Caldera Formation Research Articles

Very long period seismic signals observed before the caldera formation with the 2000 Miyake‐jima volcanic activity, Japan

Very long period seismic signals, whose waveform consists of an initial impulsive signal and a later oscillatory wave with 0.2 or 0.4 Hz in dominant frequency, were observed before the caldera formation in the 2000 activity of Miyake‐jima volcano, Japan. The results of waveform inversion show that the initial and later parts can be explained by a northward single force of 1.5 × 108 N working at a depth of 2 km beneath the summit and a moment tensor solution at a depth of 5 km below and 2 km southwest of the summit with ∼1012 Nm, respectively. A clear positive correlation of the amplitudes between the two sources strongly suggests that the shallow single force triggers the deeper moment source in spite of the several km distance between the two sources. To analyze the source time functions of the moment tensor that do not always oscillate in phase, we introduce a new method of moment tensor diagonalization which is performed in the frequency domain. According to the analysis, the two principal components have similar amplitudes and are greater than the third principal component, suggesting an axially symmetric oscillation. One of the possible systems is a combination of two cracks intersecting perpendicularly. Our interpretation is that the single force was generated when magma containing rock blocks suddenly began to move in a choked subsurface magma path, and the resultant pressure waves propagated and excited a resonance oscillation of the two cracks.

Stress fields controlling the formation of nested and overlapping calderas: Implications for the understanding of caldera unrest

Commonly, the formation of a collapse caldera does not necessarily imply the end of the volcanic activity in the area. In many cases, successive calderas may form close to the previous collapse depression or intersecting it leading to overlapping collapse structures. Occasionally, subsequent caldera collapses may take place at the interior of the first caldera creating nested collapse structures. During the last years several authors have investigated numerically how the stress field around magma chambers may favour the formation of collapse calderas assuming that the host rock surrounding the magmatic reservoir behaves as a linear elastic homogeneous medium. The numerical models presented in this work study how a caldera collapse may modify the stress field of a volcanic area and hence the conditions for the formation of future collapse calderas. Our models take into account the effect of the collapse structure considering it as a mechanical discontinuity. We also investigate the mechanical influence of the intra- and extra-caldera deposits on the formation of new calderas. All the numerical models are two-dimensional assuming plane strain and considering that the surrounding crust behaves as a linear homogeneous elastic material. The computational domain corresponds to a cross-section of the upper crust (50 × 25 km) and magma chambers are modelled as sill-like cavities located at a certain depth below the Earth's surface. The existing collapse caldera depression is 8 km wide and 2.75 km deep, however we consider the caldera infill (i.e. intra-caldera material) to be 1.25 or 1.75 km thick. We assume as loading conditions an underpressure of 10 MPa imposed at the chamber walls, that is, negative excess pressure in the chamber. In some numerical runs we have considered the existence of a previous ring fault by introducing a thin and elongate vertical weak zone at the caldera margins. We find that the stress field around shallow-level magma chambers favouring the formation of either nested or overlapping caldera structures depend on: 1) the dimensions of the previous caldera collapse structure, 2) the size of the new magma chamber susceptible to generate the subsequent caldera-forming event, and 3) the mechanical properties of the surrounding host rock and the syn- and post-collapse materials. Moreover, results obtained in this paper may be relevant to the interpretation of unrest episodes in active caldera systems. Our numerical results confirm that the formation of multiple calderas (nested, overlapping, or separate) imply the development of new shallow magma chambers different from the one that lead to the first collapse caldera. In consequence, interpretation of magmatic unrest episodes in active collapse calderas should not be based on the assumption that the responsible for the current unrest is the same system that originated the caldera.

Andean flat-slab subduction through time

Abstract The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), three incipient flat-slab segments (‘Carnegie’, Guañacos, ‘Tehuantepec’), three older and no longer active Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Palaeozoic flat-slab segment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampean flat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processes can be attributed to slab shallowing and steepening. This pattern permits recognition of other older Cenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, two different settings of slab steepening are proposed. Slab steepening under thick crust leads to delamination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcanism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Puna segments. On the other hand, when steepening affects thin crust, extension and extensive within-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payenia in the southern Andes. This last case has very limited crustal melt along the axial part of the Andean roots, which shows incipient delamination. Based on these cases, a Palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism. The geological evolution of the Andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system.

