Conference Presentations by John Browning
Field observations and numerical models of a Pleistocene-Holocene feeder dyke swarm associated wi... more Field observations and numerical models of a Pleistocene-Holocene feeder dyke swarm associated with a fissure complex to the east of the Tatara-San Pedro-Pellado complex, Southern Volcanic Zone, Chile Magma is transported through the lithosphere as dykes which, during periods of unrest, may feed eruptions at the surface. The propagation path of dykes is influenced by the crustal stress field and can be disturbed by crustal heterogeneities such as contrasting rock units or faults. Moreover, as dykes propagate, they themselves influence the surrounding stress field through processes of stress transfer, crustal deformation and seismic failure. The result is the formation of arrested dykes, as well as contrasting strike and dip angles and dyke segmentation. Here, we study the mechanisms of dyke injection and the role played in modifying the stress field and potential propagation paths of later dyke injections. To do this we combine field data from an eroded and well-exposed shallow feeder dyke swarm with a suite of two-dimensional FEM numerical models. We mapped 35 dyke segments over a~1 km long dyke swarm exposed~5 km to the East of Pellado Volcano, in the Tatara-San Pedro-Pellado (TSPP) volcanic complex, Southern Volcanic Zone of the Andes. Detailed mapping of the swarm elucidates two preferential strike orientations, one~N80°E and the other~N60°E. Our numerical models simulate both the TSPP volcanic complex and the studied dyke swarm as zones of either magmatic excess pressure or as a rigid inclusion. The crustal segment hosting the volcanic complex and dykes is modelled using an elastic domain subjected to regional compression in select model cases. Model outputs provide the stress and strain fields resulting from the different geometries and applied boundary loads. The model results indicate that individual dyke injections can locally rotate the principal stresses such as to influence the range of orientations over which later dykes will form. The orientation of σ 1 at the dyke tip ranges over 60°(±30°either side of the dyke tip) indicating that the strike orientation of later dykes will fall within this range. The effect of adding a bulk regional compression is to locally increase the magnitude of favorably oriented tensile stresses in the bedrock but to reduce the range of σ 1 orientations to 40°(±20°). This implies that under a far-field transpressive stress regime, as is common in Andean settings, regional dyke swarms will tend to maintain their strike orientation parallel to the regional bulk stress. These results should be accounted for when studying periods of volcanic unrest in order to discern the location and orientation of potential fissure eruptions in active volcanic areas such as the Southern Volcanic Zone of the Andes.
Mechanical tests with ultrasonic data of heat-treated, dry and saturated limestones Results r... more Mechanical tests with ultrasonic data of heat-treated, dry and saturated limestones Results revealed water and temperature dependence at the brittle-ductile transition Integrated in a 2D model, results highlight broad impacts for some volcanic basements Abstract Volcanic edifices are commonly unstable, with magmatic and non-magmatic fluid circulation, and elevated temperature gradients having influence on the mechanical strength of edifice and basement rocks. We present new mechanical characterisation of the Comiso limestone of the Mount Etna Volcano (Italy) basement to constrain the effects of regional ambient conditions associated with the volcanic system: the effects of pore fluid on rock strength and the effects of distal magmatic heating (~20°C to 600°C) at a range of simulated depths (0.2 to 2.0 km). The presence of water promotes ductile behaviour at shallow depths and causes a significant reduction in brittle rock strength compared to dry conditions. Thermal stressing, in which specimens were heated and cooled before mechanical testing at room temperature, has a variable effect for dry and saturated cases. In dry conditions, thermal stressing up to 450°C homogenises the strength of the specimen such that the majority of the specimens exhibit the same peak stress; at 600°C, the brittle failure is promoted at lower differential stress. The presence of water in thermally-stressed specimens promotes ductile behaviour and reduces peak strength. Acoustic emission monitoring suggests that accumulated damage is associated with the heating-cooling sequence, particularly in the 300-450-600°. Based on conduction modelling, we estimate this temperature range could affect basement rocks up to 300 m away from minor sheet intrusions and much further with larger bodies. Considering the dyke spacing beneath Etna, these conditions may apply to a significant percentage of the basement, promoting ductile behaviour at relatively shallow depths.
On certain rare occasions, the physical processes in a volcano encourage chamber-roof failure and... more On certain rare occasions, the physical processes in a volcano encourage chamber-roof failure and vertical collapse
along bounding ring-faults. The conditions which lead to caldera forming collapse are still poorly constrained. As
there have only been four, possibly five well documented caldera forming events, the geodetic signals produced
from chamber failure and collapse are not well understood. We present results from numerical models designed to
simulate the failure and subsidence of a magma chamber roof. Our aim is to present the resultant surface deformation
expected from such collapse events. All shallow magma chambers reside in crustal segments which to a
first approximation can be considered to behave as a linear elastic material. The crustal response due to deflation
and inflation cycles at caldera volcanoes is often considered using a point-pressure (Mogi) source; however, such
models are not suitable for constraining magma chamber failure and collapse volumes. We consider roof displacement
for a number of magma chamber depths, geometries, and sizes using the numerical Finite Element software,
COMSOL. In addition, we investigate the role of crustal heterogeneities and anisotropies, as well as the surface
cover (such as an ice sheet) on the results obtained. Initial models indicate significant vertical displacement (> 0.5
m), several tens of kilometres from the collapse area. Results are significantly affected by the mechanical properties
of the host rock, the magma chamber geometry, and the collapse volume (the volume of the subsidence). The
models can be used to estimate the amount of vertical and horizontal far-field surface displacements that would
be expected to be recorded by geodetic monitoring networks for a given caldera subsidence. Such results may
be useful for interpreting signals from ice-covered volcanoes, such as Bardarbunga and many other volcanoes in
Iceland.
