Wim Degruyter

Convection in a volcanic conduit recorded by bubbles

Microtextures of juvenile pyroclasts from Kīlauea’s (Hawai‘i) early A.D. 2008 explosive activity record the velocity and depth of convection within the basaltic magma-filled conduit. We use X-ray microtomography (μXRT) to document the spatial distribution of bubbles. We find small bubbles (radii from 5 μm to 70 μm) in a halo surrounding larger millimeter-size bubbles. This suggests that dissolved water was enriched around the larger bubbles—the opposite of what is expected if bubbles grow as water diffuses into the bubble. Such volatile enrichment implies that the volatiles within the large bubbles were redissolving into the melt as they descended into the conduit by the downward motion of convecting magma within the lava lake. The thickness of the small bubble halo is ∼100–150 μm, consistent with water diffusing into the melt on time scales on the order of 103 s. Eruptions, triggered by rockfall, rapidly exposed this magma to lower pressures, and the haloes of melt with re-dissolved water became sufficiently supersaturated to cause nucleation of the population of smaller bubbles. The required supersaturation pressures are consistent with a depth of a few hundred meters and convection velocities of the order of 0.1 m s−1, similar to the circulation velocity observed on the surface of the Halema‘uma‘u lava lake.

Magma reservoir response to transient recharge events: The case of Santorini volcano (Greece)

Igneous bodies are built incrementally by episodic magma injection events that vary in rate and duration. Such recharge events can either grow an upper crustal magma reservoir or trigger an eruption. This bifurcation in behavior remains poorly constrained but is essential to volcanic hazard assessment. Here we use a numerical model that couples the thermal and mechanical processes in a magma reservoir to study the evolution of the Santorini magmatic system (Greece) over the past 20 k.y. Our results constrain the recharge rate and duration that are necessary to trigger an eruption for the known long-term average inflow rate of 10–3 km3/yr. The size of the chamber and its exsolved volatile content are dominant controls on the critical recharge rate and duration that will trigger an eruption. Our model successfully reproduces the main features of the Minoan eruption and Nea Kameni activity, providing volume estimates for the active part of the current subvolcanic reservoir as well as information regarding the presence of exsolved volatiles. Thermomechanical models offer a new framework to integrate the historic eruption record with geodetic measurements and provide a context to understand the past, present, and future of active volcanic centers.

Magma Ascent and Degassing at Shallow Levels

The most dramatic change that magma undergoes while ascending to the surface is volatile loss because of pressure drop. This chapter reviews the main mechanisms by which magma can lose its volatile components to reach the volcanic vent either as a mostly degassed lava that oozes quietly or as a fragmented mixture of gases and particles that explosively rushes out into the atmosphere. These mechanisms include volatile behavior, the formation and evolution of gas bubbles and crystals during ascent, magma rheology, volcanic conduit geometry, and gas escape. Interactions between these mechanisms are explored thanks to a simplified conduit flow model.

The mechanics of shallow magma reservoir outgassing

Magma degassing fundamentally controls the Earths volatile cycles. The large amount of gas expelled into the atmosphere during volcanic eruptions (i.e. volcanic outgassing) is the most obvious display of magmatic volatile release. However, owing to the large intrusive:extrusive ratio, and considering the paucity of volatiles left in intrusive rocks after final solidification, volcanic outgassing likely constitutes only a small fraction of the overall mass of magmatic volatiles released to the Earths surface. Therefore, as most magmas stall on their way to the surface, outgassing of uneruptible, crystal-rich magma storage regions will play a dominant role in closing the balance of volatile element cycling between the mantle and the surface. We use a numerical approach to study the migration of a magmatic volatile phase (MVP) in crystal-rich magma bodies (“mush zones”) at the pore-scale. Our results suggest that buoyancy driven outgassing is efficient over crystal volume fractions between 0.4 and 0.7 (for mm-sized crystals). We parameterize our pore-scale results for MVP migration in a thermo-mechanical magma reservoir model to study outgassing under dynamical conditions where cooling controls the evolution of the proportion of crystal, gas and melt phases and to investigate the role of the reservoir size and the temperature-dependent visco-elastic response of the crust on outgassing efficiency. We find that buoyancy-driven outgassing allows for a maximum of 40-50% volatiles to leave the reservoir over the 0.4-0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7) through veining and/or capillary fracturing.

Cryoclastic origin of particles on the surface of Enceladus

Analogous to volcanic deposits on Earth, we can infer eruption characteristics on Enceladus from the relationship between particle size and distance from the vent. We develop a model in which ice particles feeding plumes are accelerated by the gas. We consider two cases: drag-limited and collision-limited acceleration, which link particle size to exit velocity. After being ejected at the vent, particles follow ballistic trajectories. We fit the model to observations of particle size on the surface inferred from modeled VIMS data collected by the Cassini spacecraft. We obtain a relationship between gas temperature and characteristic acceleration length, whereby lower gas temperatures require longer acceleration lengths. The model shows that the large size of particles on the surface is consistent with the size of particles observed with the CDA and VIMS instruments at heights of Cassini flybys, and the size of particles that reach escape velocity and are found in Saturns E-ring.

Influence of Exsolved Volatiles on Reheating Silicic Magmas by Recharge and Consequences for Eruptive Style at Volcan Quizapu (Chile)

The two most recent eruptions of Volcan Quizapu (southern Andes, Chile), only 86 years apart, were both triggered by magma recharge and extruded the same volume (about 5 km3) of the same volatile-rich dacitic magma, but showed a remarkable shift from effusive (1846-47) to explosive (1932) behavior. We demonstrate, using a newly developed model, that the presence or absence of an exsolved volatile phase in the reservoir strongly influences its mechanical and thermal response to new inputs of magma. We propose that, prior to the 1846-47 effusive eruption, gas bubbles damped the build-up of excess pressure and allowed recharge of a significant volume of magma before triggering the 1846-1847 eruption. The strong temperature increase that resulted enhanced syn-eruptive outgassing leading to an effusive eruption. In contrast, during the repose period between the 1847 and 1932 eruptions new recharges found a much less compressible host reservoir as the exsolved gas phase was largely removed in response to the prior eruption, yielding rapid pressurization, minor reheating, and comparatively less syneruptive outgassing. The combination of these effects culminated in an explosive eruption.

Near-Real-Time Detection of Tephra Eruption Onset and Mass Flow Rate Using Microwave Weather Radar and Infrasonic Arrays

During an eruptive event, the near-real-time monitoring of volcanic explosion onset and its mass flow rate (MFR) is a key factor to predict ash plume dispersion and to mitigate risk to air traffic. Microwave (MW) weather radars have proved to be a fundamental instrument to derive eruptive source parameters. We extend this capability to include an early-warning detection scheme within the overall volcanic ash radar retrieval methodology. This scheme, called the volcanic ash detection (VAD) algorithm, is based on a hybrid technique using both fuzzy logic and conditional probability. Examples of VAD applications are shown for some case studies, including the Icelandic Grímsvötn eruption in 2011, the Eyjafjallajökull eruption in 2010, and the Italian Mt. Etna volcano eruption in 2013. Estimates of the eruption onset from the radar-based VAD module are compared with infrasonic array data. One-dimensional numerical simulations and analytical model estimates of MFR are also discussed and intercompared with sensor-based retrievals. Results confirm in all cases the potential of MW weather radar for ash plume monitoring in near real time and its complementarity with infrasonic array for early-warning system design.