Transitions In Eruptive Style Research Articles

Conduit armouring preceding explosive activity at an andesitic stratovolcano, an example from Taranaki Mounga, New Zealand

The strength and permeability of volcanic conduits can directly influence eruption dynamics via moderating the outgassing of ascending magma and the density of eruption plumes. Lithic clasts in pyroclastic ejecta can be used to understand the dynamic evolution of conduit walls because they are incorporated into the ascending melt-gas-particle mixture during volcanic eruptions. We examine the 1655 CE Burrell eruption of Taranaki Mounga, which transitioned from effusive activity to an explosive sub-Plinian phase and ended in unsteady columns. This episode was followed by a series of effusive eruptions of lower explosivity. Using textural analysis and physical properties, we distinguish five dominant lithic clast types within Burrell deposits that represent different regions of the shallow conduit and vent. Lithic types 1–3 represent juvenile (‘intrusive cognate’) and older (‘intrusive accessory’) conduit-filling plug materials. Lithic type 4 represents juvenile (‘extrusive cognate’) vent-filling lava dome extruded at the eruption onset, while Type 5 lithics (‘extrusive cognate’) represent sintered/compacted cognate material from the shallow vent accumulated during transitions in eruptive style. Crystalline andesite lithics (type 1) show a microlite-dominated groundmass. Hydrothermally altered andesite lithics (type 2) show breakdown of phenocrysts and increased seismic velocity relative to type 1 lithics. Brecciated andesite lithics (type 3) comprise fractured and sintered clasts of crystalline andesite. Glassy andesite lithics (type 4) show sub-rounded vesicles and glass-hosted microlites. Banded vitrophyre lithics (type 5) show bands of varying vesicularity, crystallinity and clast load. Physical property data reveals porosity, fracturing, sintering and alteration extent dictate dynamic changes in conduit permeability and potentially strength. Our results show how, during the explosive phase of the Burrell eruption, the conduit was lined with juvenile and remnant shallow plug material that was variably fractured, sintered and altered before being eroded and ejected. Comparison with previous work on Taranaki and dome-plug material from around the world shows how fracturing and sintering of conduit walls, combined with lining with dense juvenile material, cause overall permeability reduction and strengthening of the conduit. This inhibits outgassing and preserves conduit structure, facilitating the transition to explosive activity and the establishment of a stable eruption column.

Direct observation of degassing during decompression of basaltic magma.

Transitions in eruptive style during volcanic eruptions strongly depend on how easily gas and magma decouple during ascent. Stronger gas-melt coupling favors highly explosive eruptions, whereas weaker coupling promotes lava fountaining and lava flows. The mechanisms producing these transitions are still poorly understood because of a lack of direct observations of bubble dynamics under natural magmatic conditions. Here, we combine x-ray radiography with a novel high-pressure/high-temperature apparatus to observe and quantify in real-time bubble growth and coalescence in basaltic magmas from 100 megapascals to surface. For low-viscosity magmas, bubbles coalesce and recover a spherical shape within 3 seconds, implying that, for lava fountaining activity, gas and melt remain coupled during the ascent up to the last hundred meters of the conduit. For higher-viscosity magmas, recovery times become longer, promoting connected bubble pathways. This apparatus opens frontiers in unraveling magmatic/volcanic processes, leading to improved hazard assessment and risk mitigation.

A lower bound on the rheological evolution of magma in the 2021 Fagradalsfjall Fires

As magma temperature and composition drift and change, respectively, throughout an eruption, so does its rheology. These changes may span orders of magnitude in magma viscosity and result in orders of magnitude flow velocity changes, as well as transitions in eruptive style. In this study, we present a systematic high precision quantification of the rheological variations that occurred during the 2021 Fagradalsfjall Fires. In the field, we collected a suite of 22 representative samples emplaced between day 2 and 183 of the 2021 eruption. In the laboratory, we measured the melt viscosity of each sample in a concentric cylinder viscometer. Temperatures were initially raised to 1392 °C, and then lowered stepwise to eruptive temperatures as determined through syn-eruptive radiometric measurements. The resulting dataset is analyzed as a time series. An overall trend of viscosity decrease emerges. As the eruption progressed, melt viscosity decreased by 25%, from 40 Pa s to 30 Pa s at a constant temperature of 1200 °C. However, this trend is not monotonous. At least 3 positive spikes in viscosity can be identified, at day 80, 120, and 138 of the eruption. This trend tracks with geochemical variations.