Rheological conditions at magma chamber boundaries remain poorly understood. Many field observati... more Rheological conditions at magma chamber boundaries remain poorly understood. Many field observations
of deeply-eroded and well-exposed plutons, for example Slaufrudalur and Geitafell in SE Iceland, exhibit a
sharp transition between what may have been a partially or fully molten magma chamber and its surrounding
brittle host rock. Some studies have suggested a more gradual change in the rheological properties of chamber
boundaries, marked by a ductile halo, which is likely to exert a significant impact on their rheological response.
Understanding the state and rheological conditions of magma-rock interface and interaction is essential for
constraining chamber-boundary failure conditions leading to dyke propagation, onset of volcanic eruption as well
as caldera fault formation.
We present results from a series of thermal stressing experiments in which we attempt to recreate the
likely conditions at magma-chamber boundaries. Cores of volcanic material (25 mm diameter x 65 mm long)
were heated to magmatic temperatures under controlled conditions in a horizontal tube furnace (at atmospheric
pressure) and then held at those temperatures over variable dwell times. At the maximum temperatures reached,
the inner part of the samples undergoes partial melting whilst the outer part remains solid. After cooling the brittle
shells commonly exhibit axial, fissure-like fractures with protruded blobs of solidified melt. This phenomenon
is interpreted as being the result of volume expansion during partial melting. The internal melt overpressure
generates fluid-driven fractures analogous to filter-pressing textures or on a large scale, dykes. We complement
our observations with acoustic emission and seismic velocity data obtained from measurements throughout the
experiments. These complementary data are used to infer the style and timescale of fracture formation. Our results
pinpoint the temperature ranges over which brittle fractures form as a result of internal melt overpressure in several
different rock types. Application of these observations and experiments will be useful for 1) determining the
influence of volume increase due to melting on rock behaviour and melt migration 2) constraining the rheological
conditions at magma chamber boundaries.
Several hypotheses have been proposed regarding the role of thermo-mechanical contraction in prod... more Several hypotheses have been proposed regarding the role of thermo-mechanical contraction in producing cracks
and joints in volcanic rocks. Nevertheless, most studies of thermally-induced cracking to date have focused
on the generation of cracks formed during heating. In this latter case, the cracks are formed under an overall
compressional regime. By contrast, cooling cracks are formed under an overall tensile regime. Therefore, both the
nature and mechanism of crack formation during cooling are hypothesised to be different from those for crack
formation during heating. Furthermore, it remains unclear whether cooling simply reactivates pre-existing cracks,
induces the growth of new cracks, or both.
We present results from experiments based on a new method for testing ideas on cooling-induced cracking.
Cored samples of volcanic rock (basaltic to dacitic in composition) were heated at varying rates to different
maximum temperatures inside a tube furnace. In the highest temperature experiments samples of both rocks were
raised to the liquidus temperature appropriate to their composition, forcing melt interaction and crack annealing.
We present in-situ seismic velocity and acoustic emission data, which were recorded throughout each heating and
cooling cycle. It is found consistently that the rate of acoustic emission is much higher during cooling than during
heating. In addition, acoustic emission events produced on cooling tend to be significantly higher in energy than
those produced during heating. We therefore suggest that cracks formed during cooling are significantly larger
than those formed during heating. Thin-section and crack morphology analysis of our cyclically heated samples
provide further evidence of contrasting fracture morphologies. These new data are important for assessing the
contribution of cooling-induced damage within volcanic structures and layers such as sills and lava flows. Our
observations may also help to constrain evolving ideas regarding the formation of columnar joints.
The Askja volcanic system lies on the boundary between the Eurasian and North American tectonic p... more The Askja volcanic system lies on the boundary between the Eurasian and North American tectonic plates and is
an example of a multiple caldera formed in an extensional regime. Askja is composed of at least three calderas,
the last of which formed during an explosive eruption in A.D. 1875. The caldera floor has been subsiding almost
continuously since 1983; total subsidence in this period is around 1.1 metres. Perhaps surprisingly, there has been
no slip or movement on the caldera bounding ring-faults during this subsidence period. Various models have been
proposed to explain this unusual signal. Previous models suggest two magma sources, one shallow at around 3
km depth and one much larger at around 16 km depth. In this model, subsidence is caused by depressurisation
in both sources as a result of cooling contraction and crystallisation. In other models subsidence results from
magma being squeezed out of the shallow chamber laterally; or somehow draining back into a deep seated reservoir.
