Page 332 - Caldera Volcanism Analysis, Modelling and Response

Structural Development of Calderas 307

Piecemeal collapse is the only established caldera end-member that has probably
not been satisfactorily simulated experimentally. In fact, experimental piecemeals
have been obtained during inflation-deflation cycles (Troll et al., 2002), but not
during the reactivation of pre-existing regional structures (Acocella et al., 2004), as
suggested by field data (Moore and Kokelaar, 1998). The poor evidence for
experimental piecemeal collapses is also mirrored by field studies: (a) there is only a
small number of observed piecemeal calderas (Lipman, 1997) and (b) even at type
locality of piecemeal calderas (Glencoe, Scafell), collapse was mainly achieved by
downsag and, only to a lesser extent, by faulting (Branney and Kokelaar, 1994;
Moore and Kokelaar, 1998). These facts suggest that the occurrence of piecemeal
collapse is a minor mechanism during caldera formation. As piecemeal calderas
require pre-existing faulting, they may, in principle, belong to any of the last three
evolutionary stages (Figure 12).
The possible domains for the established caldera end-members show that more
than one end-member may correspond to a certain evolutionary stage (Figure 12).
This means that, while a caldera evolution stage is precisely defined by its
structural features (Figure 10), elements from multiple caldera end-members
(Figure 12, right portion) may be present as well, even though not being diagnostic
of the collapse stage.
It thus appears that the five established caldera end-members classically described
in the literature can be easily reconciled with the four experimental stages of caldera
evolution, accordingly with the evolutionary scheme proposed in Figure 12
(Acocella, 2006a, 2007). This relies on the evolution, amount of subsidence and
diameter of the calderas and identifies a caldera accordingly with its overall surface
structure, mechanism of formation and evolution. It therefore not only incorporates
the geometric features, but more importantly, also the genetic features of calderas.

7. Conclusions

The overview of the analogue models of calderas permits to define the
following aspects on their structure and evolution:

(1) All the experiments simulating caldera collapse point out to a consistent
scenario defining the evolution and structure of calderas, regardless of any
experimental boundary condition (materials, apparatus, scaling, volcanic
edifices and regional stresses).
(2) In these experiments, a complete cycle of caldera collapse develops in four main
stages, proportional to the amount of subsidence, progressively characterised by:
(a) a downsag, (b) a reverse ring fault; (c) a peripheral downsag, (d) a peripheral
normal ring fault.
(3) Most of the available surface and subsurface geological and geophysical data at
known worldwide calderas are consistent with the experimental structural
features, as well as with their progression.
(4) The evolutionary stages explain the architecture and development of
the established caldera end-members along a continuum, where one or more