Conduit Margins Research Articles

Plug flow and brecciation in volcanic conduits can emerge from shear-induced crystal migration in otherwise Newtonian magmas

In the Earth, the flow of crystal-bearing magma is thought to be non-Newtonian and shear thinning, but the physical origin for this is poorly understood. We use hydro-granular theory to show that the decoupled migration of crystals toward conduit cores during magma ascent is a tenable microphysical mechanism for plug flow, emergent in an otherwise purely Newtonian crystal-bearing magma. We use a numerical conduit model to define the flow development length beyond which crystal migration dominates and strain localises near conduit margins. Applied to magma ascent scenarios, we show that this crystal-migration strain localisation only develops in high crystallinity magmas or magmas ascending in very narrow cracks/conduits. In all other scenarios, crystals do not contribute to non-Newtonian behaviour and such magmas are usually strictly Newtonian. The ascent of very crystal-rich dome magma could be associated with strain localisation and crystal depletion at the conduit margins, lubricating ascent through the crust.

Evidence for the formation of silicic lava by pyroclast sintering

Silicic lavas can be produced by the sintering of pyroclasts in the volcanic sub-surface, and then advected out of the vent. Here, we provide evidence for this mechanism preserved in the exposed post-glacial remnants of a silicic volcanic conduit at Hrafntinnuhryggur, Krafla volcano, Iceland. We show that the conduit margins are a clast-supported pumice lapilli tuff deposit that grades continuously into dense obsidian and that the obsidian contains cuspate relict clast boundaries and country rock lithic fragments throughout. Transects of H2O concentrations across the conduit show that the magma was degassed to different degrees laterally with systematic spatial variation that is consistent with progressive conduit clogging and final gas pressurisation. Textures in the overlying effusive lavas record the variably sheared and brecciated remnant of the same in-conduit sintering. This record of a silicic conduit system connected to upper eruptive deposits provides support for the ‘cryptic fragmentation model’ for effusive silicic volcanism.

The exposed Mule Creek vent deposits record the structure of a volcanic conduit during a hybrid explosive–effusive eruption

Silicic volcanic eruptions commonly begin with the explosive ejection of pyroclastic material, before transitioning to gentler effusion-dominated activity. Well-exposed dissected silicic systems are scarce and poorly studied, hindering the advances in our understanding of the explosive–effusive transition needed to improve interpretations of volcanic unrest and hazard forecasting. The Mule Creek vent (New Mexico, USA) is a dissected silicic conduit that records the processes controlling conduit formation and evolution, and the role tuffisites (fractures filled with variably welded pyroclasts) play in conduit dynamics. Here, we use decimeter-scale photo-mapping of lithostratigraphic units and thin section analysis to differentiate and interpret three dominant emplacement styles during vent evolution. First, there was repeated deposition and erosion of pyroclastic material at the conduit walls, recorded by erosive surfaces in pyroclastic breccia and agglomerates at the conduit margins. Second, sub-vertical domains of dense melt-dominated magma were emplaced and preserved as glass-dominated vitrophyre and brecciated vitrophyre, with the textural hallmarks of assembly from welding of pyroclasts. Finally, the sub-horizontal fracturing of previously deposited lithologies produced laterally cross-cutting tuffisites. The vent deposits track the widening and then narrowing of the conduit through time and reflect progressive insulation and generally higher temperatures towards the conduit center as pyroclasts accumulate. Welding of pyroclastic fill and the formation of dense vitrophyres towards the conduit center lowers deposit porosity and effective wall permeability. This drives localized gas pressure increases and results in gas-driven fracturing, generating tuffisites, which act as transient outgassing pathways. The structure of the Mule Creek vent records an explosive–effusive transition, constraining the processes controlling conduit evolution and aiding our interpretation of volcanic unrest.

