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Poppe et al. An Inside Perspective on Magma Intrusion KEYPOINTS -Cutting-edge dynamic wide beam X-ray Computed Tomography and Digital Volume Correlation allows an inside view on the temporal behavior of analog magma intrusion in granular host material in laboratory experiments. -Intrusion-induced 3D displacement and strain can be quantified over time. -Thick cryptodomes form with distributed strain and mixedmode fracturing in weak host materials. -Thin dikes form with localized strain and opening-mode fracturing in strong host materials. -A continuum of cone sheet geometries occurs in between these end-members. Frontiers in Earth Science | www.frontiersin.org

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Two end‐member conceptual models are used to describe deformation of the Earth's crust induced by magma intrusion. “Mode I” fracturing assumes tensile or opening‐mode, elastic deformation, while “Mode II” fracturing assumes plastic shear‐mode deformation around a viscous indenter. Field observations of both mechanisms exist, but it remains unclear which mechanism dominates in which conditions. We describe intrusion geometries, host rock deformation, and geochemical magma‐host rock interactions around 53 exceptionally preserved, tephrite‐basanite Permian dike segments of 0.5‐ to 30‐cm thickness. These thin dikes, that is, “dikelets,” intruded Late‐Ordovician carbonate‐rich sedimentary rocks on Hovedøya island, Oslo Rift, Norway. Dikelets emplaced in preexisting fractures dominantly created cavities ahead of their narrow, tapering tips and are associated with bent host rock, broken bridges, and stepped segmented geometries. Other tips are blunt with dense brittle fracturing around them. Also, cross‐sectional intrusion segment opening profiles deviate from parabola‐shaped profiles typical for elastic media. The observations demonstrate that dominant opening‐mode host rock deformation can coexist with shear‐mode deformation locally. Alignment of most dikelet segments along the dominant host rock fracture directions highlights the control of local structural orientations on magma emplacement. Analysis of bulk major and trace element compositions, in situ micro‐XRF sample analysis and carbon and oxygen stable isotope compositions, suggests that thermochemical interactions between magma and the carbonate‐rich host rock produced a low‐viscosity mixture of magma, pore water, and gas. We propose that such low‐viscosity hybrid fluid may assist in the intrusion of magma in sedimentary rocks by filling the cavity ahead of propagating sheet intrusion tips.

Abstract. Natural disasters are too often presented as resulting from extreme natural phenomena affecting helpless populations, with people being insufficiently aware of the factors leading to disasters and of the existing strategies to mitigate their impacts. We developed a board game aimed at raising awareness about geohazards and disaster risk reduction strategies. The target groups are (1) secondary school students and citizens and (2) scientists and stakeholders involved in risk management activities. For the first group, the aim is to induce a better understanding of the geohazards and disasters they are confronted with in the media or in their daily lives; for the second, the objective is to generate discussion about risk management strategies. The game was tested with students in Belgium and with citizens, earth scientists, and risk managers in several African countries. Based on analysis of the most common game strategies observed, the players' reactions during the game, and their answers to a short questionnaire, we analyzed the main learning outcomes conveyed by this game. The game Hazagora appears to positively enhance the players' insights into processes involved in disasters. As such, the game is an effective, fun learning tool to introduce participants to the concepts of geohazards and disasters and to generate discussion.

Classical mechanisms of volcanic eruptions mostly involve pressure buildup and magma ascent towards the surface1. Such processes produce geophysical and geochemical signals that may be detected and interpreted as eruption precursors1–3. On 22 May 2021, Mount Nyiragongo (Democratic Republic of the Congo), an open-vent volcano with a persistent lava lake perched within its summit crater, shook up this interpretation by producing an approximately six-hour-long flank eruption without apparent precursors, followed—rather than preceded—by lateral magma motion into the crust. Here we show that this reversed sequence was most likely initiated by a rupture of the edifice, producing deadly lava flows and triggering a voluminous 25-km-long dyke intrusion. The dyke propagated southwards at very shallow depth (less than 500 m) underneath the cities of Goma (Democratic Republic of the Congo) and Gisenyi (Rwanda), as well as Lake Kivu. This volcanic crisis raises new questions about the mechanisms controlling such eruptions and the possibility of facing substantially more hazardous events, such as effusions within densely urbanized areas, phreato-magmatism or a limnic eruption from the gas-rich Lake Kivu. It also more generally highlights the challenges faced with open-vent volcanoes for monitoring, early detection and risk management when a significant volume of magma is stored close to the surface.

Inferences about sheet intrusion emplacement mechanisms have been built largely on field observations of intrusion tip zones: magmatic systems that did not grow beyond their observed state. Here we use finite element simulation of elliptical to superelliptical crack tips, representing observed natural sill segments, to show the effect of sill tip shape in controlling local stress concentrations, and the potential propagation pathways. Stress concentration magnitude and distribution is strongly affected by the position and magnitude of maximum tip curvature κmax. Elliptical tips concentrate stress in-plane with the sill, promoting coplanar growth. Superelliptical tips concentrate maximum tensile stress (σmax) and shear stress out-of-plane of the sill, which may promote non-coplanar growth, vertical thickening, or coplanar viscous indentation. We find that σmax =  Pe(1+ 2(√[aκmax]), where Pe is magma excess pressure and a is sill half length. At short length-scales, blunted tips locally generate large tensile stresses; at longer length-scales, elliptical-tipped sills become more efficient at concentrating stress than blunt sills.