Page 269 - Caldera Volcanism Analysis, Modelling and Response

244 J. Martı ´ et al.

the importance of this perturbation and considered that these edge effects did not
significantly affect the experimental results.
The use of silicone as a magma chamber analogue circumvents the restrictions
imposed by elastic balloon walls, but introduces other limitations such as an
inappropriate geometry of the silicone container to simulate a magma chamber.
With regard to the simulation of the host rock, all analogue experiments use
granular materials (e.g. dry quartz sand, flour). Although these materials have the
ability to deform in a brittle manner and do expose a Mohr–Coulomb behaviour
alike natural rocks, they are also susceptible to slump and readjustments by small-
scale grain flow. These processes lead to structures that may not have an analogy in
natural systems and must be regarded as permanent plastic deformation (Martı ´ et al.,
1994). This effect can be reduced by decreasing the grain size and increasing the
cohesion of the material.
None of the experimental devices and setups used, at the time of this writing,
succeeds in simulating dyke injection during tumescence or collapse. The high
viscosity of silicone and the granular nature of sand prohibit intrusion of silicone
and the formation of ‘ring dikes’ during experiments (Roche et al., 2000). Dyke
injection is likely to be important during the evolution of shallow magmatic system,
as dykes may regulate the thermodynamic equilibrium inside the magma chamber.
For example, in situations of overpressure, repeated dyke intrusions can decrease
the magma chamber pressure and may avoid the initiation of an eruptive event.
Also, dyke injection will change the strength of the host rock, one of the key
parameters in controlling the structural evolution of the whole system and,
therefore, possible caldera-collapse events.
Before concluding this section, it is also important to mention that the studied
experimental models have been carried out with homogenous roofs above the
magma chamber analogue. This is not a correct approximation to natural system,
since country rocks around magma chambers are normally heterogeneous in
composition and mechanical behaviour. Previous volcanic eruptions or sedimenta-
tion processes create a roof composed of materials with very different physical
properties such as pyroclasts and lavas. Compositional heterogeneities can influence
the stress field and consequently fracture propagation and structure development
(Gudmundsson, 2008). Therefore, it is important to introduce compositional
heterogeneities in order to mimic realistic phenomena.

3. Theoretical Models on Collapse Calderas Formation
Theoretical models based on solid and fluid mechanics are a valuable tool
for understanding the physics behind many volcanic processes and, in particular,
those during collapse caldera formation (Martı ´ and Folch, 2005). The governing
equations behind the models can be quite complex and often do not have analytical
solutions. As a consequence, numerical techniques are needed to solve such
problems. For this reason, the terms ‘numerical model’ or ‘computational model’
have become, by abuse of language, synonymous with ‘theoretical model’. In this