Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landsc... more Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landscapes to devastate everything in their path 1,2. The internal dynamics underpinning these hazards cannot be directly observed 3. Here we present a quantitative view inside pyroclastic density currents by synthesizing their natural flow behaviour in large-scale experiments. The experiments trace flow dynamics from initiation to deposi-tion, and can explain the sequence and evolution of real-world deposits. We show that, inside pyroclastic density currents, the long-hypothesized non-turbulent underflow and fully turbulent ash-cloud regions 4,5 are linked through a hitherto unrecognized middle zone of intermediate turbulence and concentration. Bounded by abrupt jumps in turbulence, the middle zone couples underflow and ash-cloud regions kinematically. Inside this zone, strong feedback between gas and particle phases leads to the formation of mesoscale turbulence clusters. These extremely fast-settling dendritic structures dictate the internal stratification and evolution of pyroclastic density currents and allow the underflows to grow significantly during runout. Our experiments reveal how the underflow and ash-cloud regions are dynamically related—insights that are relevant to the forecasting of pyroclastic density current behaviour in volcanic hazard models. For more than 50 years, the transport and deposition of pyroclastic density currents (PDCs) have remained amongst the most hotly debated issues in volcanology 1,5–7. Due to their unpredictability and extreme violence, PDC deposits have been relied upon to infer flow dynamics 4,8. PDC deposits range from massive to highly stratified types 9,10. These extremes led to the conceptualization of two endmembers of PDC transport, one as a dry granular or gas-fluidized granular flow of high particle concentration 5,11,12 , and the other as a dilute fully turbulent gravity current 13,14. Attempts have been made to unify these endmembers, both theoretically 15 and with numerical modelling studies 16. However, large uncertainties remain about the multiphase physics of coupled concentrated/dilute particle-gas transport 17. Thus, even conceptual paradigms of the internal structure of PDCs remain highly controversial 4 , and envisage either rigid, coexisting zones of concentrated laminar and dilute fully turbulent transport regimes 6–8 , or a continuous gradation between them 9,10. Here we report on gas–particle flows produced by the gravitational collapse of 1.5 tons of volcanic material (Methods, Supplementary Fig. 1 and Supplementary Movie 1). As in nature, experimental currents of pumice, ash and air were synthesized by a t = 0.04 s t = 0.08 s t = 0.16 s t = 0.12 s t = 0.30 s t = 0.39 s t = 0.01 s C U b U C GP CL c CU Figure 1 | Synthesizing pyroclastic density currents in large-scale experiments. a, Side view of an experimental pyroclastic density current at the eruption simulator PELE. b, Lower 0.95 m of the flow at a runout distance of 3 m at diierent times. c, Passage of the head region at 3 m. Arrows highlight the interfaces between underflow (U, lower zone) and ash-cloud regions (C) (black), and between middle (CL) and upper (CU) zones of the pyroclastic density current, respectively. Rapid sedimentation of dendritic clusters of mesoscale turbulence entraps gas pockets (GP) in the underflows. Vertical scaling bars are 0.3-m long.
ABSTRACT Central Andean deformation history in southern Peru is recorded in Neogene volcanic unit... more ABSTRACT Central Andean deformation history in southern Peru is recorded in Neogene volcanic units of Ocoña and Cotahuasi canyons that cut across the western Cordillera. Acceleration (<25 Ma) of uplift in the region is reflected in the Neogene epiclastic deposits with interspersed and subsequent rhyolitic ignimbrites between 24.6 and 1.37 Ma. Large-volume (>100 km3) Nazca (c.24.6 Ma), Alpabamba (19.4-18.0 Ma), and Huaylillas (14.25-12.7 Ma) ignimbrite sheets preceded the canyon incision, whereas sheets of smaller volume (<50 km3), Caraveli (9.5-8.9 Ma), Lower (5.13-3.6 Ma) and Upper Sencca (c.2 Ma) and Las Lomas (c.1.56-1.37 Ma), were deposited during canyon incision and are interspersed with Lower and Upper Barroso lava flows. The Alpabamba compound ignimbrite sheets comprise a vitrophyre at the