Models of ice melting and edifice growth during subglacial basaltic eruptions.

Models of the early stages of basaltic eruptions beneath temperate glaciers are presented that consider the evolving sizes of volcanic edifices emplaced within subglacial cavities. The cavity size reflects the competing effects of enlargement by melting and closure by downward ductile deformation of the ice roof, which occurs when the cavity pressure is less than glaciostatic due to meltwater drainage. Eruptions of basaltic magma from fissures and point sources are considered, which form either hemicylindrical or hemispherical cavities. The rate of roof closure can therefore be estimated using Nye's law. The cavity size, edifice size, and depth of meltwater above the edifice are predicted by the model and are used to identify two potential eruption mechanisms: explosive and intrusive. When the cavity is considerably larger than the edifice, hydroclastic fragmentation is possible via explosive eruptions, with deposition of tephra by eruption-fed aqueous density currents. When the edifice completely fills the cavity, rising magma is likely to quench within waterlogged tephra in a predominantly intrusive manner. The models were run for a range of magma discharge rates, ice thicknesses and cavity pressures relevant to subglacial volcanism in Iceland. Explosive eruptions occur at high magma discharge rates, when there is insufficient time for significant roof closure. The models correctly predict the style of historic and Pleistocene subglacial fissure eruptions in Iceland and are used to explain the contrasting sedimentology of basaltic and rhyolitic tuyas. The models also point to new ways of unraveling the complex coupling between eruption mechanisms and glacier response during subglacial eruptions

How will melting of ice affect volcanic hazards in the 21st century?

Glaciers and ice sheets on many active volcanoes are rapidly receding. There is compelling evidence that melting of ice during the last deglaciation triggered a dramatic acceleration in volcanic activity. Will melting of ice this century, which is associated with climate change, similarly affect volcanic activity and associated hazards? This paper provides a critical overview of the evidence that current melting of ice will increase the frequency or size of hazardous volcanic eruptions. Many aspects of the link between ice recession and accelerated volcanic activity remain poorly understood. Key questions include how rapidly volcanic systems react to melting of ice, whether volcanoes are sensitive to small changes in ice thickness, and how recession of ice affects the generation, storage and eruption of magma at stratovolcanoes. A greater frequency of collapse events at glaciated stratovolcanoes can be expected in the near future, and there is strong potential for positive feedbacks between melting of ice and enhanced volcanism. Nonetheless, much further research is required to remove current uncertainties about the implications of climate change for volcanic hazards in the 21st century

Subglacial rhyolite volcanism at Torfajökull, Iceland

Subglacial rhyolite eruptions at Torfajökull, Iceland have produced a variety of volcanic edifices during the last glacial period (115-11 ka). These range from small-volume (3) volcanoes, such as Bláhnúkur and Dalakvíslfell, to larger volume (~1 km3) flat-topped tuyas such as South-east Rauđufossafjöll. Lithofacies associations at each volcano record distinct phases of volcano-ice interaction beneath temperate glaciers at least 350 m thick. All eruptions began with the construction of a pile of glassy fragmental material within a subglacial cavity. Fragmentation at Bláhnúkur was primarily caused by quenching, when rising magma encountered meltwater. Fragmentation at Southeast Rauđufossafjöll was apparently more energetic, and generated phreatomagmatic ash over 300 m thick. Dalakvíslfell is intermediate between the other two localities. Most fragmental deposits are massive, suggesting that a sustained meltwater lake did not develop during eruptions, in contrast with evidence from many basaltic volcanoes. Instead, meltwater drained away in a number of discrete channels, some of which have been identified. The eruption at Bláhnúkur apparently terminated before the glacier surface had been pierced, whereas the eruption at South-east Rauđufossafjölll produced a cap of flat-lying subaerial lava flows about 1.5 km in length. Numerical models are presented, in which simple patterns of ice melting and deformation are used to simulate the evolving size of subglacial cavities during eruptions. The radius of the cavity is compared to the radius of the growing subglacial volcano. The models predict that, at low magma discharge rates and beneath thick ice, cavities will become completely filled with volcanic debris and the eruption will be dominantly intrusive, forming the types of lithologies observed at Bláhnúkur. Cavities never become filled at higher magma discharge rates, and an explosive phreatomagmatic eruption is predicted, which would form the types of lithologies observed at South-east Rauđufossafjöll