V01-08 ROBOTIC APPENDICEAL ONLAY FOR THE TREATMENT OF URETEROENTERIC STRICTURE

You have accessJournal of UrologyCME1 Apr 2023V01-08 ROBOTIC APPENDICEAL ONLAY FOR THE TREATMENT OF URETEROENTERIC STRICTURE Joseph J. Boyle, Ellen Cahill, Sai Krishnaraya Doppalapudi, and Sammy E. Elsamra Joseph J. BoyleJoseph J. Boyle More articles by this author , Ellen CahillEllen Cahill More articles by this author , Sai Krishnaraya DoppalapudiSai Krishnaraya Doppalapudi More articles by this author , and Sammy E. ElsamraSammy E. Elsamra More articles by this author View All Author Informationhttps://doi.org/10.1097/JU.0000000000003221.08AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookLinked InTwitterEmail Abstract INTRODUCTION AND OBJECTIVE: Ureteroenteric anastomotic strictures (UES) are the most common complication requiring repeat surgery following radical cystectomy, occuring in up to 10% of patients. Untreated, they may result in loss of renal function and predispose patients to infection. Open revision of the stricture has been the gold standard, but is technically challenging. Minimally invasive surgical approaches (MIS) in stricture disease can afford greater visibility and maneuverability, while minimizing blood loss. Options for extended strictures or those located in the proximal or midureter have been historically limited to ileal ureter replacement or autologous kidney transplant, both having considerable morbidity. Onlay techniques utilizing buccal mucosa or appendix are options to repair rather than replace longer ureteral strictures. Appendiceal onlay flaps are an encouraging yet relatively under-described alternative for select patients in this group. In this video, we present a case of a 6 cm proximal right UES treated with robotic appendiceal onlay flap repair. METHODS: Procedure performed with Da Vinci Xi system. Patient placed in left lateral decubitus position. Veress needle technique used to insufflate the abdomen. Ports placed in standard position for robotic laparoscopic renal surgery, ensuring placement outside conduit margins. Right ureterolysis performed. 5-French ureteral catheter placed over a wire positioned through previously placed ureteral stent. Ureteroenteric anastomosis identified, with ureteral detubularization from anastomosis to just above stricture with a length of 6 cm. Increased ureteral caliber and vigorous saline efflux marked proximal stricture end. Appendix stapled at cecum and detubularized with preserving mesentery. Appendiceal flap sewn to detubularized segment of ureter. Intraoperative ureteroscopy demonstrated patent repair. JJ stent placed. RESULTS: No intraoperative complications. Case duration was 200 minutes. EBL was 25 mL. He was discharged POD 2 after an uncomplicated post-operative stay. Stent removed at 6 weeks. 3-month ultrasound and renal scan showed no obstruction. He was seen every 6 months with no signs or symptoms of UTI, flank pain, or hydronephrosis on ultrasound. Latest follow-up at 16 months. He reported resolution of chronic malodorous urine. CONCLUSIONS: UES remain a challenge following radical cystectomy. Robotic repair with appendiceal onlay is a safe, durable, and often overlooked treatment option. Source of Funding: None © 2023 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 209Issue Supplement 4April 2023Page: e81 Advertisement Copyright & Permissions© 2023 by American Urological Association Education and Research, Inc.MetricsAuthor Information Joseph J. Boyle More articles by this author Ellen Cahill More articles by this author Sai Krishnaraya Doppalapudi More articles by this author Sammy E. Elsamra More articles by this author Expand All Advertisement PDF downloadLoading ...

