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Magma in the Earth's crust is commonly transported through dikes. Fractures and faults, which are common in the shallow crust, form structural weaknesses that can act as energy-efficient propagation pathways. Although examples of this are known from active and extinct volcanoes in varying host rocks, the conditions and mechanisms of how and when dikes are influenced by these structures are not yet fully understood. This study investigates how basaltic dikes propagating through hyaloclastite in the shallow crust interact with pre-existing fractures. Using virtual 3D-models from drone-based photogrammetry, we mapped basaltic dikes exposed in a caldera-filling hyaloclastite in the extinct Dyrfjoll volcano, NE-Iceland, to measure the orientations of fractures and dikes, and quantify their interactions. We observe 39 changes in strike among 45 dikes and found a strong control of the governing stress field on orientations and interactions. Three types of dike-fracture interaction were identified: (1) Dikes propagating along pre-existing fractures. This is most frequently observed for dikes following the tectonic stress field. (2) Dikes with an abrupt change in strike occurring near or at a crosscutting fracture, but without magma flow into the fracture. (3) Dikes arrested at a crosscutting fracture. Such dikes may develop offshoots near the dike tip, which may approach the fracture at different angles and be able to cut across. Understanding how dikes interact with pre-existing fractures in moderately fractured host rock such as hyalo-clastite is relevant for hazard assessment and monitoring of volcanically active areas.

Field observations of extinct and exposed magma reservoirs shed light on processes operating in the roots of presently active volcanoes. The Reyðarártindur pluton, Southeast Iceland is an example of a fossil shallow magma reservoir that fed eruptions. The different chapters in this thesis examine the accumulation of magma, and processes occurring during the development and evolution of the magma reservoir from different methodological perspectives. A final model for the evolution of the Reyðarártindur pluton is then presented.

The majority of the pluton consists of one voluminous rock unit, the Main Granite, that formed by rapid magma emplacement. However, a local zone of geochemically distinct, but related further Granite Enclaves and Quartz Monzonite Enclaves attest to variations in the composition of the underlying source reservoir. Space for the ca. 2.5 km³ of magma in the pluton was made by piecemeal floor subsidence, which began with multiple dykes that then propagated laterally to form flat-roofed intrusions at different depths. During the first stages of magma emplacement, shattering, sintering and sanidinite-facies contact metamorphism affected a ca. 10 m thick zone of the basalt host rock at the magma reservoir roof. The resulting hornfels was stronger than the original altered basalt, and contained zero porosity and permeability. It thus formed a ‘cap-rock’ to the magma reservoir, limiting heat, volatile and fluid transfer until fractured and faulted at a later stage. 

The magma reservoir erupted at least once, causing local subsidence of the roof, which would have been observable at the Earth’s surface. Recharge of the magma reservoir by the same Quartz Monzonite and further Granite as exposed in the Reyðará River led to overpressure and eruption. We envisage that cooling and sealing of the piecemeal subsidence network preceded eruption, causing overpressure with magma recharge. The eruptive lifetime of the magma reservoir was limited to ca. 1000 years. This timeframe is much less than the duration of silicic magmatism in a typical Icelandic central volcano, or at other rhyolite-erupting volcanoes worldwide, which is in the order of hundreds of thousands to millions of years. Hence, the Reyðarártindur pluton likely represents a small, ephemeral part of a wider magmatic plumbing system that feeds a central volcano.

The results from these studies can provide volcano-monitoring personnel with scenarios for magma emplacement, and processes leading to eruption, which they can then use as a framework for interpreting detectable signals of magma movement and volcanic unrest.

Shallow magmatic reservoirs have been identified at many volcanoes worldwide. However, questions still remain regarding their size, dynamics and longevity. The Reyðarártindur Pluton exposed in Southeast Iceland provides a superb example to investigate the above questions. Here, we use field mapping, sampling, geochemistry, 3D pluton shape modelling and a numerical thermal model to reconstruct the assembly and eruptive history of the shallow magma body.

