Page 266 - Caldera Volcanism Analysis, Modelling and Response

A Review on Collapse Caldera Modelling 241

high-angle reverse ring faults. The outer depression is defined by inward-dipping
normal ring faults. During the collapse, the caldera walls break down causing the
caldera rim to propagate outwards.
The structural pattern controlling caldera collapse inferred from these experi-
ments can be considered as the most common one, and it is generally believed to
represent the same structural sequence that would control the formation of natural
calderas (Acocella, 2008). However, some changes in this general behaviour can be
observed as soon as other aspects are taken into account including previous doming,
topographic loading, or deviatoric tectonic stresses.
Most caldera-collapse experiments are concerned with the final stage in the
formation of a collapse caldera or with the structural evolution of such a volcanic
process, but ignore its relationship with the magma chamber decompression.
Recently, Geyer et al. (2006) have determined experimentally and semi-
quantitatively the fraction of magma ( f ) necessary to be removed in order to form
the different structural features that lead to caldera subsidence. They performed
analogue experiments to correlate the structural evolution of a collapse with the
erupted magma chamber volume fraction. The experimental device is similar to
the one used by Martı ´ et al. (1994) but water was used instead of air to inflate the
balloon (see Figure 1). This setup enables the temporal evolution of the collapse to
be recorded and the tracking of the evolution of fractures and faults. Geyer et al.
(2006) study the appearance and development of specific brittle structures, such as
surface fractures or internal reverse faults, and correlate each brittle structure with
the corresponding magma volume fraction removed from the chamber (Figure 4).
Geyer et al. (2006) also determine the critical conditions for the onset of caldera
formation.
Experimental results show that, at any characteristic structural event (e.g. first
appearance of surface fractures), the experimental relationship between volume
fraction ( f ) and chamber roof aspect ratio (R) fits a logarithmic curve (Figure 5).
This implies that volume fractions required to trigger collapse are lower for
chambers with low aspect ratios (shallow and wide) than for chambers with high
aspect ratios (deep and small). These results are in agreement with natural examples
and theoretical studies.

2.3. Restrictions of the experimental modelling
Before discussing any experimental results and their geological implications for
the study of collapse calderas, it is necessary to determine the restrictions of the
experimental setup and, if possible, try to minimise their effects for future analogue
models. In this section, we describe the restrictions and limitations imposed
by individual experimental setups (Figure 1). Analogue materials used to mimic
the host rocks and the magma chamber analogue pose major limitations on
experimental results. The main restrictions include (1) unrealistic deformation
of the experimental magma chamber, (2) existence of grain-flow processes, (3)
impossibility of dyke injection, and (4) homogeneous magma chamber roof.
The choice of the magma chamber analogue is an important part of the
experimental design. Its physical properties, as well as its deformation pattern, have