Dynamics of Caldera Formation During a Large-scale Silicic Eruption

Dynamics of Caldera Formation During a Large-scale Silicic Eruption

Inflated flows on Daedalia Planum (Mars)? Clues from a comparative analysis with the Payen volcanic complex (Argentina)

Inflation is an emplacement process of lava flows, where a thin visco-elastic layer, produced at an early stage, is later inflated by an underlying fluid core. The core remains hot and fluid for extended period of time due to the thermal-shield effect of the surface visco-elastic crust. Plentiful and widespread morphological fingerprints of inflation like tumuli and lava rises are found on the Payen volcanic complex (Argentina), where pahoehoe lava flows extend over the relatively flat surface of the Pampean foreland and reach at least 180 km in length. The morphology of the Argentinean Payen flows were compared with lava flows on Daedalia Planum (Mars), using Thermal Emission Imaging System (THEMIS), Mars Orbiter Laser Altimeter (MOLA), Mars Orbiter Camera (MOC), Mars Reconnaissance Orbiter (MRO)/High-Resolution Imaging Science Experiment (HiRISE). THEMIS images were used to map the main geological units of Daedalia Planum and determine their stratigraphic relationships. MOLA data were used to investigate the topographic surface over which the flows propagated and assess the thickness of lava flows. Finally, MOC and MRO/HIRISE images were used to identify inflations fingerprints and assess the cratering age of the Daedalia Planum’ s youngest flow unit which were found to predate the caldera formation on top of the Arsia Mons. The identification of similar inflation features between the Daedalia Planum and the Payen lava fields suggests that moderate and long lasting effusion rates coupled with very efficient spreading processes could have cyclically occurred in the Arsia Mons volcano during its eruptive history. Consequently the effusion rates and rheological proprieties of Daedalia lava flows, which do not take into account the inflation process, can be overestimated. These findings raise some doubts about the effusion rates and lava rheological properties calculated on Martian flows and recommends that these should be used with caution if applied on flows not checked with high-resolution images and potentially affected by inflation. Further HiRISE data acquisition will permit additional analysis of the flow surfaces and will allow more accurate estimates of effusion rates and rheological properties of the lava flows on Mars particularly if this data is acquired under a favourable illumination.

The April 2007 eruption and the Dolomieu crater collapse, two major events at Piton de la Fournaise (La Réunion Island, Indian Ocean)

After 10 years of high activity and 24 eruptions, Piton de la Fournaise (PdF) produced on April 2 to May 1, 2007 one of its most voluminous and intense eruption since at least one century. The eruption focused at 590 m elevation in the Grand Brûlé on the south east flank of Piton de la Fournaise, 7 km away from summit. It was located close to the southern cliff of the Grand Brûlé volcano tectonic collapse, the Rempart du Tremblet, and to the nearby village called Le Tremblet, and it highly rattled and incommoded the inhabitants there. Eleven hours after the beginning of the eruption the lava flows reached the sea and created 0.45 km 2 of new land until the end of eruption. In the night of April 5 the rock column beneath Dolomieu crater started to collapse with a piston like mechanism into the magma chamber, forming within less than 24 h the most recent and well documented caldera. We report in this paper the chronology of these two related major events in the recent activity of PdF seen by the scientific networks of the volcanological observatory (OVPF/IPGP) and field observations, which allowed to back up the scenario of the caldera formation and to quantify these two exceptional episodes.

Location of silicic caldera formation in arc settings

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.

Upward migration of Vesuvius magma chamber over the past 20,000 years

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16 km (refs 3-7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8-9 km depth, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7-8 km to 3-4 km depth between the ad 79 (Pompeii) and ad 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago and the 1944 Vesuvius eruption are included, the total upward migration of the reservoir amounts to 9-11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation or volcano spreading. Reservoir migration, and the possible influence on feeding rates, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.