base, grading into a strongly welded, eutaxitic, crystal-rich facies overlain by a thick, multi-bedded ash-flow tuff and a lithic-rich, indurated flow unit. The Huaylillas ignimbrite sheet comprises a strongly welded, crystal-rich, lithic-poor, columnar lithofacies, with devitrified pumice. The Caraveli ignimbrite sheet has a jointed vitrophyre overlain by a welded, blocky, crystal-rich flow unit. A vacuolar, saccharolytic unit forms the top of the sequence. The Lower Sencca ignimbrite sheet comprises of a basal vitrophyre and a slightly welded, fibrous pumice-rich flow unit, which grades into a welded, vapor-phase unit that contains more crystals than pumice and lithics. The Upper Sencca ignimbrite sheet consists of a black vitrophyre, grading into a strongly welded, crystal-rich, eutaxitic cooling unit. The latter is capped by a slightly welded unit, and an indurated pumice-rich, crystal-poor vapour-phase facies. Quaternary valley-fill termed Las Lomas consists of unwelded, crystal-poor pumice-flow deposits. Eighteen new 40Ar/39Ar analyses have been carried out on feldspar/glass separates from pumice and lavas. Results for the Caraveli ignimbrite (9.35±0.06 Ma), Upper Barroso lavas (2.24±0.45 Ma) and Upper Sencca ignimbrite (2.00-2.06±0.09 Ma) are in good agreement with previous data. New ages for the Lower Barroso lavas (7.32±0.05; 5.36±0.12 Ma) and Lower Sencca ignimbrites (5.13±0.01, 5.09±0.03, 4.65±0.11, 4.36 ± 0.16) extend their temporal history towards older times. Lower Barroso lava flow activity occurred as early as~7.3 and lasted until 5.4 Ma just before the Lower Sencca eruptions. The Lower Sencca ignimbrites spanned at least 1.5 Ma and are larger and more widespread than the Upper Sencca ignimbrites, which may have formed in one pulse around 2 Ma. The Anisotropy of Magnetic Susceptibility (AMS) method and the magnetic fabric at 24 sites have been used to indicate flow directions and to infer the source location of the rhyolitic ignimbrites. Mean directions and confidence cones were calculated using Bingham statistics. Most AMS ellipsoids are oblate with near-vertical, slightly imbricated minimum susceptibility directions. The magnetic signal is dominantly carried by coarse multi-domain and a few fine single-domain grains of titanomagnetite. Inferred transport directions based on AMS results reveal that the source of Huaylillas, Lower Sencca and Upper Sencca units is probably located below the Nevado Coropuna volcano. The source of the Caraveli unit points to a depression W of Nevado Sara Sara. The Alpabamba source could not be clearly determined. Keywords: ignimbrites, Peru, stratigraphy, chronology, AMS, fabric, Neogene, Quaternary.
The 2012 eruption of Tongariro volcano (New
Zealand) produced highly mobile, low-temperature, bla... more The 2012 eruption of Tongariro volcano (New Zealand) produced highly mobile, low-temperature, blast-derived pyroclastic density currents after partial collapse of the western flank of the Upper Te Maari crater. Despite a low volume (340,000 m3), the flows traveled up to 2.5 km from source, covering a total area of 6.1 km2. Along one of the blast axes, freshly exposed, proximal-to-distal sedimentary structures and grain-size data suggest emplacement of the fining upward tripartite depositional sequence (massive, stratified, and laminated) under a dilute and strongly longitudinally zoned turbulent density current. While the zoning formed in the deposit in the first 1500 m of runout, the current progressively waned to the extent where it transported a nearly homogenous grain-size mixture at the liftoff position. Our data indicate that after the passage of an erosive flow front, massive unit Awas deposited under a rapid-suspension sedimentation regime. Unit B was deposited under a traction-dominated regime generated by a subsequent portion of the flow moving at lower velocities and with lower sediment transport capacity than the portion depositing unit A. The final and slowest flow zone deposited the finest particles under weakly tractive conditions. Transport and emplacement dynamics inferred in this study show strong similarities between hydrothermal explosions, magmatic blasts, and high-energy dilute PDCs. The common occurrence of hydrothermal fields on volcanic flanks points to this hazard being an under-appreciated one at stratovolcanoes worldwide.
