Search for: All records

The 2021 eruption at Tajogaite (Cumbre Vieja) volcano (La Palma, Spain) was characterized by Strombolian eruptions, Hawaiian fountaining, white gas-dominated and grey ash-rich plumes, and lava effusion from multiple vents. The variety of eruptive styles displayed simultaneously and throughout the eruption presents an opportunity to explore controls on explosivity and the relationship between explosive and effusive activity. Explosive eruption dynamics were recorded using ground-based thermal photography and videography. We show results from the analysis of short ($<$5 min) near-daily thermal videos taken throughout the eruption from multiple ground-based locations and continuous time-lapse thermal photos over the period November 16 to November 26. We measure the apparent radius, velocity, and volume flux of the high-temperature gas-and-ash jet and lava fountaining behaviors to investigate the evolution of the explosive activity over multiple time scales (seconds-minutes, hours, and days-weeks). We find fluctuations in volume flux of explosive material that correlate with changes in volcanic tremor and hours-long increases in explosive flux that are immediately preceded by increases in lava effusion rate. Correlated behavior at multiple vents suggests dynamic magma ascent pathways connected in the shallow (tens to hundreds of meters) sub-surface. We interpret the changes in explosivity and the relative amounts of effusive and explosivity to be the result of changes in gas flux and the degree of gas coupling.

 

The rheology of lavas and magmas exerts a strong control on the dynamics and hazards posed by volcanic eruptions. Magmas and lavas are complex mixtures of silicate melt, suspended crystals, and gas bubbles. To improve the understanding of the dynamics and effective rheology of magmas and lavas, we performed dam‐break flow experiments using suspensions of silicone oil, sesame seeds, and N₂O bubbles. Experiments were run inside a magnetic resonance imaging (MRI) scanner to provide imaging of the flow interior. We varied the volume fraction of sesame seeds between 0 and 0.48, and of bubbles between 0 and 0.21. MRI phase‐contrast velocimetry was used to measure liquid velocity. We fit an effective viscosity to the velocity data by approximating the stress using lubrication theory and the imaged shape of the free surface. In experiments with both particles and bubbles (three‐phase suspensions), we observed shear banding in which particle‐poor regions deform with a lower effective viscosity and dominate flow propagation speed. Our observations demonstrate the importance of considering variations in phase distributions within magmatic fluids and their implications on the dynamics of volcanic eruptions.

 

Three-phase suspensions, of liquid that suspends dispersed solid particles and gas bubbles, are common in both natural and industrial settings. Their rheology is poorly constrained, particularly for high total suspended fractions (≳0.5). We use a dam-break consistometer to characterize the rheology of suspensions of (Newtonian) corn syrup, plastic particles and CO 2 bubbles. The study is motivated by a desire to understand the rheology of magma and lava. Our experiments are scaled to the volcanic system: they are conducted in the non-Brownian, non-inertial regime; bubble capillary number is varied across unity; and bubble and particle fractions are 0 ≤  ϕ gas  ≤ 0.82 and 0 ≤  ϕ solid  ≤ 0.37, respectively. We measure flow-front velocity and invert for a Herschel–Bulkley rheology model as a function of ϕ gas , ϕ solid , and the capillary number. We find a stronger increase in relative viscosity with increasing ϕ gas in the low to intermediate capillary number regime than predicted by existing theory, and find both shear-thinning and shear-thickening effects, depending on the capillary number. We apply our model to the existing community code for lava flow emplacement, PyFLOWGO, and predict increased viscosity and decreased velocity compared with current rheological models, suggesting existing models may not adequately account for the role of bubbles in stiffening lavas. 

Abstract. Lava flows present a significant natural hazard to communities around volcanoes and are typically slow-moving (<1 to 5 cm s−1) and laminar. Recent lava flows during the 2018 eruption of Kīlauea volcano, Hawai'i, however, reached speeds as high as 11 m s−1 and were transitional to turbulent. The Kīlauea flows formed a complex network of braided channels departing from the classic rectangular channel geometry often employed by lava flow models. To investigate these extreme dynamics we develop a new lava flow model that incorporates nonlinear advection and a nonlinear expression for the fluid viscosity. The model makes use of novel discontinuous Galerkin (DG) finite-element methods and resolves complex channel geometry through the use of unstructured triangular meshes. We verify the model against an analytic test case and demonstrate convergence rates of P+1/2 for polynomials of degree 𝒫. Direct observations recorded by unoccupied aerial systems (UASs) during the Kīlauea eruption provide inlet conditions, constrain input parameters, and serve as a benchmark for model evaluation. 

Most lava flows carry bubbles and crystals in suspension. From earlier works, it is known that spherical bubbles increase the effective viscosity while bubbles deformed by rapid flow decrease it. Changes in the spatial distribution of bubbles can lead to variable rheology and flow localization and thus modify the resulting lava flow structure and morphology. To understand the roles of bubble and solid phase crystal distributions, we conducted a series of analog experiments of high bubble fraction suspensions. We poured the analog lava on an inclined slope, observed its shape, calculated the velocity field, and monitored its local thickness. A region of localized rapid flow and low vesicularity, whose thickness is thinner than the surrounding area, develops at the center of the bubbly flows. These features suggest that the locally higher liquid fraction decreases the effective viscosity, increases the fluid density, and accelerates the flow. We also found that a halted particle‐bearing bubbly flow can resume flowing. We interpret this to result from the upward vertical separation of bubbles, which generates a liquid‐rich layer at the bottom of the flow. In our experiment, bubbles are basically spherical and decrease the flow velocity, while our estimate suggests that bubbles in natural lava flows could increase or decrease flow velocity. Downstream decreases in flow velocity stops the bubble deformation and can cause a sudden increase of effective viscosity. The vertical segregation of the liquid phase at the slowed flow front may be a way to generate a cavernous shelly paho’eho’e.

 

Blasting experiments were performed that investigate multiple explosions that occur in quick succession in unconsolidated ground and their effects on host material and atmosphere. Such processes are known to occur during phreatomagmatic eruptions at various depths, lateral locations, and energies. The experiments follow a multi‐instrument approach in order to observe phenomena in the atmosphere and in the ground, and measure the respective energy partitioning. The experiments show significant coupling of atmospheric (acoustic)‐ and ground (seismic) signal over a large range of (scaled) distances (30–330 m, 1–10 m J^−1/3). The distribution of ejected material strongly depends on the sequence of how the explosions occur. The overall crater sizes are in the expected range of a maximum size for many explosions and a minimum for one explosion at a given lateral location. As previous research showed before, peak atmospheric over‐pressure decays exponentially with scaled depth. An exponential decay rate ofwas measured. At a scaled explosion depth of 4 × 10⁻³ m J^−1/3ca. 1% of the blast energy is responsible for the formation of the atmospheric pressure pulse; at a more shallow scaled depth of 2.75 × 10⁻³ m J^−1/3this ratio lies at ca. 5.5%–7.5%. A first order consideration of seismic energy estimates the sum of radiated airborne and seismic energy to be up to 20% of blast energy. Finally, the transient cavity formation during a blast leads to an effectively reduced explosion depth that was determined. Depth reductions of up to 65% were measured.