Caldera Formation Research Articles

Bulk chemical trends at arc volcanoes are not liquid lines of descent

Although the magmatic output of arc volcanoes defines clear chemical trends, the compositional sequence of eruption is often chaotic. At best, support for the concept of progressive evolution by fractionation/assimilation in a central chamber is ambiguous. Rather than defining a liquid line of descent, we suggest that chemical trends in arc systems are populated by discrete magma batches. These have been derived from a limited set of sources and undergone a common family of processes, but in different proportions. Crystal–liquid fractionation may occur at the mafic end of the spectrum in classical fashion by gravitational crystal extraction from melt-rich magma. However, at the silicic end where melt viscosity is high, fractionation may occur by forceful expulsion of interstitial melt from near-solidus arc plutons and adjacent crust. The andesite/dacite middle ground is commonly occupied by relatively crystal-rich magma batches with complex mixing and mingling histories. Such intermediate magmas appear to be stable residents of shallow arc crust—mechanically because of their neutral buoyancy, thermally because they are frequently recharged with basalt, and chemically because they resist large shifts in composition. Stored magma that “regresses” in an evolutionary sense by mixing or mingling during recharge is preferentially erupted and forward evolution during quiescence is inhibited by high viscosity of melt. The surface expression is the building of a stratocone or dome complex. In contrast, silicic magmas, once they have coalesced from interstitial melt in their plutonic sources through coupled porous flow and diking, tend to be erupted promptly, flushing out resident andesite/dacite magma batches in their path. The surface expression is caldera formation, which commonly terminates one episode of cone building and serves as a prelude to another.

The Petrology and Geochemistry of the Aniakchak Caldera-forming Ignimbrite, Aleutian Arc, Alaska

Aniakchak caldera, Alaska, produced a compositionally heterogeneous ignimbrite 3400 years ago, which changes from rhyodacitic at the base to andesitic at the top of the eruptive sequence. Interpretations of compositionally heterogeneous ignimbrites typically include either in situ fractional crystallization of mafic magma and generation of a stratified magma body or replenishment of a silicic magma chamber by mafic inputs. Another possibility, silicic replenishment of a more mafic chamber, exists. Geochemical characteristics of the caldera-forming rhyodacite and several late precaldera rhyodacites indicate independent origins for each, within a maximum of 5000 years prior to caldera formation. Isotopic considerations preclude derivation of the caldera-forming rhyodacite from the caldera-forming andesite. However, the caldera-forming rhyodacite can be explained as the residual liquid of a mostly crystallized basalt, with addition of crustal material. The Aniakchak andesite probably formed in a shallow chamber by successive mixing events involving small volumes of basalt and rhyodacite, together with contamination. The pre-caldera rhyodacites represent erupted portions of intruding silicic magma, whereas another portion homogenized with the resident mafic magma. The caldera-forming event reflects a large influx of rhyodacite, which erupted before significant mixing occurred and also triggered draining of much of the andesitic magma from the chamber.

Submarine flank eruption preceding caldera subsidence during the 2000 eruption of Miyakejima Volcano, Japan

During the early part of a seismic swarm preceding eruption and caldera formation at Miyakejima Volcano, discoloured sea surfaces were observed ∼1.5 km off the western coast of Miyakejima on 27 June 2000. A later survey of the area using a multi-beam side scan sonar and a remotely operated small submarine revealed four craters of 20–30 m diameter aligned east-west in a 100×10–30 m area on the seafloor, with hot water at ∼140°C being released from one of the centres. Each crater consists of submarine spatter overlain in part by scoria lapilli. Dredged spatter from the craters was fresh, and there was no evidence of activity of marine organisms on the spatter surface, indicating that the discoloured sea surface resulted from magmatic eruption on the seafloor. This eruption occurred when a westward-propagating seismic swarm, initiated beneath Miyakejima’s summit, passed through the area. Finding new magma on the seafloor demonstrates that this seismic swarm was associated with intruding magma, moving outward from beneath Miyakejima. Submarine spatter shows flattened shapes with a brittle crust formed by cooling in water, and its composition is aphyric andesite of ∼54 wt% SiO2. The spatter is similar in whole rock and mineral composition to spatter erupted in 1983. However, the wide range of Cl in melt inclusions in plagioclase of the 27 June submarine spatter shows that it is not simply a remnant of the 1983 magma, which has only high Cl melt inclusions in plagioclase. The mixed character of melt inclusions suggests involvement of a magma with low Cl melt inclusions. The magma erupted explosively on 18 August from Miyakejima’s summit, considered as the second juvenile magma in this eruption, contains low Cl melt inclusions in plagioclase. Based on these observations and the eruption sequence, we present the following model: (1) A shallow magma chamber was filled with a remnant of 1983 magma that had evolved to a composition of 54–55 wt% SiO2. (2) Injection of the 18 August magma into this chamber generated a mixed magma having a wide range of Cl in melt inclusions contained plagioclase. The magma mixing might have occurred shortly before the submarine eruption and could have been a trigger for the initiation of the removal of magma from the chamber as an extensive dyke, which eventually led to caldera subsidence.

