Eoghan P Holohan | University College Dublin (original) (raw)

Papers by Eoghan P Holohan

Journal of Geophysical Research: Solid Earth, 2013

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Scientific Reports, 2017

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Solid Earth, 2019

Enclosed topographic depressions are characteristic of karst landscapes on Earth. The development... more Enclosed topographic depressions are characteristic of karst landscapes on Earth. The developmental relationship between depression types, such as sinkholes (do-lines) and uvalas, has been the subject of debate, mainly because the long developmental timescales in classical limestone karst settings impede direct observation. Here we characterize the morphometric properties and spatio-temporal development of ∼ 1150 sinkholes and five uvalas formed from ∼ 1980 to 2017 in an evaporite karst setting along the eastern coast of the hypersaline Dead Sea (at Ghor Al-Haditha, Jordan). The development of sinkhole populations and individual uvalas is intertwined in terms of onset, evolution and cessation. The sinkholes commonly develop in clusters, within which they may coalesce to form compound or nested sinkholes. In general, however, the uvalas are not defined by coalescence of sinkholes. Although each uvala usually encloses several clusters of sinkholes, it develops as a larger-scale, gentler and structurally distinct depression. The location of new sinkholes and uvalas shows a marked shoreline-parallel migration with time, followed by a marked shoreline-perpendicular (i.e. seaward) growth with time. These observations are consistent with theoretical predictions of karstifi-cation controlled by a laterally migrating interface between saturated and undersaturated groundwater, as induced by the 35 m fall in the Dead Sea water level since 1967. More generally , our observations indicate that uvalas and the sink-hole populations within them, although morphometrically distinct, can develop near-synchronously by subsidence in response to subsurface erosion.

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Remote Sensing, 2019

Regions of temperate oceanic climate have historically represented a challenge for the applicatio... more Regions of temperate oceanic climate have historically represented a challenge for the application of satellite-based multi-temporal SAR interferometry. The landscapes of such regions are commonly characterized by extensive, seasonally-variable vegetation coverage that can cause low temporal coherence and limit the detection capabilities of SAR imagery as acquired, for instance, by previous ERS-1/2 and ENVISAT missions. In this work, we exploited the enhanced resolution in space and time of the recently deployed Sentinel-1A/B SAR satellites to detect and monitor ground motions occurring in two study areas in the Republic of Ireland. The first, is a ~1800 km 2 area spanning the upland karst of the Clare Burren and the adjacent mantled lowland karst of east Galway. The second, is an area of 100 km 2 in Co. Meath spanning an active mine site. The available datasets, consisting of more than 100 images acquired in both ascending and descending orbits from April 2015 to March 2018, were processed by using the Permanent Scatterer approach. The obtained results highlight the presence of small-scale ground motions in both urban and natural environments with displacement rates along the satellite line of sight up to −17 mm/year. Localized subsidence was detected in recently built areas, along the infrastructure (both roads and railways), and over the mine site, while zones of subsidence, uplift, or both, have been recorded in a number of peatland areas. Furthermore, several measured target points indicate the presence of unstable areas along the coastline. Many of the detected movements were previously unknown. These results demonstrate the feasibility of adopting multi-temporal interferometry based on Sentinel-1 data for the detection and monitoring of mm-scale ground movements even over small areas (<100 m 2) in environments influenced by temperate oceanic climate.

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Frontiers in Earth Science, 2019

