No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Evolution of the Quaternary silicic volcanic complex of Shiribetsu, Hokkaido, Japan: an example of ignimbrite shield volcanoes in an island arc setting
Abstract
This paper describes the geology and eruptive history of a Quaternary silicic volcanic complex at Shiribetsu, Hokkaido, Japan, with a focus on volcanic landforms produced by silicic explosive eruptions that were not associated with caldera collapse. The Shiribetsu volcanic complex comprises a dacitic pyroclastic plateau and an overlying dacitic central dome complex. The pyroclastic plateau is 22 × 25 km in size, < 200 m high, and consists of two ignimbrite units (the Km-1 and Km-2 deposits; SiO2 = 67–68 wt%; total volume ~ 10 km3). The central dome complex is 4–5 km wide, rises to 650 m above the pyroclastic plateau, and comprises two adjoining edifices, which are East and West Shiribetsu (SiO2 = 62–64 wt%; total volume = 1.6 km3). East Shiribetsu has a conical morphology, whereas West Shiribetsu comprises lava flows and a dome, and has a large amphitheater. A debris avalanche deposit (volume = 1 km3) extends from the amphitheater to the western foot of West Shiribetsu. A dacitic lava dome (volume = 0.2 km3) occurs within the amphitheater. According to the stratigraphic relationships and new fission-track dating, the Shiribetsu volcanic complex evolved as follows: (1) dacitic explosive eruptions at ca. 130 ka (Km-2) and 70 ka (Km-1) produced the pyroclastic plateau; (2) effusive dacitic eruptions at 70 ka (East Shiribetsu) and 60 ka (West Shiribetsu) produced the central dome complex; (3) sector collapse occurred at West Shiribetsu at 50–60 ka; and (4) lava dome formation within the amphitheater at 50 ka. The evolution of the Shiribetsu volcanic complex resembles that of ignimbrite shield volcanoes in the central Andes. Shiribetsu may be the first identified example of ignimbrite shield volcanoes in an island arc setting. We suggest that these are an independent class of explosive silicic volcanoes, distinct from calderas, and form above small magma chambers.
... Km-2 and Km-1 tephras (Kimobetsu ignimbrite 2 and 1) were derived from the Shiribetsu-dake volcanic complex Goto et al., 2020). Km-2 and Km-1 deposits around the Shiribetsudake complex are 10-20 m thick and have estimated bulk volumes of 7.0 and 3.4 km 3 , respectively. ...
... Km-2 and Km-1 deposits around the Shiribetsudake complex are 10-20 m thick and have estimated bulk volumes of 7.0 and 3.4 km 3 , respectively. Both Km-2 and Km-1 comprise a crystal rich rhyodacite (bulk SiO 2 = 67-68 wt%) containing approximately 1-3 mm long phenocrysts of amphibole and quartz (Goto et al., 2020). On the basis of stratigraphic relationships, the inferred ages of Km-1 and Km-2 are 70-80 and 110-130 ka, respectively (Amma-Miyasaka et al., 2020). ...
... On the basis of stratigraphic relationships, the inferred ages of Km-1 and Km-2 are 70-80 and 110-130 ka, respectively (Amma-Miyasaka et al., 2020). Zircon fission track ages of 70 ± 20 ka and 130 ± 30 ka for units Km-1 and Km-2, respectively, coincide with the ages determined from stratigraphy (Goto et al., 2020). The Km-2 and Km-1 tephras are separated by the Toya tephra, which is an important stratigraphic marker horizon in Japan (e.g., Machida, 1999). ...
Zircon double dating utilises (UTh)/He dating coupled with UTh disequilibrium or UPb dating to determine eruption ages for volcanic rocks between ca. 2 ka to 1 Ma. This approach depends on understanding the crystallisation history of each zircon crystal analysed. For lack of better constraints, zircon crystallisation is generally assumed to be represented by a single crystallisation age, which is routinely determined on the rim of the grain by spot analyses. While zircon crystallisation is often protracted, interrogating the crystallisation history of a zircon crystal usually requires grinding the grain, which was previously assumed to introduce uncertainty on the alpha ejection (Ft) correction critical for accurate (UTh)/He ages. This study shows that grinding a zircon crystal to exactly 50% of its original width, to a plane of symmetry, leaves the Ft correction factor unchanged to that of the whole crystal. This result is verified by a new computer program – GriFt, which also allows the calculation of accurate Ft correction factors for a range of different grinding depths. The grinding to 50% without altering the Ft correction thus opens up the opportunity to measure both the core and rim crystallisation ages and integrate these into a more robust disequilibrium correction of (UTh)/He data.
The feasibility of this approach is tested here in a case study of zircon crystals with protracted crystallisation histories from the Shikotsu-Toya volcanic field in Hokkaido, Japan. A maximum of 15% difference in overall eruption age is calculated between rim- and core-corrected (UTh)/He ages. Eruption ages were also determined for two tephra – Kimobetsu 1 (59–79 ka) and Kimobetsu 2 (96 ± 5 ka). The geological implication from these dates is that a regionally important tephra, Toya, may be younger (<96 ± 5 ka) than previously reported (109 ± 3 ka). In addition, the maximum eruption ages determined from crystallisation age distributions calculated for samples from eruptions at Shikotsu and Kuttara (48 ± 17 and 49 ± 21 ka, respectively) are within uncertainty of previous measurements (44–41 ka and >43 ka, respectively).
... The four papers belonging to this Special Issue present an updated general overview of the progress in volcanic slope instability analysis, monitoring, and modelling from multi-disciplinary efforts, from slope to edifice and regional scale. The topics discussed range from the geological/ volcanological evolution of a Quaternary silicic volcanic complex (Shiribetsu) in Japan (Goto et al. 2020), the analysis of two very famous case studies for tsunamigenic landslides at Stromboli (Italy; Casalbore et al. 2020) and Ritter island (Papua Nuova Guinea, Karstens et al. 2020), to studies of grain-size distribution and sedimentological features of volcaniclastic mass flows (Makris et al. 2020). ...
