Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy

Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma ³⁹Ar/⁴⁰Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-³⁹Ar/⁴⁰Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-³⁹Ar/⁴⁰Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-³⁹Ar/⁴⁰Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km³) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km³) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km³) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

The Karthala caldera,Grande Comore

The active Karthala volcano is found on Grande Comore, the most westerly of four volcanic islands comprising the Comores Archipelago, between northern Madagascar and Mozambique. The caldera, roughly elliptical in outline, is 4 km long and 3 km wide, with outer walls around 100 m high. It is dominated by a large central pit crater, Chahale, which is 1300 m long, 800 m wide, and 300 m deep. A smaller cylindrical pit crater 250 m in diameter and 30 m deep, Changomeni, is found one km north of Chahale. The vertical walls of both pit craters show excellent sections of the ponded flows which form the caldera floor, and the minor faults and intrusions which affected these flows. The youngest lava on the island was produced on July 12th, 1965, as single aa basalt flow emitted from a fissure halfway between the two pit craters. Small fumaroles are still active on this flow, as well as in the pit craters and at several small cinder cones in the caldera. Alignment of pyroclastic cones and fissure eruptions forms a radial pattern centering on Chahale pit crater, suggesting that these radial fissures are locally controlled. Location of the caldera at the intersection of two regional fissure systems implies that its location is controlled by regional stresses. The present size and form of the caldera is a result of the coalescence of at least four smaller calderas. Although the visible walls of these smaller calderas do not show any outward dip, the theoretical considerations ofRobson andBarr (1964), if applicable, require that at depth these are outward-dipping ring dyke type of fractures.     

Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province,USA

Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.     

The caldera of Pantelleria

The central part of the Island of Pantelleria is occupied by a caldera depression of ellipsoidal form, the major diameter of which is about 7 kilometers. The collapse causing the caldera to form seems to have taken place after a complex eruption which led to the extrusion of a great endogenous dome, the formation of which was followed by a localized explosive activity. The succession of the acid volcanites of Pantelleria shows periodical evolutions of the magmas, undergoing various cycles of differentiation. After a period of quiescence a new cycle of differentiation started again. This may be explained by admitting that the differentiation was accompanied by an increase in the gas content. The violent degassing re-established conditions analogous to those which had existed at the beginning of the previous cycle.     

An oxygen isotope study of hydrothermal alteration in the Lake City caldera, San Juan Mountains, Colorado

A 23-m.y.-old, fossil meteoric-hydrothermal system in the Lake City caldera (11 × 14 km) has been mapped out by measuring δ ¹⁸O values of 300 rock and mineral samples. δ ¹⁸O varies systematically throughout the caldera, reaching values as low as −2. Great topographic relief, regional tilting, and variable degrees of erosion within the caldera all combine to give us a very complete section through the hydrothermal system, from the surface down to a depth of more than 2000 m. The initial δ ¹⁸O value of the caldera-fill Sunshine Peak Tuff was very uniform (+7.2 ± 0.1), making it easy to determine the exact amount of ¹⁸O depletion experienced by each sample during hydrothermal alteration. Also, we have excellent stratigraphic control on depths beneath the mid-Tertiary surface, quantitative information on mineralogical alteration products, and accurate data on the shape of the central resurgent intrusion, which was the principal ‘heat engine’ that drove the hydrothermal circulation. Major conclusions are: (1) Although pristine mid-Tertiary meteoric waters in this area had δ ¹⁸O −14, these fluids were ¹⁸O-shifted upward to about δ¹⁸O = −8 to −5 prior to entering the shallow convective system associated with the resurgent intrusive rocks. Although there was undoubtedly radial inflow toward the caldera from all directions, the highly fractured Eureka Graben, southwest of the caldera, was probably the principal source of recharge groundwater for the Lake City system. (2) Fluid flow within the caldera was dominated by three major categories of permeable zones: the porous megabreccia units (which dip outward from the resurgent dome), vertical fractures and faults related to resurgence, and the caldera ring fault itself. All of these zones exhibit marked ¹⁸O depletions, and they are also typically intensely mineralogically altered. (3) The resurgent intrusive stock and its contact metamorphic aureole of hornfels both experienced water/rock ratios lower than the permeable zones; however, they have similarly low δ ¹⁸O values because they were altered at higher temperatures. (4) Throughout the caldera, the δ ¹⁸O of Sunshine Peak Tuff decreases with increasing depth (about 6 per mil/km), indicative of a shallow thermal gradient, typical of a convective hydrothermal system. The near-surface portion of this gradient was controlled by the temperature drop associated with boiling in the uprising fluid. (5) Deeply circulating meteoric water rose along permeable ring fractures 3 to 5 km beneath the mid-Tertiary surface. These fluids were drawn into the shallow convective system through the lower, porous, megabreccia units. Near the resurgent intrusions, fluid flow was again directed upward where resurgence-related, near-vertical fractures intersect the megabreccia units.     

