Table of contents

Caldera-formation is one of the most awe-inspiring and powerful displays of nature's force. Resultant deposits may cover vast areas and significantly alter the immediate topography. Post-collapse activity may include resurgence, unrest, intra-caldera volcanism and potentially the start of a new magmatic cycle, perhaps eventually leading to renewed collapse. Since volcanoes and their eruptions are the surface manifestation of magmatic processes, calderas provide key insights into the generation and evolution of large-volume silicic magma bodies in the Earth's crust.

Despite their potentially ferocious nature, calderas play a crucial role in modern society's life. Collapse calderas host essential economic deposits and supply power for many via the exploitation of geothermal reservoirs, and thus receive considerable scientific, economic and industrial attention. Calderas also attract millions of visitors world-wide with their spectacular scenic displays.

To build on the outcomes of the 2005 calderas workshop in Tenerife (Spain) and to assess the most recent advances on caldera research, a follow-up meeting was proposed to be held in Mexico in 2008.

This abstract volume presents contributions to the 2nd Calderas Workshop held at Hotel Misión La Muralla, Querétaro, Mexico, 19–25 October 2008. The title of the workshop `Reconstructing the evolution of collapse calderas: Magma storage, mobilisation and eruption' set the theme for five days of presentations and discussions, both at the venue as well as during visits to the surrounding calderas of Amealco, Amazcala and Huichapan. The multi-disciplinary workshop was attended by more than 40 scientist from North, Central and South America, Europe, Australia and Asia. Contributions covered five thematic topics: geology, geochemistry/petrology, structural analysis/modelling, geophysics, and hazards.

The workshop was generously supported by the International Association of Volcanology and the Chemistry of The Earth's Interior (IAVCEI), the International Union of Geodesy and Geophysics (IUGG), the Universidad Nacional Autónoma de México (UNAM) through the Coordinación de la Investigación Científica, the Institute of Physics Publishing services, the Consejo Nacional de Ciencia y Tecnología de Querétaro (CONCYTEQ). The workshop represented the key activity of the IAVCEI Commission on Collapse Calderas in 2008.

We, the workshop organizers, would like to express our gratitude to all workshop participants, the staff of Misión La Muralla, the Centro de Geociencias of UNAM for administrative and logistic support, to Adelina Geyer for web support, to María Inés Rojano for organization of logistics, the Universidad Autónoma de San Luis Potosí for logistics support, the Comisiòn Nacional de Electricidad for authorizing a visit to Los Azufres geothermal field, and to all sponsors that provided financial support.

We expect these proceedings to stimulate further fruitful discussions, which we hope will be continued at a future meeting.

Jo Gottsmann and Gerardo Aguirre-Diaz October 2008

Compositional zonation in ignimbrites is relatively common, and is often inferred to record gradual withdrawal by an eruption of a density-stratified magma chamber (with silicic magma towards the top and more dense, mafic magma at the bottom). We show that this model does not match observations at the ca. 0.1 Ma Zaragoza ignimbrite from Los Humeros caldera in central Mexico. Detailed petrologic studies reveal a more complex scenario: the ignimbrite exhibits a `double' vertical zonation based on the compositions of pumice lapilli. We present evidence for mingling and limited mixing occurred during or immediately before the caldera-forming eruption. One possibility to explain the observations is that the ignimbrite eruption occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma chamber.

Three silicic calderas have been newly identified in northwest Nevada west of McDermitt caldera. This volcanism is interpreted to have formed during a short interval at 16.5-15.5 Ma, during the waning stage of Steens flood basalt volcanism after the initial impingement of the Yellowstone hot spot. New mapping demonstrates that the area affected by this mid-Miocene silicic volcanism is significantly larger than previously appreciated in the western U.S.

We recognize the most eastern and oldest collapse caldera structure in the southern portion of the Central Volcanic Zone of the Andes. A description of Middle-Upper Miocene successions related to explosive- effusive events is presented. The location of this centre close to Cerro Galn Caldera attests a recurrence in the volcanism between 12 and 2 Ma in this portion of the Altiplano - Puna Plateau.

The Chacana caldera, located immediately east of Quito, capital of Ecuador, forms the most-northern edifice of Ecuadorõs rhyolite province. It is a 50X30 km Pleistocene structure that has remained active into historic times. Vitrophyres, welded tuffs, and ignimbrites of rhyolitic and dacitic composition constitute the outer flanks, meantime syngenetic breccias and tuffs, capped later by extensive dacite lava flows and basin sediments, filled the calderaõs depression. A notable resurgence occurred that lifted quiet-water sediments to over 4000 m in elevation. The area has numerous hot springs, and little seismic activity.

