Soufrière Hills Volcano had two periods of repetitive Vulcanian activity in 1997. Each explosion discharged the contents of the upper 0.5–2
km of the conduit as pyroclastic flows and fallout: frothy ...pumices from a deep, gas-rich zone, lava and breadcrust bombs from a degassed lava plug, and dense pumices from a transition zone. Vesicles constitute 1–66
vol.% of breadcrust bombs and 24–79% of pumices, all those larger than a few tens of µm being interconnected. Small vesicles (<
few tens of µm) in all pyroclasts are interpreted as having formed syn-explosively, as shown by their presence in breadcrust bombs formed from originally non-vesicular magma. Most large vesicles (>
few hundreds of µm) in pumices are interpreted as pre-dating explosion, implying pre-explosive conduit porosities up to 55%. About a sixth of large vesicles in pumices, and all those in breadcrust bombs, are angular voids formed by syn-explosive fracturing of amphibole phenocrysts. An intermediate-sized vesicle population formed by coalescence of the small syn-explosive bubbles. Bubble nucleation took place heterogeneously on titanomagnetite, number densities of which greatly exceed those of vesicles, and growth took place mainly by decompression.
Development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion–decompression and surface quench in contact with air. Lava-plug fragments entered the air quickly after fragmentation (∼
10
s), so the interiors continued to vesiculate once the rinds had quenched, forming breadcrust bombs. Deeper, gas-rich magma took longer (∼
50
s) to reach the surface, and vesiculation of resulting pumice clasts was essentially complete prior to surface quench. This accounts for the absence of breadcrusting on pumice clasts, and for the textural similarity between pyroclastic flow and fallout pumices, despite different thermal histories after leaving the vent. It also allowed syn-explosive coalescence to proceed further in the pumices than in the breadcrust bombs. Uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn-explosive vesiculation even prior to magma fragmentation, probably in a zone of steep pressure gradient beneath the descending fragmentation front. Syn-explosive decompression rates estimated from vesicle number densities (>
0.3–6.5
MPa
s
−
1
) are consistent with those predicted by previously published numerical models.
Textural parameters such as density, porosity, pore connectivity, permeability, and vesicle size distributions of vesiculated and dense pyroclasts from the 9.4-ka eruption of Kilian Volcano, were ...quantified to constrain conduit and eruptive processes. The eruption generated a sequence of five vertical explosions of decreasing intensity, producing pyroclastic density currents and tephra fallout. The initial and final phases of the eruption correspond to the fragmentation of a degassed plug, as suggested by the increase of dense juvenile clasts (bimodal density distributions) as well as non-juvenile clasts, resulting from the reaming of a crater. In contrast, the intermediate eruptive phases were the results of more open-conduit conditions (unimodal density distributions, decreases in dense juvenile pyroclasts, and non-juvenile clasts). Vesicles within the pyroclasts are almost fully connected; however, there are a wide range of permeabilities, especially for the dense juvenile clasts. Textural analysis of the juvenile clasts reveals two vesiculation events: (1) an early nucleation event at low decompression rates during slow magma ascent producing a population of large bubbles (>1 mm) and (2) a syn-explosive nucleation event, followed by growth and coalescence of small bubbles controlled by high decompression rates immediately prior to or during explosive fragmentation. The similarities in pyroclast textures between the Kilian explosions and those at Soufrière Hills Volcano on Montserrat, in 1997, imply that eruptive processes in the two systems were rather similar and probably common to vulcanian eruptions in general.
Experimental study of caldera formation Roche, O.; Druitt, T. H.; Merle, O.
Journal of Geophysical Research,
10 January 2000, Letnik:
105, Številka:
B1
Journal Article
Recenzirano
Odprti dostop
Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using ...a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (≤1), subsidence starts by flexure of the roof and the formation of outward dipping, reverse ring faults, which in turn trigger formation of peripheral inward dipping, normal ring faults. The subsidence always occurs asymmetrically. In cross section the reverse faults delimit a coherent piston, bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets. The surface depression consists of a nondeformed area (piston) surrounded by an annular extensional zone (tilted strata). For high aspect ratios (>1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2‐D) or cones (3‐D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.