Geologic evolution of the Donguinyó-Huichapan caldera complex,central Mexican Volcanic Belt,Mexico

The Donguinyó-Huichapan caldera complex is located 110 km to the NNW of Mexico City, in the central sector of the Mexican Volcanic Belt. It is a 10 km in diameter complex apparently with two overlapping calderas, each one related to an ignimbrite sequence that contrasts in composition, mineralogy, welding, distribution, and physical aspect. The geologic evolution of this complex includes the following phases, 1) A first caldera formed at 5.0 ± 0.3 Ma, with the eruption of several discrete pulses of andesitic to trachydacitic pyroclastic flows that produced a series of densely welded ignimbrites; 2) At 4.6 ± 0.3 Ma, several small shield volcanoes and cinder cones built the rim of this caldera and erupted basaltic-andesite and andesitic lava flows; 3) At 4.2 ± 0.2 Ma, a second caldera was formed associated to the eruption of the Huichapan Tuff, which is a rhyolitic pyroclastic sequence consisting of minor unwelded ignimbrites, pumice fall and surge deposits, and a voluminous welded ignimbrite; 4) Also yielding an age of 4.2 ± 0.2 Ma, several trachydacitic lava domes were extruded along the new ring fracture and formed the rim of the Huichapan caldera, as well as five intra-caldera domes of dacitic and trachydacitic composition. Peripheral volcanism includes a large 2.5 ± 0.1 Ma shield volcano that was emplaced on the Huichapan caldera rim.The two calderas that form the Donguinyó-Huichapan complex have contrasting differences in volcanic styles that were apparently due to their differences in composition. Products erupted by the Donguinyó caldera are basaltic-andesite to trachydacitic in composition, whereas Huichapan caldera products are all high-silica rhyolites.     

La Pacana caldera and the Atana Ignimbrite — a major ash-flow and resurgent caldera complex in the Andes of northern Chile

The recently discovered La Pacana caldera, 60 × 35 km, is the largest caldera yet described in South America. This resurgent caldera of Pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the Central Andes of northern Chile at about 23° S latitude. Collapse of La Pacana caldera was initiated by the eruption of about 900 km³ of the rhyodacitic Atana Ignimbrite. The Atana Ignimbrite was erupted from a composite ring fracture system and formed at least four major ash-flow tuff units that are separated locally by thin air-fall and surge deposits; all four sheets were emplaced in rapid succession about 4.1 ± 0.4 Ma ago. Caldera collapse was followed closely by resurgent doming of the caldera floor, accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures. The resurgent dome is an elongated, asymmetrical uplift, 48.5 × 12 km, which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben. Later, postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3.5 Ma ago and persisting into the Quaternary. Hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome, but no ore deposits are known. A few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system.     

The Kari Kari caldera, Bolivia, and the Cerro Rico stock

The rocks comprising the Kari Kari massif southeast of the city of Potosi, Bolivia, consist entirely of welded ignimbrites. It is argued that the massif constitutes the resurgent centre of a 20-m.y.-old resurgent caldera. Plutonic rocks are exposed in the south, but volcanic rocks of the caldera rim are exposed in the north, and indicate a shallower erosion level there. The volcanic rocks consist of a coarse moat deposit, consisting of angular fragments of basement material and juvenile clasts, overlain by an extensive garnet-bearing ignimbrite. A plant-fossil-bearing lacustrine deposit was laid down in a lake within the caldera. The Cerro Rico stock, noted for its silver-tin mineralisation, may be a late intrusion along the caldera ring fractures.     

