Tephra Fall Research Articles

Regional stratigraphy and human occupation of the upper Susitna River basin, central Alaska Range

AbstractThis study presents geomorphological, paleoenvironmental, and archaeological data from the upland upper Susitna River basin in the central Alaska Range. The goal of this study is to establish the landscape history and record of human use of the study area and test regional land‐use models. The results indicate that the upper Susitna was first occupied in the early Holocene at least 2,000 years after the end of full‐glacial conditions and 1,000 years after evidence of landscape recovery. Early Holocene land‐use consisted of short‐term task‐specific camps occupied by highly mobile groups on logistical forays. Upland land use intensified in the middle and late Holocene after modern vegetation patterns were established; during this time hunter‐gatherers occupied both residential and task‐specific camps in the study area in a logistical land‐use system. These results support a shift in land use in the early Holocene to include upland landscapes and intensification of upland landscape use in the late–middle Holocene. A shift in residential base camp location in the late Holocene may be tied to changing subsistence activities. There is evidence for a hiatus in human occupation following Holocene tephra fall, but it is unclear whether this was related to tephra deposition or broader climate instability.

Landscape evolution on the eastern part of Lombok (Indonesia) related to the 1257 CE eruption of the Samalas Volcano

One of the most powerful eruptions of the Holocene, known as the Samalas eruption, had a VEI of 7 and took place on Lombok, eastern Indonesia in 1257 CE. Thick tephra fall covered the entire island, and pyroclastic density currents (PDCs) up to 50 m thick buried almost half of Lombok. Until now, there has been no detailed study of the geomorphological evolution of Lombok related to this eruption, especially for the eastern part of Lombok. The main goal of this paper is therefore to reconstruct the paleo-topography on the eastern part of Lombok before the eruption of the Samalas volcano in 1257 CE, and to analyse the subsequent landscape evolution following this eruption. Data were collected from more than 1300 points in order to gather geomorphological, geological, and geochemical information for the eastern part of Lombok over the last millennium. Data were obtained from wells (which are widespread in the studied area), natural outcrops (mainly cliffs along the shoreline and the rivers), and human-made outcrops in quarries, as well as from the two-dimensional resistivity profiling (Dipole-dipole array) carried out on eastern Lombok in 2016 and 2017. We estimate that 4435 ± 5.5 × 106 m3 of pumice-rich PDC, over a surface of 171 km2, buried the eastern part of Lombok in the aftermath of the 1257 CE eruption, and as a result of this rapid pumice-rich PDC deposition there has been some aggradation of the shoreline at some points, resulting in most of the pre-1257 coral reef being buried. After several intense rainfall events, the deposited volcanic material was probably eroded during the first decades following the 1257 CE eruption by mountain streams, producing lahars. We estimate that the remaining volume of pumice-rich PDC deposits following this erosion is 625 ± 5.5 × 106 m3 over a surface of 89 km2, which is equivalent to 14% of the initial volume.

Assessing Impact to Infrastructures Due to Tephra Fallout From Öræfajökull Volcano (Iceland) by Using a Scenario-Based Approach and a Numerical Model

Mt. Oraefajokull is one of the most dangerous volcanoes in Iceland with potential for a VEI6 eruption and the generation of many severe associated hazards. It is not a frequently erupting volcano with two eruptions in the last 1100 years, in 1362 and 1727-28. During the 1362 eruption 10 km3 of freshly fallen tephra was emitted, the eruption plume reached the stratosphere and was dispersed offshore towards mainland Europe. In this study we investigate the possible impact due to tephra fallout to critical infrastructures in Iceland namely – roads, airports, electrical power-lines – in case of a new eruption at Oraefajokull of similar intensity as in 1362. The analysis is done by running several times the VOL-CALPUFF dispersal model to simulate the dispersal of ash in the atmosphere and its deposition on the ground. The resulting maps show the probability of exceeding critical thickness of the tephra fall. Critical infrastructures have been added to the analysis to get a quantitative assessment of the potential impact. The results indicate that in case of an event similar to the 1362 eruption, the tephra fallout could be expected over most of the country, with higher likelihood on the eastern side. The tephra fallout is likely to have a severe impact in the proximity of the volcano, generating a deposit with a load of up to 1000 kg/m2. The likelihood of failure for more than 160 km of the electrical power-line and for critical driving conditions on about 900 km of the main ring road is between 50-100%. The probability that the tephra fall will affect three of the main domestic airports is higher than 50%. An eruption of this magnitude is likely to affect commuting and communication between the greater Reykjavik area, where the government resides, and the rest of the country. Our analysis also reveals the limitations of current knowledge and understanding of the Oraefajokull volcano and highlights the need for further studies on past activity to better characterize its future behavior.