In this study we examine the contribution of regional extension and structural discontinuities to the current
subsidence of Askja caldera. Using a finite element numerical analysis, we ascertain the state of stresses at Askja
caldera over time based on several different magma body geometries.We calculate surface displacements expected
from extension around a shallow magma body, and place these findings in the context of Icelandic calderas. In
addition we investigate the likely stress effects of the Askja caldera on the associated part of the Northern Volcanic
Zone. The proposed model seeks to understand the volcano-tectonic conditions at Askja during caldera formation,
as well as during rifting episodes. The models presented will be useful in assessing likely future rifting events and
fissure swarm activity in Askja caldera, and neighbouring volcanoes.
Collapse calderas are a surface deformation resulting from failure of a magma chamber roof, and i... more Collapse calderas are a surface deformation resulting from failure of a magma chamber roof, and in the case of piston-like subsidence, result in slip along the bounding ring faults. Understanding collapse-caldera dynamics is vital because of the potential for large destructive eruptions. Ring faults bounding collapse calderas have been observed in geophysical and analogue studies. The role of ring-fault attitude, however, on the development of collapse calderas is not well constrained. Steeply inward-dipping normal ring faults are those commonly found bounding calderas, although outward-dipping reverse ring faults do also occur and are favoured in some models of caldera formation. Here we present the results of many new finite element numerical models which investigate how the stress conditions for ring-fault formation depend on the dip of the resulting ring faults. In these models, an oblate ellipsoidal (sill-like) magma chamber, 8 x 2 km, is located in a homogenous crustal layer at 3 km depth below the surface. The dip of inward and outward dipping faults is altered between models to investigate the effects of different dips on the stress conditions needed for ring-fault formation or reactivation. The stress conditions most likely to initiate slip on a caldera fault are those whereby (1) the maximum tensile stress peaks at the surface, (2) the maximum shear stress peaks at the chamber margin (above the lateral ends of the sill-like chamber), and (3) the maximum tensile stress at the surface peaks above the lateral ends of the associated chamber. The boundary conditions most common for ring fault formation and caldera slip are minor doming and external extension. It is easier (requires less energy) to generate ring faults in a basaltic edifice, where individual layers have similar mechanical properties and therefore promote stress field homogenisation, than in stratovolcanoes composed of layers with widely different mechanical properties. The present results indicate that vertical to steeply inward-dipping ring faults do not significantly alter the near-surface stress field. By contrast, outward-dipping reverse faults have large effects on the near-surface stresses, increasing the maximum surface tensile stress by up to several mega-pascals under certain conditions. The formation of shallow-dipping ring faults is generally easier than the formation of (the commonly observed) steeply to vertical dipping faults. These results may help to explain the apparent rarity of caldera-forming unrest periods, as most natural examples exhibit steeply dipping ring faults.
Stratovolcanoes are tall and long-lived structures characterised by steeply dipping slopes and ma... more Stratovolcanoes are tall and long-lived structures characterised by steeply dipping slopes and made up of numerous layers of rocks with widely different origins and compositions. In contrast, Basaltic edifices are generally much smaller with gentle slopes constructed of relatively homogenous material. There exists a remarkable difference in the frequency of vertical (caldera forming) and lateral (landslide) collapses between these two main types of volcanic structures, whereby stratovolcanoes appear more resistant to collapse than basaltic edifices. This study focuses on the apparent contrast in failure frequencies by firstly assessing the strain energy, stress conditions and material properties needed for lateral and vertical collapses to occur. A key aim is to understand how much of the difference in failure frequencies between composite and basaltic edifices can be explained in terms of the difference in material toughness of the edifices - a difference that is partly due to variation in lithologies and contact properties and, therefore, related to the compositional range of magmas in the edifices. Some initial results from numerical models of caldera bounding (ring) faults and their effects on the prevailing stress conditions are presented. Future work will combine analytical, analogue, and numerical modelling of fault propagation, deflection, and arrest at layer contacts within edifices. It is hoped that the results from this study will be useful for assessing the likelihood of the formation of collapse calderas and large landslides during periods of unrest in volcanic edifices.
Although pumice is an end-member product of gas-rich explosive volcanism, the process of bubble g... more Although pumice is an end-member product of gas-rich explosive volcanism, the process of bubble growth which leads to the formation of pumiceous textures are not well constrained. Vesiculation in rhyolitic melts is a primary control on some of the largest explosive eruptions. This study presents the results of a series of experiments which have utilised hot-stage microscopy techniques to track vesicle growth in an initially vesicle-poor rhyolitic melt. Using rhyolitic obsidian erupted from Chaiten, Chile in 2008 (containing ~1.38 wt. % H2O), thin wafers were held at atmospheric pressure for periods of between 5 minutes and 2 days in the hot-stage, at temperatures between 575 oC and 875 oC. In-situ vesiculation was directly observed and the growth of individual bubbles measured using image tracking code in MATLAB. It was found that bubble growth rates increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 oC is ~1.27 m s-1, compared with the lowest observed growth rate of ~0.02 m s-1 at 725 oC; below this temperature, no growth was observed. Average growth rate Vr follows an exponential relationship with temperature and melt viscosity where Vr ≈ exp (0.0169T) and Vr ≈ exp (-1.202). The extent of diffusive degassing from wafer surfaces was estimated with simple diffusion models. Diffusive loss was found to be negligible during brief high-temperature experiments but became increasingly important in slower, lower temperature experiments. Several stages of bubble growth were directly observed, including initial relaxation of deformed existing bubbles into spheres, extensive growth of spherical bubbles, and, at higher temperatures, close packing and foam formation. An advantage of the imaging techniques used here is that bubble-bubble interactions can be observed in-situ at a scale of 2 to 3 microns. Evolving bubble number densities (BND) with time were determined, allowing nucleation rates to be estimated. Maximum observed BNDs were 3.4 × 1012 m-3 with maximum increases of around 160 % observed in samples with lower initial vesicularity (< 5.7 × 1011 m-3). Experimentally determined rates of nucleation, growth and coalescence assist in the reconstruction and vesiculation history of quenched products and in models of magma vesiculation at shallow levels.