Magma intrusions grow to their final geometries by deforming the Earth's crust internally and by ... more Magma intrusions grow to their final geometries by deforming the Earth's crust internally and by displacing the Earth's surface. Interpreting the related displacements in terms of intrusion geometry is key to forecasting a volcanic eruption. While scaled laboratory models enable us to study the relationships between surface displacement and intrusion geometry, past approaches entailed limitations regarding imaging of the laboratory model interior or simplicity of the simulated crustal rheology. Here we apply cutting-edge medical wide beam X-ray Computed Tomography (CT) to quantify in 4D the deformation induced in laboratory models by an intrusion of a magma analog (golden syrup) into a rheologically-complex granular host rock analog (sand and plaster). We extract the surface deformation and we quantify the strain field of the entire experimental volume in 3D over time by using Digital Volume Correlation (DVC). By varying the strength and height of the host material, and intrusion velocity, we observe how intrusions of contrasting geometries grow, and induce contrasting strain field characteristics and surface deformation in 4D. The novel application of CT and DVC reveals that distributed strain accommodation and mixed-mode (opening and shear) fracturing dominates in low-cohesion material overburden, and leads to the growth of thick cryptodomes or cup-shaped intrusions. More localized strain accommodation and opening-mode fracturing dominates in high-cohesion material overburden, and leads to the growth of cone sheets or thin dikes. The results demonstrate how the combination of CT and DVC can greatly enhance the utility of optically non-transparent crustal rock analogs in obtaining insights into shallow crustal deformation processes. This unprecedented perspective on the spatio-temporal interaction of intrusion growth coupled with host material deformation provides a conceptual framework that can be tested by field observations at eroded volcanic systems and by the ever increasing spatial and temporal resolution of geodetic data at active volcanoes.

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Solid Earth, 2019

The 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of s... more The 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of subsidence-related karst landforms, such as single and clustered sinkholes, and associated larger-scale depressions. Sub-surface material in the DEM model is removed progressively to produce an array of cavities; this simulates a network of subsurface groundwater conduits growing by chem-ical/mechanical erosion. The growth of the cavity array is coupled mechanically to the gravitationally loaded surroundings , such that cavities can grow also in part by material failure at their margins, which in the limit can produce individual collapse sinkholes. Two end-member growth scenarios of the cavity array and their impact on surface subsidence were examined in the models: (1) cavity growth at the same depth level and growth rate; (2) cavity growth at progressively deepening levels with varying growth rates. These growth scenarios are characterised by differing stress patterns across the cavity array and its overburden, which are in turn an important factor for the formation of sinkholes and uvala-like depressions. For growth scenario (1), a stable compression arch is established around the entire cavity array, hindering sinkhole collapse into individual cavities and favouring block-wise, relatively even subsidence across the whole cavity array. In contrast, for growth scenario (2), the stress system is more heterogeneous, such that local stress concentrations exist around individual cavities, leading to stress interactions and local wall/overburden fractures. Consequently, sinkhole collapses occur in individual cavities, which results in uneven, differential subsidence within a larger-scale depression. Depending on material properties of the cavity-hosting material and the overburden, the larger-scale depression forms either by sinkhole coalescence or by widespread subsidence linked geometrically to the entire cavity array. The results from models with growth scenario (2) are in close agreement with surface morphological and subsurface geo-physical observations from an evaporite karst area on the eastern shore of the Dead Sea.

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Solid Earth, 2018

Mechanical and/or chemical removal of material from the subsurface may generate large subsurface ... more Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth / diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth / diameter values in each material type may partly reflect sinkhole growth trends.

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Solid Earth , 2018

Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for u... more Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development. For this purpose, high frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor AlHaditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area. The most surprising result of the survey is the absence of evidence of a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200 m depth.
Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate mineral as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and
physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings.

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Earth Science Reviews, 2018

Advancing our knowledge of caldera volcanoes enables better assessment of hazard and more efficie... more Advancing our knowledge of caldera volcanoes enables better assessment of hazard and more efficient harnessing of resources. In this paper we review developments in concepts of magma storage and transport during the life cycle of caldera plumbing systems. We draw together: a) geological, geochemical and petrological data from intrusions and eruption deposits; b) geophysical and geochemical data from modern restless calderas; and c) geological and structural evidence from ancient calderas as well as insights from numerical and analogue models.

Overall, magma plumbing systems beneath calderas develop incrementally as magma rises, intrudes and rejuvenates. Eventually accumulation and eruption of a sufficient magma volume drives subsidence of the plumbing system roof to form a caldera. The magma plumbing system may then reside relatively unchanged or continue to re-intrude on a variety of scales. Consequences include continued eruptions, crustal resurgence, or new cycles of caldera formation. Large magma volumes characteristic of calderas may evolve as a single progressively-enlarging reservoir or through the rapid amalgamation of small, initially-independent magma pockets. Eruptible magmas may reside at depths of up to 17 km, but typically lie at shallower depths as a caldera system evolves. Timescales of subcaldera magma residence reveal two remarkable concepts: (1) portions of melt within a magma may remain molten for> 106 years, and (2) melt can be created and mobilized in a few thousand years or less.

