Distal Tephra Deposits Research Articles

Linking proximal ignimbrites and coeval distal tephra deposits to establish a record of voluminous Early Quaternary (2.4–1.9 Ma) volcanism of the Tauranga Volcanic Centre, New Zealand

The Tauranga Volcanic Centre (TgaVC) of the North Island, New Zealand, was active from 2.95 to 1.9 Ma. It lies temporally and spatially between the currently active Taupō Volcanic Zone, one of the most productive regions of Quaternary silicic volcanism globally, and its predecessor, the Coromandel Volcanic Zone. This study provides an enhanced chronology for pyroclastic volcanism of the TgaVC with four locally widespread units named and defined as follows: Welcome Bay and Wharo ignimbrites (together formerly known as Lower Pāpāmoa Ignimbrite); Otawera Ignimbrite; and Arateka Ignimbrite (formerly known as Upper Pāpāmoa Ignimbrite). These eruptives were followed by the eruption of the large-volume Waiteariki Ignimbrite at 2.1 Ma. In northern Hawke's Bay (150–170 km southeast of TgaVC), several distal tephra-fall horizons and an ignimbrite (Hikuroa Pumice Member) are preserved in marine-hosted sediments that are stratigraphically equivalent to the five ignimbrites of the TgaVC. Using new major- and trace-element data on glass in conjunction with new zircon-derived U-Pb ages, a correlation between the Hawke's Bay tephras and the TgaVC ignimbrites is established and the Hikuroa Pumice Member is confirmed as the distal deposit of the Waiteariki Ignimbrite. The integrated proximal and distal records show that at least eight eruptions occurred in the TgaVC between 2.4 and 1.9 Ma and provide a maximum repose period between explosive eruptions of ca. 50 kyrs.

Evidence for a large-magnitude Holocene eruption of Mount Rittmann (Antarctica): A volcanological reconstruction using the marine tephra record

In Antarctica, the near-source exposures of volcanic eruption deposits are often limited as they are not well preserved in the dynamic glacial environment, thus making volcanological reconstructions of explosive eruptions extremely challenging. Fortunately, pyroclastic deposits from explosive eruptions are preserved in Southern Ocean sediments surrounding Antarctica, and the tephrostratigraphy of these sequences offers crucial volcanological information including the timing and tempo of past eruptions, their magnitude, and eruption dynamics. Here we report the results of a tephrostratigraphy and tephrochronology study focused on four sediment cores recovered from the Wood Bay area in the western Ross Sea, Antarctica. In all these sedimentary sequences, we found a well-stratified primary tephra of considerable thickness, up to 80 cm, hereafter named the Aviator Tephra (AVT). According to the characteristics of the tephra deposit and its distribution, the AVT was associated with an eruption of considerable intensity, potentially representing one of the largest Holocene eruptions recorded in Antarctica. Based on the major and trace element geochemistry and the mineral assemblage of the tephra, Mount Rittmann was identified as the source of the AVT. A Holocene age of ∼11 ka was determined by radiocarbon dating organic material within the sediments and 40Ar-39Ar dating of alkali-feldspar crystals included in the tephra. Eruption dynamics were initially dominated by hydromagmatic magma fragmentation conditions producing a sustained, relatively wet and ash-rich eruptive cloud. The eruption then evolved into a highly energetic, relatively dry magmatic Plinian eruption. The last phase was characterized by renewed efficient magma-water interaction and/or collapse of the eruptive column producing pyroclastic density currents and associated co-ignimbritic plumes. The distal tephra deposits might be linked to the widespread lag breccia layer previously identified on the rim of the Mount Rittmann caldera which share the same geochemical composition. Diatoms found in the sediments surrounding the AVT and the primary characteristics of the tephra indicate that the Wood Bay area was open sea at the time of the eruption, which is much earlier than previously thought. AVT is also an excellent tephrostratigraphic marker for the Wood Bay area, in the Ross Sea, and a useful marker for future synchronization of continental ice and marine archives in the region.

