Density Stratification and Buoyancy Evolution in Pyroclastic Density Currents

Sean B. O’Donnell

doi:10.1029/2024JB029208

Density Stratification and Buoyancy Evolution in Pyroclastic Density Currents

2024-06-21

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DOI：10.1029/2024JB029208

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Pyroclastic density currents (PDCs) are density-stratified along their vertical axis, with the near-bed portion being denser than the upper portion, resulting from particle settling and ambient air entrainment at current margins. Whereas vertical density stratification likely influences mixing, sedimentation, and buoyancy of PDCs, many depth-averaged models of PDC dynamics assume currents are well-mixed. We investigated this discrepancy by performing sub-aqueous laboratory experiments and conducted complementary numerical simulations to interrogate current dynamics at finer scales. Currents with small temperature difference with the ambient fluid become density-stratified during propagation. The dynamics of such currents resemble two-phase flows, in which particles move freely and particle concentration becomes stratified, but fluid density remains constant. Currents with large temperature difference with the ambient fluid, however, do not develop density stratification during propagation, due to current dynamics becoming dominated by the fluid phase and the lessening importance of particles. Currents that develop density stratification do not lift off from the bed within the domain of the setup, whereas poorly stratified currents do lift off, forming a rising plume. Strong density stratification within currents inhibits turbulence production, preventing entrained ambient fluid on current edges from mixing into current interiors. Poorly stratified currents are highly turbulent, have vigorous internal mixing, thereby achieving lift-off. The strongly stratified currents are analogous to PDCs that result from eruption column collapse, maintaining fast velocity, low internal mixing, and high temperature over long distances. The poorly stratified currents are analogous to dilute ash-cloud surges that develop atop basal avalanches, having short runout distances. Pyroclastic density currents are incredibly destructive volcanic flows made up of hot rock and gases, and they pose a major threat to human populations in the vicinity of active volcanoes. Evidence from PDC deposits suggests that the top of such flows is less dense than the bottom, but mathematical models of such flows often inadequately assume they have unvarying density. We performed laboratory experiments and computer simulations to investigate how the difference in density between the top and bottom of such flows affects how far they travel. We find that hotter currents are less likely than colder currents to develop a density difference between the top and bottom portions. Currents that do not develop a density difference do develop more energetic mixing, which in turn makes them lift-off from the ground and rise vertically as a plume in the shape of a mushroom thermal. These experimental currents are like PDCs that result from the collapse of vertical eruption columns. Currents that do develop a density difference do not energetically mix and do not lift-off into a plume. These experimental currents are like dilute PDCs that result from the collapse of lava domes. Laboratory experiments and numerical simulations were used to study the effects of density stratification in pyroclastic density currents The temperature difference between currents and ambient fluid affects the development of density stratification during propagation Turbulence production depends on density stratification, impacting current buoyancy evolution and has implications for natural currents

Keyword:

pyroclastic density current

density stratification

turbulence production

buoyancy evolution

laboratory experiments

numerical simulations

Density Stratification and Buoyancy Evolution in Pyroclastic Density Currents

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