Tephra From the 3 March 2015 Sustained Column Related to Explosive Lava Fountain Activity at Volcán Villarrica (Chile)

Villarrica Volcano in the Southern Andes of Chile has sustained persistent open conduit activity, associated with a dynamic lava lake since at least 1984-85. Increased seismic activity and degassing in August 2014 was followed by increased Strombolian activity starting in February 2015 and culminated with a 1.5 km-high lava fountain on 3 March 2015. This eruption produced tephra fallout, spatter agglutination, clastogenic lavas and mixed avalanche deposits. Field work carried out in 2017 and 2018 focused upon the tephra fallout deposits from the 3 March 2015 paroxysmal eruption. Tephra fall deposits produced by lava fountain and a rapid transition to sustained eruption column at Villarrica are here described for first time. Tephra dispersion was observed mostly southeast of the crater in a narrow, elongated, nearly-elliptic area. Minimum observed tephra loading was estimated to be 80-120 g/m2 at crosswind locations, while maximum load estimation was about 11,500 g/m2, at 7-9 km distance from the vent. At 6-8 km downwind, the tephra fall deposit consisted of a single black scoria layer, fall varied from a lapilli blanket at medial distances (9-19 km) to a few individual particles at distal sites (20-43 km). All samples are vitrophyric in texture, with a mineral assemblage of plagioclase (3-30 %), clinopyroxene (2-5 %), olivine (1-4 %) and scarce orthopyroxene (<1 %) phenocrysts. We estimate that 1.4 x 109 kg, equivalent to ~2.4 x 106 m3 of basaltic andesite tephra (52.8 to 54.7 SiO2 wt. %), were erupted within a period of about 16 minutes at a mass eruption rate of ~1.5 x 106 kg/s. The sustained eruption column height was estimated at ~10.8 km, with magnitude and intensity of 2.11 and 9.13 respectively. We propose that a rejuvenated, volatile-rich magmatic intrusion is the probable triggering factor for the paroxysm. The Villarrica eruption is classified as a short-lived, large-scale lava fountain, similar to intense historical lava fountains at Mt. Etna and Kilauea. As evident by devastating 20th century Villarrica eruptions which also developed similar lava fountain behavior, this eruption style should be considered as frequent and a potential source of volcanic hazard in the future.

A fuzzy logic methodology for assessing the resilience of past communities to tephra fall: a Laacher See eruption 13,000 year BP case

While volcanologists are experienced in assessing present and past volcanism, and while archaeologists are experts in understanding past societies, the study of how ancient volcanic activity has impacted contemporaneous communities remains little systematised. We here present a fuzzy logic-based methodology for bringing together expert assessments in evaluating the vulnerability and, by extension, the resilience of a group of late Pleistocene foragers to the Laacher See eruption, a large explosive eruption that affected continental Europe 13,000 years ago. Based on attributes assessing human health, shelter, food supply, and water supply our analyses suggest community resilience falls from 1.0 to 0.75 – 0.8 under a 5 cm tephra fall, to 0.45 – 0.50 with a 10 cm fall, and to 0.17 – 0.2 with an 18 – 20 cm tephra fall. Our explicit assessment of different experts’ evaluation of the different attribute’s relative importance facilitates a rigour in formulating such impact scenarios. The assessment methodology is rapid and can then be matched against existing evidence or, importantly, be used to also assess contemporary communities’ potential for loss under different tephra fall conditions. The methodology can be readily transferred between case studies and, in principle, between hazards, and could contribute significantly to the design of realistic disaster scenarios, which in turn serve to build resilience in at-risk communities.