Unconsolidated volcaniclastic glass deposits on the flanks of Axial Seamount, a caldera system situated on the Juan de Fuca Ridge in the NE Pacific Ocean, demonstrate the occurrence of explosive events, in addition to effusive activity. The variety of produced glass fragments ranges from various angular forms to thin deep-sea limu o Pele, with dominantly moderately fractionated to occasionally primitive MOR basalt composition. A model involving the collapse of a magmatic foam layer may account for the observed spectrum of glass fragments.

Archean calderas provide valuable insight into internal geometries of subaqueous calderas. The New Senator caldera, Abitibi greenstone belt, Canada, is an Archean example of a subaqueous nested caldera with a basal stratigraphy dominated by gabbro-diorite dykes and sills, ponded magmas and basalt and andesite lava flows. The aim of our study is to focus on the use of physical volcanology to differentiate between the various mafic units found at the base of the New Senator caldera. Differentiation between these various mafic units is important from an exploration point of view because in modern subaqueous summit calders (e.g. Axial Seamount) margins of ponded magmas are often sites of VMS formation.

In the Ecuadorian volcanic arc a cluster of scattered rhyolitic and dacitic centers within the mainly andesitic Eastern Cordillera includes large caldera structures (Chalupas, Chacana, Cosanga) as well as smaller edifices, built upon the Paleozoic-Mesozoic metamorphic basement. At the Chacana caldera magmatism dates from 2.7 Ma to historic times. These centers erupted enormous ash flows and thick pumice lapilli falls that covered the InterAndean Valley near Quito. The role of the 50-70 km-thick crust with a notable negative gravity anomaly appears to be related to the generation of this highly silicic magmatism occurring along the crest of the Andes in the NVZ.

A new caldera, named Malpaso, is reported west of the city of Aguascalientes, Mexico. The Malpaso caldera is a volcano-tectonic depression, highly fractured and faulted, and was filled by voluminous pyroclastic products related to the caldera collapse. Due to these characteristics it as a graben caldera. It is truncated by younger normal faults of the Calvillo and Aguascalientes grabens. In this work we present a summary of the geologic and structural observations on this caldera, as well as a description of the main caldera product, the high-grade El Ocote ignimbrite.

A pyroclastic flow deposit keeps a fingerprint of the physical processes that occurred in the flow during transport and settling. Part of this information is recorded in the texture of the deposit offering an instantaneous view of the flow prior to freezing. In this work we introduce some texture's analysis techniques, based on image analysis, that we have developed or tuned during the last years.

La Peligrosa Caldera is located at Sierra Colorada (47° 15'S, 71° 40' W) in the Chon-Aike silicic LIP. It represents an unique window to understand the eruptive mechanisms that prevailed throughout the ignimbritic flare-up in Southern Patagonia during middle to late Jurassic times. Key pieces of lithologic and structural evidences are taken into account to reconstruct the volcanic structure.

The intracaldera Hannegan volcanics were erupted during two collapse episodes of the Hannegan caldera in the North Cascade mountains of Washington State. The first eruption yielded a down-to-the-north trapdoor style collapse at 3.722 ± 0.020 Ma (40Ar/39Ar) that is bounded by a horseshoe-shaped ring fault. The second collapse, most probably also trapdoor style, followed a short period of sedimentation, and completed the elliptical ring fault around the southern margin of the caldera. Post caldera plutons, with U-Pb ages of 3.42 ± 0.10 and 3.36 ± 0.20 Ma, intruded the intracaldera ignimbrite.

Hydrothermal alteration at Los Azufres geothermal held is mostly propylitic showing progressive dehydration with depth, and temperature increase. The evolution of this system is inferred to be related to deep liquid water, boiling when ascending through fractures connected to the surface.

We have performed phase equilibrium experiments to determine the pre-eruptive conditions of the largest phonolitic caldera-forming eruption (∼20 km3 of DRE) that occurred on Tenerife (Canary Islands). The Abrigo ignimbrite was erupted during the last caldera-forming episode (ca. 190 ka), from the Cañadas caldera. Comparison of the natural and experimental phase proportions and compositions indicates that the phonolite at the roof of the Abrigo magma reservoir was at 130 ± 50 MPa (corresponding to ca. 4 - 5 km below the surface), 825 ± 25 oC, with 3 ± 1 wt% dissolved H2O and fO2 at the Ni-NiO buffer ? 1 log unit. This shows that the magma that produced the largest ignimbrite on Tenerife was stored at relatively shallow depths but was water-undersaturated, and its eruption was probably triggered by input of fresh mafic magma.