It is essential to establish the timing of past sector collapse events at a volcanic edifice to evaluate not only the evolution of the volcanic system but also potential volcanic hazards. This can be done by determining the age of the collapse-generated debris avalanche deposits. However, without evidence of an associated magmatic eruption, it is impossible to recognize juvenile materials in these deposits. Thus, it is usually difficult to determine the precise age of sector collapse. Usu is a post-caldera volcano of the Toya Caldera (Hokkaido, Japan) and has been constructed since ca. 19–18 ka on top of the caldera-forming Toya pyroclastic flow deposits (Tpfl deposit: 106 ka). A sector collapse occurred after the formation of a stratovolcano edifice and produced the Zenkoji debris avalanche (ZDA) deposit with reported ages ranging from >20 to 6 ka. We investigated the tephrostratigraphy preserved in the soil above the ZDA deposit and in the surrounding area and recognized fine ash fall deposits at two locations, above and east of the ZDA deposit. The glass shards within these deposits were correlated with several tephra layers with the majority being derived from Tpfl deposits. Thus, these ash deposits should be considered reworked tephra. A considerable number of hummocks in the ZDA deposit were also composed of deformed and fragmented Tpfl deposits, which suggests that the ZDA bulldozed and partially incorporated the Tpfl deposit on the flank of the volcano. Deformation and fragmentation of the non-welded soft silicic Tpfl deposit during the transport and emplacement of the ZDA produced an accompanying ash cloud, which deposited the observed glassy, fine, ash fall units. Radiocarbon dating of soil samples directly below and above the reworked ash deposits allowed dating the sector collapse to ca. 8 ka. This age is much younger than previously proposed results. Based on our findings, the transport and emplacement mechanism of the sector collapse should be revised. Our study shows that reworked ash layers caused by the flow of a debris avalanche can be used as an indicator of the timing of a sector collapse of the volcano.
Cryptodomes are shallow-level intrusions that cause updoming of overlying sediments or other rocks. Understanding the formation of cryptodomes is important for hazard assessment, as cryptodome-forming eruptions are one of the major triggering factors in sector collapse. This paper describes internal structures of a Quaternary subaerial rhyodacite cryptodome at Ogariyama, Usu volcano, Japan (the Ogariyama dome), and examines the textural differences between subaerial and subaqueous cryptodomes to extend our knowledge of these phenomenon. The Ogariyama dome, which is one of the youngest subaerial cryptodomes in the world (<0.4 ka), can be viewed in cross-section because a vertical fault formed during the 1977–1978 eruption and cut through the center of the cryptodome, exposing its interior. The morphology of the cryptodome is scalene triangular in shape, with rounded corners in cross-section, and it is 150 m across and 80 m high. The internal structure of the dome is concentrically zoned, with a massive core, jointed rim, and brecciated border, all of which are composed of uniform, feldspar-phyric rhyodacite (SiO2 = 71–72 wt.%). The massive core (130 m across) consists of coherent rhyodacite that has indistinct, large-scale flow banding and rectangular joints that are spaced 50–200 cm apart. The jointed rim (8–12 m wide) surrounds the massive core and consists of coherent rhyodacite that is characterized by distinct rectangular joints that are 30–80 cm apart and radiate outward. The outermost brecciated border (7–10 m wide) comprises monolithological breccia, consisting of angular rhyodacite clasts (5–30 cm across) and a cogenetic matrix. These internal structures suggest that the Ogariyama dome was formed by endogenous growth, involving continuous magma supply during a single intrusive event and simple expansion from its interior. The massive core formed by slow cooling of homogeneous rhyodacite magma. The jointed rim formed by fracturing of solidifying rhyodacite magma in response to cooling–contraction and dynamic stress driven by continued movement of the less viscous core. The brecciated border formed by fragmentation of the solidified rim of the dome in response to dynamic stress. The growth style of the Ogariyama dome closely resembles that of subaqueous cryptodomes. However, the morphology and internal structures of the Ogariyama dome differ from those of subaqueous cryptodomes, given its asymmetric morphology and absence of radial columnar joints and large-scale flow banding. These differences might reflect the well-consolidated and inhomogeneous physical properties of the host sediment and the slow cooling rate and high viscosity of the Ogariyama dome. The Ogariyama dome is probably the best cross-sectional example of a subaerial cryptodome in the world. Our descriptive study of the cryptodome provides invaluable information for hazard assessment.
The generation and evolution of basaltic magmas at Usu volcano, located at the junction between the NE Japan arc and the Kuril arc, have been investigated. The mafic products, which form the somma edifice of the volcano, consist of basalt (49·6–51·3 wt % SiO2) and basaltic andesite (52·0–54·9 wt % SiO2) lavas. The basaltic lavas show relatively tight compositional trends, and 87Sr/86Sr ratios tend to decrease with increasing whole-rock SiO2 content. The water content of the basaltic magmas was determined to be ∼4·8 wt % based on plagioclase–melt thermodynamic equilibrium. Using this information and an olivine maximum fractionation model, the water content of the primary Usu magma was estimated to be 3·9 wt %. Multi-component thermodynamic calculations suggest that the primary magma was generated by ∼23% melting of the source mantle with ∼0·94 wt % H2O at ∼1300°C and ∼1·4 GPa. The 0·94 wt % water content of the source mantle is significantly higher than that beneath volcanoes in the main NE Japan arc (generally <0·7 wt % H2O); this implies that the wedge mantle at the arc–arc junction is intensively hydrated. The temperature of the wedge mantle of ∼1300°C at ∼1·4 GPa is also significantly higher than that of the mantle in the main NE Japan arc. Unlike the basaltic lavas, the whole-rock compositions of the basaltic andesite lavas are scattered in Harker variation diagrams. This observation suggests that the compositional diversity was produced by at least two independent processes. To elucidate the processes responsible for this compositional diversity, principal component analysis was applied to the major element compositions of the samples. This suggests that 47% of the diversity of the whole-rock compositions can be explained by mixing with partial melts of lower crustal materials, 25% is explained by redistribution of plagioclase phenocrysts, and 16% is explained by fractionation of accessory minerals.
The term collapse caldera refers to those volcanic depressions resulting from the sinking of the chamber roof due to the rapid withdrawal of magma during the course of an eruption. During the last three decades, collapse caldera dynamics has been the focus of attention of numerous, theoretical, numerical, and experimental studies. Nonetheless, even if there is a tendency to go for a general and comprehensive caldera dynamics model, some key aspects remain unclear, controversial or completely unsolved. This is the case of ring fault nucleation points and propagation and dip direction. Since direct information on calderas' deeper structure comes mainly from partially eroded calderas or few witnessed collapses, ring faults layout at depth remains still uncertain. This has generated a strong debate over the detailed internal fault and fracture configuration of a caldera collapse and, in more detail, how ring faults initiate and propagate. We offer here a very short description of the main results obtained by those analog and theoretical/mathematical models applied to the study of collapse caldera formation. We place special attention on those observations related to the nucleation and propagation of the collapse-controlling ring faults. This summary is relevant to understand the current state-of-the-art of this topic and it should be taken under consideration in future works dealing with collapse caldera dynamics.
