Dilute Pyroclastic Density Currents Research Articles

IMEX_SfloW2D v2: a depth-averaged numerical flow model for volcanic gas–particle flows over complex topographies and water

Abstract. We present developments to the physical model and the open-source numerical code IMEX_SfloW2D (de' Michieli Vitturi et al., 2019). These developments consist of a generalization of the depth-averaged (shallow-water) fluid equations to describe a polydisperse fluid–solid mixture, including terms for sedimentation and entrainment, transport equations for solid particles of different sizes, transport equations for different components of the carrier phase, and an equation for temperature/energy. Of relevance for the simulation of volcanic mass flows, vaporization and entrainment of water are implemented in the new model. The model can be easily adapted to simulate a wide range of volcanic mass flows (pyroclastic avalanches, lahars, pyroclastic surges), and here we present its application to transient dilute pyroclastic density currents (PDCs). The numerical algorithm and the code have been improved to allow for simulation of sub- to supercritical regimes and to simplify the setting of initial and boundary conditions. The code is open-source. The results of synthetic numerical benchmarks demonstrate the robustness of the numerical code in simulating transcritical flows interacting with the topography. Moreover, they highlight the importance of simulating transient in comparison to steady-state flows and flows in 2D versus 1D. Finally, we demonstrate the model capabilities to simulate a complex natural case involving the propagation of PDCs over the sea surface and across topographic obstacles, through application to Krakatau volcano, showing the relevance, at a large scale, of non-linear fluid dynamic features, such as hydraulic jumps and von Kármán vortices, to flow conditions such as velocity and runout.

Plio-Pleistocene rear-arc volcanism in the Southern Volcanic Zone: Eruptive styles of the Varvarco Volcanic Field

The Varvarco Volcanic Field (VVF) is located in the southern part of the Las Loicas Trough, as part of the Late Pliocene-Early Pleistocene rear-arc volcanic belt in the Transitional Southern Volcanic Zone (34.5–37°S). Its volcanic products show an elliptical distribution, elongated parallel to NW-SE main structures that regionally controlled the Las Loicas Trough. A detailed field and petrographic study was carried out to identify main lithofacies and establish its eruptive styles.The VVF magmatic evolution is initially characterized by a voluminous explosive stage represented in the area by dense and dilute pyroclastic density currents (PDC) deposits (massive lapilli tuffs, cross-stratified lapilli tuffs and diffuse-stratified tuffs). Afterwards, it evolved into an effusive stage represented by basaltic lava flows (coherent basalts), associated with Hawaiian to Strombolian eruptive style, which constitutes most of the VVF volume. The final stage of the VVF history was linked to a stratovolcano-type activity where both effusive (coherent basalts and andesites, and rhyolitic coulees) and explosive lithofacies (such as massive lithic breccias and massive lapilli tuffs) are described. Within this stage, the uppermost effusive levels were intruded by dacitic and rhyolitic domes and basaltic dykes. Available ages allow to conclude that the VVF emplacement was developed during Plio-Pleistocene times, linked to the re-steepening of the Nazca plate, after the Late Miocene Payenia shallow subduction regime.

Alternating Subplinian and phreatomagmatic phases during the construction of a phonolitic maar-diatreme volcano (Caldera del Rey, Tenerife, Canary Islands)

The Early Pleistocene, well exposed, Caldera del Rey maar-diatreme volcano, Tenerife, Canary Islands was constructed during a ∼ VEI 4 phonolitic eruption that involved two cycles of magmatic-to-phreatomagmatic activity and resulted in two overlapping craters aligned NE-SW. Magmatic phases fed unsteady Subplinian eruption columns that reached 8–12 km altitude and dispersed tephra to the west and southwest of the volcano and shed pyroclastic density currents. Phreatomagmatic phases, driven by explosive interactions between magma and groundwater, constructed an extensive tephra ring via deposition from ballistic curtains, pyroclastic density currents, and tephra fall. Near-optimal-scaled depth phreatomagmatic explosions (strong and/or shallow) excavated a substantial diatreme beneath the north crater and constructed a substantial tephra ring. This abruptly transitioned to deeper-than-optimal scaled depth explosions (weak and/or deep) that erupted mostly fine ash which was dispersed by dilute pyroclastic density currents and fallout and filled the south crater. At distances of >4 km from the volcano, over a metre of ash and pumice accumulated during the phreatomagmatic phases. The Caldera del Rey volcano provides an instructive study on how interaction between ascending felsic magma and groundwater can modify Subplinian eruptions.

