Page 311 - Caldera Volcanism Analysis, Modelling and Response

286 Valerio Acocella

structures, as well as their development, are commonly observed, even at various
scales. Such a consistency between models and nature suggests a general applicability
of experimental results. The four evolutionary stages adequately explain the
architecture and development of the established caldera end-members (downsag,
piston, funnel, piecemeal, trapdoor) along a continuum, where one or more end-
members may correspond to a specific stage. While such a continuum is controlled by
progressive subsidence, specific collapse geometries result from secondary contributory
factors (roof aspect ratio, collapse symmetry, pre-existing faults). The proposed
evolutionary scheme incorporates not only the geometric features of calderas, but more
importantly, also their genetic features.

1. Introduction

Collapse calderas are subcircular collapses formed during volcanic eruptions,
whose diameter is larger than that of explosive vents and craters (Williams, 1941).
Accordingly, only those volcanic depressions wider than B1 km should be
considered as calderas. Calderas may be characterised by significant variations in
diameter (km to tens of km), subsidence (m to km), shape (circular, elliptic or
polygonal, nested, overlapping), composition of erupted products (mafic to felsic)
and tectonic setting (extensional, strike-slip or compressional) (Gudmundsson and
Nilsen, 2006, and references therein).
Calderas are usually considered as the surface expression of the emptying of the
magma chamber during effusive or explosive eruptions. As a result of an
underpressure within the magma chamber, the roof of the reservoir collapses,
forming a depression at surface (Williams, 1941; Druitt and Sparks, 1984; Branney,
1995; Lipman, 1984, 1997; Martı ` et al., 2000). The common association between
caldera formation and eruption is one of the main motivations to study their
structure and development; in fact, predicting the possible structural control on the
behaviour of calderas during periods of unrest may prove crucial. In addition,
understanding the structure of calderas is important in geothermal and ore
exploration (Stix et al., 2003).
One approach toward systematic analysis of calderas has been to define discrete
end-member types with distinct geometric, evolutionary and eruptive character-
istics (Walker, 1984; Lipman, 1997; Kennedy and Styx, 2003; Cole et al., 2005).
Primarily based on field data, five end-member caldera geometries or styles (piston,
piecemeal, trapdoor, downsag, funnel) have been proposed (Figure 1; e.g. Lipman,
1997) and commonly referred to in the literature (e.g. Cole et al., 2005, and
references therein). However, such an end-member classification may be too
restrictive and not as useful in documenting a collapse style, which may correspond
to different morphologies at the surface (Cole et al., 2005). In addition, there is
still fragmented or very poor information on the main subsurface structures
and how these develop (Figure 1). Currently unanswered major structural
questions include the: (a) relationships between caldera morphology and caldera
structure; (b) resolution of the ‘‘room problem’’, that is how the subsidence of the