Geophysical and geochemical data illustrate the present state of active sub-caldera plumbing systems and their development on timescales of hours to years. These studies commonly reveal aseismic, low-velocity zones at depths> 6 km with spatial extents that can be larger than the caldera. The seismic attributes are consistent with rock hosting magma bodies of variable volume and melt content. These are commonly overlain by shallower low-velocity zones linked with ground deformation. The exact nature of these shallower zones is unclear, but interpretations often include shallow sills and laccoliths, and hydrothermal circulation is likely a key process as well.

Seismicity and geodetic data record the interplay between magma movement and crustal deformation at calderas. Together with evidence from field studies, numerical simulations and analogue models, such data show that magma migration at calderas may involve considerable lateral transport through dykes or sills to a site of eruption. While caldera-related magma intrusions commonly exploit structures produced by caldera subsidence, they may also follow regional tectonic structures that extend well beyond the border of the caldera. The increased structural complexity that occurs as a caldera evolves increases the permeability of the crust. This may promote small volume eruptions and shallow storage of magma in the post-collapse phase.

Our review highlights the significant progress made in understanding the range of intrusion styles and timescales that define the magma plumbing beneath calderas. Yet there are still key questions that limit the application of this understanding to directly benefit humanity: (1) What are the limits to the detection of shallow magma bodies, and can we assess the eruption risks associated with magma bodies of different sizes, depths, compositions and crystal contents? (2) Is it viable to generate electricity by extracting heat directly from magma through the sub-caldera plumbing system?

Geophysical and drillhole data from Krafla caldera, Iceland, show that rhyolite can exist undetected at shallow levels within the caldera and may not represent a hazard even when intersected by a borehole. Current work at Krafla is assessing the possibility of extracting geothermal energy from shallow magma bodies and/or superheated steam zones directly above. However, the extent and connectivity of this magma to larger volumes in time and space, as well as its applicability to other systems, may only be answered with continued focused magma drilling, geophysical experimentation and geological exploration.

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Volcanoes commonly inflate or deflate during episodes of unrest or eruption. Continuum mechanics ... more Volcanoes commonly inflate or deflate during episodes of unrest or eruption. Continuum mechanics models that assume linear elastic deformation of the Earth's crust are routinely used to invert the observed ground motions. The source(s) of deformation in such models are generally interpreted in terms of magma bodies or pathways, and thus form a basis for hazard assessment and mitigation. Using discontinuum mechanics models, we show how host-rock fracturing (i.e. non-elastic deformation) during drainage of a magma body can progressively change the shape and depth of an elastic-deformation source. We argue that this effect explains the marked spatio-temporal changes in source model attributes inferred for the March-April 2007 eruption of Piton de la Fournaise volcano, La Reunion. We find that pronounced deflation-related host-rock fracturing can: (1) yield inclined source model geometries for a horizontal magma body; (2) cause significant upward migration of an elastic-deformation source, leading to underestimation of the true magma body depth and potentially to a misinterpretation of ascending magma; and (3) at least partly explain underestimation by elastic– deformation sources of changes in sub-surface magma volume.