Scandone, R., 1990. Chaotic collapse of calderas. J. Volcanol. Geotherm. Res., 42:285-302 A study of the formation of Krakatoan-type calderas indicates that they probably form by chaotic col-lapse. The energy required to form these calderas by explosive decapitation of the volcano is seldom available during caldera-forming eruptions. Gravity anomalies resulting from different mechanisms of collapse show that chaotic-collapse calderas are characterized by a negative residual Bouguer anomaly with a gentle gradient toward the center of the caldera. Calderas of the Valles type have steeper gradients near of the caldera edges. The residual anomaly derives from the low-density shattered rocks that make up the chimney above the collapsed magma-chamber. The chimney flares toward the top giving the substructure of the caldera a funnel-shaped form. During eruptions forming calderas of the chaotic-collapse type, the eruption and emplacement of co-ignimbrite breccia is indicative of explosions due to a pressure decrease within the magma chamber. The pressure decrease is due both to drainage of magma and to collapse of the roof of the chamber both of which cause a decrease of the lithostatic load on the magma chamber. In some cases water can gain access to the magma chamber through collapse of entire sections of the hydrothermal system of the volcano with consequent explosive vaporization within the chamber itself.
The Lípez region of southwest Bolivia is the locus of a major Neogene ignimbrite flare-up, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laser-fusion 40Ar/39Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbrite-producing eruptions during the ~10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km3) of magma from sources distributed throughout the volcanic complex. The first major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa, 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km3 of magma. Pulse 3, the largest, produced at least 3100 km3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more volcanic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km3 dense rock equivalent [DRE]) and 2.89 Ma Pastos Grandes (1500 km3 DRE) ignimbrites. In addition to these large calderarelated eruptions, new age determinations refine the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40Ar/39Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.
Compositional data for >400 pumice clasts, organized according to eruptive sequence, crystal content, and texture, provide
new perspectives on eruption and pre-eruptive evolution of the >600 km3 of zoned rhyolitic magma ejected as the Bishop Tuff during formation of Long Valley caldera. Proportions and compositions
of different pumice types are given for each ignimbrite package and for the intercalated plinian pumice-fall layers that erupted
synchronously. Although withdrawal of the zoned magma was less systematic than previously realized, the overall sequence displays
trends toward greater proportions of less evolved pumice, more crystals (0·5–24 wt %), and higher FeTi-oxide temperatures
(714–818°C). No significant hiatus took place during the 6 day eruption of the Bishop Tuff, nearly all of which issued from
an integrated, zoned, unitary reservoir. Shortly before eruption, however, the zoned melt-dominant portion of the chamber
was invaded by batches of disparate lower-silica rhyolite magma, poorer in crystals than most of the resident magma but slightly
hotter and richer in Ba, Sr, and Ti. Interaction with resident magma at the deepest levels tapped promoted growth of Ti-rich
rims on quartz, Ba-rich rims on sanidine, and entrapment of near-rim melt inclusions relatively enriched in Ba and CO2. Varied amounts of mingling, even in higher parts of the chamber, led to the dark gray and swirly crystal-poor pumices sparsely
present in all ash-flow packages. As shown by FeTi-oxide geothermometry, the zoned rhyolitic chamber was hottest where crystal-richest,
rendering any model of solidification fronts at the walls or roof unlikely. The main compositional gradient (75–195 ppm Rb;
0·8–2·2 ppm Ta; 71–154 ppm Zr; 0·40–1·73% FeO*) existed in the melt, prior to crystallization of the phenocryst suite observed,
which included zircon as much as 100 kyr older than the eruption. The compositions of crystals, though themselves largely
unzoned, generally reflect magma temperature and the bulk compositional gradient, implying both that few crystals settled
or were transported far and that the observed crystals contributed little to establishing that gradient. Upward increases
in aqueous gas and dissolved water, combined with the adiabatic gradient (for the ∼ 5 km depth range tapped) and the roofward
decline in liquidus temperature of the zoned melt, prevented significant crystallization against the roof, consistent with
dominance of crystal-poor magma early in the eruption and lack of any roof-rind fragments among the Bishop ejecta, before
or after onset of caldera collapse. A model of secular incremental zoning is advanced wherein numerous batches of crystal-poor
melt were released from a mush zone (many kilometers thick) that floored the accumulating rhyolitic melt-rich body. Each batch
rose to its own appropriate level in the melt-buoyancy gradient, which was self-sustaining against wholesale convective re-homogenization,
while the thick mush zone below buffered it against disruption by the deeper (non-rhyolitic) recharge that augmented the mush
zone and thermally sustained the whole magma chamber. Crystal–melt fractionation was the dominant zoning process, but it took
place not principally in the shallow melt-rich body but mostly in the pluton-scale mush zone before and during batchwise melt
extraction.
Structures at calderas may form as a result of precursory tumescence, subsidence due withdrawal of magmatic support, resurgence, and regional tectonism. Structural reactivation and overprinting are common. To explore which types of structures may derive directly from subsidence without other factors, evidence is reviewed from pits caused by the melting of buried ice blocks, mining subsidence, scaled subsidence models, and from over 50 calderas. This review suggests that complex patterns of peripheral deformation, with multiple ring and arcuate fractures both inside and outside caldera rims, topographic embayments, arcuate graben, and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history. Downsag is a feature of many calderas and it does not indicate subsidence on an inward-dipping ring fault, as has been inferred previously. Where magmatic inflation is absent or slight, initial arcuate faults formed during collapse are likely to be multiple, and dip outwards to vertical. Associated downsag causes the peripheries of calderas undergo radial (centripetal) extension, and this accounts for some of the complex peripheral fractures, arcuate crevasses, graben, and some topographic moats. The structural boundary of a caldera, defined here as the outermost limits of subsidence and related deformation including downsag, commonly lies outside ring faults and outside the embayed topographic wall. It is likely to be funnel-shaped, i.e. inward-dipping, even though ring and arcuate fractures within it may dip outward. Inward-dipping arcuate normal faults at shallow levels and steep inward-dipping contacts between a caldera's fill and walls may both occur at a caldera that has initially subsided on outward-dipping ring faults. They arise due to peripheral surficial extension, gravitational spreading and scarp collapse. Topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag, respectively. These processes may occur in addition to surficial degradation of oversteep ring-fault scarps.
A collapsed caldera, 1.6 km in diameter and 450 m in depth, was formed at the summit of Miyakejima Volcano during the 2000 eruption. The collapsed caldera appeared on 8 July, with a minor phreatic eruption, 12 days after seismic activity and magma intrusion occurred northwest of the volcano. Growth of the caldera took from 8 July to the middle of August, with seismic swarms associated with the continuous intrusion of magma northwest of the volcano. The growth rate of the caldera was about 1.4쎻 m3/day, and the final volume of the collapsed caldera was about 6쎼 m3. Major phreatomagmatic eruptions produced a total of about 1.6ꔒ kg (1.1쎻 m3) of volcanic ash after caldera growth. The caldera structure, and the nature of the eruptive materials of the first collapse on 8 July, suggest that the surface subsidence was caused by the upward migration of a steam-filled cavity, with stoping of the roof rock above the magma reservoir. The diameter of the stoping column was estimated to be 600-700 m from circumferential faults that developed in the caldera floor, and the collapse of the caldera wall enlarged the diameter of the caldera to 1.6 km. The total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1.6-2.1 km.
