Caldera Formation Research Articles

Formation of caldera periphery faults: an experimental study

Changing stresses in multi-stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism(s) associated with caldera formation and the corresponding zones of structural weakness. We evaluate characteristic structures resulting from doming (chamber inflation), evacuation collapse (chamber deflation) and cyclic resurgence (inflation and deflation), and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry. Doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation. The structural dome so produced is characterised bysteepening in the periphery, whereas the broadening apex subsides. Pure evacuation causes the chamber roof to collapse along adjacent bell-shaped reverse faults. The distribution of concentric faults is influenced by the initial edifice morphology; steep and irregular initial flanks result in a tilted or chaotic caldera floor. The third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation. Following doming, caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome. The flanks of the edifice relax, resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults; the latter form locally uplifted and tiltedwedges (half-grabens) that grade into horst-and-graben structures. This superimposed fault pattern also extends inside the caldera. We suggest that major pressure deviations in magma chamber(s) are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first-order indication of the processes and mechanisms involved in caldera formation.

Geology and geochemistry of lavas at Nekoma volcano: Implications for origin of Quaternary low‐K andesite in the north‐eastern Honshu arc, Japan

Abstract Nekoma volcano forms part of the arc axis volcanic array of the North‐eastern Honshu arc, Japan, which is commonly characterized by medium‐K lava suites. However, Nekoma is exceptional because many of its lavas are low‐K. This anomaly has been a matter of debate. Nekoma was active from 1.1 to 0.35 Ma. The volcano consists of thick andesite flows and domes associated with block and ash flow deposits produced during lava dome formation. A horseshoe‐shaped collapse caldera was formed at the summit and small lava domes extruded into the caldera. Stratigraphy, published K–Ar ages, and tephrochronology define three stages of volcanic activity, about 1.1 Ma (Stage 1), 0.8–0.6 Ma (Stage 2) and 0.45–0.35 Ma (Stage 3; post caldera stage). Low‐K andesites occur in all stages. Extremely low‐K andesite was also associated in Stage 2 and medium‐K andesite was dominant in Stage 3. In general, lavas changed from low‐K to medium‐K after caldera formation. Geochemical study of the Nekoma lavas shows that both low‐K and medium‐K lavas are isotopically similar and were derived from a common source. Adatara and Azuma volcanoes, which lie close to Nekoma, also have both low‐K and medium‐K andesites. However, Sr isotope ratios or temporal‐spatial variations in K‐level lava classification vary between the three centers. Comparisons of K suites and Sr isotope ratios with frontal arc volcanoes in North‐east–Honshu suggest source heterogeneity existed in both medium‐ and low‐K suites. The K contents of lavas and their Sr isotopes are not simply related. This requires re‐examination of models for chemical variation of andesites in arcs.

Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes

The evolution of Andean volcanism including the formation of late Miocene to Recent collapse calderas on the Puna plateau is generally interpreted in terms of the kinematic framework of the Nazca and South American Plates. We present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first-order NW–SE-striking zones of left-lateral transtension on the local and regional scales. Consequently, ages of collapse calderas indicate activity of these fault zones which initiated at about 10 Ma on the Puna plateau. The onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen-parallel stretching upon reaching maximum crustal thickness and critical surface elevation. Horizontal magma sheets that formed at mid-crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub-vertical faults. This accounts well for the observed tectono-magmatic phenomena at surface. It follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the Andean plateau rather than to changes in the geometry of the Wadati–Benioff zone or plate boundary kinematics.

Volcanological implications of crystal-chemical variations in clinopyroxenes from the Aeolian Arc, Southern Tyrrhenian Sea (Italy)