Total grain size distribution of components of fallout deposits and implications for magma fragmentation mechanisms: examples from Campi Flegrei caldera (Italy)

In this paper, we present results of the reconstruction of the total grain size distribution (TGSD) of the material erupted during explosive volcanic eruptions at Campi Flegrei (Italy) considering all components (juvenile, lithic and crystal clasts). To date, the few TGSDs made available have been mostly reconstructed by assuming that the tephra deposits consisted of only one component. This simplification can introduce substantial bias in the interpretation of magma fragmentation mechanisms and significantly affect ash dispersion forecasts, since each tephra component has specific aerodynamic characteristics. By means of field investigations and laboratory analyses on samples collected from deposits of the Agnano-Monte Spina and Astroni eruptions, we reconstructed the TGSDs of juvenile, lithic and crystal components via the Voronoi tessellation method. Our results show how the systematic reconstruction of a TGSD, from the component-specific to the bulk TGSD, can provide important information on magma fragmentation mechanisms and wall-rock erosion processes. Results confirm that the bulk TGSD is the combination of the merging of different component subpopulations, according to their own TGSD, density and relative mass proportions. In addition, the integration of component analysis, TGSD and settling velocity data allowed characterization of the aerodynamic behaviour of each component at variable distances from the vent, which can be related to their own parent grain size distribution. The integration of new data from distal tephra deposits for the considered eruptions has thus allowed a reassessment of the erupted volumes of eruptions considered in this research, which are now 3.17 and 0.63 km3, for Agnano-Monte Spina and Astroni, respectively.

Sources of uncertainty in the Mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions

Estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale. This study uses the tephra from the caldera forming eruption of Mount Mazama (Crater Lake, OR, USA) to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude. We combine > 250 Mazama tephra occurrences, reported by a range of disciplines (including archaeology, paleoclimatology and volcanology), with new field studies to better understand the extent of the distal tephra. We find that the Mazama tephra has been remobilised to varying degrees over the past 7000 years, so each tephra locality was appraised for the likelihood that it records primary tephra fallout. We designated 45 of the distal (> 100 km from source) tephra sites as suitable for use in the production of isopachs using a spline fitting method. The new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic Mazama eruption to ~ 176 km3 (~ 61 km3 dense-rock equivalent; DRE). This study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit. Interpreting remobilised deposits can provide insight into post-eruptive processes that could potentially pose secondary hazards following large magnitude eruptions. We also show that in some circumstances, remobilised deposits preserve important physical properties such as grain size.

Geochemical characterization of mid-distal Nisyros tephra on Datça peninsula (southwestern Anatolia)

We present new distal records of tephra deposits that overly the Kos ignimbrite in seven locations of Datça peninsula. Tephra in one of these locations were previously associated with Nisyros Kyra sub-unit based only on the field characteristics. We use different proxies such as field observations, petrography, mineral, glass, and whole-rock chemistry in order to characterize and correlate the previously and recently identified pumice fall deposits on Datça. The total thickness of the fall deposit reaches to 3.5m. The size of the pumice clasts is generally within the range of lapilli, and they have vitrophyric texture consisting mainly of plagioclase (andesine to labradorite) with scarce clinopyroxene (diopside to augite), olivine (Fo48–50), amphibole (magnesio-hastingsite), and biotite crystals. Amphibole is a ubiquitous phenocryst in all Datça tephra units and used as a criterion for the correlation. Glass major element analyses by EMPA reveal two different groups with andesitic and dacitic compositions. Difference in silica content (up to ca. 4wt%) detected in the same specimen also designates the heterogeneity in pumice glass. This heterogeneity in glass composition is also supported by the frequent occurrence of banded pumice clasts in Datça tephra. Whole-rock composition of the pumice is mainly andesitic with calc-alkaline affinity. Multi-element patterns on primitive-mantle normalized diagram display typical arc-magmatism signature (i.e. depletion in Nb, Ta, Ti, and P).In order to check and eliminate the potential alternatives, we compared the distal deposits on Datça not only with Kyra, but also with other Nisyros tephra units. Yet, Kyra is the only unit that has comparable depositional characteristics, calcic amphibole crystals, andesitic-dacitic glass and whole-rock chemistry, and distal tephra deposits on neighboring islands (Tilos and Chalki). Therefore, we associate Datça tephra deposits with some proximal Kyra subunits of intermediate composition. However, if further geochemical (especially glass and mineral chemistry) data are provided for the Lakkí and Melisserí tephra units, a more thorough tephra correlation will be possible. Finally, our results may have implications on Nisyros tephra dispersal and geochemical characterization.

