Large Silicic Caldera Research Articles

Gathering insights into volcanic risk from Auckland to the Andes, Antilles, and Arabia: an unexpected journey to professorship in volcanology

In 2020 I was promoted to Professor at Waipapa Taumata Rau the University of Auckland (UoA), joining the small circle of women (now three, the others being Kathleen Campbell and Philippa Black, also from UoA) who had achieved promotion to Professor in Geology in Aotearoa New Zealand, and the first ever in the field of volcanology. This promotion was a gratifying and somewhat unexpected achievement for me considering that I had started out studying languages and linguistics! In this contribution, which is based on my “inaugural” lecture as Professor in 2021, I provide an overview of my journey, framed primarily through the work of my doctoral students. I pay particular focus on the Andes where I worked on large silicic caldera systems, the Lesser Antilles, characterised by andesite and dacitic dome complexes and stratovolcanoes, and the distributed basaltic volcanism in Auckland and Saudia Arabia. I share some insights gained along the way and describe how these have influenced a shift in my research from more fundamental petrology to more applied volcanology over the course of my career.

Understanding caldera degassing from a detailed investigation at Lake Rotoiti, Okataina Volcanic Centre, New Zealand

Volcanic lakes in large silicic caldera volcanoes are an important source of CO2 emissions. However, quantifying CO2 output is challenging due to the lack of observed historical CO2 flux records and the large size of the volcanic and hydrothermal systems. Twenty percent of the surface area of Okataina Caldera, Taupo Volcanic Zone, New Zealand is covered with lakes. Geothermal expressions, predominantly on the Okataina caldera margin, occur in at least six different locations, with surface expressions both on land and under water. Lake Rotoiti is located at the northwest edge of the Okataina Caldera, spans across the Tikitere and Taheke geothermal fields, and has inputs from on-land thermal springs, lake floor hydrothermal vents, and from Lake Rotorua. CO2 flux from Lake Rotoiti was assessed using the accumulation chamber method and three techniques are used to process the data for total CO2 emission of the lake: (1) sequential Gaussian simulation (sGs) method allows quantification of CO2 emission with spatial control, (2) graphical statistical approach (GSA) allows the quantification of CO2 emission from different degassing regimes, and (3) a method based on water chemistry of the lake. We find CO2 is mostly emitting at Tumoana Bay and Central Basin, and the emission rate is 271 ± 38 t d−1 of CO2 (based on sGs). We then added this estimated emission data to the existing CO2 data for individual geothermal systems distributed around the active Okataina volcanic centre and calculate that the entire caldera is emitting at least 1856 t d−1 of CO2. The total emission is dominated by the lakes, where the topography is lower. We discuss the implications of this study in terms of preferential degassing locations and the amount of degassing, in particular for CO2, in an active caldera setting.

Giant silicic ignimbrites and comagmatic domes at the southeastern end of the Sierra Madre Occidental, México

The Sierra Madre Occidental (SMO) ignimbrite province includes giant ignimbrites related to large silicic calderas and dome complexes. Previous work in the SMO has paid little attention to the ignimbrites and domes exposed in the southeasternmost portion of the volcanic province, in the Guanajuato and Querétaro States. This work focuses on two widespread Oligocene ignimbrite sheets and numerous lava domes of this part of the SMO, and provides data on their distribution, stratigraphy, geochronology, textures, and geochemistry. Two large ignimbrite sheets and several domes were identified, which according to the stratigraphy they were classified in pre- and post-ignimbrites. We have named the ignimbrites as Victoria I and Victoria II. Victoria I is purple to light grey, with strong welding, horizontal foliation, and well-developed columnar jointing. It contains quartz, plagioclase and sanidine; fiamme is a common feature. Victoria II ignimbrite conformably overlies Victoria I. It is dark pink to brown, densely welded but less welded than Victoria I; in most locations it has developed columnar jointing. Victoria II's mineral assemblage includes biotite and hornblende in addition to quartz, plagioclase and sanidine; it has a higher content of lithics fragments and fiamme than Victoria I. A black vitrophyre forms base of each ignimbrite and marks the contact between them. These ignimbrites are stratigraphic markers due to their wide distribution and distinctive field and textural characteristics. We have obtained zircon U–Pb ages of 31.65 ± 0.17 to 30.57 ± 0.14 Ma (1σ) for Victoria I ignimbrite and 31.31 ± 0.21 to 29.73 ± 0.18 Ma for Victoria II ignimbrite. Silicic lava domes identified in the study area are either pre- or post-ignimbrite; the former are dacitic, and the latter are rhyolitic or rhyodacitic. We focus on the post-ignimbrite domes with the hypothesis that they were comagmatic with the Victoria ignimbrites. The post-ignimbrite domes are devitrified rocks with phenocrysts of plagioclase, quartz, sanidine, biotite and hornblende; and zircon U–Pb ages yielded 30.81 ± 0.21 Ma to 28.02 ± 0.19 Ma.

