What Triggers Caldera Ring‐Fault Subsidence at Ambrym Volcano? Insights From the 2015 Dike Intrusion and Eruption
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Abstract Surface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. Here, we show that an average of ∼40 cm of slip occurred on a normal caldera ring‐fault during this moderate‐sized (VEI < 3) event, which intruded a volume of ∼24 × 10 6 m 3 and erupted ∼9.3 × 10 6 m 3 of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym's main craters. After estimating magma compressibility, we provide an upper bound on the critical fraction ( f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate‐sized dike intrusions.Keywords:
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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.
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Dike
Dome (geology)
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Magma chamber
Igneous differentiation
Fractional crystallization (geology)
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Doming
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[1] The incremental caldera collapses of Fernandina (1968), Miyakejima (2000), and Piton de la Fournaise (2007) are analyzed in order to understand the collapse dynamics in basaltic setting and the associated edifice deformation. For each caldera, the collapse dynamics is assessed through the evolution of the (1) time interval T between two successive collapse increments, (2) amount of vertical displacement during each collapse increment, and (3) magma outflow rate during the whole collapse caldera process. We show from the evolution of T that Piton de la Fournaise and Fernandina were characterized by a similar collapse dynamics, despite large differences in the caldera geometry and the duration of the whole collapse caldera process. This evolution significantly differs from that of Miyakejima where T strongly fluctuated throughout the whole collapse process. Quantification of the piston vertical displacements enables us to determine the magma outflow rates between each collapse increment. Displacement data (tiltmeter and/or GPS) for Piton de la Fournaise and Miyakejima are used to constrain the edifice overall deformation and the edifice deformation rates. These data reveal that both volcanoes experienced edifice inflation once the piston collapsed into the magma chamber. Such a deformation, which lasts during the first collapse increments only, is interpreted as the result of larger volume of piston intruded in the magma chamber than magma withdrawn before each collapse increment. Once the effect of the collapsing rock column vanishes, edifice deflates. We also determine for each caldera the critical amount of magma evacuated before collapse initiation and compare it to analog models. The significant differences between models and nature are explained by the occurrence of preexisting weak zones in nature, i.e., the ring faults, that are not taken into account in analog models. Finally, we show that T at Piton de la Fournaise and Fernandina was equally controlled by the frictional resistance along the ring faults and the magma outflow rate. In addition to these two parameters, the collapse dynamics of Miyakejima was also influenced by variations of the magma bulk modulus, which changed after the influx of deep gas-rich magma into the collapse-related magma chamber. Altogether, our results show that the dynamics of caldera collapse in basaltic volcanoes proceeds in two phases: Phase 1, starting with the first collapse, is characterized by the largest collapse amplitude, an incremental edifice inflation, and a step-by-step increase of the rate of magma outflow. Phase 2 shows a rapid decrease of the magma discharge rate to a low level concomitant with the continuous edifice deflation. If deep magma is injected into the magma chamber, as at Miyakejima, an additional phase occurs (phase 3).
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The caldera structure after collapse of the magma chamber was estimated by numerical simulation. In the simulation, the collapse of the magma chamber was approximated by the contraction of a small sphere in the elastic medium, and the distribution of plastic and/or rupturing area was calculated using the Coulomb failure criterion under the assumption of an elastic‐perfectly‐plastic material. Given an undefined or isotropic regional stress field, the plastic area (caldera) was found to develop as a circular depression on the surface, appearing funnelform in cross section. Under an anisotropic regional stress field, the caldera developed in the direction of the maximum compression (or minimum extension) axis. In all simulations, the collapse of the magma chamber resulted in the formation of an outward dipping reverse ring fault around the area above the chamber, and an inward dipping normal ring fault at the periphery of the caldera.
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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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Dome (geology)
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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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