Effusive-explosive Transition Research Articles

Conduit processes in crystal-rich dacitic magma and implications for eruptive cycles at Guagua Pichincha volcano, Ecuador

Stratovolcanoes are commonly characterised by cyclic eruptive activity marked by transitions between dome-forming, Vulcanian, Subplinian and Plinian eruptions. Guagua Pichincha volcano (Ecuador) has been a location of such cyclicity for the past ~ 2000 years, with Plinian eruptions in the first and tenth centuries AD (Anno Domini/after Christ), and CE (Common Era) 1660, which were separated by dome-forming to Subplinian eruptions, such as the recent 1999–2001 eruption. These cycles are therefore a prominent example of effusive-explosive transitions at varying timescales. Here, we investigate the reasons for such shifts in activity by focusing on degassing and outgassing processes within the conduit. We have coupled a petrophysical and textural analysis of dacites from the CE 1660 Plinian eruption and the 1999–2001 dome-forming/Vulcanian eruption, with different percolation models in order to better understand the role of degassing on eruptive style. We demonstrate that the transition from dome-forming to Plinian activity is correlated with differences in phenocryst content and consequently in bulk viscosity. A lower initial phenocryst content and viscosity is inferred for the Plinian case, which promotes faster ascent, closed-system degassing, fragmentation and explosive activity. In contrast, dome-forming phases are promoted by a higher magma viscosity due to higher phenocryst content, with slower ascent enhancing gas escape and microlite crystallization, decreasing explosivity and yielding effusive activity.ResumenLos estratovolcanes se caracterizan comúnmente por presentar actividad eruptiva cíclica, marcada por transiciones entre erupciones formadoras de domos y erupciones de tipo Vulcanianas, Subplinianas y Plinianas. El volcán Guagua Pichincha (Ecuador) ha dado lugar a tal ciclicidad durante los últimos ~ 2000 años, con erupciones Plinianas tanto en los siglos Primero y Décimo, como en el año 1660, las cuales estuvieron intercaladas por erupciones formadoras de domos y de tipo Subplinianas, tal como ocurrió durante la erupción reciente de 1999–2001. Estos ciclos son, por lo tanto, ejemplos destacados de transiciones eruptivas de tipo efusiva-explosiva a escalas de tiempo variadas. En este trabajo, investigamos las razones de tales cambios de actividad enfocándonos en procesos de exsolución y pérdida de gases del magma en el conducto (desgasificación en sistemas cerrado y abierto). Hemos acoplado análisis petrofísicos y texturales tanto de dacitas de la erupción Pliniana de 1660, como de la erupción formadora de domos/Vulcaniana de 1999–2001, junto con diferentes modelos de percolación, para así comprender mejor el rol de la exsolución de volátiles en el estilo eruptivo. Demostramos que la transición desde una actividad efusiva formadora de domos a una Pliniana está correlacionada con diferencias en el contenido de fenocristales y, subsecuentemente, con la viscosidad total del magma. Un contenido inicial menor de fenocristales y una menor viscosidad se infiere para el caso Pliniano, lo que promueve un ascenso más rápido, desgasificación en sistema cerrado, fragmentación y finalmente actividad explosiva. Por el contrario, las fases formadoras de domos son promovidas por una viscosidad mayor debido a un contenido mayor de fenocristales, con ascenso más lento promoviendo a su vez el escape de gases y la cristalización de microlitos, disminuyendo la explosividad y produciendo actividad efusiva.

Rheological change and degassing during a trachytic Vulcanian eruption at Kilian Volcano, Cha\xeene des Puys, France

