Collapse caldera walls: Mechanical controls on slope failure and morphology

Abstract

Volcanic calderas are enclosed topographic depressions that are commonly delimited by steep annular cliffs (caldera walls). The walls represent degraded scarps of ring faults that accommodated caldera collapse. Here, we assess the mechanical controls on caldera wall morphology and failure by: (i) analysing slope properties of recently formed calderas; (ii) examining analytical solutions for slope failure; and (iii) simulating progressive slope failure via Distinct Element Method (DEM) numerical modelling. Average slope angles of the caldera walls in nature range from 20° to 65° and slope heights range from 99 m to 1080 m. Smaller slope heights are weakly tied to greater slope inclinations. Almost 60% of sampled caldera wall profiles show a steeper upper section and a gentler lower section, which in numerical models are reproduced as an upper slope comprising intact wall rock and a lower slope comprising failed material inclined at the angle of repose. Moreover, modelled material with a low angle of internal friction and low cohesion exhibits shear failure, whereas material with high friction and high cohesion shows tension cracking, buckling and toppling. The slope height and inclination data from the numerical models agree well with those in nature. The numerical model data correspond more closely to analytically-predicted failure planes than critical (i.e. limit equilibrium) slopes because they are short-lived and therefore unlikely to be sampled in either model or nature. If rock mass cohesions or friction angles constrained analytically from natural slopes are determined by assuming the slope is critical, they are likely to be underestimates. The homogeneous numerical models here give an estimated natural rock mass cohesion of 0.5 MPa or less. This is much lower than typical laboratory-scale cohesion values, and it highlights the importance of strength reduction when upscaling mechanical parameters from laboratory experiments for modelling slope stability and volcano deformation.