Influence of regional tectonics and pre-existing structures on the formation of elliptical calderas in the Kenyan Rift

Calderas are formed by the collapse of large magma reservoirs and are commonly elliptical in map view. The orientation of elliptical calderas is often used as an indicator of the local stress regime; but, in some rift settings, pre-existing structural trends may also influence the orientation. We investigated whether either of these two mechanisms controls the orientation of calderas in the Kenyan Rift. Satellite-based mapping was used to identify the rift border faults, intra-rift faults and orientation of the calderas to measure the stress orientations and pre-existing structural trends and to determine the extensional regime at each volcano. We found that extension in northern Kenya is orthogonal, whereas that in southern Kenya is oblique. Elliptical calderas in northern Kenya are orientated NW–SE, aligned with pre-existing structures and perpendicular to recent rift faults. In southern Kenya, the calderas are aligned NE–SW and lie oblique to recent rift faults, but are aligned with pre-existing structures. We conclude that, in oblique continental rifts, pre-existing structures control the development of elongated magma reservoirs. Our results highlight the structural control of magmatism at different crustal levels, where pre-existing structures control the storage and orientation of deeper magma reservoirs and the local stress regime controls intra-rift faulting and shallow magmatism. Supplementary material: Details of the Standard Deviation Ellipse function and statistical methods are available at http://www.geolsoc.org.uk/SUP18849

Analogue experiments on the rise of large bubbles through a solids-rich suspension:A “weak plug” model for Strombolian eruptions

Physical interactions between bubbles and crystals affect gas migration and may play a major role in eruption dynamics of crystal-rich magmas. Strombolian eruptions represent an end member for bubble-crystal interactions, in which large bubbles (significantly larger than the crystal size) rise through a crystal-rich near-surface magma. Indeed, volcanoes that produce Strombolian eruptions often generate ejecta with > 30 vol% (often > 45 vol%) average crystallinity. At Stromboli Volcano, Italy, average crystallinity can reach 55 vol%, which is approaching the eruptibility limit for magmas. At such high crystallinities the solids interact mechanically with each other and with bubbles. This complex rheology complicates the two-phase (liquid-gas) slug flow model often applied to Strombolian eruptions. To examine the effect of crystals on bubble rise, we performed analogue experiments in which large bubbles rise in a vertical tube filled with silicone oil and polypropylene particles. The particles have a slightly lower density than the oil, and therefore form a layer of oil + particles at the upper surface. We varied surface pressure, particle volume fraction, length of the particle-bearing cap, and bubble size to examine the ways in which these parameters influence Strombolian-type eruptions. We show that in experiments, suspended solids begin to affect bubble rise dynamics at particle volume fractions as low as 30 vol% (or, when divided by the random close packing value, a normalized particle fraction φ=0.64). Bubbles in experiments with higher particle contents deform as they rise and burst through a small aperture, generating surface fountains that begin abruptly and decay slowly, and longer-lasting acoustic signals of lower amplitude than in particle-poor experiments. Particle fractions > 38 vol% (φ>0.80) generated strong deformations on fast-expanding bubbles that applied a high stress on the cap, but they trapped bubbles that were less overpressured. Qualitatively, the gas release behavior observed in particle-rich experiments is consistent with observations of Strombolian eruptions. Moreover, we estimate that the observed crystallinity of pyroclasts at Stromboli volcano represents φ>0.8. From this we suggest a “weak plug” model for Strombolian eruptions that evolves towards a low-viscosity equivalent of Vulcanian-style plug failure with a more crystalline, stronger, and less permeable plug. Importantly, this model allows the rise of several bubbles in the conduit at the same time and suggests that longer-lasting, more pulsatory and complex eruptions may reveal an increase in near-surface crystallinity, shedding some light on changing conduit conditions that could help determine the different gas rise regimes involved in passive degassing, puffing, and different expressions of Strombolian explosions

A little data goes a long way: automating seismic phase arrival picking at Nabro Volcano with transfer learning

Supervised deep learning models have become a popular choice for seismic phase arrival detection. However, they do not always perform well on out-of-distribution data and require large training sets to aid generalization and prevent overfitting. This can present issues when using these models in new monitoring settings. In this work, we develop a deep learning model for automating phase arrival detection at Nabro volcano using a limited amount of training data (2,498 event waveforms recorded over 35 days) through a process known as transfer learning. We use the feature extraction layers of an existing, extensively trained seismic phase picking model to form the base of a new all-convolutional model, which we call U-GPD. We demonstrate that transfer learning reduces overfitting and model error relative to training the same model from scratch, particularly for small training sets (e.g., 500 waveforms). The new U-GPD model achieves greater classification accuracy and smaller arrival time residuals than off-the-shelf applications of two existing, extensively-trained baseline models for a test set of 800 event and noise waveforms from Nabro volcano. When applied to 14 months of continuous Nabro data, the new U-GPD model detects 31,387 events with at least four P-wave arrivals and one S-wave arrival, which is more than the original base model (26,808 events) and our existing manual catalog (2,926 events), with smaller location errors. The new model is also more efficient when applied as a sliding window, processing 14 months of data from seven stations in less than 4 h on a single graphics processing unit