The geology and geophysics of the ambrym caldera,New Hebrides

Ambrym Island has an unusually large, well-preserved basaltic caldera 13 km across. The caldera occurs in the central region of an early broad composite cone, which formed a north-south line with three smailer volcanoes. Alter the caldera was formed volcanism occurred within it and along fissure lines running nearly east-west. Two volcanic cones are active almost continuously and historic fissure cruptions have been recorded. The caldera formed by quiet subsidence, or by subsidence accompanied by eruption of scoria lappili similar to that erupted prior and subsequent to caldera formation. The collapse was at least 600 metres and radiocarbon dating suggests it took place less than 2000 years ago. The caldera is detined by gravity anomalies 10 to 14 milligals lower than those at its rim suggesting predominantly ash infilling. Aeromagnetic anomalies show a prominent. nearly east-west lineation, with normally magnetised bipole anomalies over the centre of the caldera and over fissure lines east of it. The source of the present volcanic activity is believed to be located along dyke fissures, with a perched magma chamber beneath the caldera. The geophysical evidence on Ambrym, together with that of regional east trending magnetic anomalies and recent bathymetric results, suggests that the volcanic activity is localised by the intersection of an east-west fracture zone with the axis of the New Hebrides island are.     

The hydrothermal system of the Calabozos caldera,central Chilean Andes

Active thermal springs associated with the late Pleistocene Calabozos caldera complex occur in two groups: the Colorado group which issues along structures related to caldera collapse and resurgence, and the Puesto Calabozos group, a nearby cluster that is chemically distinct and probably unrelated to the Colorado springs. Most of the Colorado group can be related to a hypothetical parent water containing ∼400 ppm Cl at ∼250°C by dilution with ≥50% of cold meteoric water. The thermal springs in the most deeply eroded part of the caldera were derived from the same parent water by boiling.The hydrothermal system has probably been active for at least as long as 300,000 years, based on geologic evidence and calculations of paleo-heat flow. There is no evidence for economic mineralization at shallow depth. The Calabozos hydrothermal system would be an attractive geothermal prospect were its location not so remote.     

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.     

Gravity changes and passive SO2 degassing at the Masaya caldera complex, Nicaragua

An understanding of the mechanisms responsible for persistent volcanism can be acquired through the integration of geophysical and geochemical data sets. By interpreting data on micro-gravity, ground deformation and SO₂ flux collected at Masaya Volcano since 1993, it is now clear that the characteristically cyclical nature of the activity is not driven by intrusion of additional magma into the system. Rather, it may be due in large part to the blocking and accumulation of gas by restrictions in the volcano substructure. The history of crater collapse and formation of caverns beneath the crater floor would greatly facilitate the trapping and storage of gas in a zone immediately beneath San Pedro and the other craters. Another mechanism that may explain the observed gravity and gas flux variations is the convective overturn of shallow, pre-existing, degassed, cooled, dense magma that is replaced periodically by lower density, hot, gas-rich magma from depth. Buoyant gas-rich magma rises from depth and is emplaced near the surface, resulting in the formation and fluctuation of a low-density gas-rich layer centred beneath Nindirí and Santiago craters. As this magma vigorously degasses, it must cool, increase in density and eventually sink. Five stages of activity have been identified at Masaya since 1853 and the most recent data suggest that the system may have been entering another period of reduced degassing in 2000. This type of analysis has important implications for hazard mitigation because periods of intense degassing are associated with poor agricultural yields and reduced quality of life. A better understanding of persistent cyclically active volcanoes will allow for more effective planning of urban development and agricultural land use.     