In 3D, the c. 2.5 km3 pluton has a castle-like shape characterised by flat roof segments that are vertically offset along steep faults. The exposed pluton is constructed largely of a single rock unit, the Main Granite (69.9 to 77.6 wt.% SiO2). Two additional units occur only as enclaves: the Granite Enclaves (67.4 to 70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8 to 67.3 wt.% SiO2). However, geochemistry clearly indicates that the units are related and hence were likely derived from the same source reservoir. 

In two locations, the pluton roof displays depressions associated with large dykes. Within these two dykes the rock is partially to wholly tuffisitic, and geochemical compositions range from quartz monzonite to granite. We interpret these dykes as eruption-feeding conduits from the pluton. Additionally, we speculate that the mingling of magmatic units with compositional ranges from quartz monzonite to granite within the conduits indicates that injection of new magma into the reservoir triggered eruption. 

Rapid pluton construction is indicated by ductile contacts between units in the pluton and a thermal model calculates the top 75 m would have rheologically locked up within 1000 years. Hence, we argue that the pluton was a short-lived part of the wider magmatic system that fed the associated volcano, and that timeframes from emplacement to eruption were limited to 1000 years.

Rhodes, E. Barker, A. K. Burchardt, S. et al. (2021). Rapid assembly and eruption of a shallow silicic magma reservoir, Reyðarártindur Pluton, Southeast Iceland. G-Cubed. DOI: 10.1029/2021GC009999

Although it is widely accepted that shallow silicic magma reservoirs exist, and can feed eruptions, their dynamics and longevity are a topic of debate. Here, we use field mapping, geochemistry, 3D pluton reconstruction and a thermal model to investigate the assembly and eruptive history of the shallow Reyoarartindur Pluton, southeast Iceland. Primarily, the exposed pluton is constructed of a single rock unit, the Main Granite (69.9-77.7 wt.% SiO2). Two further units are locally exposed as enclaves at the base of the exposure, the Granite Enclaves (67.4-70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8-67.3 wt.% SiO2). Geochemically, the units are related and were likely derived from the same source reservoir. In 3D, the pluton has a shape characterized by flat roof segments that are vertically offset and a volume of >2.5 km(3). The pluton roof is intruded by dikes from the pluton, and in two locations displays depressions associated with large dikes. Within these particular dikes the rock is partially to wholly tuffisitic, and rock compositions range from quartz monzonite to granite. We interpret these zones as eruption-feeding conduits from the pluton. A lack of cooling contacts throughout the pluton indicates rapid magma emplacement and a thermal model calculates the top 75 m would have rheologically locked up within 1,000 years. Hence, we argue that the Reyoarartindur Pluton was an ephemeral part of the wider plumbing system that feeds a volcano, and that timeframes from emplacement to eruption were rapid.

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

Reyðarártindur is one of several felsic plutons exposed in Southeast Iceland, interpreted to be the shallow plumbing systems of late Neogene volcanic centres (Cargill et al., 1928; Furman et al., 1992; Padilla, 2015). These plutons are considered to preserve analogous plumbing systems to the central volcanoes active in Iceland today (Furman et al., 1992). Reyðarártindur is the oldest pluton in Southeast Iceland at 7.30 ± 0.06 Ma (Padilla, 2015), and has been conveniently incised by the Reyðará River, making it ideal for an in-depth study of the external and internal geometry of a shallow rift-zone magma plumbing system.

In order to analyse mechanisms of magma emplacement, we have conducted detailed structural mapping of the pluton and its basaltic host rock using drone-based photogrammetry. To complement this, we have also extensively sampled and analysed the geochemistry and petrology of the pluton interior. An outline of the pluton is shown in Figure 1, highlighting that the pluton is NNW-SSE trending, which is in contrast to the NE-SW regional dyke trend. A total thickness of 500 m and a calculated volume of 1.5 km3 is exposed. While the pluton walls are steeply-dipping, the pluton roof is mostly flat. Deviations from the flat roof occur in the form of areas that are cut by steep dip-slip faults with displacements of up to 100 m. Roof faulting creates both structural highs (horsts) and lows (grabens, as well as a monoclinal structure) in the roof. Many of the faults are intruded by felsic dykes, some of them seem to have been the feeders of surface eruptions.

