Poroelastic responses of confined aquifers to subsurface strain and their use for volcano monitoring

Well water level changes associated with magmatic unrest can be interpreted as a result of pore pressure changes in the aquifer due to crustal deformation, and so could provide constraints on the subsurface processes causing this strain. We use finite element analysis to demonstrate the response of aquifers to volumetric strain induced by pressurized magma reservoirs. Two different aquifers are invoked – an unconsolidated pyroclastic deposit and a vesicular lava flow – and embedded in an impermeable crust, overlying a magma chamber. The time-dependent, fully coupled models simulate crustal deformation accompanying chamber pressurization and the resulting hydraulic head changes as well as flow through the porous aquifer, i.e. porous flow. The simulated strain leads to centimetres (pyroclastic aquifer) to metres (lava flow aquifer) of hydraulic head changes; both strain and hydraulic head change with time due to substantial porous flow in the hydrological system. Well level changes are particularly sensitive to chamber volume, shape and pressurization strength, followed by aquifer permeability and the phase of the pore fluid. The depths of chamber and aquifer, as well as the aquifer's Young's modulus also have significant influence on the hydraulic head signal. While source characteristics, the distance between chamber and aquifer and the elastic stratigraphy determine the strain field and its partitioning, flow and coupling parameters define how the aquifer responds to this strain and how signals change with time. We find that generic analytical models can fail to capture the complex pre-eruptive subsurface mechanics leading to strain-induced well level changes, due to aquifer pressure changes being sensitive to chamber shape and lithological heterogeneities. In addition, the presence of a pore fluid and its flow have a significant influence on the strain signal in the aquifer and are commonly neglected in analytical models. These findings highlight the need for numerical models for the interpretation of observed well level signals. However, simulated water table changes do indeed mirror volumetric strain, and wells are therefore a valuable addition to monitoring systems that could provide important insights into pre-eruptive dynamics

Impact of channelized flow on temperature distribution and fluid flow in restless calderas:Insight from Campi Flegrei caldera, Italy

Magmatic hydrothermal systems develop by the imposition of a magmatically derived heat flux upon a shallow groundwater system. As such their dynamics can be intermittently perturbed by changing conditions within the associated magmatic system. Understanding the nature of the coupling between the magmatic and groundwater systems is thus key to discriminating geophysical signals of magmatic unrest from purely hydrothermal ones.Using a series of numerical groundwater models run with TOUGH2, we simulate the coupled groundwater–magmatic system at Campi Flegrei caldera, with particular emphasis on the impact of permeability developed within local fault systems and the dynamics of the system during magmatic unrest. Simulation results suggest that faults can play an important role in controlling the dynamics of recharge and heat transport within the shallow hydrothermal reservoir. Results specifically highlight that contrasts in permeability between faults and surrounding rock impact local temperature gradients, with faults either acting as preferential routes for recharge or discharge of groundwater, depending on fault/caldera fill permeability contrast and the vertical extent of the fault. Simulations of magmatic unrest with a step-wise increase in basal heat flux suggest that periodic geophysical and chemical signals may stem from the interaction between the development of gas at depth and the recharge–discharge dynamics of the reservoir. These results highlight the potential for the dynamics of magmatic–hydrothermal systems to be significantly impacted by the presence and nature of local fault systems. Where dynamic groundwater systems are involved, it is thus important to understand the impact of such geological elements when interpreting monitoring data such as ground deformation, seismicity and gas emissions

Prince Rupert’s Drops: An analysis of fragmentation by thermal stresses and quench granulation of glass and bubbly glass