A model is presented accounting for many features of the Altiplano-Puna volcanic complex situated in the Central Volcanic Zone of the Andes which contains 50 recently active volcanoes. The dominant elements of the complex are several large nested caldera complexes which are the source structures for the major regionally distributed ignimbrite sheets that characterize the complex. The study of the complex reveals the importance of the intersection of subsidiary axis-oblique tectonic trends related to regional stress fields peculiar to individual oceanic ridge sections with the axis-parallel trends predominant at all spreading centers in localizing hydrothermal discharge zones.
The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced ~1 km³ of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted ~22.5 km³ of magma in the form of a composite coulee. Phase 3 produced a 6-km-long, 3-km-wide flow that emanated from a collapsed dome. Chao's anomalous size is a function of both the relatively steep local slope (20° to 3°) and the available volume of magma. The eruption duration for Chao's empalcement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 m³ s⁻¹ for short periods. Four other lavas in the vicinity with volumes of ~5 km³ closely resemble Chao and are probably comagmatic. -from Authors
We present a new LA–ICP–MS system for zircon fission‐track (FT) and U–Pb double dating, whereby a femtosecond laser combined with galvanometric optics simultaneously ablates multiple spots to measure average surface U contents. The U contents of zircon measured by LA–ICP–MS and standardized with the NIST SRM610 glass are comparable to those measured by the induced fission track method, and have smaller analytical errors. LA–ICP–MS FT dating of seven zircon samples including three IUGS age standards is as accurate as the external detector method, but can give a higher‐precision age depending on the counting statistics of the U content measurement. Double dating of the IUGS age standards gives FT and U–Pb ages that are in agreement. A chip of the Nancy 91,500 zircon has a homogeneous U content of 84 ppm, suggesting the possibility of using this zircon as a matrix‐matched U standard for FT dating. When using the Nancy 91,500 zircon as a U standard, a zeta calibration value of 42–43 yr cm−2 for LA–ICP–MS FT dating is obtained. While this value is strictly only valid for the particular session, it can serve as a reference for other studies.
The Shikotsu-Toya volcanic field (STVF) in southwestern (SW) Hokkaido (Japan) comprises three caldera volcanoes (Toya, Kuttara, and Shikotsu calderas and their post-caldera volcanoes) and Yotei and Shiribetsu stratovolcanoes. The stratigraphy, chronology, petrography, and glass geochemistry of tephra layers in the Ishikari pyroclastic terrain are carefully examined and reassessed. On the basis of the presence of volcaniclastic soil horizons and common petrological features, individual eruptive episodes are defined and the source volcanoes of each tephra layer are identified by correlation with proximal deposits. 28 eruptive episodes are distinguished, including six that are newly discovered. Based on K-Ar ages of older effusive lavas around the STVF, effusive andesitic volcanism, which was active since the Middle Miocene in SW Hokkaido, would have converged in the STVF by 0.9–0.5 Ma. After a dormant period of 400 kyr, explosive eruptions began at Toya 122.5 ka, and at Shiribetsu before 120–115 ka. A catastrophic caldera-forming eruption ca. 106 ka resulted in the formation of Toya caldera. Explosive activity then migrated eastward, and Kuttara and Shikotsu volcanoes began erupting explosively before 90 ka and 85 ka, respectively. Subsequently, the large stratovolcano Yotei was constructed after ca. 75 ka in the back-arc side of the STVF. Following repeated explosive eruptions of these five volcanoes, the largest caldera-forming eruption occurred ca. 43.8 ka at Shikotsu volcano. The STVF has produced VEI* (volcanic explosivity index) 7 eruptions every 20–40 kyr since explosive eruptions began, and the total volume of magma erupted in the STVF is estimated to be ca. 700 km³ DRE (dense rock equivalent). These findings suggest that the STVF is one of the youngest ignimbrite flare-up suites in Japan, and that its eruption rate is up to ca. 5.6 km³ DRE/kyr since 125 ka.
Santorini Volcano is an outstanding natural laboratory for studying arc volcanism, having had twelve Plinian eruptions over the last 350,000 years, at least four of which caused caldera collapse. Periods between Plinian eruptions are characterized by intra-caldera edifice construction and lower intensity explosive activity. The Plinian eruptions are fed from magma reservoirs at 4–8 km depth that are assembled over several centuries prior to eruption by the arrival of high-flux magma pulses from deeper in the sub-caldera reservoir. Unrest in 2011–2012 involved intrusion of two magma pulses at about 4 km depth, suggesting that the behaviour of the modern-day volcano is similar to the behaviour of the volcano prior to Plinian eruptions. Emerging understanding of Santorini's plumbing system will enable better risk mitigation at this highly hazardous volcano.
Catastrophic sector collapse is a destructive volcanic process that occurs during the growth history of a volcano. Understanding the timing and causal mechanisms of sector collapse at individual volcanoes is essential for evaluating long-term volcano evolution and associated hazards. This paper describes the lithofacies, tephrostratigraphy, and radiocarbon dating of a debris avalanche deposit at Usu volcano, Japan, based on detailed field geological surveys, and proposes that uplift of soft substratum played an important role in destabilizing the young volcanic edifice. Usu is a Quaternary basaltic to andesitic stratovolcano that has an amphitheater at the summit. The basement of the volcano has been uplifted by 300–400 m relative to the surrounding area. The debris avalanche deposit is > 0.3 km³ in volume and displays well-preserved hummocky topography. The hummocks are composed of andesite blocks derived from the stratovolcano, and non-welded pyroclastic flow and fluvial deposits derived from the basement of the stratovolcano. Tephrostratigraphy and radiocarbon dating of charcoal samples from the debris avalanche deposit indicate that the sector collapse occurred at ~ 16 ka. The age of sector collapse, combined with previous geochronological data, enables reconstruction of the eruptive history of Usu as follows: (1) andesitic explosive eruption at 18–19 ka, (2) stratovolcano growth between 18 and 16 ka, (3) sector collapse at ~ 16 ka, (4) hiatus in volcanic activity for ~15,000 years, (5) rhyolitic Plinian eruption in AD 1663, and (6) dacitic dome-forming eruptions after AD 1663. This eruptive history indicates that the sector collapse occurred within ~3000 years of the onset of stratovolcano growth, much sooner than at other volcanoes where the timing of sector collapse is known. We infer that the sector collapse of such a young volcanic edifice was caused by a combination of dome-like uplifting/tilting (forced folding) of soft substratum, overloading of the volcanic edifice on the soft substratum, a NE–SW-trending fault system that deeply cuts the substratum, and possible magma intrusion within the edifice and associated earthquakes. Usu provides an excellent example of edifice instability that is characterized by uplift of soft substratum. Our new view of causal mechanisms of the sector collapse provides invaluable information for hazard assessment of Quaternary volcanoes.