Volcanology and facies analysis of the youngest phreatomagmatic deposits on O'ahu, Hawai'i

The area known as the Lānaʻi Lookout has Oʻahu's youngest and best exposures of deposits from pyroclastic density currents (PDCs), generated by explosions involving magma-water interaction. This work presents the first detailed facies analysis of the deposits at the Lānaʻi Lookout, fundamental to understanding the explosion dynamics, post-fragmentation transport and depositional processes, and to unravel the eruptive history. The lithofacies identified include planar-bedded, low-angle, and megaripple, cross-bedded ash to lapilli, accretionary lapilli-rich falls and isolated ballistic blocks and bombs. These features are interpreted as generated by mostly dilute PDCs, co-PDC fall and sedimentation from eruptive clouds and ballistic-charged jets. Overall, at least four eruptive stages formed the pyroclastic succession: (i) frequent, brief explosions from Kahauloa and Koko craters, producing fallout deposits and ballistics; (ii) a dominant stage of sustained passing to short-lived production of PDCs from Kahauloa crater, followed by a short break in volcanic activity; (iii) return of explosive activity dominated by fallout deposits likely sourced at Kahauloa; (iv) intermittent PDCs from Kahauloa alternating with debris flows coming down Koko Crater flanks. This stage is followed by lava flow emplacement that marked the end of the explosive eruptions at the Koko Rift system. In summary, the lithofacies analysis and stratigraphic relations of this complex multi-vent system informs on the dynamics, eruptive style, and frequency of eruptions of the late-stage volcanism on Oʻahu.

Explosive sequence of La Soufrière, St Vincent, April 2021: insights into drivers and consequences via eruptive products

Abstract This paper forensically reconstructs the timings, impacts and processes that drove the sequence of explosive eruptions of La Soufrière, St Vincent in April 2021 using a combination of field-based stratigraphy and textural dissection of the deposit character together with contemporary visual observations. Explosive activity on 9 April and early on 10 April involved destruction of almost all of the 2020/2021 lava dome, c. 60% of the 1979 dome and formation of a 600 m diameter crater by 2pm UTC on 10 April. Following the initial explosion, plumes rose to altitudes of c. 15 km and pyroclastic density currents (PDCs), formed by column collapse, first occurred on 10 April, only after >24 h of explosive activity. Dense PDCs reached the sea only in the Larikai and Roseau valleys, and dilute PDCs were restricted to within 2.5 km of the Summit Crater rim. The tephra fallout deposits are stratified, composed of numerous layers of both lapilli-rich and ash-rich layers, which we have grouped into at least 7 units, based on their common characteristics (Units 1 to 7). Volume estimates, using a range of techniques to constrain uncertainties, indicate that the bulk volume of tephra (fallout and PDC) is 1.19 × 10 8 m 3 ± 20% making this a VEI 4 eruption.

Large-scale volcanic deposit fluidization by dilute pyroclastic density currents

Large-scale volcanic deposit fluidization by dilute pyroclastic density currents

Characteristics and controls of the runout behaviour of non-Boussinesq particle-laden gravity currents – A large-scale experimental investigation of dilute pyroclastic density currents