Resistivity structure of western Taupo Volcanic Zone, New Zealand

data from two deep‐penetration electrical resistivity surveys crossing the western margin of the Taupo Volcanic Zone (TVZ) near Tihoi, northwest of lake Taupo, North Island, New Zealand, have been used to identify the elements of the margin. In this vicinity the volcanologically defined boundary of the TVZ includes the andesitic cones of Pureora and Titiraupenga. The faulted margin of the TVZ lies east of these cones and is marked in the resistivity data by a dipping interface (faulted margin) between the resistive greywacke rocks to the west and the relatively conductive volcanic rocks that fill the upper few kilometres of the TVZ. The resistivity and gravity signatures of the margin are coincident in this vicinity. To the immediate east of the margin is a low‐resistivity zone c. 10 km wide and 2.5 km deep. The low‐resistivity material within this region is identified as old (>1 Ma) volcaniclastics that contain clays produced by low‐temperature alteration and devitrification. This material is reversely magnetised. We suggest that this zone was formed following the onset of active extension of the TVZ and has since remained relatively undisturbed while the focus of volcanism and deformation has moved to the east. The eastern boundary of this low‐resistivity zone coincides with the western edge of the whakamaru caldera (the boundary of the “young TVZ"). Although the resistivity data can be matched using an interface with a dip to either the east or west, an eastward dip would be consistent with the boundary having been formed during the collapse associated with the formation of the Whakamaru caldera. The resistivity signature within the Whakamaru caldera is consistent with >2 km thickness of younger, more resistive volcanic sequences, suggesting the low‐resistivity layers found to the west have been disrupted and downfaulted to be replaced with young, resistive caldera infill.

Refinement of the late Quaternary geologic history of Erebus volcano, Antarctica using 40Ar/ 39Ar and 36Cl age determinations

Six new 40Ar/ 39Ar and three cosmogenic 36Cl age determinations provide new insight into the late Quaternary eruptive history of Erebus volcano. Anorthoclase from 3 lava flows on the caldera rim have 40Ar/ 39Ar ages of 23 ± 12, 81 ± 3 and 172 ± 10 ka (all uncertainties 2 σ). The ages confirm the presence of a second, younger, superimposed caldera near the southwestern margin of the summit plateau and show that eruptive activity has occurred in the summit region for 77 ± 13 ka longer than previously thought. Trachyte from “Ice Station” on the eastern flank is 159 ± 2 ka, similar in age to those at Bomb Peak and Aurora Cliffs. The widespread occurrences of trachyte on the eastern flank of Erebus suggest a major previously unrecognized episode of trachytic volcanism. The trachyte lavas are chemically and isotopically distinct from alkaline lavas erupted contemporaneously in the summit region < 5 km away. The three 36Cl ages are the first exposure ages reported for rocks from Erebus volcano. At an assumed erosion rate of 1.5 mm/ka 36Cl cosmogenic ages are 6.0 ± 0.5 ka for the Ice Tower Ridge flow, 4.9 ± 0.4 ka for Lower Hut flow, and 6.5 ± 0.6 ka for the Northeast flow. The 36Cl ages overlap within analytical uncertainty or are slightly younger than 40Ar/ 39Ar ages for these lava flows, and confirm the overall young age (< 25 ka) of the summit lava flows. One interpretation of these uniformly young ages is that older eruptions infilled the topographic depression caused by caldera formation and subsequently was covered by younger lavas.

Structure and petrology of newly discovered volcanic centers in the northern Kermadec–southern Tofua arc, South Pacific Ocean