Effects of tephra falls on vegetation: A Late‐Quaternary record from southern Italy

Abstract Impacts of tephra deposition on vegetation are recorded in a series of 10 high temporal resolution absolute pollen diagrams from Lago Grande di Monticchio, each diagram spanning a single tephra deposition event during the last glacial–interglacial cycle. Sediment accumulation rates determined by counting and measurement of annual laminations (“varves”) provide an accurate and precise chronology, enabling the minimum recovery time after a tephra fall to be determined. In most cases, pollen accumulation rate was reduced after the tephra fall, indicating reduced vegetation productivity. Tephra deposition events also led to changes in vegetation composition, although these varied in magnitude. The magnitude and duration of the impacts upon the vegetation were related to the thickness of the tephra layer deposited, the thickest layers examined (&gt;250 mm) having minimum recovery times of up to a century and thicker layers generally having greater impacts upon pollen productivity and vegetation composition. Tephra chemistry also influenced the persistence of the impact. The nature of the prevailing vegetation prior to the tephra fall influenced the degree and persistence of the impact. Tephra layers &lt;30 mm thick had minimum recovery times of up to 90 years when they fell on wooded steppe vegetation, whereas cold steppe recovered much more quickly, as did forest. The relative sensitivity of wooded steppe was contrary to our expectations. Of individual pollen taxa, Cupressaceae emerged as particularly sensitive to tephra deposition. Synthesis. Applying absolute pollen analytical methods to a sediment record with a well‐supported and precise chronology obtained from a lake in a volcanic region where the vegetation has been subject to numerous tephra deposition events enabled us to explore the impacts of such events. Our results provide evidence of differential impacts upon individual plant taxa and of differential sensitivity of three vegetation types that have prevailed in the region during the last glacial–interglacial cycle. The influences of tephra thickness and chemistry on minimum recovery time were substantial. Our results are relevant to forecasting the potential impacts upon ecosystems of volcanic eruptions.

Forensic assessment of the 1999 Mount Cameroon eruption, West-Central Africa

The 28 March to 22 April 1999 eruption of Mount Cameroon volcano in southwest Cameroon occurred from multiple vents along fissures at two sites. Vents opened first at the upper site 1 (2650 m) and were more explosive than vents at the lower site 2 (1500 m), which were more effusive. Earthquakes, lava flows, tephra fall, ballistics and gas emissions affected the volcano's west and south flanks, including forests, plantations, stock animals, water supplies, coastal communities and their people. Through an analysis of existing published data and new interview data, we provide an overview of the environmental, social and economic consequences of these hazards on infrastructure, human health, and socio-economic and agricultural activities of the four coastal communities most affected by the eruption: Batoke, Bakingili, Debundscha and Idenau. The collected data provide a more detailed description of the short- and long-term direct and indirect effects of the eruption and response than has been provided to date. Sedimentation of tephra and ballistics from site 1 produced both short-term and long-lasting impacts on people, through the contamination of plants and water supplies, which induced impacts on human health and commercial activities. A ~9.2 km long lava flow erupted from site 2 received significant short-term attention as it severed the only arterial coastal highway, forced the evacuation of some 600 residents of Bakingili, and interrupted commerce between communities. The agricultural sector also suffered due to burning of crops and soils. The only obvious significant benefit of the eruption appears to be that the long lava flow has become a tourist attraction, responsible for bringing in money for food, drink and lodging. However, the long-term cascading effects caused by the hazards have proven to be more severe than the immediate direct and indirect effects during the eruption.