One of the most dangerous aspects of explosive volcanism is the occurrence of dilute pyroclastic density currents that move at high velocities of tens to about a hundred of metres per second outwards from volcanic vents. Predicting the runout behaviour of these turbulent flows of hot particles and air is complicated by strong changes in the flow density resulting from entrainment of ambient air, sedimentation of particles, as well as heating and expansion of the gas phase. Current hazard models that are based on the behaviour of aqueous gravity currents cannot capture all aspects of the flow dynamics, and thus pyroclastic density current dynamics remain comparatively poorly understood. Here we interrogate the runout behaviour of dilute pyroclastic density currents in large-scale experiments using hot volcanic material and gas. We demonstrate that the flows transition through four dynamic regimes with distinct density and force characteristics. The first, inertial regime is characterized by strong deceleration under high density differences between the flow and ambient air where suspended particles carry a main proportion of the flows' momentum. When internal gravity waves start to propagate from the flow body into the advancing flow front, the currents transition into a second, inertia-buoyancy regime while flow density continues to decline. In this regime, subsequent arrivals of fast-moving internal gravity waves into the front replenish momentum and lead to sudden short-lived front accelerations. In the third regime, when the density ratio between flow and ambient air decreases closer to a value of unity, buoyancy forces become negligible, but pressure drag forces are large and constitute the main flow retarding force. In this inertia-pressure drag regime, internal gravity waves cease to reach the front. Finally, and with the density ratio decreasing below 1, the current transitions into a buoyantly rising thermal in regime 4.Unlike for aqueous gravity currents, the Froude number is not constant and viscous forces are negligible in these gas-particle gravity currents. We show that, in this situation, existing Boussinesq and non-Boussinesq gravity current models strongly underpredict the front velocity for most of the flow runout for at least half of the flow propagation. These results are not only important for hazard mitigation of pyroclastic density currents but are also relevant for other turbulent gas-particle gravity currents, such as powder snow avalanches and dust storms.

Mid-Holocene lateral collapse of Antuco volcano (Chile): debris avalanche deposit features, emplacement dynamics, and impacts

Antuco (37.4°S, 71.4°W; Chile) is a dominantly basaltic stratovolcano whose original ~ 3300 m altitude main cone experienced a catastrophic sector collapse at ~ 7.1 cal ka BP, producing a volcanic debris avalanche deposit (VDAD) with hummocky surface and ~ 6.4 km3 of volume. We carried out geological studies of its debris avalanche deposit, which was distributed to the W and displays a longitudinal facies transformation from edifice’s megablocks and block to mixed facies in distal areas (up to 25 km from the scar). Our observations support the behavior of the avalanche beginning as a translational slide, and then as plug flow when confined within the Laja River valley. Clay abundance and high content of hydrothermally altered material may suggest active participation of water; flow velocities are estimated to ~100 m s−1. We primarily identify the steep-sided flanks of the cone, and hydrothermal alteration promoted the edifice instability, while basement seismogenic structures may have ultimately triggered the landslide. Subsequent landslide-led events include the transformation of the volcanic activity with explosive eruptions producing a sequence of dilute pyroclastic density currents (PDCs) ending ~3.4 ky BP, and extensive lava effusion rapidly reconstructing the collapsed edifice. Moreover, the Antuco VDAD also blocked the natural output of the Laja Lake, increasing its level by ~200 m and then triggering cataclysmic outburst floods by dam rupture, preserved as high-energy alluvial beds with ages between 2.8 and 1.7 ky BP. The Antuco constitutes an excellent example of a critical chain of events initiated by a stratovolcano lateral collapse and warns for detailed hazard investigations to better comprehend its related impacts.

Inverting sediment bedforms for evaluating the hazard of dilute pyroclastic density currents in the field

Pyroclastic density currents are ground hugging gas-particle flows associated to explosive volcanic eruptions and moving down a volcano's slope, causing devastation and deaths. Because of the hostile nature they cannot be analyzed directly and most of their fluid dynamic behavior is reconstructed by the deposits left in the geological record, which frequently show peculiar structures such as ripples and dune bedforms. Here, a set of equations is simplified to link flow behavior to particle motion and deposition. This allows to construct a phase diagram by which impact parameters of dilute pyroclastic density currents, representing important factors of hazard, can be calculated by inverting bedforms wavelength and grain size, without the need of more complex models that require extensive work in the laboratory.