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A B S T R A C T Ground subsidence and sinkhole collapse are phenomena affecting regions of karst ... more A B S T R A C T Ground subsidence and sinkhole collapse are phenomena affecting regions of karst geology worldwide. The rapid development of such phenomena around the Dead Sea in the last four decades poses a major geological hazard to the local population, agriculture and industry. Nonetheless many aspects of this hazard are still incompletely described and understood, especially on the eastern Dead Sea shore. In this work, we present a first low altitude (< 150 m above ground) aerial photogrammetric survey with a Helikite Balloon at the sinkhole area of Ghor Al-Haditha, Jordan. We provide a detailed qualitative and quantitative analysis of a new, high resolution digital surface model (5 cm px −1) and orthophoto of this area (2.1 km 2). We also outline the factors affecting the quality and accuracy of this approach. Our analysis reveals a kilometer-scale sinuous depression bound partly by flexure and partly by non-tectonic faults. The estimated minimum volume loss of this subsided zone is 1.83 • 10 6 m 3 with an average subsidence rate of 0.21 m yr −1 over the last 25 years. Sinkholes in the surveyed area are localized mainly within this depression. The sinkholes are commonly elliptically shaped (mean eccentricity 1.31) and clustered (nearest neighbor ratio 0.69). Their morphologies and orientations depend on the type of sediment they form in: in mud, sinkholes have a low depth to diameter ratio (0.14) and a long-axis azimuth of NNE–NE. In alluvium, sinkholes have a higher ratio (0.4) and are orientated NNW–N. From field work, we identify actively evolving artesian springs and channelized, sediment-laden groundwater flows that appear locally in the main depression. Consequently, subrosion, i.e. subsurface mechanical erosion, is identified as a key physical process, in addition to dissolution, behind the subsidence and sinkhole hazard. Furthermore, satellite image analysis links the development of the sinuous depression and sinkhole formation at Ghor Al-Haditha to preferential groundwater flow paths along ancient and current wadi riverbeds.

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The Dead Sea region has faced substantial environmental challenges in recent decades, including w... more The Dead Sea region has faced substantial environmental challenges in recent decades, including water resource scarcity, ~1m annual decreases in the water level, sinkhole development, ascending-brine freshwater pollution, and seismic disturbance risks. Natural processes are significantly affected by human interference as well as by climate change and tectonic developments over the long term. To get a deep understanding of processes and their interactions, innovative scientific approaches that integrate disciplinary research and education are required. The research project DESERVE (Helmholtz Virtual Institute Dead Sea Research Venue) addresses these challenges in an interdisciplinary approach that includes geophysics, hydrology, and meteorology. The project is implemented by a consortiumof scientific institutions in neighboring countries of the Dead Sea (Israel, Jordan, Palestine Territories) and participating German Helmholtz Centres (KIT, GFZ, UFZ). A new monitoring network of meteorological, hydrological, and seismic/geodynamic stations has been established, and extensive field research and numerical simulations have been undertaken. For the first time, innovative measurement and modeling techniques have been applied to the extreme conditions of the Dead Sea and its surroundings. The preliminary results show the potential of these methods. First time ever performed eddy covariance measurements give insight into the governing factors of Dead Sea evaporation. High-resolution bathymetric investigations reveal a strong correlation between submarine springs and neo-tectonic patterns. Based on detailed studies of stratigraphy and borehole information, the extension of the subsurface drainage basin of the Dead Sea is now reliably estimated. Originality has been achieved in monitoring flash floods in an arid basin at its outlet and simultaneously in tributaries, supplemented by spatio-temporal rainfall data. Low-altitude, high resolution photogrammetry, allied to satellite image analysis and to geophysical surveys (e.g. shear-wave reflections) has enabled a more detailed characterization of sinkhole morphology and temporal development and the possible subsurface controls thereon. All the above listed efforts and scientific results take place with the interdisciplinary education of young scientists. They are invited to attend joint thematic workshops and winter schools as well as to participate in field experiments.

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Advances in Volcanology, 2015

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Geological Society, London, Special Publications, 2014