Explosive eruption of El Chichón volcano (Mexico) disrupted 6th century Maya civilization and contributed to global cooling

Abstract A remarkably long period of Northern Hemispheric cooling in the 6th century CE, which disrupted human societies across large parts of the globe, has been attributed to volcanic forcing of climate. A major tropical eruption in 540 CE is thought to have played a key role, but there is no consensus about the source volcano to date. Here, we present evidence for El Chichón in southern Mexico as the most likely candidate, based on a refined reconstruction of the volcano’s eruption history. A new chronological framework, derived from distal tephra deposits and the world’s largest Holocene beach ridge plain along the Gulf of Mexico, enabled us to positively link a major explosive event to a prominent volcanic sulfur spike in bipolar ice core records, dated at 540 CE. We speculate that voluminous tephra fall from the eruption had a severe environmental impact on Maya societies, leading to temporary cultural decline, site abandonment, and migration within the core area of Maya civilization.

New insights into the evolution of the magmatic system of a composite andesite volcano revealed by clasts from distal mass-flow deposits: Ruapehu volcano, New Zealand

Stratovolcanoes characteristically build large composite edifices over long periods with stacked lavas intercalated with pyroclastic deposits. In most cases, only the most recent volcanic products are exposed on the flanks of the volcano, and consequently the search for deposits recording an older eruptive and magmatic history is typically focussed far from the cone, within distal tephra deposits. Clasts within lahar and debris avalanche deposits may also provide unique insights into the earliest eruptive and magmatic history of long-lived volcanoes, especially when widespread fallout is absent. Careful sampling and subsequent petrological and geochemical analyses of lava and pumice clasts from six distal mass-flow deposit sequences (hyperconcentrated flow, debris flows and debris avalanche deposits) from Mt. Ruapehu (New Zealand), combined with detailed stratigraphic studies and radiometric age dating, give new perspectives on the pre-50 ka magmatic system of this complex volcano. A conglomerate emplaced between 340 and 310 ka contains evidence for the oldest episode of Mt. Ruapehu volcanism, and unusually for the composite cone, pumice clasts from this unit contain amphibole-bearing xenoliths. Chemical and petrological data for these oldest Ruapehu clasts indicate that a deep (∼40 km) crustal storage system had already developed under Mt. Ruapehu before ∼340 ka. From the very earliest stages, evolution was largely controlled by magma mixing, along with decoupled assimilation and fractional crystallization within numerous isolated small-scale magma batches stored throughout the crust. From around 340 to 160 ka, there was a progressive shift towards more primitive compositions, suggesting that during this period large-scale replenishment events involving mantle-derived basaltic magmas occurred within the mid- to upper crustal storage system. Subsequent magmas became progressively more evolved due to decoupled fractional crystallization and assimilation processes accompanied by magma recharge events, which triggered major phases of eruptive activity.

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.

High-resolution palynological evidence for vegetation response to the Laacher See eruption from the varved record of Meerfelder Maar (Germany) and other central European records

The Laacher See eruption (LSE) was one of the larger Plinian eruptions of the late Quaternary in Europe. Distal tephra deposits of the LSE are found in three major directions and have been encountered over an area of at least 230,000km2. Large volcanic explosions such as the LSE can have direct as well as indirect effects on vegetation and climate. Previous studies suggest that the effects of the LSE probably were of short duration only, in the order of several decades at the longest. The varved sediment record of Meerfelder Maar provides an excellent natural archive to study the impact of the LSE on the vegetation. Using a sub-decadal sampling resolution, we are able to reconstruct an abrupt decrease in the abundance of pine pollen to only 5% in the last ~5years prior to the LST. A concurrent increase in charcoal strongly suggests that the disappearance of pine is related to fire-activity before the eruption. All taxa show decreased pollen productivity directly following the LSE, although most taxa recover within 1–2 decades. Pinus takes the longest to recover to high values after the LSE. Mechanisms that could explain this longer-lasting minimum include short-scale climate changes such as an increase in precipitation or a decrease in temperature, soil acidification, and an increase in wildfires. Variance partitioning shows that the pollen data (both percentage and PAR) show a statistically significant response to the LSE. A review of other high-resolution palynological records shows that sites that are situated relatively close (101–102km) to the Laacher See show a distinct change in the pollen diagrams, commonly mainly a decrease in Pinus, just above the LST, while sites that are located further away only show minor fluctuations or no change at all. The MFM record is the only dataset that shows a decrease in the abundance of pine pollen prior to the LST.