Autobrecciation and fusing of mafic magma preceding explosive eruptions

Abstract Bubble and crystal textures evolve during magma ascent, altering properties that control ascent such as permeability and viscosity. Eruption style results from feedbacks between ascent, bubble nucleation and growth, microlite crystallization, and gas loss, all processes recorded in pyroclasts. We show that pyroclasts of the mafic Curacautín ignimbrite of Llaima volcano, Chile, record a history of repeated autobrecciation, fusing, and crystallization. We identified pyroclasts with domains of heterogeneous vesicle textures in sharp contact with one another that are overprinted by extensive microlite crystallization. Broken crystals with long axes (l) >10 μm record fragmentation events during the eruption. A second population of unbroken microlites with l ≤10 μm overprint sutures between fused domains, suggesting the highly crystalline groundmass formed at shallow depths after autobrecciation and fusing. Nearly all pyroclasts contain plutonic and ancestral Llaima lithics as inclusions, implying that fusing occurs from a few kilometers depth to as shallow as the surface. We propose that Curacautín ignimbrite magma autobrecciated during ascent and proto-pyroclasts remained melt rich enough to fuse together. Lithics from the conduit margins were entrained into the proto-pyroclasts before fusing. Autobrecciation broke existing phenocrysts and microlites; rapid post-fusing crystallization then generated the highly crystalline groundmass. This proposed conduit process has implications for interpreting the products of mafic explosive eruptions.

A Petrologic Insight into Transitioning Eruption Styles from the Devil’s Rock Region, Ambae, Vanuatu

AbstractAmbae Island is the largest volcano in the New Hebrides Arc with recent eruptive activity occurring primarily at the summit and along the island’s rift zone. The Devil’s Rock area forms a prominent outcrop on the SW coast. Eruptive deposits here are derived from both strombolian and phreatomagmatic eruptions that contain a similar olivine- and clinopyroxene-rich juvenile basaltic component. This study focuses on a particular transition from strombolian to phreatomagmatic activity to understand if the change in eruption style is a function of magmatic processes or properties (e.g. different composition, ascent rate, degassing history) or if it is driven purely by external factors (e.g. magma–water interaction and/or vent migration). Melts from the strombolian to phreatomagmatic phase record the same melt compositions and volatile contents, suggesting the same magma batch is involved throughout the eruption. More broadly, similarities in H2O, CO2 and S concentrations between olivine- and pyroxene-hosted melt inclusions from Devil’s Rock melt inclusions and those erupted during the 2017–2018 summit eruptions may indicate that a long-term shared magmatic reservoir exists beneath Ambae. Physical characteristics of juvenile tephra including groundmass crystallinity and porosity are combined with melt inclusion compositions to better understand the degassing and crystallisation history and melt evolution of this volcanic system across the transitioning eruptive sequence. Groundmass crystallisation is variable and negatively correlated with connected porosity of erupted scoria reflecting mixing of materials at the vent and inclusion of dense clasts from conduit margins. A direct comparison of crystallinities between strombolian and phreatomagmatic phases reveals higher crystallinity in the strombolian deposits, which is reflective of post-fragmentation crystallisation of clasts. This is particularly evident in the proximal strombolian materials. Qualitative crystallisation textures of melt inclusions are used in a similar fashion to groundmass crystallinities to assess the relative timing of cooling. These trends mirror those of the groundmass and suggest longer cooling times and more effective degassing for samples of the transitional materials. Based on our analysis of deposits at Devil’s Rock, the transition from a strombolian to a phreatomagmatic eruption style was likely driven by groundwater or seawater incursion into the shallow conduit, close to modern-day sea level. Overall, these results suggest a dynamic system where different magmatic cooling histories for strombolian versus phreatomagmatic eruptive phases are reflected in changing groundmass crystallinity. This highlights the propensity for transitions in eruption style over seemingly short time intervals and significantly enhancing eruption explosivity.

Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

Abstract. The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2 m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ∼ 1 m wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20 cm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3 m long by ∼ 9 cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with permeability enhanced by ∼ 3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at ∼ 500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Hot Sintering of Melts, Glasses and Magmas

