Tephra Sequence Research Articles

Case study of a cluster of simple and complex monogenetic volcanoes in the north‐east part of the Michoacan–Guanajuato Volcanic Field, Central Mexico: Nomenclature implications

With the advent of new terminologies to categorize and characterize the simple and complex monogenetic volcanoes, also came the semantic issues, which caused a predicament for the usage of terms like simple, complex, polycyclic, polymagmatic, complex monogenetic volcanoes with polygenetic inheritance. To analyse and validate this nomenclature, we studied an overlapping volcanic structure located south of the present‐day town of Irapuato, Central Mexico, that appears to be a monogenetic complex at first sight. Field observations, tephra stratigraphy, petrography and geochemistry of the tephra deposits confirms that the structure is in fact a cluster of three simple (San Joaquin tuff ring and two scoria mounds) and one complex, polycyclic, polymagmatic (La Sanabria‐San Roque tuff ring) monogenetic volcanoes formed by independent events, governed by distinct conduits and magma bodies of different origin (subduction‐related, OIB and E‐MORB origin) and separated by different tephra sequences of dissimilar components and depositional characteristics. We estimate the magma volumes (using the juvenile content and their vesicularity percentage) to be at 0.40–1.31 × 108, 0.25 × 108 and 0.42–0.90 × 108 m3 for San Joaquin, La Sanabria and San Roque, reckoning an eruption duration of 77 and 48 and 81 days, respectively (considering an average eruption rate of 6 m3/s from the well‐documented shallow crater (<30 m), Ukinrek maars in Alaska) occurring within the age range of 40–70 k years (crater diameter and depth ratio). This study not only aided to validate the above‐mentioned terms for monogenetic volcanoes, but also reconsider a few of them and avoid confusion with the polygenetic counterparts.

Radiocarbon dating, eruptive dynamics, and hazard implications of four Holocene Plinian-type eruptions recorded in Manizales city, Colombia

Urban centers settled under the influence of volcanic activity are common in the Colombian Andes, particularly those such as Manizales city (ca. 400,000 inhabitants) and other smaller towns on the western flank of the Central Cordillera. Several pumice-rich tephra layers widely dispersed beneath the city suggest multiple Plinian-type eruptions have occurred nearby. From this geological record, the four youngest, thickest, and most persistent layers in the region are the most representative of this activity. This work uses mapping, physical descriptions, radiocarbon dating, and calculation of physical parameters to: (1) define and characterize the tephra sequence, (2) determine the age of the eruptions, (3) define the source and eruptive dynamics associated with each of the events, and (4) give insights into the volcanic hazard for Manizales city. The deposits were named in this study as Lower Manizales (Lm), Middle-lower Manizales (Mlm), Middle-upper Manizales (Mum) and Upper Manizales (Um) tephra layers, together forming the “Manizales tephra sequence”. Mapping of the deposits revealed the lower layer originated from the little known Plazuela volcano (~ 15 km east of Manizales), while the others originated in the Cerro Bravo volcano (~ 25 km east of Manizales). The radiometric analysis indicates that the lowest tephra layer originated at 7879.5 ± 55.5 cal. years BP, while the three overlying layers were deposited at 7730 ± 55 cal. years BP, 5294 ± 19 cal. years BP, and 2759 ± 20 cal. years BP. All of these deposits were formed by Plinian-type eruptions, with column heights varying between 18 and 29 km. The minimum tephra volume emitted by each eruption was between 0.21 and 0.38 km3, and the emission rate 21.6 × 107 kg/s for the Lm layer, 3.38 × 107 kg/s for the Mlm layer, 6.27 × 107 kg/s for the Mum layer, and 5.16 × 107 kg/s for the Um layers. The results suggest that the region is strongly threatened by pyroclastic falls associated with several active volcanoes located in the Central Cordillera.

A 6.2 Ma‐Long Record of Major Explosive Eruptions From the NW Pacific Volcanic Arcs Based on the Offshore Tephra Sequences on the Northern Tip of the Emperor Seamount Chain

AbstractWe present a continuous ∼6.2 Ma long record of explosive activity from the Northwest Pacific volcanic arcs based on a composite tephra sequence derived from Ocean Drilling Program Sites 882A and 884B, and core MD01‐2416 on the Detroit Seamount. Geochemical fingerprinting of tephra glass using major and trace element analyses and correlations of tephra layers between the three cores allowed the identification of 119 unique tephras, suggesting eruptions of magnitude (M) of 5.8–7.8. Age estimates for all the identified eruptions were obtained with the help of published and further refined age models for the studied cores, direct 40Ar/39Ar dating of four ash layers, and Bayesian age modeling. The glass compositions vary from low‐ to high‐K2O basaltic andesite to rhyolite and exhibit typical subduction‐related affinity. The majority of the tephras originated from Kamchatka, only a few tephras—from the neighboring Kuril and Aleutian arcs. The glass compositions revealed no temporal trends but made it possible to identify their source volcanic zones in Kamchatka and, in some cases, to determine their source eruptive centers. Our data indicates episodes of explosive activity recorded in the Detroit tephra sequence at ∼6,200, 5,600–5,000, 4,300–3,700 ka, and almost continuous activity since ∼3,000 ka. Within the latter episode, the most active intervals can be identified at 1,700–1,600, 1,150–1,050, and 600–50 ka. Geochemically fingerprinted and dated Detroit tephra sequence form a framework for dating and correlating diverse paleoenvironmental archives across the Northwest Pacific and for studies of geochemical evolution of the adjacent volcanic arcs.