When volcanic eruptions involve interaction with external water (hydrovolcanism), the result is an ash-rich and energetic volcanic plume, as illustrated dramatically by the January 2022 Tonga eruption. The origin of the high explosive energy of these events remains an important question. We investigate this question by studying Prince Rupert's Drops (PRDs)-tadpole-shaped glass beads formed by dripping molten glass into water-which have long fascinated materials scientists because the great strength of the head contrasts with the explosivity of the metastable interior when the tail is broken. We show that the fragment size distribution (FSD) produced by explosive fragmentation changes systematically with PRD fragmentation in air, water, and syrup. Most FSDs are fractal over much of the size range, scaling that can be explained by the repeated fracture bifurcation observed in three-dimensional images from microcomputed tomography. The shapes of constituent fragments are determined by their position within the original PRD, with platey fragments formed from the outer (compressive) shell and blocky fragments formed by fractures perpendicular to interior voids. When molten drops fail to form PRDs, the glass disintegrates by quench granulation, a process that produces fractal FSDs but with a larger median size than explosively generated fragments. Critically, adding bubbles to the molten glass prevents PRD formation and promotes quench granulation, suggesting that granulation is modulated by heterogeneous stress fields formed around the bubbles during sudden cooling and contraction. Together, these observations provide insight into glass fragmentation and potentially, processes operating during hydrovolcanism

Immersion freezing of supercooled water drops containing glassy volcanic ash particles

AbstractThe freezing temperatures of hundreds of water drops with radii 20–50μm containing known average concentrations of suspended, mostly micron- to submicron-sized, volcanic ash particles composed of SiO2-rich glass were recorded using optical microscopy. As expected, the ash suppresses supercooling, and in contrast to earlier studies of much larger ash particles, the median freezing temperature clearly scales with the available ash surface area per drop. The heterogeneous nucleation rate coefficient per unit mass of ash (jm) increases exponentially with decreasing temperature (T) (increasing supercooling) with a possible change in the slope of a plot of logjm against T at T=245±1K. Although uncertainties in the ash surface area limit quantitative comparisons, we conclude that volcanic glass is a less effective ice-nucleating agent than feldspar crystals and more similar to other minerals previously studied

Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit

International audienceThe 18 May 1980 eruption of Mount St. Helens started with a lateral blast that fed a pyroclastic surge, which then uplifted to form a co-blast plume. Thirty minutes later, Plinian activity started at the vent and fluctuated in intensity for ~9 h. The resulting fallout deposit, documented to > 600 km from vent, presents some striking features: (1) displacement of the overall deposit to the north of the vent, (2) a secondary thickness and mass maximum at ~300 km from vent, (3) a total grain size distribution dominated by fine ash (62 wt % of the deposit < 63 μm), and (4) individual grain size distributions that vary dramatically in the crosswind direction from strongly bimodal in the south to skewed unimodal in the north. Results from a new deconvolution of the individual grain size distributions show that they are a combination of a coarse subpopulation that decreases in size with distance from vent and a constant fine subpopulation with a mean of ∼15 μm. Relative proportions of each subpopulation vary asymmetrically in the crosswind directions, with the fine subpopulation preponderant toward the north and the coarse one dominating the south of the deposit, both reach their absolute maxima in mass on the deposit axis. Componentry analyses of selected samples show that blast-derived material is greatly enriched toward the north of the deposit. These results indicate that the co-blast plume dispersed fine-grained material over great distances and dominated the fine subpopulation. Comparison with reanalysis data of atmospheric wind fields and satellite images of the spreading ash cloud suggests contrasting ash transport and depositional processes for the (early) co-blast plume and the (later) vent-derived Plinian plumes. The co-blast plume is displaced to the north; it had a high overshoot height, and eastward dispersion via strong winds low in the stratosphere (~10-15 km). The Plinian plumes were lower and dispersed most of the material to the southeast as the direction of high-velocity winds shifted just before the late climactic Plinian eruptive phase. Fine ash (fine subpopulation) was deposited continuously throughout the deposit, with an increase of sedimentation rate ~300 km from the vent where there is a secondary maximum in the deposit mass and thickness. Fine ash probably settled by a combination of enhanced sedimentation mechanisms, including not only aggregation but also gravitational convective instabilities of cloud base, hydrometeor formation and destruction, and entrainment of small particles by larger ones. Finally, we show that half of the deposit (by mass) in the medial area was deposited by the co-blast plume, and that a significant proportion of the Mount St. Helens fallout deposit is nonjuvenile, which has implications for the magmatic budget of this eruption