Research Article| May 01, 2022 Hot Sintering of Melts, Glasses and Magmas Fabian B. Wadsworth; Fabian B. Wadsworth Department of Earth Sciences, Durham University, Durham, United Kingdom fabian.b.wadsworth@durham.ac.uk Search for other works by this author on: GSW Google Scholar Jérémie Vasseur; Jérémie Vasseur Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany j.vasseur@lmu.de Search for other works by this author on: GSW Google Scholar Edward W. Llewellin; Edward W. Llewellin Department of Earth Sciences, Durham University, Durham, United Kingdom ed.llewellin@durham.ac.uk Search for other works by this author on: GSW Google Scholar Donald B. Dingwell Donald B. Dingwell Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany dingwell@lmu.de Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2022) 87 (1): 801–840. https://doi.org/10.2138/rmg.2022.87.17 Article history first online: 01 May 2022 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Fabian B. Wadsworth, Jérémie Vasseur, Edward W. Llewellin, Donald B. Dingwell; Hot Sintering of Melts, Glasses and Magmas. Reviews in Mineralogy and Geochemistry 2022;; 87 (1): 801–840. doi: https://doi.org/10.2138/rmg.2022.87.17 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Magma may fragment as it ascends through the volcanic plumbing system to the Earth surface, forming volcanic ash and pyroclasts (Scheu and Dingwell 2022, this issue). If the pyroclastic products of fragmentation are deposited hot, they can stick, coalesce, and weld together, re-amalgamating into coherent magma. Subaerial deposits include welded, lava-like, and rheomorphic ignimbrites (Branney et al. 1992; Andrews and Branney 2011), and welded fall deposits proximal to an eruptive vent (Houghton and Carey 2015). Welding occurs in the sub-surface at conduit margins (Gardner et al. 2017; Wadsworth et al. 2020... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Syn-Eruptive Conditions of the AD 1530 Sub-Plinian Eruption of La Soufrière of Guadeloupe (Lesser Antilles)

A prerequisite in refining volcanic hazard at explosive volcanoes is a better quantification of the timescales of the syn-eruptive processes, such as magma degassing and crystallization prior to eruption. To this aim, new data on the matrices (microlites, residual glasses, and bubbles) of pumice, scoria, and dense clasts erupted during the AD 1530 andesitic eruption of La Soufrière of Guadeloupe are combined with published data from phase-equilibrium and kinetic experiments, in order to estimate pressures of microlite crystallization and magma ascent rates. From the timescale data, we infer that the AD 1530 eruption started with phreatomagmatic explosions tapping magmas that ascended during about 1 month (decompression rate of ∼50 Pa/s) from the coldest parts of the reservoir (∼825°C and a 74 wt% SiO2 melt). These magmas continuously crystallized microlites (∼25 vol% plagioclase, orthopyroxene, clinopyroxene, magnetite, quartz, and apatite), but did not outgas likely due to sealed conduit margins. The flank collapse (preexisting “cold” edifice) that followed the phreatomagmatic phase triggered a sub-Plinian eruption that progressively tapped the hotter main reservoir (∼875°C and 71 wt% SiO2 interstitial melt), emitting banded and homogeneous pumice. The banded pumice did not significantly outgas and mostly lack decompression-induced microlites, suggesting short ascent durations of the order of 0.5–1 day (decompression rates of 1,400–4,000 Pa/s). The following Strombolian phase emitted dark scoria that did not significantly outgas and only crystallized rare microlites, suggesting ascent duration of the order of 2 days (decompression rates of ∼550 Pa/s). The terminal lava dome growth involved fully outgassed magmas ascended during more than 1 month, giving time for microlite crystallization (∼40 vol% plagioclase, orthopyroxene, clinopyroxene, magnetite, and cristobalite). The detection of any shallow new magmatic intrusion is crucial, since it can trigger a sequence of conduit processes leading to an eruption marked by a succession of different and disastrous eruptive styles, following a scenario similar to the AD 1530 eruption. Overall, we provide a combined approach of petrological, geochemical, and experimental data that may be used to infer ascent conditions and rates at other volcanic systems.