Crystal chemistry and structural data for clinopyroxene from the Aeolian islands (Southern Tyrrhenian Sea, Italy) were determined with the aim of obtaining geobarometric information and exploring implications for the structure of volcanic plumbing systems. Cell and M1 site volumes for clinopyroxenes, which are known to decrease with increasing pressure of crystallization, revealed variable values, both within some single islands and along the entire arc, indicating polybaric conditions of crystallization. The lowest cell and M1 volumes were found at Filicudi, plotting close to values of clinopyroxenes from high-pressure ultramafic xenoliths entrained in alkali basalts. Indications of high-pressure crystallization were also found at Salina and, to a lesser extent, at Alicudi, all situated in the western sector of the Aeolian Arc. The central and eastern islands of Lipari, Vulcano, Panarea and Stromboli generally show higher values of cell parameters, suggesting crystallization in shallow magma chambers. These islands are characterized by the occurrence of large calderas, which are apparently lacking at Salina and Filicudi. Time-related variations were observed for cell and M1 volumes of clinopyroxene for some islands. At Salina, the early-erupted products display low values of cell parameters with respect to later activity, thus indicating a decrease in crystallization pressure with time. A similar, although less striking, pattern is observed at Alicudi and Lipari. An overall increase in cell parameters with time was observed at the scale of the entire arc. The observed variations in clinopyroxene structural parameters highlight the significance of pyroxene crystal chemistry for petrogenetic and volcanological interpretation. Correlation with time and the structural characteristics of volcanoes suggest significant regional and temporal modifications in the plumbing systems of Aeolian volcanoes. Clinopyroxenes from Filicudi and the older Salina crystallized at high pressure in deep magma chambers, in the lower crust or at the mantle–crust boundary. The lower crystallization pressure in the younger Salina is interpreted as evidence of upward migration of magma chambers with time. Similar evolution can be envisaged for Alicudi. Instead, the entire evolutionary history of the central and eastern islands was dominated by low-pressure crystallization, with formation of calderas and generation of abundant acid products that are scarce or absent in the western islands. Evolution of the plumbing system of single volcanoes and of the Aeolian arc in general is probably related to modification of stress regimes and/or thinning of the arc basement, due to the effect of uprising mantle material above the Ionian subduction zone.

La Pacana caldera, N. Chile: a re-evaluation of the stratigraphy and volcanology of one of the world's largest resurgent calderas

La Pacana caldera in the Central Andes of northern Chile is one of the largest and best exposed resurgent calderas in the world. The caldera had previously been recognised as the source of the regionally-extensive Atana ignimbrite, but additional field and stratigraphic evidence, along with new K–Ar age determinations and geochemical data have led to a revision of the geology and development of this major Andean caldera. In particular, this information allows more realistic estimates of eruptive volumes and has implications for the style of ignimbrite eruption. Two major ignimbrites appear to have originated from La Pacana caldera, based on their thickness variations, lateral distributions and stratigraphic relations: the crystal-poor, rhyolitic Toconao ignimbrite (4–5 Ma) and the crystal-rich, dacitic Atana ignimbrite (4 Ma). Following caldera collapse and formation of the resurgent Atana block, several crystal-rich dacitic–rhyolitic domes formed along the margin of the resurgent block. New K–Ar ages show that this post-caldera volcanism continued from 4 to 2 Ma, indicating that the La Pacana magmatic system was active for at least 2 Ma after the main eruption. The Atana ignimbrite extends west, south and east of La Pacana caldera. Our work shows that the ignimbrite sequence northeast of the caldera, formerly mapped as Atana outflow, represents two new units which we name the upper and lower Tara ignimbrites. The distribution of the Tara ignimbrites points to a source to the north. The upper Tara ignimbrite comprises four flow units with interbedded surge and fall deposits and a characteristic, heterogeneous pumice population. It occurs in the La Pacana moat and onlaps the resurgent block. These field relations and a new K–Ar age of 3.8 Ma show convincingly that this ignimbrite erupted after formation of La Pacana caldera. The lower Tara ignimbrite is a single extensive flow unit, and has an age of 5.6 Ma. Two outcrops of lag breccia occur adjacent to the caldera topographic margin and these are interpreted as vent areas for the Atana ignimbrite. This indicates that the structural caldera margin does not coincide with the resurgent block as previously thought, but is close to the topographic margin. Combining this redefinition of caldera geometry with the revised distribution of the Atana ignimbrite implies an overall trap-door-like configuration of caldera collapse, with a hinge to the north and maximum subsidence in the south. The new volume estimate of 2500 km 3 for the caldera-forming Atana ignimbrite, including intracaldera and outflow facies, places it among the most voluminous ignimbrites of the world.