The NZAPLUME III expedition of September–October 2004 to the northern Kermadec–southern Tofua (NKST) arc, between 28°52′S and 25°07′S, resulted in the discovery of at least seven new submarine volcanic centers and a substantial caldera complex adjacent to the previously known Monowai Seamount. The volcanic centers form a sublinear chain that coincides with the Kermadec Ridge crest in the south (Hinetapeka) and diverges ∼45 km westward of the ridge crest in the north (“V”) just to the south of where the Louisville Ridge intersects with the arc. All of the centers contain calderas or caldera‐like structures, as well as multiple cones, domes, fissure ridges, and vent fields. All show signs of recent eruptive and current hydrothermal activity. There are strong structural controls on edifice location, with cones and fissure ridges typically associated with faulting parallel to the regional ∼12° strike of the arc front. Several of the calderas are ellipsoidal, orientated northwest–southeast in the general direction of least compressive stress. Sampled volcanic rocks, representing the most recently erupted lavas, are all low‐K tholeiites. Two of the centers, Gamble and Rakahore, yielded only high‐silica dacite to rhyolite (69–74 wt% silica), whereas two others, Monowai and “V,” yielded only basalt to andesite (48–63 wt% silica). Mineral assemblages are plagioclase‐pyroxene dominated, with accessory Fe‐Ti oxides, apatite, olivine, and quartz/tridymite/cristobalite, typical of dry volcanic arc systems. Hornblende occurs only in a felsitic rhyolite from Hinepuia volcanic center, and zircon is absent. Glass contents range to 57% in basalts–andesites (mean 20%), and 97% in andesites–rhyolites (mean 59%) and other quench textures, including swallow‐tailed, plumose, or dendritic crystal forms and crystallites, are common. Most lavas are highly vesicular (≤63%; mean 28%) and have low volatile contents (mostly &lt;2 wt%) which, together with the occurrence of tridymite or cristobalite, indicates explosive eruption and rapid cooling. Exceptions are rocks from “U” volcanic center, which have low vesicularity and low glass contents across a wide compositional range, indicating effusive eruption. Disequilibrium mineral textures, the frequent occurrence of xenoliths and xenocrysts, and macroscopic evidence for magma mingling indicate that many of the lavas are hybrids, having resided only a short time in upper crustal reservoirs prior to eruption. Silicic magmas are major components of NKST arc volcanism and caldera formation is the dominant eruptive style. The scale of silicic magmatism is in marked contrast to the dominant basaltic–andesitic magmatism in the southern Kermadec arc. With evidence from other arcs, silicic magmatism is now recognized as a major feature of intraoceanic arcs globally.

The structure and formation of mud volcano summit calderas

Circular depressions bounded by inward-dipping faults found at the upper terminations of large mud volcano systems (&gt;500 m diameter) are termed ‘mud volcano summit calderas’. From new mapping and comparison with previously identified examples we describe a series of common structural and morphological features found at a number of calderas and propose a mechanism for caldera formation. A typical example consists of concentric deformational and volcanic zones including an outermost topographic rim, inward-dipping circular fault system, ‘moat’ and raised central ‘pedestal’ of freshly extruded mud volcanic sediment. This distinctive ‘moat and pedestal’ morphology can be explained in terms of the quantity and rheology of extruded mud, and appears characteristic of calderas from the South Caspian Basin and elsewhere. The association of fresh mud volcanic deposits with the calderas in combination with extensive literature on caldera modelling leads us to conclude that the most likely causal mechanism is subsidence as a result of evacuation of fluids and sediment from shallow structural levels during eruptions. Summit calderas mapped in this study characterize dormant periods of mud volcanism in subaerial and submarine settings, and appear to be a common structural element in the extrusive domain of mud volcano systems.

Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion

Uturuncu volcano, SW Bolivia, is a dormant stratovolcano (∼85 km³) dominated by dacitic lava domes and flows. ³⁹Ar/⁴⁰Ar ages show that the volcano was active between 890 ka and 271 ka, with the lavas becoming younger and less extensive at higher elevations. There are current signs of unrest. Between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm/yr. Deformation indicates volume changes of 400 × 10⁸ m³ over 14 years, an average of ∼1 m³/s (10⁻² km³/yr). The deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body. Deformation models indicate a source at depths of 17 to 30 km beneath current local relief. In a reconnaissance survey, persistent seismic activity (mean of 2.6 earthquakes per hour with a maximum of 14 per hour) was recorded at about 4 km depth below the center of the uplift, 4 km SW of the volcano9s summit. The seismic events have a normal b value (∼1.04) and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion. The porphyritic dacite lavas (64−68% SiO₂) have a plagioclase-orthopyroxene-biotite-magnetite-ilmenite assemblage and commonly contain juvenile silicic andesite inclusions, cognate norite nodules and crustal xenoliths. Temperature estimates are in the range 805 to 872°C for the dacites and about 980°C for the silicic andesites. The dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust. Compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite. Reversely zoned orthopyroxene phenocrysts in the andesitic end-member are explained by changing oxidation states during crystallization. Fe³⁺/Fe²⁺ ratios from orthopyroxene crystals and Fe³⁺ in plagioclase provide evidence for a relatively reduced melt that subsequently ascended, degassed and became more oxidized as a consequence of degassing. The geophysical and petrological observations suggest that dacite magma is being intruded into the Altiplano-Puna regional crustal magma body at 17 km or more depth, consistent with deformation models. In the Late Pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together. The current unrest, together with geophysical anomalies and 270 ka of dormancy, indicate that the magmatic system is in a prolonged period of intrusion. Such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation.