Based on information of enthalpies of the fluids of wells from the geothermal reservoir of Los Humeros, Puebla, Mexico, we determined the thermodynamic conditions of the reservoir comparing the values of enthalpies of the fluids of discharge of the wells with the values published in the literature for different thermodynamic state of fluids.

We present a thermodynamic model for the phase equilibria of gases and brines, which couples Henry's Law with the Soave–Redlich–Kwong equation of state to reproduce experimental data. In this communication we focus on the H₂S H₂O NaCl system.

The zoned plagioclase and quartz crystals within the multiple magmas that erupted during the ∼55 ka Rotoiti caldera forming eruption appear to have had complex thermal and compositional histories. Compositional zoning suggests that magmatic systems were open, and received multiple pulses of hotter and more mafic magma. Limited diffusion across the crystals indicates that crystals did not dwell at magmatic temperatures for prolonged periods (<100 years), and suggest that the melts were generated and erupted quickly.

A variety of source mechanisms have been proposed to account for observed caldera deformation. Here we present a systematic set of new results from numerical forward modelling using a Finite Element Method. which provides a link between measured ground deformation and the inaccessible deformation source. We simulate surface displacements due to pressure changes in a shallow oblate reservoir overlain by host rock with variable mechanical properties. We find that the amplitude and wavelength of resultant ground deformation is dependent on the distribution of mechanically stiff and soft lithologies and their relative distribution above a reservoir. In addition, we note an influence of layering on the critical ratio of horizontal over vertical displacements, a criterion employed to discriminate between different finite source geometries.

Plan-view ellipticity of a pre-caldera magma reservoir, and its influence on the development of caldera ring fracturing and eruptive behaviour, have not previously been subjected to dedicated evaluation. We experimentally simulated caldera collapse into elliptical magma chambers and found that collapse into highly-elliptical chambers produced a characteristic pattern of ring-fault localization and lateral propagation. Although results are preliminary, the general deformation pattern for elliptical resurgence shows strong similarities to elliptical collapse. Ring faults accommodating uplift again initiate around the chamberõs short axis and are reverse, but dip inward. Field and geophysical observations at several elliptical calderas of varying scale (e.g. Long Valley, Katmai, and Rabaul calderas) are consistent with a control from elliptical magma chamber geometry on ring fracturing and eruption, as predicted from our experiments.

Most collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub-vertical normal faults (ring-faults) after a decompression of the magma chamber, following eruption. Here, we present new numerical models that use a Finite Element Method to investigate the effects of variable crustal stratigraphy (lithology/thickness/order of strata) above a magma chamber, on local stress field distribution and how these in turn compare with existing criteria for ring-fault initiation. Results indicate that the occurrence and relative distribution of mechanically different lithologies may be influential in generating or inhibiting caldera collapse.

Pyroclastic density currents (PDCs) are among the most amazing, complex and dangerous volcanic phenomena. They are moving mixtures of particles and gas that flow across the ground, and originate in different ways and from various sources, during explosive eruptions or gravity-driven collapse of domes. We present results from experimental work to investigate the generation of large-scale, multiphase, gravity-driven currents. The experiments described here are particularly devoted to understanding the inception and development of PDCs under impulsive injection conditions by means of the fast application of a finite stress to a finite mass of pyroclastic particles via expansion of compressed gas.

We find that, in summary, PDC generation from collapse of pressure-adjusted or overpressurised pyroclastic jets critically depends on behaviour of injection into the atmosphere, which controls the collapsing mechanisms and then the physical parameters of the initiating current.

Three main types of collapse calderas can be defined, 1) summit caldera: those formed at the top of large volcanoes, 2) classic caldera: semi-circular to irregular-shaped large structures, several km in diameter and related to relatively large-volume pyroclastic products, and 3) graben caldera: explosive volcano-tectonic collapse structures from which large-volume, ignimbrite-forming eruptions occurred through several fissural vents along the graben master faults and the intra-graben block faults. These in turn can collapse at least with three styles: 1) Piston: when the collapse occurs as a single crustal block; 2) Trap-door: when collapse occurs unevenly along one side while the opposite side remains with no collapse; 3) Piece-meal: when collapse occurs as broken pieces of the crust on top of the magma chamber.

We present an analysis of caldera evolution at Miyakejima in 2000. The caldera changed its structure from piston to funnel subsidence during its growth. The successive subsidence of the central block induced landslides at the caldera wall, which successively enlarged the diameter of the caldera.

