1,721,232 research outputs found
Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength
Full text linkMagmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution
“Low pressure” (P≥150 MPa) experiments in piston cylinder apparatus
No full textThe piston cylinder apparatus is one of the most versatile instruments operating in laboratories of experimental petrology because it provides a safe, inexpensive, and easy-to-use technique for accessing high-pressure and high-temperature phases and processes. The operating pressure of the piston cylinder usually ranges between 0.5 and 5 GPa, which allows Earth scientists to investigate materials and processes occurring at deep crustal to upper mantle levels. Incomplete compaction of the assembly and thermocouple failure are typical problems in experiments performed at pressures lower than 0.5 GPa. In this work, we present new calibration data that demonstrates that the piston cylinder apparatus is suitable for experiments at pressure as low as to 150 MPa. Two new designs for the 25 mm assembly have been developed and calibrated using two different calibration methods: the NaCl melting curve and the solubility of H2O in albite and rhyolite melts. NaCl calibration data shows that positive friction corrections of 45, 55, and 60 MPa applied to the force of the piston are necessary to establish sample pressures of 300, 200, and 150 MPa respectively. The H2O solubility experiments in albite and rhyolite not only confirm the corrections determined using the NaCl calibration method, but also indicate that the friction correction is the same even when operating at temperatures higher than 800°C and for 24-hour durations. The accuracy of the pressure estimate associated with the calibration methods is ±25 MPa.
The major advantage of using the new design assemblies in the piston cylinder apparatus is that experiments for which low-pressure conditions as well as very fast heating and quenching rates are required (e.g. volcanic and hydrothermal systems), can be performed with the same ease and precision as for the pressure ranges for which piston cylinders are routinely employed
Towards a very low-friction assembly for piston cylinder: can we go below 150 MPa?
No full textThe piston cylinder apparatus is one of the most versatile instruments operating in laboratories of experimental petrology because it provides a safe, inexpensive, and easy-to-use technique for accessing high-pressure and high-temperature phases and processes. The operating pressure of the piston cylinder usually ranges between 0.5 and 5 GPa, which allows Earth scientists to investigate materials and processes occurring at deep crustal to upper mantle levels. Incomplete compaction of the assembly and thermocouple failure are typical problems in experiments performed at pressures lower than 0.5 GPa.
We have demonstrated that, using proper assemblies, piston cylinder apparatus can effectively be used at pressure as low as 150 MPa. At this purpose, we have developed and tested a new 25 mm furnace assembly, made up of crushable MgO, borosilicate glass and NaCl. Calibration of the new assembly yielded to a significant increase of correction at lower pressure (an additional correction +60 MPa is required when operating at 150 MPa). With the aim to further decrease the operative pressure limit of piston cylinder below 150 MPa, we are testing a new “softer” assembly, in which the MgO sample holder has been replaced with a mixture of NaCl and KCl. The use of the softer material allows a better compaction of the assembly, maintaining an hydrostatic regime even for lower hydraulic pressure. Nevertheless, the softening/melting of the salt decreases the resistivity of the assembly, implying a higher operation current. This problem can be worked out by insulating the sample holder salt from the graphite furnace.
