Are eruptions from linear fissures and caldera ring dykes more likely to produce pyroclastic flows?

Abstract

International audienceTurbulent volcanic jets are produced by highly-energetic explosive eruptions and may form buoyant plumes that rise many tens of kilometres into the atmosphere to form umbrella clouds or collapse to generate ground-hugging pyroclastic flows. Ash injected into the atmosphere can be transported for many hundreds of kilometres with the potential to affect climate, disrupt global air travel and cause respiratory health problems. Pyroclastic flows, by contrast, are potentially catastrophic to populations and infrastructure close to the volcano. Key to which of these two behaviours will occur is the extent to which the mechanical entrainment and mixing of ambient air into the jet by large (entraining) eddies forming the jet edge changes the density of the air–ash mixture: low entrainment rates lead to pyroclastic flows and high entrainment rates give rise to buoyant plumes. Recent experiments on particle-laden (multi-phase) volcanic jets from flared and straight-sided circular openings suggest that the likelihood for buoyant plumes will depend strongly on the shape and internal geometry of the vent region. This newly recognised sensitivity of the fate of volcanic jets to the structure of the vent is a consequence of a complex dynamic coupling between the jet and entrained solid particles, an effect that has generally been overlooked in previous studies. Building on this work, here we use an extensive series of experiments on multi-phase turbulent jets from analogue linear fissures and annular ring fractures to explore whether the restrictive vent geometry during cataclysmic caldera-forming (CCF) eruptions will ultimately lead a relatively greater frequency of pyroclastic flows than eruptions from circular vents on stratovolcanoes. Our results, understood through scaling analyses and a one-dimensional theoretical model, show that entrainment is enhanced where particle motions contribute angular momentum to entraining eddies. However, because the size of the entraining eddies scales approximately with vent width, the extent of entrainment is reduced as the vent width becomes small in comparison to its length. Consequently, our work shows that for specified mass eruption rates, the high length-to-width ratio vents typical of CCF events are more likely to produce pyroclastic flows. We suggest that the enigmatic trend in the geological record for the largest CCF eruptions to produce pyroclastic flows is an expected consequence of their being erupted through continuous or piece-wise continuous caldera ring fractures

Experimental Measurement of the Solid Particle Concentration in Geophysical Turbulent Gas‐Particle Mixtures

“…A first important limitation of our experiments is the range of Reynolds numbers we investigated, which was up to Re mix ~ 10 6 , while natural geophysical mixtures may have Reynolds numbers up to ~10 9 –10 10 (e.g., Andrews & Manga, ). Another limitation is the investigation of spherical particles with only one grain size and Stokes numbers larger than ≈1, whereas natural mixtures commonly contain particles of various grain sizes and shapes and with wider ranges of Stokes numbers (Burgisser et al, ; Jessop et al, ).…”

The dynamics of volcanic jets: Temporal evolution of particles exit velocity from shock‐tube experiments

“…Accordingly, the commonly assumed “pseudo‐gas” model for the gas‐particle mixture is oversimplified in most volcanic cases. Rapid decompression experiments on porous volcanic rocks have shed light on the process of magma fragmentation and ejection [e.g., Alidibirov and Dingwell , , ; Kueppers et al ., , ; Alatorre‐Ibargüengoitia et al ., , ; Montanaro et al ., ], while analogue injection experiments investigated several aspects of plume dynamics [ Chojnicki et al ., , ; Jessop et al ., ]. Beyond volcanology, the influence of different working conditions on gas and particle velocity is of interest for an enhanced understanding of general gas dynamics [ Sommerfeld , ; Peña Fernández and Sesterhenn , ] or thermal spraying [ Yin et al ., , and references therein].…”

“…Valentine [1998] summarized the boundary conditions favoring buoyant rise over collapse as (1) narrow vents, (2) high exit velocities, (3) high gas content, and possibly (4) high pressure ratio at the vent. The role of vent geometry on plume dynamics during explosive eruptions has been the focus of studies investigating ejection velocity [Wilson et al, 1980;Wilson and Head, 1981;Kieffer, 1989] and jet radius [Woods and Bower, 1995;Jessop et al, 2016]. If ejection velocity is mainly determined by gas mass fraction, gas overpressure at the vent, and magma temperature [Woods and Bower, 1995], a flaring vent can help in driving the transition between subsonic and supersonic flow [Wilson and Head, 1981;Kieffer, 1989].…”

“…If ejection velocity is mainly determined by gas mass fraction, gas overpressure at the vent, and magma temperature [Woods and Bower, 1995], a flaring vent can help in driving the transition between subsonic and supersonic flow [Wilson and Head, 1981;Kieffer, 1989]. Furthermore, vent characteristics seem to affect the mass eruption rate (MER), as it is proportional to the cross-sectional area of the jet, which is related to the vent size [Koyaguchi et al, 2010;Ogden, 2011;Saffaraval et al, 2012;Jessop et al, 2016]. In nature, a wide range of vent geometries has been observed, from circular to elongated, and these features are dynamically evolving.…”

“…Amount and size of the ejected particles ("volcanic cargo") have a great impact on jet dynamics. Two-way and four-way coupling interactions between fluid (melt or gas) and particles in volcanic systems have been demonstrated theoretically [Bercovici and Michaut, 2010], numerically [Carcano et al, 2014;Cerminara et al, 2016], observationally [Taddeucci et al, 2015], and experimentally [Burgisser et al, 2005;Jessop et al, 2016]. Accordingly, the commonly assumed "pseudo-gas" model for the gas-particle mixture is oversimplified in most volcanic cases.…”