Page 20 - Caldera Volcanism Analysis, Modelling and Response

Preface xix

Which are the mechanisms of fracture propagation during ring fault
formation?
Are caldera-forming faults outward or inward dipping ring faults?

This volume aims at providing some answers to these puzzling questions by
exploring the current understanding of these complex geological processes in the
form of 14 contributions to this volume. The book includes some of the
contributions presented at the workshop but it is also seeded with invited papers
to cover the broad spectra of studies dedicated to caldera volcanism. Although we
aimed at integrating quite diverse disciplines, it is clear that we cannot provide an
exhaustive treatment of all flavours of caldera studies. There are of course additional
and equally important disciplines of caldera research, which are not covered by this
volume and there is still a significant amount of work to be done. We intended this
volume to be another step forward in attempting to capture the full dynamics of
volcanic calderas.
The volume starts with a treatment of geological and geochemical investigations
at collapse calderas. Costa reviews the approaches to and existing data on magma
residence times and rates of igneous activity related to major calderas. The paper
also explores how this time information contributes to establishing the rates of
magmatic differentiation, mass and thermal budgets of caldera-forming reservoirs.
Residence times are highly variable for different systems and range from a few
thousand years to several hundred thousand years.
Sulpizio and Dellino describe the analysis of pyroclastic products of caldera
volcanism. They illustrate how the geological record can be read in order to deduce
the eruption mechanism and the physical processes that influence the mobility,
sedimentology and depositional mechanisms of pyroclastic density currents (PDCs)
generated during both caldera formation as well as post-caldera volcanism. In this
context, a particular emphasis is given to the influence of various types of
morphological settings and obstacles on the deposition and the mobility of PDCs.
Pittari et al. reconstruct pre- and syn-caldera-collapse processes associated
with the 186 ka caldera-forming event on Tenerife via detailed geological and
geochemical analyses of ignimbrite deposits. They show that significant lateral
variations in the proportions of lithic clast types, within depositional units, are
consistent with an eruption involving multiple vents around a caldera that
underwent piecemeal collapse. Based on evidence from vertical variations in lithic
clast proportions between depositional units, the authors infer an increasing depth
of conduit wall rock fragmentation during the eruption.
Aguirre-Diaz et al. report on the large ignimbrite province of the Sierra Madre
Occidental in Mexico, presenting a particular type of caldera formation associated
with the generation of tectonic grabens during vertical collapse and the formation
of thick ignimbrite deposits. After the main collapses and ignimbrite emplacements
during a flare-up period between 38 and 23 Ma, faulting and subsidence continued
for several millions of years displacing the intra-graben-caldera products downward
into the tectonic depression, but preserving the chaotic arrangement of the
collapsed blocks. In many cases, the graben-caldera vents are related to gold and
silver hydrothermal mineralisation and thus the findings constitute an important