Remobilization and eruption of an upper crustal cumulate mush at the Singkut caldera (North Sumatra, Indonesia)

Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpreting the signals of potential reawakening at active volcanoes. In this paper, we provide the first volcanological and petrochronological characterization of the Singkut caldera, a young volcanic system located in northern Sumatra (Indonesia), in close proximity to Medan, one of the country's most popoluous cities. Singkut formed at ∼44 ka during a VEI 6 explosive eruption that deposited at least ∼26 km3 of tephra (dense rock equivalent, DRE). The cataclysmic eruption was preceded by >200 ky of mostly effusive pre-caldera activity and followed by effusive to mildly explosive post-caldera activity. The lavas and pumices have high crystallinity (up to 62% crystals) with andesitic to dacitic bulk-rock composition and rhyolitic glass. Mineral textures and matrix glass compositions indicate resorption of quartz, plagioclase and zircon. Zircon crystallization ages show a complete overlap with the eruption ages in pre-caldera lavas, while a time gap in zircon crystallization (>50 ky) is identified in the caldera-forming tuff and post-caldera lavas. Ti-in-zircon thermometry shows that the Singkut magma body experienced a temperature increase starting approximately upon eruption of the pre-caldera lavas (∼254 ka). Such thermal perturbation determined progressive melting of mineral phases in the cumulate crystal mush, caused the resorption of the youngest zircon domains before the caldera-forming eruption, and hampered zircon crystallization between the caldera-forming eruption and the effusion of the post-caldera lavas (∼16 ka). Our data demonstrate how cumulate melting processes played a key role in leading the volcanic system towards a caldera-forming eruption and controlled the transitions in eruptive style between the effusive phases and the explosive climactic eruption.

Rapid eruptive transitions from low to high intensity explosions and effusive activity: insights from textural analysis of a small-volume trachytic eruption, Ascension Island, South Atlantic

Proximal deposits of small-volume trachytic eruptions are an under-studied record of eruption dynamics despite being common across a range of settings. The 59 ± 4 ka Echo Canyon deposits, Ascension Island, resulted from a small-volume explosive-effusive trachytic eruption. Variations in juvenile clast texture reveal changes in ascent dynamics and transitions in eruption style. Five dominant textural types are identified within the pumice lapilli population. Early Strombolian-Vulcanian eruption phases are typified by macro- and micro-vesicular equant clast types. Sheared clasts are most abundant at the eruption peak, transitioning to dense clasts in later phases due to shear-induced coalescence, outgassing and vesicle collapse. Melt densification and outgassing via tuffisite veins increased plume density, contributing to partial column collapse and the explosive-effusive transition. Bulk vesicularity distributions indicate a shift in dominant fragmentation mechanism during the eruption, from early-stage bubble interference and rupture to late-stage transient fragmentation, with a transient peak of Plinian activity. Dome and lava groundmass crystallinities of up to 70% indicate near-complete degassing during effusive phases, followed by shallow over pressurisation and a final less explosive phase. We provide textural evidence for high-intensity explosive phases and rapid transitions in eruptive style during small-volume trachytic eruptions and consider the impact of trachytic melt compositions on underlying dynamics of these short-lived, explosive events. This analysis demonstrates the value of detailed stratigraphy in understanding critical changes in eruption dynamics and the timescales over which they may occur which is of particular value in anticipating future eruptions of this type.

