The rebirth and evolution of Bezymianny volcano, Kamchatka after the 1956 sector collapse

Volcanic sector collapses, events ranging from slow creep to catastrophic failures, significantly alter volcanic landscapes and pose substantial hazards. While cone regrowth after such collapses is documented at several volcanoes, a detailed chronological account of this process has been lacking. This study addresses this gap by focusing on Bezymianny volcano in Kamchatka, Russia, which experienced a major sector collapse in 1956. Understanding the post-collapse regrowth of Bezymianny is crucial for several reasons. Firstly, it provides a unique, high-resolution dataset spanning seven decades, allowing unprecedented insights into the temporal dynamics of volcanic rebuilding. Secondly, Bezymianny’s andesitic composition makes it representative of many active volcanoes globally, enhancing the generalizability of the findings. Thirdly, analyzing the interplay between tectonic stress changes, magma pathways, and eruption style following a sector collapse is critical for accurate hazard assessments. The research question guiding this study is: How does the morphology and eruptive behavior of Bezymianny volcano evolve after the 1956 sector collapse, and what are the underlying mechanisms driving this evolution?

The literature extensively documents the impact of sector collapses on volcanic systems. These collapses mobilize vast volumes of material, generating hazards such as directed blasts, pyroclastic flows, lahars, and tsunamis. Studies highlight the significant changes in magma reservoir pressure, location, and geometry following collapses, often leading to compositional changes in erupted products. The regrowth of volcanic cones after sector collapses has been observed at various volcanoes, including Mount St. Helens, Soufriere Hills, Santa Maria, Ritter Island, Shiveluch, and Bezymianny. However, most previous studies lack the long-term, high-resolution data necessary to fully elucidate the regrowth process. Existing research suggests that regrowth may start immediately or with a delay, commonly nucleating in the center of the exposed amphitheater. Long-term regrowth is typically assumed to be relatively regular, with characteristic dome height-to-width ratios. Numerical simulations suggest that stress field redistribution following a collapse can deflect and bifurcate magma pathways, leading to shifts in eruption locations.

This study leverages a unique dataset of aerial photographs (1949-2013), high-resolution Pléiades satellite imagery (2017), and UAV imagery (2017) to analyze Bezymianny’s post-collapse evolution. The methodology involves several key steps: 1. **Photogrammetry:** Analog aerial photographs were digitized, and digital elevation models (DEMs) were created using Erdas Imagine and Photomod software. Ground control points (GCPs), derived from theodolite data and identifiable topographic features, were used for georeferencing and accuracy assessment. Point clouds were generated, filtered, and manual extraction was employed in areas with fumarolic activity. 2. **Satellite and UAV Data Integration:** Pléiades satellite imagery and UAV images were processed separately to create DEMs. The aerial and satellite-derived DEMs were then carefully aligned using CloudCompare software. This combined approach allowed us to improve point cloud resolution and address any gaps in the data. 3. **Morphological Mapping:** High-resolution stereoscopic images were used to map volcanic landforms (shear lobes, lava flows, lava plugs) and their evolution using Erdas Imagine. Visual interpretation allowed researchers to classify and quantify these features based on standard volcanological criteria. 4. **Volume and Growth Rate Estimation:** Volumetric changes between consecutive DEMs were computed using CloudCompare, enabling the calculation of growth rates for different periods. The uncertainties associated with volume and growth rate estimates were rigorously assessed and included in the analysis. 5. **Magma Pathway Simulations:** Numerical simulations were performed using a previously developed model that treats magma pathways as 2D boundary element mixed-mode cracks. Three stress scenarios were modeled: pre-collapse, post-collapse, and regrowth. The model considered magma overpressure, topographic stress, and the effects of edifice loading and unloading on magma pathways. The simulations aimed at understanding the effect of stress field redistribution on vent locations and magma pathways.