Elliptical calderas in active tectonic settings: an experimental approach

Caldera volcanoes form due to collapse of a magma chamber roof into the underlying magma chamber. Many field, theoretical and experimental studies have postulated that calderas are delimited by reverse ring faults and are surrounded by peripheral concentric normal faults. In the simplest theoretical scenario, circular magma chambers produce circular calderas. Many calderas, however, are elliptical in shape, particularly those in extensional and compressive tectonic settings. Several factors may explain elliptical calderas. The first is the presence of an elliptical magma chamber, established by, for instance, preferential intrusion along pre-existing basement structures or differential spalling of the magma chamber walls. The second is the overlap (nesting) of several discrete calderas to form a single, larger elliptical structure. The third is asymmetric subsidence. The fourth is variable pre-collapse topography. A fifth possible factor is distortion of the caldera faults by the regional stress field during caldera formation. A sixth factor is the post-collapse distortion of the caldera structure due to continued regional deformation. To better understand relationships between caldera surface expression, reservoir geometry and regional tectonic stresses, we conducted scaled analogue experiments. These experiments examined the impact of regional stress and associated structures on calderas formed during evacuation of reservoirs (circular rubber balloons) of known dimensions and depths. The results show that, in principle, calderas produced in compression/extension experiments are elongated parallel to the direction of minimum horizontal compressive stress, despite the chamber beneath being circular in plan view. As a consequence, model ring fault orientation varied from steeply dipping where striking perpendicular to the minimum horizontal regional compressive stress, to shallower dips where striking parallel to the minimum horizontal regional compressive stress. This leads us to suggest that the influence of a regional stress field on caldera fault orientation during and/or after caldera formation may be significant in the development of elliptical calderas. In addition, such variation of caldera ring fault dip from steep to relatively shallow could influence location and behaviour of ring fissure eruptions.

Effects of changing stress states on the development of caldera-bounding faults: Geological evidence from Kumano caldera, Japan

Collapse of a large caldera can cause spatial and temporal perturbations of stress, and formation of “caldera faults.” The stress variations influence the direction of slip vectors on the fault planes; hence, stress estimation is important for the study of caldera-forming processes. In our paleostress estimation, the stress variations in the collapse of the ca. 14 Ma Kumano caldera in Japan have been revealed. A stress inversion method based on the Wallace-Bott hypothesis was used to compute the orientation of the principal stress axes ( σ 1≥ σ 2≥ σ 3) and the stress ratio ϕ=( σ 2− σ 3)/( σ 1− σ 3), where 0≤ ϕ≤1. The caldera faults formed simultaneously with the caldera-forming ash-flow tuff eruption. Therefore, paleostress solutions obtained from slip data measured on such faults show the spatial and temporal changes of the stress at the time of the caldera collapse. The computed stress ratio ϕ characterizes a pair of stress fields. In the early stage, the stress field with ϕ∼1.0 shows a semi-radial trajectory of stress σ 2 and an eastern concentric trajectory of stress σ 3. This stress regime, resulting from pre-collapse tumescence, counteracts the gravitational force and thus produces smaller net vertical stress. The regional tumescence above an inflated magma chamber is the most plausible source of the stress field, and it is consistent with the timing of the caldera formation. In the late stage, the stress field with ϕ∼0.5 shows the semi-radial trajectory of stress σ 2 and the west-convex and concentric trajectory of stress σ 3. Change of the stress ratio ϕ from 1.0 to 0.5 implies that increase in the relative magnitude of the stress σ 1 caused the deeper subsidence of the caldera floor. Stress variations may be of significant value for reconstructing the structural history of the caldera.