Formation d'une caldera d'érosion et instabilité récurrente d'une île de point chaud : la caldera de Taburiente, La Palma, îles Canaries / Formation of an erosion caldera and recurring instability on a hotspot-generated island: the caldera de Taburiente, La Palma, Canary Islands

Abstract The caldera de Taburiente is an erosion caldera caused by backwearing of the walls of a giant landslide embayment that affected the south-west flanks of the island of La Palma 560 ky. ago. This study reconstitutes the geonwrphological evolution of the depression and emphasises the role of lithological and structural factors. Patterns of erosion are guided by the structural configuration, in which uplifted and deeply weathered submarine volcanic materials are overlain by denser subaerial volcanic rocks, and by the geometry of a deep-seated structure, which has been interpreted as an older central collapse caldera. Recurrent instability is a determining factor in the formation of erosion calderas on hotspot-related islands. The construction rates (0,5-2 m/ka) and destruction rates (linear incision: 0,3-3,5 m/ka; headward erosion: 0,7-20 m/ka) estimated in this study provide a time-frame for understanding the stages of growth and decay of volcanic landforms in the Canary Islands.

Pictorial 2 : Caldera Formation Process during the Miyakejima 2000 Volcanic Activity Detected by Spatio-temporal Gravity Changes

Pictorial 2 : Caldera Formation Process during the Miyakejima 2000 Volcanic Activity Detected by Spatio-temporal Gravity Changes

Caldera Formation Process during the Miyakejima 2000 Volcanic Activity Detected by Spatio-temporal Gravity Changes

Employing both absolute and relative gravimeters, we carried out a hybrid microgravity survey at Miyakejima volcano, Japan. We detected significant gravity changes, e.g., exceeding 1000 At gal, during the course of our measurements since June 1998. The spatio-temporal gravity changes provide us with strong evidence of preexisting vacant space beneath the island before the summit collapse of the volcano. Also, our data are totally in conflict with the so-called magma drainback model, suggesting that the magma flowed out laterally.

Caldera formation in the Rum Central Igneous Complex, Scotland

The Northern Marginal Zone of the Rum Central Igneous Complex in NW Scotland represents part of the early, felsic phase of the volcano. The marginal zone is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks. Its formation has been explored through field examination of the ring fracture system of the Complex and its pyroclastic and epiclastic intracaldera facies. A sequence of magmatic tumescence and chamber growth caused initial doming, followed by the formation of a collapse structure without accompanying volcanism. This collapse structure, circular in plan, is akin in origin to a salt basin formed by crustal stretching above a rising diapir. We call this the proto-caldera. Collapse breccias, which represent the slumping and sliding of megablocks, blocks and boulders of the Torridonian sandstones which form the walls of the basin, were the original infilling. Logs of these deposits reveal considerable variation in thickness of the breccias (from 80–170 m) in the Complex, indicating an uneven floor to the proto-caldera, consistent with piecemeal collapse. Following accumulation of up to >70 m thickness of breccia, thin interbedded rhyodacitic crystal tuffs (10–30 cm) record the earliest eruptions of felsic magma in the caldera. Caldera formation was then interrupted by a period of quiescence, recorded by the presence of an epiclastic sandstone of locally several metres thickness, formed by washout of fines from the breccias. Subsequent resurgence created a fracture pattern characteristic of doming, along which rhyodacite magma rose in dykes and erupted up to perhaps 10 km3 of rhyodacitic intracaldera ignimbrites. This major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber. Mafic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma.

Genetic links between ash-flow calderas and associated ore deposits as revealed by large-scale thermo-mechanical modeling

In addition to a number of epithermal ore deposits, some porphyry-type ore deposits are also hosted by ash-flow calderas. Spatial correlation between ash-flow calderas and these deposits is observed in several parts of the world, but temporal correlation between the mineralization and the episode of caldera formation remains unclear, especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation. Because caldera collapse represents only a very short event in a long time-scale evolution between a silicic magma chamber and its host rocks, we have developed a thermo-mechanical model of upper crustal and magmatic system behavior during the pre- and post-caldera formation stages. Major petrophysical properties of ash-flow caldera related rocks are incorporated into the model, which accounts for brittle–elasto-ductile rheology and heat transfer, and reproduces fault initiation and various caldera collapse scenarios. Large-scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described. In particular, we found that the outer sides of caldera border faults are associated with both anomalous heat-transfer and a high fracture density. The finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults. In most cases, the model predicts “Y-shaped” fault geometry, which is surprisingly similar to that of mineralized faults in the field. Variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20MPa, with deep overpressured zones close to the inner edge of the chamber corners. Despite the fact that hydrothermal convection and fluid geochemistry are not included in the model, these thermal and mechanical results are believed to constrain the conditions for mineralization processes. Our results show that in the presence of regional extension, the development of caldera border faults may be suppressed to the advantage of deep-seated fractures, clustered above the central part of the magma chamber, thus favoring conditions for the formation of porphyry-type ore deposits. Finally, we demonstrate that well before the eruption, the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust, resulting in breakup and pre-eruptive normal faulting and thus allowing for conditions of pre-caldera mineralization.