Correction to: The mysterious grooves of Volcán Bárcena: a review of the role of streamwise counter-rotating vortices during erosion by dilute pyroclastic density currents

Correction to: The mysterious grooves of Volcán Bárcena: a review of the role of streamwise counter-rotating vortices during erosion by dilute pyroclastic density currents

The mysterious grooves of Volcán Bárcena: a review of the role of streamwise counter-rotating vortices during erosion by dilute pyroclastic density currents

Although erosion during high-energy passage of a pyroclastic density current (PDC) causes great damage, analyses of the effects of such erosion are sparse in scientific literature compared to observations and interpretations of depositional processes. In this paper, we review observations of surfaces where PDCs have eroded sets of grooves that provide information on the erosion process. We postulate that in some cases, the grooves were carved by streamwise vortices in the boundary layer of the PDC and review possible fluid dynamic instabilities that can give rise to such vortices. For the prominent grooves at Volcán Bárcena, Mexico, we propose that a fluid dynamic instability, which we dub the “groovy instability,” occurred and caused formation of erosive counter-rotating vortices. This instability occurs when the particle concentration boundary layer thickness, δc, is larger than the velocity (shear) boundary layer thickness, δu, i.e., L=δc /δu>1. In subaqueous turbidity currents, these vortices have a typical wavelength of ~25*δc. If this relation is applied to the grooves formed on Volcán Bárcena, the inferred particle concentration boundary layer is estimated to have been <1 m thick. We postulate that a transition between erosion of grooves and deposition of dunes at Volcán Bárcena occurred when hydraulically supercritical flow on the upper flanks changed to subcritical flow about halfway down the mountain. We call attention to boundary layer dynamics in erosive pyroclastic density currents at a dimension that is difficult to scale quantitatively in laboratory experiments and is usually not resolved computationally and to the need for incorporating such dynamics into models of PDC dynamics.

Decrypting silicic magma/plug fragmentation at Azufral crater lake, Northern Andes: insights from fine to extremely fine ash morpho-chemistry

Azufral (SW Colombia) is a dangerous silicic volcano hosting a crater lake, which serves as an excellent example of an incipient plug disruption through phreatomagmatism. We studied the youngest succession of dilute pyroclastic density currents (PDCs) onlapping the north-eastern crater rim. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy was used to carry out an automated single-particle analysis of fine to extremely fine ash. We were able to obtain fast and accurate chemical analysis and imaging of 15,098 particles within the 250–63-μm and the 63–32-μm size ranges. The 2D form and roughness parameters were determined for 4895 juvenile glassy particles and validated by 3D micro-X ray computer tomography. There are two end members of high (group 1) and low (group 2) roughness juvenile glassy particles. Group 1 comprises high-roughness glass particles with solidity values as low as 0.34 in 2D and 0.33 in 3D, and convexity values as low as 0.33 in 2D and 0.26 in 3D. Group 2 comprises low-roughness glass particles with 2D solidity values > 0.79 and 3D solidity values typically > 0.58. In this group, 2D convexity values are > 0.68 and 3D convexity values are > 0.71. Both end members are mostly discriminated by the 2D Concavity Index (0.14 to 0.77 in group 1 vs. 0.05–0.35 in group 2). The remaining group 3 comprises particles of intermediate roughness values. In this study, we show how an incipient plug developed over a short repose time might be subjected to only a few cycles of vesicle nucleation, collapse and densification, retaining the characteristics of juvenile glass. Each glassy juvenile ash type, defined by a particular morphology, roughness and microtexture can be linked to a density “stratified” conduit model. In Azufral, the capping and conduit lining dense regions and the permeable zones of the incipient plug likely cracked. The newly formed cracks could allow hydraulic forcing caused by external water and induce phreatomagmatic interaction. This interaction favoured the fine fragmentation of the plug while enhancing ongoing magmatic processes. Finally, the variations of bulk componentry provided clues on dilute pyroclastic density current transport and physical fractionation processes by secondary fragmentation, elutriation and interaction with the crater rim.