Bubble growth in rhyolitic melts is a primary control on some of the largest explosive eruptions,... more Bubble growth in rhyolitic melts is a primary control on some of the largest explosive eruptions, but growth dynamics remain controversial. We have used hot-stage microscopy to directly observe vesiculation of a Chaiten rhyolite melt (containing ~1.38 wt. % H 2O) at atmospheric pressure. Thin wafers of obsidian were held from 5 minutes up to 2 days in the hot-stage at temperatures between 575 oC and 875 oC. The growth of many individual bubbles was measured using image tracking code within MATLAB. We found that bubble growth rates increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 oC is ~1.27 Î_m s-1, compared with the lowest observed growth rate of ~0.02 Î_m s-1 at 725 oC; below this temperature no growth was observed. Average growth rate V r follows an exponential relationship with temperature and melt viscosity where V r ~ exp (0.0169T) and V r ~ exp (-1.202η). Comparison of these measured rates with existing bubble growth models (e.g. Navon, Proussevitch and Sahagian) indicates slower growth than expected at the highest temperatures. The extent of diffusive degassing of H 2O and OH- from wafer surfaces during experiments was estimated with simple diffusion models. It was found to be negligible during brief high-temperature experiments but became increasingly important for slower, lower-temperature experiments. Several stages of bubble growth were directly observed, including initial relaxation of deformed existing bubbles into spheres, extensive growth of spheres, and, at higher temperatures, close packing and foam formation. An advantage of the imaging techniques used here is that bubble-bubble interactions can be observed in-situ at relatively high resolution. Bubble deformation due to bubble-bubble interaction and coalescence was observed in most experiments. Evolving bubble number densities (BND) with time were determined, allowing nucleation rates to be estimated. Maximum observed BNDs were 3.4 x 1012 m-3 with maximum increases of around 143 % observed in samples with a lower initial vesicularity <5.7 x 1011 m-3. The experiments described can be used to effectively retrace the vesiculation history of samples, providing a useful tool for aiding in the interpretation of end member products.
Abstracts by John Browning
"Oman borders the Indian Ocean on the Northern and Eastern coasts and is therefore at potential r... more "Oman borders the Indian Ocean on the Northern and Eastern coasts and is therefore at potential risk from tsunami impact. In recent time Oman has experienced 2 well documented tsunami, one being the 2004 boxing day disaster and the other a lesser well known tsunami generated from the Makran Subduction Zone, an area of high tsunamigenic potential located on the southern Iran and Pakistan coasts.
Research is presented assessing tsunami risk to the coastal towns of Muscat and Salalah using a deterministic approach to understand maximum inundation and damage caused from three scenario events. Vulnerability is assessed based on economic damage and a series of risk maps created. Salalah is predominantly at low risk from tsunami inundation. The most probable source for a tsunami in this region is from the far field Sunda-Sumatra fault, it is estimated using survivor function analysis of past seismic events that there is 80% chance of a tsunamigenic characteristic earthquake within 2.5 years is this area.
It is found that the studied area of Muscat is at high risk from tsunami generated by earthquake and sub-marine landslides on the Makran accretionary complex. A tsunami generated from these sources could potentially amplify by a factor of 1.5 in the Mutrah bay inlet to generate run-up values of 5.1-6.5m in a local area, maximum economic damage from such events is estimated to be over OMR 25 million. Finally a risk reduction strategy is suggested for the high risk area of Muscat highlighting evacuation routes, tsunami safety sites and hazard zones. It is hoped that the assessment of risk may go some way to a government lead disaster risk reduction strategy being implemented in coastal Oman.
"
Papers by John Browning
Bulletin of Volcanology
The physical processes that operate within, and beneath, a volcano control the frequency, duratio... more The physical processes that operate within, and beneath, a volcano control the frequency, duration, location and size of volcanic eruptions. Volcanotectonics focuses on such processes, combining techniques, data, and ideas from structural geology, tectonics, volcano deformation, physical volcanology, seismology, petrology, rock and fracture mechanics and classical physics. A central aim of volcanotectonics is to provide sufficient understanding of the internal processes in volcanoes so that, when combined with monitoring data, reliable forecasting of eruptions, vertical (caldera) and lateral (landslide) collapses and related events becomes possible. To gain such an understanding requires knowledge of the material properties of the magma and the crustal rocks, as well as the associated stress fields, and their evolution. The local stress field depends on the properties of the layers that constitute the volcano and, in particular, the geometric development of its shallow magma chamber...