A controlled-source audio-frequency magnetotelluric (CSAMT) survey was conducted over Toya caldera, Hokkaido, Japan, to investigate its subsurface structure. The caldera is 10-11 km in diameter and contains a freshwater lake, Lake Toya, that occupies the entire caldera floor. A post-caldera dacitic dome complex, the Nakajima Islands, is present within Lake Toya. The CSAMT survey was carried out along a 16-km-long line that crosses Toya caldera in a NE–SW direction, passing over the Nakajima Islands. The 17 receiver stations (7 stations located outside of Lake Toya, 5 stations within Lake Toya, and 5 stations on the Nakajima Islands) were distributed along the survey line. Unique on-boat measurements were performed at the stations on Lake Toya. Two-dimensional inversion of the CSAMT data revealed the resistivity structure for the upper 1500 m beneath the caldera. The resistivity structure indicates the existences of high-resistivity (> 100 Ω·m) and low-resistivity (< 30 Ω·m) domains at the northeastern and southwestern sides of Lake Toya, respectively, a medium-resistivity (30-50 Ω·m) domain beneath Lake Toya, a high-resistivity (> 100 Ω·m) layer at the Nakajima Islands, and a low-resistivity (< 10 Ω·m) domain beneath the Nakajima Islands. This resistivity structure, combined with geological and bathymetric data, suggests that the subsurface structure of Toya caldera comprises altered Tertiary to Quaternary volcanic/sedimentary rocks outside of the caldera, and a homogeneous caldera-fill deposit beneath the caldera floor. A 9-km-diameter ring fault may occur along the caldera rim. There is a conspicuous hydrothermal alteration zone beneath the Nakajima Islands that may have formed in response to heating of the caldera-fill deposit by the underlying magma during the volcanic activity that formed the Nakajima Islands.
To understand the eruptive history, structure, and magmatic evolution of Yotei Volcano, southwest Hokkaido, Japan, we investigated the geology and petrology of tephras located around the base of the volcano. We identified 43 tephra units interbedded with soils (in descending stratigraphic order, tephras Y1–Y43), and four widespread regional tephras. Ten radiocarbon ages were obtained from soils beneath the Yotei tephras. On the basis of petrologic differences and, the stratigraphic positions of thick layers of volcanic ash soil, indicative of volcanic stratigraphic gaps, the Yotei tephras are divided into four groups (in ascending stratigraphic order): Yotei tephra groups I, II-1, II-2, and II-3. We calculated the age of each eruptive deposit based on the soil accumulation rate, and estimated the volume of each eruption using isopach maps or the correlation between eruption volume and the maximum thickness at ~ 10 km from the summit crater. The results regarding eruptive activity and the rate of explosive eruptions indicate four eruptive stages at Yotei Volcano over the last 50,000 years. Stage I eruptions produced Yotei tephra group I between ca. 54 cal. ka BP and up to at least ca. 46 cal. ka BP, at relatively high average eruption rates of 0.07 km3 dense-rock equivalent (DRE)/ky. After a pause in activity of ca. 8000 years, Stage II-1 to II-2 eruptions produced Yotei tephra groups II-1 and II-2 from ca. 38 to ca. 21 cal. ka BP at high average eruption rates (0.10 km3 DRE/ky), after a pause in activity of 2000–3000 years. Finally, after another pause in activity of 4000–5000 years, Stage II-3 eruptions produced Yotei tephra group II-3 from ca. 16.5 cal. ka BP until the present day, at low average eruption rates (0.009 km3 DRE/ky). Whole-rock geochemical compositions vary within each tephra group over the entire eruption history. For example, group I and II-3 tephras contain the lowest and highest abundances, respectively, of K2O, P2O5, and Zr. Group II-1 has the highest abundances of Zr and Y. These trends indicate that the explosive activity was controlled by an evolving magma system. An understanding of these temporal changes in the chemical composition of the magma will enable future correlations of tephras with volcanic edifices, thereby revealing the full eruptive history and structure of Yotei Volcano, and constraining the timing of geological events in the region.
Four stages of stress-field change during the late Cenozoic time in northern part of southwest Hokkaido are discriminated using alignment of craters. The sequential variation in σ1 of the stage 4, N55°W (4.2-4. 0 Ma) →N70°W (2.4-2.0 Ma and 1.5 Ma)→ N33°W (1.2-0Ma) → N25°-15°W (ca. 30 000 year BP) resembles that of the Hawaiian chain. This suggests that the stress field of the area has been controlled by the movement of the Pacific plate since the Pliocene. -from English summary
Resurgent cauldrons are defined as cauldrons (calderas) in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome. Seven of the best known resurgent cauldrons are: Valles, Toba, Creede, San Juan, Silverton, Lake City, and Timber Mountain. Geologic summaries of these and Long Valley, California, a probable resurgent caldera, are presented. Using the Valles caldera as a model, but augmented by information from other cauldrons, seven stages of volcanic, structural, sedimentary, and plutonic events are recognized in the development of resurgent cauldrons. They are: (I) Regional tumescence and generation of ring fractures; (II) Calderaforming eruptions; (III) Caldera collapse; (IV) Preresurgence volcanism and sedimentation; (V) Resurgent doming; (VI) Major ring-fracture volcanism; (VII) Terminal solfatara and hot-spring activity. These stages define the terminal cycle of resurgent cauldrons, which in the Valles caldera spanned more than 1 million years. The known and inferred occurrence of the seven stages in the eight cauldrons discussed, together with some time control in four cauldrons, indicates that resurgent doming is early in the postcollapse history; hence, it seems part of a pattern and not fortuitous. Doming of the cauldron block by magma pressure is preferred to doming by stock or laccolithic intrusion, although these processes may be subsidiary. Magma rise that produces doming may be explained in several ways, but the principal cause is not known. Nor is it known why some otherwise similar calderas do not have resurgent domes, although size and thickness of the cauldron block and the degree to which it was deformed during caldera collapse may be factors. All known resurgent structures are larger than 8 miles in diameter and are associated with silicic and, presumably, high-viscosity magmas. Genetically, resurgent cauldrons belong to a cauldron group in which subsidence of a central mass takes place along ring fractures and is related to eruption of voluminous ash flows, thereby differing from Kilauean-type calderas. It is proposed that typical Krakatoan-type calderas differ in that collapse is chaotic and ring fractures are not essential to their formation. Krakatoan calderas typically occur in the andesitic volcanoes of island arcs or the eugeosynclinal environment, and their sub-volcanic analogues are not known, whereas resurgent and related Glen Coe-type cauldrons are more common in cratonic or post-orogenic environments as are their sub-volcanic analogues - granitic ring complexes. Granitic ring complexes, such as Lirue, Sande, Ossipee, and Alnsj0, are probably the closest sub-volcanic analogues of resurgent calderas. The source areas of most of the ash-flow sheets of western United States and Mexico are yet to be found. It is suggested that many of them will prove to be resurgent structures. Present evidence suggests that ore deposits are more commonly associated with resurgent cauldrons than with other cauldron types.
