New geochemical models have been developed to provide a further tool for interpreting and understanding eruptive events, also for the purpose of assessing the hazard in volcanic crises.
Combining the estimate of magma recharge at depth with measurements of volcanic degassing, a team of researchers from the Etna Observatory and the Palermo Section of the National Institute of Geophysics and Volcanology (INGV) has quantified the high pressurization in which it was located Etna before the start of the December 2018 eruption.
The results are contained in the study 'Intense overpressurization at basaltic open-conduit volcanoes as inferred by geochemical signals: the case of the Mt Etna December 2018 eruption', just published in the journal Science Advances.
Indeed, Etna was used as a case study thanks to the important extension of the monitoring and surveillance networks, also developed in the context of the cooperation between INGV and the Department of Civil Protection, aimed at measuring the outgassing of the volcano.
The study addressed the unresolved question of the balance between gases existing deep within a volcanic system and their outgassing at the surface. The scientists first evaluated the measurements of the flows and the chemical composition of the volcanic gases emitted by Etna starting a couple of years before the eruptive activity being investigated.
The innovative idea was developed by comparing the amount of gas emitted into the atmosphere and that rising from the deep crust together with the magma. The latter was calculated using an original approach identified in the recent INGV research "Temporal variations of helium isotopes in volcanic gases quantify pre-eruptive refill and pressurization in magma reservoirs:The Mount Etna case", which uses geochemical data in a physical and chemical model of magma chamber pressurization. It was therefore quantified that, at the time of the December eruption, the volcanic system had already accumulated an enormous quantity of magmatic fluids which could not have been effectively released by normal volcanic degassing. In this way, the necessary energy was made available for the intrusion of two dikes (i.e., sub-vertical fracturing of the rocks, determined by the thrust of the rising magma) and for the opening of the eruptive fracture, reactivating the sliding of the eastern flank of the mountain and finally triggering the paroxysmal explosion.
"Our study has highlighted an imbalance between the amount of gas normally rising with the magma from the mantle below a volcano and that emitted in the pre- and inter-eruptive phases", explains Antonio Paonita, a researcher at INGV. “Recognizing and quantifying this 'imbalance' and its evolution almost in real time provides a new interpretative key for assessing the volcano's "state of activity". This aspect can also be very important to contribute to the prediction of volcanic activity especially in areas where there is a high population density”.
The imbalance between gases accumulated in the depths and those emitted on the surface adds to the well-known discrepancy between the volumes of magma estimated to be stored in the crust compared to the volumes actually emitted on the surface, a topic still of lively debate in the scientific community.
"Thermodynamic models of the magmatic outgassing of the December 2018 eruption have been developed," explains the researcher. “Thus, we found that the various types of geochemical signals we monitored 'see' different windows of depth in the magma feeding system, allowing us to follow the timing of the magma upwelling and to hypothesize that the preparation for the December eruption it began almost two years before the event itself".
In fact, another interesting aspect deriving from the study concerns the estimation of the realization times of the entire process which links the deep phenomena to the observed superficial manifestations.
“We were able to observe that the overall eruptive process of the December eruption, intended as the one linking the beginning of the deep magmatic input to the eruptive manifestation on the surface, lasted about two years. This long accumulation process almost never ends in a single eruptive event, however violent, but it takes a long time to be dissipated. Etna continuously confirms this aspect with its very frequent eruptive activities, among which particularly energetic events emerge such as the 2018 eruption, and which in the long run try to restore a balance between fluid and magma inputs from the depths and lava emissions and gas on the surface".
“The results of the study”, concludes Paonita, “represent a significant advance in understanding eruption escalation in basaltic volcanism in general. Etna is the archetype of an open vented basaltic system and it is likely that similar processes operate in many volcanoes around the world."
The published research has an essentially scientific value, without immediate implications regarding the aspects of civil protection at the moment.
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The evaluation of the magmatic degassing is useful for understanding the eruptions of basaltic volcanoes
New geochemical models have been developed to provide a further tool for the interpretation and understanding of eruptive events, also for the purpose of assessing the hazard in volcanic crises
By combining the estimation of magma recharge at depth with the volcanic degassing measurements, a team of researchers from the Osservatorio Etneo and the Palermo Section of the Istituto Nazionale di Geofisica e Volcanologia (INGV) quantified the high pressurization which featured Mount Etna before the start of the December 2018 eruption.
The results are contained in the study 'Intense overpressurization at basaltic open-conduit volcanoes as inferred by geochemical signals: the case of the Mt Etna December 2018 eruption', just published in the journal Science Advances.
Etna was used as a case study thanks to the important extension of the monitoring and surveillance networks designed to measure the degassing of the volcano, developed as part of the cooperation program between INGV and the Italian Department of Civil Protection.
