Page 261 - Caldera Volcanism Analysis, Modelling and Response

236 J. Martı ´ et al.

pyroclastic flows, or volcano instability (Martı ´ and Folch, 2005). Analogue and scale
experiments can simulate volcanic processes in the laboratory under conditions that
are comparable or similar to those in nature. Such experiments have the advantage
that they help to visualise phenomena that cannot be directly observed in natural
systems. In comparison with field studies, experimental modelling, together with
theoretical modelling (see later in this paper), helps to understand the processes that
give rise to the products found in the geological record. Experimental models,
together with field observations and laboratory data, are necessary to validate
theoretical models.
Analogue and scale experiments have been developed to investigate the internal
structure of collapse calderas, but also their dynamics and structural controls
(Komuro et al., 1984; Komuro, 1987; Martı ´ et al., 1994; Roche et al., 2000;
Acocella et al., 2000, 2001, 2004; Roche and Druitt, 2001; Walter and Troll, 2001;
Kennedy et al., 2004; Lavalle ´e et al., 2004; Holohan et al., 2005; Geyer et al., 2006).
These experiments need to reproduce permanent deformation structures such
as fractures and faults so that the experimental design for most cases includes
cohesive, dry, powder mixtures (sand, fused alumina, flour, etc) simulating the
crust, and silicone, air or water into an elastic balloon to simulate magma and
magma chambers (Figure 1). As a result, analogue and scale experiments allow us to
reproduce different caldera structures depending on the geometry (depth, shape,
and size of the experimental chamber) and initial conditions (existence or not
of previous doming, regional tectonics, etc.) of each experiment. Moreover,
they provide relevant information on the structural evolution of the caldera process
as they allow to trace the evolution of fractures and faults that control caldera
subsidence (Geyer et al., 2006).
However, analogue and scale experiments also expose some significant
limitations. The principal drawback is the problem of scale. Volcanic systems
involve processes that operate over a much wider larger space and longer timescales
than those reproducible in the laboratory. The correct scaling is of utmost
importance to assess dynamics, since they may be directly dependent on processes
on different scales. In many cases, the appropriate scaling relationships are
unknown. However, rigorous scaling almost always involves scaling of both the
physical properties of the materials used and the dimensions of the system.
Therefore, fully scaled dynamical simulations will usually require the use of
appropriate analogue materials. If analogue experiments are not fully scaled, their
‘simulation’ of natural processes is unlikely to reproduce the entire spectra of
dynamic behaviour. The role of not-for-scale experiments is primarily to offer a
tool for the investigation and visualisation of caldera-collapse processes in order to
deduce which geometrical relationships between the magma chamber and its
surrounding medium may control the final results.
A detailed revision of the different experimental designs and results from caldera
studies is given in another contribution to this volume (Acocella, 2008). Here, we
briefly summarise some significant aspects of experimental modelling of collapse
calderas in order to provide a basis for a general discussion of collapse caldera
modelling.