Mechanical and geometric controls on the structural evolution of pit crater and caldera subsidence

Pit craters and calderas are volcanic depressions produced by subsidence of a magma reservoir roof. To identify how geometric and mechanical factors may influence the structural evolution of this subsidence, we used two‐dimensional distinct element method numerical models. The reservoir host rock was represented as an assemblage of bonded circular particles that interact according to elastic‐frictional laws. Varying particle and bond properties produced a range of bulk material properties characteristic of natural rock masses. Fracturing results when bonds break, once their shear or tensile strength is exceeded. The magma reservoir was represented as a region of nonbonded low‐friction particles. Withdrawal of magma was simulated by incrementally reducing the area of the reservoir particles. Resultant gravity‐driven failure and subsidence of the reservoir roof were explicitly replicated. Interaction of the roof's strength, Young's modulus, thickness/diameter ratio (T/D), and the reservoir's shape yields a variety of model structures and subsidence styles. In conceptual terms, four end‐member subsidence styles developed: (1) “central sagging” favored by low strength and low T/D; (2) “central snapping” favored by high strength, low T/D, and a sill‐like reservoir shape; (3) “single central block” favored by low to intermediate strength, high Young's modulus, and intermediate T/D; and (4) “multiple central blocks” favored by high strength, low Young's modulus, and high T/D. Most model realizations incorporated some combination of each style, however. The models provide a geomechanical framework for understanding natural pit crater or caldera structures, as at Nindiri (Nicaragua), Fernandina (Galapagos), Dolomieu (La Reunion), and Miyakejima (Japan). Key Points The structure of collapse calderas and pit craters is studied by DEM models The models directly reproduce many of the structures observed in the field The natural structures are related to model geometric and mechanical properties