The Pucón eruption was the largest Holocene explosive outburst of Volcán Villarrica, Chile. It discharged >1.0 km
3
of basaltic-andesite magma and >0.8 km
3
of pre-existing rock, forming a thin ...scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60
14
C years BP.
The activity of Convention at Montserrat Soufrière Hills Volcano, Montserrat, during the period 1995-1999 included numerous violent explosions. Two major cycles of Vulcanian explosions occurred in ...1997: a first of 13 explosions between 4 and 12 August and a second of 75 between 22 September and 21 October. The explosions were short-lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows, and buoyant plumes ascended 3-15 km into the atmosphere. Each explosion discharged on average 3×10^sup 5^ m^sup 3^ (dense-rock equivalent, DRE) of magma, draining the conduit to depths of 1-2 km. The paper focuses on the first few seconds of three explosions of the 75 that occurred in September/October 1997: 6 October 1997 at 17:50, 7 October 1997 at 16:02 and 9 October 1997 at 12:32. Physical parameters such as exit velocities, magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum-dominated finger jets visible on videos during the initial stages of each explosion. The model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent, and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters. Each explosion reveals a progressive increase in eruptive intensity with time, jet exit velocities increasing from 40 m s^sup -1^ at the beginning of the explosion up to 140 m s^sup -1^ after a few seconds. Modelling suggests that the first magma to exit was largely degassed, whereas that discharged after a few seconds contained up to 2 wt% water. Magma overpressures up to ~10 MPa are estimated to have existed in the conduit immediately prior to each explosion. Progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre-eruptive gradients in water content and magma pressure in the upper reaches (probably 100-500 m) of the conduit. Fountain collapse occurred during the first 10-20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant. Such high eruptive densities were due to the presence of partially degassed magma in the conduit.PUBLICATION ABSTRACT
Pyroclastic flows are noted for their highly ‘mobile’ behaviour, being able to flow on slopes as low as a few degrees, and this has been attributed to fluidisation or partial fluidisation by escaping ...gases. The three types of pyroclastic flows generated during the eruption of Soufrière Hills Volcano on Montserrat (dome-collapse, fountain-collapse and surge-derived) differ in their apparent ‘mobility’, suggesting different degrees of fluidisation and friction reduction in the flowing granular materials. Several possible sources of fluidising gas operate in pyroclastic flows. The purpose of this paper is to assess the feasibility of one mechanism: the release during flow of pressurised gases trapped in the vesicles of juvenile clasts. Measurements with a helium pycnometer show that all vesicles in dense lava blocks from dome-collapse pyroclastic flows are connected, so that the mechanism is probably not viable for such flows. On the other hand, pumices from fountain-collapse pyroclastic flows from Soufrière Hills and other volcanoes contain up to 10 vol% of isolated vesicles. This is confirmed by SEM imagery, which shows that Soufrière Hills pumices contain two vesicle populations, with a bimodal size distribution. The large vesicles are mostly interconnected, whereas about two thirds of the smaller population (<15 μm) are isolated. The small vesicles are located as strings, and in some cases as distinctive spheroidal clusters, in the walls between the large vesicles. Many small vesicles and spheroidal vesicle clusters inflated into the (connected) large vesicles as they grew, because they were isolated and had higher internal pressures. The populations probably record two bubble nucleation events in the ascending magma. The second nucleation event must have occurred before the pumices left the vent because the two populations are also preserved in fallout pumices ejected during the same 1997 Vulcanian explosions that generated the pyroclastic flows. One possibility supported by calculations is that the second event occurred in response to brutal decompression of the magmatic foam as it fragmented. Rupture of isolated vesicles provides a potential source of gas in fountain-collapse pyroclastic flows at Soufrière Hills and other volcanoes. Despite significant uncertainties of the relevant parameters, rough calculations show that abrasion of pumice clasts during flow transport could potentially liberate gas at a sufficient rate to fluidise or partially fluidise the material. However, this mechanism is not tenable in dome-collapse or surge-derived flows on Montserrat, the juvenile components of which lack isolated vesicles. Other gas sources, or other mechanisms of friction reduction, must be invoked for these flows.
Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected ...unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.
