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Abstract

Volcanic calderas are delimited by a ‘caldera wall’ which can be several hundred meters in height. Here, we assess the roles of friction and cohesion on caldera wall morphology by: (i) analysing the slope properties of several young natural calderas in the ALOS-3D global digital surface model (DSM), and (ii) comparing those observations to the results of analytical solutions and of new Distinct Element Method (DEM) modelling. The DEM models explicitly simulated the process of progressive caldera collapse, wall formation and destabilisation, enabling exploration of the emergence of slope morphology as a function of increasing subsidence and of mechanical properties. We compare the modelled slopes to analytical predictions for planar slope failure, showing that natural caldera slopes represent post-failure slopes rather than critical slopes. This may lead to discrepancies in comparison with analytical solutions. We therefore exploit the high temporal sampling of the DEM models to compare critical caldera slopes to existing analytical solutions. We determine that material cohesions of 2 MPa yield unrealistic slope geometries and failure mechanisms, where deep tensile cracks of several hundred meters control slope degradation. We find the upper bound of realistic rock mass cohesion of 0.5 MPa, significantly lower than those determined by laboratory testing. We identify different failure mechanisms as a result of the mechanical properties: (1) granular flow in the case of weak, altered rock, and (2) tensile cracking and block toppling in the case of strong, unaltered rock.