Kaguyak dome field and its Holocene caldera, Alaska Peninsula

Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka ( 14C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO 2). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO 2), and phreatic ejection of lakefloor sediments onto the caldera rim. CO 2 and H 2S bubble up through the lake, weakly but widely. Geochemical analyses ( n = 148), including pre-and post-caldera lavas (53–74% SiO 2), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extracaldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO 2). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.

Phreatomagmatic eruptions associated with the caldera collapse during the Miyakejima 2000 eruption, Japan

The 2000 AD eruption of Miyakejima was characterized by a series of phreatomagmatic eruptions from the subsiding caldera. Six major eruptive events occurred, and they can be divided into the first and second periods separated by a 25-day hiatus. The phreatomagmatic eruptions produced a total of ~ 2 × 10 10 kg of tephra, which mainly comprised fine-grained volcanic ash. The tephra layers could be divided into six fall units corresponding to the six major eruptive events. The grains of the 2000 tephra consisted mainly of accessory fragments derived from the previous edifice. Juvenile fragments were identified in four major units. The angular and blocky shape of the juvenile fragments showed rapid cooling of magma by external water, indicating the mechanical mixing of ground water and magma. The removal of the load on the aquifer by caldera subsidence and the intrusion of magma into the decompressed aquifer caused a series of phreatomagmatic eruptions from the subsiding caldera. The absence of surface water in the subsiding caldera during the eruption showed that groundwater participated in the phreatomagmatic eruptions. Rising magma along the ring faults intruded into the decompressed aquifer surrounding the caldera and caused phreatomagmatic explosions. The 2000 eruption of Miyakejima indicated that the fine-grained tephra abundant in hydrothermally altered rock fragments is a possible indicator of the phreatomagmatic eruptions in the hydrothermal system during the caldera formation in basaltic volcanoes.

Consequences of Explosive Supereruptions

Rare but extremely large explosive supereruptions lead to the catastrophic formation of huge calderas, devastation of substantial regions by pyroclastic flow deposits, and ash falls that cover continent-sized areas. The effects of future supereruptions will be felt globally or at least by a whole hemisphere. The most widespread effects are likely to derive from the volcanic gases released, particularly sulfur gases that are converted into sulfuric acid aerosols in the stratosphere. These will remain for several years, promoting changes in atmospheric circulation and causing surface temperatures to fall dramatically in many regions, bringing about temporary reductions in light levels and producing severe and unseasonable weather ('volcanic winter'). Major disruptions to global societal infrastructure can be expected for periods of months to years, and the cost to global financial markets will be high and sustained.

Magmatic and tectonic evolution of the Caetano caldera, north-central Nevada: A tilted, mid-Tertiary eruptive center and source of the Caetano Tuff

The Caetano Tuff is a late Eocene, rhyolite ash-flow tuff that crops out within an ∼90-km-long, east-west–trending belt in north-central Nevada, previously interpreted as an elongate graben or “volcano-tectonic trough.” New field, petrographic, geochemical, and geochronologic data show that: (1) the east half of the “trough” is actually the Caetano caldera, formed by eruption of the Caetano Tuff at 33.8 Ma and later structurally dismembered during Miocene extension; (2) the west half of the trough includes both the distinctly younger and unrelated Fish Creek Mountains caldera (ca. 24.7 Ma) and a west-trending paleovalley partly filled with outflow Caetano Tuff; and (3) the Caetano Tuff as previously defined actually consists of three distinct units, two units of the 33.8 Ma Caetano Tuff and an older (34.2 Ma) tuff, exposed north of the Caetano caldera, herein named the tuff of Cove Mine. Miocene extensional faulting and tilting has exposed the Caetano caldera over a paleodepth range of >5 km, from the caldera floor through post-caldera sedimentary rocks, providing exceptional constraints on an evolutionary model of the caldera that are rarely available for other calderas. The Caetano caldera filled with more than 4 km of intracaldera Caetano Tuff, while outflow tuff flowed west and south of the caldera, primarily down Eocene paleovalleys. Caldera fill consists of two units of Caetano Tuff. The lower compound cooling unit is as much as 3600 m thick and is separated by a complete cooling break from a 500–1000-m-thick upper unit that consists of multiple, thin, ash flows interbedded with sedimentary deposits. Multiple granite porphyries, including the 25-km 2 Carico Lake pluton, intruded and domed the center of the caldera within 0.1 Ma of caldera formation; one of these porphyries is associated with pervasive argillic and advanced argillic alteration of the western half of the caldera. All exposed caldera-related rocks are rhyolites or granites (71–77.5 wt% SiO 2 ). Caldera collapse was significantly greater than the thickness of caldera fill and created a topographic depression that served as a depocenter until at least 25 Ma, filling with nearly 1 km of sediments and distally derived, ash-flow tuffs. The caldera is presently exposed in a series of 40–50°, east-tilted blocks bounded by north-striking, west-dipping normal faults that formed after 16 Ma. Slip on these faults accommodated ∼100% E-W extension, making the restored Caetano caldera ∼20 km east-west by 10–18 km north-south. The estimated volume of intracaldera Caetano Tuff is, therefore, ∼840 km 3 , and the minimum estimated total eruptive volume is ∼1100 km 3 . Although the Caetano magmatic system was probably too young to supply heat for nearby Carlin-type gold deposits in the Cortez district, earlier nearby magmatic activity may have contributed to formation of these deposits. Reconstruction of the late Eocene, pre-Caetano caldera geologic setting, immediately prior to caldera formation, indicates that the Cortez Hills and Horse Canyon Carlin-type deposits formed at ≤1 km depths.