A first reconstruction of the evolution of Cubilche Volcanic Complex, Imbabura Province, Ecuador

Cubilche Volcanic Complex (Imbabura Province, Ecuador) includes three morphologically well-preserved eruptive centers: in increasing size, Cunrru Dome Complex (3300 m above sea level, m asl), Panga Ladera (3420 m asl) and Cubilche (3836 m asl). As this complex is poorly known, we conducted a volcanological study of its evolution by combining geomorphological, lithostratigraphic, radiocarbon (14C), and petrological analyses, along with eruption size determinations. Cunrru Dome Complex shows an eastward-opening scarp formed during a dome collapse event that emplaced a small-volume debris avalanche deposit and a pumice-rich layer from an accompanying directed explosion. The higher Panga Ladera edifice is breached towards the north, and the emplacement of the resulting debris avalanche deposit was also accompanied by a blast-forming eruption of ash and pumice. The post-collapse vent subsequently produced dilute pyroclastic density currents (PDCs) that built a low cone. The Cubilche eruptive center comprises old and young edifices. The Old Cubilche edifice emitted lavas from a summit crater and flank vents before being partly destroyed during a flank failure event associated with an explosive eruption that produced a widespread pyroclastic density current deposit dated at ca 44 ka and an associated pyroclastic fall deposit. The Young Cubilche cone grew inside the Old Cubilche avalanche crater during ~14–19 ka over five eruptive episodes that included early effusive eruptions, a dome growth period that terminated with a PDC that emplaced scoria flow deposits at ca 29.6 ka, an explosive eruption at ca 26.3 ka, and final Strombolian activity emitting more mafic products. As the adjacent Imbabura Volcanic Complex erupted in early Holocene times and the two complexes seem to share common reservoirs and plumbing systems, we cannot fully discard a possible reactivation at Cubilche. Other more short-term hazards are debris flows and landslides potentially triggered by heavy rain fall and/or seismic events.

A Miocene Phreatoplinian eruption in the North-Eastern Pannonian Basin, Hungary: The Jató Member

A Middle Miocene, ~8 m thick pyroclastic succession, reported from the Bükk Foreland Volcanic Area (BFVA) in Northern Hungary (Central Europe) specified here as the Jató Member, was produced by silicic phreatomagmatism (Phreatoplinian sensu lato). Two well-preserved outcrops ~8 km apart and inferred to be within ~10–50 km from source represent the discontinuously exposed, layered, paleosol-bounded, phreatomagmatic Jató Member. They show an identical phenocrystal assemblage of feldspar, biotite and amphibole without weathered zones or signs of erosion, that suggest deposition in one eruption phase lasting hours to months. The succession contains three subunits: 1) subunit A, 1.8 m thick, a series of well-sorted fine to coarse ash or lapilli tuff layers with constant thickness; 2) subunit B, 2.1 m thick, a series of normal-graded layers with an upper fine-grained zone containing abundant ash aggregates with a coarser-grained core and distinctively finer-grained outer rim; 3) subunit C, 4.5 m thick, a massive, poorly to well-sorted coarse ash with gas escape structures and ash aggregates at its base. The upward change of these lithofacies implies an initially sustained dry fallout-dominated deposition of ash and pumice lapilli resulting in subunit A. Subsequently, multiple wet and dilute Pyroclastic Density Currents (PDCs) dispersed subunits B and C. The general abundance of PDC-related ash aggregates in the middle-upper part of the succession (particularly in subunit B), and the transformation of a fall-dominated to a collapsing depositional regime producing wet dilute PDCs, imply the increasing influence of water during the eruption (Phreatoplinian sensu lato). The presence of water is related to an epicontinental sea during Middle to Late Miocene in the Carpatho-Pannonian region. The transition from an initial dry magmatic phase generated fallout activity followed by the emplacement of wet PDCs' rich in ash aggregates, when external water infiltrated from a surrounding lake or sea water entered the vent.

