Janine B Krippner, Alexander Belousov, Michael Ramsey

Two of the largest known historic block-and-ash flow (BAF) fields are located on Shiveluch volcano, Kamchatka. The deposits were produced during retrogressive and pulsatory dome collapse events that occurred over 6-9 hour-long eruptions on 27-28 February 2005 and 27 October 2010. The BAFs that were produced by these partial dome collapse events extend up to 19 km from the dome and inundate areas of 24.1 and 22.3 km2. We used satellite and field data to investigate the surface morphology of the BAF deposits and their impact on the surrounding area. The deposit surfaces contain overlapping lobate deposits, compaction ridges, levees and channel morphologies, degassing structures, abundant large dome blocks, and large areas of syn- or immediately-post-depositional remobilization, as shown by bench scallops and arcuate scarps. The dome-collapse events produced flows with two components: the dense block-and-ash component of the flows, and the associated dilute pyroclastic surges. The surge components traveled beyond the main flow and killed vegetation to distances of nearly 300 m. The 2005 dome collapse event produced a BAF that extended 17.8 km from the dome and emplaced a large fan deposit and associated surge, which destroyed an area of forest 10 km2 in size. The October 2010 event also produced a large fan deposit with a distal channelized deposit that extended an additional 5.4 km beyond the main 2010 fan for a total distance from the dome of 19 km. Since deposition, fluvial erosion and deposition produced large areas comprised of dendritic and braided channel deposits. The surface morphology of the deposits as revealed in satellite imagery and observed in stratigraphic sections give insight into the pulsatory nature of the dome collapse events, the final moments of deposition, and the subsequent erosion of the deposits. Preservation of the deposits is exceptional due to the high-latitude location, where snow-cover is present for much of the year and erosion is limited. This detailed study of the Shiveluch deposits provides rare insight into the processes responsible for generation of large and long run-out dome-collapse BAFs in both channelized and non-channelized environments.