From eruption scenarios to probabilistic volcanic hazard analysis: An example of the Auckland Volcanic Field, New Zealand

The dichotomy between probabilistic and scenario-based volcanic hazard assessments stems from their opposing strengths and weaknesses. The quantification of uncertainty and lack of bias in the former is balanced against the temporal narrative and communicability of the latter. In this paper we propose a novel methodology to bridge between the two, deriving a pseudo-probabilistic hazard estimate from a suite of dynamic scenarios covering multiple volcanic hazards and transitions in eruptive style, as designed for emergency management purposes, in a monogenetic volcanic field. We use existing and new models for eruption style transitions, which provide probabilities conditional on local environmental conditions, thus obtaining the relative likelihoods of each scenario at every location in the field. The results are interpreted in terms of the probability of various hazards and combinations of hazards arising from various scenarios at critical locations. Conversely, we also demonstrate that it may be possible to optimise the likelihood of the scenario allocations across desired locations for emergency management training purposes.

Volcanic conduit controls on effusive-explosive transitions and the 2010 eruption of Merapi Volcano (Indonesia)

Individual volcanoes can produce both effusive and explosive eruptions. A transition between these two eruption styles dramatically changes the hazards and can occur either between distinct eruption events or within one eruption episode. The causes of these transitions are difficult to determine due to the number of system parameters that can influence whether or not magma fragments in a runaway process. We apply a numerical model of magma ascent in a volcanic conduit to isolate and test the effects of key parameters related to magma rheology and system geometry. We find that for a given volcanic system, parameters that control magma viscosity, such as initial water mass fraction, initial crystal volume fraction, and temperature, have the greatest influence on whether or not magma fragments during ascent and erupts explosively. We also define a ‘critical condition’ for the full set of initial parameters under which a transition in eruption style, from effusive to explosive or the reverse, is more likely to occur. Under these conditions, small heterogeneities in the water or crystal content of the magma, or small perturbations to the conduit pressure gradient due to magma chamber overpressure or dome growth or collapse, can disrupt the magmatic conditions and cause a transition in eruption style. The 2010 VEI 4 eruption of Merapi Volcano included both effusive and explosive phases and was larger by an order of magnitude than its eruptions during the previous century. We constrain our model for the Merapi system using published literature values and show that between the previous eruption in 2006 and the 2010 eruption, the shallow magmatic system at Merapi reached critical conditions due to the ascent from depth of a large, hotter, more volatile-rich magma. Under these critical conditions and according to our model results, small changes in the volatile content of the magma, small dome collapses, subtle changes in degassing rate, or the addition of CO2 to the magma through decarbonation of the bedrock, are all feasible mechanisms for triggering rapid transitions between effusive and explosive activity during the 2010 eruption period.

Temporal evolution of the magmatic system at Tungurahua Volcano, Ecuador, detected by geodetic observations

Changes in the pathway and timescale of magma ascent can be responsible for variations in eruptive style during long-lived eruptions, but are only documented at a small number of well-instrumented systems. Here we integrate PS-InSAR from high resolution TerraSAR-X radar imagery with continuous GPS data from 4 sites at Tungurahua volcano, Ecuador. Our results show long-term uplift between 2011 and 2014 associated with a continuously inflating prolate spheroid reservoir at a depth of ∼ 10 km beneath the summit. Comparisons with eruptive flux, taking compressibility into account, suggest that during this time period slightly over half the magma supplied to the system was erupted. The observations span three distinct phases of eruption and in 2012–2013, an increase in eruptive activity was accompanied by uplift on the volcano's western flank. Similar episodes have previously been observed during large Vulcanian eruptions and we attribute them to intrusions into an area of mechanical weakness. A localised patch of subsidence mid-way up the Tungurahua's western flank is co-located with a swarm of shallow long-period seismicity and may represent a potential site for a development of a lateral vent. This study demonstrates that satellite geodetic techniques are capable of characterising the geodetic signature of transitions in eruptive style during long-lived andesitic eruptions even where deformation is low magnitude and measurements are challenging.

Transition of eruptive style: Pumice raft to dome-forming eruption at the Havre submarine volcano, southwest Pacific Ocean

Transitions in eruptive style are common at volcanoes. Understanding how and why these transitions occur remain open questions. The 2012 eruption of the submarine Havre volcano in the Kermadec arc (southwest Pacific Ocean) produced a raft of floating pumice followed by a pair of domes from the same vent. Here, we used measurements on erupted magmas and constraints on the eruption rate, combined with a model for magma ascent, to identify the dominant controls on the transition in eruption style. During the raft-forming stage, magma ascent was fast enough that little gas was lost. Magma reached the seafloor with great enough vesicularity to be buoyant and produce clasts that could float. As the eruption waned, the eruption rate decreased, and the conduit narrowed. Sufficient gas was then lost to the surrounding country rocks during ascent such that the erupted magma was no longer buoyant relative to seawater. Most of the original dissolved water in the magma was lost to the crust surrounding the conduit during the dome-forming stage.