Pyroclastic flow eruption large-scale experiment (PELE) is a large-scale facility for experimenta... more Pyroclastic flow eruption large-scale experiment (PELE) is a large-scale facility for experimental studies of pyroclastic density currents (PDCs). It is used to generate high-energy currents involving 500–6500m3 natural volcanic material and air that achieve velocities of 7–30ms1, flow thicknesses of 2–4.5m, and runouts of >35 m. The experimental PDCs are synthesized by a controlled “eruption column collapse” of ash-lapilli suspensions onto an instrumented channel. The first set of experiments are documented here and used to elucidate the main flow regimes that influence PDC dynamic structure. Four phases are identified: (1) mixture acceleration during eruption column collapse, (2) column-slope impact, (3) PDC generation, and (4) ash cloud diffusion. The currents produced are fully turbulent flows and scale well to natural PDCs including small to large scales of turbulent transport. PELE is capable of generating short, pulsed, and sustained currents over periods of several tens of seconds, and dilute surge-like PDCs through to highly concentrated pyroclastic flow-like currents. The surge-like variants develop a basal <0.05m thick regime of saltating/rolling particles and shifting sand waves, capped by a 2.5–4.5m thick, turbulent suspension that grades upward to lower particle concentrations. Resulting deposits include stratified dunes, wavy and planar laminated beds, and thin ash cloud fall layers. Concentrated currents segregate into a dense basal underflow of <0.6m thickness that remains aerated. This is capped by an upper ash cloud surge (1.5–3m thick) with 100 to 104 vol% particles. Their deposits include stratified, massive, normally and reversely graded beds, lobate fronts, and laterally extensive veneer facies beyond channel margins.
The 6 August 2012 Te Maari eruption produced violent and widespread "cold" Pyroclastic Density Cu... more The 6 August 2012 Te Maari eruption produced violent and widespread "cold" Pyroclastic Density Currents (PDCs) following unroofing of the pressurized hydrothermal system. Despite an erupted volume of only~5 ×10 5 m 3 , and lacking any juvenile component, the 340,000 m 3 of PDCs spread over an area of 6.1 km 2 and had mobilities that were on the order of volcanic blasts or blast-like PDCs. This great mobility was due to strong lateral focussing of explosion energy, producing jets with initial velocities N 100 m/s. We present a type-stratigraphy for these hydrothermal-derived low-temperature PDCs that show a tripartite deposit sequence. Each of the deposit units dominates respectively three outward-gradational sedimentary facies, reflecting transitions in the propagating PDC transport and depositional mechanisms. The largest PDCs, directed west and east of the Upper Te Maari area were generated from outer-cone breccias and tuffs that were mostly highly hydrothermally altered. Landsliding and the geometry of the hydrothermal area led to the directed jetting. Initial particle-laden jets laid sheets, grading into lobes of proximal massive sand to gravel-rich facies dominated by unit A and extending up to 1 km from the vents. As the jets were collapsing, a vertically and longitudinally stratified PDC developed within the first few hundred metres from source. Exponential thinning and coarse-tail grading-dominated fining with radial distance of massive unit A resulted from fast deposition and progressive depletion of the most concentrated flow region behind the PDC head. Markedly slower tractional sedimentation from the passing PDC body and tail deposited the highly stratified and ripple-bedded fine-coarse ash of unit B. This formed distinctive dune fields of the medial dunebedded ash-rich facies. Upwards in depositional sequences the waning of the current can be seen, by replacement of higher-energy bedforms to progressively lower ones. Downstream progressive waning and further depletion are characterised by the development of the distal wavy to planar-bedded ash-rich facies. This is increasingly dominated by the uppermost deposition unit C of laminated fine-med ash deposited by gently turbulent, dilute phoenix clouds. These high energy PDCs, sourced from flank hydrothermal systems should be regarded as a serious threat in any multihazard assessment of a stratovolcano, even though they may not be one of the major magmatic vent sites. In addition, further detailed studies of these hydrothermal jetting events and their deposits should be pursued in order to better understand large-volume volcanic blasts, which appear to be a larger scale sibling phenomenon.