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ABSTRACT Olympus Mons on Mars is an exceptional volcano, not only for its enormous size, but also... more ABSTRACT Olympus Mons on Mars is an exceptional volcano, not only for its enormous size, but also for its structural inventory that includes faulting and mass movements. It is a basaltic shield volcano with a height of 22 km, a diameter of about 600 km, and an average flank slope of 5°. Its characteristics include a summit caldera complex, upper- to mid-flank terraces, a basal circumferential scarp of up to 8 km height, and widespread lobate deposits that extend several hundred kilometers from the basal scarp into the surrounding plains. The formation of these major structural elements and association to gravity tectonics remained unclear, however. This study investigated the combined effects of lithospheric flexure and volcanic spreading in the evolution of Olympus Mons. For this purpose, the deformation of an elastoplastic volcanic cone under Martian gravity was simulated with axisymmetric finite element models. To reproduce observed structural complexities, these models were combined with a viscoelastic mantle and a variable coupling-decoupling behaviour at the interface between volcano and underlying lithosphere. We found that the combination of lithospheric flexure and volcanic spreading is able to account for Olympus Mons upper-flank terraces and basal overthrusting. Terraces are explained with radial compression, with an extent and expression that is related to both lithospheric flexure and the nature of a basal detachment surface. As coupling along the basal detachment decreases, and spreading increases, the zone of flank terracing migrates toward the summit area. The presence of faults on the shield depends on the edifice cohesion and the time of volcano growth relative to mantle relaxation. To produce surface faults, a high edifice cohesion has to be combined with quasi instantaneous volcano emplacement. When edifice cohesion is an order of magnitude lower, however, an instantaneous volcano emplacement is unnecessary to produce surface faults. For a load growing in equilibrium with a deforming mantle, modelled by allowing for viscous relaxation of the mantle between loading steps, faults can be produced on every incrementally-formed volcano surface. For a large shield volcano made of basaltic rock mass, a growth time of up to millions of years and a low cohesion value are more realistic assumptions in the end. Therefore, to understand the structural configuration of Olympus Mons, interfingered processes of deformation need to be considered. Flexure, spreading, and other factors may display complexities that significantly alter the position and expression of surface faults, such as those associated with unstable flanks, circumferential scarps and thrust belts, or terraces.

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Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma r... more Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations has obscured insight into the mechanical interplay between collapse and
eruption. We use multiparameter geophysical and geochemical data to show that the 110-square kilometer and 65-meter-deep collapse of Bárdarbunga caldera in 2014–2015 was initiated through withdrawal of magma, and lateral migration through a 48-kilometers-long dike, from a 12-kilometers deep reservoir. Interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual, nearexponential decline of both collapse rate and the intensity of the 180-day- long eruption.

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Journal of Geophysical Research: Solid Earth, 2013

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Scientific Reports, 2017

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Solid Earth, 2019

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Remote Sensing, 2019

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Frontiers in Earth Science, 2019

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Solid Earth, 2019

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Solid Earth, 2018

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Solid Earth , 2018

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Earth Science Reviews, 2018

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Advances in Volcanology, 2015

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Geological Society, London, Special Publications, 2014

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$Research paper thumbnail of Influences of magma chamber ellipticity on ring fracturing and eruption at collapse calderas$

… Series: Earth and …, Jan 1, 2008

Page 1. Influences of magma chamber ellipticity on ring fracturing and eruption at collapse calde... more Page 1. Influences of magma chamber ellipticity on ring fracturing and eruption at collapse calderas This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 IOP Conf. Ser.: Earth Environ. Sci. 3 012018 ...

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Volcano-related subsidence on Mars has various forms, two of which are the subject of this study.... more Volcano-related subsidence on Mars has various forms, two of which are the subject of this study. On a small scale, collapse calderas have commonly developed at the tops of volcanic edifices. On a larger scale, whole edifices and their underlying basements seem to have subsided via lithospheric flexure. Previous continuum-based numerical simulations of each of these processes revealed a similar pattern of surface stress states. These stress states are generally used in conjunction with Andersonian fault mechanics to predict the structures that should form at the Martian surface in response to subsidence. For calderas, a central zone of reverse faulting and a peripheral zone of normal faulting have been postulated and observed. For lithospheric flexure, the same zones were broadly predicted, but with the addition of an intermediate zone of strike-slip faulting. The prediction of the intermediate zone has been problematic, however, because associated strike-slip faults have not been observed in nature. We report results from experimental models of caldera subsidence and lithospheric flexure, both of which produce arrays of structures that broadly conform to those observed in nature. However, an intermediate zone of strike-slip faulting predicted by Andersonian theory is, as in nature, not observed. Instead, this zone mainly comprises oblique-reverse and oblique-normal faults; ‘pure' strike-slip faults are rare. Overall, the structures observed in analogue simulations of both subsidence types are more compatible with those predicted by 3D strain theory of faulting, rather than Andersonian theory. Our study suggests that a fuller understanding and prediction of faulting related to volcanic subsidence on Mars and other planets should therefore be guided by 3D strain theory.