The marine isotope stage 1–5 cryptotephra record of Tenaghi Philippon, Greece: Towards a detailed tephrostratigraphic framework for the Eastern Mediterranean region

The iconic climate archive of Tenaghi Philippon (TP), NE Greece, allows the study of short-term palaeoclimatic and environmental change throughout the past 1.3 Ma. To provide high-quality age control for detailed palaeoclimate reconstructions based on the TP archive, (crypto)tephra studies of a peat core ‘TP-2005’ have been carried out for the 0–130 ka interval. The results show that the TP basin is ideally positioned to receive tephra fall from both the Italian and Aegean Arc volcanic provinces. Two visible tephra layers, the Santorini Cape Riva/Y-2 (c. 22 ka) and the Campanian Ignimbrite (CI)/Y-5 (c. 39.8 ka) tephras, and six primary cryptotephra layers, namely the early Holocene E1 tephra from the Aeolian Islands (c. 8.3 ka), the Campanian Y-3 (c. 29 ka) and X-6 tephras (c. 109.5 ka), as well as counterpart tephras TM-18-1d (c. 40.4 ka), TM-23-11 (c. 92.4 ka) and TM-33-1a (c. 116.7 ka) from the Lago Grande di Monticchio sequence (southern Italy), were identified along with repeatedly redeposited Y-2 and CI tephra material. Bayesian modelling of the ages of seven of the primary tephra layers, 60 radiocarbon measurements and 20 palynological control points have been applied to markedly improve the chronology of the TP archive. This revised chronology constrains the age of tephra TM-18-1d to 40.90–41.66 cal ka BP (95.4% range). Several tephra layers identified in the TP record form important isochrons for correlating this archive with other terrestrial (e.g., Lago Grande di Monticchio, Sulmona Basin and Lake Ohrid) and marine (e.g., Adriatic Sea core PRAD 1-2 and Aegean Sea core LC21) palaeoclimate records in the Mediterranean region.

Insights into eruption dynamics from the 2014 pyroclastic deposits of Kelut volcano, Java, Indonesia, and implications for future hazards

On 13 to 14 February 2014 a ~4h long, VEI 4 eruption occurred at Kelut volcano (Java, Indonesia). Pyroclastic density currents (PDCs) and extensive ash fall led to 7 fatalities and disruption to flights across the Asia-Pacific region. New sedimentological descriptions of the pyroclastic deposits from the 2014 eruption were compared with eyewitness and satellite reports to elucidate temporal variations in eruptive dynamics. The stratigraphy of the deposits is presented in 3 stages, associated with two eruptions that occurred approximately ~15–30min apart. Stage 1 PDC deposits originate from the smaller onset eruption. The PDC deposits from Stage 2, and tephra fall deposits from Stage 3 originate from the second, plinian eruption. During the onset eruption (Stage 1), low energy PDCs were produced that ran out to <2.6km. Basal layers show characteristics of deposits similar to ash-cloud surges that carried dominantly fine ash and crystal fragments. These are capped by deposits typical of high-particle concentrated pyroclastic flows. All Stage 1 deposits have high contents of dense lithic fragments (up to 44% by vol.), sourced from the 2007–2008 lava dome and conduit walls, indicating that the eruption onset was driven by an explosive release of gas-overpressure below the vent-capping dome. Increases in the magma flux and transition to a more constant eruption led to a growing eruption column during the ~2-hour long Stage 2 plinian eruption. Pumice rich (>70% by vol.) PDC deposits ran out to 4.7km from the vent. The deposits reflect an increased output of fresh fragmented magma, and some conduit widening evidenced by dense lithic fragments. Vent instabilities and incorporation of dense material into basal margins of the plume led to the marginal collapse and formation of these PDCs. Stage 3 occurred in the final hour at the peak of the plinian eruption, around 01:00 to 02:00, and produced reversely graded lapilli fall deposits with ≤90vol% pumice from a 26km-high plume. This indicates that there was a sustained flux of juvenile magma to the now open vent system, and expansion and fragmentation of the gas-rich magma was at its most efficient. Our study of the eruptive sequence of Kelut provides some constraints on predicted patterns for future explosive activity, critical for further hazard assessment of the volcano. Since 1901 Kelut has erupted on intervals of 1 to 23years, and the 2014 event characterises a typical “explosive” style of eruption that alternates regularly with effusive dome-formation and collapse events. This pattern depends on the dynamics of magma renewal to the system, and degassing conditions in the shallow magma reservoir and upper conduit. If this pattern holds, and the next eruption occurs within the next two decades, a return of prolonged dome growth could be anticipated.