The ballistic ejection of blocks during explosive eruptions constitutes a major hazard near activ... more The ballistic ejection of blocks during explosive eruptions constitutes a major hazard near active volcanoes. Fields of ballistic clasts can provide important clues towards quantifying the energy, dynamics and directionality of explosive events, but detailed datasets are rare. During the 6 August 2012 hydrothermal eruption of Upper Te Maari (Tongariro), New Zealand, three explosions occurred in rapid succession within less than 20 s. The first two produced laterally-directed pyroclastic density currents (PDC), and the final vertical explosion generated an ash plume. Each of these explosions was associated with the ejection of ballistic blocks. We present detailed maps of the resulting 5.1 km 2 block impact field and the distribution of the N2200 impact craters with diameters N 2.5 m. There are two distinct regions of high crater concentration, where crater densities reach more than six times the average background density. These occur at distances of 500-700 m east and 1000-1350 west of a 430-m-long fissure that was created during the eruption. The high-density fields are characterized by a narrow radial spread of b 45°and are located along the proximal transport direction of the pyroclastic density currents. A provenance analysis of ballistic blocks allowed us to reconstruct two different eruptive vents for the explosions. The first two laterally-directed explosions were sourced from the fissure, while the third explosion occurred through the pre-existing Upper Te Maari Crater, generating a roughly axisymmetric shower of ballistics. Stratigraphic relationships between impact craters, PDC and fall deposits suggest that the ballistic blocks were initially coupled with the rapidly expanding gas-particle mixtures that produced the PDCs. Ballistic trajectory modeling, reproducing the lateral extent and main impact density pattern of the western impact field, allows estimation of the vertical expansion angle of the second and largest explosion. The calculations show that the largest proportion of the explosion energy was strongly focused as a narrow and extremely shallow (from −3 to 15°from the horizontal) laterally expanding hydrothermal blast. The results presented here constitute an important data set for ballistic hazard assessment at Tongariro volcano and they can provide further clues towards better understanding highly energetic laterally directed volcanic explosions at similar hydrothermal fields.
Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landsc... more Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landscapes to devastate everything in their path 1,2. The internal dynamics underpinning these hazards cannot be directly observed 3. Here we present a quantitative view inside pyroclastic density currents by synthesizing their natural flow behaviour in large-scale experiments. The experiments trace flow dynamics from initiation to deposi-tion, and can explain the sequence and evolution of real-world deposits. We show that, inside pyroclastic density currents, the long-hypothesized non-turbulent underflow and fully turbulent ash-cloud regions 4,5 are linked through a hitherto unrecognized middle zone of intermediate turbulence and concentration. Bounded by abrupt jumps in turbulence, the middle zone couples underflow and ash-cloud regions kinematically. Inside this zone, strong feedback between gas and particle phases leads to the formation of mesoscale turbulence clusters. These extremely fast-settling dendritic structures dictate the internal stratification and evolution of pyroclastic density currents and allow the underflows to grow significantly during runout. Our experiments reveal how the underflow and ash-cloud regions are dynamically related—insights that are relevant to the forecasting of pyroclastic density current behaviour in volcanic hazard models. For more than 50 years, the transport and deposition of pyroclastic density currents (PDCs) have remained amongst the most hotly debated issues in volcanology 1,5–7. Due to their unpredictability and extreme violence, PDC deposits have been relied upon to infer flow dynamics 4,8. PDC deposits range from massive to highly stratified types 9,10. These extremes led to the conceptualization of two endmembers of PDC transport, one as a dry granular or gas-fluidized granular flow of high particle concentration 5,11,12 , and the other as a dilute fully turbulent gravity current 13,14. Attempts have been made to unify these endmembers, both theoretically 15 and with numerical modelling studies 16. However, large uncertainties remain about the multiphase physics of coupled concentrated/dilute particle-gas transport 17. Thus, even conceptual paradigms of the internal structure of PDCs remain highly controversial 4 , and envisage either rigid, coexisting zones of concentrated laminar and dilute fully turbulent transport regimes 6–8 , or a continuous gradation between them 9,10. Here we report on gas–particle flows produced by the gravitational collapse of 1.5 tons of volcanic material (Methods, Supplementary Fig. 1 and Supplementary Movie 1). As in nature, experimental currents of pumice, ash and air were synthesized by a t = 0.04 s t = 0.08 s t = 0.16 s t = 0.12 s t = 0.30 s t = 0.39 s t = 0.01 s C U b U C GP CL c CU Figure 1 | Synthesizing pyroclastic density currents in large-scale experiments. a, Side view of an experimental pyroclastic density current at the eruption simulator PELE. b, Lower 0.95 m of the flow at a runout distance of 3 m at diierent times. c, Passage of the head region at 3 m. Arrows highlight the interfaces between underflow (U, lower zone) and ash-cloud regions (C) (black), and between middle (CL) and upper (CU) zones of the pyroclastic density current, respectively. Rapid sedimentation of dendritic clusters of mesoscale turbulence entraps gas pockets (GP) in the underflows. Vertical scaling bars are 0.3-m long.