Plutonic record of a caldera-forming silicic eruption: The shatter zone of the Cadillac Mountain granite, coastal Maine

Abstract Although it is widely accepted that large silicic calderas are associated with voluminous synvolcanic intrusive complexes at depth, geological evidence for caldera-forming eruptions preserved in plutonic rocks has largely been elusive. Here, we document a rare example of such evidence in the Cadillac Mountain intrusive complex, Maine (USA), where erosion has revealed a remarkable marginal “shatter zone” that records evidence for a major caldera-forming eruption. This shatter zone, up to >1 km wide, is bounded by a steep ring fault at its outer margin, which grades inward into Cadillac Mountain granite. Its outer margins are characterized by intensely brecciated and deformed country rock injected by felsite veins, reflecting explosive fragmentation associated with eruptive decompression. This marginal facies grades inward to a chaotic mélange of variably rounded and remelted country rock blocks in granitic matrix, reflecting debris eroded from ring fault conduit walls and milled in an eruptive jet before collapsing onto crystal mush. Further inward, blocks up to 80 m in size were stoped from the collapsing chamber roof and settled onto strong mush. Textural and chemical variations in the shatter zone matrix reveal syneruptive ascent of distinct silicic and more mafic magma from depth, which was likely drawn through the highly permeable shatter zone toward areas of low pressure beneath active vents. The Cadillac Mountain shatter zone provides clear evidence for a major eruption preserved in the plutonic record and supports the origin of some granites as the cumulate roots of large silicic volcanic systems.

Diffusion chronometry and the timescales of magmatic processes

Volcanic eruptions can represent major societal hazards. Placing tighter bounds on the timescales of magmatic processes that precede eruptions is, therefore, important for volcano monitoring and forecasting. Diffusion chronometry, where volcanic crystals that contain chemical gradients are treated as time capsules, allows the timescale of various magmatic processes to be constrained. In this Review, we discuss the basics of diffusion chronometry and describe how re-equilibration via chemical diffusion provides insights into the timescales of magma storage, ascent and eruption. Crystals from mafic volcanoes record timescales of days to years between magma intrusion and eruption, which broadly match those recorded by monitoring data (such as increased seismicity). The timescales recorded in crystals from large silicic calderas, however, are typically longer than those from mafic volcanoes, spanning decades to millennia, but almost two orders of magnitude shorter than the timescales obtained by U-Th isotope disequilibria in zircon. The cause of this discrepancy is debated but likely reflects the protracted magma accumulation and complex thermal history that many crystals experience before eruption. Diffusion chronometry adds the fourth dimension to volcano science (that is, time), and advances in analytical and experimental approaches (such as NanoSIMS) open up new opportunities for understanding magmatic systems. Constraining the timescale of magmatic processes in the build-up to eruption is critical for hazard assessments and informing volcano-monitoring networks. This Review discusses the application of diffusion chronometry, which uses re-equilibration of chemical zoning profiles in crystals to extract time information, to understand the timescales of magmatic processes.