Evaluating relative tephra fall hazard and risk in the Asia-Pacific region

Evaluating relative tephra fall hazard and risk in the Asia-Pacific region

Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes

The Tungurahua volcano (Northern Andean Volcanic Zone) has been erupting since 1999, with at least four eruptive phases up to present. Although a dozen of research focuses in tephra fall deposits during this period, none of them cover the full eruptive cycle. We investigated the eruptive mechanisms and tephra fall deposition processes at Tungurahua between 1999 and 2014, through systematic analyses of tephra samples collected westward of the volcano using mechanical sieving grain size analysis, lithology, scanning electron microscopy, X-Ray fluorescence and X-Ray diffraction. Tephra is compounded by varying amounts of scoria (black and brown), lithics, hydrothermally altered fragments, pumice, glass shards and free crystals. Textural analyses of juvenile grains (scoria, pumice and glass shards) revealed a diversity of features concerning to their vesicularity, shape and surface/perimeter. Initially, tephra was characterized by hydrothermally altered fragments related to a phreatic phase which then evolved to a pure magmatic activity with Strombolian eruptions. A homogeneous andesitic composition was observed between 1999 and 2003; however silicarich compositions occurred later in 2006. Similarly, the mineral assemblage contained plagioclase, pyroxene and olivine, but magnetite and akermanite were then included during 2006, thus indicating the eruption of a new, probably mixed magma. As consequence, Plinian activity occurred in August 2006. Further activity in 2007 ejected notable amounts (40-65%) of recycled material during Vulcanian eruptions. New eruptions occurred between 2008 and 2010, and juvenile ash revealed the interplay between brittle and ductile fragmentation through ash explosions, jetting events and Strombolian activity. The activity between 2010 and 2012 incorporated hydrothermally altered material at time that eruptive silences became longer and frequent, thus suggesting the development of a sporadic hydrothermal system. Finally, between 2013 and 2014 a series of Vulcanian events occurred. Observed grain size distributions allow us to propose three different processes occurring during tephra deposition: 1) deposition of multiple ash plumes, 2) contributions from elutriated pyroclastic density currents or grain size mixing due to major eruptions, and 3) the aggregation of particles due to rain and/or lighting. From mineralogy and grain size we infer that exposition to ash may produce acute human health effects.

Significance and timing of the mid-17th-century eruption of Long Island, Papua New Guinea

Tibito Tephra was first recorded in the central highlands of Papua New Guinea (PNG) in 1971. By the late 1970s, the tephra had been mapped across tens of thousands of square kilometres, traced to its source on Long Island in the Bismarck Sea, and linked to pyroclastic density currents in the Matapun Beds on the island. With a tephra fall volume &gt;10 km3, this eruption was clearly one of the 10 largest globally in the past 600 years. Although almost certainly in the AD 17th or 18th centuries, determining just when this VEI 6 eruption occurred has proved difficult. Whether this eruption occurred before or after William Dampier sailed past, named, described and drew a profile of Long Island has been debated also. Dating the Long Island eruption has implications for assessing the rate of revegetation of the island, recolonisation of the island and caldera lake, Jared Diamond’s theory of ‘supertramp’ birds, correlations between major eruptions and ice core chronologies, the constant rate of supply model of 210Pb concentration in lake sediments, reservoir effects in Lake Kutubu in the southern highlands of PNG, the timing of a prehistoric phase of agriculture in the Western Highlands and the longevity of oral histories recording a taim tudak (time of darkness) when sand fell from the sky, houses collapsed, crops were ruined and people died. This paper reviews early dates based on radiocarbon, 210Pb and paleosecular magnetic variation, historical reports and genealogical dates from oral histories, and speculation based on tree ring and ice core evidence. Analyses of 10 new radiocarbon dates from the Matapun Beds on Long Island are reported. Our best estimate places the eruption between 1651 and 1671 AD with a 95.4% probability and between 1655 and 1665 AD with a 68.2% probability.