The ability of a dense pyroclastic flow to maintain high gas pore pressure, and hence low friction, during runout is determined by (1) the strengths and longevities of gas sources, and (2) the ...ability of the material to retain residual gas once those sources become ineffective. The latter is termed the gas retention capacity. Gas retention capacity in a defluidizing granular material is governed by three timescales: one for the evacuation of bubbles (t ^sub be^ ; brief and not considered in this paper), one for hindered settling from the expanded state (t ^sub sett^), and one for diffusive release of residual pore pressure from the non-expanded state (t ^sub diff^). The relative magnitides of t ^sub sett^ and t ^sub diff^ depend on bed thickness, t ^sub sett^ dominating in thin systems and t ^sub diff^ in thick ones. Three pyroclastic flow materials, two ignimbrites and a block-and-ash flow sample, were studied experimentally to investigate expansion behaviour under gas flow and to determine gas retention times. Effects of particle size were evaluated by using two size cuts (<4 mm and <250 μm) from each sample. Careful drying of the materials was necessary to avoid effects of humidity-related cohesion. Two sets of experiments were carried out: (1) expansion in the non-bubbling regime at 50-200°C, (2) bed collapse tests from the initially bubbling state at 50-550°C. Provided that gas channelling was avoided by gentle stirring, all the samples exhibited a regime of uniform expansion prior to the onset of bubbling. Fine particle size (in particular high fines content), low particle density and high temperature all favoured smoother fluidization by increasing the maximum expansion possible in the non-bubbling state. An empirical equation describing the uniform expansion of the materials was determined. High temperature also favoured greater gas partitioning into the dense phase of the bubbling bed, as well (in finer-grained samples) as higher voidage in the settled bed. Large values of t ^sub sett^ and t ^sub diff^ were favoured by fine particle size. Temperature had less influence, suggesting that experimental results at low temperatures (50-200°C) can be extrapolated to higher temperatures. Gas retention times provide insight into the ability of pyroclastic flows in expanded (t ^sub sett^) or non-expanded (t ^sub diff^) flow states to retain gas once air ingestion or gas production have become ineffective. Finer-grained pyroclastic flows are expected to retain gas longer, and hence to have higher apparent 'mobilities', than coarser-grained ones of comparable volume, as has been observed on Montserrat.PUBLICATION ABSTRACT
Santorini caldera has had a long history of plinian eruptions and caldera collapses, separated by 20-40 kyr interplinian periods. We have carried out a study to constrain magma storage/extraction ...depths beneath the caldera. We analysed H sub(2)O in 138 olivine-, pyroxene- and plagioclase-hosted melt inclusions from plinian and interplinian products from the last 200 kyr, and CO sub(2), S, Cl, F and delta D in various subsets of these. The dataset includes 64 inclusions in products of the Minoan plinian eruption of the late 17th century BCE. All the melt inclusions were ellipsoidal and isolated, with no textural evidence for volatile leakage. Mafic melt inclusions contain 1-4 wt % H sub(2)O and up to 1200 ppm CO sub(2), 1200 ppm S, 2000 ppm Cl and 400 ppm F; silicic inclusions contain 2-7 wt % H sub(2)O, up to 150 ppm CO sub(2), up to 400 ppm S, 2000-6000 ppm Cl and 600-1000 ppm F. The delta D values of 27 representative inclusions (-37 to -104ppt) are intermediate between mantle and slab values and rule out significant H sub(2)O loss by hydrogen diffusion from olivine-hosted inclusions. H sub(2)O, S and Cl behave compatibly in melt inclusion suites varying from mafic to silicic in composition, showing that entrapment of many melt inclusions took place under volatile-saturated conditions. Most Santorini melts are saturated in a free COHSCl vapour phase at depths of less than 10 km; the only exceptions are basaltic melts from a single interplinian eruption, which were volatile-undersaturated up to K sub(2)O contents of 1 wt %. The rhyolitic melt of the Minoan eruption probably contained a free hypersaline liquid phase. H sub(2)O + CO sub(2) saturation pressures were calculated using suitably calibrated solubility models to estimate pre-eruptive magma storage depths. Magmas feeding plinian eruptions were stored at >4 km (>100 MPa) and extracted over depth intervals of several kilometres. Plagioclase phenocrysts in rhyodacitic pumice from the Minoan eruption have cores containing melt inclusions trapped at depths up to 10-12 km (320 MPa), and rims (also orthopyroxene and clinopyroxene) containing inclusions trapped at 4-6 km (100-160 MPa). This records late-stage