ABSTRACT Central Andean deformation history in southern Peru is recorded in Neogene volcanic unit... more ABSTRACT Central Andean deformation history in southern Peru is recorded in Neogene volcanic units of Ocoña and Cotahuasi canyons that cut across the western Cordillera. Acceleration (&lt;25 Ma) of uplift in the region is reflected in the Neogene epiclastic deposits with interspersed and subsequent rhyolitic ignimbrites between 24.6 and 1.37 Ma. Large-volume (&gt;100 km3) Nazca (c.24.6 Ma), Alpabamba (19.4-18.0 Ma), and Huaylillas (14.25-12.7 Ma) ignimbrite sheets preceded the canyon incision, whereas sheets of smaller volume (&lt;50 km3), Caraveli (9.5-8.9 Ma), Lower (5.13-3.6 Ma) and Upper Sencca (c.2 Ma) and Las Lomas (c.1.56-1.37 Ma), were deposited during canyon incision and are interspersed with Lower and Upper Barroso lava flows. The Alpabamba compound ignimbrite sheets comprise a vitrophyre at the base, grading into a strongly welded, eutaxitic, crystal-rich facies overlain by a thick, multi-bedded ash-flow tuff and a lithic-rich, indurated flow unit. The Huaylillas ignimbrite sheet comprises a strongly welded, crystal-rich, lithic-poor, columnar lithofacies, with devitrified pumice. The Caraveli ignimbrite sheet has a jointed vitrophyre overlain by a welded, blocky, crystal-rich flow unit. A vacuolar, saccharolytic unit forms the top of the sequence. The Lower Sencca ignimbrite sheet comprises of a basal vitrophyre and a slightly welded, fibrous pumice-rich flow unit, which grades into a welded, vapor-phase unit that contains more crystals than pumice and lithics. The Upper Sencca ignimbrite sheet consists of a black vitrophyre, grading into a strongly welded, crystal-rich, eutaxitic cooling unit. The latter is capped by a slightly welded unit, and an indurated pumice-rich, crystal-poor vapour-phase facies. Quaternary valley-fill termed Las Lomas consists of unwelded, crystal-poor pumice-flow deposits. Eighteen new 40Ar/39Ar analyses have been carried out on feldspar/glass separates from pumice and lavas. Results for the Caraveli ignimbrite (9.35±0.06 Ma), Upper Barroso lavas (2.24±0.45 Ma) and Upper Sencca ignimbrite (2.00-2.06±0.09 Ma) are in good agreement with previous data. New ages for the Lower Barroso lavas (7.32±0.05; 5.36±0.12 Ma) and Lower Sencca ignimbrites (5.13±0.01, 5.09±0.03, 4.65±0.11, 4.36 ± 0.16) extend their temporal history towards older times. Lower Barroso lava flow activity occurred as early as~7.3 and lasted until 5.4 Ma just before the Lower Sencca eruptions. The Lower Sencca ignimbrites spanned at least 1.5 Ma and are larger and more widespread than the Upper Sencca ignimbrites, which may have formed in one pulse around 2 Ma. The Anisotropy of Magnetic Susceptibility (AMS) method and the magnetic fabric at 24 sites have been used to indicate flow directions and to infer the source location of the rhyolitic ignimbrites. Mean directions and confidence cones were calculated using Bingham statistics. Most AMS ellipsoids are oblate with near-vertical, slightly imbricated minimum susceptibility directions. The magnetic signal is dominantly carried by coarse multi-domain and a few fine single-domain grains of titanomagnetite. Inferred transport directions based on AMS results reveal that the source of Huaylillas, Lower Sencca and Upper Sencca units is probably located below the Nevado Coropuna volcano. The source of the Caraveli unit points to a depression W of Nevado Sara Sara. The Alpabamba source could not be clearly determined. Keywords: ignimbrites, Peru, stratigraphy, chronology, AMS, fabric, Neogene, Quaternary.
