Influence of conduit and topography complexity on spine extrusion at Shiveluch volcano, Kamchatka

Lava spines, extrusions of low-porosity, solid magma, offer valuable insights into volcanic conduit processes. Their formation is often slow and involves pre-eruptive outgassing and crystallization. The height and stability of spines are crucial for hazard assessment, as their collapse can generate pyroclastic density currents. While numerous studies have examined spine features at volcanoes like Mount Pelée, Mount St. Helens, Soufrière Hills, and Unzen, the role of conduit geometry and subsurface architecture remains debated. Previous research suggests that inclined conduits or material heterogeneities influence spine directionality and shape. However, detailed observations of the subsurface structures and their interaction with surface topography are rare. This research focuses on the 2020 spine extrusion at Shiveluch volcano, Kamchatka, leveraging various remote sensing techniques and numerical/analog modeling to understand the influence of both conduit and topographic complexity on spine growth and its ultimate instability. The study aims to provide improved understanding of the processes controlling spine morphology, the relationship between conduit structure and spine directionality, and the hazards associated with spine growth and collapse. This is important for refining hazard assessment models and improving early warning systems for future eruptions.

Previous studies on lava spines have documented a wide range of morphologies and growth patterns. The extrusion rate significantly impacts spine geometry: low rates (below ~1 m³/s) lead to taller, more vertical spines, while higher rates result in blockier domes or pancake-shaped lobes. Studies on Mount St. Helens highlighted the role of stick-slip motion along conduit margins, generating drumbeat seismicity and producing a mantle of faulted rock. The heterogeneity of conduit materials (varying porosity, permeability, and crystallinity) significantly impacts the spine's behavior, with brittle behavior more prominent at higher extrusion rates and ductile behavior at slower rates. The influence of the underlying conduit geometry (inclined conduits potentially producing inclined extrusions) and the development of fractures on spine extrusions remain areas of ongoing research. The existing literature lacks detailed studies combining remote sensing data with detailed modeling efforts to comprehensively address the effects of both subsurface conduit complexity and surface topography on spine extrusion at a single volcano.

This study utilized a multi-faceted approach to analyze the 2020 spine extrusion event at Shiveluch volcano. Time-lapse camera data from a distance of 43 km was used to track the spine's height changes over time. Image analysis involved alignment, brightness adjustment, and manual pixel tracking to determine vertical growth rates. TerraSAR-X satellite radar data, acquired at 11-day intervals, provided high-resolution observations of the dome and spine evolution. Pixel offset calculations, using a particle image velocimetry (PIV) approach, quantified deformation and displacement. High-resolution optical imagery from helicopter surveys (October 22, 2019) and Pléiades satellite data (October 1 and 13, 2020) were used to generate digital terrain models (DTMs) and orthophotos. These allowed detailed analysis of spine morphology, striations, fractures, and their spatiotemporal evolution. The analysis involved manual tracing of features, azimuthal direction calculations, and lineament density estimations. Finally, the study employed both analog (sandbox) and numerical (PFC, Particle Flow Code) modeling to simulate spine extrusion under varying conditions. These models incorporated heterogeneous conduit material properties and topographic complexity to investigate their influence on spine directionality and growth patterns. The analog models used sand-plaster mixtures to simulate different material strengths, while the numerical models employed a 2D discrete element method (DEM) to capture particle-scale interactions.

Analysis of the time-lapse camera data revealed rapid initial spine height growth (over 100 m in two months), followed by slower growth. Radar observations confirmed the emergence of the spine on April 24, 2020, with associated widespread deformation of the dome preceding the spine extrusion. After the spine emerged, this widespread deformation ceased, suggesting mechanical decoupling between the spine and the dome. Photogrammetry and satellite data provided detailed measurements of the spine: dimensions of approximately 150 m wide and 300 m long, a height of 114 m above the talus and 220 m above the pre-existing topography. The data revealed a polished, flat spine surface and the presence of striations aligned with the spine's long axis, suggesting high strain rates and potential fault gauge formation. Fracture analysis revealed a bimodal distribution of fractures: those oriented orthogonal to the striations which increased in displacement over time, and those parallel to the 2019 collapse scar. Pixel offset calculations from satellite imagery revealed a mean daily spine displacement of ~1.7 m, corresponding to an extrusion volume of 0.3–0.7 m³/s. Both analog and numerical models successfully reproduced many aspects of the spine's growth, confirming the observations and measurements. The models indicate that the interplay of heterogeneous conduit material and complex topography can lead to asymmetric spine extrusion and inclination. Models showed that a topographic high (buttress) on one side of the conduit, coupled with varying material strength within the conduit, created a dominant shear zone that directed the lateral extrusion and the overall spine inclination. The models successfully reproduced the observation of mechanical decoupling of the spine and the surrounding dome.

The findings address the research question by demonstrating the significant roles of both subsurface conduit heterogeneity and surface topography in controlling the directionality and morphology of spine extrusion. The asymmetric extrusion of the spine, its northward inclination, and the observed striations and fractures are all well explained by the models considering material heterogeneity and topographic buttressing. The results highlight that simplified models assuming homogeneous material and simple conduit geometry are insufficient for accurately predicting spine behavior. The mechanical decoupling between the spine and the dome, observed in both the data and the models, indicates a complex interplay of forces during spine growth. The study's findings are relevant to volcano hazard assessment. Accurate prediction of spine growth and collapse requires consideration of factors beyond simple extrusion rates, including pre-existing topography and conduit architecture. The detailed kinematic analysis of the spine provides valuable constraints on shallow magma system parameters. The study also emphasizes the importance of using advanced remote sensing techniques and detailed modeling for a comprehensive understanding of volcanic processes.

This research presents a comprehensive analysis of a spine extrusion event at Shiveluch volcano, combining multiple data sources and modeling techniques. The results demonstrate the importance of subsurface conduit architecture and surface topography in controlling spine growth directionality and stability. The findings have implications for volcano hazard assessment, suggesting that future studies should focus on integrating high-resolution remote sensing data with more sophisticated models that incorporate complex subsurface and topographic features. Future research directions could include detailed analysis of the pre-eruptive state of the volcano, incorporation of detailed thermal and rheological models of the magma, and the study of seismicity to gain a better understanding of the dynamic interactions between the conduit, spine, and the surrounding edifice. More comprehensive investigations of other volcanoes, integrating similar datasets and advanced models, will be necessary to further validate these findings and enhance the accuracy of volcanic hazard assessments globally.

While this study provides a detailed analysis of the 2020 Shiveluch spine extrusion, some limitations should be acknowledged. The time-lapse camera data, due to its large distance and low resolution, provides less precise measurements than the satellite and photogrammetry data. The numerical models, while successfully reproducing key aspects of the extrusion, are simplified representations of a complex system. The models don't fully capture the detailed physics of the magmatic system, particularly aspects like degassing and crystallization, which may influence the material properties. The spatial resolution of satellite data, although high, may still not fully resolve fine-scale features. Finally, seismic data was not available to support the model inferences regarding conduit dynamics.