Sedimentology of Volcanic Debris Avalanche Deposits

The deposits of volcanic debris avalanches (VDAs) contain diagnostic features that distinguish them from those of other landslides. In this chapter, we summarize the sedimentary characteristics and the different (litho-)facies described over the past four decades, and how findings from individual case studies can be adapted as globally applicable sedimentological tools. A plethora of descriptive terms and partially conflicting definitions emerged in the ever-growing literature on VDA deposits (VDADs). These we summarize and make recommendations for future use. Different facies models that were developed at different volcanoes might point to unique emplacement conditions (e.g. dry versus wet; confined versus unconfined) and, if confirmed, the apparent ‘conflict' of terminology might help identify the paleo-settings of ancient VDAs. General observations of large unsaturated landslides of different origin show that preservation of source stratigraphy, (mega-)clasts, jigsaw-fractured clasts, and incorporation of runout path material are common features. Their unique composition, grain sizes, and abundance of matrix sets VDADs apart from deposits of large rockslides and debris flows. The latter can be associated with VDAs, and whether they formed syn- or post-VDAD emplacement is reflected in forensic evidence within the depositional sequences. Recent case studies illustrate the advances in analytical techniques and in understanding the processes of debris avalanche transport and deposition forty years after the eruption and lateral collapse of Mount St. Helens volcano

Volcanic Debris Avalanche Transport and Emplacement Mechanisms

Field observations of volcanic debris avalanche (VDA) morphology, sedimentology, and structural features have inspired several hypotheses on their dynamic behaviour. These include plug flow, translational slide, and sliding along multiple shear zones, none of which involve large-scale turbulence during transport. The plug flow model shows normal gradation in the plug, and reverse grading in the laminar boundary layers. During translational sliding, spreading of the mass is accommodated by listric normal faults that flatten into a main sliding plane at the base of or within the avalanche body. Multiple shear zones include progressive fragmentation within the avalanching mass, resulting in pockets of shear and slip. We present case studies for each model and hypotheses for the formation of flowbands on the deposit surface. Processes involved during emplacement include disintegration, dynamic fragmentation, and matrix injection. Near the base, bulldozing and incorporation of substrata change the composition and behaviour of the VDA. In extreme cases, VDAs transform into lahars if sufficient water is available for entrainment. Post-emplacement, lahars can also happen, e.g., through debris dewatering, loading of saturated substrata or in the case of landslide dam failure. VDA also create secondary slides when deflected by topographic barriers or when the margins are oversteepened.</p

A Historical Perspective on Lateral Collapse and Volcanic Debris Avalanches

In the four decades since the 1980 eruption of Mount St. Helens, debris-avalanche deposits generated by gravitational lateral collapse of volcanoes have become widely recognized. Selected regionally sequenced case studies highlight the evolution of thought regarding these events prior to 1980 in contrast to subsequent research with benefit of insights from the events of May 18, 1980. These typically hummocky deposits, of volcanic materials but lying far beyond volcanoes, had puzzled geologists for more than a century and been interpreted as a wide range of primary and secondary volcanic or non-volcanic features. Contrary to general perception, however, the volcanological literature contained multiple accounts prior to 1980 that recognized the landslide origin of some of these deposits, albeit mostly in regional publications not widely known. The burst of interest in lateral-collapse events after 1980 has led to an average of one regional or global debris-avalanche inventory annually in terrestrial or submarine settings and the recognition of a thousand events from nearly 600 volcanoes. The last major volcaniclastic process to be widely recognized and understood, large-volume debris avalanches originating from lateral collapse of volcanic edifices have been found to be a relatively common occurrence across a wide spectrum of volcanic features and settings