The structural architecture of the Los Humeros volcanic complex and geothermal field

The Los Humeros Volcanic Complex (LHVC) is a large silicic caldera complex in the Trans-Mexican Volcanic Belt (TMVB), hosting a geothermal field currently in exploitation by the Comisión Federal de Electricidad (CFE) of Mexico, with an installed capacity of ca. 95 MW of electric power. Understanding the structural architecture of LHVC is important to get insights into the interplay between the volcano-tectonic setting and the characteristics of the geothermal resources in the area. The analysis of volcanotectonic interplay in LHVC benefits from the availability of subsurface data obtained during the exploration of the geothermal reservoir that allows the achievement of a 3D structural view of the volcano system. The LHVC thus represents an important natural laboratory for the development of general models of volcano-tectonic interaction in calderas. In this study, we discuss a structural model of LHVC based on morphostructural and field analysis, integrated with well logs, focal mechanism solutions and magnetotelluric imaging. The structural analysis suggests that inherited regional tectonic structures recognized in the basement played an important role in the evolution of the magma feeding system, caldera collapses and post-caldera deformations. These inherited weak planes have been reactivated by resurgence faults and post-caldera magma-driven hydrofractures under a local radial stress field generated by the shallow LHVC magmatic/hydrothermal system. The local stress field induced caldera resurgence and volcanotectonic faulting. The results of this study are important to better constrain the structural architecture of large caldera complexes. Also, our study is useful to understand the structure of the Los Humeros geothermal field and support the exploration of deeper Super-Hot Geothermal Systems (SHGSs) and engineering of Enhanced Geothermal Systems (EGSs) for electric power production in the LHVC and other active resurgent calderas.

The Role of Tectonic Stress in Triggering Large Silicic Caldera Eruptions

AbstractWe utilize 3‐D temperature‐dependent viscoelastic finite element models to investigate the mechanical response of the host rock supporting large caldera‐size magma reservoirs (volumes >102 km3) to local tectonic stresses. The mechanical stability of the host rock is used to determine the maximum predicted repose intervals and magma flux rates that systems may experience before successive eruption is triggered. Numerical results indicate that regional extension decreases the stability of the roof rock overlying a magma reservoir, thereby promoting early‐onset caldera collapse. Alternatively, moderate amounts of compression (≤10 mm/year) on relatively short timescales (<104 years) increases roof rock stability. In addition to quantifying the affect of tectonic stresses on reservoir stability, our models indicate that the process of rejuvenation and mechanical failure is likely to take place over short time periods of hundreds to thousands of years. These findings support the short preeruption melt accumulation timescales indicated by U series disequilibrium studies.

Insights into the emplacement of upper-crustal plutons and their relationship to large silicic calderas, from field relationships, geochronology, and zircon trace element geochemistry in the Stillwater – Clan Alpine caldera complex, western Nevada, USA

Geologic mapping, new U-Pb zircon ages, and new and published 40Ar/39Ar sanidine ages document the timing and extent of Oligocene magmatism in the southern Stillwater Range and Clan Alpine Mountains of western Nevada, where Miocene extension has exposed at least six nested silicic calderas and underlying granitic plutons to crustal depths locally ≥9km. Both caldera-forming rhyolitic tuffs and underlying plutons were emplaced in two episodes, one from about 30.4–28.2Ma that included the Deep Canyon, Job Canyon, and Campbell Creek calderas and underlying plutons, and one from about 25.3–24.8Ma that included the Louderback Mountains, Poco Canyon, and Elevenmile Canyon calderas and underlying plutons. In these two 1–2m.y. periods, almost the entire Mesozoic upper crust was replaced by Oligocene intrusive and extrusive rocks to depths ≥9km over an estimated total area of ~1500km2 (pre-extension). Zircon trace element geochemistry indicates that some plutonic rock can be solidified residual magma from the tuff eruptions. Most plutons are not solidified residual magma, although they directly underlie calderas and were emplaced along the same structures shortly after to as much as one million years after caldera formation. Magma chambers and plutons grew by floor subsidence accommodated by downward transfer of country rocks. If other Great Basin calderas are similar, the dense concentration of shallowly exposed calderas in central Nevada is underlain by a complexly zoned mid-Cenozoic batholith assembled in discrete pulses that coincided with formation of large silicic calderas up to 2500–5000km3.