silicic replenishment of a <2 km thick shallow magma chamber, rather than extraction of melts syn-eruptively over the entire depth range. The plagioclase cores were carried from depth in the ascending melt, then overgrown by the rims in the shallow chamber. Exsolution of volatiles during ascent may have caused the replenishment melt to inject as a bubbly plume, causing mixing prior to eruption. This would explain (1) the homogeneity of the Minoan rhyodacitic magma, and (2) extraction of melt inclusions from the entire pressure spectrum during the first eruptive phase. Most silicic magmas feeding eruptions of the interplinian periods were stored in reservoirs at shallow depths (2-3 km) compared with those feeding the plinian eruptions (>4 km). Melt inclusions from the ad 726 eruption of Kameni Volcano yield a pre-eruptive storage depth of 4 km, which is similar to that estimated from geodetic data for the inflation source during the 2011-2012 period of caldera unrest; this supports a magmatic origin of the unrest. The level of pre-ad 726 magma storage beneath Kameni was deeper than that of earlier silicic interplinian eruptions, perhaps owing to changes in crustal stress caused by the Minoan eruption. Combined with previously published results, the melt inclusion data provide a time-integrated image of the crustal plumbing system. Mantle-derived basalts are injected into the lower crust, where they fractionate to produce evolved melts in bodies of hot crystal mush. Evolved residual melts separate from their parent mushes in the 8 to >15 km depth interval, then ascend rapidly into the upper crust, where they either crystallize or accumulate as bodies of eruptible, crystal-poor magma.
We conducted laboratory experiments on dam-break flows of sub-250-µm volcanic ash, generated by the release of gas-fluidized and variably non-expanded to expanded (up to 35%) beds, in order to gain ...insights into the internal kinematics of pyroclastic flows. The flows were typically several cm thick and had frontal speeds of up to ∼2 m s
−1
. High-speed videos taken through the transparent sidewall of the 3-m-long channel were analyzed with a particle-tracking algorithm, providing a spatial and temporal description of transport and sedimentation. The flows deposited progressively as they traveled down the flume, being consumed by sedimentation until they ran out of volume. Deposition commenced 5–20 cm rearward of the flow front and (for a given expansion) proceeded at a rate independent of distance from the lock gate. Deposit aggradation velocities were equal to those inferred beneath quasi-static bed collapse tests of the same ash at the same initial expansions, implying that shear rates of up to ∼300 s
−1
have no measurable effect on aggradation rate. The initially non-expanded (and just fluidized) flow deposited progressively at a rate indicative of an expansion of a few percent, perhaps due to shear-induced Reynolds dilation during initial slumping. The fronts of the flows slid across the flume floor on very thin basal shear layers, but once deposition commenced a no-slip condition was established at the depositional interface. Within the flows, the trajectory of the constituent particles was linear and sub-horizontal. The velocities of the particles increased with height above the depositional interface, reached a maximum, then declined slightly towards the flow surface, perhaps due to air drag. At a given location, the velocity profiles were translated upwards as the deposit aggraded. The results show that even cm-thin, poorly expanded flows of ash deposit progressively, as inferred for many pyroclastic flows. The change from (frontal) slip to (rearward) no-slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits.
Cosmic ray exposure (CRE) dating was performed on the caldera cliffs of Santorini with the aim of detecting cliff segments predating the Minoan eruption (17th century BCE). The methodology involved ...the determination of in situ-produced cosmogenic
36
Cl concentration in basaltic-to-rhyodacitic whole rocks cropping out in the cliffs. After the samples were processed following the chemical protocol of
36
Cl preparation for silicate rocks,
36
Cl concentrations were measured by accelerator mass spectrometry (AMS). Important challenges during the implementation procedure were related to large amounts of radiogenic
36
Cl, complex modeling of inherited
36
Cl, and dominance of the thermal and epithermal (low-energy) neutron capture production pathway. Nevertheless, quantitative assessments on the basis of the contribution of the low-energy neutron capture pathway percent to the total production rate validated the calculated CRE dates. Current CRE ages demonstrate that an ancient caldera existed on pre-Minoan Santorini, occupying at least the northern half of the modern-day caldera.