Geophysical Journal International, 2022
SUMMARY Crustal rocks undergo repeated cycles of stress over time. In complex tectonic environmen... more SUMMARY Crustal rocks undergo repeated cycles of stress over time. In complex tectonic environments where stresses may evolve both spatially and temporally, such as volcanoes or active fault zones, these rocks may experience not only cyclic loading and unloading, but also rotation and/or reorientation of stresses. In such situations, any resulting crack distributions form sequentially and may therefore be highly anisotropic. Thus, the tectonic history of the crust as recorded in deformed rocks may include evidence for complex stress paths, encompassing different magnitudes and orientations. Despite this, the ways in which variations in principal stresses influence the evolution of anisotropic crack distributions remain poorly constrained. In this work, we build on the previous non-linear anisotropic damage rheology model by presenting a newly developed poroelastic rheological model which accounts for both coupled anisotropic damage and porosity evolution. The new model shares the ma...
An understanding of the amount of magma available to supply any given eruption is useful for dete... more An understanding of the amount of magma available to supply any given eruption is useful for determining the potential eruption magnitude and duration. Geodetic measurements and inversion techniques are often used to constrain volume changes within magma chambers, as well as constrain location and depth, but such models are incapable of calculating total magma storage. For example, during the 2012 unrest period at Santorini volcano, approximately 0.021 kmˆ3 of new magma entered a shallow chamber residing at around 4 km below the surface. This type of event is not unusual, and is in fact a necessary condition for the formation of a long-lived shallow chamber. The period of unrest ended without culminating in eruption, i.e the amount of magma which entered the chamber was insufficient to break the chamber and force magma further towards the surface. Using continuum-mechanics and fracture-mechanics principles, we present a model to calculate the amount of magma contained at shallow depth beneath active volcanoes. Here we discuss our model in the context of Santorini volcano, Greece. We demonstrate through structural analysis of dykes exposed within the Santorini caldera, previously published data on the volume of recent eruptions, and geodetic measurements of the 2011–2012 unrest period, that the measured 0.02% increase in volume of Santorini&#39;s shallow magma chamber was associated with magmatic excess pressure increase of around 1.1 MPa. This excess pressure was high enough to bring the chamber roof close to rupture and dyke injection. For volcanoes with known typical extrusion and intrusion (dyke) volumes, the new methodology presented here makes it possible to forecast the conditions for magma-chamber failure and dyke injection at any geodetically well-monitored volcano.
Most studies of thermally-induced cracking to date have focused on the generation of cracks forme... more Most studies of thermally-induced cracking to date have focused on the generation of cracks formed during heating and thermal expansion. Both the nature and mechanism of crack formation during cooling are hypothesised to be different from those formed during heating. We present in-situ acoustic emission data recorded as a proxy for crack damage evolution throughout a series of heating and cooling experiments on samples of basalt and dacite. The results show that both the rate and energy of acoustic emission are consistently much higher during cooling than during heating. When comparing the AE during the heating phase with the AE during the cooling phase of a comparable duration heating and cooling cycle; we find that there are ∼ 150 times as many hits during cooling. Furthermore, the average energy of those AE are more than 3 times greater, resulting in a total AE energy that is almost 500 times higher during cooling than during heating. Seismic velocity comparisons and crack morphology analysis of our heated and cooled samples support the contemporaneous acoustic emission data and also indicate that thermal cracking is largely isotropic. These new data are important for assessing the contribution of cooling-induced damage within volcanic structures and layers such as dikes, sills and lava flows.
Journal of Volcanology and Geothermal Research, 2020
In active volcanic chains, the crustal redistribution and storage of hydrothermal and/or magmatic... more In active volcanic chains, the crustal redistribution and storage of hydrothermal and/or magmatic fluids can result in the development of ore deposits, geothermal reservoirs and surficial hydrothermal and volcanic expressions. Magma is transported through the lithosphere as dykes which, during periods of unrest, may feed eruptions at the surface. The propagation path of dykes is influenced by the crustal stress field and can be disturbed and whether a dyke reaches the surface to feed an eruption is influenced by the crustal stress field. The propagation path of dykes can be disturbed by crustal heterogeneities such as contrasting rock units or faults. Moreover, as dykes propagate, they themselves influence the surrounding stress field through processes of stress transfer, crustal deformation and seismic failure. The result is the formation of arrested dykes, as well as contrasting strike and dip angles and dyke segmentation. Here, we study the mechanisms of dyke injection and the role played in modifying the stress field and potential propagation paths of later dyke injections. To do this we combine field data from an eroded and well-exposed shallow feeder dyke swarm with a suite of twodimensional FEM numerical models. We mapped 35 dyke segments over a ~1 km long dyke swarm exposed ~5 km to the East of Pellado Volcano, in the Tatara-San Pedro-Pellado (TSPP) volcanic complex, Southern Volcanic Zone of the Andes. Detailed mapping of the swarm elucidates two preferential strike orientations, one ~N80ºE and the other ~N60ºE. Our numerical models simulate both the TSPP volcanic complex and the studied dyke swarm as zones of either magmatic excess pressure or as a rigid inclusion. The crustal segment hosting the volcanic complex and dykes is modelled using an elastic domain subjected to regional compression in select model cases. Model outputs provide the stress and strain fields resulting from the different geometries and applied boundary loads. The model results indicate that individual dyke injections can locally rotate the principal stresses such as to influence the range of orientations over which later dykes will form. The orientation of the maximum principal stress (1) at the dyke tip ranges over 60º (30º either side of the dyke tip) indicating that the strike orientation of later dykes will fall within this range. The effect of adding a bulk regional compression is to locally increase the magnitude of favorably oriented tensile stresses in the bedrock but to reduce the range of 1 orientations to 40º (20º). This implies that under a far-field transpressive stress regime, as is common in Andean settings, regional dyke swarms will tend to maintain their strike orientation parallel to the regional bulk stress. These results should be accounted for when studying periods of volcanic unrest in order to discern the location and orientation of potential fissure eruptions in active volcanic areas such as the Southern Volcanic Zone of the Andes.