The 2012 eruption of Tongariro volcano (New
Zealand) produced highly mobile, low-temperature, bla... more The 2012 eruption of Tongariro volcano (New Zealand) produced highly mobile, low-temperature, blast-derived pyroclastic density currents after partial collapse of the western flank of the Upper Te Maari crater. Despite a low volume (340,000 m3), the flows traveled up to 2.5 km from source, covering a total area of 6.1 km2. Along one of the blast axes, freshly exposed, proximal-to-distal sedimentary structures and grain-size data suggest emplacement of the fining upward tripartite depositional sequence (massive, stratified, and laminated) under a dilute and strongly longitudinally zoned turbulent density current. While the zoning formed in the deposit in the first 1500 m of runout, the current progressively waned to the extent where it transported a nearly homogenous grain-size mixture at the liftoff position. Our data indicate that after the passage of an erosive flow front, massive unit Awas deposited under a rapid-suspension sedimentation regime. Unit B was deposited under a traction-dominated regime generated by a subsequent portion of the flow moving at lower velocities and with lower sediment transport capacity than the portion depositing unit A. The final and slowest flow zone deposited the finest particles under weakly tractive conditions. Transport and emplacement dynamics inferred in this study show strong similarities between hydrothermal explosions, magmatic blasts, and high-energy dilute PDCs. The common occurrence of hydrothermal fields on volcanic flanks points to this hazard being an under-appreciated one at stratovolcanoes worldwide.
Pyroclastic flow eruption large-scale experiment (PELE) is a large-scale facility for experimenta... more Pyroclastic flow eruption large-scale experiment (PELE) is a large-scale facility for experimental studies of pyroclastic density currents (PDCs). It is used to generate high-energy currents involving 500–6500m3 natural volcanic material and air that achieve velocities of 7–30ms1, flow thicknesses of 2–4.5m, and runouts of >35 m. The experimental PDCs are synthesized by a controlled “eruption column collapse” of ash-lapilli suspensions onto an instrumented channel. The first set of experiments are documented here and used to elucidate the main flow regimes that influence PDC dynamic structure. Four phases are identified: (1) mixture acceleration during eruption column collapse, (2) column-slope impact, (3) PDC generation, and (4) ash cloud diffusion. The currents produced are fully turbulent flows and scale well to natural PDCs including small to large scales of turbulent transport. PELE is capable of generating short, pulsed, and sustained currents over periods of several tens of seconds, and dilute surge-like PDCs through to highly concentrated pyroclastic flow-like currents. The surge-like variants develop a basal <0.05m thick regime of saltating/rolling particles and shifting sand waves, capped by a 2.5–4.5m thick, turbulent suspension that grades upward to lower particle concentrations. Resulting deposits include stratified dunes, wavy and planar laminated beds, and thin ash cloud fall layers. Concentrated currents segregate into a dense basal underflow of <0.6m thickness that remains aerated. This is capped by an upper ash cloud surge (1.5–3m thick) with 100 to 104 vol% particles. Their deposits include stratified, massive, normally and reversely graded beds, lobate fronts, and laterally extensive veneer facies beyond channel margins.
The 6 August 2012 Te Maari eruption produced violent and widespread "cold" Pyroclastic Density Cu... more The 6 August 2012 Te Maari eruption produced violent and widespread "cold" Pyroclastic Density Currents (PDCs) following unroofing of the pressurized hydrothermal system. Despite an erupted volume of only~5 ×10 5 m 3 , and lacking any juvenile component, the 340,000 m 3 of PDCs spread over an area of 6.1 km 2 and had mobilities that were on the order of volcanic blasts or blast-like PDCs. This great mobility was due to strong lateral focussing of explosion energy, producing jets with initial velocities N 100 m/s. We present a type-stratigraphy for these hydrothermal-derived low-temperature PDCs that show a tripartite deposit sequence. Each of the deposit units dominates respectively three outward-gradational sedimentary facies, reflecting transitions in the propagating PDC transport and depositional mechanisms. The largest PDCs, directed west and east of the Upper Te Maari area were generated from outer-cone breccias and tuffs that were mostly highly hydrothermally altered. Landsliding and the geometry of the hydrothermal area led to the directed jetting. Initial particle-laden jets laid sheets, grading into lobes of proximal massive sand to gravel-rich facies dominated by unit A and extending up to 1 km from the vents. As the jets were collapsing, a vertically and longitudinally stratified PDC developed within the first few hundred metres from source. Exponential thinning and coarse-tail grading-dominated fining with radial distance of massive unit A resulted from fast deposition and progressive depletion of the most concentrated flow region behind the PDC head. Markedly slower tractional sedimentation from the passing PDC body and tail deposited the highly stratified and ripple-bedded fine-coarse ash of unit B. This formed distinctive dune fields of the medial dunebedded ash-rich facies. Upwards in depositional sequences the waning of the current can be seen, by replacement of higher-energy bedforms to progressively lower ones. Downstream progressive waning and further depletion are characterised by the development of the distal wavy to planar-bedded ash-rich facies. This is increasingly dominated by the uppermost deposition unit C of laminated fine-med ash deposited by gently turbulent, dilute phoenix clouds. These high energy PDCs, sourced from flank hydrothermal systems should be regarded as a serious threat in any multihazard assessment of a stratovolcano, even though they may not be one of the major magmatic vent sites. In addition, further detailed studies of these hydrothermal jetting events and their deposits should be pursued in order to better understand large-volume volcanic blasts, which appear to be a larger scale sibling phenomenon.