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Collapse calderas are delimited by reverse ring faults and surrounded by peripheral concentric no... more Collapse calderas are delimited by reverse ring faults and surrounded by peripheral concentric normal faults. In the simplest scenario, circular magma chambers produce circular calderas. Many calderas are elliptical in shape, however, particularly those in highly active tectonic settings. Several factors may explain caldera ellipticity in such regimes: 1) Initial geometry of magma chamber(s) 2) Distribution and orientation of pre-existing regional faults, and 3) Influence of the regional stress field on caldera fault geometries. To better understand relationships between caldera morphology, reservoir geometry and regional tectonics, we conducted two analogue experimental series: One series investigated the influence of orthogonal tectonic stresses on caldera and chamber shapes. In all cases where tectonic stress was applied across circular chambers (balloons), elliptical calderas were produced. Pre-existing basement structures also influenced the shape of calderas, either increasing or reducing elongation. Intrusion of silicon gel into tectonically active sand piles showed that silicon gel chambers responded systematically to applied tectonic stress, and that associated calderas would be elliptical in shape. A second series examined the effect of strike slip faulting on magma chambers and associated calderas. We used sand to simulate brittle crust and cream honey to simulate granitic magma. With a sufficiently high transtensive component, pull-apart-like half grabens formed above the passive honey chamber. Chamber evacuation following strike-slip deformation produced arcuate reverse faults that were again occasionally affected by regional structures. From our results, we identify a number of controls for elliptical caldera formation in tectonically active settings, including initial chamber geometry, caldera fault distortion, and interaction with pre-existing structures. Our results indicate that the final caldera surface expression will be the result of interplay between these processes, and may deviate significantly from the underlying plan-view chamber geometry.

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Olympus Mons is the biggest volcano on Mars, towering 22 km above the NW flank of the Tharsis-Syr... more Olympus Mons is the biggest volcano on Mars, towering 22 km above the NW flank of the Tharsis-Syria upland. Olympus has a characteristic morphology featuring a flat summit, convex upper flanks, concave lower flanks, and a basal escarpment. The summit hosts a six-caldera collapse complex. The upper flanks are terraced in a circumferential, imbricate pattern, whilst the lower flanks are covered in leveed lava channels and lava fans. The basal scarp surrounds much of the shield and reaches heights of 6 km in places. Beyond the escarpment lie aureole deposits, particularly prominent to the NW, downslope from the Tharsis Rise. Although the caldera complex probably developed due to multi-cyclic recharge and evacuation of magma chambers within the edifice, the origin of the remaining features is contentious. Flank terraces have been ascribed to lithospheric flexure, magma chamber tumescence, or volcano spreading. The basal scarp has been linked to tectonic, eruptive, and mass movement processes. The aureole deposits are generally regarded as causally related to the escarpment, but they too have been attributed to a range of formation mechanisms. Previous attempts to explain the genesis of this suite of structures have primarily invoked lithospheric flexure or volcano spreading, but no holistic model yet exists. Here we show that lithospheric flexure, with coeval slip along a basal décollement, is the leading candidate mechanism for the formation of the terraces, scarp, and aureole deposits of Olympus Mons. In a set of scaled analogue experiments, we loaded a ductile silicone putty “lithosphere” with a brittle sand cone “edifice”; a thin layer of putty below the cone served as a detachment surface. Flexure of the underlying silicone produced imbricate, outward-verging convexities on the cone’s upper- and mid flanks, a concave-upward annular trough on its lower flanks, and a prominent scarp at its base. The convexities closely resembled the geometry of terraces on Olympus, and the scarp bore a structural similarity to the basal escarpment encircling the volcano. We therefore propose that the distinctive features of Olympus Mons were generated as follows. Flexure-induced constriction of the volcano was accommodated on its mid- to upper flanks by distributed terracing, and at its base by flexural slip along a décollement; movement along this slip surface produced the basal scarp. Oversteepening of the escarpment led it to fail along localised listric normal faults, forming landslide deposits in an aureole about the volcano. These deposits probably helped fill the attendant flexural trough and obscure associated surface fractures. A detachment has long been suspected beneath Olympus, and may consist of incompetent material entrained within the edifice, e.g. (ice-rich) volcanoclastics, aeolian deposits, and argillaceous sediments. Beyond shaping this volcano, flexural slip might also have influenced the structure of other volcanoes, accounting for basal scarps surrounding the nearby Arsia and Ascraeus Montes, and Mauna Loa on Earth.