The petrology and geochemistry of the Handkerchief Mesa mixed magma complex, San Juan Mountains, Colorado

The Handkerchief Mesa mixed magma complex is one of several late Cenozoic volcanic complexes in the southeastern San Juan Mountains characterized by mingling and limited mixing of basalt and rhyodacite. Stratigraphy in the dissected vent complex at Handkerchief Mesa records three phases of volcanism, the first and third displaying evidence for coeruption of mafic and silicic magmas. Phases 1 and 2 erupted silicic pyroclastics and basaltic lava flows, respectively. Phase-3 eruptions were dominated by rhyodacite lava flows, rhyodacite dikes, and abundant mingled and mixed hybrid lavas.Pre- and syneruptive basalt-rhyodacite mixing of phase-3 eruptions is shown by: (1) inclusions of quenched basalt in rhyodacite; (2) partially disaggregated basalt inclusions in mixed hybrids and rhyodacites; (3) interfingering lenses of mixed hybrid lavas and rhyodacite. Whole-rock major- and trace-element analyses support a two-component mixing model whereby intermediate hybrids are produced by mixing of basalt and rhyodacite (up to 30% basalt: 70% rhyodacite). Disequilibrium phenocryst textures and mineral compositions are consistent with multistage mixing culminating in an eruptive mixing event. Protracted mixing along a boundary zone at the base of a rhyodacite magma chamber may be responsible for stabilizing Fe-rich olivine phenocrysts in some hybrids.Basalt-rhyodacite mixing is inhibited by rapid crystallization in the basalt shortly after inclusion within the lower temperature melt. The degree to which mechanical dispersion and blending ensues is a critical function of the initial temperature contrast (ΔT_i) between the two magmas. Thermal models, simulating the conductive cooling histories for basalt spheres in rhyodacite reservoirs, suggest that at large ΔT_i's (> 200°) rapid cooling of the inclusion leads to disequilibrium crystallization with concomitant depression of equilibrium solidi, grain boundary wetting by residual liquids, and limited disaggregation of the inclusion imposed by movement of the host. For small ΔT_i's (< 100°) temperatures within the inclusion can be maintained above the solidus for prolonged time periods, enhancing the possibility of producing homogeneous mixed hybrids through mechanical blending and diffusion. Both mechanisms operated at Handkerchief Mesa and contributed to the range of observed textures and compositions.     

2.8-Ma ash-flow caldera at Chegem River in the northern Caucasus Mountains (Russia), contemporaneous granites, and associated ore deposits

Diverse latest Pliocene volcanic and plutonic rocks in the north-central Caucasus Mountains of southern Russia are newly interpreted as components of a large caldera system that erupted a compositionally zoned rhyolite-dacite ash-flow sheet at 2.83 ± 0.02 Ma (sanidine and biotite ⁴⁰Ar/³⁹Ar). Despite its location within a cratonic collision zone, the Chegem system is structurally and petrologically similar to typical calderas of continental-margin volcanic arcs. Erosional remnants of the outflow Chegem Tuff sheet extend at least 50 km north from the source caldera in the upper Chegem River. These outflow remnants were previously interpreted by others as erupted from several local vents, but petrologic similarities indicate a common origin and correlation with thick intracaldera Chegem Tuff. The 11 × 15 km caldera and associated intrusions are superbly exposed over a vertical range of 2,300 m in deep canyons above treeline (elev. to 3,800 m). Densely welded intracaldera Chegem Tuff, previously described by others as a rhyolite lava plateau, forms a single cooling unit, is > 2 km thick, and contains large slide blocks from the caldera walls. Caldera subsidence was accommodated along several concentric ring fractures. No prevolcanic floor is exposed within the central core of the caldera. The caldera-filling tuff is overlain by andesitic lavas and cut by a 2.84 ± 0.03-Ma porphyritic granodiorite intrusion that has a cooling age analytically indistinguishable from that of the tuffs. The Eldjurta Granite, a pluton exposed low in the next large canyon (Baksan River) 10 km to the northwest of the caldera, yields variable K-feldspar and biotite ages (2.8 to 1.0 Ma) through a 5-km vertical range in surface and drill-hole samples. These variable dates appear to record a prolonged complex cooling history within upper parts of another caldera-related pluton. Major W-Mo ore deposits at the Tirniauz mine are hosted in skarns and hornfels along the roof of the Eldjurta Granite, and associated aplitic phases have textural features of Climax-type molybdenite porphyries in the western USA. Similar ⁴⁰Ar/³⁹Ar ages, mineral chemistry, and bulk-rock compositions indicate that the Chegem Tuff, intracaldera intrusion, and Eldjurta Granite are all parts of a large magmatic system that broadly resembles the middle Tertiary Questa caldera system and associated Mo deposits in northern New Mexico, USA. Because of their young age and superb three-dimensional exposures, rocks of the Chegem-Tirniauz region offer exceptional opportunities for detailed study of caldera structures, compositional gradients in volcanic rocks relative to cogenetic granites, and the thermal and fluid-flow history of a large young upper-crustal magmatic system.     

