Page 259 - Caldera Volcanism Analysis, Modelling and Response

234 J. Martı ´ et al.

relevant but atomised topics. From a methodological point of view, and in addition to
essential field and petrological studies, collapse calderas have also been investigated
through analogue and theoretical models and geophysical imaging. Each approach
presents advantages and disadvantages. We review the most significant contributions,
summarise the relevant outcomes, and highlight the strong points and weaknesses of each
approach. Analogue models enable a qualitative study of the structural evolution of a
collapse process and allow us to infer which geometric factors play a relevant role.
Differences among employed models lie in the applied experimental devices, the host rock
analogue material (dry quartz sand, flour, etc.), and the magma chamber analogue (water
or air-filled balloons, silicone reservoirs, etc.). However, the results obtained from different
experimental setups are not substantially different if basic input parameters are kept
similar in the experiments. Discrepancies in results mainly stem from restrictions of
experimental designs. Theoretical (mathematical) models have grown in importance during
the past decades, in combination with the development of computational resources.
Nowadays, these models constitute a significant source of information on caldera-forming
processes and can predict semi-quantitatively general conditions for fault formation and
propagation. Theoretical studies can be classified in two groups according to their
objectives. One group focuses on the evolution of pressure within the magmatic reservoir
during a caldera-forming eruption. The second looks more into the structural conditions for
caldera collapse and hence relate to analogue models. Both analogue and theoretical
models are employed to gain a fundamental understanding of caldera processes and their
resulting structures. Additionally, geophysical imaging helps to construct a regional image
of the subsurface at active calderas, thus imposing constraints on the structural
investigations based on analogue and mathematical modelling. A revision of each of these
three complementary approaches to the study of collapse calderas is given in this paper,
together with a combined analysis of their main findings and restrictions.

1. Introduction

Collapse calderas form by subsidence of the magma chamber roof during
magma removal from a reservoir primarily in the form of a volcanic eruption but
also by lateral magma migration. They represent one of the most enigmatic
geological structures we can recognise on Earth and other terrestrial planets
(Francis, 2003; Lipman, 2000). Their tremendous destructive potential, commonly
implying significant atmospheric impacts, as well as their association with ore
deposits and geothermal resources, have made calderas one of the main subjects of
interest of modern and traditional volcanology.
Like any other volcanic phenomena, caldera-forming eruptions represent the
culmination of a long-lived geological process involving the generation of magma
at depth, its ascent and differentiation, and finally its eruption on the Earth surface.
They occur in nearly all volcanic environments and are associated with most magma
types including basalts and more evolved compositions. However, caldera-forming
eruptions are rare compared to volcanic eruptions without caldera formation, as
they require very specific stress and thermodynamic conditions for collapse to occur
(Druitt and Sparks, 1984; Gudmundsson, 1998; Gudmundsson et al., 1997; Martı ´