Spatiotemporal sediment transport and deposition processes in experimental dilute pyroclastic density currents

Dilute pyroclastic density currents (dilute PDCs) are frequent and highly dangerous flows of hot gas and particles occurring at explosive volcanoes. The study and interpretation of the sedimentary characteristics of their associated deposits is one of the most important approaches to better understand these violent phenomena and to characterise their dynamics, frequency and magnitude in the geological record of volcanoes. Current strategies are based on sediment transport principles developed for fluvial and aeolian systems. How well these analogies capture the sediment transport behaviour of dilute PDCs remains poorly understood, as the hot and hostile conditions of these fast-moving volcanic flows hamper direct measurements.Here we report the results of large-scale experiments that aim to synthesise the behaviour of dilute PDCs in order to investigate the transport and sedimentation processes inside the hot flows. These flows reproduce the spatiotemporal deposit facies variations seen in natural deposits of dilute PDCs. Through measurements of the evolving flow structure (velocity, particle concentration and flow grain size distribution), we show that the lower region of the density stratified current can be subdivided from the aggrading deposit upwards into a dynamic bedload region with particle concentrations of c. 0.5 to several vol%, a transient region with particle concentrations of c. 0.1–1.5 vol%, a dilute, fully turbulent region with formation of mesoscale clusters, and an upper dilute turbulent region absent of mesoscale clusters. We show that the particle feeding mechanisms of the transient region is related to the occurrence of mesoscale clusters. This process has a key role in modifying the sediment transport modes inside the bedload region. These modifications cause a variation of the dynamics of the lower flow boundary, including erosion/deposition events, and variations of the deposition rate over at least three orders of magnitude. We present images and video footage of these processes that include the formation of shifting sandwaves, rolling and saltation of particles inside the experimental dilute PDC, and analyse how these relate to the development of massive, stratified and laminated deposit structures. Through mapping the isochrones of deposition across the flow length and measuring the propagating flow above, we capture a spatiotemporal view of the aggrading deposit, which can guide the interpretation of natural deposits.

Stratigraphy and evolution of the Holocene Aljojuca Maar volcano (Serdán-Oriental basin, Eastern Trans-Mexican Volcanic Belt), and implications for hazard assessment

Aljojuca maar is located in the eastern Trans-Mexican Volcanic Belt, within the Serdán-Oriental Basin, which is characterized by Quaternary bimodal monogenetic volcanism. Aljojuca maar has an irregular shape, with an eastern embayment structure that forms an E-W alignment with at least three older scoria cones to the east. The crater walls expose a sequence of pre-maar volcaniclastic deposits, with a lava flow linked to the older scoria cones and a paleosol at its top; the latter indicates that a significant hiatus occurred between the growth of the scoria cones and the eruption of the Aljojuca maar, whose deposits overlie the soil. Strong interactions of the ascending magma with groundwater led to the emplacement of a ~30-m-thick maar sequence. Our detailed stratigraphic study shows that the eruption can be grouped into two main stages, comprising eight stratigraphic units deposited during five eruptive phases. The first stage reflects the development of the main crater (a maar sensu stricto) and is recorded by six stratigraphic units (from A to F). The eruption began with a strombolian pulse, followed by phreatomagmatic activity (unit A), forming dilute pyroclastic density currents. In the second phase, phreatomagmatic explosions prevailed with different degrees of fragmentation, from juvenile and lithic-rich lapilli fallout (unit B), to less diluted pyroclastic density currents (unit C), and a spatially restricted strombolian pulse. The third phase includes a main-vent enlargement event represented by a chaotic and heterolithic breccia (unit D). In the fourth phase, dilute pyroclastic density current deposits (unit E) dominate, with minor chaotic breccias (unit F). The second stage (fifth phase) includes the occurrence of a possibly fissural E-W structure, which formed a scallop in the eastern flank of the maar (eastern embayment structure), breaking the previously formed main crater's eastern flank and producing a 4-m-chaotic breccia (unit G) followed by the occurrence of a dilute pyroclastic density current (unit H), whose energy enabled the current to climb to the closest scoria cone. The migration of the explosive loci toward the eastern flank of the maar reflects a tectonic control by the dominant regional east-trending structural pattern. Several radiocarbon ages support that the eruption of Aljojuca took place between 2.7 ka (age of charcoal fragments under the ejecta ring) and ~2.2 ka (age of the oldest lake sediments in the maar crater). This has important implications for the hazard assessment of volcanism in this area, which had not been considered previously as a potentially active system.