Incursion of a large-volume,spatter-bearing pyroclastic density current into a caldera lake: Pavey Ark ignimbrite,Scafell caldera,England

The Scafell caldera-lake volcaniclastic succession is exceptionally well exposed. At the eastern margin of the caldera, a large andesitic explosive eruption (>5 km³) generated a high-mass-flux pyroclastic density current that flowed into the caldera lake for several hours and deposited the extensive Pavey Ark ignimbrite. The ignimbrite comprises a thick (≤125 m), proximal, spatter- and scoria-rich breccia that grades laterally and upwards into massive lapilli-tuff, which, in turn, is gradationally overlain by massive and normal-graded tuff showing evidence of soft-state disruption. The subaqueous pyroclastic current carried juvenile clasts ranging from fine ash to metre-scale blocks and from dense andesite through variably vesicular scoria to pumice (<10³ kg m⁻³). Extreme ignimbrite lithofacies diversity resulted via particle segregation and selective deposition from the current. The lacustrine proximal ignimbrite breccia mainly comprises clast- to matrix-supported blocks and lapilli of vesicular andesite, but includes several layers rich in spatter (≤1.7 m diameter) that was emplaced in a ductile, hot state. In proximal locations, rapid deposition of the large and dense clasts caused displacement of interstitial fluid with elutriation of low-density lapilli and ash upwards, so that these particles were retained in the current and thus overpassed to medial and distal reaches. Medially, the lithofacies architecture records partial blocking, channelling and reflection of the depletive current by substantial basin-floor topography that included a lava dome and developing fault scarps. Diffuse layers reflect surging of the sustained current, and the overall normal grading reflects gradually waning flow with, finally, a transition to suspension sedimentation from an ash-choked water column. Fine to extremely fine tuff overlying the ignimbrite forms ∼25% of the whole and is the water-settled equivalent of co-ignimbrite ash; its great thickness (≤55 m) formed because the suspended ash was trapped within an enclosed basin and could not drift away. The ignimbrite architecture records widespread caldera subsidence during the eruption, involving volcanotectonic faulting of the lake floor. The eruption was partly driven by explosive disruption of a groundwater-hydrothermal system adjacent to the magma reservoir.     