The study addressed the unsolved question of the balance between gases existing at depth in a volcanic system and their degassing at the surface. Scientists first of all considered the measurements of the output and chemical composition of the volcanic gases emitted by Etna, starting from some years before the investigated eruptive activity. The innovative idea consisted in comparing the amount of gas emitted into the atmosphere and that coming from the deep crust together with the magma. The latter was calculated by using an original approach developed in the recent INGV research "Temporal variations of helium isotopes in volcanic gases quantify pre-eruptive refill and pressurization in magma reservoirs: The Mount Etna case", which uses geochemical data into a physical and chemical model of pressurization of the magma chamber. It was quantified that, at the time of the December eruption, the volcanic system had already accumulated an enormous amount of magmatic fluids that could not have been effectively released by normal volcanic degassing. In this way, the required energy was made available for the intrusion of two dikes (sub-vertical fractures of the rocks determined by the thrust of the rising magma) and for the opening of the eruptive fracture, also reactivating the sliding of the eastern flank of the mountain and finally triggering the paroxysmal explosion.
"Our study highlighted an imbalance between the amount of gas normally rising with the magma from the mantle beneath a volcano and that emitted in pre- and inter-eruptive phases", explains Antonio Paonita, researcher at INGV. "Recognizing and quantifying this 'imbalance' and its evolution almost in real time provides a new interpretative key for evaluating the 'state of activity' of the volcano. This aspect can also be very important in contributing to the prediction of volcanic activity, especially in areas where there is a high population density".
The imbalance between accumulated gases at depth and those emitted at the surface parallels the well-known discrepancy between the volumes of magma that are estimated to be stored in the crust compared to the volumes actually emitted to the surface, a subject still of lively debate in the scientific community.
“Thermodynamic models of the magmatic degassing of the December 2018 eruption have been developed", explains the researcher. "In doing so, we found that the various types of geochemical signals we monitored 'see' different depth windows in the magma supply system, allowing us to follow the timing of the magma rise and to hypothesize that the preparation of the December eruption started almost two years before the event itself”.
Another interesting aspect that in fact arose from the study concerns the estimation of the preparatory times of the entire process that links the onset of the magma dynamics at depth to the observed phenomena at the surface.
“We observed that the overall eruptive process of the December eruption, regarded as the one that links the beginning of the deep magmatic input to the eruptive manifestation on the surface, lasted about two years. This long process of accumulation almost never ends by a single eruptive event, even if very violent, but requires a long time to be dissipated. Etna continuously confirms this aspect with its very frequent eruptive activities, among which we observe particularly energetic events such as the eruption of 2018. In the long run they try to restore a balance between the inputs of fluids and magma from the depths versus lava and gas emissions at the surface".
“The results of the study”, concludes Paonita, “represent a significant step forward in understanding the escalation of eruptions in basaltic volcanism in general. Etna is the archetype of an open-conduit basaltic system and it is likely that similar processes work in many volcanoes around the world".
The published research has an essentially scientific value, currently devoid of immediate implications regarding the aspects of civil protection.
link:
Fig. 1 - Cumulative input and output of CO2. Cumulative amount of CO2 introduced into the magma system from depth (blue line) and emitted by plume outgassing (brown line). The He isotope ratio signal of the P39 site (dashed pink) is also shown.
Fig. 1 - Cumulative input and output of CO2. Cumulative amount of CO2 inputted into the magmatic system from depth (blue line) and outputted from plume degassing (brown line). The isotope signal from the P39 site is also shown (dashed pink).
Fig 2 - Scheme of the Etna magma system that fueled the December 2018 eruption. From above, the long recharge of magma and fluids at depth, the transfer of magma throughout the intermediate system to the base of the volcanic edifice since June in November 2018, and finally the pressurization of the accumulation system in the ducts (2-3 km below the craters) and the decompression of the deep and intermediate supply system.
Fig 2 - Sketch of the Mt Etna magmatic system feeding the December 2018 eruption. From the top, the long-lasting magma and fluid recharge at depth, the magma transfer throughout the intermediate system up to the base of the volcanic edifice from June to November 2018, and finally the coupled pressurization of the conduit storage system (2-3 km below the craters) and decompression of the deep and intermediate reservoirs.
Fig 3 - Geochemical signals in the period July 2017 - February 2019. From bottom to top: a) 3He/4He isotope ratio in peripheral gases (such as R/Ra, where R=sample isotope ratio and Ra=atmosphere isotope ratio); b) Normalized curve of the total fluxes of CO2 emitted by the soils, recorded by the EtnaGAS network (daily average and moving average on a weekly basis, respectively as gray and blue lines); c) CO2/SO2 ratio measured in the Etna plume, and d) daily and weekly SO2 flux measured in the volcanic plume (grey and black line, respectively). The volcanic activity of the period studied is highlighted by distinguishing eruptive events and ash emission (green, gray bands) and the eruptive phase of 24 December (red band).
Fig 3 - Investigated geochemical signals in the period July 2017 - February 2019. From bottom to the top: a) 3He/4He isotope ratio in peripheral gas vents (as R/Ra, being R=the isotope ratio in the sample and Ra=the isotope ratio in air); b) Normalized curve of the total flows of CO2 exhaling from the soils recorded by the EtnaGAS network (daily and running average on weekly basis as gray and blue line, respectively); c) CO2/SO2 ratio measured in the Etna plume, and d) the daily and weekly bulk SO2 flux measured in the volcanic plume (grey and black line, respectively). The volcano activity of the studied period is also shown by distinguishing eruptive and ash emission events (green, gray headbands) and the eruptive phase of December 24 (red band).