Distal tephras of the eastern Lake Victoria basin, equatorial East Africa: correlations, chronology and a context for early modern humans

The tephrostratigraphic framework for Pliocene and Early Pleistocene paleoanthropological sites in East Africa has been well established through nearly 50 years of research, but a similarly comprehensive framework is lacking for the Middle and particularly the Late Pleistocene. We provide the first detailed regional record of Late Pleistocene tephra deposits associated with artifacts or fossils from the Lake Victoria basin of western Kenya. Correlations of Late Pleistocene distal tephra deposits from the Wasiriya beds on Rusinga Island, the Waware beds on Mfangano Island and deposits near Karungu, mainland Kenya, are based on field stratigraphy coupled with 916 electron microprobe analyses of eleven major and minor element oxides from 50 samples. At least eight distinct distal tephra deposits are distinguished, four of which are found at multiple localities spanning >60 km over an approximately north to south transect. New optically stimulated luminescence dates help to constrain the Late Pleistocene depositional ages of these deposits. Our correlation and characterization of volcaniclastic deposits expand and refine the current stratigraphy of the eastern Lake Victoria basin. This provides the basis for relating fossil- and artifact-bearing sediments and a framework for ongoing geological, archaeological and paleontological studies of Late Pleistocene East Africa, a crucial time period for human evolution and dispersal within and out of Africa.

Violent Strombolian or not? Using ground-penetrating radar to distinguish deposits of low- and high-energy scoria cone eruptions

Scoria cones often grow by the accumulation of individual particles transported ballistically in Strombolian-type eruptions. Alternative models of cone formation suggest that cones are built primarily of fallout from the eruption column in more explosive eruptions, often termed violent Strombolian. Currently, the distinction between normal Strombolian and violent Strombolian is based on direct observations of eruptions or by inference of eruption characteristics from mapping of tephra fall deposits. Unfortunately, medial to distal tephra fall deposits erode rapidly, leaving behind only the near-vent facies of scoria cones which are thicker and less easily eroded. Therefore, a tool that is capable of delineating differences between low-energy Strombolian deposits and higher energy violent Strombolian deposits from investigation of the preserved scoria edifice is necessary. Ground-penetrating radar imaging of Cerro Negro, an active basaltic scoria cone in Nicaragua, has revealed details of cone deposits at depths of up to 12 m. The record of the 1992 eruption, which was observed to be violent Strombolian, shows quantifiable differences from normal Strombolian near-vent facies, including reflections in the downwind profile that are continuous for hundreds of meters and through the slope break. The ability to differentiate between tephra fallout and ballistically emplaced deposits at Cerro Negro suggests ground-penetrating radar imaging may be useful in distinguishing eruptive style in older scoria cones, where the medial to distal tephra deposits are eroded or buried.

Late glacial explosive activity on Mount Etna: Implications for proximal–distal tephra correlations and the synchronisation of Mediterranean archives

Plinian and Ignimbrite deposits represent explosive activity (ca. 17–19calka) within the predominantly effusive and mildly explosive (Strombolian) volcanic history of Mount Etna (Italy). Proximal glasses from the Biancavilla Ignimbrites and Unit D Plinian fall deposits are characterised. Fall deposits recorded at Acireale (D1b and D2b) and Giarre (D1a and D2a) are geochemically distinct confirming they relate to different eruptions. The Acireale Plinian fall (D1b and D2b) deposits compositionally overlap with the Biancavilla Ignimbrite deposits. These explosive eruptions from Etna are considered responsible for widespread ash dispersals throughout the central Mediterranean region, producing the marker tephra layers (Y-1/Et-1) recorded in marine and lacustrine sedimentary archives. Stratigraphically these distal tephras occur at or close to the onset of the last deglaciation (Termination 1) within their respective palaeoenvironmental records, therefore making them potentially crucial tephrostratigraphic markers. This study investigates distal tephra deposits thought to be from Etna recorded in the Ionian Sea (Y-1), Lago Grande di Monticchio (LGdM, Italy; tephras TM-11 and TM-12-1), Lago di Mezzano (Italy) and the Haua Fteah cave (Libya). The glass chemistry of Y-1 tephras recorded in the Ionian Sea and at Haua Fteah is consistent with the Biancavilla Ignimbrites (16,965–17,670calyrs BP) and the upper Acireale Plinian fall (D2b). The LGdM record indicates that explosive activity on Etna associated with Unit D spans a minimum of 1540±80 varve years. TM-12-1 (19,200–19,804calyrs BP) in LGdM appears to represent the oldest distal counterpart of Etna Unit D explosive activity and is associated with the lower Acireale (D1b) Plinian eruption. The proximally undefined TM-11 (17,640–18,324calyrs BP) and distal correlatives are geochemically distinct from the Ionian Sea Y-1 tephra. Such significant compositional differences seen between distal tephra layers are not observed within individual proximal units and are likely to indicate that the distal tephras relate to separate eruptive phases. Until proximal relationships can be established, the TM-11 type Y-1 equivalents should be termed TM-11. Great care should be exercised when using these distal ash layers to synchronise sedimentary records during a crucial period of environmental change.