Graben structure in the Las Cañadas edifice (Tenerife, Canary Islands): implications for active degassing and insights on the caldera formation

A graben structure has been identified at the western area of the Las Cañadas caldera wall, here referred as the Los Azulejos Graben. This graben is 1 km wide and is bounded by two major normal faults trending NE–SW, the Los Azulejos Fault and the Ucanca Fault. The graben was active for at least 0.5 Ma, from the end of the Ucanca Fm to the end of the Guajara Fm, and before the collapse of the Las Cañadas edifice that formed the western caldera. A right-lateral transtension regime operated in the graben as suggested by small fault orientations and kinematics. The prolongation of the NE rift zone of Tenerife to the Cañadas edifice is the most likely volcano-tectonic scenario for the graben. In this context, inflation of phonolitic shallow magma chambers may have produced reverse faults and reactivation of normal faults. An intense and widespread hydrothermal alteration, here called Azulejos-type, occurred mainly before the graben formation, while a fault-related hydrothermal alteration occurred during and after the graben. Diffuse carbon dioxide and hydrogen degassing in and around the Las Cañadas caldera show relatively enriched values along a NE–SW trend suggesting that faults in the Los Azulejos Graben act as a pathway for deep-seated gases to the surface. Diffuse degassing and hydrothermalism indicate that the graben area has been a zone of intense fluid circulation during the evolution of the Las Cañadas edifice.

Caldera formation by magma withdrawal from a reservoir beneath a volcanic edifice

Caldera formation has been explained by magma withdrawal from a crustal reservoir, but little is known about the conditions that lead to the critical reservoir pressure for collapse. During an eruption, the reservoir pressure is constrained to lie within a finite range: it cannot exceed the threshold value for eruption, and cannot decrease below another threshold value such that feeder dykes get shut by the confining pressure, which stops the eruption. For caldera collapse to occur, the critical reservoir pressure for roof failure must therefore be within this operating range. We use an analytical elastic model to evaluate the changes of reservoir pressure that are required for failure of roof rocks above the reservoir with and without a volcanic edifice at Earth's surface. With no edifice at Earth's surface, faulting in the roof region can only occur in the initial phase of reservoir inflation and affects a very small part of the focal area. Such conditions do not allow caldera collapse. With a volcanic edifice, large tensile stresses develop in the roof region, whose magnitude increase as the reservoir deflates during an eruption. The edifice size must exceed a threshold value for failure of the roof region before the end of eruption. The largest tensile stresses are reached at Earth's surface, indicating that faulting starts there. Failure affects an area whose horizontal dimensions depend on edifice and chamber dimensions. For small and deep reservoirs, failure conditions cannot be achieved even if the edifice is very large. Quantitative predictions are consistent with observations on a number of volcanoes.

Large-scale failures on domes and stratocones situated on caldera ring faults: sand-box modeling of natural examples from Kamchatka, Russia

Edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs. Evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized. We studied morphologies of six calderas in Kamchatka, Russia, with diameters of 4 to 12 km. Edifices affected by caldera subsidence have residual heights of 250–800 m, and typical amphitheater-like depressions opening toward the calderas. The amphitheaters closely resemble horseshoe-shaped craters formed by large-scale flank failures of volcanoes with development of debris avalanches. Where caldera boundaries intersect such cones, the caldera margins have notable outward embayments. We therefore hypothesize that in the process of caldera formation, these eccentrically situated edifices were partly displaced and destabilized, causing large-scale landslides. The landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins. To test this hypothesis, we carried out sand-box analogue experiments, in which caldera formation (modeled by evacuation of a rubber balloon) was simulated. The deformation of volcanic cones was studied by placing sand-cones in the vicinity of the expected “caldera” rim. At the initial stage of the modeled subsidence, the propagating ring fault of the caldera bifurcates within the affected cone into two faults, the outermost of which is notably curved outward off the caldera center. The two faults dissect the cone into three parts: (1) a stable outer part, (2) a highly unstable and subsiding intracaldera part, and (3) a subsiding graben structure between parts (1) and (2). Further progression of the caldera subsidence is likely to cause failure of parts (2) and (3) with failed material sliding into the caldera basin and with formation of an amphitheater-like depression oriented toward the developing caldera. The mass of material which is liable to slide into the caldera basin, and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone. A cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone, having a small opening angle. Accordingly, the caldera exhibits a prominent outward embayment. By contrast, collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle, i.e. the embayment of the caldera rim is negligible. The relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera/cone interactions. Thus, our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large-scale failures with formation of indicative amphitheaters oriented toward the caldera basins. More generally, the scalloped appearance of boundaries of calderas in contact with pre-caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation.

