Page 316 - Caldera Volcanism Analysis, Modelling and Response

Structural Development of Calderas 291

3.2. Caldera collapse without regional stress field
Komuro (1987) is the first attempt to specifically simulate a collapse caldera. This is
achieved using dry ice evaporating beneath a mixture of sand and clay. The
spherical dry ice simulates a contracting reservoir extruding magma at surface,
whereas the sand-clay mixture simulates the upper crust. The length ratio between

5
model and nature is L B10 . As stated by the author, because of the cohesive
clay, this mixture probably has a strength one order of magnitude larger than
required to simulate the upper crust. Evaporation of dry ice results in the
overburden collapse, which propagates upwards to surface, where subcircular ring
fractures are observed (Figure 2). These are outward dipping in the central part,
formed at early stages, and subvertical at the periphery of the collapse, formed at
later stages. The outward dip of the ring faults may permit the intrusion of magma
during collapse (Figure 2b). No radial fractures are observed.
Marti et al. (1994) simulate caldera collapse by deflating an elastic balloon
(magma chamber analogue) within fused alumina powder (upper crust analogue)
(Figure 3a). Different balloon shapes (spherical, cylindrical, flat, penny-shaped) are
used to reproduce a wide range of magma chambers. The length ratio between
5
model and nature is L B10 . To simulate collapse, air is removed from the
balloon. The resulting subsidence is mainly achieved by means of subvertical faults,
which confine an area characterised by several concentric collapsing blocks; these
are all bordered by outward dipping reverse faults, each accommodating a minor
amount of displacement and dying out towards the subvertical faults (Figure 3b).
Given these concentric ring faults at surface, the authors suggest that nested calderas
may result from the activity of a single magma chamber. The area of collapse
increases with the size of the balloon and decreases with its depth.
Roche et al. (2000) is probably the most detailed and comprehensive
experimental study focused on collapse calderas so far. Collapses are achieved by
sinking Newtonian silicone putty (magma analogue) at the base of a sand-pack
(brittle crust analogue). The sand-silicone interface is flat. Sinking is obtained by

Figure 2 Experiments from Komuro (1987). (a) In£ation+de£ation; (b) de£ation only
(modi¢ed after Komuro,1987). AU :1