Improved age estimates for Holocene Ko-g and Ma-f~j tephras in northern Japan using Bayesian statistical modelling

The Ko-g and Ma-f~j tephras are two key isochronous marker layers in northern Japan, which are from the largest Plinian eruptions of Komagatake volcano (VEI = 5) and Mashu caldera (VEI = 6), respectively. Despite extensive radiocarbon studies associated with the two tephras, individual calibrated results show considerable variations and thus accurate ages of these important eruptions remain controversial. Bayesian statistical approaches to calibrating radiocarbon determinations have proven successful in increasing accuracy and sometimes precision for dating tephras, which is achieved through the incorporation of additional stratigraphic information and the combination of evidence from multiple records. Here we use Bayesian approaches to analyse the proximal and distal information associated with the two tephra markers. Through establishing phase and deposition models, we have taken into account all of the currently available stratigraphic and chronological information. The cross-referencing of phase models with the deposition model allows the refinement of eruption ages and the deposition model itself. Using this we are able to provide the most robust current age estimates for the two tephra layers. The Ko-g and Ma-f~j tephras are hereby dated to 6657-6505 (95.4%; 6586±40, μ±σ) cal yr BP, and 7670-7395 (95.4%; 7532±72, μ±σ) cal yr BP, respectively. These updated age determinations underpin the reported East Asian Holocene tephrostratigraphic framework, and allow sites where the tephra layers are present to be dated more precisely and accurately. Our results encourage further applications of Bayesian modelling techniques in the volcanically active East Asian region.     

The 1982 eruptions of El Chichon volcano,Mexico (2): Observations and numerical modelling of tephra-fall distribution

Tephra fallout from the A-1 (March 29, 0532 UT), B (April 4, 0135 UT), and C (April 4, 1122 UT) 1982 explosive eruptions of El Chichon produced three tephra fall deposits over southeastern Mexico. Bidirectional spreading of eruption plumes, as documented by satellite images, was due to a combination of tropospheric and stratospheric transport, with heaviest deposition of tephra from the ENE tropospheric lobes. Maximum column heights for the eruptions of 27, 32, and 29 km, respectively, have been determined by comparing maximum lithic-clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns. These column heights suggest peak mass eruption rates of 1.1 × 10⁸, 1.9 × 10⁸, and 1.3 × 10⁸ kg/s. Maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits. Sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration. New estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2.19 km³ (1.09 km³ DRE, dense rock equivalent) or roughly twice the amount of the deposit mapped on the ground. Up to one-half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra.     

The immediate environmental effects of tephra emission

The Earth’s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere. These tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents. The interaction of tephra with or in receiving environments may induce an array of physical, chemical and biological effects. The consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent, small eruptions, while larger and rarer events may have acute, regional-scale impacts. It is, therefore, necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems. We collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post-eruptive effects of tephra emission into the atmosphere, onto vegetation, soil or ice/snow surfaces and in aquatic systems. We further consider the repercussions that may be induced in the weeks to years afterwards. In the atmosphere, tephra can influence cloud properties and air chemistry by acting as ice nuclei (IN) or by offering sites for heterogeneous reactions, respectively. Tephra on vegetation causes physical damage, and sustained coverage may elicit longer-term physiological responses. Tephra deposits on soils may alter their capacity to exchange gas, water and heat with the atmosphere or may have a specific chemical effect, such as nutrient input or acidification, on sensitive soils. Tephra deposition onto snow or ice may affect ablation rates. Rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged (months or years) erosion from the tephra-covered catchment. In the first weeks after deposition, tephra leaching may affect river chemistry. The abundance and speciation of phytoplankton populations in lakes may be altered by tephra-induced changes in water chemistry or sediment–water nutrient cycling. In the oceans, tephra deposition may fertilise Fe-limited waters, with potential impacts on the global carbon cycle. Embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies, combined with consideration of the interconnectivity of induced impacts within and between different receiving environments.     