Quartz and feldspar zoning in the eastern Erzgebirge volcano-plutonic complex (Germany, Czech Republic): evidence of multiple magma mixing

Zoned quartz and feldspar phenocrysts of the Upper Carboniferous eastern Erzgebirge volcano-plutonic complex were studied by cathodoluminescence and minor and trace element profiling. The results verify the suitability of quartz and feldspar phenocrysts as recorders of differentiation trends, magma mixing and recharge events, and suggest that much heterogeneity in plutonic systems may be overlooked on a whole-rock scale. Multiple resorption surfaces and zones, element concentration steps in zoned quartz (Ti) and feldspar phenocrysts (anorthite content, Ba, Sr), and plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the eastern Erzgebirge volcano-plutonic complex. Generally, feldspar appears to be sensitive to the physicochemical changes of the melt, whereas quartz phenocrysts are more stable and can survive a longer period of evolution and final effusion of silicic magmas. The regional distribution of mixing-compatible textures suggests that magma mingling and mixing was a major process in the evolution of these late-Variscan granites and associated volcanic rocks. Quartz phenocrysts from 14 magmatic phases of the eastern Erzgebirge volcano-plutonic complex provide information on the relative timing of different mixing processes, storage and recharge, allowing a model for the distribution of magma reservoirs in space and time. At least two levels of magma storage are envisioned: deep reservoirs between 24 and 17 km (the crystallisation level of quartz phenocrysts) and subvolcanic reservoirs between 13 and 6 km. Deflation of the shallow reservoirs during the extrusion of the Teplice rhyolites triggered the formation of the Altenberg-Teplice caldera above the eastern Erzgebirge volcano-plutonic complex. The deep magma reservoir of the Teplice rhyolite also has a genetic relationship to the younger mineralised A-type granites, as indicated by quartz phenocryst populations. The pre-caldera biotite granites and the rhyodacitic Schönfeld volcanic rocks represent temporally and spatially separate magma sources. However, the deep magma reservoir of both is assumed to have been at a depth of 24–17 km. The drastic chemical contrast between the pre-caldera Schönfeld (Westfalian B–C) and the syn-caldera Teplice (Westfalian C–D) volcanic rocks is related to the change from late-orogenic geotectonic environment to post-orogenic faulting, and is considered an important chronostratigraphic marker.

The Negra Muerta Volcanic Complex, southern Central Andes: geochemical characteristics and magmatic evolution of an episodically active volcanic centre

This petrologic analysis of the Negra Muerta Volcanic Complex (NMVC) contributes to understanding the magmatic evolution of eruptive centres associated with prominent NW-striking fault zones in the southern Central Andes. Specifically, the geochemical characteristics and magmatic evolution of the two eruptive episodes of this Complex are analysed. The first one occurred as an explosive eruption at 9 Ma and is represented by a strongly welded, fiamme-rich, andesitic to dacitic ignimbrite deposit. The second commenced with an eruption of a rhyolitic ignimbrite at 7.6 Ma followed by effusive discharge of hybrid lavas at 7.3 Ma and by emplacement of andesitic to rhyodacitic dykes and domes. Both explosive and effusive eruptions of the second episode occurred within a short time span, but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber. Our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber, pressurising the magmatic system and triggering explosive eruption of rhyolitic magma. Chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition, which could have been strong enough to inhibit the interaction between the two magmas. After the initial explosive stages of the eruption at 7.6 Ma, the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation. Restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber. At 7.3 Ma, increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera. Assimilation during turbulent ascent (ATA) is invoked to explain a conspicuous reversed isotopic signature ( 87Sr/ 86Sr and 143Nd/ 144Nd) in the entire volcanic series. Therefore, the 7.6 to 7.3 Ma volcanic rocks of the NMVC resulted from synchronous and mutually interacting petrological processes such as recharge, fractional crystallization, hybridisation, and Assimilation during Turbulent Ascent (ATA). Geochemical characteristics of both volcanic episodes show diverse type and/or depth in the sources and variable influence of upper crustal processes, and indicate a recurrence in the magma-forming conditions. Similarly, other minor volcanic centres in the transversal volcanic belts of the Central Andes repeated their geochemical signatures throughout the Miocene.

