Scales of columnar jointing in igneous rocks: field measurements and controlling factors

Columnar jointing is a common feature of solidified lavas, sills and dikes, but the factors controlling the characteristic stoutness of columns remain debated, and quantitative field observations are few in number. In this paper, we provide quantitative measurements on sizing of columnar joint sets and our assessment of the principal factors controlling it. We focus on (1) chemistry, as it is the major determinant of the physical (mechanical and thermal) properties of the lava, and (2) geology, as it influences the style of emplacement and lava geometry, setting boundary conditions for the cooling process and the rate of heat loss. In our analysis, we cover lavas with a broad range of chemical compositions (from basanite to phonolite, for six of which we provide new geochemical analyses) and of geological settings. Our field measurements cover 50 columnar jointing sites in three countries. We provide reliable, manually digitized data on the size of individual columns and focus the mathematical analysis on their geometry (23,889 data on side length, of which 17,312 are from full column sections and 3,033 data on cross-sectional area and order of polygonality). The geometrical observations show that the variation in characteristic size of columns between different sites exceeds one order of magnitude (side length ranging from 8 to 338cm) and that the column-bounding polygons' average order is less than 6. The network of fractures is found to be longer than required by a minimum-energy hexagonal configuration, indicating a non-equilibrium, geologically quick process. In terms of the development and characteristic sizing of columnar joint sets, our observations suggest that columns are the result of an interplay between the geological setting of emplacement and magma chemistry. When the geological setting constrains the geometry of the emplaced body, it exerts a stronger control on characteristic column stoutness. At unconstrained geometries (e.g. unconfined lava flows), chemistry plays the major role, resulting in stouter columns in felsic lavas and slenderer columns in mafic lava

Effects of basal drag on subduction dynamics from 2D numerical models

Subducting slabs are an important driver of plate motions, yet the relative importance of different forces in governing subduction motions and styles remains incompletely understood. Basal drag has been proposed to be a minor contributor to subduction forcing because of the lack of correlation between plate size and velocity in observed and reconstructed plate motions. Furthermore, in single subduction system models, low basal drag leads to subduction behaviour most consistent with the observation that trench migration velocities are generally low compared to convergence velocities. By contrast, analytical calculations and global mantle flow models indicate basal drag can be substantial. In this study, we revisit this problem by examining the drag at the base of the lithosphere, for a single subduction system, in 2D models with a free trench and composite non-linear rheology. We compare the behaviour of short and long plates for a range of asthenospheric and lithospheric rheologies. We reproduce results from previous modelling studies, including low ratios of trench over plate motions. However, we also find that any combination of asthenosphere and lithosphere viscosity that produces Earth-like subduction behaviour leads to a correlation of velocities with plate size, due to the role of basal drag. By examining Cenozoic plate motion reconstructions, we find that slab age and plate size are positively correlated: higher slab pull for older plates tends to be offset by higher basal drag below these larger plates. This, in part, explains the lack of plate velocity-size correlation in observations, despite the important role of basal drag in the subduction force balance.Lior Suchoy was supported by the Engineering and Physical Sciences Research Council (EPSRC) (grant no. EP/N509486/1). Ben Maunder and Saskia Goes were supported by the Natural Environment Research Council (NERC) (grant no. NE/K010743/1). Rhodri Davies was supported by the Australian Research Council (grant no. DP170100058)

Transient Neuronal Populations Are Required to Guide Callosal Axons: A Role for Semaphorin 3C

Neurons, glia, and callosal axons operate as a “ménage à trois” in the development of the corpus callosum

Lithosphere as a constant-velocity plate: Chasing a dynamical LAB in a homogeneous mantle material

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Workshop booklet - 2024 Ada Lovelace Workshop on Modelling Mantle and Lithosphere Dynamics

Organisation committee of the 2024 Ada Lovelace workshop (Fanny Garel, Catherine Thoraval) supported by the European Geological UnionSee the website : https://meetings.copernicus.org/2024AdaLovelaceWorkshop/International audienceThe workshop series was initiated in 1987 in Neustadt an der Weinstrasse, Germany, and workshops usually take place every two years. The last workshops were held in Hévíz, Hungary (2022), Sienna, Italy (2019), and Putten, the Netherlands (2017).The workshops, previously known as International Workshops on Mantle and Lithosphere Dynamics, were renamed in 2018 by the EGU Topical Events Committee in honour of the 19th century English mathematician Ada Lovelace. The 2024 workshop will take place at Domaine du Lazaret in Sète, France on the mediterranean sea, 40 km from Montpellier. The meeting is co-sponsored by European Geosciences Union (EGU), Université de Montpellier, Centre National pour la Recherche Scientifique (CNRS), ANR project Rheobreak (PI Fanny Garel), Géosciences Montpellier and Computational Infrastructure for Geodynamics (CIG)