The 184 ka Lower Pumice 1 eruption sequence records a complex history of eruption behaviours denoted by two significant eruptive phases: (1) a minor precursor (LP1-Pc) and (2) a major Plinian phase ...(LP1-A, B, C). The precursor phase produced 13 small-volume pyroclastic fallout, surge and flow deposits, which record the transition from a dominantly magmatic to a phreatomagmatic eruptive style, and exhibit a normal (dacite to andesitic-dacite) to reverse (andesitic-dacite to dacite) compositional zonation of juvenile pyroclasts in the stratigraphy. Incipient bioturbation and variability in unit thickness and lithology reflect multiple time breaks and highlight the episodic nature of volcanism prior to the main Plinian eruption phase. The Plinian magmatic eruption phase is defined by three major stratigraphic divisions, including a basal pumice fallout deposit (LP1-A), an overlying valley-confined ignimbrite (LP1-B) and a compositionally zoned (rhyodacite to basaltic andesite) lithic-rich lag breccia (LP1-C), which caps the sequence. This sequence records the initial development of a buoyant convective eruption column and the transition to eruption column and catastrophic late-stage caldera collapse events. Similarities in pyroclast properties (i.e., chemistry, density), between the Plinian fallout (LP1-A) and pyroclastic flow (LP1-B) deposits, indicate that changes in magma properties exerted no influence on the dynamics and temporal evolution of the LP1 eruption. Conversely, lithic breccias at the base of the LP1-B ignimbrite suggest that the transition from a buoyant convective column to column collapse was facilitated by mechanical erosion of the conduit system and/or the initiation of caldera collapse, leading to vent widening, an increase in magma discharge rate and the increased incorporation of lithics into the eruption column, causing mass overload. Lithic-rich lag breccia deposits (LP1-C), which cap the eruption sequence, record incremental, high-energy caldera collapse events, whereby downfaulting occurred in discrete jumps, resulting in variable magma discharge rates and the development of a fissure vent system.
We use the deposit sequence resulting from the first catastrophic caldera collapse event recorded at Santorini (associated with 184 ka Lower Pumice 1 eruption), to study the shallow conduit dynamics ...at the peak of caldera collapse. The main phase of the Lower Pumice 1 eruption commenced with the development of a sustained buoyant eruption column, producing a clast-supported framework of rhyodacitic white pumice (LP1-A). The clasts have densities of 310–740 kg m
−3
, large coalesced vesicles that define unimodal size distributions and moderate to high vesicle number densities (1.2 × 10
9
–1.7 × 10
9
cm
−3
). Eruption column collapse, possibly associated with incipient caldera collapse, resulted in the development of pyroclastic flows (LP1-B). The resulting ignimbrite is characterised by rhyodacitic white pumice with a narrow density range (250–620 kg m
−3
) and moderate to high vesicle number densities (1.3 × 10
9
–2.1 × 10
9
cm
−3
), comparable to clasts from LP1-A. An absence of deep, basement-derived lithic clast assemblages, together with the occurrence of large vesicles and relatively high vesicle number densities in pumice from the fallout and pyroclastic flow phases, suggests shallow fragmentation depths, a prolonged period of bubble nucleation and growth, and moderate rates of decompression prior to fragmentation (7–11 MPa s
−1
). Evacuation of magma during the pyroclastic flow phase led to under-pressurisation of the magma reservoir, the propagation of faults (associated with the main phase of caldera collapse) and the formation of 20 m thick lithic lag breccias (LP1-C). Rhyodacitic pumices from the base of the proximal lithic lag breccias show a broader range of density (330–990 kg m
−3
), higher vesicle number densities (4.5 × 10
9
–1.1 × 10
10
cm
−3
) and higher calculated magma decompression rates of 15–28 MPa s
−1
than pyroclasts from the pre-collapse eruptive phases. In addition, the abundance of lithic clasts, including deeper, basement-derived lithic assemblages, records the opening of new vents and a deepening of the fragmentation surface. These data support numerical simulations which predict rapid increases in magma decompression and mass discharge rates at the onset of caldera collapse.
The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at ...170°C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3‐m‐long, horizontal lock‐exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h0 were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with , like other types of dam‐break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s−1, aggradation rates were identical to those determined independently, at the same value of E, in quasi‐static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h0/x0 in the lock reservoir and (2) the ratio tsett/tgrav of two timescales: a particle settling time tsett and a gravitational acceleration time tgrav.
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