Conduit processes during the climactic phase of the Shinmoe-dake 2011 eruption (Japan): Insights into intermittent explosive activity and transition in eruption style of andesitic magma

The climactic phase of the Shinmoe-dake 2011 eruption was characterized by sub-Plinian events (January 26p.m., 27a.m., and 27p.m.) and lava accumulation in the crater (late on January 27 to January 29), all of which were accompanied by Vulcanian events. The magma discharge rate for lava accumulation was close to that of the sub-Plinian events. We discuss evolution of syneruptive magma ascent through the climactic phase on the basis of decompression-induced crystallization of groundmass microlites, in order to reveal how the final eruption style was determined at the explosive–effusive transition. We examined pumice and lava-like pyroclasts from three sub-Plinian events, the January 28 Vulcanian event, and a lava block ballistically ejected during the February 1 Vulcanian event. The samples exhibit variable groundmass textures due to variable syneruptive ascent conditions, not due to pre-eruptive inhomogeneity of the andesitic magma and different degrees of syneruptive volatile exsolution preceding quenching.Eruptive units were first characterized according to bulk density, assuming that slower ascent resulted in higher density. A decrease in the magma ascent rate led to a gradual shift to an effusive eruption style. There was little variation in ascent rate among magmas erupted simultaneously during the first and second sub-Plinian events (bulk density 0.8–2.1g/cm3), but differences increased in the third sub-Plinian and January 28 Vulcanian eruptions with minor appearance of slow magma (0.9–2.8g/cm3). Only slow magma occupied the shallow conduit at the completion of lava accumulation (February 1 lava; 2.1g/cm3). Among the first and second sub-Plinian units, the early stage of the second sub-Plinian event had the greatest variation in ascent rate, with extension to a slightly higher bulk density (1.0–2.1g/cm3) than the other events (0.8–1.7g/cm3). Ascent-rate variations in the early stage of the second sub-Plinian event, the third sub-Plinian event, and the January 28 Vulcanian event are all due to the preceding calm phase in the intermittent explosive activity. The moving magma column, which showed a vertical zonation in the degree of degassing, formed in the shallow conduit during the calm phase due to a decrease in the discharge rate, and this resulted in the co-ejection of magmas with variable degassing upon the start of explosive events.The crystal size distribution (CSD) of plagioclase microlites constrains the syneruptive branching (i.e., a point of divergence) in ascent rate and eruption style, which occurred after the start of microlite crystallization. The CSDs of the different samples show differences only in small crystal sizes; slope at these size fractions is smallest for low-density (<2.0g/cm3) pyroclasts from all eruptive phases, medium for high-density pyroclasts and lava (>2.0g/cm3) of the January 28 and February 1 events, and greatest for high-density pyroclasts (>2.0g/cm3) of the third sub-Plinian event. We propose, based on the similarity in CSD, that the shallow magma-feeding system was common for the two craters active during the January 28 explosive–effusive hybrid activity, where the Vulcanian event and emplacement of lava of the later February 1 event occurred in different parts of the original crater. The samples have variable plagioclase microlite number densities (1.4×105–3.5×106mm−3), based on variations in small crystal sizes among different CSDs. This variability resulted not only from the usual increasing trend in number density with increasing degree of undercooling, but also the decreasing trend associated with the strong undercooling during explosive eruptions.

