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Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at ∼ 700 K, at a rate of between 7 and 80 K.min−1. We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved concentration of wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved concentration of wt%. Our results show that at a storage temperature of 1123–1193 K, the viscosity of the stored magma is ∼ 3×105 Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to ∼ 2×109 Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface.

Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at similar to 700 K, at a rate of between 7 and 80 K.min(-1). We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved H2O concentration of 2.12 wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved H2O concentration of 0.12 wt%. Our results show that at a storage temperature of 1123-1193 K, the viscosity of the stored magma is similar to 3x10(5) Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to similar to 2x10(9) Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface.

Geothermal exploration targets large magmatic intrusions as heat sources because of their size, longevity, and amount of stored energy, but as shallow volcanic plumbing systems comprise numerous smaller intrusions, their geothermal potential warrants consideration. Here, we evaluate the geothermal impact of dykes and sills on caldera-infill rocks. We present geological data and geothermometry on intrusions in the eroded Breiðuvík caldera in Northeast Iceland, which serves as an analogue to the active, and geothermally exploited, Krafla volcano. These data inform 2D finite element models of dyke and sill intrusions that consider heat transfer in porous media. Our results indicate that small intrusions create considerable thermal anomalies in their immediate vicinity. These anomalies are larger-magnitude and longer-lasting for individual thick sills and dykes, but networks of smaller sills and dykes emplaced close in time and space can create more widespread thermal anomalies that may be viable economic targets for decades after their emplacement.

Ignimbrites within calderas host intrusions with hazardous and/or economically significant hydrothermal systems. The Hvítserkur ignimbrite at Breiðuvík caldera, north-eastern Iceland, is intruded by basaltic dykes. Our data show that the ignimbrite immediately adjacent to the dyke is hard, dark-coloured, recrystallised quartz, plagioclase, and alkali feldspar with a low permeability and porosity and frequent macrofractures. At 1-2 m from the dyke, the ignimbrite is hard, dominantly glassy with pervasive perlitic microfractures, has high permeability, but low porosity and frequent macrofractures. A narrow zone of pervasive unlithified clay exists 2 m from the dyke. Beyond this, the ignimbrite is soft and zeolite-rich, has low permeability, high porosity and fewer macrofractures. The dyke intrusion promoted a narrow zone of welding, fracturing and perlitisation in the ignimbrite resulting in high permeability and focussed alteration. Our study shows how intrusions and their thermal aureoles create vertical pathways for, and horizontal barriers to, geothermal fluid flow.

Understanding the processes that drive explosive-effusive transitions during large silicic eruptions is crucial to hazard mitigation. Conduit models usually treat magma ascent and degassing as a gradual, unidirectional progression from bubble nucleation through magmatic fragmentation. However, there is growing evidence for the importance of bi-directional clastogenic processes that sinter fragmented materials into coherent clastogenic magmas. Bombs that were ejected immediately before the first emergence of lava in the 2011-2012 eruption at Cordon Caulle volcano (Chile) are texturally heterogeneous composite assemblages of welded pyroclastic material. Although diverse in density and appearance, SEM and X-ray tomographic analysis show them all to have been formed by multi-generational viscous sintering of fine ash. Sintering created discrete clasts ranging from obsidian to pumice and formed a pervasive clast-supporting matrix that assembled these clasts into a conduit-sealing plug. An evaluation of sintering timescales reveals texturally disparate bomb components to represent only minutes of difference in residence time within the conduit. Permeability modelling indicates that the plug was an effective conduit seal, with outgassing potential-even from high-porosity regions-being limited by the inability of gas to flow across tendrils of densely sintered inter-clast matrix. Contrary to traditional perspectives, declining expressions of explosivity at the surface need not be preceded or accompanied by a decline in fragmentation efficiency. Instead, they result from tips in balance between the opposing processes of fragmentation and sintering that occur in countless cycles within volcanic conduits. These processes may be particularly enhanced at silicic fissure volcanoes, which have laterally extensive subsurface plumbing systems that require complex magma ascent pathways. The textures investigated here reveal the processes occurring within silicic fissures to be phenom, Correction in: BULLETIN OF VOLCANOLOGY, Volume:83, Issue:3, Article Number:15, DOI:10.1007/s00445-021-01441-8