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Super-volcano eruptions are intimately associated with kilometre-scale subsidence of an over 10 k... more Super-volcano eruptions are intimately associated with kilometre-scale subsidence of an over 10 km wide magma chamber roof and formation of a collapse caldera. Understanding the initiation, development, and interrelation of structures formed during caldera subsidence is therefore essential for more accurately assessing the eruptive threat and economic potential presented by super-volcanoes. Currently, structural architectures and subsidence styles of collapse calderas are each viewed as end- members, which usefully convey idealised structural geometries and subsidence styles. However, natural calderas rarely, if ever, unequivocally conform to one end-member model. Instead, natural calderas often include structural elements of several end-members. The occurrence and relative importance of each end- member has therefore been intensely debated. In addition, the possibility of some form of continuum between these end-members has been raised, but this has not been clearly defined. We synthesise recent advances in caldera volcano research from two extensively applied, but thus far poorly integrated, scientific approaches - analogue experimental modelling and detailed field-based studies. By methodically relating evidence from field and experimental studies of caldera formation, we show that calderas on all scales on Earth, including those associated with super-volcano eruptions, most likely share the same overall structural evolution and architecture. Moreover, we also show that the characteristic structural elements of each of the five end-member caldera types may accumulate through gradational phases of a single collapse evolution. This phenomenon explains why most natural calderas display structural elements of several idealised end-members. It also relates the putative continuum between idealised caldera end-members to progressive subsidence and evolving structural maturity.

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Subsidence of the roof of a magma reservoir commonly produces volcanic depressions, which, depend... more Subsidence of the roof of a magma reservoir commonly produces volcanic depressions, which, depending on scale, are termed pit craters or collapse calderas. To identify what geometric and mechanical factors influence the structural evolution of this subsidence, we used Distinct Element Method (DEM) numerical models. The models comprise an assemblage of gravitationally-loaded circular particles that interact at their contacts according to elastic-frictional laws. Cohesion is provided by inter-particle bonds that break once their shear or tensile strength is exceeded. Varying particle and bond micro-properties yields assemblage macro-properties characteristic of natural rock masses (e.g. elasticity, failure, and strain softening), with fracture localization simulated by bond breakage. The magma reservoir is represented as a region of non-bonded, low-fiction particles. Withdrawal of magma is simulated by incrementally reducing the area of the reservoir particles. The resultant gravity-driven failure and subsidence of the overlying reservoir roof is explicitly replicated. We investigated the effect on subsidence of several initial mechanical and geometric properties. Of these, the strength (cohesion) and thickness/diameter ratio (T/D) of the roof exerted strongest influences, whilst the roof's Young's Modulus and the reservoir's ceiling curvature imparted secondary influences. These properties interacted to produce four ‘end-member' subsidence styles, although elements of several styles tended to occur in any one model realization: (1) Predominantly central sagging at low T/D and low strength; (2) hitherto unrecognized ‘central snapping' at low T/D and high strength; (3) predominantly single central block subsidence at low to intermediate strength and at intermediate T/D ratios; (4) multiple central block subsidence at high strength and high T/D ratios. The critical T/D ratio, at which subsidence changes from single central block to multiple central block, was strongly dependent on material strength. Our model results provide a more quantitative understanding of the genesis of subsidence styles observed or inferred in the field: e.g. at Nindiri crater, Nicaragua; Dolomieu caldera, Reunion; and Miyakejima caldera, Japan.