Locating magma reservoirs using InSAR and petrology before and during the 2011–2012 Cordón Caulle silicic eruption

We combine petrological and InSAR observations to probe the origin of deformation of Cordón Caulle volcano in Southern Chile before and during the 2011–2012 eruption. The erupted rhyodacite is crystal-poor and was stored at about 900 °C with ca. 4 wt% H2O and 220 ppm CO2 in the melt, corresponding to a storage depth of about 6–7 km. In addition to a previously documented 2007–2008 deformation episode, we find three pre-eruptive deformation episodes: the first between 2008 and 2009 that covers ∼400 km2 with a maximum uplift of 18.5 cm, the second between February and March 2010 that covers ∼20 km2 with a maximum uplift of 10 cm, and the third between March and May 2011 that covers ∼40 km2 with a maximum uplift of 6 cm. Modeling of the 2008–2009 deformation episode suggests source depths of about 5–9 km, consistent with the petrologically determined magma storage depth. The sum of modeled source volume changes from 2007 to 2009 is about 12 times smaller than the observed erupted volume. Our calculation of magma compressibility shows that there must have been a significant volume of pre-existing magma in the reservoir to account for this difference. Co-eruptive subsidence shows evidence for three distinct magma sources, and pre-eruptive inflation is seen at two of the three sources. Modeled deformation sources are located 2–10 km from the 2011–2012 eruptive vent and coincide with the vents from previous historical eruptions (1921–22, 1960) and with the neighboring Puyehue volcano. This implies the existence of an interconnected magma plumbing system that allows for pressure transfer and which covers an area of at least 20 km2. Thus, the deformation and geochemical characteristics of Cordón Caulle resemble those of a large silicic caldera system. The presence of a mafic crystal clot demonstrates the existence of basaltic melts in the silicic reservoir, but we find that the mafic magmas reached the reservoir at least several hundred years before eruption. Therefore, if the pre-eruptive deformation was caused by replenishment to a shallow reservoir, the new magma was silicic and similar to the pre-existing magma, and/or there was no interaction between the two. We find that pre-eruptive deformation can occur in pulses and reach its peak years before eruption, and thus lack of deformation does not necessarily imply that eruption is not imminent.

The magmatic evolution of three Venusian coronae

Abstract The volcanic and tectonic histories of Venusian coronae appear to be intricately linked. We explore that link through the construction of geological maps of three coronae using Magellan synthetic aperture radar (SAR) imagery and altimetry radar data. Each examined corona – Aramaiti, Bhumidevi and Zemire Coronae – is characterized by an annulus of concentric fractures, lava flows, tholi and clusters of small shields. Radial fractures occur at one corona (Bhumidevi Corona), whereas the other two coronae (Aramaiti and Zemire Coronae) occur within linear fracture belts. Based on observed timing relationships, we propose that the evolution of these three coronae is similar to large silicic caldera formation on Earth in that the formation of concentric fractures facilitated a mass evacuation of a magma reservoir residing underneath each corona resulting in subsequent corona collapse. Our model emphasizes the late-stage evolution of these coronae and predicts that all the erupted products could be basaltic. Such predictions are testable through continued geological mapping of coronae, as well as through in situ spectral analyses by Venusian landers.

Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico

Los Humeros is the largest caldera volcano in the Mexican volcanic belt. Its second largest caldera-forming eruption, the ca. 0.1 Ma Zaragoza eruption, is recorded by two Plinian pumice-fall layers and a zoned intra-Plinian ignimbrite. Diverse pumice types within the ignimbrite provide insights about the way that different magmas within a single magmatic system interact, and the way in which this can give rise to a major explosive ignimbrite-forming eruption. Normal-and-reverse compositional zoning in the ignimbrite is defined by vertical variations in the relative abundance of rhyodacitic (69–71 wt%% SiO 2 ) and andesitic (54–63 wt% SiO 2 ) pumice lapilli: Lower parts are dominated by rhyodacite and pass gradationally up into a central part with andesitic and rhyodacite pumice, and this passes up into a rhyodacitic uppermost part, with no andesite. Petrographic and microprobe analyses of coexisting glass and phenocrysts provide mixed evidence of equilibrium and disequilibrium conditions in the magmas at the time of eruption. The Fe-Ti oxides record magma temperatures of ∼850 °C (andesite) and 780 °C (rhyodacite). The andesitic pumice contains euhedral labradorite (∼An 60 ), and orthopyroxene and clinopyroxene, in a dacitic glass groundmass, which yield equilibrium Na-Ca K d pl/liq and Fe-Mg K d pl/liq ratios. It also contains highly calcic plagioclase (to An 82 ) that in some cases is highly resorbed and mantled by the more sodic plagioclase, which may record early mixing between andesitic and plagioclase-bearing basaltic magmas, followed by equilibrium crystallization within the hybrid magma. The rhyodacite contains euhedral crystals of more-evolved plagioclase (∼An 30-40 ) and euhedral pyroxenes in a rhyolitic glass groundmass (74–75 wt% SiO 2 ). The pyroxenes yield disequilibrium Fe-Mg K d pl/liq ratios and indicate formation from a liquid that was more mafic than the liquid that formed the glass groundmass of the dacitic pumice. Subordinate pumices with interbanded rhyodacite and a scarcity of intermediate-composition pumices indicate that the magmas remained separate for most of the time, and mingled only immediately prior to, and during eruptive quenching. Rather than a simple density-stratified magma chamber, the Zaragoza eruption may have occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma reservoir, possibly arranged as semiconnected high-melt lenses or zones within a partially consolidated crystal mush. However, contrary to assumptions of simple replenishment, tapping, and fractionation-type systems, the Zaragoza magmas contain no record of previously erupted highly evolved rhyolites that developed when zircon joined the fractionating assemblage. This absence indicates that the highly evolved rhyolites had either been completely tapped or solidified prior to the Zaragoza eruption eruption, or that interaction was prevented by contrasting magma densities and viscosities.

Geochemical and isotopic development of the Coromandel Volcanic Zone, northern New Zealand, since 18 Ma

Cenozoic subduction-related volcanism in northern New Zealand developed from mainly basaltic and andesitic volcanisms in the Early Miocene to mainly silicic volcanism in the present Taupo Volcanic Zone. The first explosive silicic eruptions occurred at 12Ma and large silicic calderas formed from 8Ma. This transition is recorded by the 18–1.9Ma Coromandel Volcanic Zone (CVZ) succession, which provides an outstanding opportunity to investigate the origins of the modern North Island volcanism. New major and trace element data on a rock suite representing CVZ andesites, rhyolites and basalts indicate a consistent subduction-related origin for the entire succession. The CVZ andesites generally define a consistent medium-potassic differentiation trend, however, distinct andesites with relatively sodic compositions occur at the bases of two consecutive eruptive series at ~18 and ~10Ma. The distinct andesites are geochemically similar to basalts that erupted sporadically near the oceanic margin of the CVZ. CVZ rhyolites are medium- to high-potassic, peraluminous rocks with initial 87Sr/86Sr values comparable to or somewhat more radiogenic than coeval andesites (≥0.7049). Their bulk rock composition suggests derivation from sources comparable to least-differentiated andesites or basalts. We interpret the consecutive andesitic successions to represent stages in CVZ development during which distinct arc segments developed consecutively. The early stage in each segment yielded mainly intermediate volcanism whereas in the later stage volcanism locally proceeded rapidly (within ~1Myr of initial andesitic activity) to silicic activity. Taken together, the succession records the rapid development and migration of arc system in a dynamic plate boundary setting.