Holocene development of mountain wetlands within and outside of landslide in the Hachimantai volcanic group, northeastern Japan

In warm and humid, tectonically active regions like Japan, landslides are an important influence on the development of mountain wetlands, but data bearing upon the relationship of Quaternary landslides to wetland development are scarce. We used lithological analysis, 14C dating, tephra age data, and carbon contents of wetland cores to compare the evolution of four wetlands, one (the Oyachi wetland) within a landslide area transformed by mass movement and three (the Appi Highland wetlands) outside of a landslide area, in the heavy snowfall region of the Hachimantai Mountains of northeastern Japan. Evidence from the Oyachi wetland shows that its transition from peatland to forest was interrupted at about 5500 cal BP by slope movement leading to the development of a lake that was drained by streams at about 3300 cal BP, after which a peatland environment has persisted until the present. We suggest that the evolution of this type of wetland is primarily influenced by landslide movements and stream dissection rather than climate change. In the Appi Highland wetlands, peatlands appeared much later, around the time of the To-a tephra fall in 915 AD, during the Medieval Warm Period, and have persisted with little change until the present. We suggest that the development of mountain wetlands outside of landslide areas is primarily related to climate changes rather than topographic changes. Sediment analyses of mountain wetlands within landslide areas may be useful for tracing the development of Quaternary landslides and subsequent topographic changes that may have implications for biodiversity in mountainous regions.

Holocene tephrochronology of the lower Río Cisnes valley, southern Chile

Sediment cores from lakes and bogs in the Río Cisnes valley contain tephra from explosive eruptions of volcanoes in the southern part of the Andean Southern Volcanic Zone (SSVZ). These tephra, which thicken and coarsen to the west, are attributed to eruptions from Melimoyu, Mentolat, Hudson, and potentially either Macá, Cay or one of the many minor eruptive centers (MEC) located both along the Liquiñe-Ofqui Fault Zone (LOFZ) and surrounding the major volcanoes. Correlation of the tephra between two new cores in the lower Río Cisnes valley, and amongst other cores previously described from the region, and source volcano identification for the tephra, has been done using lithostratigraphic data (tephra layer thickness and grain size), petrography (tephra glass color, vesicle morphology, and type and abundance of phenocryst phases), and by comparison of bulk tephra trace-element characteristics with previously published whole-rock and bulk tephra chemical analysis. Four tephras in these cores are attributed to eruptions of Mentolat, four to eruptions from Melimoyu, one possibly to Hudson, and six cannot be assigned to a specific source volcano. Some of these tephra correspond to pyroclastic tephra fall deposits previously observed in outcrop, including the MEL2 eruption of Melimoyu and the MEN1 eruption of Mentolat. However, others have not been previously observed and represent the products of newly identified small to medium sized eruptions from volcanoes of the SSVZ. These results provide new information concerning the frequency and magnitude of explosive eruption of SSVZ volcanoes and contribute to the evaluation of volcanic hazards in the region.

Framework for developing volcanic fragility and vulnerability functions for critical infrastructure