The ballistic ejection of blocks during explosive eruptions constitutes a major hazard near activ... more The ballistic ejection of blocks during explosive eruptions constitutes a major hazard near active volcanoes. Fields of ballistic clasts can provide important clues towards quantifying the energy, dynamics and directionality of explosive events, but detailed datasets are rare. During the 6 August 2012 hydrothermal eruption of Upper Te Maari (Tongariro), New Zealand, three explosions occurred in rapid succession within less than 20 s. The first two produced laterally-directed pyroclastic density currents (PDC), and the final vertical explosion generated an ash plume. Each of these explosions was associated with the ejection of ballistic blocks. We present detailed maps of the resulting 5.1 km 2 block impact field and the distribution of the N2200 impact craters with diameters N 2.5 m. There are two distinct regions of high crater concentration, where crater densities reach more than six times the average background density. These occur at distances of 500-700 m east and 1000-1350 west of a 430-m-long fissure that was created during the eruption. The high-density fields are characterized by a narrow radial spread of b 45°and are located along the proximal transport direction of the pyroclastic density currents. A provenance analysis of ballistic blocks allowed us to reconstruct two different eruptive vents for the explosions. The first two laterally-directed explosions were sourced from the fissure, while the third explosion occurred through the pre-existing Upper Te Maari Crater, generating a roughly axisymmetric shower of ballistics. Stratigraphic relationships between impact craters, PDC and fall deposits suggest that the ballistic blocks were initially coupled with the rapidly expanding gas-particle mixtures that produced the PDCs. Ballistic trajectory modeling, reproducing the lateral extent and main impact density pattern of the western impact field, allows estimation of the vertical expansion angle of the second and largest explosion. The calculations show that the largest proportion of the explosion energy was strongly focused as a narrow and extremely shallow (from −3 to 15°from the horizontal) laterally expanding hydrothermal blast. The results presented here constitute an important data set for ballistic hazard assessment at Tongariro volcano and they can provide further clues towards better understanding highly energetic laterally directed volcanic explosions at similar hydrothermal fields.
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Papers by Eric Breard
Zealand) produced highly mobile, low-temperature, blast-derived pyroclastic density currents after partial collapse of
the western flank of the Upper Te Maari crater. Despite a low
volume (340,000 m3), the flows traveled up to 2.5 km from
source, covering a total area of 6.1 km2. Along one of the blast
axes, freshly exposed, proximal-to-distal sedimentary structures and grain-size data suggest emplacement of the fining upward tripartite depositional sequence (massive, stratified, and laminated)
under a dilute and strongly longitudinally zoned turbulent
density current. While the zoning formed in the deposit in
the first 1500 m of runout, the current progressively waned to
the extent where it transported a nearly homogenous grain-size
mixture at the liftoff position. Our data indicate that after the
passage of an erosive flow front, massive unit Awas deposited
under a rapid-suspension sedimentation regime. Unit B was
deposited under a traction-dominated regime generated by a
subsequent portion of the flow moving at lower velocities and
with lower sediment transport capacity than the portion depositing unit A. The final and slowest flow zone deposited the
finest particles under weakly tractive conditions. Transport
and emplacement dynamics inferred in this study show strong
similarities between hydrothermal explosions, magmatic blasts,
and high-energy dilute PDCs. The common occurrence of hydrothermal fields on volcanic flanks points to this hazard being an under-appreciated one at stratovolcanoes worldwide.