Cyclical summit collapse events at Ascraeus Mons, Mars

The sequential development of multiple summit calderas of shield volcanoes gives clues about how the spatial location of shallow magma chambers evolves with time and how the magma supply into these chambers evolves. We re-interpret the sequence in which caldera collapse events took place at the Martian shield volcano Ascraeus Mons and argue that the formation of the central caldera, usually accepted to be the most recent, was followed by two further collapse events. We also present structural evidence for an early, large central caldera and, therefore, also an early large magma reservoir. This challenges the current hypothesis for Ascraeus Mons that successive magma reservoirs have increased in size as this volcano matured and that a reservoir occupied a central location within the volcano only at the end of its active lifespan. We propose that there has been a cyclical oscillation in the site of shallow storage of magma at Ascraeus Mons between central and peripheral magma reservoirs. The earliest reservoir formed centrally, but magma was subsequently redirected into peripheral reservoirs, over which the peripheral calderas formed. Later, the central location was utilized again, only to be followed once more by a development of peripheral storage reservoirs. This alternating pattern implies substantial variations with position and time in both the magma supply rate from the Martian mantle and the distribution of stresses within the volcanic edifice.

K-Ar dating of Mesta volcanics (SW Bulgaria)

The Mesta volcanic massif (SW Bulgaria) belongs to the Macedonian-Rhodope-North Aegean Magmatic Zone. It crops out along the middle course of Mesta River and covers about 200 km2 of the homonymous graben. The bаsеmеnt of Mesta graben and the neighbouring elevated blocks of Pirin and Western Rhodope mountains are composed of high-grade metamorphics, hosting granitoidic plutons of diverse ages, including the Paleogene Central Pirin and Teshevo plutons. Apart from the volcanic bodies, the graben-fill consists of complexly interfingered Paleogene stratified continental terrigenous sedimentary rocks, volcanogenic-sedimentary rocks and volcaniclastics (both руrо-, and epiclastics), overlain unconformably bу Neogene and Quaternary deposits. The Mesta volcanics are represented by two main types: high-K calc-alkaline to shoshonitic dacites (in some places with а transition to latites) and high-K calc-alkaline rhyodacites. The magmatic bodies of subvolcanic and extrusive facies are represented by а number of morphologic varieties, which belongs to two polygonal caldera structures (Кrеmеn and Banichan) and two linear magmato-tectonic zones (Gostoun and Dobrinishte). This paper presents 12 К-Аг ages obtained for 11 lауа bodies of different geological position and for one re-deposited volcanic clast from the middle part of the stratified Paleogene section. The ages are in support of the suggestion that the Mesta volcanics originated from а zoned magma chamber. More silicic (rhyodacitic) eruptions and subvolcanic intrusions are related to earlier volcanic events in the northern part of the massif - оn the territories of the Кrеmеn caldera, Gostoun and Dobrinishte zones. These events (approximately 33-31 Ма) are рге-, and sincollapse ones in respect to the Кrеmеn caldera formation. The later (sin- and post-collapse) rhyodacitic eruptions were accompanied and followed by eruptions and subvolcanic intrusions of less silicic, dacitic melts (31-28 Ма). The main portion of the dacites is related to the centres in the Banichan caldera (29-28 Ма). The time-span obtained covers the events of the paroxysmal volcanic activity in the Mesta graben only. Thus it is shorter than the real time-interval of the volcanism. This time-span is comparable with that of the voluminous Раlеоgene volcanism in the Smolyan region (Central Rhodopes). The Rb-Sr ages of neighbouring Teshevo and Central Pirin plutons plot in its boundaries as well. Some recommendations for more detailed and precise future radiometric studies are proposed to define the age of the initial and the final volcanic events in the Paleogene Mesta graben.

