Multi-Parametric Field Experiment Links Explosive Activity and Persistent Degassing at Stromboli

Visually unattainable magmatic processes in volcanic conduits, such as degassing, are closely linked to eruptive styles at the surface, but their roles are not completely identified and understood. To gain insights, a multi-parametric experiment at Stromboli volcano (Aeolian Islands, Italy) was installed in July 2016 focusing on the normal explosive activity and persistent degassing. During this experiment, gas-dominated (type 0) and particle-loaded (type 1) explosions, already defined by other studies, were clearly identified. A FLIR thermal camera, an Ultra-Violet SO₂ camera and a scanning Differential Optical Absorption Spectroscopy were deployed to record pyroclast and SO2 masses emitted during individual explosions, as well as persistent SO₂ fluxes, respectively. An ASHER instrument was also deployed in order to collect ash fallouts and to measure the grain size distribution of the samples. SO2 measurements confirm that persistent degassing was far greater than that emitted during the explosions. Further, we found that the data could be characterized by two periods. In the first period (25–27 July), activity was mainly characterized by type 0 explosions, characterized by high velocity jets. Pyroclast mass fluxes were relatively low (280 kg/event on average), while persistent SO2 fluxes were high (274 t/d on average). In the second period (29–30 July), activity was mainly characterized by type 1 explosions, characterized by low velocity jets. Pyroclast mass fluxes were almost ten times higher (2,400 kg/event on average), while persistent gas fluxes were significantly lower (82 t/d on average). Ash characterization also indicates that type 0 explosions fragments were characterized by a larger proportion of non-juvenile material compared to type 1 explosions fragments. This week-long field experiment suggests that, at least within short time periods, Stromboli’s type 1 explosions can be associated with low levels of degassing and the mass of particles accompanying such explosive events depends on the volume of a degassed magma cap sitting at the head of the magma column. This could make the classic particle-loaded explosions of Stromboli an aside from the true eruptive state of the volcano. Instead, gas-dominated explosions can be associated with high levels of degassing and are indicative of a highly charged (with gas) system. We thus suggest that relatively deep magmatic processes, such as persistent degassing and slug formation can rapidly influence the superficial behavior of the eruptive conduit, modulating the presence or absence of degassed magma at the explosion/fragmentation level

The physics of dancing peanuts in beer

In Argentina, some people add peanuts to their beer. Once immersed, the peanuts initially sink part way down into the beer before bubbles nucleate and grow on the peanut surfaces and remain attached. The peanuts move up and down within the beer glass in many repeating cycles. In this work, we propose a physical description of this dancing peanuts spectacle. We break down the problem into component physical phenomena, providing empirical constraint of each: (i) heterogeneous bubble nucleation occurs on peanut surfaces and this is energetically preferential to nucleation on the beer glass surfaces; (ii) peanuts enshrouded in attached bubbles are positively buoyant in beer above a critical attached gas volume; (iii) at the beer top surface, bubbles detach and pop, facilitated by peanut rotations and rearrangements; (iv) peanuts containing fewer bubbles are then negatively buoyant in beer and sink; and (v) the process repeats so long as the beer remains sufficiently supersaturated in the gas phase for continued nucleation. We used laboratory experiments and calculations to support this description, including constraint of the densities and wetting properties of the beer–gas–peanut system. We draw analogies between this peanut dance cyclicity and industrial and natural processes of wide interest, ultimately concluding that this bar-side phenomenon can be a vehicle for understanding more complex, applied systems of general interest and utility

Les dynamismes éruptifs et mécanismes associés en contexte basaltique : étude du Piton de la Fournaise, île de La Réunion, France

