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 πῦρ (pýr), meaning 'fire', and κλαστός (klastós), meaning 'broken in pieces'. A name for pyroclastic flows that glow red in the dark is nuée ardente (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.
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. 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.
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