The subsurface structures of caldera ring faults are often inferred from numerical and analog mod... more The subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies. All of these inferred structures need to be compared with actual ring faults so as to test the model implications. Here, we present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland. At the time of collapse caldera formation, over 200 m of vertical displacement was accommodated along a ring fault, which is exceptionally well exposed at a depth of approximately 1.2 km below the original surface of the volcano. There are abrupt changes in the ring fault attitude with depth, but its overall dip is steeply inward. Several inclined sheets within the caldera became arrested at the ring fault; other sheets became deflected up along the fault to form a multiple ring dike. We present numerical models showing stress fields that encourage sheet de...
Uploads
Conference Presentations by John Browning
along bounding ring-faults. The conditions which lead to caldera forming collapse are still poorly constrained. As
there have only been four, possibly five well documented caldera forming events, the geodetic signals produced
from chamber failure and collapse are not well understood. We present results from numerical models designed to
simulate the failure and subsidence of a magma chamber roof. Our aim is to present the resultant surface deformation
expected from such collapse events. All shallow magma chambers reside in crustal segments which to a
first approximation can be considered to behave as a linear elastic material. The crustal response due to deflation
and inflation cycles at caldera volcanoes is often considered using a point-pressure (Mogi) source; however, such
models are not suitable for constraining magma chamber failure and collapse volumes. We consider roof displacement
for a number of magma chamber depths, geometries, and sizes using the numerical Finite Element software,
COMSOL. In addition, we investigate the role of crustal heterogeneities and anisotropies, as well as the surface
cover (such as an ice sheet) on the results obtained. Initial models indicate significant vertical displacement (> 0.5
m), several tens of kilometres from the collapse area. Results are significantly affected by the mechanical properties
of the host rock, the magma chamber geometry, and the collapse volume (the volume of the subsidence). The
models can be used to estimate the amount of vertical and horizontal far-field surface displacements that would
be expected to be recorded by geodetic monitoring networks for a given caldera subsidence. Such results may
be useful for interpreting signals from ice-covered volcanoes, such as Bardarbunga and many other volcanoes in
Iceland.
of deeply-eroded and well-exposed plutons, for example Slaufrudalur and Geitafell in SE Iceland, exhibit a
sharp transition between what may have been a partially or fully molten magma chamber and its surrounding
brittle host rock. Some studies have suggested a more gradual change in the rheological properties of chamber
boundaries, marked by a ductile halo, which is likely to exert a significant impact on their rheological response.
Understanding the state and rheological conditions of magma-rock interface and interaction is essential for
constraining chamber-boundary failure conditions leading to dyke propagation, onset of volcanic eruption as well
as caldera fault formation.
We present results from a series of thermal stressing experiments in which we attempt to recreate the
likely conditions at magma-chamber boundaries. Cores of volcanic material (25 mm diameter x 65 mm long)
were heated to magmatic temperatures under controlled conditions in a horizontal tube furnace (at atmospheric
pressure) and then held at those temperatures over variable dwell times. At the maximum temperatures reached,
the inner part of the samples undergoes partial melting whilst the outer part remains solid. After cooling the brittle
shells commonly exhibit axial, fissure-like fractures with protruded blobs of solidified melt. This phenomenon
is interpreted as being the result of volume expansion during partial melting. The internal melt overpressure
generates fluid-driven fractures analogous to filter-pressing textures or on a large scale, dykes. We complement
our observations with acoustic emission and seismic velocity data obtained from measurements throughout the
experiments. These complementary data are used to infer the style and timescale of fracture formation. Our results
pinpoint the temperature ranges over which brittle fractures form as a result of internal melt overpressure in several
different rock types. Application of these observations and experiments will be useful for 1) determining the
influence of volume increase due to melting on rock behaviour and melt migration 2) constraining the rheological
conditions at magma chamber boundaries.
and joints in volcanic rocks. Nevertheless, most studies of thermally-induced cracking to date have focused
on the generation of cracks formed during heating. In this latter case, the cracks are formed under an overall
compressional regime. By contrast, cooling cracks are formed under an overall tensile regime. Therefore, both the
nature and mechanism of crack formation during cooling are hypothesised to be different from those for crack
formation during heating. Furthermore, it remains unclear whether cooling simply reactivates pre-existing cracks,
induces the growth of new cracks, or both.
We present results from experiments based on a new method for testing ideas on cooling-induced cracking.