Quantifying volcanic ash fall hazard to electricity infrastructure

Quantifying the potential ash fall hazards from re-awakening volcanoes is a topic of great interest. While methods for calculating the probability of eruptions, and for numerical simulation of tephra dispersal and fallout exist, event records at most volcanoes that re-awaken sporadically on decadal to millennial cycles are inadequate to develop rigorous forecasts of occurrence, much less eruptive volume. Here we demonstrate a method by which eruption records from radiocarbon-dated sediment cores can be used to derive forecasting models for ash fall impacts on electrical infrastructure. Our method is illustrated by an example from the Taranaki region of New Zealand. Radiocarbon dates, expressed as years before present (B.P.), are used to define an age-depth model, classifying eruption ages (with associated errors) for a circa 1500–10 500 year B.P. record at Mt. Taranaki (New Zealand). In addition, data describing the youngest 1500 years of eruption activity is obtained from directly dated proximal deposits. Absence of trend and apparent independence in eruption intervals is consistent with a renewal model using a mix of Weibulls distributions, which was used to generate probabilistic forecasts of eruption recurrence. After establishing that interval length and tephra thickness were independent in the record, a thickness–volume relationship (from [Rhoades, D.A., Dowrick, D.J., Wilson, C.J.N., 2002. Volcanic hazard in New Zealand: Scaling and attenuation relations for tephra fall deposits from Taupo volcano. Nat. Hazards, 26:147–174]) was inverted to provide a frequency–volume relationship for eruptions. Monte Carlo simulation of the thickness–volume relationship was then used to produce probable ash fall thicknesses at any chosen site. Several critical electrical infrastructure sites in the Taranaki Region were analysed. This region, being the only gas and condensate-producing area in New Zealand, is of national economic importance, with activities in and around the area depending on uninterrupted power supplies. Forecasts of critical ash thicknesses (1 mm wet and 2 mm dry) that may cause short-circuiting, surges or power shutdowns in substations show that the annual probabilities of serious impact are between ~ 0.5% and 27% over a 50 year period. It was also found that while large eruptions with high ash plumes tend to affect “expected” areas in relation to prevailing winds, the direction impacts of small ash falls are far less predictable. In the Taranaki case study, areas out of normal downwind directions, but close to the volcano, have probabilities of impact for critical thicknesses of 1–2 mm of around half to 60% of those in downwind directions and therefore should not be overlooked in hazard analysis. Through this method we are able to definitively show that the potential ash fall hazard to electrical infrastructure in this area is low in comparison to other natural threats, and provide a quantitative measure for use in risk analysis and budget prioritisation for hazard mitigation measures.     

龙岗金龙顶子火山空降碎屑物数值模拟及概率性灾害评估

空降碎屑物为爆炸式火山喷发产生的一种重要的灾害类型,数值模拟已成为一个快速有效地确定火山灰扩散和沉积范围的方法。本文根据改进的Suzuki(1983)二维扩散模型,编写了基于Windows环境下的火山灰扩散程序。通过对前人资料的分析,模拟了龙岗火山群中最新火山喷发——金龙顶子火山喷发产生的空降碎屑物扩散范围,与实测结果具有很好的一致性,证实了模型的可靠性和参数的合理性。根据该区10年的风参数,模拟了7021次不同风参数时金龙顶子火山灰的扩散范围,以此制作了火山灰沉积厚度超过1cm和0.5cm时的概率性空降碎屑灾害区划图。本文的研究可为龙岗火山区火山危险性分析和灾害预警与对策提供重要的科学依据。     