An estimated 95% of the pluton volume is rhyolitic in composition, with 73-76 wt.% SiO2. Geochemically, the magma in the majority of the pluton is similar, but hand samples and thin sections show a large variety of textures. In the lower part of the exposure there is a zone of mingling and mixing between a matrix magma and several different types of silicic enclaves (Figure 1). The matrix magma is more mafic with an SiO2 content of 68-73 wt.% and the enclaves vary in nature with no systematic shape, size or aspect ratio. There are at least two types of enclaves, and the predominant type is a coarse grained trachydacite with 64-69 wt.% SiO2. These less evolved compositions are limited to a 1 km stretch of the riverbed in the centre of the pluton. Closer to the wall contacts (i.e. to the north and south of the mingling zone), the composition of the magma returns to that of the main magma body, as observed at higher elevations.

Our poster aims to summarise our results and present interpretations of the magmatic processes preserved in the Reyðarártindur pluton. Our preliminary results indicate that the pluton was emplaced by a combination of floor subsidence and roof doming, and that the pluton structure was modified during further magma intrusion into, and eruption from, the pluton.

 Fig. 1 – Map of the Reyðarártindur Pluton, South-East Iceland.

References

Cargill, H., Hawkes, L., and Ledeboen, J. (1928). The major intrustions of South-Eastern Iceland. Quarterly Journal of the Geological Society of London 84, 505–539.

Furman, T., Meyer, P. S., and Frey, F. (1992). Evolution of Icelandic central volcanoes: evidence from the Austurhorn intrusion, southeastern Iceland. Bulletin of Volcanology. 55, 45–62.

Padilla, A. (2015). Elemental and isotopic geochemistry of crystal-melt systems: Elucidating the construction and evolution of silicic magmas in the shallow crust, using examples from southeast Iceland and southwest USA [PhD Dissertation: Vanderbilt University].

A team of scientists from the Centre of Natural Hazards and Disaster Science (www.cnds.se) and Uppsala University are planning a study on the role of Wikipedia during volcanic eruptions.

Pageview statistics show a spike of up to 200 000 views (e.g. Anak Krakatau eruption and associated tsunami in December 2018) at the onset of a volcanic event. In acknowledgement of this, the team want to explore the role of Wikipedia as a disaster response resource during such events, when accurate and readily accessible information can save lives. The poster will outline our current approach to this project, propose suitable methods and illustrate the work in progress.

Climate change, globalization, urbanization, social isolation, and increased interconnectedness between physical, human, and technological systems pose major challenges to disaster risk reduction (DRR). Subsequently, economic losses caused by natural hazards are increasing in many regions of the world, despite scientific progress, persistent policy action, and international cooperation. We argue that these dramatic figures call for novel scientific approaches and new types of data collection to integrate the two main approaches that still dominate the science underpinning DRR: the hazard paradigm and the vulnerability paradigm. Building from these two approaches, here we propose a research framework that specifies the scope of enquiry, concepts, and general relations among phenomena. We then discuss the essential steps to advance systematic empirical research and evidence-based DRR policy action. Plain Language Summary The recent deadly earthquake in Iran-Iraq has been yet another reminder of the topicality of natural hazards, and it has come just after an unprecedented series of catastrophic events, including the extensive flooding in South Asia and the string of devastating hurricanes in the Americas. He we identify three main puzzles in the nexus of natural hazards and vulnerabilities, and demonstrate how novel approaches are needed to solve them with reference to a flood risk example. Specifically, we show how a new research framework can guide systematic data collections to advance the fundamental understanding of socionatural interactions, which is an essential step to improve the development of policies for disaster risk reduction.

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