We conducted laboratory dam-break experiments on initially fluidized granular flows using two different fine-grained powders (mean grain sizes 47 and 67μm) down a smooth, horizontal channel with an ...impermeable base. The powders were first fluidized and expanded to a known degree in the flume reservoir, then released down the channel by opening a sliding gate. The mixture formed rapidly moving flows that defluidized and deposited progressively as they propagated. The experiments were similar to those carried out previously using volcanic ash by Girolami et al. (2008, 2010) but explored a much larger range of initial aspect ratios (height-to-length ratio, a=0.25 to 4). They were designed to investigate the effects of initial expansion (up to 50vol.% above loose packing) and aspect ratio on the dynamics of flow propagation and deposition, and to explore different scalings in order to determine the physical parameters governing these processes. The flows exhibit a similar behaviour to other types of transient granular flows, including three well defined propagation phases (acceleration, constant velocity, and stopping phases) and the progressive aggradation of a basal static layer during emplacement. The deposit aggradation velocity depends only on the initial powder expansion and is similar to that of a collapsing bed of the same powder, expanded by the same amount, under quasi-static, non-shearing conditions. At a given initial expansion, the maximum runout distance scales with the initial bed height h0, the runout duration with (h0/g)1/2 and the maximum velocity with (gh0)1/2. However, runout distance and duration both increase with increasing initial expansion. This is attributed to the effect of hindered settling in delaying defluidization of the dense, but slightly expanded, suspension. The data enable us to identify an additive scaling law providing a smooth transition from non-expanded to expanded flows.
•Synthetical powders behave similarly than natural volcanic ash.•Particle sedimentation depends only on the initial mixture expansion.•Initial aspect ratio of the mixture has a little effect on the flow dynamics.•A new scaling law provides a smooth transition from non-expanded to expanded flows.
Summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal. We investigate the ...mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples. Models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir. Experiments are carried out using different roof aspect ratios (roof thickness/roof width) of 0.2–2. They reveal two basic collapse mechanisms, dependant on the roof aspect ratio. One occurs at low aspect ratios (≤1), as illustrated by aspect ratios of 0.2 and 1. Outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block. Collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface (aspect ratio of 0.2) or by the creation of inward dipping normal faults delimiting a terrace (aspect ratio of 1). At an aspect ratio of 1, overhanging pit walls are the surface expressions of the reverse faults. Experiments at high aspect ratio (>1.2) reveal a second mechanism. In this case, collapse occurs by stopping, which propagates upwards by a complex pattern of both reverse faults and tension fractures. The initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it. An intermediate mechanism occurs at aspect ratios of 1.1–1.2. In this case, stopping leads to the formation of a cavity with a thin and unstable roof, which collapses suddenly. The newly formed depression then exhibits overhanging walls. Surface morphology and structure of natural examples, such as the summit pit craters at Masaya Volcano, Nicaragua, have many of the features created in the models, indicating that the internal structural geometry of experiments can be applied to real examples. In particular, the surface area and depth of the underpressured reservoir can be roughly estimated. We present a morphological analysis of summit pit craters at volcanoes such as Kilimanjaro (Tanzania), San Cristobal, Telica and Masaya (Nicaragua), and Ubinas (Peru), and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case.
The Soufriere Hills Volcano on Montserrat has produced avalanche‐like pyroclastic flows formed by collapse of the unstable lava dome or explosive activity. Pyroclastic flows associated with dome ...collapse generate overlying dilute surges which detach from and travel beyond their parent flows. The largest surges partially transform by rapid sedimentation into dense secondary pyroclastic flows that pose significant hazards to distal areas. Different kinds of pyroclastic density currents display contrasting mobilities indicated by ratios of total height of fall H, run‐out distance L, area inundated A and volume transported V. Dome‐collapse flow mobilities (characterised by either L/H or A/V2/3) resemble those of terrestrial and extraterrestrial cold‐rockfalls (Dade and Huppert, 1998). In contrast, fountain‐fed pumice flows and fine‐grained, secondary pyroclastic flows travel slower but, for comparable initial volumes and heights, can inundate greater areas.