Source models of long-period seismic events at Galeras volcano, Colombia

SUMMARY Long-period (LP) seismic events have occurred repeatedly at Galeras volcano, Colombia, during the transition from effusive dome formation to explosive Vulcanian eruptions. Since 1989, two types of LP events have been observed there: one characterized by long-lasting, decaying harmonic oscillations (NLP events) and the other by non-harmonic oscillatory features (BLP events). NLP events are attributed to resonances of a dusty gas-filled crack in the magma plugging the eruptive conduit. Sixteen episodes of NLP events occurred at Galeras during 1992–2010, each characterized by systematic temporal variations in the frequencies and quality factors of NLP events. Our and previous estimates of crack model parameters during three of those NLP episodes indicate that the similar temporal variations in crack geometry and fluid properties can be explained by an increase in the ash content within the crack and a decrease in crack volume. We found that NLP events, associated with low SO2 fluxes, are anticorrelated with BLP events, which are accompanied by high SO2 emissions. From our observations and analytical results, we inferred that BLP events are generated by resonances of open cracks in the uppermost magma plug, corresponding to tuffisite veins, that efficiently transfer volcanic gases. After sufficient degassing and densification, the magma plug effectively seals the conduit. The growing overpressure in the deeper magma is then released through a shear fracture along the conduit margin. The intrusion of deeper, vesiculated magma into the shear fracture depressurizes and fragments the magma, producing a dusty gas and triggering the crack resonances that generate NLP events. Our results thus indicate that the evolution of the properties of the magma plug controls the occurrences of BLP and NLP events at Galeras. Although NLP events do not always precede explosive eruptions, they indicate that an important overpressure is building in the shallow conduit.

Micro-porous pyroclasts reflecting multi-vent basaltic-andesite Plinian eruptions at Mt. Tongariro, New Zealand

Mt. Tongariro is a multi-vent volcanic complex in the central North Island of New Zealand, constructed along the axis of an active graben. Its five largest-known explosive eruption episodes occurred 11,000 years ago during a ~200 year-long paroxysm. During these, stable Plinian and collapsing sub-Plinian columns erupted from multiple sequential and synchronous vents along the graben axis. We examined the spatially varying eruption dynamics during the 200-year paroxysm through the lens of pyroclastic textures. Each eruption episode produced micro-vesicular basaltic-andesitic juvenile lapilli with high vesicle number densities and high groundmass crystallinity. Textural characteristics indicate that pressurisation via second boiling drove eruptions and rhyolite-like melt viscosities promoted brittle failure. Highly variable and mingled textures within individual juvenile lapilli evidence diverse conditions developed across the conduit during the eruption. Foamy magma rose rapidly through the conduit core, driving explosive vigour, and dragged and sheared gas-poor crystallised magma from the conduit margins. Consistent with the northward-widening graben, the most stable Plinian eruptions occurred at northern vents, under highest extensional stresses and possibly widest most stable conduits. By contrast, collapsing, unstable eruption columns occurred at southern vents, where mainly degassed and crystalline magma erupted at lower ejection rates through narrower, unstable and shifting conduits. These data collectively show how spatial variations in structure and tectonics may strongly influence explosive eruption style.

Feasibility and usability of real-time intraoperative quantitative fluorescent-guided perfusion assessment during resection of gastroesophageal junction cancer.

Anastomotic leakage after resection of gastroesophageal junction cancer is a dangerous complication, and leakage rates have remained stable for decades. Perfusion is crucial for anastomotic healing, but traditional perfusion assessment is limited in a minimally invasive environment. New methods as indocyanine green fluorescence angiography (ICG-FA) have proven promising, but quantitative analysis has been challenging. This study aimed to demonstrate the feasibility and usability of real-time intraoperative quantitative fluorescence angiography (q-ICG) with a touchscreen tablet. A software for q-ICG was previously developed and validated. Ten patients underwent perfusion assessment in white light (WL), with ICG-FA, and with q-ICG during Ivor-Lewis esophageal resection. The usability of the tablet-based software was tested with the System Usability Scale (SUS®). Furthermore, we investigated the differences in perfusion assessment as the distance from the conduit margin to a surgeon selected point of sufficient perfusion for anastomosis using the different modalities. Q-ICG was successful in all patients, with an excellent median SUS® of 82.5 (77.5-93.8). Significant differences in distances from the conduit margin to points of sufficient perfusion selected by the surgeons were found: ICG: WL = 14.1mm (p = 0.048), q-ICG: WL = 32.08mm (p < 0.001), and q-ICG: ICG = 17.95mm (p = 0.002). Furthermore, significant differences of perfusion were found between the points, when q-ICG was performed retrospectively in the surgeon selected areas (p = 0.008-0.013). Real-time intraoperative touchscreen-based q-ICG was feasible with excellent usability, and differences in sufficient perfusion points selected by the surgeons between modalities were found. Further studies should focus on clinical relevance and determine cutoff values associated with anastomotic leakage.