Basaltic eruptions are increasingly studied in a lot of active volcanoes around the world. Yet, their pre- and syn-eruptive processes are not fully understood. In order to characterize and understand the set of mechanisms occurring from the shallow magmatic system to the surface, this thesis is focused on the study of the eruptive products of Piton de la Fournaise volcano (La Réunion Island, France), which is one of the most active volcano in the world. Integrated textural and chemical analysis on tephra and lava are performed to identify some key parameters which can be related to major magmatic processes, eruptive dynamics and fragmentation processes. When it is available and relevant, these interpretations are integrated with pre- and syn-eruptive monitoring measurements acquired by ground-based and remote sensing observation systems.The first objective of this integrated study is to gain insight on the magmatic overpressure, conduit and eruptive dynamics during basaltic dyke eruptions. During the June 2014 eruption, eruptive triggers and syn-eruptive transitions from Hawaiian-style to mild Strombolian-style activities, associated with a decreasing of the lava flux, have been related to a second boiling mechanism and a pre-eruptive pressurization which was indirectly evidenced by golden pumice emission (highly vesicular and glassy tephra). On the contrary, the July 2015 eruption (relatively scarce golden pumice emission) was triggered by a shallow magmatic input that was tracked in the textural and chemical features of the eruptive products.On year later, another short-lived eruption occurred on September 2016, which emitted a lot of golden pumice. However, this eruption, dominated by Hawaiian fountaining, behaved atypically with an increase in lava flux associated with uncommon ash emission the last day of the eruptive activity. Thus, the second objective of the thesis is to improve our comprehension on the transition from Hawaiian-style (associated with golden pumice and fluidal scoria) to more explosive activities (associated with dense, crystal-rich opaque scoria and ash emission). The integrated study on this eruption supports the hypothesis that the opaque scoria and ash were inherited from the sub-surface cooling and degassing of the golden pumice material. This suggests the formation of a low-permeable plug, which modulated overpressure pulses at shallow levels and transient explosive activity under the late input of a relatively undegassed and ascending magma. It is commonly accepted that effusive activity represents the main emitted magmatic volume in basaltic shield volcanoes. However, the September 2016 eruption reminded us that basaltic eruptions can sometime produce hazards related to ash-dominated events. Therefore, the third purpose of the thesis is to bring together a comprehensive sampling of tephra from major historical and recent eruptions at Piton de la Fournaise and Karthala (Ngazidja Island, Comoros) volcanoes. The study of these different deposits reveals very different textural characteristics reflecting different fragmentation processes, as ductile fragmentation, brittle fragmentation (triggered by syn-eruptive crystallization of the juvenile magma), interactions between magma and external fluids, high magma ascent velocities and shear-induced fragmentation mechanisms during caldera collapse events.All these insights expose a lot of different magmatic and fragmentation processes which highlight the complexity of the mechanisms driving such basaltic eruptions. This thesis only represent a part of the research possibilities on basaltic eruptions. In the future, many additional laboratory studies and real-time monitoring of the eruptive activity can be performed in order to even better understand and better predict the change in eruptive activity than can represent unexpected threats to the environing populations.Les éruptions basaltiques sont de plus en plus étudiées au sein de nombreux volcans. Cependant, les processus pré- et syn-éruptifs gouvernant ces éruptions ne sont pas entièrement compris. Dans le but de caractériser et comprendre l’ensemble des mécanismes opérant depuis le système magmatique superficiel jusqu’à la surface, cette thèse est