Cerro Purico is a 1.3 m.y.-old ignimbrite shield volcano. The first erupted unit was the rhyolitic Toconao ignimbrite (87Sr/86Sr 0.7100), followed, after an erosional interval, by the dacitic Purico ignimbrite with the same radiometric age (87Sr/86Sr 0.7085). Andesitic to dacitic lavas and extrusive domes were later emplaced in the summit region. The upper part of the Purico ignimbrite contains banded and mafic (andesitic) pumices isotopically indistinguishable from the hosts. The younger dome (Chascon) contains basaltic xenoliths characterized by Sr isotope ratios (0.7059) lower than the host dacite (0.7073). The rocks of the Cerro Purico complex as a whole are characterized as low TiO2, high K, calc-alkaline types with major and trace-element contents closely similar to other Central Andean volcanoes. A complex magmatic history is required to explain the origin of mafic pumice clasts and isotopically contrasted mafic xenoliths. Simple crustal anatexis cannot account for the range of compositions observed.-J.M.H.
The Rusutsu plateau covered with volcanic ash forms table-lands of 300-400m in elevation, being bounded by Mts. Yotei and Shiribetsu in directions of N. W. and S. E. In this area, no underwater and any river were found and men have never lived. After the 2nd World War, the government researched the underground water for a purpose of reclamation. Numbers of wells were excavated to 20-40m in depth through volcanic ash stratum to comply with the result of research. But before long, the wells were dried out due to the underground water-level being lowered below the bottom of wells. The reason was that the wells had not been reached to the impermeable layer. The writer engaged in the research works in this area since 1961 and concluded the followings. (1) No underground water exists in volcanic ash but it flows in lava under the ash stratum. (2) Test borings were made in 1963 after geological investigation. Next year, to follow the result, a deep well was excavated to 100m at the same site, obtaining more than the water of 400m³/day to fall down 10 cm of the waterlevel. This means that in case where the water-level falling down to 50cm, 1, 800m³/day of the water can be resulted. © 1965, The Japanese Society of Limnology. All rights reserved.
The authors have developed and demonstrated an independent calibration of zircon fission track dating with the external detector method. During the past two decades, investigations have continued concerning the issues of the absolute age calibration of fission track dating. The derivation of accurate fission track age equations has been made possible by taking into account the relationships between the latent and etched track lengths. Our dating exercise involving 10 zircon reference samples supports our claim that absolute age determinations can be carried out based on the following three parameters: (1) the 238U spontaneous fission decay constant of 8.5 × 10−17 a−1 (λf) which is recommended by the IUPAC; (2) thermal neutron fluence values based on pre‐irradiated IRMM540 dosimeter glass, which is equivalent to the Au and Co metal activation monitors in the 1990 IUGS recommendation; and (3) a [GQR] correction value of 1.35 which has been experimentally determined for the combination of zircon external surfaces irradiated with mica external detectors. Independent measurements of uranium concentration by the fission track method were also tested. A new method of fission track dating that uses mass spectrometry for uranium measurements was simulated with success using our zircon data. We propose that the Fission Track Community reassesses the situation and formulates a new recommendation, in which the established independent method is placed on at least an equal footing with the standard‐based method.
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.
Catastrophic collapse of volcanic edifices and the accompanying rockslide-debris avalanches drastically change landforms and cause disasters around volcanoes. Rapid modification of the landforms created by these events makes it difficult to estimate the magnitudes of prehistoric events and evaluate damage. However, the widespread preservation of hummocks along the course of rockslide-debris avalanches is useful for understanding the physical characteristics of these landslides. We analyzed data on hummocks from seven prehistoric events in northern Japan to derive the relationship between hummock size and distance from landslide source, and interpreted the geomorphic significance of the intercept and slope coefficients of the observed functional relationships. Hummock size decreases as an exponential function of distance for volcanic rockslide-debris avalanches, although each event has its own distinct distribution pattern. The intercept coefficient, α, which corresponds to the initial average size of hummocks (blocks) at the origin of the landslide, shows a strong correlation with the volume of the collapsed mass, indicating that the initial size of blocks at the source may be determined by the volume of the collapsed mass. The slope coefficient, β, which describes the rate of decrease in size of hummocks with distance, shows a strong correlation with the coefficient of friction of the rockslide-debris avalanche, indicating that the attrition or size decrease rate of hummocks is controlled by the mobility of the avalanche. These relationships enable us to estimate the volume of the collapsed mass and the travel distance of an avalanche. Because it is sometimes difficult to obtain the evidence directly indicating the volume of collapses and the damage they caused, the findings are significant also for hazard assessment that the size–distance relationships of hummocks can be obtained from fragmentary remnants of a rockslide-debris avalanche to help reveal the characteristics of the events.
A wide variety of Upper Cenozoic ignimbrite centres have been identified and studied on Landsat imagery of the Central Andes: some have been investigated in the field. Lower Miocene ignimbrite centres are distributed along an arc which, in north Chile, diverges markedly from the line of the present trench and parallels an arc of Lower Miocene plutons in the Eastern Cordillera of Bolivia. Much of the ignimbrite erupted since the late Miocene has been erupted around the faulted margins of the Altiplano basin of western Bolivia, particularly where the Western and Eastern Cordilleras converge.Typical examples of ignimbrite centres are described. Some centres take the form of ignimbrite shield volcanoes. Other ignimbrite sheets are more widely dispersed. The degree of dispersal probably relates to the height of the eruption column. Partial or total caldera formation sometimes took place and a few of these calderas have resurgent centres. Late-stage lava extrusions are common and some of these are located on caldera ring fractures.
Sources of large-volume ignimbrites in the Central Andes are difficult to identify by conventional means. MSS band 7 LANDSAT imagery of the region was obtained with the specific objective of using the synoptic view to identify large ignimbrites and their sources. Two are described. The Guataquina ignimbrite covers some 2300 km2 and probably has a volume of some 70 km3. It appears to have a source in Cerro Guacha, a complex caldera-graben structure 25 km across. The Cerro Galan ignimbrite covers an area of some 2000 km' on the flanks of a major resurgent caldera some 30 km by 20 km across. Younger volcanic rocks have been erupted from two points on the caldera wall, and the structure appears to have had a geological history broadly similar to that of the Valles caldera.
