Crustal deformation at Ambrym (Vanuatu) imaged with satellite geodesy: constraints on magma storage, migration, and outgassing
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Les systemes volcaniques rift-caldera basaltiques fournissent les conditions propices a l’etude de plusieurs processus volcaniques, comme le transport de magma, les effondrements de caldera et le remplissage magmatique. Certaines des plus grandes calderas dans le monde, cependant, sont situees dans des regions isolees dont l’acces peut etre dangereux ou logistiquement complexe. La teledetection des deformations du sol, du degazage et des anomalies thermiques, offre une alternative pour y suivre l’activite volcanique. Ambrym, une ile volcanique du Vanuatu isolee mais tres active, a subi de nombreux episodes de deformation du sol au cours des 20 dernieres annees. Depuis 2015, deux eruptions ont eu lieu a l’interieur de sa caldera de 12 km de diametre. La premiere eruption a eu lieu apres 15 ans de degazage passif et d’activite des lacs de lave. L’eruption la plus recente, en decembre 2018, a vidange les lacs de lave des crateres sommitaux, provoquant l’intrusion d’un volume >0.4 km3 dans la zone de rift sud-est. Le dike engendre a parcouru une distance de plus de 20 km, et s’est ouvert de plus de 4 metres en profondeur. La vidange du magma a produit une subsidence de la caldera a grande echelle, associee a une activation des failles bordant la caldera, et a alimente une eruption sous-marine de ponces basaltiques. Une eruption plus modeste a eu lieu en fevrier 2015, activant egalement une portion de la caldera, et extrayant du magma depuis une chambre situee a une profondeur de ∼4.1 km. Ces deux evenements confirment que les failles bordieres de la caldera d’Ambrym sont des structures actives. L’activite de ces failles contribue a la topographie de la caldera d’Ambrym, dont le mecanisme de formation est discute (eruption Plinienne initiale a 2ka, suivie d’eruptions phreatomagmatiques). La detection d’une activation des failles de caldera par la geodesie spatiale nous permet de formuler l’hypothese que des centaines d’intrusions de tailles moderees a grandes peuvent contribuer a un approfondissement de la caldera, en drainant le magma stocke temporairement sous la caldera d’Ambrym. Outre ces deux evenements eruptifs majeurs, nous mettons en evidence deux episodes (2004–2007, 2015–2017) de subsidence rapide (∼10 cm an-1), mesures par InSAR. Aucune de ces deux periodes n’est associee a une eruption repertoriee. A partir des informations glanees au cours des eruptions de 2015 et 2018 (e.g., la profondeur des zones de stockage, le volume des intrusions, la relation entre l’activite eruptive et les lacs de lave), nous explorons les mecanismes physiques pouvant expliquer cette subsidence inter-eruptive. L’episode de 2004–2007 est probablement associe a une intrusion de dike (en l’absence d’eruption), engendrant la depressurisation d’un sill superficiel, hydrauliquement connecte aux lacs de lave. A partir d’un modele theorique propose par Girona et al, 2014, en couplant le degazage passif (mesure par spectroscopie satellitaire) et la depressurisation du reservoir magmatique (deduite de la geodesie spatiale), nous proposons que l’episode de 2015–2017 ait pour origine la depressurisation d’un reservoir magmatique de grande taille (>10 km3). Par contraste, de courtes periodes de soulevement pourraient etre limite aux periodes de temps pendant lesquelles le systeme est ferme, par exemple en 2019–2020 apres l’episode de vidange des lacs de lave en 2018, et potentiellement en 2007–2010 a la suite d’un evenement d’intrusion non repertorie en 2005. En comparant les phases de regain d’activite d’Ambrym avec celles observees dans les autres systemes rift-caldera basaltiques (Kīlauea, Barðarbunga, Sierra Negra, etc.), les resultats obtenus dans cette dissertation permettent de mieux comprendre l’activite des lacs de lave, le developpement de la caldera, les processus de deformation induits par le degazage, le remplissage magmatique, et la geometrie et le fonctionnement des systemes rift-caldera basaltiques.Keywords:
Caldera
Magma chamber
Caldera
Magma chamber
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Annual surveys of trilateration and leveling networks in and around Long Valley caldera in the 1982–1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right‐lateral strike slip on a vertical fault in the south moat of the caldera. The rate of inflation of the magma chamber seems to have been roughly constant (0.02 km 3 /yr) in the 1982–1985 interval, but the slip rate on the south moat fault has decreased substantially. In addition, there is evidence for a shallow source of dilatation (possibly dike intrusion) beneath the south moat of the caldera in 1983 and less certain evidence for a deep source (possibly magma chamber inflation beneath Mammoth Mountain) in the western caldera in 1983–1985. Deformation in the 1985–1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984–1985.