principalement axée sur l'étude des produits éruptifs du Piton de la Fournaise (île de la Réunion, France), qui est l'un des volcans les plus actifs au monde. Des analyses texturales et géochimiques sur les pyroclastes et les laves sont conjointement réalisées dans le but d’identifier des paramètres clés pouvant être reliés à des processus magmatiques majeurs, à des dynamiques éruptives et à des mécanismes de fragmentation. Lorsque cela est pertinent, ces interprétations sont intégrées à des données de surveillance pré- et syn-éruptives acquises au sol ou par télédétection.Le premier objectif de cette étude intégrée est de mieux comprendre les mécanismes à l’origine des surpressions magmatiques et des dynamiques éruptives lors des éruptions récentes du Piton de la Fournaise. Au cours de l’éruption de Juin 2014, les déclenchements éruptifs ainsi que les transitions syn-éruptives d’un régime hawaiien vers un régime strombolien relativement faible, associés à une diminution du flux lavique, ont été liés à une vésiculation et à une surpression pré-éruptive du magma qui furent indirectement mises en évidence par l’émission de pyroclastes de type golden pumice (pyroclastes très vésiculés et vitreux). Au contraire, l’éruption de Juillet 2015 (émission de golden pumice relativement rare) a été déclenchée par un apport magmatique superficiel qui a pu être identifié au sein des signatures texturales et géochimiques des produits éruptifs.Un an plus tard, une autre éruption de courte durée s'est produite en Septembre 2016, produisant beaucoup de pyroclastes de type golden pumice. Cependant, cette éruption dominée par des fontaines de lave, se comporta de manière atypique et fut associée à une augmentation du flux lavique et d’une émission inhabituelle de cendres lors du dernier jour de l’activité éruptive. Le deuxième objectif de la thèse est donc d’améliorer nos connaissances sur la transition d’une activité hawaiienne (associée à l’émission de pyroclastes de type golden pumice et fluidal scoria) vers une activité plus explosive associée à l’émission de scories et de cendres relativement denses et riches en cristaux, de type transitional et opaque. Les analyses texturales et chimiques des produits éruptifs confirment l'hypothèse selon laquelle ces scories et cendres denses sont issues du refroidissement et du dégazage à pression ambiante du magma initialement vésiculé et peu cristallisé. Il est donc possible de former un bouchon faiblement perméable, provoquant des impulsions magmatiques pressurisées à faible profondeur et une activité explosive transitoire sous l’influence et la remontée tardive d’un magma relativement peu dégazé. Il est communément admis que l’activité effusive représente la majorité du volume magmatique émis au sein des volcans boucliers basaltiques. Cependant, l’éruption de Septembre 2016 a montré que les éruptions basaltiques peuvent parfois produire des aléas liés à des émissions de cendres non-négligeables. Par conséquent, le troisième but de la thèse est de rassembler une série d’échantillons correspondant aux principales éruptions historiques et récentes des volcans du Piton de la Fournaise et du Karthala (île de Ngazidja, Comores). L’étude de ces différents dépôts cendreux révèle des caractéristiques texturales très différentes suggérant différents processus de fragmentation, à savoir la fragmentation ductile, la fragmentation fragile (provoqué par la cristallisation syn-éruptive du magma juvénile), des interactions entre le magma et des fluides externes, des vitesses de remontée magmatiques élevées ou encore la fragmentation par cisaillement pendant l’effondrement de calderas.Toutes ces connaissances exposent de nombreux processus magmatiques et de fragmentation qui mettent en évidence la complexité des mécanismes à l'origine de telles éruptions basaltiques. Ces travaux de thèse ne représentent qu’une partie des possibilités de recherche sur les éruptions basaltiques. Dans le futur, de nombreuses études de laboratoire supplémentaires et de surveillance de l’activité en temps-réel pourront être réalisées pour encore mieux comprendre et mieux prévoir l'évolution de l’activité éruptive en surface qui peut présenter des risques inattendus pour les populations environnantes