The thermal properties of volcanic rocks are crucial to accurately model heat transfer in volcanoes and in geothermal systems located within volcanic deposits. Here we provide laboratory measurements of thermal conductivity and thermal diffusivity for variably porous andesites from Mt. Ruapehu (New Zealand) and variably altered basaltic-andesites from Merapi volcano (Indonesia) measured at ambient laboratory pressure and temperature using the transient hot-strip method. The specific heat capacity of each sample was then calculated using these measured values and the bulk sample density. Thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, but specific heat capacity does not vary systematically with porosity. For a given porosity, saturation with water increases thermal conductivity and specific heat capacity, but decreases thermal diffusivity. Measurements on samples from Merapi volcano show that, compared to the unaltered samples from Mt. Ruapehu, hydrothermal alteration deceases thermal conductivity and thermal diffusivity, and increases specific heat capacity. We use an effective medium approach to parameterise these data, showing that when the porosity and pore-fluid properties are scaled for, the measured values agree well with theoretical predictions. We find that despite the microstructural complexity of the studied andesites, porosity is the principal parameter dictating their thermal properties. To understand whether the measured changes in thermal properties are sufficient to influence natural processes, we model heat transfer from magma to the surrounding host-rock by solving Fick's second law cast in 1D Cartesian (dyke geometry) and cylindrical (conduit geometry) coordinates. We provide models for different host-rock porosities (0-0.6), different initial magmatic temperatures (800-1200 degrees C), and different levels of host-rock alteration. Our modelling shows how the cooling of a dyke and conduit is slowed by a h

Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarðvík-Dyrfjöll, Krafla and Húsafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks.We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability.Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation.

Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarðvík-Dyrfjöll, Krafla and Húsafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks.We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability.Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation.

The structures and textures preserved in lava domes reflect underlying magmatic and eruptive processes, and may provide evidence of how eruptions initiate and evolve. This study explores the remarkable cycles in lava extrusion style produced between 1922 and 2012 at the Santiaguito lava dome complex, Guatemala. By combining an examination of eruptive lava morphologies and textures with a review of historical records, we aim to constrain the processes responsible for the range of erupted lava type and morphologies. The Santiaguito lava dome complex is divided into four domes (El Caliente, La Mitad, El Monje, El Brujo), containing a range of proximal structures (e.g., spines) from which a series of structurally contrasting lava flows originate. Vesicular lava flows (with a'a like, yet non-brecciated flow top) have the highest porosity with interconnected spheroidal pores and may transition into blocky lava flows. Blocky lava flows are high volume and texturally variable with dense zones of small tubular aligned pore networks and more porous zones of spheroidal shaped pores. Spines are dense and low volume and contain small skeletal shaped pores, and subvertical zones of sigmoidal pores. We attribute the observed differences in pore shapes to reflect shallow inflation, deflation, flattening, or shearing of the pore fraction. Effusion rate and duration of the eruption define the amount of time available for heating or cooling, degassing and outgassing prior to and during extrusion, driving changes in pore textures and lava type. Our new textural data when reviewed with all the other published data allow a cyclic model to be developed. The cyclic eruption models are influenced by viscosity changes resulting from (1) initial magmatic composition and temperature, and (2) effusion rate which in turn affects degassing, outgassing and cooling time in the conduit. Each lava type presents a unique set of hazards and understanding the morphologies and dome progression is useful