The caldera-forming eruption of Volcán Ceboruco, Mexico

of magma erupted, ∼95% of which was deposited as fall layers. During most of the deposition of P1, eruptive intensity (mass flux) was almost constant at 4–8×107 kg s−1, producing a Plinian column 25–30 km in height. Size grading at the top of P1 indicates, however, that mass flux waned dramatically, and possibly that there was a brief pause in the eruption. During the post-P1 phase of the eruption, a much smaller volume of magma erupted, although mass flux varied by more than an order of magnitude. We suggest that caldera collapse began at the end of the P1 phase of the eruption, because along with the large differences in mass flux behavior between P1 and post-P1 layers, there were also dramatic changes in lithic content (P1 contains ∼8% lithics; post-P1 layers contain 30–60%) and magma composition (P1 is 98% rhyodacite; post-P1 layers are 60–90% rhyodacite). However, the total volume of magma erupted during the Jala pumice event is close to that estimated for the caldera. These observations appear to conflict with models which envision that, after an eruption is initiated by overpressure in the magma chamber, caldera collapse begins when the reservoir becomes underpressurized as a result of the removal of magma. The conflict arises because firstly, the P1 layer makes up too large a proportion (∼75%) of the total volume erupted to correspond to an overpressurized phase, and secondly, the caldera volume exceeds the post-P1 volume of magma by at least a factor of three. The mismatches between model and observations could be reconciled if collapse began near the beginning of the eruption, but no record of such early collapse is evident in the tephra sequence. The apparent inability to place the Jala pumice eruptive sequence into existing models of caldera collapse, which were constructed to explain the formation of calderas much greater in volume than that at Ceboruco, may indicate that differences in caldera mechanics exist that depend on size or that a more general model for caldera formation is needed.

The geology and petrology of Volcán San Juan (Nayarit, México) and the compositionally zoned Tepic Pumice

Volcán San Juan is the westernmost Quaternary composite volcano in the Mexican Volcanic Belt. Its activity is divided into three stages of decreasing eruptive volume. During stage-1 activity, the main cone of San Juan and the adjacent Cerro Alto grew to a combined volume of ∼60 km 3 through eruption of hornblende-bearing andesites, dacites, and rhyodacites. Stage 1 included explosive eruptions of hornblende-dacite and an andesite dated at 33,750 a (years ago) and 23,010 a. Stage-2 activity involved a major Plinian eruption 14,770 a that produced the hypersthene–hornblende-bearing, compositionally zoned, rhyodacitic–andesitic Tepic Pumice and led to formation of an oval-shaped caldera 4×1 km in diameter. Tepic, the capital city of Nayarit State, with a population of ∼250,000, lies at the NE foot of Volcán San Juan. This location was buried under 2–9 m of rhyodacitic pumice-fall deposits from the Tepic Pumice eruption. Erosion has since removed virtually all exposures of Tepic Pumice >15 km from the caldera. The estimated volume of erupted tephra is 5.6 km 3, equivalent to 1.2 km 3 of magma. Modeling of the eruption dynamics indicates that as the eruption evolved the column height increased from ∼15 km to ∼24 km and maximum mass discharge rate increased from ∼3×10 6 kg/s to ∼6×10 7 kg/s. During stage-3 activity, a hornblende-andesitic lava dome partially filled the caldera and sent block lava flows down the north slope of the cone; total erupted volume was ∼0.55 km 3. Compared to other composite volcanoes of the western Mexican Volcanic Belt, the products from San Juan have the highest Sr contents and the lowest Rb, Y, Yb, and Lu contents, resulting in high La/Yb and Sr/Y, with low Rb/Sr. These observations indicate a strong slab-melt component at San Juan, consistent with its proximity to the Middle America Trench where young, hot lithosphere is subducting. The San Juan andesites, dacites, and rhyodacites also have relatively high 87Sr/ 86Sr, 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb, and low ε Nd values compared to these other volcanoes, which appear to reflect significant crustal contamination of San Juan's magmas.