The major advantage of using the new design 25 mm assemblies in the piston cylinder apparatus is that experiments for which low-pressure conditions as well as very fast heating and quenching rates are required (e.g. volcanic and hydrothermal systems), can be performed with the same ease and precision as for the pressure ranges for which piston cylinders are routinely employed
Merapi Volcano: Geology, Eruptive Activity, and Monitoring of a High-Risk Volcano
No full textThis book provides the first comprehensive compilation of cutting-edge research on Merapi volcano on the island of Java, Indonesia, one of the most iconic volcanoes in the world. It integrates results from both the natural (geology, petrology, geochemistry, geophysics, physical volcanology) and social sciences, and provides state-of-the-art information on volcano monitoring, the assessment of volcanic hazards, and risk mitigation measures.As one of Indonesia’s most active and dangerous volcanoes, Merapi is perhaps best known for its pyroclastic density currents, which are produced by gravitational or explosive lava dome failures (commonly referred to as Merapi-type nuées ardentes). Merapi’s eruptions have posed a persistent threat to life, property and infrastructure within the densely populated areas on the volcano’s flanks, as demonstrated most recently by catastrophic eruptions, which attracted worldwide media interest
Three-dimensional geometry of concentric intrusive sheet swarms in the Geitafell and the Dyrfjöll Volcanoes, Eastern Iceland
No full textSheet intrusions (inclined sheets and dykes) in the deeply eroded volcanoes of Geitafell and Dyrfjöll,eastern Iceland, were studied at the surface to identify the location, depth, and size of their magmaticsource(s). For this purpose, the measured orientations of inclined sheets were projected in three dimensionsto produce models of sheet swarm geometries. For the Geitafell Volcano, the majority of sheetsconverge toward a common focal area with a diameter of at least 4 to 7 km, the location of which coincideswith several gabbro bodies exposed at the surface. Assuming that these gabbros represent part of the magmachamber feeding the inclined sheets, a source depth of 2 to 4 km below the paleoland surface is derived.A second, younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm.The source of this second swarm is estimated to be located to the SE of the source of Swarm 1, below thepresent‐day level of exposure and deeper than the source of the first swarm. For the Dyrfjöll Volcano,we show that the sheets can be divided into seven different subsets, three of which can be interpretedas swarms. The most prominent swarm, the Njardvik Sheet Swarm, converges toward a several kilometerswide area in the Njardvik Valley at a depth of 1.5 to 4 km below the paleoland surface. Two additionalmagmatic sources are postulated to be located to the northeast and southwest of the main source. Crosscuttingrelationships indicate contemporaneous, as well as successive activity of different magma chambers,but without a resolvable spatial trend. The Dyrfjöll Volcano is thus part of a complex volcanic cluster thatextends far beyond the study area and can serve as fossil analog for nested volcanoes such as Askja, whereasin Geitafell, the sheet swarms seem to have originated from a single focus at one time, thus defining a singlecentral volcanic complex, such as Krafla Volcano
An experimental study of the geometry and kinematics of caldera collapse
No full textTHESIS 8606Calderas are 1 - 100 km diameter, volcanic depressions that form primarily through m-scale to km?scale subsidence of a magma reservoir roof into an underlying magma reservoir. This study addresses several fundamental geometric and kinematic aspects of the structure of caldera volcanoes via a series of scaled physical (or analogue) modelling experiments. Results of these models are combined in detail with observations from natural calderas recorded in the literature, in order to provide as strong an element of ?ground-truthing? for the experimental results as possible.
The basic experimental materials and methods are common to all the results chapters (2-6), and so for convenience are summarised together in Chapter 1. An exception to some extent is the experiment set described is Chapter 5; these were made with different materials and apparatus, the particulars of which are described in that chapter. Each of the results chapters is otherwise self-contained, with its own specific introduction to the problems addressed, summary of data, discussion, and conclusions. The contents of Chapters 1, 5, and 6 have been published in two peer-reviewed articles. The remaining chapters are presently unpublished, although Chapter 3 and part of Chapter 4 have been submitted for peer review, whilst the rest of Chapter 4 and a part of Chapter 2 will be submitted for publication at a later date. Given the assistance of co-authors in the preparation of manuscripts now integrated into the thesis, my contribution to the individual chapters as presented here may be summarised as: Chapter 1: 85%, Chapter 2: 90%, Chapter 3: 75%, Chapter 4: 80%, Chapter 5: 50%, Chapter 5: 75%. The structure of the thesis, the major aspects of caldera evolution investigated, and the main results are as follows:
Chapter 1: Introduction and Methodology - outlines the current understanding of caldera structure as achieved to date by previous authors and supplies the rationale for using physical models to help unravel caldera evolution. This section also summarises the insights for how these. Details of the experimental methodology employed in this study, as well as scaling considerations for the application of small-scale models to the study of large-scale deformation associated with caldera subsidence in nature, are also provided.