Caldera
Magma chamber
Dike
Dome (geology)
Trilateration
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Caldera
Magma chamber
Igneous differentiation
Fractional crystallization (geology)
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Abstract Simulating magma propagation pathways requires both a well‐calibrated model for the stress state of the volcano and models for dike advance within such a stress field. Here, we establish a framework for calculating computationally efficient and flexible magma propagation scenarios in the presence of caldera structures. We first develop a three‐dimensional (3D) numerical model for the stress state at volcanoes with mild topography, including the stress induced by surface loads and unloading due to the formation of caldera depressions. Then, we introduce a new, simplified 3D model of dike propagation. Such a model captures the complexity of 3D magma trajectories with low running time, and can backtrack dikes from a vent to the magma storage region. We compare the new dike propagation model to a previously published 3D model. Finally, we employ the simplified model to produce shallow dike propagation scenarios for a set of synthetic caldera settings with increasingly complex topographies. The resulting synthetic magma pathways and eruptive vent locations broadly reproduce the variability observed in natural calderas.
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Magma chamber
Stress field
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One hundred and ninety‐two small earthquakes which occurred recently near the southern edge of Long Valley caldera at depths from 1 to 13 km are used to map the subsurface geometry of magma bodies in the caldera. Seismograms of these events recorded northwest, north, northeast, and east of Long Valley with ray paths through the caldera are often anomalous in that S‐wave arrivals are absent and high frequency P‐wave energy is missing. These anomalous ray paths intersect in an area of the south‐central caldera which we interpret to be a region of molten or partially molten rock. This magma body lies between about 4 1/2 and at least 13 km depth and is 10 km long by 5 km wide beneath 7 km. The magma appears to be relatively contiguous below 7 km and more dispersed above. Another magma body may be present in the northwest caldera coincident with an interpreted 7‐8 km deep reflection from the top of a magma chamber.
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Explosive caldera-forming eruptions eject voluminous magma during the gravitational collapse of the roof of the magma chamber. Caldera collapse is known to occur by rapid decompression of a magma chamber at shallow depth, however, the thresholds for magma chamber decompression that promotes caldera collapse have not been tested using examples from actual caldera-forming eruptions. Here, we investigated the processes of magma chamber decompression leading to caldera collapse using two natural examples from Aira and Kikai calderas in southwestern Japan. The analysis of water content in phenocryst glass embayments revealed that Aira experienced a large magmatic underpressure before the onset of caldera collapse, whereas caldera collapse occurred with a relatively small underpressure at Kikai. Our friction models for caldera faults show that the underpressure required for a magma chamber to collapse is proportional to the square of the depth to the magma chamber for calderas of the same horizontal size. This model explains why the relatively deep magma system of Aira required a larger underpressure for collapse when compared with the shallower magma chamber of Kikai. The distinct magma chamber underpressure thresholds can explain variations in the evolution of caldera-forming eruptions and the eruption sequences for catastrophic ignimbrites during caldera collapse.
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The La Primavera caldera lies close to the triple junction of the Tepic-Zacoalco, Colima, and Chapala rifts in the western part of the Mexican Volcanic Belt. It is a promising geothermal field with 13 deep wells already drilled. We calculated solute geothermometric temperatures (Na–K, Na–Li, and SiO2) from the chemistry of geothermal water samples; determined values are generally between 99°C and 202°C for springs and between 131°C and 298°C for wells. Thermal modelling is an important geophysical tool as documented in the study of this and other Mexican geothermal areas. Using the computer program TCHEMSYS, we report new simulation results of three-dimensional (3-D) thermal modelling of the magma chamber underlying this caldera through its entire eruptive history. Equations (quadratic fit) describing the simulated temperatures as a function of the age, volume and depth of the magma chamber are first presented; these indicate that both the depth and the age of the magma chamber are more important parameters than its volume. A comparison of 3-D modelling of the La Primavera and Los Humeros calderas also shows that the depth of the magma chamber is more important than its volume. The best model for the La Primavera caldera has 0.15 million years as the emplacement age of the magma chamber, its top at a depth of 4 km, and its volume as 600 km3. Fresh magma recharge events within the middle part of the magma chamber were also considered at 0.095, 0.075, and 0.040 Ma. The simulation results were evaluated in the light of actually measured and solute geothermometric temperatures in five geothermal wells. Future work should involve a smaller mesh size of 0.050 or 0.10 km on each side (instead of 0.25 km currently used) and take into account the topography of the area and all petrogenetic processes of fractional crystallization, assimilation, and magma mixing as well as heat generation from natural radioactive elements.
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Magma chamber
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Magma chamber
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Abstract Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km 3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.
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Effusive eruption
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