Caldera-forming eruptions of mushy magma modulated by feedbacks between ascent rate, gas retention/loss and bubble/crystal framework interaction

Caldera-forming eruptions of mushy silicic magma are among the most catastrophic natural events on Earth. In such magmas, crystals form an interlocking framework when their content reaches critical thresholds, resulting in the dramatic increase in viscous resistance to flow. Here, we propose a new mechanism for the ascent of mushy magma based on microstructural observations of crystal-rich silicic pumices and lavas from the Central Andes and decompression experiments. Microstructural data include spherical vesicles and jigsaw-puzzle association of broken crystals in pumices, whereas there is limited breakage of crystals in lavas. These observations insinuate that shearing of magma during ascent was limited. Decompression experiments reveal contrasting interaction between growing gas bubbles and the crystal framework in crystal-rich magma. Under slow decompression typical of effusive eruptions, gas extraction is promoted, whereas under rapid decompression, bubbles are retained and the crystal framework collapses. This feedback between decompression rate, retention of gas bubbles, and integrity of the crystal framework leads to strong non-linearity between magma decompression rate and eruption explosivity. We extend these findings to caldera-forming eruptions of crystal-rich magma where large overpressures are induced by caldera-collapse, resulting in magma plug-flow, rapid decompression facilitated by shear-localization at conduit margins, and explosive eruption.

Using microlites to gain insights into ascent conditions of differing styles of volcanism at Soufrière Hills Volcano

Microlite textural characteristics are analyzed in the products of Vulcanian explosions, ash venting occurring synchronously with lava effusion (syn-extrusive ash venting), a precursory explosion, and lava dome effusion associated with Phases 3, and 5 of the current Soufrière Hills Volcano (SHV) eruption. 2D microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism. In addition, magma ascent rates for each eruptive style were calculated using microlite number density (Toramaru et al. 2008).Strong differences between pyroclastic and effusive microlites are observed in 2D population statistics and crystal size distributions, indicating higher amounts of smaller microlites (<0.0054 mm) in the lava dome compared with pyroclastic products. Crystal size distributions show that the lava dome and Vulcanian explosion samples have similar amounts of large microlites (>0.0084 to 0.012 mm). However, syn-extrusive ash venting samples have comparatively greater amounts of large microlites. The similarity between lava dome and Vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system (>2 km depth). The contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at SHV. The greater amounts of large microlites in syn-extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit, inferred to result from differences in ascent rate. Sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn-extrusive ash venting compared with the lava dome.Calculated magma ascent rates for Vulcanian explosions and syn-extrusive ash venting have similar ranges, while lava effusion is up to a factor of 10 faster. Differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style. Ascent rate is calculated at microlite nucleation depth using the method of Toramaru et al. (2008). However, the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent. More small microlites in the lava dome suggest greater nucleation rates in the shallow conduit, recording an on average shallower magma ascent rate (<1 km depth). Rapid decompression during pyroclastic eruptions restricts shallow crystallization, suggesting the microlite population is on average recording the deeper conduit system (>2 km depth).