Tectonic controls on the nature of large silicic calderas in volcanic arcs

Research Article| August 01, 2008 Tectonic controls on the nature of large silicic calderas in volcanic arcs Gwyneth R. Hughes; Gwyneth R. Hughes * 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA *E-mail: gwyneth@stanford.edu. Search for other works by this author on: GSW Google Scholar Gail A. Mahood Gail A. Mahood 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Gwyneth R. Hughes * 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA Gail A. Mahood 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA *E-mail: gwyneth@stanford.edu. Publisher: Geological Society of America Received: 20 Jan 2008 Revision Received: 23 Apr 2008 Accepted: 25 Apr 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (8): 627–630. https://doi.org/10.1130/G24796A.1 Article history Received: 20 Jan 2008 Revision Received: 23 Apr 2008 Accepted: 25 Apr 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gwyneth R. Hughes, Gail A. Mahood; Tectonic controls on the nature of large silicic calderas in volcanic arcs. Geology 2008;; 36 (8): 627–630. doi: https://doi.org/10.1130/G24796A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Silicic magma bodies stabilize the continental margin by redistributing low-density material into the upper crust. We examined the tectonic traits of 91 young, arc-related, silicic calderas to test previous assertions that the nature of voluminous silicic arc volcanism depends on specific tectonic characteristics. We find that caldera occurrence positively correlates with convergence rate except in arcs with backarc spreading, which have few or no calderas. Calderas located on oceanic and young continental crust are predominantly dacitic, whereas rhyolitic calderas are mainly located on Mesozoic or older continental crust under extension. Occurrence does not increase with greater local subduction obliquity or duration of present arc activity. Our determination of controls on the formation and composition of silicic calderas in volcanic arcs contributes to the study of continental evolution. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Control of the aspect ratio of the chamber roof on caldera formation during silicic eruptions

Several factors may control the development of a silicic caldera during an eruption: here we match previous studies of caldera forming eruptions with analogue experiments to consider the role of the aspect ratio (thickness/width) of the magma chamber roof. These data suggest that large silicic caldera formation is controlled by: (1) the availability of magma; (2) initial explosive eruption through a conduit connecting the reservoir and the surface; (3) fast depressurization of the reservoir; and (4) chamber roof collapse, with newly‐formed fractures connecting the reservoir with the surface and creation of a continuous ring conduit feeding annular vents, capable of sustaining the eruption. The occurrence of the latter condition may depend upon the aspect ratio of the chamber roof: lower ratios (<1.6) form coherent collapse, ensuring connection from the chamber to the surface; higher ratios (>1.6) form incoherent collapse, which may hinder the continuation of the eruption after an initial partial emptying of the chamber. The aspect ratio of the chamber roof should therefore be considered as a further factor, capable of controlling the formation of large silicic calderas.

The role of magma buoyancy in Caldera‐forming eruptions

Large calderas are formed by collapse of the crust into magma chambers during the eruption of vast quantities of silicic magma. Here I show that magmatic buoyancy can play a key role in attaining the conditions necessary for these eruptions to occur. During an eruption, magma buoyancy allows the simultaneous conditions of the chamber becoming underpressurized, whilst the conduit remains open, consequently the eruption continues and reduces chamber pressures further. The maximum chamber underpressures attained, and the associated stresses imposed on the surrounding crust, are dependent on the magma‐crust density contrast and chamber size. Sufficiently large imposed stresses cause the crust surrounding the chamber to collapse, stimulating a voluminous eruption and caldera formation. The increase in magma buoyancy associated with the compositional evolution of a magma body accounts for the common transition from initial small or moderate sized intermediate composition eruptions, to ultimately large silicic caldera collapse events.

