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.