Cored samples of volcanic rock (basaltic to dacitic in composition) were heated at varying rates to different
maximum temperatures inside a tube furnace. In the highest temperature experiments samples of both rocks were
raised to the liquidus temperature appropriate to their composition, forcing melt interaction and crack annealing.
We present in-situ seismic velocity and acoustic emission data, which were recorded throughout each heating and
cooling cycle. It is found consistently that the rate of acoustic emission is much higher during cooling than during
heating. In addition, acoustic emission events produced on cooling tend to be significantly higher in energy than
those produced during heating. We therefore suggest that cracks formed during cooling are significantly larger
than those formed during heating. Thin-section and crack morphology analysis of our cyclically heated samples
provide further evidence of contrasting fracture morphologies. These new data are important for assessing the
contribution of cooling-induced damage within volcanic structures and layers such as sills and lava flows. Our
observations may also help to constrain evolving ideas regarding the formation of columnar joints.
an example of a multiple caldera formed in an extensional regime. Askja is composed of at least three calderas,
the last of which formed during an explosive eruption in A.D. 1875. The caldera floor has been subsiding almost
continuously since 1983; total subsidence in this period is around 1.1 metres. Perhaps surprisingly, there has been
no slip or movement on the caldera bounding ring-faults during this subsidence period. Various models have been
proposed to explain this unusual signal. Previous models suggest two magma sources, one shallow at around 3
km depth and one much larger at around 16 km depth. In this model, subsidence is caused by depressurisation
in both sources as a result of cooling contraction and crystallisation. In other models subsidence results from
magma being squeezed out of the shallow chamber laterally; or somehow draining back into a deep seated reservoir.
In this study we examine the contribution of regional extension and structural discontinuities to the current
subsidence of Askja caldera. Using a finite element numerical analysis, we ascertain the state of stresses at Askja
caldera over time based on several different magma body geometries.We calculate surface displacements expected
from extension around a shallow magma body, and place these findings in the context of Icelandic calderas. In
addition we investigate the likely stress effects of the Askja caldera on the associated part of the Northern Volcanic
Zone. The proposed model seeks to understand the volcano-tectonic conditions at Askja during caldera formation,
as well as during rifting episodes. The models presented will be useful in assessing likely future rifting events and
fissure swarm activity in Askja caldera, and neighbouring volcanoes.
Abstracts by John Browning
Research is presented assessing tsunami risk to the coastal towns of Muscat and Salalah using a deterministic approach to understand maximum inundation and damage caused from three scenario events. Vulnerability is assessed based on economic damage and a series of risk maps created. Salalah is predominantly at low risk from tsunami inundation. The most probable source for a tsunami in this region is from the far field Sunda-Sumatra fault, it is estimated using survivor function analysis of past seismic events that there is 80% chance of a tsunamigenic characteristic earthquake within 2.5 years is this area.
It is found that the studied area of Muscat is at high risk from tsunami generated by earthquake and sub-marine landslides on the Makran accretionary complex. A tsunami generated from these sources could potentially amplify by a factor of 1.5 in the Mutrah bay inlet to generate run-up values of 5.1-6.5m in a local area, maximum economic damage from such events is estimated to be over OMR 25 million. Finally a risk reduction strategy is suggested for the high risk area of Muscat highlighting evacuation routes, tsunami safety sites and hazard zones. It is hoped that the assessment of risk may go some way to a government lead disaster risk reduction strategy being implemented in coastal Oman.
"
Papers by John Browning
along bounding ring-faults. The conditions which lead to caldera forming collapse are still poorly constrained. As
there have only been four, possibly five well documented caldera forming events, the geodetic signals produced
from chamber failure and collapse are not well understood. We present results from numerical models designed to
simulate the failure and subsidence of a magma chamber roof. Our aim is to present the resultant surface deformation
expected from such collapse events. All shallow magma chambers reside in crustal segments which to a
first approximation can be considered to behave as a linear elastic material. The crustal response due to deflation
and inflation cycles at caldera volcanoes is often considered using a point-pressure (Mogi) source; however, such
models are not suitable for constraining magma chamber failure and collapse volumes. We consider roof displacement
for a number of magma chamber depths, geometries, and sizes using the numerical Finite Element software,
COMSOL. In addition, we investigate the role of crustal heterogeneities and anisotropies, as well as the surface
cover (such as an ice sheet) on the results obtained. Initial models indicate significant vertical displacement (> 0.5
m), several tens of kilometres from the collapse area. Results are significantly affected by the mechanical properties
of the host rock, the magma chamber geometry, and the collapse volume (the volume of the subsidence). The
models can be used to estimate the amount of vertical and horizontal far-field surface displacements that would
be expected to be recorded by geodetic monitoring networks for a given caldera subsidence. Such results may
be useful for interpreting signals from ice-covered volcanoes, such as Bardarbunga and many other volcanoes in
Iceland.
of deeply-eroded and well-exposed plutons, for example Slaufrudalur and Geitafell in SE Iceland, exhibit a
sharp transition between what may have been a partially or fully molten magma chamber and its surrounding
brittle host rock. Some studies have suggested a more gradual change in the rheological properties of chamber
boundaries, marked by a ductile halo, which is likely to exert a significant impact on their rheological response.