Subglacial rhyolite volcanism at Torfajökull, Iceland

Subglacial rhyolite eruptions at Torfajökull, Iceland have produced a variety of volcanic edifices during the last glacial period (115-11 ka). These range from small-volume (3) volcanoes, such as Bláhnúkur and Dalakvíslfell, to larger volume (~1 km3) flat-topped tuyas such as South-east Rauđufossafjöll. Lithofacies associations at each volcano record distinct phases of volcano-ice interaction beneath temperate glaciers at least 350 m thick. All eruptions began with the construction of a pile of glassy fragmental material within a subglacial cavity. Fragmentation at Bláhnúkur was primarily caused by quenching, when rising magma encountered meltwater. Fragmentation at Southeast Rauđufossafjöll was apparently more energetic, and generated phreatomagmatic ash over 300 m thick. Dalakvíslfell is intermediate between the other two localities. Most fragmental deposits are massive, suggesting that a sustained meltwater lake did not develop during eruptions, in contrast with evidence from many basaltic volcanoes. Instead, meltwater drained away in a number of discrete channels, some of which have been identified. The eruption at Bláhnúkur apparently terminated before the glacier surface had been pierced, whereas the eruption at South-east Rauđufossafjölll produced a cap of flat-lying subaerial lava flows about 1.5 km in length. Numerical models are presented, in which simple patterns of ice melting and deformation are used to simulate the evolving size of subglacial cavities during eruptions. The radius of the cavity is compared to the radius of the growing subglacial volcano. The models predict that, at low magma discharge rates and beneath thick ice, cavities will become completely filled with volcanic debris and the eruption will be dominantly intrusive, forming the types of lithologies observed at Bláhnúkur. Cavities never become filled at higher magma discharge rates, and an explosive phreatomagmatic eruption is predicted, which would form the types of lithologies observed at South-east Rauđufossafjöll

Examining rhyolite lava flow dynamics through photo-based 3D reconstructions of the 2011–2012 lava flowfield at Cordón-Caulle, Chile

During the 2011–2012 eruption at Cordón-Caulle, Chile, an extensive rhyolitic flowfield was created (in excess of 0.5 km3 in volume), affording a unique opportunity to characterise rhyolitic lava advance. In 2012 and 2013, we acquired approximately 2500 digital photographs of active flowfronts on the north and east of the flowfield. These images were processed into three-dimensional point clouds using structure-from-motion and multi-view stereo (SfM–MVS) freeware, from which digital elevation models were derived. Sequential elevation models—separated by intervals of three hours, six days, and one year—were used to reconstruct spatial distributions of lava velocity and depth, and estimate rheological parameters. Three-dimensional reconstructions of flowfronts indicate that lateral extension of the rubbly, 'a'ā-like flowfield was accompanied by vertical inflation, which differed both spatially and temporally as a function of the underlying topography and localised supply of lava beneath the cooled upper carapace. Compressive processes also drove the formation of extensive surface ridges across the flowfield. Continued evolution of the flowfield resulted in the development of a compound flowfield morphology fed by iterative emplacement of breakout lobes. The thermal evolution of flow units was modelled using a one-dimensional finite difference method, which indicated prolonged residence of magma above its glass transition across the flowfield. We compare the estimated apparent viscosity (1.21–4.03 × 1010 Pa s) of a breakout lobe, based on its advance rate over a known slope, with plausible lava viscosities from published non-Arrhenian temperature–viscosity models and accounting for crystallinity (~ 50 vol.%). There is an excellent correspondence between viscosity estimates when the lava temperature is taken to be magmatic, despite the breakout being located > 3 km from the vent, and advancing approximately nine months after vent effusion ceased. This indicates the remarkably effective insulation of the lava flow interior, providing scope for significant evolution of rhyolitic flow fields long after effusive activity has ceased

Examining rhyolite lava flow dynamics through photo-based 3D reconstructions of the 2011–2012 lava flowfield at Cordón-Caulle, Chile