Petrology of Aoso volcano, northeast Japan arc: temporal variation of the magma feeding system and nature of low-K amphibole andesite in the Aoso-Osore volcanic zone

The Aoso volcano is a member of the newly defined volcanic front of Northeast Japan, characterized by the occurrence of low-K and hornblende andesites. Its activity can be divided into three stages: the early, caldera-forming, and late stages. While petrographic and geochemical data show all products underwent magma mixing or co-mingling, Sr and Nd isotopic ratios indicate that all are consanguineous. The end-member magmas are basaltic and andesitic in the early stage, but basaltic and dacitic in the late stage. In the caldera-forming stage, hornblende-free and hornblende-rich andesites co-mingled, which triggered an explosive eruption leading to caldera formation. Hornblende occurs also in the dacite from the early part of the late stage. These hornblende andesites and dacites are lower in magmatic temperature compared to hornblende-free andesites. The estimated basaltic end-member is low-K and high in magmatic temperature, and can be derived by a high degree of partial melting of mantle under the volcanic front. The estimated andesitic and dacitic end-members cannot be derived from the basaltic end-member magma through fractional crystallization, but can be derived from partial re-melting of the solidified low-K basalt, leaving amphibolitic and gabbroic residues, respectively.

The Elbrus Caldera in the Northern Caucasus: Geological structure and time of formation

Complex field work in combination with the detailed interpretation of aerial and satellite photographs allowed us to discovered for the first time a large collapsed Elbrus Caldera (17◊14 km along the edge of outer escarpment with a total area of 230 km 2 ). The Elbrus Volcano is located in its southern part. The mass ejecta of rhyodacitic and rhyolitic pyroclastics and the formation of the tu and ignimbrite coves are related to the caldera formation. Time of the caldera origin, according U-Pb (SHRIMP) dating on zircon from fiamme of ignimbrites is between 689±30-722±15 Ka. Based on comprehensive investigation of the rocks of the Elbrus volcanic center, including K-Ar, 39 Ar/ 40 Ar and EPR dating of rock-forming quartz, we identified in its evolution precaldera, caldera and postcaldera cycles. Two last cycles were subdivided into early and late stages.

Elbrus active Volcano and its geological history

Using the new geological map of the Elbrus Volcano (scale 1:50 000) and the new isotopic datings of the volcanic rocks, the formation history of Elbrus Volcano was found to include a pre-caldera, a caldera, and a post-caldera cycle, during which the modern Elbrus stratovolcano was formed. The Early and The Late period of the volcano's evolution have been identified in the two latter cycles. ignimbrite horizons, as well as breaks in volcanic activity, represented by moraine deposits resting on the surface of lava flows in the same volcanic rock sequence. This map shows that the formation history of the Elbrus Volcano included a pre-caldera cycle, a caldera cycle, the onset of which was marked by a powerful explosive eruption, responsible for the caldera formation, and a post-caldera cycle which resulted in the formation of the present-day Elbrus stratovolcano. It is important to note that each of the two latter cycles in- cluded The Early and The Late period of the evolution of the volcano. No pre-caldera volcanic rocks have been found confidently thus far. These are supposed to include the outliers of tra- chyandesite lava flows found in the mouth of the Khudes River (up to 200 m thick) and of trachybasalt lava flows (120-150 m thick) in the upper reaches of the Tyzyl River.

Calderas and caldera structures: a review

Calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation. Yet, it is only in the last 25 years that a thorough three-dimensional study of calderas has been carried out, utilising studies of eroded calderas, geophysical analysis of their structures and analogue modelling of caldera formation. As more data has become available on calderas, their individuality has become apparent. A distinction between ‘caldera’, ‘caldera complex’, ‘cauldron’, and ‘ring structure’ is necessary, and new definitions are given in this paper. Descriptions of calderas, based on dominant composition of eruptives (basaltic, peralkaline, andesitic–dacitic, rhyolitic) can be used, and characteristics of each broad group are given. Styles of eruption may be effusive or explosive, with the former dominant in basaltic calderas, and the latter dominant in andesitic–dacitic, rhyolitic and peralkaline calderas. Four ‘end-member’ collapse styles occur—plate or piston, piecemeal, trapdoor, and downsag—but many calderas have multiple styles. Features of so-called ‘funnel’ and ‘chaotic’ calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary. Ground deformation comprises subsidence or collapse (essential characteristics of a caldera) and uplifting/doming and fracturing due to tumescence and/or resurgence (frequent, but not essential). Collapse may occur on pre-existing structures, such as regional faults or on faults created during the caldera formation, and the shape of the collapse area will be influenced by depth, size and shape of the magma chamber. The final morphology of a caldera will depend on how the caldera floor breaks up; whether collapse takes place in one event or multiple events, whether vertical movement is spasmodic or continuous throughout the eruptive sequence, and whether blocks subside uniformly or chaotically at one or more collapse centres. A meaningful description of any caldera should therefore include; number of collapse events, presence or absence of resurgence, caldera-floor coherency, caldera-floor collapse geometry, and dominant composition of eruptives.