Understanding the state and rheological conditions of magma-rock interface and interaction is essential for
constraining chamber-boundary failure conditions leading to dyke propagation, onset of volcanic eruption as well
as caldera fault formation.
We present results from a series of thermal stressing experiments in which we attempt to recreate the
likely conditions at magma-chamber boundaries. Cores of volcanic material (25 mm diameter x 65 mm long)
were heated to magmatic temperatures under controlled conditions in a horizontal tube furnace (at atmospheric
pressure) and then held at those temperatures over variable dwell times. At the maximum temperatures reached,
the inner part of the samples undergoes partial melting whilst the outer part remains solid. After cooling the brittle
shells commonly exhibit axial, fissure-like fractures with protruded blobs of solidified melt. This phenomenon
is interpreted as being the result of volume expansion during partial melting. The internal melt overpressure
generates fluid-driven fractures analogous to filter-pressing textures or on a large scale, dykes. We complement
our observations with acoustic emission and seismic velocity data obtained from measurements throughout the
experiments. These complementary data are used to infer the style and timescale of fracture formation. Our results
pinpoint the temperature ranges over which brittle fractures form as a result of internal melt overpressure in several
different rock types. Application of these observations and experiments will be useful for 1) determining the
influence of volume increase due to melting on rock behaviour and melt migration 2) constraining the rheological
conditions at magma chamber boundaries.
and joints in volcanic rocks. Nevertheless, most studies of thermally-induced cracking to date have focused
on the generation of cracks formed during heating. In this latter case, the cracks are formed under an overall
compressional regime. By contrast, cooling cracks are formed under an overall tensile regime. Therefore, both the
nature and mechanism of crack formation during cooling are hypothesised to be different from those for crack
formation during heating. Furthermore, it remains unclear whether cooling simply reactivates pre-existing cracks,
induces the growth of new cracks, or both.
We present results from experiments based on a new method for testing ideas on cooling-induced cracking.
Cored samples of volcanic rock (basaltic to dacitic in composition) were heated at varying rates to different
maximum temperatures inside a tube furnace. In the highest temperature experiments samples of both rocks were
raised to the liquidus temperature appropriate to their composition, forcing melt interaction and crack annealing.
We present in-situ seismic velocity and acoustic emission data, which were recorded throughout each heating and
cooling cycle. It is found consistently that the rate of acoustic emission is much higher during cooling than during
heating. In addition, acoustic emission events produced on cooling tend to be significantly higher in energy than
those produced during heating. We therefore suggest that cracks formed during cooling are significantly larger
than those formed during heating. Thin-section and crack morphology analysis of our cyclically heated samples
provide further evidence of contrasting fracture morphologies. These new data are important for assessing the
contribution of cooling-induced damage within volcanic structures and layers such as sills and lava flows. Our
observations may also help to constrain evolving ideas regarding the formation of columnar joints.
an example of a multiple caldera formed in an extensional regime. Askja is composed of at least three calderas,
the last of which formed during an explosive eruption in A.D. 1875. The caldera floor has been subsiding almost
continuously since 1983; total subsidence in this period is around 1.1 metres. Perhaps surprisingly, there has been
no slip or movement on the caldera bounding ring-faults during this subsidence period. Various models have been
proposed to explain this unusual signal. Previous models suggest two magma sources, one shallow at around 3
km depth and one much larger at around 16 km depth. In this model, subsidence is caused by depressurisation
in both sources as a result of cooling contraction and crystallisation. In other models subsidence results from
magma being squeezed out of the shallow chamber laterally; or somehow draining back into a deep seated reservoir.
In this study we examine the contribution of regional extension and structural discontinuities to the current
subsidence of Askja caldera. Using a finite element numerical analysis, we ascertain the state of stresses at Askja
caldera over time based on several different magma body geometries.We calculate surface displacements expected
from extension around a shallow magma body, and place these findings in the context of Icelandic calderas. In
addition we investigate the likely stress effects of the Askja caldera on the associated part of the Northern Volcanic
Zone. The proposed model seeks to understand the volcano-tectonic conditions at Askja during caldera formation,
as well as during rifting episodes. The models presented will be useful in assessing likely future rifting events and
fissure swarm activity in Askja caldera, and neighbouring volcanoes.
Research is presented assessing tsunami risk to the coastal towns of Muscat and Salalah using a deterministic approach to understand maximum inundation and damage caused from three scenario events. Vulnerability is assessed based on economic damage and a series of risk maps created. Salalah is predominantly at low risk from tsunami inundation. The most probable source for a tsunami in this region is from the far field Sunda-Sumatra fault, it is estimated using survivor function analysis of past seismic events that there is 80% chance of a tsunamigenic characteristic earthquake within 2.5 years is this area.
It is found that the studied area of Muscat is at high risk from tsunami generated by earthquake and sub-marine landslides on the Makran accretionary complex. A tsunami generated from these sources could potentially amplify by a factor of 1.5 in the Mutrah bay inlet to generate run-up values of 5.1-6.5m in a local area, maximum economic damage from such events is estimated to be over OMR 25 million. Finally a risk reduction strategy is suggested for the high risk area of Muscat highlighting evacuation routes, tsunami safety sites and hazard zones. It is hoped that the assessment of risk may go some way to a government lead disaster risk reduction strategy being implemented in coastal Oman.
"