Transition in eruption style during the 2011 eruption of Shinmoe-dake, in th Kirishima volcanic group: Implications from a steady conduit flow model

Mount Shinmoe-dake, in the Kirishima volcanic group (in southern Kyushu, Japan), erupted in January 2011. The eruption style was initially phreatomagmatic, and then underwent a series of transitions from sub-plinian explosions to an extrusion of lava from the summit crater. The purpose of the present study is to investigate the cause of such changes in eruption styles, focusing on the conditions for the eruption to be non-explosive and for the lava effusion to cease. To examine the conditions in the conduit and magma chamber, a numerical code is devised, based on the one-dimensional steady flow model of Kozono and Koyaguchi (2010), who modeled a dome-forming eruption. We systematically search for a condition in which the magma would not be fragmented, but the initial volatile content in the magma chamber would remain constant and unchanged. We find that the high magma permeability and/or the high degree of lateral gas escape was needed for the eruption to be effusive, and we estimate the pressure decrement at the cessation of lava extrusion.

Estimating rates of decompression from textures of erupted ash particles produced by 1999-2006 eruptions of Tungurahua volcano, Ecuador

Persistent low- to moderate-level eruptive activity of andesitic volcanoes is difficult to monitor because small changes in magma supply rates may cause abrupt transitions in eruptive style. As direct measurement of magma supply is not possible, robust techniques for indirect measurements must be developed. Here we demonstrate that crystal textures of ash particles from 1999 to 2006 Vulcanian and Strombolian eruptions of Tungurahua volcano, Ecuador, provide quantitative information about the dynamics of magma ascent and eruption that is difficult to obtain from other monitoring approaches. We show that the crystallinity of erupted ash particles is controlled by the magma supply rate (MSR); ash erupted during periods of high magma supply is substantially less crystalline than during periods of low magma supply. This correlation is most easily explained by efficient degassing at very low pressures (<<50 MPa) and degassing-driven crystallization controlled by the time available prior to eruption. Our data also suggest that the observed transition from intermittent Vulcanian explosions at low MSR to more continuous periods of Strombolian eruptions and lava fountains at high MSR can be explained by the rise of bubbles through (Strombolian) or trapping of bubbles beneath (Vulcanian) vent-capping, variably viscous (and crystalline) magma.

Multiple levels of magma storage during the 1980 summer eruptions of Mount St. Helens, WA

Transitions in eruptive style—explosive to effusive, sustained to pulsatory—are a common aspect of volcanic activity and present a major challenge to volcano monitoring efforts. A classic example of such transitions is provided by the activity of Mount St. Helens, WA, during 1980, where a climactic Plinian event on May 18 was followed by subplinian and vulcanian eruptions that became increasing pulsatory with time throughout the summer, finally progressing to episodic growth of a lava dome. Here we use variations in the textures, glass compositions and volatile contents of melt inclusions preserved in pyroclasts produced by the summer 1980 eruptions to determine conditions of magma ascent and storage that may have led to observed changes in eruptive activity. Five different pyroclast types identified in pyroclastic flow and fall deposits produced by eruptions in June 12, July 22 and August 7, 1980, provide evidence for multiple levels of magma storage prior to each event. Highly vesicular clasts have H2O-rich (4.5–5.5 wt%) melt inclusions and lack groundmass microlites or hornblende reaction rims, characteristics that require magma storage at P≥160 MPa until shortly prior to eruption. All other clast types have groundmass microlites; PH20 estimated from both H2O-bearing melt inclusions and textural constraints provided by decompression experiments suggest pre-eruptive storage pressures of ∼75, 40, and 10 MPa. The distribution of pyroclast types within and between eruptive deposits can be used to place important constraints on eruption mechanisms. Fall and flow deposits from June 12, 1980, lack highly vesicular, microlite-free pyroclasts. This eruption was also preceded by a shallow intrusion on June 3, as evidenced by a seismic crisis and enhanced SO2 emissions. Our constraints suggest that magma intruded to a depth of ≤4 km beneath the crater floor fed the June eruption. In contrast, eruptions of July and August, although shorter in duration and smaller in volume, erupted deep volatile-rich magma. If modeled as a simple cylinder, these data require a step-wise decrease in effective conduit diameter from 40–50 m in May and June to 8–12 m in July and August. The abundance of vesicular (intermediate to deep) clast types in July and August further suggests that this change was effected by narrowing the shallower part of the conduit, perhaps in response to solidification of intruded magma remaining in the shallow system after the June eruption. Eruptions from July to October were distinctly pulsatory, transitioning between subplinian and vulcanian in character. As originally suggested by Scandone and Malone (1985), a growing mismatch between the rate of magma ascent and magma disruption explains the increasingly pulsatory nature of the eruptions through time. Recent fragmentation experiments Spieler et al. (2004) suggest this mismatch may have been aided by the multiple levels at which magma was stored (and degassed) prior to these events.

