Page 284 - Caldera Volcanism Analysis, Modelling and Response

A Review on Collapse Caldera Modelling 259

controversies will last until the application of a model, which also considers dyke
injection, will be available (see also Walter, 2008).
The stress field around a magma chamber is the most important controlling
factor of caldera-collapse-formation processes. In natural systems, this stress field has
contributions from three main sources: the stress perturbation associated with
the magma chamber itself (over- or underpressure), the regional or far-field stress,
and finally the topography loading stresses. In fact, at some volcanic complexes, the
topographic load may constitute the principal upper-crustal stress field and is able
to modify the regional fault patterns, increase the fault throw, and induce extension
(Lavalle ´e et al., 2004, and references therein). Despite this fact, most numerical
models use a flat horizontal topography, although calderas usually form in volcanic
fields with significant topographic relief. There are only a few studies on the effect
of a volcanic edifice on magma chamber emplacement and dyke propagation
(e.g. Pinel and Jaupart, 2000, 2003, 2004).
Another important restriction of existing models is that they are unable to
introduce regional faults or previously formed structures. Thus numerical models
are unable to simulate multi-cycling processes of inflation and deflation, although in
some of them (e.g. Burov and Guillou-Frottier, 1999) it is possible to observe the
reactivation of structures formed during the uplift stages. Clearly, the existence
of regional faults or previously formed structures strongly affects the morphology
and, in some cases, the mechanism of caldera collapse (Acocella et al., 2001)as
well as the post-collapse behaviour (Folch and Gottsmann, 2006; Gottsmann and
Battaglia, 2008).
Finally, it is important to remark that there is still no consensus on the most
adequate rheology for the magmas and host rock, and models dealing with different
rheologies may give rise to markedly different results.

4. Geophysical Imaging and Its Value for
Caldera Studies

As we have seen in the previous sections, mathematical and analogue models
of processes leading to caldera formation and post-collapse evolution provide
generalised insights into the evolution of volcanoes. These studies certainly have
implications in their own right as described earlier, but in order to validate their
applicability modelling results need to be critically assessed against the results
obtained from other studies such as field observations. The investigation of deeply
eroded caldera complexes can reveal information on the shape, depth, and dynamics
of regional magma reservoirs (preserved as plutons or batholiths) as well as on the
geometry of bounding faults (Lipman, 2000). The analysis of ancient successions
thus provides a critical validation of such models.
Obtaining information on the subsurface structure at modern active calderas is
less straightforward, as direct large-scale probing of the subsurface is impossible.
Petrological investigations of erupted materials as well as the chemical analysis of
volcanic gases help constraining an image of the caldera subsurface (Todesco, 2008).