Holocene explosive activity of Hudson Volcano, southern Andes

 Fallout deposits in the vicinity of the southern Andean Hudson Volcano record at least 12 explosive Holocene eruptions, including that of August 1991 which produced ≥4 km³ of pyroclastic material. Medial isopachs of compacted fallout deposits for two of the prehistoric Hudson eruptions, dated at approximately 3600 and 6700 BP, enclose areas at least twice that of equivalent isopachs for both the 1991 Hudson and the 1932 Quizapu eruptions, the two largest in the Andes this century. However, lack of information for either the proximal or distal tephra deposits from these two prehistoric eruptions of Hudson precludes accurate volume estimates. Andesitic pyroclastic material produced by the 6700-BP event, including a  1 10-cm-thick layer of compacted tephra that constitutes a secondary thickness maximum over 900 km to the south in Tierra del Fuego, was dispersed in a more southerly direction than that of the 1991 Hudson eruption. The products of the 6700-BP event consist of a large proportion of fine pumiceous ash and accretionary lapilli, indicating a violent phreatomagmatic eruption. This eruption, which is considered to be the largest for Hudson and possibly for any volcano in the southern Andes during the Holocene, may have created Hudson's 10-km-diameter summit caldera, but the age of the caldera has not been dated independently. Received: 31 January 1997 / Accepted: 29 October 1997     

Chronology and dispersal characteristics of recently (last 5000 years) erupted tephra of Cotopaxi (Ecuador): implications for long-term eruptive forecasting

Cotopaxi, the highest active volcano on earth and one of the most dangerous of Ecuador is constituted by a composite cone made up of lava and tephra erupted from the summit crater. The activity of the present volcano begun with large-volume plinian eruptions followed by a succession of small-volume lava emissions and pyroclastic episodes which led to the edification of a symmetrical cone. The growth of the cone was broken by an episode of slope failure, the scar of which is now obliterated by recent and historical products. Volcanic history, eruptive frequency and characteristics of the activity were investigated by studying the stratigraphy of tephra and carrying out fifteen new ¹⁴C dating on paleosols and charcoals. The investigated period is comprised between the slope failure and the present. The deposit of the volcanic landside (dry debris avalanche of Rio Pita), previously believed to be between 13,000 and 25,000 yr B.P., is now considered to have an age slightly older than 5000 yr B.P. The stratigraphy of tephra of the last 2000 years reveals the existence of 22 fallout layers. Seven of them were dated with ¹⁴C whereas three were ascribed to the eruptions of 1534, 1768 and 1877 on the basis of comparison with historical information.Maximum clast size distribution (isopleths) of 9 tephra layers points out that the sustained explosive eruptions of Cotopaxi during the last 2000 years are characterized by very high dispersive power (plinian plumes with column heights between 28 and 39 km) and high intensity (peak mass discharges from 1.1 to 4.1 × 10⁸kg/s). The magnitude (mass) of tephra fallout deposits calculated from distribution of thickness (isopaches) are, however, moderate (from 0.8 to 7.2 × 10¹¹ kg). The limited volume of magma erupted during each explosive episode is consistent with the lack of caldera collapses. Small-volume pyroclastic flows and surges virtually accompanied all identified tephra fallouts. During such an activity large scale snow/ice melting of the summit glacier produced devastating mudflows comparable in scale to those of 1877 eruption. By assuming a 1:1 correspondence between fallout episodes and generation of large-scale lahar, we have estimated an average recurrence of one explosive, lahartriggering event every 117 years over the last two millennia. This value compares well with that calculated by considering the period since Spanish Conquest. The probability of having an eruption like this in 100 or 200 years is respectively of 0.57 and 0.82. Such an high probability underscores the need for quick actions aimed at the mitigation of Cotopaxi lahar hazard along all the main valleys which originate from the volcano.     

Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska

 A discontinuous pumiceous sand, a few centimeters to tens of centimeters thick, is located up to 15 m above mean high tide within Holocene peat along the northern Bristol Bay coastline of Alaska. The bed consists of fine-to-coarse, poorly to moderately well-sorted, pumice-bearing sand near the top of a 2-m-thick peat sequence. The sand bed contains rip-up clasts of peat and tephra and is unique in the peat sequence. Major element compositions of juvenile glass from the deposit and radiocarbon dating of enclosing peat support correlation of the pumiceous sand with the caldera-forming eruption of Aniakchak Volcano. The distribution of the sand and its sedimentary characteristics are consistent with emplacement by tsunami. The pumiceous sand most likely represents redeposition by tsunami of climactic fallout tephra and beach sand during the approximately 3.5 ka Aniakchak caldera-forming eruption on the Alaska Peninsula. We propose that a tsunami was generated by the sudden entrance of a rapidly moving, voluminous pyroclastic flow from Aniakchak into Bristol Bay. A seismic trigger for the tsunami is unlikely, because tectonic structures suitable for tsunami generation are present only south of the Alaska Peninsula. The pumiceous sand in coastal peat of northern Bristol Bay is the first documented geologic evidence of a tsunami initiated by a volcanic eruption in Alaska. Received: 3 December 1997 / Accepted: 11 April 1998     

One Million Years tephra record at IODP Sites U1436 and U1437: Insights into explosive volcanism from the Japan and Izu arcs

The 1 Myr tephra records of IODP (International Ocean Discovery Program) Holes U1436A and U1437B in the Izu‐Bonin fore‐ and reararc were investigated in order to assess provenance and eruptive volumes, respectively. In total, 304 tephra samples were examined and 260 primary tephra layers were identified. Tephra provenance was determined by means of major and trace element compositions of glass shards and distinguished between Japan and Izu‐Bonin arc origin of the tephra layers. A total of 33 marine tephra compositions were correlated to the Japan arc and 227 to the Izu arc. Twenty marine tephra layers were correlated between the two drilling sites. Additionally, we defined eleven correlations of marine tephra deposits to major widespread Japanese eruptions; from the 1.05 Ma Shishimuta‐Pink Tephra to the 30 ka Aira‐Tn Tephra, both from Kyushu Island. These eruptions provide independent time markers within the sediment record and six correlations were used to date tephra layers from Japan in Hole U1436A to establish an alternative age model for this hole. Furthermore, the minimum distal tephra volumes of all detected events were calculated, which enabled the comparison of the tephra volumes that derived from the Japan and the Izu‐Bonin arcs. For some of the major Japanese eruptions these are the first volume estimations that also include distal deposits. All of the Japanese tephras derived from events with eruption magnitude M_v ≥ 5.6 and three of the investigated eruptions reach magnitudes M_v ≥ 7. Volcanic events of the Izu‐Bonin arc have mostly eruption magnitudes M_v ≤ 5.     

TEPHRA RECORD FROM QUANYANG PEAT OF THE CHANGBAISHAN MILLENNIUM ERUPTION

Tephra, usually produced by explosive eruptions, is deposited rapidly, hence, it can serve as a distinctive and widespread synchronous marker horizon correlating terrestrial, marine and ice core records. The tephra from Changbaishan Millennium eruption, a widely distributed tephra, is an important marker bed across the Japan Sea, Japan Islands and even in the Greenland ice cores 9000km away from volcanic vent. In this study, a discrete tephra was identified in the Quanyang peat~45km northeast to the Changbaishan volcano. Radiocarbon ¹⁴ C dating on the plant remains constrains an age of 886-1013calAD(95.4%)to the tephra layer, which can correspond to the Millennium eruption of Changbaishan in time. In addition, there was no similar volcanic eruption in the surrounding areas except Changbaishan at the same time. This tephra shows rhyolitic glass shards major element compositions similar to those rhyolitic tephra from Millennium eruption. This study illustrates that tephra from Millennium eruption has been transported to Quanyang peat~45km northwest to the Changbaishan volcano. Additionally, the diameter of the pumice lapilli is up to 0.3cm, implying that the tephra must be transported more distal away from Quanyang peat and formed a widely distributed isochronic layer. Glass geochemistry of the Quanyang tephra, different from the distal tephra recorded at Sihailongwan, Japan, and Greenland ice, shows a close affinity to the pyroclastic flow deposits of the Millennium eruption while not from fall deposits. This may indicate that distribution of the Millennium eruption of Changbaishanin in different directions may be controlled by different stages of eruption. This layer with well-defined annual results can be used to optimize the chronological framework of the corresponding sedimentary environment, thus facilitating more accurate discussion of corresponding environmental changes, which can achieve the contrast of the ancient climate records in the whole Northeast China-Japan and arctic regions.     