Redefining active volcanoes: a discussion

The analysis of the current definitions of active volcanoes indicates that they are empirical, conventional, inaccurate, nongeological, and arbitrarily constraining. Redefinition is therefore needed. One possible approach is to refine the current empirical definitions. A statistically reasonable and practical redefinition using a geologically based time convention-Holocene or 10000 years-is suggested. A set of time conditions according to volcano typology-i.e. 1000; 10000 and 100000 years for high-frequency basaltic shields, andesitic-dacitic composite volcanoes and low-frequency large silicic calderas, respectively-as further refinement of the empirical definition is also envisaged. Devising a phenomenological definition as a theoretical approach is another possibility, but in practice extant “diagnostic” means are still unsatisfactory to discriminate accurately between dormant and extinct volcanoes. As a consequence of the redefinition, a classification of volcanoes according to their eruptive status is proposed. Redefinition of active volcanoes might increase accuracy in the usage of basic terms in volcanology and influence volcanic hazard assessment and risk mitigation projects.

Mechanisms of crustal uplift and subsidence at the Yellowstone caldera, Wyoming

Leveling surveys in 1923, 1976, and each year from 1983 to 1993 have shown that the east-central part of the Yellowstone caldera, near the base of the Sour Creek resurgent dome, rose at an average rate of 14±1 mm/year from 1923 to 1976 and 22±1 mm/year from 1976 to 1984. In contrast, no detectable movement occurred in the same area from 1984 to 1985 (-2±5 mm/year), and from 1985 to 1993 the area subsided at an average rate of 19±1 mm/year. We conclude that uplift from 1923 to 1984 was caused by: (1) pressurization of the deep hydrothermal system by fluids released from a crystallizing body of rhyolite magma beneath the caldera, then trapped beneath a self-sealed zone near the base of the hydrothermal system; and (2) aseismic intrusions of magma into the lower part of the sub-caldera magma body. Subsidence since 1985 is attributed to: (1) depressurization and fluid loss from the deep hydrothermal system, and (2) sagging of the caldera floor in response to regional crustal extension. Future intrusions might trigger renewed eruptive activity at Yellowstone, but most intrusions at large silicic calderas seem to be accommodated without eruptions. Overpressurization of the deep hydrothermal system could conceivably result in a phreatic or phreatomagmatic eruption, but this hazard is mitigated by episodic rupturing of the self-sealed zone during shallow earthquake swarms. Historical ground movements, although rapid by most geologic standards, seem to be typical of inter-eruption periods at large, mature, silicic magma systems like Yellowstone. The greatest short-term hazards posed by continuing unrest in the Yellowstone region are: (1) moderate to large earthquakes (magnitude 5.5–7.5), with a recurrence interval of a few decdes; and (2) small hydrothermal explosions, most of which affect only a small area (<0.01 km2), with a recurrence interval of a few years.

Gravity-height correlations for unrest at calderas

Calderas represent the sites of the world's most serious volcanic hazards. Although eruptions are not frequent at such structures on the scale of human lifetimes, there are nevertheless often physical changes at calderas that are measurable over periods of years or decades. Such calderas are said to be in a state of unrest, and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors. Here we review combined gravity and elevation data from several restless calderas, and present new data on their characteristic signatures during periods of inflation and deflation. We find that unless the Bouguer gravity anomaly at a caldera is extremely small, the free-air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient, and not the theoretical gradient, use of which may introduce significant errors. In general, there are two models that fit most of the available data. The first involves a Mogi-type point source, and the second is a Bouguer-type infinite horizontal plane source. The density of the deforming material (usually a magma chamber) is calculated from the gravity and ground deformation data, and the best fitting model is, to a first approximation, the one producing the most realistic density. No realistic density is obtained where there are real density changes, or where the data do not fit the point source or slab model. We find that a point source model fits most of the available data, and that most data are for periods of caldera inflation. The limited examples of deflation from large silicic calderas indicate that the amount of mass loss, or magma drainage, is usually much less than the mass gain during the preceding magma intrusion. In contrast, deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms.