Complex changes in eruption dynamics during the 79 AD eruption of Vesuvius

The 79 AD eruption of Vesuvius included 8 eruption units (EU1–8) and several complex transitions in eruptive style. This study focuses on two important transitions: (1) the abrupt change from white to gray pumice during the Plinian phase of the eruption (EU2 to EU3) and (2) the shift from sustained Plinian activity to the onset of caldera collapse (EU3 to EU4). Quantification of the textural features within individual pumice clasts reveals important changes in both the vesicles and groundmass crystals across each transition boundary. Clasts from the white Plinian fall deposit (EU2) present a simple story of decompression-driven crystallization followed by continuous bubble nucleation, growth and coalescence in the eruptive conduit. In contrast, pumices from the overlying gray Plinian fall deposit (EU3) are heterogeneous and show a wide range in both bubble and crystal textures. Extensive bubble growth, coalescence, and the onset of bubble collapse in pumices at the base of EU3 suggest that the early EU3 magma experienced protracted vesiculation that began during eruption of the EU2 phase and was modified by the physical effects of syn-eruptive mingling-mixing. Pumice clasts from higher in EU3 show higher bubble and crystal number densities and less evidence of bubble collapse, textural features that are interpreted to reflect more thorough mixing of two magmas by this stage of the eruption, with consequent increases in both vesiculation and crystallization. Pumice clasts from a short-lived, high column at the onset of caldera collapse (EU4) continue the trend of increasing crystallization (enhanced by mixing) but, unexpectedly, the melt in these clasts is more vesicular than in EU3 and, in the extreme, can be classified as reticulite. We suggest that the high melt vesicularity of EU4 reflects strong decompression following the partial collapse of the magma chamber.

The change of eruption styles of Martian volcanoes and estimates of the water content of the Martian mantle

Estimated water contents in the Martian mantle range from 36 ppm to more than 1%. These values are based on the chemical analyses such as hydrous minerals in SNC meteorites and formation models of Mars. This study evaluates the water content of the Martian mantle using the change with time of volcanic eruption style on Mars as an observational constraint. Styles of volcanic activity depend on the volatile content of the magma and the atmospheric pressure. Because a low atmospheric pressure leads to a more explosive volcanic eruption, it has been believed that the volcanism on the current Martian environment would be very explosive. Our calculations, however, show that, under the current Martian atmospheric conditions, erupted magma cannot entrain the ambient air effectively, so the decrease in temperature of the magma during ascent is small. Consequently, the erupted magma may form a lava-like deposit when it falls back on the ground. This effusive-like style of eruption is a counterpart of clastogenic lava on Mars. On the other hand, numerical calculations under a thick CO 2 atmosphere, which may correspond to an ancient Martian atmosphere, reveal a rather explosive eruption style. Geological features of earlier stages of Martian history in the Noachian and Hesperian eras suggest that the volcanic eruptions on Mars were explosive then. Effusive eruptions, however, became dominant in more recent times. It has been widely accepted that Mars experienced a major climate change. In addition, the release factor of volatiles on Mars has been suggested to be as small as 0.017–0.112. This may imply that the volatile content has been almost constant throughout Martian history. Consequently, we assume that this change in eruption style was caused by the change in atmospheric pressure. For a given water content of magma, a major climatic change may lead to a transition in eruption style. If we know the atmospheric pressure at the time of this transition, we can calculate the possible range of the volatile content of the mantle using our numerical simulations. If the atmospheric pressure on Mars around late Hesperian era is about 1 bar, the estimated values for a typical Martian magma are 0.05–0.25 wt.%, which is within the range of the water content of typical terrestrial basaltic magmas.