Evaluating suitability of a tephra dispersal model as part of a risk assessment framework

In volcanic risk assessment it is necessary to determine the appropriate level of sophistication for a given predictive model within the contexts of multiple sources of uncertainty and coupling between models. A component of volcanic risk assessment for the proposed radioactive waste repository at Yucca Mountain (Nevada, USA) involves prediction of dispersal of contaminated tephra during violent Strombolian eruptions and the subsequent transport of that tephra toward a hypothetical individual via surface processes. We test the suitability of a simplified model for volcanic plume transport and fallout tephra deposition (ASHPLUME) coupled to a surface sediment-transport model (FAR) that calculates the redistribution of tephra, and in light of inherent uncertainties in the system. The study focuses on two simplifying assumptions in the ASHPLUME model: 1) constant eruptive column height and 2) constant wind speed and direction during an eruption. Variations in tephra dispersal resulting from unsteady column height and wind conditions produced variations up to a factor of two in the concentration of tephra in sediment transported to the control population. However, the effects of watershed geometry and terrain, which control local remobilization of tephra, overprint sensitivities to eruption parameters. Because the combination of models used here shows limited sensitivity to the actual details of ash fall, a simple fall model suffices to estimate tephra mass delivered to the hypothetical individual.     

Estimation of tephra volumes from sparse and incompletely observed deposit thicknesses

We present a Bayesian statistical approach to estimate volumes for a series of eruptions from an assemblage of sparse proximal and distal tephra (volcanic ash) deposits. Most volume estimates are of widespread tephra deposits from large events using isopach maps constructed from observations at exposed locations. Instead, we incorporate raw thickness measurements, focussing on tephra thickness data from cores extracted from lake sediments and through swamp deposits. This facilitates investigation into the dispersal pattern and volume of tephra from much smaller eruption events. Given the general scarcity of data and the physical phenomena governing tephra thickness attenuation, a hybrid Bayesian-empirical tephra attenuation model is required. Point thickness observations are modeled as a function of the distance and angular direction of each location. The dispersal of tephra from larger well-estimated eruptions are used as leverage for understanding the smaller unknown events, and uncertainty in thickness measurements can be properly accounted for. The model estimates the wind and site-specific effects on the tephra deposits in addition to volumes. Our technique is exemplified on a series of tephra deposits from Mt Taranaki (New Zealand). The resulting estimates provide a comprehensive record suitable for supporting hazard models. Posterior mean volume estimates range from 0.02 to 0.26 km ³. Preliminary examination of the results suggests a size-predictable relationship.     

Compositional heterogeneity of distal tephra deposits from the 1912 eruption of Novarupta, Alaska

Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An₂₅ plagioclase, Fs₄₂ orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An₄₅ plagioclase and Fs₃₅, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.     

The exceptional magnitude and intensity of the Toba eruption,sumatra: An example of the use of deep-sea tephra layers as a geological tool

The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km². The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 10⁶ km². We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km³ of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km³ respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 10⁶ m³/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.     

Explosive eruptive record in the Katmai region,Alaska Peninsula: an overview

At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin and from Shelikof Strait to Bristol Bay (∼8,500 km²). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy Mountain summit dome (<250 ¹⁴C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 ¹⁴C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two tephras (∼3,700 and ∼2,700 ¹⁴C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time (mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 ¹⁴C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite ash from Kaguyak’s caldera-forming event (5,800 ¹⁴C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 ¹⁴C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500 ¹⁴C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 ¹⁴C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available.     