Ash clouds temperature estimation. Implication on dilute and concentrated PDCs coupling and topography confinement

Pyroclastic density currents (PDCs) are among the most hazardous of all volcanic processes in terms of high speeds and unpredictable extent. While concentrated PDCs are usually topographically confined, the dilute counterpart (ash cloud) is able to overrun topographic barriers, with unexpected trajectories posing a high risk for human settlements around the volcano. Here, for the first time, the temperature of an ash could, for a PDC originated during the 11 July, 2015 Volcán de Colima eruption, is determined, without pre-installed instruments, based on the degree of charcoaling of trees affected by the ash cloud. Temperature estimations were performed using Reflectance analysis and microtomography images processing of pine wood charred fragments. The combination of these two independent and well-established methods to organic matter charred in a volcanic environment constitutes a pioneering attempt for the indirect temperature estimation of dilute pyroclastic density currents (PDCs). Charcoal fragments were sampled at different heights along tree trunks outstanding from the PDC deposit. Both the temperatures obtained from charcoal analyses (reflectance and microtomography) and observation of damages to the tree trunks allowed to distinguish: (i) a lower Zone A, which extends 150–180 cm above the top of the PDC deposit, where trunks show peeled bark and multiple lithic impacts; temperature values are equal or slightly higher than the underlying deposit for the entire length of the valley; (ii) an upper Zone B, developed above 150–180 cm from the top of the PDC deposit, where trees are only burned without any block impact marks; temperature estimations for Zone B are comparable with the PDC deposit temperature range from proximal to distal areas. The temperature data indicate that the 11 July, 2015 Colima PDC event, the ash cloud was always thermally coupled with the under-running concentrated flow for the entire length of the ravine, explaining the observed strong vertical uplift of the ash cloud and the substantial absence of ash cloud detachments along flow. A corollary of our study is that, should a detachment have occurred, the ash cloud surge would have had initial temperatures as high as the one carried by the high concentration part of the PDC. A major outcome of our study is that the temperature estimation of ash clouds bears important implication in terms of hazard assessment for pyroclastic density currents along narrow valleys that usually cut the steep slopes of stratovolcanoes.

Recognizing unsteadiness in the transport systems of dilute pyroclastic density currents

Laboratory density currents generated with unsteady source conditions provide insight into the distances and timescales over which unsteadiness at the vent should persist in natural dilute pyroclastic density currents (PDCs). The laboratory experiments comprise heated 20-μm talc particles turbulently suspended in air and introduced as density currents into an 8.5 × 6 × 2.6 m air-filled chamber. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers are all similar to those of natural dilute PDCs. Although the experiments’ Reynolds numbers are several orders of magnitude less than natural PDCs, the experiments are fully turbulent and are thus dynamically similar to some dilute natural PDCs. “Unsteadiness” in the experiments is generated by adding two pauses of duration t to the eruption (e.g., a 100 s experiment comprising three ~ 33 s pulses separated by two pauses of t = 10 s). Propagation distance of the leading head is proportional to the square root of time; positions of the trailing pulses have similar time dependence, but they generally travel slightly faster than, and thus catch and merge with the leading current. Trailing pulses are more easily distinguished from the body of the preceding current when t is large. Analysis of turbulent structures through space and time shows that unsteadiness at the eruption source can be distinguished from turbulent fluctuations in the currents when t is greater than the integral turbulent timescale of the current body τbody, a statistical measure of the characteristic timescale of unsteadiness within a turbulent flow. The distances over which unsteadiness in the transport system persist scale with t/τbody and the ratio of the thermal Richardson numbers of the trailing and leading pulses. As t/τbody increases from ~ 2 to ~ 4, trailing pulses remain distinct over an increasing fraction of the lead runout distance. When subsequent pulses have higher RiT than the leading current, they coalesce with the leading pulse at more proximal distances, but when RiT is much lower, they remain distinct over distances similar to that of the leading head even when t is similar to τbody. For currents that have depositional timescales comparable to or shorter than τbody, deposits should be expected to preserve a record of unsteadiness over the distances that unsteady pulses persist.

