Fissure eruptions are commonly linked to magma dikes at depth and are associated with elastic and inelastic surface deformation. Elastic deformation is well described by subsidence occurring above the dike plane and uplift and lateral widening occurring perpendicular to the dike plane. Inelastic deformation is associated with the formation of a graben, which is bordered by graben parallel faults that might express as sets of fractures at the surface. Additionally, secondary structures, such as push-ups, bends and step overs, yield information about the deforming domain. However, once these structures are formed during fissure eruptions, they are rarely preserved in nature, due to the effects of rapid erosion, sediment coverage or overprinting by other faulting events. Therefore, simple normal fault displacements are commonly assumed at dikes. At the 2014/2015 Holuhraun eruption sites (Iceland), increasing evidence suggests that developing fractures exhibited variations in their displacement modes. In an attempt to investigate these variations, a fieldwork mapping project combining Terrestrial Laser Scanning (TLS) and Unmanned Aerial Vehicle (UAV)-based aerophoto analysis was undertaken. Using these data, we generated local high-resolution Digital Elevation Models (DEMs) and a structural map that facilitated the identification of kinematic indicators and the assessment of the observed structures. We identified 315 fracture segments from these satellite data. We measured the strike directions of single segments, including the amount of opening and opening angles, which indicate that many of the measured fractures show transtensional dislocations. Of these, ∼81% exhibit a significant left-lateral component and only ∼17% exhibit a right-lateral component. Here, we demonstrate that the local complexities in these fracture traces and geometries are closely related to variations in their transtensional opening directions. Moreover, we identified local changes in fracture azimuths and offsets close to eruption sites, which we speculate are associated with geometric changes in the magma feeder itself. The results highlight that the opening of fractures associated with an erupting fissure Müller et al. Structures at Holuhraun may record transtensional modes with both, left-lateral and right-lateral components. These results further highlight the value of using UAV-based high-resolution data to contribute to the integrity of the observations of the structural complexities produced by local geologic events.

The eruption frequency of geysers can be studied easily on the surface. However, details of the internal structure including possible water and gas filled chambers feeding eruptions and the driving mechanisms often remain elusive. We used a multidisciplinary network of seismometers, video cameras, water pressure sensors and one tiltmeter to study the eruptive cycle, internal structure, and mechanisms driving the eruptive cycle of Strokkur geyser in June 2018. An eruptive cycle at Strokkur always consists of four phases: (1) Eruption, (2) post‐eruptive conduit refilling, (3) gas filling of the bubble trap, and (4) regular bubble collapse at shallow depth in the conduit. For a typical single eruption 19 ± 4 bubble collapses occur in Phase 3 and 8 ± 2 collapses in Phase 4 at a mean spacing of 1.52 ± 0.29 and 24.5 ± 5.9 s, respectively. These collapses release latent heat to the fluid in the bubble trap (Phase 3) and later to the fluid in the conduit (Phase 4). The latter eventually reaches thermodynamic conditions for an eruption. Single to sextuple eruptions have similar spacings between bubble collapses and are likely fed from the same bubble trap at 23.7 ± 4.4 m depth, 13–23 m west of the conduit. However, the duration of the eruption and recharging phase linearly increases likely due to a larger water, gas and heat loss from the system. Our tremor data provides documented evidence for a bubble trap beneath a pool geyser.

Catastrophic lava dome collapse is considered an unpredictable volcanic hazard because the physical properties, stress conditions, and internal structure of lava domes are not well understood and can change rapidly through time. To explain the locations of dome instabilities at Merapi volcano, Indonesia, we combined geochemical and mineralogical analyses, rock physical property measurements, drone-based photogrammetry, and geoinformatics. We show that a horseshoe-shaped alteration zone that formed in 2014 was subsequently buried by renewed lava extrusion in 2018. Drone data, as well as geomechanical, mineralogical, and oxygen isotope data suggest that this zone is characterized by high-porosity hydrothermally altered materials that are mechanically weak. We additionally show that the new lava dome is currently collapsing along this now-hidden weak alteration zone, highlighting that a detailed understanding of dome architecture, made possible using the monitoring techniques employed here, is essential for assessing hazards associated with dome and edifice failure at volcanoes worldwide.

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