During the 2011–2012 eruption at Cordón-Caulle, Chile, an extensive rhyolitic flowfield was created (in excess of 0.5 km3 in volume), affording a unique opportunity to characterise rhyolitic lava advance. In 2012 and 2013, we acquired approximately 2500 digital photographs of active flowfronts on the north and east of the flowfield. These images were processed into three-dimensional point clouds using structure-from-motion and multi-view stereo (SfM–MVS) freeware, from which digital elevation models were derived. Sequential elevation models—separated by intervals of three hours, six days, and one year—were used to reconstruct spatial distributions of lava velocity and depth, and estimate rheological parameters. Three-dimensional reconstructions of flowfronts indicate that lateral extension of the rubbly, 'a'ā-like flowfield was accompanied by vertical inflation, which differed both spatially and temporally as a function of the underlying topography and localised supply of lava beneath the cooled upper carapace. Compressive processes also drove the formation of extensive surface ridges across the flowfield. Continued evolution of the flowfield resulted in the development of a compound flowfield morphology fed by iterative emplacement of breakout lobes. The thermal evolution of flow units was modelled using a one-dimensional finite difference method, which indicated prolonged residence of magma above its glass transition across the flowfield. We compare the estimated apparent viscosity (1.21–4.03 × 1010 Pa s) of a breakout lobe, based on its advance rate over a known slope, with plausible lava viscosities from published non-Arrhenian temperature–viscosity models and accounting for crystallinity (~ 50 vol.%). There is an excellent correspondence between viscosity estimates when the lava temperature is taken to be magmatic, despite the breakout being located > 3 km from the vent, and advancing approximately nine months after vent effusion ceased. This indicates the remarkably effective insulation of the lava flow interior, providing scope for significant evolution of rhyolitic flow fields long after effusive activity has ceased

Geochemical constraints on the role of tuffisite veins in degassing at the 2008–09 Chaitén and 2011–12 Cordón Caulle rhyolite eruptions

Hybrid activity during the rhyolitic eruptions of Chaitén (2008-09) and Cordón Caulle (2011-2012) in Chile has offered unprecedented insights into the enigmatic and complex degassing processes occurring during eruptions of silicic magma. Highly permeable, transient fracture networks within the conduit can act as outgassing channels. Their interaction with deeper volatile-rich melt can account for both punctuated explosive activity and large-scale degassing of the system, leading towards predominantly effusive behaviour. In this study we characterise trace element concentrations and 210Pb-226Ra systematics within pyroclastic material from the recent eruptions at Chaitén and Cordón Caulle volcanoes. Results reveal how gas fluxing from deep, volatile-rich reservoirs to the surface, within magmatic conduits, can be recorded by trace elements and 210Pb-226Ra disequilibria in tuffisite veins. Tuffisite veins (particle-filled fracture networks) are present in volcanic bombs from both eruptions. Trace element heterogeneity associated with tuffisites preserves evidence for degassing. At Chaitén, enrichments (e.g. Cu) and depletions (e.g. Mo, Li and Bi) are identified in vein material and clasts transported within veins, and record multiple degassing events. At Cordón Caulle, enrichments of volatiles in an early vein (e.g. Tl and Bi) and depletions in a later vein (e.g. Cd, In, Pb and Tl) reflect interactions between glassy clasts and the carrier gas phase that transported them. In contrast, 210Pb and 226Ra, which can be fractionated during degassing, are mostly in secular equilibrium. Modelling suggests that the disparity between the signals preserved in these two types of chemical signatures reflects the brevity of degassing events and the relative volumes of tuffisite veins and the bodies of degassing magma that they source gas from. The lack of preserved 210Pb enrichments in tuffisite veins at both volcanoes places an upper limit on the mass of deeper, bubble-rich magma outgassed via tuffisites during their lifetime. This study shows that both the presence, and absence, of sample-scale geochemical heterogeneity can be used to place constraints on syn-eruptive physical processes and underlines the value of analysing a wide suite of trace element species

Emplacing a cooling-limited rhyolite lava flow: similarities with basaltic lava flows