Southern Kermadec submarine caldera arc volcanoes (SW Pacific): caldera formation by effusive and pyroclastic eruption

Three newly identified submarine caldera volcanoes (Brothers, Healy, and Rumble II West), of the southern Kermadec arc (within water depths <1500 m), provide new insights into modes of submarine arc caldera formation. Swath MR1 data, seafloor photography, and rock dredge sampling establish the edifice and caldera morphology, outcrop volcanology, magma compositions, and petrography and rock properties of erupted components for each volcano. These data reveal the Brothers and Rumble II West volcanoes have: (1) concentric, steep and 50–450 m high escarpments, that are interpreted as caldera wall ring-faults, encircling ca. 3-km wide calderas and associated central cones, (2) heterogeneous flank-draping lava flows, and reworked pyroclastic deposits, and (3) the predominance of incipiently to poorly vesiculated (mostly <40%), moderately dense (1.6–2.4 g cm −3) eruptive flows and clasts. Erupted lavas from Brothers and Rumble II West are predominantly dacitic (64–67 wt.% SiO 2), and basalt–basaltic–andesite (64–67 wt.% SiO 2), respectively. Morphology of the Brothers and Rumble II West volcanoes are interpreted (although the former less confidently) as evidence of incremental caldera collapse formed by prolonged, episodic, effusive magma withdrawal from summit and rift flank vents. In contrast, the Healy volcano has: (1) a flat ca. 2.5-km wide “caldera” floor flanked by a 250–400 m high, and a relatively shallowly inclined “caldera” wall, (2) the presence of a near ubiquitous mantle of relatively well sorted, blocks and lapilli over the edifice flanks and caldera, with rare seafloor outcrop, and no obvious evidence of lava flows, (3) the predominance of rhyodacitic compositions (66–70 wt.% SiO 2) contents, and (4) extremely vesiculated (>80%) and low density (0.3–0.5 g cm −3) erupted pumiceous clasts. These observations are interpreted as evidence of subaqueously quenched, pyroclastic eruption(s), sheathed by ambient water flashing to steam, discharging some 5 km 3 of pyroclastic material from Healy volcano. Caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption(s), although it is possible formation as an explosive crater occurred, but we consider the latter unlikely given the volume of erupted material and water depth. Extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas, and syn-eruptive caldera collapse, is possible within water depths of <1500–1000 m. In contrast, incremental, post-eruptive caldera collapse is more likely to occur with repetitive effusive eruptions of more mafic compositions, at both summit and rift flank sites, within water depths of >500–1000 m.

The Povoação Ignimbrite, Furnas Volcano, São Miguel, Azores

The Povoação Ignimbrite Formation (PIF) was emplaced by one of the larger explosive trachytic eruptions of Furnas Volcano, São Miguel, Azores. Trachytic ignimbrites are common in the products of Furnas Volcano and examples of welding occur in at least three ignimbrites of which the Povoação Ignimbrite is the most extensive. The PIF may correlate with the formation of the main caldera of Furnas. In the Povoação Ignimbrite, the welded horizons thicken, without evidence of rheomorphism, into palaeovalleys and can be seen to thin and in some places become completely attenuated over old ridges. The welded horizons are intimately associated with non-welded ignimbrites and in some places there is an alternation between welded and non-welded horizons. On interfluves, the ignimbrite is stratified and some of the welded horizons show pinch and swell and occasional cross-bedding. The welding is interpreted as a primary depositional feature with the clasts sintering on emplacement. It is argued that this ignimbrite was emplaced from a turbulent pulsatory pyroclastic flow. Some pulses were hotter which enabled more extensive development of welding. The flows became more concentrated and denser down valleys favouring the emplacement of thicker welded units.

A Proposal of Caldera‐Related Genesis for the Roseki Deposits in the Mitsuishi Mining Area, Southwest Japan

Abstract: Geology of the largest Roseki (pyrophyllite–clay)–mineralized, Mitsuishi mining area was re–examined, and a composite caldera model (Wake caldera) was proposed. Evidence for the caldera formation is (1) Existence of huge volume of rhyolitic tuffs in the mining area, (2) A basin structure in rhyolitic tuffs surrounded by the Permo‐Triassic basement rocks, (3) Arc‐like distribution of the Roseki deposits along the eastern edge of the proposed caldera, (4) Tectonic disturbance and intrusive bodies along the caldera wall, and (5) Presence of meso– and mega‐breccias at the bottom of the caldera wall. The proposed caldera, sizing NNW‐SSE 23 km by ENE‐WSW 15 km, may have younger, Inner caldera (15 by 15 km), defined by Lower Member of the late Cretaceous rhyolitic tuffs, thus composite in character. The Roseki deposits were formed after the Inner caldera collapse by hydrothermal fluids ascended through the caldera wall, then spread into the permeable horizon. This caldera proposal bears a significant change in the regional exploration strategy for the Roseki deposits.