Evolution of Aoba caldera volcano,New Hebrides

Aoba is a basalt volcano situated in the northern part of a chain containing all the active volcanoes in the New Hebrides. The chain extends the length of the New Hebrides. Growing from a depth of 2,400 meters on the sea floor, the volcano probably emerged above sea level in the late Pliocene or early Pleistocene. The age of the oldest exposed rocks is unknown. Relatively fluid lavas with autobrecciated surfaces probably issued from tissures, initiating a shield-building stage as the volcano emerged. Airfall pyroclastics increase towards the top of these lavas and are overlain by agglomerates marking a more explosive episode. Activity continued with the effusion of picrite basalt, accompanied by spasms of ash emission that formed crystal tuff. Subsequently a more explosive episode produced agglomerate and tuff with occasional tongues of lava. The two oval summit calderas are apparently related to deep-seated subsidence. Lack of pumice deposits, and the basic nature of the magma suggest that the foundering of the calderas was a quiet event, possibly due to massive outpourings of lava at a lower level, although a substantial volume also erupted from the summit volcanoes at this time. A broad pyroclastic cone, which was still growing 360 years ago, occupies the centre of the inner caldera. It is surmounted by a wide crater, or possibly small caldera, containing a lake in which palagonite tuff cones have formed. The western end of the inner caldera is occupied by an explosion crater, and the eastern end by a semicircular lake. A thermal area containing a solfatara on the southeast shore of the eastern lake, and staining in the crater lake suggestive of fumarole activity, are the only evidence of vulcanicity at the present time. It is difficult to correlate events at the centre of the volcano with those at the lateral fissures. Later episodes at the centre are probably broadly contemporaneous with activity along the fissures, the inner ends of which are mantled by younger deposits of the central volcano. Accumulation of material about this axial fiissure system, marked by no less than 64 cruptive foci, mainly spatter cones, and phreatic explosion craters where they intersect the coast, has extended the island to the northeast and southwest, producing the present oval shape. Numerous flows spilled from these fissures, the last reaching the sea at N’dui N’dui only 300 years ago according to local legend. Abundant ash was emitted from both the summit calderas and flank fissures at a late stage, forming a tuff mantle with layers of accretionary lapilli. The last volcanic event was the formation of a lahar which destoyed a village on the northeast slope of the volcano about 100 years ago. No consistent variation with time is evident in the composition of the magma, although plagiophyric and aphyric lava erupted during the later stages. All the rocks are basaltic, and differ only in the presence or absence of phenocryst-forming minerals, and the proportions in which they occur. Picrite basalt and ankaramite erupted from the central volcano and flank fissures, respectively.     

Lithic types in ignimbrites as a guide to the evolution of a caldera complex, Taupo volcanic centre, New Zealand

Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.     

Propagation, linkage, and interaction of caldera ring-faults: comparison between analogue experiments and caldera collapse at Miyakejima, Japan, in 2000

The formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash-flow eruptions. Caldera collapse occurred in 2000 at Miyakejima Island (Japan) in response to a lateral intrusion. Based on geophysical data it is inferred that a set of caldera ring faults was propagating upward. To understand the kinematics of ring-fault propagation, linkage, and interaction, we describe new laboratory sand-box experiments that were analyzed through Digital Image Correlation (DIC) and post-processed using 2D strain analysis. The results help us gain a better understanding of the processes occurring during caldera subsidence at Miyakejima. We show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards. Then a set of normal faults initiates from tension fractures at the surface, propagating downwards to link with the reverse faults at depth. With increasing amounts of subsidence, interaction between the reverse- and normal-fault segments results in a deactivation of the reverse faults, while displacement becomes focused on the outer normal faults. Modeling results show that the area of faulting and collapse migrates successively outward, as peak displacement transfers from the inner ring faults to later developed outer ring faults. The final structural architecture of the faults bounding the subsiding piston-like block is hence a consequence of the amount of subsidence, in agreement with other caldera structures observed in nature. The experimental simulations provide an analogy to the observations and seismic records of caldera collapse at Miyakejima volcano, but are also applicable to caldera collapse in general.     

The new worldwide collapse caldera database (CCDB): A tool for studying and understanding caldera processes

Collapse calderas are one of the most important volcanic structures not only because of their hazard implications, but also because of their high geothermal energy potential and their association with mineral deposits of economic interest. The objective of this work is to describe a new general worldwide Collapse Caldera DataBase (CCDB), in order to provide a useful and accessible tool for studying and understanding caldera collapse processes. The principal aim of the CCDB is to update the current field based knowledge on calderas, merging together the existing databases and complementing them with new examples found in the bibliography, and leaving it open for the incorporation of new data from future studies. This database does not include all the calderas of the world, but it tries to be representative enough to promote further studies and analyses. We have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references, and their information has been summarized in a database linked to a Geographical Information System (GIS) application. Thus, it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database (e.g. age, structure). The information recorded in the CCDB can be grouped in seven main information classes: caldera features, properties of the caldera-forming deposits, magmatic system, geodynamic setting, pre-caldera volcanism, caldera-forming eruption sequence and post-caldera activity. Additionally, we have added two extra classes. The first records the references consulted for each caldera. The second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin. A further purpose of this work is to construct the CCDB web page. In this web page where registered users can acquire the current database version, as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes. Additionally, the CCDB includes a formulary that will facilitate the incorporation of new calderas into the database.