Potentially active volcanoes of Peru-Observations using Landsat Thematic Mapper and Space Shuttle imagery

Landsat Thematic Mapper (TM) images and Space Shuttle color photograph have been used to make a synoptic study of the volcanoes of southern Peru (∼14°–17° S), the northernmost portin of the Central Volcanic Zone (CVZ ∼14°–28° S) of the Andes. Apart from providing consistent coverage, the chief merit of the TM for this study has been the spatial resolution provided by the ∼30-m pixel size. The optimal ∼20-m resolution, variable lighting and viewing geometry, and stereo capability of the Shuttle photography provided an invaluable ancilliary data set. At the resolution available, subtle glacial-morphological features such as valley and terminal moraines can be confidently identified, and these features have been used to determine the relative ages of volcanoes. Volcanoes have been classified as potentially active if they have; (i) a well-preserved summit crater, (ii) pristine lava flow texture and morphology, (iii) flank lava flows with low albedo, and (iv) evidence of postglacial (<10 000 years) activity. Eight major volcanoes are postulated to be potentially active. Most are large, dominantly andesitic, composite cones with edifice heights of up to 2500 m; some of which threaten nearby settlements. One of them, Sabancaya, was active as recently as July 1988. Other, little-known, postglacial volcanic features include Huaynaputina, site of a major explosive rhyolitic eruption in 1600 a.d., and several fields of monogenetic scoria cones and lava flows. The active volcanic front is some 200 km east of the Peru-Chile trench, and the volcanoes lie on a trenchparallel trend oblique to the EW subduction. This narrow volcanic zone is thought to reflect the steep dip of the Nazca plate through the zone of magma generation. The break in the trend of the volcanic front in the northern extremity of the volcanic zone is thought to reflect the complexity of the crustal stress field above a major segment boundary in the subducting plate. The fields of mafic monogenetic centers also occur in this region. In comparison with the southern part of the CVZ, the general paucity of older volcanic edifices north of ∼17° S suggests a more recent onset of volcanism north of this latitude probably resulting from the oblique subduction of the Nazca ridge and the consequent northward migration of its intersection with the Pere-Chile trench. This, coupled with the lack of any large silicic caldera systems and youthful dacite domes, like those found further south, suggest that there are real differences between the volcanic evolution of different parts of the CVZ.

Monitoring unrest in a large silicic caldera, the long Valley-inyo craters volcanic complex in east-central California

Recent patterns of geologic unrest in long Valley caldera in east-central California emphasize that this large, silicic volcanic system and the adjacent, geologically youthful Inyo-Mono Craters volcanic chain are still active and capable of producing locally hazardous volcanic eruptions. A series of four magnitude -6 earthquakes in May 1980 called attention to this current episode of unrest, and subsequent activity has included numerous earthquake swarms in the south moat of the caldera accompanied by inflation of the resurgent dome by more than 50 cm over the last five years. The seismicity associated with this unrest is currently monitored by a network of 31 telemetered seismic stations with an automatic processing system that yelds hypocentral locations and earthquake magnitudes in near-real time. Deformation of the ground is monitored by a) a series of overlapping trilateration networks that provide coverage ranging from annual measurements of regional deformation to daily measurements of deformation local to the active, southern section of the caldera, b) a regional network of level lines surveyed annually, c) a regional network of precise gravity stations occupied annually, d) local, L-shaped level figures surveyed every few months, and e) a network of fourteen borehole tiltmeter clusters (two instruments in each cluster) and a borehole dilatometer, the telemetered signals from which provide continuous data on deformation rates. Additional telemetered data provide continuous information on fluctuations in the local magnetic field, hydrogen gas emission rates at three sites, and water level and temperatures in three wells. Continuous data on disharge rates and temperatures from hot springs and fumaroles are collected by several on-site recorders within the caldera, and samples for liquid and gas chemistry are collected several times per year from selected hot springs and fumaroles.