Eruptive dynamics and associated mechanisms in a basaltic context : insights from Piton de la Fournaise volcano, La Réunion Island, France

Les éruptions basaltiques sont de plus en plus étudiées au sein de nombreux volcans. Cependant, les processus pré- et syn-éruptifs gouvernant ces éruptions ne sont pas entièrement compris. Dans le but de caractériser et comprendre l’ensemble des mécanismes opérant depuis le système magmatique superficiel jusqu’à la surface, cette thèse est principalement axée sur l'étude des produits éruptifs du Piton de la Fournaise (île de la Réunion, France), qui est l'un des volcans les plus actifs au monde. Des analyses texturales et géochimiques sur les pyroclastes et les laves sont conjointement réalisées dans le but d’identifier des paramètres clés pouvant être reliés à des processus magmatiques majeurs, à des dynamiques éruptives et à des mécanismes de fragmentation. Lorsque cela est pertinent, ces interprétations sont intégrées à des données de surveillance pré- et syn-éruptives acquises au sol ou par télédétection.Le premier objectif de cette étude intégrée est de mieux comprendre les mécanismes à l’origine des surpressions magmatiques et des dynamiques éruptives lors des éruptions récentes du Piton de la Fournaise. Au cours de l’éruption de Juin 2014, les déclenchements éruptifs ainsi que les transitions syn-éruptives d’un régime hawaiien vers un régime strombolien relativement faible, associés à une diminution du flux lavique, ont été liés à une vésiculation et à une surpression pré-éruptive du magma qui furent indirectement mises en évidence par l’émission de pyroclastes de type golden pumice (pyroclastes très vésiculés et vitreux). Au contraire, l’éruption de Juillet 2015 (émission de golden pumice relativement rare) a été déclenchée par un apport magmatique superficiel qui a pu être identifié au sein des signatures texturales et géochimiques des produits éruptifs.Un an plus tard, une autre éruption de courte durée s'est produite en Septembre 2016, produisant beaucoup de pyroclastes de type golden pumice. Cependant, cette éruption dominée par des fontaines de lave, se comporta de manière atypique et fut associée à une augmentation du flux lavique et d’une émission inhabituelle de cendres lors du dernier jour de l’activité éruptive. Le deuxième objectif de la thèse est donc d’améliorer nos connaissances sur la transition d’une activité hawaiienne (associée à l’émission de pyroclastes de type golden pumice et fluidal scoria) vers une activité plus explosive associée à l’émission de scories et de cendres relativement denses et riches en cristaux, de type transitional et opaque. Les analyses texturales et chimiques des produits éruptifs confirment l'hypothèse selon laquelle ces scories et cendres denses sont issues du refroidissement et du dégazage à pression ambiante du magma initialement vésiculé et peu cristallisé. Il est donc possible de former un bouchon faiblement perméable, provoquant des impulsions magmatiques pressurisées à faible profondeur et une activité explosive transitoire sous l’influence et la remontée tardive d’un magma relativement peu dégazé. Il est communément admis que l’activité effusive représente la majorité du volume magmatique émis au sein des volcans boucliers basaltiques. Cependant, l’éruption de Septembre 2016 a montré que les éruptions basaltiques peuvent parfois produire des aléas liés à des émissions de cendres non-négligeables. Par conséquent, le troisième but de la thèse est de rassembler une série d’échantillons correspondant aux principales éruptions historiques et récentes des volcans du Piton de la Fournaise et du Karthala (île de Ngazidja, Comores). (...)Basaltic eruptions are increasingly studied in a lot of active volcanoes around the world. Yet, their pre- and syn-eruptive processes are not fully understood. In order to characterize and understand the set of mechanisms occurring from the shallow magmatic system to the surface, this thesis is focused on the study of the eruptive products of Piton de la Fournaise volcano (La Réunion Island, France), which is one of the most active volcano in the world. Integrated textural and chemical analysis on tephra and lava are performed to identify some key parameters which can be related to major magmatic processes, eruptive dynamics and fragmentation processes. When it is available and relevant, these interpretations are integrated with pre- and syn-eruptive monitoring measurements acquired by ground-based and remote sensing observation systems.The first objective of this integrated study is to gain insight on the magmatic overpressure, conduit and eruptive dynamics during basaltic dyke eruptions. During the June 2014 eruption, eruptive triggers and syn-eruptive transitions from Hawaiian-style to mild Strombolian-style activities, associated with a decreasing of the lava flux, have been related to a second boiling mechanism and a pre-eruptive pressurization which was indirectly evidenced by golden pumice emission (highly vesicular and glassy tephra). On the contrary, the July 2015 eruption (relatively scarce golden pumice emission) was triggered by a shallow magmatic input that was tracked in the textural and chemical features of the eruptive products.On year later, another short-lived eruption occurred on September 2016, which emitted a lot of golden pumice. However, this eruption, dominated by Hawaiian fountaining, behaved atypically with an increase in lava flux associated with uncommon ash emission the last day of the eruptive activity. Thus, the second objective of the thesis is to improve our comprehension on the transition from Hawaiian-style (associated with golden pumice and fluidal scoria) to more explosive activities (associated with dense, crystal-rich opaque scoria and ash emission). The integrated study on this eruption supports the hypothesis that the opaque scoria and ash were inherited from the sub-surface cooling and degassing of the golden pumice material. This suggests the formation of a low-permeable plug, which modulated overpressure pulses at shallow levels and transient explosive activity under the late input of a relatively undegassed and ascending magma. It is commonly accepted that effusive activity represents the main emitted magmatic volume in basaltic shield volcanoes. However, the September 2016 eruption reminded us that basaltic eruptions can sometime produce hazards related to ash-dominated events. Therefore, the third purpose of the thesis is to bring together a comprehensive sampling of tephra from major historical and recent eruptions at Piton de la Fournaise and Karthala (Ngazidja Island, Comoros) volcanoes. The study of these different deposits reveals very different textural characteristics reflecting different fragmentation processes, as ductile fragmentation, brittle fragmentation (triggered by syn-eruptive crystallization of the juvenile magma), interactions between magma and external fluids, high magma ascent velocities and shear-induced fragmentation mechanisms during caldera collapse events.All these insights expose a lot of different magmatic and fragmentation processes which highlight the complexity of the mechanisms driving such basaltic eruptions. This thesis only represent a part of the research possibilities on basaltic eruptions. In the future, many additional laboratory studies and real-time monitoring of the eruptive activity can be performed in order to even better understand and better predict the change in eruptive activity than can represent unexpected threats to the environing populations

Basaltic dyke eruptions at Piton de La Fournaise: characterization of the eruptive products with implications for reservoir conditions, conduit processes and eruptive dynamics

