Mechanical controls on caldera slope morphology and failure

Volcanic calderas are delimited by a ‘caldera wall’ which can be several hundred meters in height. This represents the degraded scarp of a fault that accommodates roof subsidence. 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 a text-book analytical solution and of new Distinct Element Method (DEM) modelling.

Our analysis of the DSM suggest that caldera wall heights are not as closely linked to slope angle as previously suggested. Slope angles range from 20 – 65° and slope heights range from 99 m - 1085 m. We find that the smaller slope heights are not robustly tied to greater slope angle. When compared to analytical predictions, these slope-height data yield expected rock mass cohesion values of less than 0.25 MPa for all calderas, which is 2-3 orders of magnitude less than typical laboratory-scale values.

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. Results confirm that low bulk cohesion values <0.5 MPa are required to reproduce the observed ranges of slope angles and slope heights, and they indicate that friction is the dominant control on slope evolution. Different failure mechanisms resulted as a function of cohesion and friction during early collapse: (1) granular flow with low friction and cohesion, and (2) block toppling at high friction and cohesion. During later collapse, shear failure dominates regardless of cohesion. At higher cohesion and/or friction values, the models resulted in non-linear concave-upward slope profiles that are seen at many natural calderas.