The analysis revealed a distinct multi-stage evolution of Bezymianny volcano following the 1956 sector collapse: 1. **Endogenous Growth (1956-1967):** Initially, two endogenous lava domes formed ~400 m apart within the collapse amphitheater. The average growth rate during this phase was approximately 56,600 m³/day. 2. **Exogenous (Extrusive) Growth (1967-1976):** The growth style shifted to exogenous, with shear lobes extruding from multiple open craters and accumulating at different sectors of the dome complex. The average growth rate decreased to 43,600 m³/day in 1967-1968. 3. **Exogenous (Extrusive-Effusive) Growth (1977-2006):** A transition to extrusive-effusive activity occurred, marked by the emplacement of lava flows alongside shear lobes and lava plugs. Growth rates continued to decrease, ranging from 15,500 to 17,000 m³/day. 4. **Stratocone Formation (2006-2017):** The dome complex evolved into a symmetric stratocone with a stable summit crater, characterized by interbedded lava flows and pyroclastic deposits. The growth rate stabilized to 15,500 to 17,000 m³/day. The total volume of the rebuilt central cone between 1956 and 2017 was 0.591 km³, yielding an overall average growth rate of 26,400 m³/day. Vent migration was observed, initially scattered but gradually focusing towards the center of the new cone. Numerical modeling demonstrated that stress changes caused by the sector collapse and subsequent regrowth played a significant role in directing magma pathways and vent locations. The model highlights that the collapse unloading initially caused vent shifting towards the collapse embayment, while subsequent dome loading caused the vents to centralize. The long-term average growth rate of 26,400 m³/day is comparable to similar rates at other volcanoes but significantly lower than some others, likely reflecting differences in eruption styles and magma viscosity. Petrological studies indicated a gradual decrease in silica content over time, potentially linked to the activation of a deeper magma reservoir, which is also potentially a factor for the shift from predominantly extrusive to extrusive-effusive activity. The study also found that the lava flows act as an armor preventing flank collapses and stabilizing the new edifice.

The findings demonstrate that the regrowth of Bezymianny volcano after the 1956 sector collapse was a complex, multi-stage process involving significant changes in eruption style and vent location. The initial endogenous growth gave way to exogenous growth, characterized by the extrusion of shear lobes and the effusion of lava flows. The gradual decrease in growth rates was likely related to the increasing stability of the edifice and a shift to a more effusive eruption style, potentially influenced by changes in magma viscosity and source. The numerical simulations successfully linked the vent migration pattern to the stress changes caused by the sector collapse and the regrowth of the volcanic edifice. These results highlight the importance of considering the dynamic interactions between stress fields, magma pathways, and topographic changes when assessing volcanic hazards. While the model provides important insights, it simplifies the complex system by neglecting factors such as rock heterogeneities and magma viscosity changes. Future research should incorporate these factors to improve the model's predictive capabilities. The relative speed of the regrowth at Bezymianny compared to other volcanoes indicates the potential for rapid rebuilding in certain cases.

This study presents a comprehensive analysis of Bezymianny volcano's regrowth after its 1956 sector collapse using high-resolution, long-term photogrammetric data. The results reveal a multi-stage process with distinct changes in growth style and vent migration. Numerical modeling shows that stress changes associated with the collapse and regrowth are key factors in shaping magma pathways and eruption locations. The rapid regrowth rate highlights the potential for relatively fast edifice reconstruction after large-scale collapses. Future research should focus on incorporating additional factors into the numerical models to improve predictions of volcanic activity and associated hazards. Monitoring the continued activity at Bezymianny and its potential for flank instability is crucial for hazard assessment.

The study's reliance on photogrammetric data means the analysis is limited to surface observations. Internal processes and deeper subsurface magma dynamics were not directly observed and needed to be inferred using numerical modeling. The numerical model simplifies several aspects of the volcanic system, including rock heterogeneity and magma viscosity changes, which could impact the accuracy of the simulations. The limited number of GCPs for the early aerial photographs may impact the accuracy of volume estimations and growth rate calculations for earlier periods. Although an attempt was made to account for these limitations through error estimation and other means, future studies could leverage more advanced techniques.