Page 260 - Caldera Volcanism Analysis, Modelling and Response

A Review on Collapse Caldera Modelling 235

et al., 2000; Jellinek and De Paolo, 2003; Folch and Martı ´, 2004; Lavalle ´e et al.,
2006). It is generally assumed that collapse calderas form after a significant
decompression of the associated magma chamber following a central vent eruption
(Druitt and Sparks, 1984; Martı ´ et al., 2000), but conditions for caldera formation
may also develop under overpressure in the magma chamber (Gudmundsson et al.,
1997; Gudmundsson, 1998).
Traditionally, our knowledge on collapse calderas has been based on field
studies (Williams, 1941; Williams and McBirney, 1979; Lipman, 1984, 1997, 2000;
Walker, 1984), which allow us to identify the caldera-forming products and
the resulting caldera structure. However, the different degree of preservation of
calderas precludes, in most cases, a complete view of its internal structure and the
corresponding caldera-forming deposits. Young calderas tend to have good
exposures of extra-caldera deposits but do not show the caldera interior, while in
old calderas erosion and, occasionally, tectonism, expose to some extent the internal
caldera structure (see Mueller et al., 2008) but may not preserve the whole sequence
of caldera products or the original caldera wall. We must appreciate that field
studies can provide vital insights into the final stage of caldera formation and thus
enable an indirect way to infer the caldera-forming process mostly through the
analysis of the succession of caldera deposits. However, active calderas field
studies are not capable of deducing the structural evolution of calderas, their deep
structure, or the conditions that lead to caldera formation.
A significant advance in the understanding of caldera formation has been
provided in recent years by the application of experimental (analogue and scale) and
theoretical (mathematical) modelling. These studies offer an easy way to infer
and visualise the structural evolution of a caldera and to predict the mechanical and
thermodynamic conditions that control its formation. Also, geophysical sounding
is progressively providing improved images of the interior structure of some calderas
and their associated subvolcanic systems. In combination with field studies,
experimental, theoretical and geophysical imaging is required to capture the full
complexity of caldera-forming processes. This multidisciplinary approach is
particularly vital in order to obtain a reliable image of caldera structures, not
only for prospecting mineral deposits or geothermal exploitation, but also for the
forecasting of future behaviour of calderas at unrest or to analyse the potential for
caldera-forming eruptions in active volcanic systems.
The main goal of this paper is to revise the existing experimental, theoretical,
and geophysical studies of collapse calderas, comparing their results and analysing
their advantages and limitations, and to list the main advances that the application of
such models has provided on our understanding of this type of volcanic structures.

2. The Role of Experimental Models in Caldera Studies
During recent years, analogue and scale modelling has become a useful tool
to study volcanic processes, in particular those related to the dynamics of shallow
magma chambers, eruption mechanisms, dynamics of volcanic plumes and