Page 267 - Caldera Volcanism Analysis, Modelling and Response

242 J. Martı ´ et al.

Figure 4 Sketches showing di¡erent stages in the structural evolution of the collapse process.
Notice that, for constant values of f, structural development is strongly dependent on the roof
aspect ratio R (modi¢ed after Geyer et al., 2006).

a significant bearing on ground subsidence and the resulting structures, which
eventually determine whether results are deemed realistic. The most widespread
magma chamber analogues are latex balloons (filled with either air or water) and
silicone reservoirs. Other analogue materials such as a rigid ball (Komuro et al.,
1984) or a dry ice ball (Komuro, 1987) were used on one occasion, but their use
was subsequently abandoned.
The use of a latex balloon as a magma chamber analogue imposes several
restrictions. The main limitations are wall effects of the elastic material. The balloon
can be deflated almost indefinitely, allowing the system to steadily pass through
different stages of collapse. This is certainly not the case in natural systems, where
the withdrawal process from the chamber can be interrupted as soon as magma is
unable to flow out, for example, when the eruption conduit is blocked (Martı ´ et al.,
2000). Furthermore, during doming processes, the elastic walls permit an almost
infinite expansion to geologically improbable levels (Martı ´ et al., 1994). In natural
systems, once expansion reaches a certain level, the tensile strength of the country
rock is exceeded, fracturing is induced, and an eruption can ensue (Tait et al., 1989).
Collapse and doming structures can be overstated compared to natural cases. Some
authors (Lavalle ´e et al., 2004; Geyer et al., 2006) avoid these edge effects by
stopping the experiment while the induced ground deformation is within realistic
limits.