Chapter 2: The Geometry and Evolution of Circular Caldera Collapse Structures - details the results of experimental subsidence of mechanically isotropic reservoir roofs that are circular in plan view. This baseline experiment set illustrates the generalised kinematic evolution of caldera subsidence. Structures resolving radial and concentric strains are documented, the latter for the first time. The control of the roof?s thickness/diameter ratio upon these structures? development is also highlighted. The kinematics of caldera ring faults, the generation of polygonal caldera outlines and the development of piecemeal-type subsidence are also discussed in light of the evidence from experiment and nature.
Chapter 3: The Caldera Collapse Continuum - systematically integrates results of simple circular collapses in Chapter 2 with those from past experimental, numerical and field studies to produce a ?state-of-the-art? synthesis of the mechanics and kinematics of caldera collapse. This synthesis sheds new light on the long-standing but poorly-defined continuum between end-member caldera collapse styles. Structural elements characteristic of each end-member are shown to reflect progressively overlapping structural sub-processes in a single collapse event. The continuum between end-member collapse styles is thus defined primarily as a progressive evolution related to increasing subsidence, although the regulatory role of roof thickness/diameter and caldera subsidence/diameter ratios in the development of structural elements of each collapse end-member is also highlighted.
Chapter 4: The Role of Magma Chamber Ellipticity in Caldera Collapse ? documents the results of experimental collapse of mechanically isotropic reservoir roofs that are varyingly elliptical in plan view. Physical models demonstrate that an elliptical reservoir roof predictably fails first at the ends of its shorter principal axis, and show that lateral propagation (?unzipping?) patterns vary systematically with ellipticity. The models also highlight a characteristic geometric arrangement of elliptical ring structures and provide a first comprehensive view of the elliptical caldera structural architecture in 3D.
Chapters 5 & 6: Regional tectonic influences on the evolution of caldera collapse ? outlines the results of experimental collapse of reservoir roofs that contain pre-existing mechanical anisotropies in the form of regional-tectonic faults and collapse under regional tectonic stress. Although disregarded in most previous physical and numerical models of calderas, regional faults have long been thought to play a major role in caldera development, principally through their reactivation during collapse. In Chapter 5, caldera formation in areas of orthogonal regional deformation is examined; in Chapter 6, caldera collapse in strike-slip regimes is investigated. In physical models, regional faults reactivated during collapse where they coincided with and ran parallel to the magma reservoir margins. Here, where syn-collapse strain usually tends to localise, pre-collapse faults are optimally orientated for reactivation. Contrary to inferences from some previous field studies, differential movement on pre-collapse faults was not observed in central parts of the subsiding reservoir roof. Physical models thus confirm that regional fault reactivation is an important process, and help clarify conditions favouring it.
Chapter 7: Summary of Main Conclusions and Outlook - provides a brief overview of the main findings of this thesis, notes their general implications for our understanding of calderas, and outlines some potential avenues for future work.
An electronic appendix (DVD) is attached to the back of the thesis. This appendix contains photos of all experiments (in jpeg format), animations of the experiments (in MS PowerPoint files), and copies of the two published articles (in PDF format)
Magmatic differentiation and bimodality in oceanic island settings - implications for the petrogenesis of magma in Tenerife, Spain
No full textTHESIS 9028The Tenerife post-Icod-collapse succession, comprised of the Teide-Pico Viejo central complex and its adjacent rift zones, marks the latest eruptive cycle on Tenerife (200-0 ka) that broadly evolved from primitive lavas to differentiated and partly explosive volcanism. At the same time, primitive lavas continued to erupt from dyke complexes in the rift zones, while intermediate lavas effused in the geographical transition from rift zone to central complex. To constrain the magmatic processes, that gave rise to the observed temporal and spatial patterns, several types of geochemical analyses of these rocks were applied and results embedded into a detailed, pre-existing framework of radiometric ages and whole-rock data.