Similarities and differences in the historical records of lava dome-building volcanoes: implications for understanding magmatic processes and eruption forecasting

A key question for volcanic hazard assessment is the extent to which information can be exchanged between volcanoes. This question is particularly pertinent to hazard forecasting for dome-building volcanoes, where effusive activity may persist for years to decades, and may be punctuated by periods of repose, and sudden explosive activity. Here we review historical eruptive activity of fifteen lava dome-building volcanoes over the past two centuries, with the goal of creating a hierarchy of exchangeable (i.e., similar) behaviours. Eruptive behaviour is classified using empirical observations that include patterns of SO2 flux, eruption style, and magma composition. We identify two eruptive regimes: (i) an episodic regime where eruptions are much shorter than intervening periods of repose, and degassing is temporally correlated with lava effusion; and (ii) a persistent regime where eruptions are comparable in length to periods of repose and gas emissions do not correlate with eruption rates. A corollary to these two eruptive regimes is that there are also two different types of repose: (i) inter-eruptive repose separates episodic eruptions, and is characterised by negligible gas emissions and (ii) intra-eruptive repose is observed in persistently active volcanoes, and is characterised by continuous gas emissions. We suggest that these different patterns of can be used to infer vertical connectivity within mush-dominated magmatic systems. We also note that our recognition of two different types of repose raises questions about traditional definitions of historical volcanism as a point process. This is important, because the ontology of eruptive activity (that is, the definition of volcanic activity in time) influences both analysis of volcanic data and, by extension, interpretations of magmatic processes. Our analysis suggests that one identifying exchangeable traits or behaviours provides a starting point for developing robust ontologies of volcanic activity. Moreover, by linking eruptive regimes to conceptual models of magmatic processes, we illustrate a path towards developing a conceptual framework not only for comparing data between different volcanoes but also for improving forecasts of eruptive activity

Crystal controls on permeability development and degassing in basaltic andesite magma

Understanding degassing of mafic magmas is important for modeling eruptions and examining controls on eruption style. We conducted high-pressure–high-temperature isothermal decompression experiments to investigate the effects of decompression-induced crystallization on permeability development and magma degassing. Experiments were performed on hydrous basaltic andesite (54 wt% SiO2) decompression rates equivalent to magma ascent velocities of ∼1–3 m s–1. We measured the gas flux of the quenched samples using a bench-top permeameter and calculated the Darcian (k1) and inertial (k2) permeabilities using the Forchheimer equation. The experimental samples developed permeability at a critical vesicularity (Φc) of 56.4 ± 2.7 vol% (at 0.125 MPa s–1) and 50.76 ± 5.6 vol% (at 0.083 MPa s–1), considerably lower than the Φc > 63 vol% permeability threshold determined for crystal-free basaltic andesite melts. The percolation threshold decrease is observed when the microlites comprise ≥∼20 vol% and can be explained by the onset of yield strength, which occurs when the crystals form a loosely packed, touching framework

Historical analysis and stratigraphy of the post-1534 ad pyroclastic deposits at Cotopaxi Volcano, Ecuador: implications for lahar hazard assessment

Abstract, COV4 2006, Quito

Compositional evolution of magma from Parícutin Volcano, Mexico: The tephra record

The birth of Parícutin Volcano, Mexico, in 1943 provides an unprecedented opportunity to document the development of a monogenetic cinder cone and its associated lava flows and tephra blanket. Three ‘type' sections provide a complete tephra record for the eruption, which is placed in a temporal framework by comparing both bulk tephra and olivine phenocryst compositions to dated samples of lava and tephra. Our data support the hypothesis of Luhr (2001) that the first four months of activity were fed by a magma batch (Phase 1) that was distinct from the magma that supplied the subsequent eight years of activity. We further suggest that the earliest erupted (vanguard) magma records evidence of temporary residence at shallow levels prior to eruption, suggesting early development of a dike and sill complex beneath the vent. Depletion of this early batch led to diminished eruptive activity in June and July of 1943, while arrival of the second magma batch (Phase 2) reinvigorated activity in late July. Phase 2 fed explosive activity from mid-1943 through 1946, although most of the tephra was deposited by the end of 1945. Phase 3 of the eruption began in mid-1947 with rapid evolution of magma compositions from basaltic andesite to andesite and dominance of lava effusion. The combined physical and chemical characteristics of the erupted material present a new interpretation of the physical conditions that led to compositional evolution of the magma. We believe that syn-eruptive assimilation of wall rock in a shallow complex of dikes and sills is more likely than pre-eruptive assimilation within a large magma chamber, as previously assumed. We further suggest that waning rates of magma supply from the deep feeder system allowed evolved, shallowly stored magma to enter the conduit in 1947, thus triggering the rapid observed change in the erupted magma composition. This physical model predicts that assimilation should be observable in other monogenetic eruptions, particularly those with low pressure melt inclusions and with eruption durations of months to years