The rhyodacitic 172ka Lower Pumice 2 (LP2) eruption terminated the first magmatic cycle at Santorini (Greece), producing a proximal <50m thick succession of pyroclastic fall deposits, ...diffusely-stratified to massive ignimbrites and multiple lithic breccias. The eruption commenced with the development of a short-lived precursory eruption column, depositing a <15cm blanket of 1–2cm sized pumice fragments at near vent localities (LP2-A1). The precursor deposits are conformably overlain by a <30m thick sequence of reversely-graded/ungraded pumice fall deposits that reflect opening and widening of a point-source vent, increasing mass discharge rates up to 108kgs−1, and the development of a 36km high Plinian eruption column (LP2-A2, A3). The progressive increase in maximum vesicle number density (NVF) in rhyodacitic pumice, from 3.2×109cm−3 in the basal fall unit of LP2-A2-1 to 9.2×109cm−3 in LP2-A3, translates to an increase in magma decompression rate from 18 to 29MPas−1 over the course of the initial Plinian phase. This is interpreted to be a consequence of progressive vent widening and a deepening of the fragmentation surface. Such interpretations are supported by the increase in lithic clast abundance vertically through LP2-A, and the occurrence of basement-derived (deep) lithic components in LP2-A3. The increasing lithic clast content and the inability to effectively entrain air into the eruption column, due to vent widening, resulted in column collapse and the development of pyroclastic density currents (PDCs; LP2-B). A major vent excavation event or the opening of new vents, possibly associated with incipient caldera collapse, facilitated the ingress of water into the magmatic system, the development of widespread PDCs and the deposition of a <20m thick massive phreatomagmatic tuff (LP2-C). The eruption cumulated in catastrophic caldera collapse, the enlargement of a pre-existing flooded caldera and the discharge of lithic-rich PDCs, depositing proximal <9m thick lithic lag breccias (LP2-D).
•We revise the stratigraphy of the 172 ka Lower Pumice 2 eruption sequence.•The LP2 eruption commenced with the development of an initial precursor eruption column.•Vent widening throughout the initial Plinian phase resulted in increased magma decompression rates, a deepening of the fragmentation surface and the development of pyroclastics density currents.•The geometry and structure of the vent system have a significant role in the course and temporal evolution of Plinian eruptions.
Numerous pyroclastic flows were produced during 1996–97 by collapse of the growing andesitic lava dome at Soufriere Hills Volcano, Montserrat. Measured deposit volumes from these flows range from 0.2 ...to 9 × 106 m³. Flows range from discrete, single pulse events to sustained large scale dome collapse events. Flows entered the sea on the eastern and southern coasts, depositing large fans of material at the coast. Small runout distance (<1 km) flows had average flow front velocities in the order of 3–10 m/s while flow fronts of the larger runout distance flows (up to 6.5 km) advanced in the order of 15–30 m/s. Many flows were locally highly erosive. Field relations show that development of the fine grained ash cloud surge component was enhanced during the larger sustained events. Periods of elevated pyroclastic flow productivity and sustained dome collapse events are linked to pulses of high magma extrusion rates.
The abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification. We conducted two series of experiments using ash from the nonwelded ...base of the rhyolitic Rattlesnake Tuff of Oregon, USA, to examine conditions of welding. One series of experiments was conducted at atmospheric pressure (1 At) in a muffle furnace with variable run times and temperature and another series was conducted at 5 MPa and 600 °C in a cold seal apparatus with variable run times and water contents. We compared the results to a suite of incipiently to densely welded, natural samples of the Rattlesnake Tuff.
Experiments at 1 At required a temperature above 900 °C to produce welding, which is in excess of the estimated pre-eruptive magmatic temperature of the tuff. The experiments also yielded globular clast textures unlike the natural tuff. During the cold-seal experiments, the gold sample capsules collapsed in response to sample densification. Textures and densities that closely mimic the natural suite were produced at 5 MPa, 600 °C and 0.4 wt.% H
2O, over run durations of hours to 2 days. Clast deformation and development of foliation in 2-week runs were greater than in natural samples. Both more and less water reduced the degree of welding at otherwise constant run conditions.
For 5 MPa experiments, changes in the degree of foliation of shards and of axial ratios of bubble shards and non-bubble (mainly platy) shards, are consistent with early densification related to compaction and partial rotation of shards into a foliation. Subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards. Sintering (local fusion of shard–shard contacts) was increasingly important with longer run times, higher temperatures, and greater pressures. During runs with high water concentrations, sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces.
A few tenths wt.% H
2O in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity. Large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor-phase sublimates seal permeability in the tuff.
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