Magma ascent during silicic dome-forming eruptions is characterized by significant changes in magma viscosity, permeability, and gas overpressure in the conduit. These changes depend on a set of parameters such as ascent rate, outgassing and crystallization efficiency, and magma viscosity, which in turn may influence the prevailing conditions for effusive versus explosive activity. Here, we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive-explosive transitions during Vulcanian phases of the 9.4 ka eruption of Kilian Volcano, Chaîne des Puys, France. Our results suggest that effusive activity at the onset of Vulcanian episodes at Kilian Volcano was promoted by (i) rapid ascent of initially crystal-poor and volatile-rich trachytic magma, (ii) a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization, and (iii) efficient degassing/outgassing in a crystal-rich magma at shallow depths. Trachytic magma repeatedly replenished the upper conduit, and variations in the amount of decompression and cooling caused vertical textural stratification, leading to variable degrees of crystallization and outgassing. Outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles, fractures, and tuffisite veins, fostering the formation of cristobalite in the carapace and talus regions. Build-up of overpressure was likely caused by closing of pore space (bubbles and fractures) in the dome through a combination of pore collapse, cristobalite formation, sintering in tuffisite veins, and limited pre-fragmentation coalescence in the dome or underlying hot vesicular magma. Sealing of the carapace may have caused a transition from open- to closed- system degassing and to renewed explosive activity. We generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity, rapid microlite growth, and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents. We show that trachytes may erupt with a similar style (e.g., Vulcanian) but at significantly higher ascent rates than their andesitic, dacitic, and rhyolitic counterparts. This suggests that the periodicity of effusive-explosive transitions at trachytic volcanoes may differ from that observed at the well-monitored andesitic, dacitic, and rhyolitic volcanoes, which has implications for hazard assessment associated with trachytic eruptions.

U-Th zircon dating reveals a correlation between eruptive styles and repose periods at the Nisyros-Yali volcanic area, Greece

Water-rich silicic magmas are capable of erupting effusively and explosively, and this drastic change in eruptive styles, termed effusive-explosive transition, has important implications in managing volcanic hazards. Some volcanoes exhibit effusive-explosive transitions during the same eruptive event, while others show this behavior between different eruptions. In the latter case, magma chamber processes induce physical-chemical changes in the magma, which can favor either effusivity or explosivity. This is the case for the Nisyros-Yali volcanic center, from the South Aegean Sea. In the recent stages of activity (past 120 ky), the volcanic area generated eight rhyolitic effusive and explosive events (five on the island of Nisyros and three on the island of Yali), including two caldera-forming eruptions. Changes of water content, temperature and pre-eruptive water-saturation between effusive and explosive deposits point to a potential time-dependency between the two eruptive styles. We investigate this time-dependency by applying UTh disequilibrium dating to zircon crystals. Our eruptive age estimates of the investigated units range from 118.7 ± 10 ka to 19.9 ± 1.5 ka for Nisyros, and from 40 ± 5.2 ka to 22.7 ± 1.6 ka for Yali. Yali volcano has developed after the two caldera-forming events on Nisyros, which occurred at 63.1 ± 4.7 ka and 58.4 ± 2.7 ka. Yali marks the transition to a more geometrically complex system, where the upper-crustal silicic mush hosts at least two eruptible magma chambers (one under Yali, and one under Nisyros). The eruptive styles at both volcanoes seem to be correlated with the length of the repose periods. Effusive events occur after longer periods of volcanic quiescence, while explosive events are generated after shorter periods of repose of ~5–10 ky, which can be extended based on eruption age uncertainty to <18 ky for Nisyros and <12 ky for Yali. This observation is explained by the physical state of the volatiles in the magma chamber, with longer repose periods favoring volatile build-up. This can lead to water-supersaturation at storage pressures which was shown to favor effusivity. Based on this interpretation, both Nisyros and Yali volcanoes are presently in the effusive time window, which makes it probable for the next eruptions to be non-explosive.

Effusive-explosive transitions of water-undersaturated magmas. The case study of Methana Volcano, South Aegean Arc