Volcanic risk assessment using probabilistic models is increasingly desired for risk management, particularly for loss forecasting, critical infrastructure management, land-use planning and evacuation planning. Over the past decades this has motivated the development of comprehensive probabilistic hazard models. However, volcanic vulnerability models of equivalent sophistication have lagged behind hazard modelling because of the lack of evidence, data and, until recently, minimal demand. There is an increasingly urgent need for development of quantitative volcanic vulnerability models, including vulnerability and fragility functions, which provide robust quantitative relationships between volcanic impact (damage and disruption) and hazard intensity. The functions available to date predominantly quantify tephra fall impacts to buildings, driven by life safety concerns.We present a framework for establishing quantitative relationships between volcanic impact and hazard intensity, specifically through the derivation of vulnerability and fragility functions. We use tephra thickness and impacts to key infrastructure sectors as examples to demonstrate our framework. Our framework incorporates impact data sources, different impact intensity scales, preparation and fitting of data, uncertainty analysis and documentation. The primary data sources are post-eruption impact assessments, supplemented by laboratory experiments and expert judgment, with the latter drawing upon a wealth of semi-quantitative and qualitative studies. Different data processing and function fitting techniques can be used to derive functions; however, due to the small datasets currently available, simplified approaches are discussed. We stress that documentation of data processing, assumptions and limitations is the most important aspect of function derivation; documentation provides transparency and allows others to update functions more easily. Following our standardised approach, a volcanic risk scientist can derive a fragility or vulnerability function, which then can be easily compared to existing functions and updated as new data become available.To demonstrate how to apply our framework, we derive fragility and vulnerability functions for discrete tephra fall impacts to electricity supply, water supply, wastewater and transport networks. These functions present the probability of an infrastructure site or network component equalling or exceeding one of four impact states as a function of tephra thickness.

Dynamics and style transition of a moderate, Vulcanian-driven eruption at Tungurahua (Ecuador) in February 2014: pyroclastic deposits and hazard considerations

Abstract. The ongoing eruptive cycle of Tungurahua volcano (Ecuador) since 1999 has been characterised by over 15 paroxysmal phases interrupted by periods of relative calm. Those phases included one Subplinian as well as several Strombolian and Vulcanian eruptions and they generated tephra fallouts, pyroclastic density currents (PDCs) and lava flows. The 1 February 2014 eruption occurred after 75 days of quiescence and only 2 days of pre-eruptive seismic crisis. Two short-lived Vulcanian explosions marked the onset of the paroxysmal phase, characterised by a 13.4 km eruptive column and the trigger of PDCs. After 40 min of paroxysm, the activity evolved into sporadic Strombolian explosions with discrete ash emissions and continued for several weeks. Both tephra fall and PDCs were studied for their dispersal, sedimentology, volume and eruption source parameters. At large scale, the tephra cloud dispersed toward the SSW. Based on the field data, two dispersal scenarios were developed forming either elliptical isopachs or proximally PDC-influenced isopachs. The minimum bulk tephra volumes are estimated to 4.55 × 106 m3, for an eruption size estimated at volcanic explosivity index (VEI) 2–3. PDCs, although of small volume, descended by nine ravines of the NNW flanks down to the base of the edifice. The 1 February 2014 eruptions show a similar size to the late 1999 and August 2001 events, but with a higher intensity (I 9–10) and shorter duration. The Vulcanian eruptive mechanism is interpreted to be related to a steady magma ascent and the rise in over-pressure in a blocked conduit (plug) and/or a depressurised solidification front. The transition to Strombolian style is well documented from the tephra fall componentry. In any of the interpretative scenarios, the short-lived precursors for such a major event as well as the unusual tephra dispersion pattern urge for renewed hazard considerations at Tungurahua.

A Bayesian approach to linking archaeological, paleoenvironmental and documentary datasets relating to the settlement of Iceland (Landnám)