A case study of the composite lava flow of Montana Reventada allowed to investigate magma mixing as one potential mechanism to generate intermediate magma on Tenerife. The two end-members were a basanite and a phonolite, which erupted one after another, the basanite before the phonolite. The phonolite carries a considerable amount of mafic enclaves. Based on field evidence, the magma mixing event was constrained to a short interval before the eruption. A detailed geochemical dataset was used to confirm the mixed nature of the inclusions and to determine mixing ratios. Not all elements and oxides could be modelled, which is explained by observed crystal exchange between basanite and phonolite and by interdiffusion of trace elements between enclaves and phonolite. It thus appears that intermediate magma may form by magma mixing on Tenerife
Consequences of giant landslides on ocean island magmatism: volcanic and geochemical evolution of the Teno massif, Tenerife, and El Hierro Island (Canary Archipelago)
No full textTHESIS 878
Quantitative studies of rock fabrics and textures in layered mafic intrusions of the British tertiary igneous province : implications for magma system emplacement and evolution
No full textTHESIS 8289This thesis examines and considers the processes which led to magmatic layering and associated planar and linear fabrics in mafic and ultramafic rocks of four igneous centres of the British (and Irish) Palaeogene Igneous Province. The intrusions studied are the Ardnamurchan Centre 3 Gabbros, the Rum Layered Suite, the Skye Druim Hain Layered Gabbros (all in NW Scotland) and the Carlingford Later Gabbros (in NE Ireland). The interpretations of layer formation have important implications for magma chamber processes, and for intrusion geometries and emplacement mechanisms.
The study utilizes detailed field observations, modem methods of quantitative textural (Crystal Size Distribution; CSD) and rock microfabric (Anisotropy of Magnetic Susceptibility; AMS) analysis together with mineral-chemical data. AMS fabrics are typically carried by magnetite and are generally oriented parallel to visible macroscopic layer-parallel foliations. In the case of the Ardnamurchan and Rum intrusions, deformed structures in the layered rocks and consistently plunging orientations of magnetic lineations are interpreted as strong evidence for significant central subsidence of each body following initial emplacement. For the Ardnamurchan Centre 3 rocks, the combined evidence suggests that the intrusion is a composite lopolith and not a ring-dyke, as previously suggested. The close association of syn- magmatically deformed layering with inward-plunging magnetic lineations in parts of the Carlingford Later Gabbros suggests that this intrusion may also have undergone central subsidence, though less than at Ardnamurchan or Rum
The structure of Guadeloupe, Maderas and Mt Cameroon volcanoes and the impact of strike-slip movements
No full textTHESIS 8982There are three kinds of strike-slip faults: pure strike-slip, transtensional and transpressional. They have been recognized in all geodynamic environments and are the most common fault type associated with volcanic activity. Many volcanic edifices are built in the vicinity of a fault with strike-slip motion. The impact of strike-slip fault movements on a volcanic cone has been addressed by several studies over the last decade. This study considers a broad range of fault and volcano geometries through three natural examples: the Guadeloupe volcanoes in the Lesser Antilles, Mt Cameroon in West Africa and Maderas volcano in Nicaragua. Detailed field and remote sensing studies are used to establish structural maps of these three little studied volcanoes. These maps are then compared with experimental structures that have developed in analogue cones deformed by strike-slip, transtensional and transpressional faults. The study of Guadeloupe volcanoes leads to a new interpretation of its constructional phases. A regional NW-SE striking sinistral transtensional fault on which the Guadeloupe volcanoes have been built is responsible for their alignment, for the dyke strikes and for the major collapse events. On Mt Cameroon, the rift zone and elongated morphology are controlled by the inactive strike-slip fault on which the volcano has been built. This study also reveals that Mt Cameroon volcano has spread over its weak sedimentary substratum. On Maderas volcano, the summit graben, vent alignment, lower flank half-grabens and summit lineaments are related to gravitational spreading and to regional tectonic movements. The orientation of these structures indicates that the Maderas volcano was built above a NW-SE striking dextral transtensional fault. The theoretical model of strike-slip motion and volcanic cone interaction established with the analogue models can be applied to these and to other volcanoes to determine the location, slip, kinematics and strike of structures hidden by recent eruptions and intense erosion
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