Most arc volcanoes erupt intermediate to silicic magmas that have sufficient volatile contents to behave explosively. Despite this explosive potential, low-energy effusive eruptions of such viscous water-rich magmas are common occurrences. Hence, predicting the style of the next eruption, with its obvious importance on hazard mitigation, remains one of the main challenges of volcanology. Here, we investigate the changes in eruptive styles at Methana volcano, from the South Aegean Arc. This volcano has generated multiple andesite-to-dacite eruptions, with the last event occurring in historical times (~2250 BP). We focus on 14 eruptive events, 3 being explosive. We compare the petrology and geochemistry of the deposits to reconstruct the magma chamber events that preceded both types of eruptions. We estimate the thermo-chemical conditions that characterized each eruptive event, and highlight the causes that promoted or inhibited magma fragmentation: temperature, pressure, composition, relative volumes of different magma batches, dissolved water content, crystallinity, melt and bulk viscosities. The results indicate that Methana harbors an upper-crustal silicic reservoir stored at ~2 kbar (likely dacitic to rhyodacitic in composition) that is in a highly crystalline state (>50 vol% crystals, mostly uneruptible). All the eruptions are triggered by deeper magma recharge ascending from storage pressures of ~4–5 kbar, leading to some rejuvenation of the shallow silicic mush. The most mafic recharges (~55 wt% SiO2, ~1000 °C, >3 wt% H2O) promote effusive eruptions after mixing and hybridizing the upper-crustal reservoir (hybrids of ~57–64 wt% SiO2, ~900–950 °C, generally 3–4 wt% H2O). Slightly colder, wetter and more differentiated recharges (~62 wt% SiO2; ~900 °C, ~4 wt% H2O) trigger explosive events after having minimal interaction with the upper-crustal reservoir. However, the dissolved water content is not the only factor influencing explosivity, as some of the wettest magmas (>4.5 wt% H2O) generate lava flows. Our data indicate that a key control on effusive-explosive transitions at Methana is the crystallinity of the erupted material. A high crystallinity (40–55 vol%) increases the bulk-viscosity, which results in the magma having a slower ascent velocity. In addition, crystallinity enhances the formation of permeable pathways for the gas. Longer ascent timescales and enhanced permeability in the conduit improve the outgassing potential of the magmas and lead to effusive behavior. The opposite occurs for explosive events. Here, the lower crystallinity of the magmas (30 vol%) translates into lower bulk-viscosities and faster ascent rates, which inhibit outgassing. This case study highlights the importance of obtaining better constraints on the state of subvolcanic magma reservoirs, in particular concerning crystallinity and volatile concentrations, which has to become a priority in the years to come.

A connection between magma chamber processes and eruptive styles revealed at Nisyros-Yali volcano (Greece)

Arc volcanoes generally emit water-rich, high-viscosity silicic magmas, which are prone to erupt explosively. However, effusive behavior is a common occurrence despite the high-H2O, high viscosity conditions. The contrasting shift from effusive to explosive behavior (and vice-versa) at any individual volcano raises the question on what controls eruptive style. Permeability development in conduits allows magma to outgas and is clearly a key factor. However, an important question is whether magma reservoir processes can also have an influence on eruptive styles. The answer could have direct impact on predicting eruptive behavior. Hence, we explore this potential connection by analyzing nine alternating effusive and explosive silicic deposits that were emplaced during distinct eruptions at the active Nisyros-Yali volcanic center. The lavas and pyroclastic deposits are compositionally similar. This indicates a negligible influence of the bulk rock composition on different eruptive styles. The crystal contents vary between units, without any clear correlation with eruptive style (from nearly aphyric to ~45 vol% crystals). Mineral textures and chemistry do show variations between effusive and explosive eruptions, with a larger proportion of resorbed plagioclase and, in most cases, more evolved amphiboles present in the lava flows. Mineral thermo-barometry and hygrometry show that the storage zones of magmas generating effusive eruptions evolved towards colder and more water-rich conditions (710–790 °C; 5.6–6.5 wt% H2O) than their explosive counterparts (815–850 °C; 4.2–4.6 wt% H2O). At storage pressures of 1.5–2 kbar, relevant for Nisyros-Yali, the volatile saturation level is reached at >5 wt% H2O. Therefore, it is likely that the magmas reached water-saturation before generating effusive eruptions, and were undersaturated before explosive events. We hypothesize that the presence of exsolved volatiles in the subvolcanic reservoir can enhance the outgassing potential of the magma during conduit ascent. Hence, the rhyolitic effusive-explosive transitions can be influenced by the pre-eruptive exsolved versus dissolved state of the volatiles in the magma chamber. This can lead to the less explosive eruptions for the most water-rich reservoir conditions.

The effusive-explosive transitions at Rokatenda 2012–2013: unloading by extrusion of degassed magma with lateral gas flow

Between October 2012 and August 2013, Rokatenda, one of the most poorly understood volcanoes in Indonesia, entered a phase of intense eruptive activity which involved extrusion of viscous lava, gas discharge and explosive activity. During the 10-month-long eruption, a lava volume of 2–5 × 106 m3 was extruded at mean output rate of 0.3 m3 s−1, with 2 to 3-month-long high extrusion rate phases being terminated by explosive events. Extrusion built a lava dome attaining a maximum height of ∼80 m above the crater rim, with a basal width of about 250 m. The composition of the 2012–2013 lava dome is comparable to that of the 1980 lava dome, both being andesite-trachydacite. Mineralogically, the 2012–2013 lava dome is mainly composed of plagioclase, pyroxene and an undetermined opaque mineral. Halogens released during eruption are consistent with the extrusion being fed, at least in the first eruption phase, by a degassed magma. This resulted in the formation of a dense, viscous plug in the conduit that led to a lateral gas flow, with gasses escaping around the plug to form multiple craters surrounding the dome. During the course of the eruptive activity, degassed magma was progressively forced out of the vent to unload deeper magma and force the system into an explosive phase. Such a scenario has occurred in the past at Rokatenda and is likely to be repeated in the future and creates an activity pattern that may be used to characterize such systems.