Lithosphere-asthenosphere transition tracked in models of plate and mantle dynamics: the “constant-velocity plate” deforms at its base

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Modelling of the dynamics and cooling of lava flows (towards a framework for thermal remote-sensing as an eruption monitoring tool)

Pendant une éruption effusive, le principal risque est posé par l'avancée des coulées de lave depuis l'évent. Le débit éruptif contrôle en grande partie cette dynamique, et sa détermination en temps réel est un point-clé dans la gestion du risque volcanique. Un suivi de l'éruption peut être réalisé par satellite, avec des mesures régulières du signal thermique (radiance) émis en surface par le champ de lave. Cette thèse étudie l'évolution du signal thermique de surface pour des courants de gravité qui avancent et se refroidissent simultanément. Pour un fluide dont la viscosité ne dépend pas de la température et injecté à débit constant, des expériences de laboratoire et un modèle théorique montrent que la radiance augmente pendant un premier stade transitoire avant d'atteindre un palier. Pour des paramètres de coulées de lave, la théorie prédit que le signal radié stationnaire dépend principalement du débit éruptif, et la puissance prédite est en bon accord avec des données de terrain. Des expériences avec un matériel de type cire montrent que la solidification ralentit l'étalement du courant, qui se fait au travers de débordements ponctuels. Il n'y a alors pas d'état stationnaire dans le signal thermique de surface, mais le matériel fluide (le plus chaud) radie un flux de chaleur constant, dont la valeur augmente avec le débit imposé à la source. La thèse explore également l'effet de la topographie et de la variation temporelle du débit sur la dynamique et le signal thermique de l'écoulement. L'utilisation quantitative de la télédétection thermique nécessite une modélisation appropriée de la rhéologique effective et de la structure thermique interne des coulées de lave.During an effusive eruption, the main hazard is related to how fast and how far lava will spread from the volcanic vent before it stops. The effusion rate exerts an important control on the advance of lava flows, and its real-time estimate during an eruption is a key issue for hazard assessment. The eruption can be monitored by remote-sensing, with periodic satellite acquisitions of the thermal signal (radiance) emitted by the surface of the lava flow field. This thesis investigates the evolution of the surface thermal signal of gravity currents, simultaneously spreading and cooling. For a fluid with a temperature-independent viscosity, supplied at a constant rate, laboratory experiments and a simple theoretical model show that the surface thermal signal first increases during a transient stage before reaching a plateau value. For lava flow parameters, the theoretical model predicts that the steady radiant heat flux mostly dépends on the effusion rate; and the predited power concurs at first-order with data from natural lava flows. Experiments using a wax-like material show that solidification slows down the spreading of the current, which advances with episodic overflow events. There is no steady state for the bulk surface thermal signal, and only the fluid part of the flow (the hottest material) radiates a constant heat flux, whose value increases with increasing input rate. The quantitative use of thermal remote-sensing requires an appropriate modelling of the effective rheology and of the internal thermal structure of lava flows.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Benchmarking lava-flow models

International audiencePrediction of the emplacement of volcanic mass flows (lava flows, pyroclastic density currents, debris avalanches and debris flows) is required for hazard and risk assessment, and for the planning of risk-mitigation measures. Numerical computer-based models now exist that are capable of approximating the motion of a given volume of volcanic material from its source to the deposition area. With these advances in technology, it is useful to compare the various codes in order to evaluate their respective suitability for real-time forecasting, risk preparedness and post-eruptive response. A ‘benchmark’ compares codes or methods, all aimed at simulating the same physical process using common initial and boundary conditions and outputs, but using different physical formulations, mathematical approaches and numerical techniques. We set up the basis for a future general benchmarking exercise on volcanic mass-flow models and, more specifically, establish a benchmark series for computational lava-flow modelling. We describe a set of benchmarks in this paper, and present a few sample results to demonstrate output analysis and code evaluation methodologies. The associated web-based communal facility for sharing test scenarios and results is also described