Event stratigraphy and emplacement mechanisms of the last major caldera eruption on Sete Cidades Volcano (São Miguel, Azores): the 16 ka Santa Bárbara Formation

Sete Cidades is an active yet dormant central volcano located in the westernmost part of Sao Miguel Island (Azores archipelago), characterized by a 5-km-wide multi-stage caldera formed in the last 36 ka. In this work, we present new stratigraphic, grain size, petrographic, and geochemical data of the 16 ka Santa Barbara Formation, related to the last stage of caldera formation. We define the lithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position, lithofacies, and distribution of the deposits. The preserved deposits outcrop in a narrow area (over < 5 km between the caldera rim and the coastal cliffs) and include ash beds with dispersed lapilli (Lower Member), coarse-grained pumice and lithic-rich lapilli tuffs (Middle Member), and pumice fall deposits (Upper Member). The minimum bulk volume preserved on land is about 0.03 km3; however, the majority of the pyroclastic products were deposited in the ocean. The presence of magmatic mafic enclaves (48 wt% SiO2) within the juvenile pumice clasts (63 wt% SiO2) is testimony to the injection of trachybasaltic magma into the trachytic magma reservoir shortly prior to eruption. The mingling of these two magmas acted as the eruption trigger. The eruption history can be divided into three main phases each characterized by different eruption styles and deposit emplacement mechanisms, accompanied by enlargement of the caldera. The eruption started with a sequence of short-lived explosions generating unstable columns that produced ash fallout and generated dilute pyroclastic density currents, mostly deposited in the SW sector of the volcano. The climax of the eruption was marked by successive low fountaining events that generated high particle concentration pyroclastic density currents emplaced on the SW and NE flanks. The final phase of the eruption was characterized by establishment of a sub-Plinian eruption column that produced pumice fall deposits on the NE flank of the volcano. Our study allows delineation of eruptive scenarios for future major explosive eruptions at Sete Cidades, a volcano which is located on the most populated island of the Azores (Sao Miguel Island), and can provide a framework for the assessment of volcanic hazards on oceanic islands elsewhere.

PYFLOW_2.0: a computer program for calculating flow properties and impact parameters of past dilute pyroclastic density currents based on field data

This paper presents PYFLOW_2.0, a hazard tool for the calculation of the impact parameters of dilute pyroclastic density currents (DPDCs). DPDCs represent the dilute turbulent type of gravity flows that occur during explosive volcanic eruptions; their hazard is the result of their mobility and the capability to laterally impact buildings and infrastructures and to transport variable amounts of volcanic ash along the path. Starting from data coming from the analysis of deposits formed by DPDCs, PYFLOW_2.0 calculates the flow properties (e.g., velocity, bulk density, thickness) and impact parameters (dynamic pressure, deposition time) at the location of the sampled outcrop. Given the inherent uncertainties related to sampling, laboratory analyses, and modeling assumptions, the program provides ranges of variations and probability density functions of the impact parameters rather than single specific values; from these functions, the user can interrogate the program to obtain the value of the computed impact parameter at any specified exceedance probability. In this paper, the sedimentological models implemented in PYFLOW_2.0 are presented, program functionalities are briefly introduced, and two application examples are discussed so as to show the capabilities of the software in quantifying the impact of the analyzed DPDCs in terms of dynamic pressure, volcanic ash concentration, and residence time in the atmosphere. The software and user’s manual are made available as a downloadable electronic supplement.