Unravelling the effusive-explosive transitions and the construction of a volcanic cone from geological data: The example of Monte dei Porri, Salina Island (Italy)

The volcanic activity that built up the Monte dei Porri stratocone (Salina Island) was reconstructed using new stratigraphic data, which allowed seven eruption units to be distinguished. Alternating Strombolian/Vulcanian to sub-Plinian/Plinian explosive and effusive activity emplaced fall and pyroclastic density current deposits and lava flows that formed the volcanic cone. The minimum erupted bulk volumes were assessed at 100×106m3 each for EU1, EU2, EU3 and EU6, while that of EU4 is ca. 200×106m3. Rough estimation of EU7 volume yields values around 150×106m3. The calculation of volume was not possible for the EU5 deposits. The magmas that fed the different eruption units of the Monte dei Porri succession range in composition from basalt to andesite, with the exception of dacites erupted in the initial phase of activity. SEM image analyses on coarse ash from the different pyroclastic units suggest that hydromagmatic fragmentation cannot be the cause of the large variations in explosivity observed throughout the stratigraphic succession. Based on the lithic component of pyroclastic deposits and xenolith contents of lava flows, the plumbing system that fed the different eruption units of Monte dei Porri was split into a deep magma storage level (15–20km) and shallower magma batches (3–5km). Our calculations indicate that the volumes of erupted material can account for magmatic triggering (injection of new magma) of eruptive units from the shallower feeding system, but they are not sufficient for suggesting magmatic initiation of the eruption units from the deeper feeding system. It is therefore assumed that the eruptions from the deep magma reservoir necessitate a favourable lithostatic stress, likely calling for a reduction of the local tectonic forces. A qualitative model explaining the eruptive style transitions among and within the different eruption units is presented, taking into account the relation between magmatic overpressure and lithostatic stress.

Thermal vesiculation during volcanic eruptions.

Terrestrial volcanic eruptions are the consequence of magmas ascending to the surface of the Earth. This ascent is driven by buoyancy forces, which are enhanced by bubble nucleation and growth (vesiculation) that reduce the density of magma. The development of vesicularity also greatly reduces the 'strength' of magma, a material parameter controlling fragmentation and thus the explosive potential of the liquid rock. The development of vesicularity in magmas has until now been viewed (both thermodynamically and kinetically) in terms of the pressure dependence of the solubility of water in the magma, and its role in driving gas saturation, exsolution and expansion during decompression. In contrast, the possible effects of the well documented negative temperature dependence of solubility of water in magma has largely been ignored. Recently, petrological constraints have demonstrated that considerable heating of magma may indeed be a common result of the latent heat of crystallization as well as viscous and frictional heating in areas of strain localization. Here we present field and experimental observations of magma vesiculation and fragmentation resulting from heating (rather than decompression). Textural analysis of volcanic ash from Santiaguito volcano in Guatemala reveals the presence of chemically heterogeneous filaments hosting micrometre-scale vesicles. The textures mirror those developed by disequilibrium melting induced via rapid heating during fault friction experiments, demonstrating that friction can generate sufficient heat to induce melting and vesiculation of hydrated silicic magma. Consideration of the experimentally determined temperature and pressure dependence of water solubility in magma reveals that, for many ascent paths, exsolution may be more efficiently achieved by heating than by decompression. We conclude that the thermal path experienced by magma during ascent strongly controls degassing, vesiculation, magma strength and the effusive-explosive transition in volcanic eruptions.