Peralkaline magma evolution and the tephra record in the Ethiopian Rift

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.

Marine-continental tephra correlations: Volcanic glass geochemistry from the Marsili Basin and the Aeolian Islands, Southern Tyrrhenian Sea, Italy

Major, minor and trace element analysis of volcanic glass in proximal and distal (<2mm) tephra deposits underpins tephrochronology. This approach has been tested in the Aeolian Islands and the Tyrrhenian Sea using juvenile clasts in pyroclastic fall and flow deposits. Geochemical data are used to link marine tephras in the Marsili Basin (core TIR2000-C01) to explosive eruptions of (1) Lipari (Monte Pilato; 776calAD); (2) Vulcano; and (3) Campi Flegrei (Soccavo 1; 11,915–12,721calyears BP). Whether a polymictic coarse grained volcaniclastic turbidite in the Marsili Basin originated from collapse on Salina remains unresolved because multi-elemental analysis raises doubt about the published correlation to the Pollara region.It is evident that correlation of proximal continental and distal marine tephras, at a high level of confidence, requires a full complement of major, minor and trace element data. In conjunction with considerations of the mineralogy and morphology of juvenile deposits these data help define petrological lineages such that precise provenance can be established. Whilst a precise proximal–distal match must be based on identical major, minor and trace element concentrations it is clear that resurgent activity from a single volcano can produce magmas with identical compositions. In such cases stratigraphic relationships must complement any geochemical study. Occasionally proximal stratigraphies may be unrepresentative of the complete eruptive history because of a lack of exposure due to burial by more recent effusive and explosive activity, or sector collapse which can remove vital stratigraphy particularly on volcanic islands.

Ultra-distal tephra deposits from super-eruptions: Examples from Toba, Indonesia and Taupo Volcanic Zone, New Zealand

Voluminous rhyolitic eruptions from Toba, Indonesia, and Taupo Volcanic Zone (TVZ), New Zealand have dispersed volcanic ash over vast areas in the late Quaternary. The ∼74 ka Youngest Toba Tuff (YTT) eruption deposited ash over the Bay of Bengal and the Indian subcontinent to the west. The ∼340 ka Whakamaru eruption (TVZ) deposited the widespread Rangitawa Tephra, dominantly to the southeast (in addition to occurrences northwest of vent), extending across the landmass of New Zealand, and the South Pacific Ocean and Tasman Sea with distal terrestrial exposures on the Chatham Islands. These super-eruptions involved ∼2500 km3 and ∼1500 km3 of magma (dense-rock equivalent; DRE), respectively.Ultra-distal terrestrial exposures of YTT at two localities in India, Middle Son Valley, Madhya Pradesh, and Jurreru River Valley, Andhra Pradesh, at distances of >2000 km from the source caldera, show a basal ‘primary’ ashfall unit ∼4 cm thick, although deposits containing reworked ash are up to ∼3 m in total thickness. Exposures of Rangitawa Tephra on the Chatham Islands, >900 km from the source caldera, are ∼15–30 cm thick. At more proximal localities (∼200 km from source), Rangitawa Tephra is ∼55–70 cm thick and characterized by a crystal-rich basal layer and normal grading. Both distal tephra deposits are characterized by very-fine ash (with high PM10 fractions) and are crystal-poor.Glass chemistry, stratigraphy and grain-size data for these distal tephra deposits are presented with comparisons of their correlation, dispersal and preservation. Using field observations, ash transport and deposition were modeled for both eruptions using a semi-analytical model (HAZMAP), with assumptions concerning average wind direction and strength during eruption, column shape and vent size. Model outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated with tephra fallout. Modeling based on observed YTT distal tephra thicknesses indicate a relatively low (<40 km high), very turbulent eruption column, consistent with deposition from a co-ignimbrite cloud extending over a broad region. Similarly, the Whakamaru eruption was modeled as producing a predominantly Plinian column (∼45 km high), with dispersal to the southeast by strong prevailing winds. Significant ash fallout of the main dispersal direction, to the northwest of source, cannot be replicated in this modeling. The widespread dispersal of large volumes of fine ash from both eruptions may have had global environmental consequences, acutely affecting areas up to thousands of kilometers from vent.