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.

Origin and Evolution of Silicic Magmatism at Yellowstone Based on Ion Microprobe Analysis of Isotopically Zoned Zircons

The origin of large-volume Yellowstone ignimbrites and smallervolume intra-caldera lavas requires shallow remelting of enormous volumes of variably O-depleted volcanic and sub-volcanic rocks altered by hydrothermal activity. Zircons provide probes of these processes as they preserve older ages and inherited dO values.This study presents a high-resolution, oxygen isotope examination of volcanism at Yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size.We report 357 analyses of cores and rims of zircons, and isotope profiles of 142 single zircons in 11 units that represent majorYellowstone ignimbrites, and post-caldera lavas. Many zircons from these samples were previously dated in the same spots by sensitive high-resolution ion microprobe (SHRIMP), and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination. We additionally report oxygen isotope analyses of quartz crystals in three units.The results of this work provide the following new observations. (1) Most zircons from post-caldera low-dO lavas are zoned, with higher dO values and highly variable U^Pb ages in the cores that suggest inheritance from pre-caldera rocks exposed on the surface. (2) Many of the higher-dO zircon cores in these lavas have U^Pb zircon crystallization ages that postdate caldera formation, but pre-date the eruption age by 10^20 kyr, and represent inheritance of unexposed post-caldera sub-volcanic units that have dO similar to the Lava Creek Tuff. (3) Young and voluminous 0 25^0 1Ma intra-caldera lavas, which represent the latest volcanic activity atYellowstone, contain zircons with both high-dO and lowdO cores surrounded by an intermediate-dO rim. This implies inheritance of a variety of rocks from high-dO pre-caldera and low-dO post-caldera units, followed by residence in a common intermediate-dO melt prior to eruption. (4) Major ignimbrites of Huckleberry Ridge, and to a lesser extent the Lava Creek and Mesa Falls Tuffs, contain zoned zircons with lower-dO zircon cores, suggesting that melting and zircon inheritance from the lowdO hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse. (5) The dO zoning in the majority of zircon core^rim interfaces is step-like rather than smoothly inflected, suggesting that processes of solution^reprecipitation were more important than intracrystalline oxygen diffusion. Concave-downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals.This study suggests that silicic magmatism atYellowstone proceeded via rapid, shallow-level remelting of earlier erupted and hydrothermally alteredYellowstone source rocks and that pulses of basaltic magma provided the heat for melting. Each postcaldera Yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and dO values.The commonly held model of a single, large-volume, super-solidus, mushy-state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data. Rather, the source rocks for theYellowstone volcanism were cooled below the solidus, hydrothermally altered by heated meteoric waters that caused low dO values, and then remelted in distinct pockets by intrusion of basic magmas. Each packet of new melt inherited zircons that retained older age and dO values.This interpretation may have significance for interpreting seismic data for crustal low-velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side. New basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas. We also suggest that the Lava Creek Tuff

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 (&lt;1.6) form coherent collapse, ensuring connection from the chamber to the surface; higher ratios (&gt;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.