According to present concepts, a caldera is a more or less circular volcanic depression larger than a crater which is caused by subsidence. It is commonly considered that the subsided mass consists of a block or blocks encircled by a ring fracture. Caldera collapse is generally correlated with a major explosive eruption. The present investigation is concerned with six features which do not conform well with the favored caldera model. Attention is given to downsagged calderas, the distribution of postcaldera vents in calderas, vent rings, the size of calderas and cauldrons, incremental caldera growth, and caldera-forming events. It is found that no single structural or genetic model applies to all calderas. Thus, the fact of subsidence may be the only common feature. It is pointed out that most known ring dikes occur in Precambrian crust. This may mean that the subsiding piston mechanism operates best where the crust is sufficiently rigid and strong.
The Toya pyroclastic flow deposit is one of the largest tephras of late Pleistocene in northern Japan, covering wide areas of the western part of Hokkaido around the Toya caldera. We have extensively investigated a fine-grained vitric ash layer which has very similar petrographic and chemical properties to the Toya pyroclastic flow deposit in many localities in northern Japan. The stratigraphic position of this ash in the standard sequence of Hachinohe area, northern Honshu, shows that it was emplaced after the last interglacial culmination, represented by the Takadate marine terrace, and also immediately after the Tagadai interstade (c.105 ka) and before the Nejo interstade (c.80 ka), presumably around 90-100 ka. -from English summary
Lithic fragments of precaldera basement rocks in the Plinian fallout deposits of the Bishop Tuff indicate that the eruption began in what is now the S-central part of Long Valley caldera, along or adjacent to the Hilton Creek fault. The change from a single-vent Plinian mode of eruption to multiple vents along the ring fault took place after emplacement of as little as 20% of the total Bishop ejecta. Approximately 66% of the total eruptive volume was trapped within the subsiding caldera.-from Authors
A high concentrated glass bead, with a flux to sample ratio of two to one, was prepared to evaluate the potential for major and trace element analyses of various silicate rocks by X-ray fluorescence spectrometry. Careful instrument set up, matrix effect and peak overlap corrections allowed enhanced accuracy of analysis. The accuracy reached a level comparable to that of using five to one glass beads for major elements and pressed powder pellets for trace elements. Using this method, major and trace element analyses were possible using one single glass bead.
The 1.3 Ma Purico complex is part of an extensive Neogene-Pleistocene ignimbrite province in the central Andes. Like most other silicic complexes in the province, Purico is dominated by monotonous intermediate ash-flow sheets and has volumetrically minor lava domes. The Purico ignimbrites (total volume 80-100 km3) are divided into a Lower Purico Ignimbrite (LPI) with two extensive flow units, LPI I and LPI II; and a smaller Upper Purico Ignimbrite (UPI) unit. Crystal-rich dacite is the dominant lithology in all the Purico ignimbrites and in the lava domes. It is essentially the only lithology present in the first LPI flow unit (LPI I) and in the Upper Purico Ignimbrite, but the LPI II flow unit is unusual for its compositional diversity. It constitutes a stratigraphic sequence with a basal fall-out deposit containing rhyolitic pumice (68-74 wt% SiO2) overlain by ignimbrite with dominant crystal-rich dacitic pumice (64-66 wt% SiO2). Rare andesitic and banded pumice (60-61 wt% SiO2) are also present in the uppermost part of the flow unit. The different compositional groups of pumice in LPI II flow unit (rhyolite, andesite, dacite) have initial Nd and Sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice ()Nd=-6.7 to -7.2 and 87Sr/86Sr=0.7085-0.7090). However, two lines of evidence show that the andesite, dacite and rhyolite pumices do not represent a simple fractionation series. First, melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples. Second, mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre-eruptive temperatures (780-800 C), whereas the rhyolites and andesites yield consistently higher temperatures (850-950 C). Hornblende geobarometry and pressure constraints from H2O and CO2 contents in melt inclusions indicate upper crustal (4-8 km) magma storage conditions. The petrologic evidence from the LPI II system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than, and chemically unrelated to, the underlying dacite. We suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic Purico magma chamber. Rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite. Isotopic and compositional variations in the Purico system are typical of those seen throughout the Neogene ignimbrite complexes of the Central Andes. These characteristics were generated at moderate crustal depths (<30 km) by crustal melting, mixing and homogenization involving mantle-derived basalts. For the Purico system, assimilation of at least 30% mantle-derived material is required.
The Central Volcanic Zone (CVZ) of the Andes, which extends from 14° to 28° S is one of the largest provinces of Late Tertiary to Recent ignimbrite volcanism in the world. In the 21°30′S to 23°30′S portion of the CVZ in northern Chile, ignimbrite volcanism was initiated at the beginning of the Late Miocene, ∼10.4 Ma ago, and continued until the Recent. The stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions (> 100 km³); the Artola ignimbrite member (9.4 Ma), the Sifon ignimbrite member (8.3 Ma), the Pelon ignimbrite member (5.5 Ma), the Puripicar ignimbrite member (4.2 Ma) and the Atana ignimbrite member (4.1 Ma). At least three of these represent only portions of larger ignimbrite sheets erupted from centres of SW Bolivia. Two of the ignimbrites, the Sifon and Atana ignimbrites, must represent erupted volumes in excess of 1000 km³ each. The major units, which represent ∼90% of the total volume of ignimbrite in this area, are intercalated with smaller (< 50 km³), less extensive volcanic units, some of which were erupted from source structures within the study area.
Understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation. Geological data, including that from analysis of deeply eroded examples, allow the overall surface setting of calderas to be defined, whereas deep drillings and geophysical investigations provide insights on their subsurface structure. Collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types (downsag, piston, funnel, piecemeal, trapdoor), being viewed as end-members. Despite its importance, such a classification does not adequately examine: (a) the structure of calderas (particularly the nature of the caldera's bounding faults); and (b) how this is achieved (including the genetic relationships among the five caldera types). Various sets of analogue models, specifically devoted to study caldera architecture and development, have been recently performed, under different conditions (apparatus, materials, scaling parameters, stress conditions).