Petrological Architecture of a Magmatic Shear Zone: A Multidisciplinary Investigation of Strain Localisation During Magma Ascent at Unzen Volcano, Japan

Shearing of magma during ascent can promote strain localisation near the conduit margins. Any mechanical and thermal discontinuities associated with such events may alter the chemical, physical and rheological stability of the magma and thus its propensity to erupt. Lava spines can record such processes, preserving a range of macroscopic and microscopic deformation textures, attributed to shearing and friction, as magma ascends through the viscous-brittle transition. Here, we use a multidisciplinary approach combining petrology, microstructures, crystallography, magnetics and experimentation to assess the evidence, role and extent of shearing across a marginal shear zone of the 1994–1995 lava spine at Unzen volcano, Japan. Our results show that crystals can effectively monitor stress conditions during magma ascent, with viscous remobilisation, crystal plasticity and comminution all systematically increasing towards the spine margin. Accompanying this, we find an increase in mineral destabilisation in the form of pargasitic amphibole breakdown displaying textural variations across the shear zone, from symplectitic to granular rims towards the spine margin. In addition, the compaction of pores, chemical and textural alteration of interstitial glass and magnetic variations all change systematically with shear intensity. The strong correlation between the degree of shearing, crystal deformation and disequilibrium features, together with distinct magnetic properties, implies a localised thermal input due to shear and frictional processes near the conduit margin during magma ascent. This was accompanied by late-stage or post-emplacement fluid- and gas-induced alteration of the gouge, as well as oxidation and glass devitrification. Understanding and recognising evidence for strain localisation during magma ascent may, therefore, be vital when assessing factors that regulate the style of volcanic eruptions, which may provide insights into the cryptic shifts from effusive to explosive activity as observed at many active lava domes.

Hot pressing in conduit faults during lava dome extrusion: Insights from Mount St. Helens 2004–2008

Rhyodacitic volcanoes such as Mount St. Helens (MSH), Soufrière Hills, Mount Unzen and Mount Pelée erupt spines mantled by layers of magma-derived cataclasite and fault gouge. MSH produced seven lava spines from 2004–2008 composed of low-porosity, compositionally uniform, crystalline dacite. Dome extrusion was attended by continuous ‘drumbeat’ seismicity, derived from faulting along the conduit margin at 0.5–1 km depth, and evidenced by the enveloping gouge layers. We describe the properties of the gouge-derived fault rocks, including laboratory measurements of porosity and permeability. The gouge varies from unconsolidated powder to lithified low-porosity low-permeability fault rocks. We reconstruct the subsurface ascent of the MSH magma using published field observations and create a model that reconciles the diverse properties of the gouge with conditions in the conduit during ascent (i.e. velocity, temperature). We show lithification of the gouge to be driven by ‘hot pressing’ processes, wherein the combination of elevated temperature, confining pressure and dwell-time cause densification and solid-state sintering of the comminuted, crystal-rich (glass-poor) gouge. The degree of gouge lithification corresponds with residence time in the conduit such that well-lithified materials reflect extended times in the subsurface due to slower ascent rates. With this insight, we suggest that gouge competence can be used as a first-order estimate of lava ascent rates. Furthermore we posit gouge lithification, which reduces porosity and permeability, inhibits volcanic outgassing thereby increasing the potential for explosive events at spine-producing volcanoes.

Permeability Evolution in Variably Glassy Basaltic Andesites Measured Under Magmatic Conditions

AbstractHeat from inflowing magma may act to seal permeable networks that assist passive outgassing at volcanic conduit margins and in overlying domes, reducing the efficiency of overpressure dissipation. Here we present a study of the evolution of permeability—measured under magmatic conditions—with increasing temperature in glassy and glass‐poor basaltic andesites from Merapi volcano (Indonesia). Whereas the permeability of glass‐poor rocks decreases little up to a temperature of 1,010°C, glassy specimens experience a pronounced decrease in permeability above the glass transition once the viscosity of the crystal suspension is low enough to relax under external stresses. Changes in temperature alone are thus not enough to significantly modify the permeability of the glass‐poor rocks that commonly form Merapi's dome. However, the presence of glass‐rich domains in a dome may lead to local sealing of the volcanic plumbing between eruptions, exacerbating localized overpressure development that could contribute to explosivity.