Icelandic settlement (Landnám) period farmsteads offer opportunities to explore the nature and timing of anthropogenic activities and environmental impacts of the first Holocene farming communities. We employ Bayesian statistical modelling of archaeological, paleoenvironmental and documentary datasets to present a framework for improving chronological robustness of archaeological events. Specifically, we discuss events relevant to the farm Hrísbrú, an initial and complex settlement site in southwest Iceland. We demonstrate that tephra layers are key in constraining reliable chronologies, especially when combined with related datasets and treated in a Bayesian framework. The work presented here confirms earlier interpretations of the chronology of the site while providing increased confidence in the robustness of the chronology. Most importantly, integrated modelling of AMS radiocarbon dates on Hordeum vulgare grains, palynological data, documented evidence from textual records and typologically diagnostic artefacts yield increased dating reliability. The analysis has also shown that AMS radiocarbon dates on bone collagen need further scrutiny. Specifically for the Hrísbrú farm, first anthropogenic footprint palynomorph taxa are estimated to around AD 830–881 (at 95.4% confidence level), most likely before the tephra fall out of AD 877 ± 1 (the Landnám tephra layer), demonstrating the use of arable fields before the first known structures were built at Hrísbrú (AD 874–951) and prior to the conventionally accepted date of the settlement of Iceland. Finally, we highlight the importance of considering multidisciplinary factors for other archaeological and paleoecological studies of early farming communities of previously uninhabited island areas.

The opening subplinian phase of the Hekla 1991 eruption: properties of the tephra fall deposit

The 1991 Hekla eruption started on 17th of January with an intense 50-min-long explosive phase that transitioned into fire fountain activity lasting for 2 days. The eruptive plume rose to maximum height in about 10 min and the total mass of tephra deposited from the opening phase was 8.6 × 109 kg (VEI 3 event). The principal axis of tephra fall is to the NNE of Hekla and grain-size analysis reveals a systematic decrease in grain-size away from source. Majority of sample sites show typically unimodal grain-size distributions, although a few have bimodal distributions where the secondary mode is a subtle finer peak. The calculated total grain-size distribution (TGSD) is bimodal, with a coarse primary peak (−3.5 to −2.5 φ) and a subordinate fine peak (2.5 to 3.5 φ). The coarse peak is lapilli-dominated and was deposited within the first 25 km of transport, whereas the fine peak is coarse-ash-dominated and fits well with the modal grain-size of samples deposited >65 km from Hekla. Ascent rate of the magma and conditions for vesiculation in the shallow conduit became increasingly uniform with time through the 1991 opening phase.

霧島火山新燃岳2011年3月～2012年2月の降灰量調査 : 即時的降灰量推定の予察的検討

霧島火山新燃岳2011年3月～2012年2月の降灰量調査 : 即時的降灰量推定の予察的検討

Unifying tephrostratigraphic approaches to redefine major Holocene marker tephras, Mt. Taranaki, New Zealand

In this study, geochemical fingerprinting of glass shards and titanomagnetite phenocrysts was used to match twenty complex pyroclastic deposits from the flanks of Mt. Taranaki to major tephra fall “marker beds” in medial and distal deposition sites. These correlations hinged upon identifying time-bound compositional changes (a chemostratigraphy) in distal Taranaki tephra-fall sequences preserved in lake and peat sediment records around the volcano. The current work shows that previous soil-stratigraphy based studies led to miscorrelations, because they relied upon radiocarbon dates, a “counting back” approach, and an underestimate of the number of eruptions that actually occurred in any time frame. The new tephrostratigraphy proposed at Mt. Taranaki resulted from stratigraphic rearranging of several earlier-defined units. Some tephra units are older than previously determined (e.g., Waipuku, Tariki, and Mangatoki; ~6 to 9calkaBP), while one of the most prominent Taranaki marker tephra deposit, the Korito, is shown to lie stratigraphically above a widespread rhyolitic marker bed from Taupo volcano, the Stent Tephra (also known as unit Q; ~4.3calkaBP). Pyroclastic tephra deposits previously dated between ~6 to 4calkaBP at a key tephra section, c. 40km NE of Mt. Taranaki's summit, were misidentified and are now shown to comprise new marker tephra deposits, including the Kokowai (~4.7calkaBP), which is a prominent marker horizon on the eastern flanks of the volcano. A new local proximal stratigraphy for <5calkaBP tephra units can be well correlated to tephra layers within distal lake and peat sequences, but the differences between the two records indicates an overall larger number of eruptions have occurred at this volcano than previously thought. This study additionally demonstrates the utility of titanomagnetite chemistry for discrimination and correlation of groups or sequences of tephra deposits – even if unique compositions cannot be identified.