The Cerro Panizos ignimbrite center, in the central Andes Mountains, produced two ignimbrite sheets and many lava flows. The ignimbrite of Quebrada Cienago was erupted at 7.9 Ma, and effusive eruptions continued until the two cooling units of the crystal-rich dacitic Cerro Panizos Ignimbrite were emplaced at 6.7 Ma. The lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge. Lithic fragments reach significant concentrations (>5%) only in the uppermost two meters of this cooling unit, where they document vent-wall collapse or the opening of a new vent. The upper cooling unit contains many flow units with variations in welding, thicknesses, and lithic fragment concentrations, implying an unsteady eruption column, the opening of many vents, and probable caldera collapse. Triangulation of anisotropy of magnetic susceptibility flow-direction measurements locate a single vent for the lower cooling unit, whereas the upper cooling unit had many vents within the present dome cluster. A 15-km diameter topographic depression, marked by inward dips of 4–8 at the cooling unit contact, is centered on the vent area of the lower cooling unit. The depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit. Collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit. Resurgent magmatism occurred as lava flows that mundated the caldera area. A ring of dacite domes, erupted until at least 6.1 Ma, in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit. Maximum caldera subsidence (353 km3) is not enough to account for the erupted volume (652 km3 DRE minimum).
Large volcanic debris avalanches, often exceeding a cubic kilometer in volume, create massive amphitheater-shaped reentrants into the volcanic edifice that differ in morphology and origin from normal collapse calderas. The volume of debris avalanche deposits at the base of these breached craters or calderas often correlates closely with the volumes of the missing sectors of the volcanic edifices, indicating that the dominant process in the formation of these depressions is massive slope failure of a portion of the volcanic cone. Debris avalanche deposits display a hummocky topography with numerous small hills and closed depressions, longitudinal and transverse ridges, and locally homogeneous debris with a jigsaw fit, features that are typical of landslide deposits. The size of the hummocks and the maximum size of breccia blocks within them tends to decrease away from the source. Data on travel distance (L) of debris avalanches as a function of vertical drop (H) demonstrates the great mobility (median H/L = 0.11) of these avalanches, which are emplaced at calculated velocities often exceeding 100 km/hr.Volcanic debris avalanches and associated formation of “avalanche calderas” have occurred at roughly four per century in historic time, several times the historic rate for the formation of Krakatau-type calderas. These depressions often show a preferred orientation normal to the dominant direction of dike emplacement. The differential stress produced by the emplacement of parallel dike swarms is an important factor among the many factors that contribute to large-scale volcanic slope movements. In addition to hazards from rapid emplacement of the avalanches and possible associated directed blasts, a major secondary volcanic hazard from these events is tsunamis produced by the rapid impact of debris avalanches from coastal volcanoes into the sea.
Prominent among the pyroclastic deposits of Santorini are several thick, widespread lithic breccia deposits, which are found in intimate association with ignimbrite. At least three of these breccias are interpreted, on the basis of field and grain-size criteria, as having originated by the segregation of lithic clasts from active pyroclastic flows. They therefore record the occurrence of three large, previously unrecognized ignimbrite-forming eruptions of the volcano.The breccias of the 18,500 yr. B.P. Cape Riva eruption include two types. The first type is a thin, basal ground breccia, which overlies a strong erosion surface. This breccia shows pinch and swell structures and is strongly enriched in lithic and crystal components. It is considered to have formed by strong fluidization due to incorporation of air into the head of an active pyroclastic flow.The second, and predominant, type consists of thick co-ignimbrite lag breccias (up to 25 m), which overlie the ground breccia. These deposits are generally clast-supported, poorly sorted breccias which in places grade both vertically and laterally into non-welded pumiceous ignimbrite. They consist of well-defined, normally graded units which show coarse tail grading of lithic and pumice clasts. Each breccia unit is underlain by a thin, inversely graded ignimbrite basal layer, and correlates laterally with a flow unit of the associated ignimbrite. The lag breccias are therefore thick equivalents of the 2b lithic concentration zones of Sparks et al. The lag breccias and ignimbrite contain abundant lithic segregation structures that are characteristic of strong gas fluidization. These structures, the presence of basal layers, and the gradation into normal ignimbrite, suggest that the lag breccias originated by the segregation of lithic clasts within the bodies of dense, but strongly fluidized pyroclastic flows.The Cape Riva breccias occur within a few kilometers of their source vent and are interpreted as proximal facies of their associated ignimbrite. The presence of the ground breccia indicates that within this distance, the pyroclastic flows had developed the head and body regions characteristic of gravity currents.The deflation of the pyroclastic flow bodies, within a few kilometers from source, to particle concentrations sufficient to permit the generation of basal layers and coarse tail grading, is incompatible with present theories of column collapse. It is postulated that high pressures at the base of the collapsing Cape Riva eruption column were sufficient to significantly compress the dilute particle-gas mixture of the column close to the source vent. Subsequent sedimentation, as the pyroclastic flows moved laterally, increased the density further to the point where the observed sedimentary features could form. Simultaneous decompression of the gas phase resulted in strong fluidization, and the segregation of the lag breccias.
New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber ∼ 7000 yr B.P.
Seventy-one debris avalanche deposits are identified from 52 Japanese Quaternary volcanoes. The structures of these volcanoes are mostly stratovolcanoes and lava domes. No avalanche deposit is found in calderas, pyroclastic cones or maar volcanoes. Debris avalanche deposits are found in 18% of all Quaternary volcanoes or 25% of Quaternary stratovolcanoes and lava domes. The ratio rises to 49% when considering only active stratovolcanoes and lava domes. At least five debris avalanche deposits have formed since 9th century. The maximum height difference of sliding (H) for each debris avalanche ranges from 200 to 2400 meters and the maximum runout distance of sliding (L) ranges from 1.6 to 32 km. The ratio H/L ranges from 0.2 to 0.06, and becomes smaller in larger debris avalanches. They are more mobile than landslides in non-volcanic areas. Volume is within a range of 0.03 and 9 km3. There is no relation between the direction of sliding and the regional horizontal compressional stress axis at the site of a volcano. Two-dimensional computer simulations of avalanches using a simplified physical model are made. The maximum velocity is mainly controlled by the length of steep slope.
- S de Silva
Igneous rocks: a classification and glossary of terms. Recommendations of the international union of geological sciences subcommission on the systematics of igneous rocks
- Le Maitre
- R W Streckeisen
- A Zanettin
- Le Bas
- M J Bonin
- B Bateman
- P Bellieni
- G Dudek
- A Efremova
- S Keller
- J Lameyre
- J Sabine
- P A Schmid
- R Sorensen
- H Woolley
Stratigraphy and lithofacies of the Toya Ignimbrite in southwestern Hokkaido, Japan: Insights into the caldera-forming eruption at Toya caldera
- Y Goto
- K Suzuki
- T Shinya
- A Yamauchi
- M Miyoshi
- T Danhara
- A Tomiya
The Cenozoic system and its geotectonic feature at the southern foot of Mt. Yotei, southwestern Hokkaido
- S Ohtsu
Japan: Implications for island arc caldera evolution
- C A Feebrey
- Toya Petrology
- Caldera
- Hokkaido