In 2009 the IDDP-1 borehole at Krafla, Iceland unexpectedly intersected rhyolitic magma at 2.1 km depth [1,2], providing unprecedented opportunities to investigate silicic melt formation and storage, and potential for powerful geothermal energy production. A key objective is to constrain the nature of the rhyolitic melt and its response to drilling. As no intact core was extracted, evidence is fragmental - from glassy rhyolitic clasts retrieved from the cuttings. These exhibit a range of glass colours, vesicularities and phenocryst contents [1,2]. Here we use benchtop infra-red spectroscopy and petrological microscopy to characterise the H2O concentrations and bubble number densities within diverse glassy clasts, complemented by 1 Atm bubble growth experiments with a heated stage to investigate vesicle growth. Juvenile glassy clasts were divided into three categories (brown>banded>very dark glass). H2O concentrations within clasts showed some spatial variability, with enrichment towards bubble-rich areas that may be resorption-related but could not be adequately characterised with a benchtop source. However, mean values ranged from 1.41-1.68 wt %, with no statistically significant difference between clast types. This is broadly consistent with previous studies [1,2]. Bubble growth rates in all clast types were determined during isothermal dwells at 600, 650 and 700 C, for which bubbles grew at 0.03-0.09, 0.11-0.31, and 0.46-0.82 m s-1 respectively. The highest growth rates were measured for the most water-rich clast analysed - a banded clast with mean H2O of 1.68 wt %, and initially-larger bubbles also grew more rapidly. Measured bubble number densities (BNDs) range from 10[11.7] m-3 in banded clasts to 10[13.1] m-3 in very dark clasts, corresponding to decompression rates of 0.1-1 MPa/s [3], although experimentation on IDDP-1 magma is needed to properly calibrate BNDs as a decompression rate meter. Nonetheless, such decompression rates suggest nucleation occurred o

Fourier transform infrared spectroscopy (FTIR) is an analytical technique utilized to measure the concentrations of H and C species in volcanic glasses. Water and CO2 are the most abundant volatile species in volcanic systems. Water is present in magmas in higher concentrations than CO2 and is also more soluble at lower pressures, and, therefore it is the dominant volatile forming bubbles during volcanic eruptions. Dissolved water affects both phase equilibria and melt physical properties such as density and viscosity, therefore, water is important for understanding magmatic processes. Additionally, quantitative measurements of different volatile species using FTIR can be achieved at high spatial resolution. Recent developments in analytical equipment such as synchrotron light sources and the development of focal plane array (FPA) detectors allow higher resolution measurements and the acquisition of concentration maps. These new capabilities are being used to characterize spatial gradients (or lack thereof) around bubbles and other textural features, which in turn lead to new insights into the behavior of volcanic feeder systems. Here, practical insights about sample preparation and analysis of the distribution and speciation of volatiles in volcanic glasses using FTIR spectroscopy are discussed. New advances in the field of FTIR analysis produce reliable data at high spatial resolution that can be used to produce datasets on the distribution, dissolution and diffusion of volatiles in volcanic materials.

Fourier transform infrared spectroscopy (FTIR) is an analytical technique utilized to measure the concentrations of H and C species in volcanic glasses. Water and CO2 are the most abundant volatile species in volcanic systems. Water is present in magmas in higher concentrations than CO2 and is also more soluble at lower pressures, and, therefore it is the dominant volatile forming bubbles during volcanic eruptions. Dissolved water affects both phase equilibria and melt physical properties such as density and viscosity, therefore, water is important for understanding magmatic processes. Additionally, quantitative measurements of different volatile species using FTIR can be achieved at high spatial resolution. Recent developments in analytical equipment such as synchrotron light sources and the development of focal plane array (FPA) detectors allow higher resolution measurements and the acquisition of concentration maps. These new capabilities are being used to characterize spatial gradients (or lack thereof) around bubbles and other textural features, which in turn lead to new insights into the behavior of volcanic feeder systems. Here, practical insights about sample preparation and analysis of the distribution and speciation of volatiles in volcanic glasses using FTIR spectroscopy are discussed. New advances in the field of FTIR analysis produce reliable data at high spatial resolution that can be used to produce datasets on the distribution, dissolution and diffusion of volatiles in volcanic materials.