Extended Abstract -
Pit craters and collapse calderas are volcanic depressions formed when a magma reservoir roof subsides, as magma is withdrawn and/or erupted from the reservoir. The two are distinguished mainly by scale; the term ‘caldera’ is usually arbitrarily applied to depressions with a diameter greater than one kilometre. Such depressions are near-ubiquitous features of volcanic fields or edifices. They occur in all regional tectonic settings and in association with volcanism of all compositions. Available field and geophysical evidence indicates a variety of mechanisms by which caldera subsidence may be accommodated and thus a diversity of styles of subsidence. To gain a quantitative understanding of this variety, and to identify what geometric and mechanical factors influence it, we used Distinct Element Method (DEM) numerical models.
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. By varying particle and bond micro-properties, we generated several DEM materials. Virtual biaxial rock tests then constrained material macro-properties, which are characteristic of natural rock masses. 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. Resultant gravity-driven failure and large-strain subsidence of the overlying reservoir roof is explicitly replicated.
The DEM models produce subsidence structures that develop in sequences closely matching those observed in analogue models and inferred at natural case studies. The DEM models also show that these structures and their development can be strongly affected by an interaction of several geometric (principally roof T/D) and mechanical (strength, Young’s Modulus) factors. At low material strengths, and especially for low T/D ratios, distributed strain accommodation in the form of sagging of roof strata is prevalent. With increased strength, and with increased T/D, the subsidence-related strain becomes more localised along discrete faults. In addition, near-surface inward rotations that are accommodated by sagging at low strength are accommodated by rigid block tilting at high strength. Increased strength and T/D act to change the failure style from a single central block to multiple central blocks. At high strengths, ephemeral cavities may form above successively formed blocks. Higher Young’s modulus causes a rapid shattering of the material and thus to produce a ‘weaker’ overall structural behaviour of the model when compared with equivalent Low Young’s modulus models. At low to intermediate strengths, sagging is thus more prevalent. At high strengths, multiple central block failure style is less well-developed.
DEM numerical simulations show that the variety of structures observed at natural pit craters and collapse calderas arises through a complex interaction of T/D ratio, strength and Young’s modulus. These simulations can thus provide a more quantitative understanding of the genesis of different structural styles observed or inferred in the field - e.g. at Nindiri crater, Nicaragua, Dolomieu caldera, Reunion and Miyakejima caldera, Japan.
Abstract of talk given to:
Collapse Caldera Workshop 2010, Reunion Island, October 2010.