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Pit craters and collapse calderas are volcanic depressions produced by subsidence of the roof of ... more Pit craters and collapse calderas are volcanic depressions produced by subsidence of the roof of a magma reservoir. They are near-ubiquitous features of volcanic fields or edifices on Earth and other terrestrial planets. Pit crater and caldera subsidence may occur by diverse structural mechanisms, each previously linked with various initial controlling factors. We present Distinct Element Method (DEM) numerical models of this subsidence process to shed new light on such mechanisms and the factors controlling them. Our models comprise an assemblage of gravitationally-loaded circular particles that interact at their contacts according to elastic-frictional laws. Cohesion is provided by inter-particle bonds that break once their shear or tensile strength is exceeded. Varying particle and bond micro-properties yielded assemblage macro-properties and rheological responses characteristic of natural rock masses (e.g. elasticity, failure, and strain softening), with fracture localization simulated by bond breakage. The magma reservoir is represented as a region of non-bonded, low-friction particles. Withdrawal of magma is simulated by incrementally reducing the area of the reservoir particles. The resultant gravity-driven failure and subsidence of the overlying reservoir roof is explicitly replicated. We have investigated the effect on subsidence of initial mechanical and geometric properties. The former included Young’s Modulus and strength (cohesion), whilst the latter included the thickness/diameter ratio (T/D) of the roof and the ceiling curvature imparted by the shape of the underlying reservoir. These initial mechanical and geometric properties interacted to produce several ‘end-member’ subsidence mechanisms: (1) Predominantly central sagging at low strength and low T/D; (2) predominantly single central block subsidence at low to intermediate strength and at intermediate T/D; (3) multiple central block subsidence at high strength and high T/D; (4) a hitherto unrecognized ‘central snapping’ mechanism at high strength, low T/D, and with a low ceiling curvature. In terms of material properties, the occurrence of these mechanisms was weakly dependent on Young’s Modulus, but strongly dependent on material strength. In terms of geometric properties, a moderate dependency on reservoir shape at low T/D diminished at intermediate and high T/D. One important outcome of the interaction of mechanical and geometric properties is that the critical T/D ratio, above which the single central block mechanism changes to the multiple central block mechanism, decreases from > 1 to < 0.6 with increasing material strength. Our model results agree well with those of analogue studies of pit crater and caldera subsidence, and provide a more quantitative understanding of the genesis of different mechanisms observed in the field - e.g. at Nindiri crater, Nicaragua and Miyakejima caldera, Japan.

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Collapse calderas are structurally delimited by reverse ring faults and are surrounded by periphe... more Collapse calderas are structurally delimited by reverse ring faults and are surrounded by peripheral concentric normal faults. In the simplest scenario, circular magma chambers produce circular calderas. Many calderas however, are elliptical in shape, particularly those in extensional and compressional tectonic settings. Two possible mechanisms may explain elliptical calderas. The first is the presence of an elliptical magma chamber, established by preferential growth along pre-existing basement structures or multicyclicity. The second is distortion of the caldera faults by the regional stress field during and/or after caldera formation. To better understand the relationship between caldera morphology, reservoir size, reservoir depth and regional tectonic stresses, we conducted scaled analogue experiments. We used circular reservoir shapes (simulated with rubber balloons) and placed them at specific depths in a sand-filled deformation rig. Under non-stress conditions calderas were predictably circular in plan. However, within regional stress fields of increasing strength, calderas appeared increasingly elliptical in plan, despite the underlying magma chambers being circular. Overall, calderas produced from circular magma chambers in compression/extension experiments were elongate parallel to the direction of least compressive stress. This observation suggests that for caldera formation in regional tectonic regimes, distortion of the caldera ring faults may be of great significance. Consequently, the orientation of the ring fault varies from steeply dipping at the caldera margin that faces the maximum compressive stress, to shallower dips along the section of the margin that faces the maximum extensional stress. This variation of caldera fault dip may influence the eruptive behaviour at different locations along such an elliptical ring fissure. Distinction between caldera fault distortion and preexisting weaknesses/elliptical chamber growth, may be difficult in nature due to the complementary effects of those mechanisms. Only detailed analysis of further field and experimental data will resolve the relative importance of either mechanism.

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