Formation of, and slip on existing, collapse calderas is much more common in basaltic edifices than in composite edifices. I suggest that this difference is partly due to the composite edifices being tougher and more resistant to ring-fault formation than a basaltic edifices. The high matieral toughness of composite edifices is related to their being composed of rock layers with widely different elastic properties, the elastic mismatch promoting deflection and/or arrest of potential ring faults at layer contacts.

Silicic calderas are the surface expressions of silicic magma chambers, and thus their study may yield information about what tectonic and crustal features favor the generation of evolved magma. The goal of this study is to determine whether silicic calderas in arc settings are preferentially located behind the volcanic front. After a global analysis of young, arc-related calderas, we find that silicic calderas at continental margins do form over a wide area behind the front, as compared to other types of arc volcanoes.

In April 2007, Piton de la Fournaise volcano experienced a caldera collapse during its largest historical eruption. We present here the resulting deformation and a synthesis of the seismicity recorded during recent caldera collapses. It allows us to propose a unifying mechanism that explains the pulsating collapse dynamics.

The volcano-tectonic events at the Villa de Reyes Graben (VRG), in the southern Sierra Madre Occidental, Mexico, include 1) a regional NNE fault system developed before 32 Ma, 2) this pre-32 Ma faulting controlled the emplacement of 31.5 Ma dacitic domes, 3) NE faulting at 28 Ma that displaced the 31.5 Ma dacitic domes and formed the VRG, as well as the oblique grabens of Bledos and Enramadas oriented NW, 4) emplacement of Panalillo ignimbrite at 28 Ma filling the VRG and erupting from fissures related to the oblique grabens, and eruption of Placa basalt apparently also from fault-controlled vents.

Despite a profusion of analogue models relatively little is known about the internal structure and temporal evolution of explosive caldera volcanoes. So how can modellers test their predictions given that the internal structures of many young calderas are concealed? Mapping ancient exhumed calderas has proven advantageous, yet this requires a large investment of time and expertise to constrain the structural evolution in sufficient detail. We aim to investigate the interplay between the structural evolution and eruption style over time at a modern caldera. We have selected Los Humeros (Mexico) because it is thought to be an example of a caldera with some piecemeal development, and it also has a well-exposed pyroclastic succession and abundant borehole data.

A possible interaction of (volcano-) tectonic earthquakes with the continuous seismic noise recorded in the volcanic island of Tenerife was recently suggested, but existing catalogues seem to be far from being self consistent, calling for the development of automatic detection and classification algorithms. In this work we propose the adoption of a methodology based on Hidden Markov Models (HMMs), widely used already in other fields, such as speech classification.

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain. The high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera (LCC) in greater detail than previously available. Mathematical construction of a subsurface model from the local anomaly data, employing a 3-D inversion enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture of the complex, and shedding light on its evolution.

We report on a joint gravimetric and ground deformation study on Montserrat, with the aim of quantifying mass and/or density changes beneath the island related to the volcanic activity at Soufrire Hills Volcano (SHV). Our observations coupled with 3-D data inversion indicate the existence of a previously unrecognised NNW-SSE trending zone of structural weakness (i.e. fault) that is located at shallow depths beneath the Centre Hills of Montserrat, along which active fluid migration is coupled to magmatic stressing at SHV.

The central Mediterranean area comprises some of the most active volcanoes of the northern hemisphere. Some of their names recall myths or events in human history: Somma-Vesuvius, Etna, Stromboli, Vulcano, Ischia and Campi Flegrei. These volcanoes are still active today, and produce both effusive and explosive eruptions. In particular, explosive eruptions can produce and disperse large amount of volcanic ash, which pose a threat to environment, economy and human health over a large part of the Mediterranean area. We present and discuss data of ash dispersal from some explosive eruptions of southern Italy volcanoes, which dispersed centimetre -thick ash blankets hundred of kilometres from the source, irrespective of the more limited dispersal of the respective coarse grained fallout and PDC deposits. The collected data also highlight the major role played by lower atmosphere winds in dispersal of ash from weak plumes and ash clouds that accompany PDC emplacement.

We present an analysis of volcanic threat of Nisyros island (Greece) based on a catalogue of questions compiled for the USGS National Volcano Early Warning System (NVEWS). We find that the score puts Nisyros in the league of volcanoes posing a very high threat. US volcanoes with a comparable threat level include Mt. St. Helens, Augustine and the Long Valley caldera.