A historical pyroclastic flow emplaced within a pre-existing Pleistocene lava tube: Silidong, Tianchi Volcano, Changbaishan, northeastern China

Abstract

Pyroclastic flow deposits recently found within a pre-existing lava tube at Tianchi Volcano represent, to the best of our knowledge, the only such reported occurrence worldwide. In this case, pyroclastic flow of Tianchi’s “Millennium eruption” (~1 ka) traveled about 18 km from the summit eruptive source and poured successively into the ~560-m-long accessible segment of Silidong lava tube. Mapping of tube morphology, combined with detailed characterization of the features associated with the pumice flow deposits (e.g., adhering of pyroclastic materials on tube walls, fumarole pipes, rootless vents, and flow fronts of the deposit surface) has enabled plausible inferences regarding the original within-tube conditions and dynamic flow regime during emplacement. We propose a model of an aggrading pyroclastic flow which locally varies its sedimentation rate. The pyroclastic deposit is thicker in locations of reduced flow mobility, and the resultant variations in deposit thickness appear to control the distribution of fumarole pipes and rootless vents.