Abstract - Pit craters and collapse calderas are volcanic depressions produced by subsidence of the roof of a magma reservoir. They are near-ubiquitous features of volcanic fields or edifices on Earth and other terrestrial planets. Pit crater and caldera subsidence may occur by various structural mechanisms, each previously linked with various initial controlling factors. We present Distinct Element Method (DEM) numerical models of this subsidence process to shed new light on such mechanisms and the factors controlling them. Our models comprise an assemblage of gravitationally-loaded circular particles that interact at their contacts according to elastic-frictional laws. Cohesion is provided by inter-particle bonds that break once their shear or tensile strength is exceeded. Varying particle and bond micro-properties yielded assemblage macro-properties and rheological responses characteristic of natural rock masses (e.g. elasticity, failure, and strain softening), with fracture localization simulated by bond breakage. The magma reservoir is represented as a region of non-bonded, low-fiction particles. Withdrawal of magma is simulated by incrementally reducing the area of the reservoir particles. The resultant gravity-driven failure and subsidence of the overlying reservoir roof is explicitly replicated. We have investigated the effect on subsidence of initial mechanical and geometric properties. The former included Young’s Modulus and strength (cohesion), whilst the latter included the thickness/diameter ratio (T/D) of the roof and the ceiling curvature imparted by the shape of the underlying reservoir. These initial mechanical and geometric properties interacted to produce several ‘end-member’ subsidence mechanisms: (1) Predominantly central sagging at low strength and low T/D; (2) predominantly single central block subsidence at low to intermediate strength and at intermediate T/D ratios; (3) multiple central block subsidence at high strength and high T/D ratios; (4) a hitherto unrecognized ‘central snapping’ mechanism at high strength, low T/D, and with a low ceiling curvature. In terms of material properties, the occurrence of these mechanisms was weakly dependent on Young’s Modulus, but strongly dependent on material strength. In terms of geometric properties, a moderate dependency on reservoir shape at low T/D ratios diminished at intermediate and high T/D ratios. One important outcome of the interaction of mechanical and geometric properties is that the critical T/D ratio, at which the single-block mechanism changes to multiple-block subsidence, was strongly dependent on material strength. Our model results agree well with those of analogue studies of pit crater and caldera subsidence, and provide a more quantitative understanding of the genesis of different mechanisms observed in the field - e.g. at Nindiri crater, Nicaragua and Miyakejima caldera, Japan.
Abstract of talk given to:
AGU annual conference, San Francisco, December 2009.