studies of pyroclastic density currents (PDCs). It is used to generate high-energy currents involving 500–6500m3
natural volcanic material and air that achieve velocities of 7–30ms1, flow thicknesses of 2–4.5m, and runouts
of >35 m. The experimental PDCs are synthesized by a controlled “eruption column collapse” of ash-lapilli
suspensions onto an instrumented channel. The first set of experiments are documented here and used to
elucidate the main flow regimes that influence PDC dynamic structure. Four phases are identified: (1) mixture
acceleration during eruption column collapse, (2) column-slope impact, (3) PDC generation, and (4) ash cloud
diffusion. The currents produced are fully turbulent flows and scale well to natural PDCs including small to large
scales of turbulent transport. PELE is capable of generating short, pulsed, and sustained currents over periods
of several tens of seconds, and dilute surge-like PDCs through to highly concentrated pyroclastic flow-like
currents. The surge-like variants develop a basal <0.05m thick regime of saltating/rolling particles and
shifting sand waves, capped by a 2.5–4.5m thick, turbulent suspension that grades upward to lower
particle concentrations. Resulting deposits include stratified dunes, wavy and planar laminated beds, and thin
ash cloud fall layers. Concentrated currents segregate into a dense basal underflow of <0.6m thickness that
remains aerated. This is capped by an upper ash cloud surge (1.5–3m thick) with 100 to 104 vol% particles.
Their deposits include stratified, massive, normally and reversely graded beds, lobate fronts, and laterally
extensive veneer facies beyond channel margins.
Zealand) produced highly mobile, low-temperature, blast-derived pyroclastic density currents after partial collapse of
the western flank of the Upper Te Maari crater. Despite a low
volume (340,000 m3), the flows traveled up to 2.5 km from
source, covering a total area of 6.1 km2. Along one of the blast
axes, freshly exposed, proximal-to-distal sedimentary structures and grain-size data suggest emplacement of the fining upward tripartite depositional sequence (massive, stratified, and laminated)
under a dilute and strongly longitudinally zoned turbulent
density current. While the zoning formed in the deposit in
the first 1500 m of runout, the current progressively waned to
the extent where it transported a nearly homogenous grain-size
mixture at the liftoff position. Our data indicate that after the
passage of an erosive flow front, massive unit Awas deposited
under a rapid-suspension sedimentation regime. Unit B was
deposited under a traction-dominated regime generated by a
subsequent portion of the flow moving at lower velocities and
with lower sediment transport capacity than the portion depositing unit A. The final and slowest flow zone deposited the
finest particles under weakly tractive conditions. Transport
and emplacement dynamics inferred in this study show strong
similarities between hydrothermal explosions, magmatic blasts,
and high-energy dilute PDCs. The common occurrence of hydrothermal fields on volcanic flanks points to this hazard being an under-appreciated one at stratovolcanoes worldwide.
studies of pyroclastic density currents (PDCs). It is used to generate high-energy currents involving 500–6500m3
natural volcanic material and air that achieve velocities of 7–30ms1, flow thicknesses of 2–4.5m, and runouts
of >35 m. The experimental PDCs are synthesized by a controlled “eruption column collapse” of ash-lapilli
suspensions onto an instrumented channel. The first set of experiments are documented here and used to
elucidate the main flow regimes that influence PDC dynamic structure. Four phases are identified: (1) mixture
acceleration during eruption column collapse, (2) column-slope impact, (3) PDC generation, and (4) ash cloud
diffusion. The currents produced are fully turbulent flows and scale well to natural PDCs including small to large
scales of turbulent transport. PELE is capable of generating short, pulsed, and sustained currents over periods
of several tens of seconds, and dilute surge-like PDCs through to highly concentrated pyroclastic flow-like
currents. The surge-like variants develop a basal <0.05m thick regime of saltating/rolling particles and
shifting sand waves, capped by a 2.5–4.5m thick, turbulent suspension that grades upward to lower
particle concentrations. Resulting deposits include stratified dunes, wavy and planar laminated beds, and thin
ash cloud fall layers. Concentrated currents segregate into a dense basal underflow of <0.6m thickness that
remains aerated. This is capped by an upper ash cloud surge (1.5–3m thick) with 100 to 104 vol% particles.
Their deposits include stratified, massive, normally and reversely graded beds, lobate fronts, and laterally
extensive veneer facies beyond channel margins.