Accurate forecasts of lava flow length rely on estimates of eruption andmagma properties and, potentially more challengingly, on an understanding of the relative influence of characteristics such as the apparent viscosity, the yield strength of the flow core, or the strength of the lava’s surface crust. For basaltic lavas, the relatively high frequency of eruptions has resulted in numerous opportunities to test emplacement models on such low silica lava flows. However, the flow of high silica lava is much less well understood due to the paucity of contemporary events and, if observations of flow length change are used to constrain straightforward models of lava advance, remaining uncertainties can limit the insight gained. Here, for the first time, we incorporatemorphological observations from during and after flow field evolution to improve model constraints and reduce uncertainties. After demonstrating the approach on a basaltic lava flow (Mt. Etna 2001), we apply it to the 2011–2012 Cordón Caulle rhyolite lava flow, where unprecedented observations and syn-emplacement satellite imagery of an advancing silica-rich lava flow have indicated an important influence from the lava flow’s crust on flow emplacement. Our results show that an initial phase of viscosity-controlled advance at Cordón Caulle was followed by later crustal control, accompanied by formation of flow surface folds and large-scale crustal fractures. Where the lava was unconstrained by topography, the cooled crust ultimately halted advance of the main flow and led to the formation of breakouts from the flow front and margins, influencing the footprint of the lava, its advance rate, and the duration of flow advance. Highly similar behavior occurred in the 2001 Etna basaltic lava flow. In our comparison of these two cases, we find close similarities between the processes controlling the advance of a crystal-poor rhyolite and a basaltic lava flow, suggesting common controlling mechanisms that transcend the profound rheological and compositional differences of the lavas

Basalt, Unveiling Fluid-filled Fractures, Inducing Sediment Intra-void Transport, Ephemerally:Examples from Katla 1918

This article documents textures within basaltic Katla 1918 pyroclasts, where particle-filled fractures and bubbles have been observed. These features are analogous to tuffisite veins; particle-filled fractures which represent the preserved remains of transient degassing pathways in shallow conduits. Such fractures have long been considered restricted to high viscosity silicic melts. However, through BSE images and compositional maps, we have identified similar tuffisite-like features in crystal-poor basalt pyroclasts from the 1918 CE subglacial eruption of Katla, Iceland (K1918). Clast textures record transient mobility of juvenile/lithic particles, melt droplets and gas through magmatic fractures and connected vesicles. Key evidence includes (1) the presence of variably sintered fine-ash particles within variably healed fractures and vesicles (present in >80% of clasts analysed), (2) compositional maps that reveal the presence of foreign particles within preserved and healed permeable pathways, and (3) lower vesicularities immediately surrounding ‘fracture’ walls, suggestive of diffusive volatile loss into a permeable network. The 1918 CE eruption of Katla occurred under a thick glacier, however the ice was quickly breached, owing partly to the explosive nature of the eruption. We propose that the formation and preservation of these transient permeable networks have been facilitated by rapid decompression of a relatively volatile-rich magma, in a confined subglacial environment, with combined magmatic and phreatomagmatic fragmentation, followed by rapid quenching by meltwater. Tuffisite veins in rhyolite demonstrate repeated fracture-healing cycles, which drive incremental release of overpressured gas and help to defuse explosive eruptions. Interestingly, the permeable network at Katla failed to defuse the 1918 CE eruption, which involved a particularly violent subglacial eruptive phase. It is unclear whether this demonstrates an inability of mafic tuffisite-like features to efficiently degas magma (perhaps owing to the especially transient nature of permeable pathways in low viscosity magmas) or an ability to enhance fragmentation by providing infiltration pathways for external water. The latter scenario may explain the rapid melting of the overlying glacier as the large surface area-to-volume ratio of fractured magma would allow rapid heat transfer. Nevertheless, we document a previously unrecognised texture in basaltic magmas. It is intriguing why it has not, to the best of our knowledge, been documented elsewhere. Have these permeable pathways been overlooked in the past (e.g. mistaken for bad sample preparation or not noticed without high magnification BSE images) and are in fact a widespread phenomenon in subglacial (and other?) basalts; or do our samples in fact represent a rarely preserved texture? Either way, they offer a new insight into the degassing and fragmentation of subglacial basalt