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.

Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions

New geochronological, geochemical, and Sr-isotopic data on volcanics erupted before the Campanian Ignimbrite (CI, 37 ka) and the Neapolitan Yellow Tuff (NYT, 12 ka) caldera-forming eruptions at Campi Flegrei (CF) have allowed us to investigate the behavior and temporal evolution of the Phlegraean magmatic system. The most prominent feature of the CF magmatic system was the existence of a large, trachytic magma chamber, episodically recharged, which fed eruptions for tens of thousands years before the CI and NYT eruptions. During the pre-CI caldera activity, magmas were episodically erupted from vents located outside the present caldera structure. These magmas ranged in composition from trachyte to alkali-trachyte, with Sr-isotope ratios increasing through time, and becoming identical to that of the CI magma, at about 44 ka ago. This suggests that the Phlegraean magmatic system before the CI eruption was acting as an open system. It was being progressively replenished by new batches of magma that mixed with the resident less radiogenic, fractionating trachytic magmas and was periodically tapped. The magma chamber evolution culminated in the catastrophic eruption of the voluminous (150 km 3 DRE), chemically and isotopically zoned CI trachytic magmas, and in the resultant CI caldera formation. Subsequent to the CI eruption, during a period of moderate subaereal volcanic activity of about 20 ka duration, magmas predominantly trachytic to alkali-trachytic in composition and isotopically similar to the last emitted CI magma were erupted from vents located inside the CI caldera. The temporal trend shown by Sr-isotope ratios provides evidence for a new input of alkali-trachytic magma, at ca. 15 ka, with 87 Sr/ 86 Sr ratio identical to that of the alkali-trachytic magma feeding the first phase of the NYT eruption. These data testify to the arrival in a short time span of a new trachytic to alkali-trachytic magma in the system, isotopically distinct from the CI magma, that gave rise about 3 ka later to eruption of the NYT (40 km 3 DRE).

The Denham Caldera on Raoul Volcano: dacitic volcanism in the Tonga–Kermadec arc

Denham Caldera, on Raoul Volcano in the Tonga–Kermadec arc, is a simple collapse structure. It is 6.5×4 km 2 across, >0.3 km deep, represents an erupted magma volume of 8–16 km 3, and is considerably larger than previously estimated. Caldera formation was contemporaneous with an eruption of homogeneous dacitic magma at 2.2 ka. Tephra from this event is preserved as a pumice fall and pyroclastic flow sequence up to 120 m thick on Raoul Island. Offshore, this sequence forms a submarine deposit diminishing in thickness from 120 m near the northern caldera rim to 80 m at a distance of 3.5 km, where it rests upon an erosional planation surface of 17–20 ka age incised into older volcaniclastic rocks. Both the caldera volume and ejecta distribution of the 2.2 ka eruption are comparable to the caldera volume and ejecta distribution of the 1883 Krakatau eruption. Resurgent volcanism has built at least six submarine pyroclastic cones along a north–northeast trending lineament crossing Denham Caldera. Redistribution of pumiceous dacite from these cones, including ephemeral islands that emerged during the 1814 and 1870 eruptions, has contributed to recent shoaling in Denham Bay. A smaller eruption occurred in 1964–65, and hydrothermal activity persists at several sites along the lineament. Voluminous felsic volcanism at intra-oceanic arcs is usually associated with crustal extension, and was unexpected in the Tonga–Kermadec arc. However, because the tectonic fabric within the active back-arc Havre Trough is oblique to the Kermadec arc, rifts within the trough apparently propagate to the volcanic front and transfer extensional strain to the arc, thereby promoting the ascent of felsic magma. The orientation of Denham Caldera, as well as that of Macauley Caldera 110 km further south, thus reflect the stress field of the Havre Trough and not the predominant field of the arc.