International audienceSmall-volume proximal-summit eruptions, sometimes with only little precursors, are common at Piton de la Fournaise (La Reunion Island, France). Their eruptive style ranges from Hawaiian to Strombolian, but their eruptive mechanisms are still not completely understood. To gain insight, we combined syn-eruptive field work, textural and geochemical measurements on the short-lived eruption of July 2015 and we compared it with the 2014 eruption, which opened the ongoing eruptive cycle at Piton de la Fournaise. Crystal-poor and moderately vesicular components were predominant at the beginning of the eruptions, during the lava flux peak and the Hawaiian-style activity. Their abundance decreased with time in favour of less vesiculated and crystal-rich components during the decrease of the lava flux and the Strombolian-style activity. Physical, textural and chemical characterization of the pyroclasts, as well as thermometry calculations, permit relating the different texture and chemical features of the eruptive products with the pre- and syn-eruptive processes. Geophysical precursors detected several weeks before the July 2015 eruption, as well as some evidences of magmatic reheating recorded in bulk and crystal compositions highlight that the July 2015 magma results from a pre-eruptive mixing between more and less dif-ferentiated magmas. We finally deduced that the observed syn-eruptive textural variations are related to the same batch of magma undergoing different cooling and degassing dynamics

Multi-parametric field experiment links explosive activity and persistent degassing at Stromboli

International audienceA multi-parametric experiment at Stromboli volcano (Aeolian Islands, Italy) was installed in July 2016 focusing on the normal explosive activity and persistent degassing. A FLIR thermal camera, an UV SO₂ camera and a scanning DOAS, were deployed to record pyroclast and SO2 masses emitted during individual explosions, as well as persistent SO₂ fluxes, respectively. An ASHER instrument was also deployed in order to collect ash fallouts and to measure the grain size distribution of the samples. SO2 measurements confirm that persistent degassing was far greater than that emitted during the explosions. Further, we found that the data could be characterized by two periods. In the first period (25-27 July), activity was mainly characterized by gas-dominated (type 0) explosions, characterized by high velocity jets. Pyroclast mass fluxes were relatively low (280 kg/event on average), while persistent SO2 fluxes were high (274 t/d on average). In the second period (29-30 July), activity was mainly characterized by particle-loaded (type 1) explosions, characterized by velocity jets. Pyroclast mass fluxes were almost ten times higher (2400 kg/event on average), while persistent gas fluxes were significantly lower (82 t/d on average). Ash characterization also indicates that type 0 explosions fragments were characterized by a larger proportion of non-juvenile material compared to type 1 explosions fragments. These data suggest that Stromboli's type 1 explosions can be associated with low levels of degassing and the mass of particles accompanying such explosive events depends on the volume of a degassed magma cap sitting at the head of the magma column. This could make the classic particle-loaded explosions of Stromboli an aside from the true eruptive state of the volcano. Instead, gas-dominated explosions can be associated with high levels of degassing and are indicative of a highly-charged (with gas) system. We thus suggest that relatively deep magmatic processes, such as persistent degassing and slug formation can rapidly influence the superficial behavior of the eruptive conduit, modulating the presence or absence of degassed magma at the explosion/fragmentation level

Magma fragmentation and particle size distributions in low intensity mafic explosions: the July/August 2015 Piton de la Fournaise eruption

Co-auteur étrangerInternational audienceUnderstanding magma fragmentation mechanisms in explosive eruptions is a key requirement for volcanic hazard assessment, eruption management and risk mitigation. This paper focuses on a type case small explosivity eruption (July–August 2015 eruption of Piton de la Fournaise). These eruptions, despite being often overlooked, are exceedingly frequent on local-to-global scales and constitute a significant hazard in vent-proximal areas, which are often populated by guides, tourists and, indeed, volcanologists due to their accessibility. The explosions presented here are ideal cases for the study of the dynamics of magma fragmentation and how it relates to the size distribution of scoria generated at the vent. We documented these events visually and thermally, and characterised the products through sample-return. This allowed us to describe small-scale gas bursts sending ejecta up to 30 m during intermittent lava fountains. Surface tension instabilities and inertial forces played a major role in fragmentation processes and generated particles with coarse-skewed distributions and median diameters ranging from − 8 to − 10 ϕ. However, with time distributions of particles in the most energetic fountains shifted towards more symmetrical shapes as median grains sizes became finer. Analyses of sequences of images demonstrate that the evolution of particle size distributions with time is due to instability of magma droplets and (in-flight) fragmentation

Monitoring of Eruptive Products: Pyroclastic Density Currents and their Deposits

International audienc

Monitoring of Eruptive Products: Pyroclastic Density Currents and their Deposits

International audienc

Monitoring of Eruptive Products: Deposits Associated with Pyroclastic Fallout

International audienc