Page 286 - Caldera Volcanism Analysis, Modelling and Response

A Review on Collapse Caldera Modelling 261

1996), and electric and electromagnetic imaging (Wait, 1982; Parasnis, 1996).
Here, we aim at reviewing existing work from geophysical imaging and to assess
the value of geophysical information to provide insights into the evolution of
collapse calderas including key issues such as magma generation, storage, and the
distribution of subsurface discontinuities. We restrict this evaluation to relatively
well-studied calderas that either showed historic eruptions or are currently
undergoing unrest.
We particularly assess published work to analyse its potential for providing
answers to the two most controversially discussed issues at active calderas. First, the
question of shape, size, and depth of underlying magmatic bodies (see previous
sections), and second, the question of faults and their role in the evolution of
calderas (Acocella, 2008; Gudmundsson, 2008; and previous sections). The first
question has two important implications for the dynamics at active calderas:

(i) A widely accepted hypothesis to explain that the generation of a collapse caldera
is a considerable emptying of a horizontally elongated (sill-shaped) magma
body, which results in the formation of a surface depression with a diameter
approximately equal to the diameter of the reservoir. The identification of
magmatic systems at depth using geophysical imaging provides essential
constraints on their geometry and thermodynamic state. This information is
critical for the assessment of potential caldera collapse when such volcanoes
eventually undergo magmatic reactivation.
(ii) Magma reservoirs identified by geophysical imaging can be assessed if they
qualify as candidates of causative bodies (magmatic or hydrothermal) responsible
for periods of unrest inferred from dynamic investigations as explained above.

The second question relates directly to the issue of bounding fault geometry
(inclination, length, strike). The vertical collapse of crustal roof rocks along
bounding faults into an emptying magma reservoir is widely regarded as the prime
mechanism for the generation of a volcanic caldera as outline in the previous
sections. Faults determine the style of collapse as well as the structure of the
resulting depression (Acocella, 2008; Gudmundsson, 2008; and previous sections).
Post-caldera eruptions and resurgence are also attributed to activity along bounding
faults (Saunders, 2001, 2004). Other studies emphasise the influence of bounding
faults on ground deformation during caldera unrest (De Natale et al., 1997; Folch
and Gottsmann, 2006). Geophysical information on fault geometries at calderas not
only provide insights into the stress regime leading to collapse but also provide
important constraints for the evaluation of signals during caldera unrest.
We review the work published on the calderas of Rabaul (Papua New Guinea),
Campi Flegrei (Italy), Taupo (New Zealand), Toba (Indonesia), Las Can ˜adas de
Tenerife (Spain), Valles (USA), and Long Valley (USA). We have selected these
calderas predominantly because of the availability of a relative wealth of geophysical
data from multi-parametric imaging and their spread of geotectonic settings.
Table 2 provides an overview of geophysical techniques applied at the selected
calderas as well as key results on their subsurface structure. Note that we focus on
information that provides insights into subsurface reservoirs and faults.