A Laacher See-eruption supplement to Tephrabase: Investigating distal tephra fallout dynamics

Tephrabase ( www.tephrabase.org) is an online data repository and research tool for tephrochronological investigations in northern Europe. This paper presents and discusses an update to Tephrabase that contains geographic and, as far as available, geochemical data on 452 data-points for the widespread isochronous Laacher See tephra. This tephra is dated to c. 12,920 calendar years BP and acts as an important chronostratigraphic marker for the Lateglacial GI-1a/late Allerød chronozone in terrestrial and lacustrine palaeoenvironmental archives as well as in archaeological stratigraphies. Based on these data, a revised map of the total tephra distribution is presented, suggesting that the north-eastern fallout lobe might contain a secondary fallout thickness maximum, 230–340 km distant from the eruptive centre. In addition, new geochemical data from a series of distal tephra deposits spanning most of the north-eastern fallout lobe are used to evaluate previous suggestions regarding the distribution of fallout from distinct eruption phases. The aim of the study is that future tephrochronological investigations can use this database and the revised distribution map for effectively targeting sites for analysis.

Scale in tephrostratigraphic correlation: An example from Turkish Pleistocene archaeological sites

Issues of scale are persistent but rarely made explicit in tephrostratigraphy. Eruption magnitude, the nature of available outcrop, and the resolution and type of data available define the scale of any proposed correlation. Different scales require different approaches to demonstrate, or more accurately, to estimate the likelihood of correlation among two or more deposits. This is emphasized through a probabilistic approach to the correlation of proximal and distal tephra deposits associated with Lower and Middle Paleolithic archaeological sites from central Turkey through the use of discriminant analyses and multiple analyses of variance.

Distal cryptotephra found in a Viking boathouse: the potential for tephrochronology in reconstructing the Iron Age in Norway

Distal tephra deposits from Icelandic volcanic eruptions have been found in Norway and can be used to precisely date a variety of sedimentary environments. Tephrochronology has not yet been applied to archaeological investigations in Norway because tephra are generally not found as visible layers, but are present as very low concentrations of glass shards (i.e. cryptotephra). In this study, we present results from the analysis of cryptotephras found in an Iron Age boathouse in northern Norway. The boathouse was associated with the chieftain center at Borg on Vestvågøy in the Lofoten Islands. In 2003, a trench was excavated and the stratigraphy of the boathouse was described. Radiocarbon ages from cultural deposits show that it was constructed in the Early Iron Age c. AD 540–660 and the main period of use was at the end of the Iron Age between c. AD 1030 and AD 1270. Volcanic glass shards were isolated from sediment samples collected above and below the cultural deposit representing the main period of use. Electron microprobe analysis of the glass shards showed that the lower sample resembles the AD 860 Layer B tephra and the upper sample resembles tephra erupted from the Hekla volcanic system between AD 1104 and AD 1300. These tephrochronologic dates agree with the radiocarbon-derived dates and possibly further constrain the boathouse’s main period of use to c. AD 1030–1104. Our results demonstrate the value of using tephrochronology for archaeological studies in Norway and the potential for finding cryptotephra from other large explosive volcanic eruptions during the Iron Age.

Geochemical fingerprinting of basaltic tephra deposits in the Auckland Volcanic Field

Small volume (<2 km3) basaltic volcanoes have been active throughout the late Quaternary in the Auckland Volcanic Field (AVF) in northern New Zealand. The main pyroclastic products of these centres are phreatomagmatic surges containing abundant accidental ejecta and magmatic fall deposits of limited aerial extent. Previously they had received no attention from a tephrostratigraphic perspective. Despite extensive weathering, many of these deposits contain basaltic glass that can be used to geochemically fingerprint the volcanic source and individual eruptive events. The glasses are predominantly basanites with SiO2 contents in the range 42–50 wt%. Many individual emplacement units are compositionally homogeneous on the basis of electron microprobe analysis (SiO2 ± 0.5 wt%), and can be distinguished on the basis of TiO2, CaO, K2O, and P2O5 contents. Individual tephra beds can easily be characterised, however volcanoes are more compositionally diverse. Although the volcanoes are considered monogenetic, they display a wide range of different eruptive styles, and their pyroclastic deposits can display a compositional range (SiO2 45–50 wt%) within short stratigraphic sequences that show no evidence of hiatus. This increases the difficulty in matching distal tephra deposits to their source. Glass compositional data from 18 volcanoes show no evidence of spatial or temporal trends within the AVF.

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.