Constraining the Magmatic System at Mount St. Helens (2004–2008) Using Bayesian Inversion With Physics‐Based Models Including Gas Escape and Crystallization

AbstractPhysics‐based models of volcanic eruptions track conduit processes as functions of depth and time. When used in inversions, these models permit integration of diverse geological and geophysical data sets to constrain important parameters of magmatic systems. We develop a 1‐D steady state conduit model for effusive eruptions including equilibrium crystallization and gas transport through the conduit and compare with the quasi‐steady dome growth phase of Mount St. Helens in 2005. Viscosity increase resulting from pressure‐dependent crystallization leads to a natural transition from viscous flow to frictional sliding on the conduit margin. Erupted mass flux depends strongly on wall rock and magma permeabilities due to their impact on magma density. Including both lateral and vertical gas transport reveals competing effects that produce nonmonotonic behavior in the mass flux when increasing magma permeability. Using this physics‐based model in a Bayesian inversion, we link data sets from Mount St. Helens such as extrusion flux and earthquake depths with petrological data to estimate unknown model parameters, including magma chamber pressure and water content, magma permeability constants, conduit radius, and friction along the conduit walls. Even with this relatively simple model and limited data, we obtain improved constraints on important model parameters. We find that the magma chamber had low (&lt;5 wt %) total volatiles and that the magma permeability scale is well constrained at ∼10−11.4m2 to reproduce observed dome rock porosities. Compared with previous results, higher magma overpressure and lower wall friction are required to compensate for increased viscous resistance while keeping extrusion rate at the observed value.

Time-dependent permeability evolution in compacting volcanic fracture systems and implications for gas overpressure

Volcanic eruptions are driven by the ascent of volatile-laden magma. The capacity of a volcano system to outgas these volatiles—its permeability—controls the explosive potential, and fractures at volcanic conduit margins play a crucial role in tempering eruption explosivity by acting as outgassing pathways. However, these fractures are often filled with hot volcanic debris that welds and compacts over time, meaning that these permeable pathways have a finite lifetime. While numerous studies emphasize that permeability evolution is important for regulating pressure in shallow volcanic systems, how and when this occurs remains an outstanding question in volcanology. In this contribution, we show that different pressure evolution regimes can be expected across a range of silicic systems as a function of the width and distribution of fractures in the system, the timescales over which they can outgas (a function of depth and temperature), and the permeability of the host material. We define outgassing, diffusive relaxation, and pressure increase regimes, which are distinguished by comparing the characteristic timescales over which they operate. Moreover, we define a critical permeability threshold, which determines (in concert with characteristic timescales of diffusive mass exchange between the pore and melt phases) whether systems fracture and outgas efficiently, or if a volcano will be prone to pressure increases, incomplete healing, and explosive failure.

Rapid crystallization during recycling of basaltic andesite tephra: timescales determined by reheating experiments

Microcrystalline inclusions within microlite-poor matrix are surprisingly common in low intensity eruptions around the world, yet their origin is poorly understood. Inclusions are commonly interpreted as evidence of crystallization along conduit margins. Alternatively, these clasts may be recycled from low level eruptions where they recrystallize by heating within the vent. We conducted a series of experiments heating basaltic andesite lapilli from temperatures below the glass transition (~690 °C) to above inferred eruption temperatures (>1150 °C) for durations of 2 to >60 minutes. At 690 °C < T < 800 °C, crystallization is evident after heating for ~20 minutes; at T > 800 °C, crystallization occurs in <5 minutes. At T ≥ 900 °C, all samples recrystallize extensively in 2–10 minutes, with pyroxenes, Fe-oxides, and plagioclase. Experimental crystallization textures closely resemble those observed in natural microcrystalline inclusions. Comparison of inclusion textures in lapilli from the active submarine volcano NW Rota-1, Mariana arc and subaerial volcano Stromboli suggest that characteristic signatures of clast recycling are different in the two environments. Specifically, chlorine assimilation provides key evidence of recycling in submarine samples, while bands of oxides bordering microcrystalline inclusions are unique to subaerial environments. Correct identification of recycling at basaltic vents will improve (lower) estimates of mass eruption rate and help to refine interpretations of eruption dynamics.