A major resurgent caldera in southern Mexico: the source of the late Eocene Tilzapotla ignimbrite

The Tilzapotla caldera constitutes the first discovery of a major Tertiary collapse volcanic structure south of the Mexican Volcanic Belt. Although it is spatially associated with silicic ignimbrites in a region relatively distant from the extensive ignimbritic province of the Sierra Madre Occidental (SMO), it is among the largest collapse calderas documented in Mexico. The caldera is defined by a 33×24 km semi-elliptical structure that encircles the largest exposures of the Tilzapotla ignimbrite and corresponds to the structural margin rather than the topographic rim. A central uplifted block limited by NW-trending faults is the main indication of a resurgent stage. The caldera structural margin is surrounded by extensive exposures of Cretaceous marine sequences that structurally define a broad elliptical dome (45×35 km) originated in the first stage of the caldera evolution. There is evidence showing that the 34 Ma Tilzapotla ignimbrite represents the climatic event of the caldera collapse. It is constituted by a massive sequence of crystal vitric tuff with conspicuous euhedral biotite and abundant quartz. The intra-caldera facies is intercalated with mega- and meso-breccias of limestone and anhydrite fragments derived from the slumping of the caldera wall during the caldera collapse. The overlying sequence includes post-collapse ignimbrites as well as amphibole and pyroxene bearing dacitic to andesitic lava flows. The age (33 to 32 Ma) and isotopic signatures of these lava flows indicate a resurgent event related with the input of more primitive magmas into the magma chamber. The rectilinear northeastern and southwestern segments of the structural margin of the caldera correspond to NW-trending tectonic lineaments that are part of a regional strike-slip system, active at the time of the caldera formation. We interpret that the NW tectonic structures defined zones of weakness that accommodated the caldera collapse in the northeastern and southwestern segments of the caldera structural margin.

Subsurface cyclic magma sill expansions in the 2000 Miyakejima volcano eruption: Possibility of two‐phase flow oscillation

Step‐like tilt changes were observed at intervals of dozens of hours during the caldera formation stage occurred during the 2000 Miyakejima volcano eruption. Our analysis suggests that these tilt steps were caused by the cyclic expansion of a subsurface sill‐like magma plumbing system trending from SE to NW of the volcano. In the same period a caldera gradually grew at the summit area to a very large 1.6 km in diameter and 450 m in depth. The source mechanism of these tilt steps is fairly well explained by a two‐phase flow instability, called a pressure drop oscillation. This model indicates that the gas‐phase capacity in the upper part of the conduit acts as a pump that controls the flux of two‐phase magma in the sill‐like reservoir. Caldera formation as well as magma uplift and descent in the conduit may reflect temporal changes in the intervals between tilt steps, which are themselves proportional to the volume of this gas capacity.

Geometrical and mechanical constraints on the formation of ring-fault calderas

Ash-flow, plate-subsidence (piston-like) calderas are bounded by a set of arcuated sub-vertical collapse faults named ring-faults. Experimental studies on caldera formation, performed mostly using spherical or cylindrical magma chamber geometries, find that the resulting ring-faults correspond to steeply outward dipping reverse faults, and show that pre-existing fractures developed during pre-eruptive phases of pressure increase may play a major role in controlling the final collapse mechanism, a situation that should be expected in small to medium sized ring-fault calderas developed on top of composite volcanoes or volcanic clusters. On the other hand, some numerical experiments indicate that large sill-like, elongated magma chambers may induce collapse due to roof bending without fault reactivation, as seems to occur in large plate-subsidence calderas formed independently of pre-existing volcanoes. Also, numerical experiments allow the formation of nearly vertical or steeply inward dipping normal ring-faults, in contrast with most of the analogue models. Using a thermoelastic model, we investigate the geometrical and mechanical conditions to form ring-fault calderas, in particular the largest ones, without needing a previous crust fracturing. Results are given in terms of two dimensionless geometrical parameters, namely λ and e. The former is the chamber extension to chamber depth ratio, whereas the latter stands for the chamber eccentricity. We propose that the ( λ, e) pair determinates two different types of ring-fault calderas with different associated collapse regimes. Ring-fault region A is related to large plate-subsidence calderas (i.e. Andean calderas or Western US calderas), for which few depressurisation is needed to set up a collapse initially governed by flexural bending of the chamber roof. In contrast, ring-fault region B is related to small to moderate sized calderas (i.e. composite volcano calderas), for which much depressurisation is needed. Our opinion is that collapse requires, in the latter case, reactivation of pre-existing fractures and it is therefore more complex and history dependent.

Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse

Research Article| May 01, 2004 Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse Kohei Kazahaya; Kohei Kazahaya 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Hiroshi Shinohara; Hiroshi Shinohara 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Kozo Uto; Kozo Uto 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Masanobu Odai; Masanobu Odai 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Yasuhiro Nakahori; Yasuhiro Nakahori 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Hirokazu Mori; Hirokazu Mori 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Hideki Iino; Hideki Iino 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Makoto Miyashita; Makoto Miyashita 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Junichi Hirabayashi Junichi Hirabayashi 3Volcano Fluid Research Centre, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-0033, Japan Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (5): 425–428. https://doi.org/10.1130/G20399.1 Article history received: 10 Dec 2003 rev-recd: 10 Jan 2004 accepted: 12 Jan 2004 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 Kohei Kazahaya, Hiroshi Shinohara, Kozo Uto, Masanobu Odai, Yasuhiro Nakahori, Hirokazu Mori, Hideki Iino, Makoto Miyashita, Junichi Hirabayashi; Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse. Geology 2004;; 32 (5): 425–428. doi: https://doi.org/10.1130/G20399.1 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 An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO2 emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO2 emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO2 emission rate from nonerupting volcanoes evaluated before this activity. The SO2 emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO2 emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Controls on caldera structure: Results from analogue sandbox modeling

We conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas. Dry sand contained in a 1-m-diameter cylinder served as a crustal rock analogue, and a water-filled 0.6-m-diameter rubber bladder served as an analogue magma chamber. Scaling parameters included a length ratio ( L *) of 2.5 × 10 –5 and a stress ratio (σ*) of 1.8–2.4 × 10 –5 . In contrast to some previous analogue models, the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled. Generally, deformation began with broad sagging, followed by an arcuate or linear outward-dipping fault that formed on one side of the caldera. This fault propagated laterally around the caldera in both directions, sometimes joining other faults, and typically forming an overall polygonal structure. As subsidence continued, the caldera grew incrementally outward and progressively formed a series of concentric outward-dipping faults. Lastly, a peripheral zone of extension and pronounced sagging, and commonly an inward- dipping outer fault related to extension, developed at the surface. As the depth of the chamber increased, (1) the area of faulting decreased, (2) the symmetry of the caldera was affected, and (3) the coherence of the subsiding block decreased. Tilting the chamber caused highly asymmetric subsidence to occur. In this case, faults formed first where the bladder was shallowest. Subsidence then shifted rapidly to where the bladder was deepest, producing an elongate trapdoor caldera that was deepest where the bladder was deepest. Our experiments highlight the roles of sagging and faulting during caldera subsidence. Surface fault patterns both in our experiments and at natural calderas are frequently not circular. The aspect ratio of the block above the magma chamber controls the shape of the caldera, which is frequently polygonal. The faults at natural calderas determine locations and migration of eruptive vents, the degree of subsidence, the style of postcal dera resurgent magmatism, and the extent of hydrothermal circulation. Our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas.

Workshop weighs climate change studies at Valles caldera

The 22‐km‐diameter Valles caldera is the world's type resurgent caldera and is host to a 280°C liquid‐dominated hydrothermal system [Smith and Bailey, 1968; Goffand Gardner, 1994]. The caldera also contains various lacustrine sediments and hydromagmatic deposits dating from the inception of caldera formation to roughly 50 ka. More recent bog deposits are also present. Many of these deposits are presumably buried within the caldera moat and overlain and interbedded with post‐caldera moat rhyolite eruptions. New geologic mapping at the 1:24,000 scale shows that the best exposures of lacustrine rocks occur on the uplifted flanks of the central resurgent dome [Goff et al., 2003] and as eroded remnants within the encircling valleys.