Pre-eruptive reheating during magma mixing at Quizapu volcano and the implications for the explosiveness of silicic arc volcanoes

Effusive-explosive transitions are observed in volcanoes erupting water-rich magmas that lack mineralogical evidence of degassing, suggesting that gas loss occurred rapidly and ascent was fast despite the high viscosity of rhyodacitic to rhyolitic melts. Here, we show that pre-eruptive heating of water-rich, silicic magma by mixing with hot recharge in subvolcanic chambers controls the effusive-explosive transition. Volcan Quizapu (Chile) emitted a 5 km3 mingled dacite-andesite lava flow in A.D. 1846–1847 that was ∼130 °C hotter than the 5 km3 of the same dacite that lead to a Plinian eruption in A.D. 1932. We explain the suppression of explosive fragmentation in the 1846–1847 lavas by enhanced syneruptive magma degassing as a consequence of late reheating following mixing with andesite recharge. Higher magma temperatures led to the transitioning into effusive eruptive behavior by significantly accelerating volatile diffusion and lowering melt viscosity, which facilitated bubble nucleation, growth, and coalescence, while also inhibiting brittle fragmentation. Thus, recharge by mafic magmas in subvolcanic magma chambers may reduce the risk of explosive eruptions in cases where heat can be efficiently transferred to cold, volatile-rich silicic magmas.

Injection of vesicular magma into an andesitic dome at the effusive–explosive transition

A single, vesicular pyroclast collected from a major (11×106m3) dome collapse eruption of the Soufrière Hills volcano, Montserrat (21st September 1997) was found to contain textural and Raman evidence for multiple decompression events and magma injection into the dome just prior to the onset of dome collapse. Quartz, plagioclase and hornblende phenocrysts contain closed and burst melt inclusions (MIs), the latter in the form of either foam or cylindrical melt extrusions from cavities on fracture surfaces. Fractures within ruptured quartz phenocrysts were infilled with glass, whereas those within plagioclase are entirely free of glass, indicating relatively early fracturing (decompression) of the quartz. This interpretation is supported by slightly higher measured H2OT in burst MIs in quartz (0.44wt.%±0.07, 1s.d.) compared with plagioclase (0.31wt.%±0.05, 1s.d.), indicating equilibration on rupturing of the quartz at higher PH2O. H2OT levels in glass from burst MIs in plagioclase are consistent with plagioclase fragmentation at H2O saturation pressures <5MPa, at an estimated depth of <210m, within the dome. The fragmentation occurred whilst the groundmass glass was still plastic, as evident from the presence of stretched vesicles in glass at fracture terminations. The fractures in plagioclase are empty of vapour phase precipitates (mainly cristobalite that infills matrix vesicles) which constrains the fracturing of plagioclase to the last depressurisation event recorded in the pyroclast, which was the dome collapse which led to the pyroclast being ejected from the dome. We suggest that our study provides the first direct (i.e. non seismic) evidence for the injection of gas-charged magma into the dome at the onset of dome collapse, marking the transition from effusive to explosive eruption, and that the magma injection could have provided the triggering mechanism for dome collapse.

Volcaniclastic processes on modern submarine arc stratovolcanoes: sidescan and photographic evidence from the Rumble IV and V volcanoes, southern Kermadec Arc (SW Pacific)

Studies of volcaniclastic processes on modern, submarine, arc stratovolcanoes are rare. Facies models of submarine volcanism and associated volcaniclastic deposits have to date emphasized general and/or regional processes. Two andesite-alkali basaltic volcanoes (Rumble IV and V volcanoes) from the southern Kermadec arc are “mapped” with swath bathymetry and sidescan imagery, and photographic data, to characterise volcaniclastic processes on these modern, submarine, arc stratovolcanoes, and their interaction with ambient marine processes. The principal observation from the Rumble IV and V data is the fine-scale spatial variability in the distribution of effusive lavas, various volcaniclastic deposits, and hemipelagic-pelagic sediments. Mass-movement of volcaniclastic deposits is common, and includes (1) epiclastic debris flows, (2) pillow breccia talus deposition penecontemporaneous with lava eruption, (3) traction particulate gravity flows, and possibly (4) sector collapse. Epiclastic redeposition is inferred to be mobilised by episodic, but frequent, localised, small volume flows, which largely redeposit debris on the edifice. Winnowed and rippled volcaniclastic sediment indicates bottom-current activity is common, but localised, over the entire height of both volcanoes, down to a water depth of ~ 2200 m. These currents also possibly entrain detritus for subsequent mobilisation by particulate gravity flows. Irregular and ragged spatter agglutinates on Rumble IV, interpreted as evidence of lava fountaining, and wellsorted, fines poor, scoriaceous material on Rumble V, interpreted as evidence of explosive eruption, indicate the transition between effusive and explosive volcanism is between 650 and 700 m water depth. Other southern Kermadec volcanoes (e.g. Tangaroa) have scoriaceous detritus at 960–970 m water depth. Positions of these effusive-explosive transition zones are likely to oscillate up and down the volcano during glacioeustatic fluctuations.