Clarifying the distal to proximal tephrochronology of the Millennium (B–Tm) eruption,Changbaishan Volcano,northeast China

Tephra dispersed during the Millennium eruption (ME), Changbaishan Volcano, NE China provides one of the key stratigraphic links between Asia and Greenland for the synchronization of palaeoenvironmental records. However, controversy surrounds proximal-distal tephra correlations because (a) the proposed proximal correlatives of the distal ME tephra (i.e. B–Tm) lack an unequivocal chronostratigraphic context, and (b) the ME tephra deposits have not been chemically characterized for a full spectrum of element using grain-specific techniques. Here we present grain-specific glass chemistry, including for the first time, single grain trace element data, for a composite proximal sequence and a distal tephra from Lake Kushu, northern Japan (ca. 1100 km away from Changbaishan). We demonstrate a robust proximal-distal correlation and that the Kushu tephra is chemically associated with the ME/B–Tm. We propose that three of the proximal pyroclastic fall units were erupted as part of the ME. The radiocarbon chronology of the Kushu sedimentary record has been utilised to generate a Bayesian age-depth model, providing an age for the Kushu tephra which is consistent with high resolution ages determined for the eruption and therefore supports our geochemical correlation. Two further Bayesian age-depth models were independently constructed each incorporating one of two ice-core derived ages for the B–Tm tephra, providing Bayesian modelled ages of 933–949 and 944–947 cal AD (95.4%) for the Kushu tephra. The high resolution ice-core tephra ages imported into the deposition models help test and ultimately constrain the radiocarbon chronology in this interval of the Lake Kushu sedimentary record. The observed geochemical diversity between proximal and distal ME tephra deposits clearly evidences the interaction of two compositionally distinct magma batches during this caldera forming eruption.     

PRELIMINARY VOLCANIC HAZARD ZONATION IN JINLONGDINGZI VOLCANO,LONGANG VOLCANO AREA,JILIN PROVINCE,CHINA

Longgang volcano cluster is 150km away from the Tianchi volcano, located in Jingyu and Huinan Counties, Jilin Province, China. It had a long active history and produced hundreds of volcanoes. The latest and largest eruption occurred between 1 500 and 1 600 years ago by Jinlongdingzi(JLDZ)volcano which had several eruptions in the history. This paper discusses the volcanic hazard types, and using the numerical simulations of lava flow obtained with the Volcflow model, proposes the hazard zonation of JLDZ volcano area. JLDZ volcano eruption type is sub-plinian, which produced a great mass of tephra fallout, covering an area of 260km². The major types of volcanic hazards in JLDZ area are lava flow, tephra fallout and spatter deposits. Volcflow is developed by Kelfoun for the simulation of volcanic flows. The result of Volcflow shows that the flows are on the both sides of the previous lava flows which are low-lying areas now. According to the physical parameters of historical eruption and Volcflow, we propose the preliminary volcanic hazard zonation in JLDZ area. The air fall deposits are the most dangerous product in JLDZ. The highly dangerous region of spatter deposits is limited to a radius of about 2km around the volcano. The high risk area of tephra fallout is between 2km to 9km around the volcano, and between 9km to 14km is the moderate risk area. Out of 14km, it is the low risk area. Lava flow is controlled by topography. From Jinchuan Town to Houhe Village near the volcano is the low-lying area. If the volcano erupts, these areas will be in danger.     

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km² reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (>150 km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105 km³ of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350 km³ of deposits. Thus, the total volume of this tephra amounts over 380–490 km³. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.     

GPR investigation of tephra fallout, Cerro Negro volcano, Nicaragua: a method for constraining parameters used in tephra sedimentation models

Most tephra fallout models rely on the advection–diffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection–diffusion equation to tephra sedimentation using data collected on the proximal (350 to ~1,200?m from the vent) to medial (greater than ~1,200?m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and high-resolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection–diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2–3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700–1,600?m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13?km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of ~500?m²?s^?1 estimated for the deposits, confirming that the advection–diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350?m) to ~1,200?m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than ~2?km; and (3) a medial zone, in which particles fall from ~4 to 7?km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud—a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of ~400?s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.