a cloud that 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. Gravitational collapse of a lava dome or spine, with subsequent avalanches and flows 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 collapses or explodes (e.g., the eruption of Mount St. Helens on May 18, 1980). As distance from the volcano increases, this rapidly transforms into a gravity-driven current.
Size, speed, temperature, and effects
Flow volumes range from a few hundred cubic metres to more than . Larger flows can travel for hundreds of kilometres, although none on that scale has occurred for several hundred thousand years. Most pyroclastic flows are around and travel for several kilometres. Flows can deposit from less than 1 metre up to 200 metres in depth of loose rock fragment. 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).
Effects of pyroclastic flows with historical examples