Redox-controlled differential weathering of andesitic pumice: Possible catalytic effects of ferrous/ferric iron on rapid halloysite accumulation in a Holocene tephra layer

Differential weathering process of Holocene andesitic tephra in Atsuma, Hokkaido, northern Japan, were investigated with a specific focus on the flame-like occurrence of halloysite bands and their formation mechanism. The parent material was coarse-grained pumice (Ta-d2) deposited 9 ka to form a highly permeable metre-thick layer atop fine-grained impervious periglacial loam, which is covered by humic soil–tephra sequences. The shallower part of the Ta-d2 layer was weathered to a reddish or yellowish brown colour, which is characterized by intensive leaching of Si and Al, with abundant precipitation of iron (hydr)oxide and amorphous secondary minerals in an oxidizing environment. A reductive grey colour was observed only at the Ta-d2 bottom below the halloysite band, where pumice grains survived significant elemental leaching and contained ferrous iron in the solid phase with abundant dissolved Fe in the liquid phase. Between these oxidized and preserved zones, an irregularly shaped, fine-grained, white iron-rich halloysite layer with a decimetre-scale thickness developed. In this clay band, the molar ratio of Si to (Al + Fe) was nearly 1:1, with a small amount of remnant Ca. Mössbauer analysis revealed that Fe2+ was initially incorporated into the aluminosilicate sheet via Al3+ substitution and transformed into Fe3+ within the structure. The halloysite band was formed at depths in the tephra layer corresponding to the redox front. The abundance of Si and Al in pore water in any zone of the tephra layer meets the stability requirements of halloysite, indicating that the redox condition controls the rapid local accumulation of halloysite through the isomorphous substitution followed by valence change of iron. We propose a new hypothesis stating that iron catalyses halloysite nucleation via a stepwise function, including layer charging to absorb hydrated cations and subsequent charge loss resulting in cation desorption and passive intercalation of water molecules within the 1:1 layer.

Eruptive history of Mt. Fuji over the past 8000 years based on integrated records of lacustrine and terrestrial tephra sequences and radiocarbon dating

Reliable records of past volcanic activities are essential for assessing future eruption probabilities and establishing effective volcanic disaster mitigation plans. We integrated the lacustrine and terrestrial sequences of tephra deposits at the northeastern foot of Mt. Fuji based on high-resolution radiocarbon (14C) dating to elucidate its eruptive history over the past 8000 years. Accurate chronologies of 29 tephra layers in the Lake Yamanaka sediments (core YA-1) were determined using the age-depth model of the 14C of bulk organic matter after variability correction of lake reservoir ages. The following ages were compared to those of major tephra deposits in the proximal trench section (MF20-01) and in the literature. From ∼7300 to ∼3200 cal yr BP, four tephra layers in core YA-1 (i.e., YA-T1, 2, 7, and 10) were likely correlated with the scoria fall deposits S-3, 5, 6, and 10 respectively, as reported in the outcrops near the lake. The core also features six undetected eruptions from ∼5050 to ∼3900 cal yr BP, demonstrating the advantages of lake sediments proximal to the volcano in identifying previously undetected eruptions. After ∼3200 cal yr BP, five tephra layers in core YA-1, ranging from ∼2900 to ∼1600 cal yr BP (YA-T11, 12, 14, 20, and 22), showed similar age ranges as the major tephra layers in MF20-01 (IY5, 8, 11, 25, and 26), with identical lithological characteristics and stratigraphic orders. This suggests that the tephra sequence in the trench section positively reflects the major eruptive events over the past 2900 years, providing a basis for the comprehensive reconstruction of volcanic eruption histories in the region.

Millennial-scale coseismic landslide history inferred from topographic and stratigraphic features of a post-caldera cone of Aso Volcano in southwestern Japan

The 2016 Mw 7.0 Kumamoto earthquake triggered numerous landslides of fallout tephra deposits around Aso Volcano in southwestern Japan. Although rainfall-induced landslides have been repeatedly observed on the tephra-mantled slopes of this region, the spatial patterns and frequency of coseismic landslides remain unclear because of the scarce landslide evidence associated with palaeoseismicity. To examine the distribution of landslides induced by the 2016 Kumamoto earthquake and possibly by prior large earthquakes, we conducted geomorphological analyses using airborne light detection and ranging (LiDAR) data, together with geological investigations of tephra deposits in the Takanoobane lava dome (TLD), a post-caldera cone of Aso Volcano. Repeated LiDAR surveys performed before and after the earthquake allowed us to quantify the volume and geometry of five coseismic landslides (K1–K5), as well as three component blocks of the K1 landslide (K1a–c). The surveys also allowed identification of 12 older landslides (O1–O12) fully covered by vegetation. Source areas of recent coseismic landslides ranged from 1050 to 14,920 m2; those of older landslides were estimated to be 1110–15,730 m2. The H/L ratios of the K1 landslide and its component blocks K1a–c ranged from 0.139 to 0.165, despite their volumes ranging from 104 to 105 m3. The K1 landslide occurred on the southwest-facing slope, where older landslides were not recognised before the earthquake. Its slip surface was formed at the base of the 30 ka pumice layer. The largest one of the older landslides (O1) is located on the south-facing slope, adjacent to the K1 landslide. Stratigraphic investigations revealed that most of the tephra sequence, including the 30 ka pumice layer, has already been eroded at the centre of the O1 landslide. Radiocarbon dating further constrained the landslide age to ca. 7.0 ka. Because its topographic and stratigraphic features are similar to the features of the K1 landslide, the O1 landslide may have been triggered by a palaeoearthquake approximately 7000 years ago, which was previously unknown around the Aso caldera region.

Testing less-conventional methods to date a late-pleistocene to Holocene eruption: Radiocarbon dating of paleosols and 36Cl exposure ages at Pelado volcano, Sierra Chichinautzin, Central Mexico

Despite their significance for estimating hazards and forecasting future activity, dating young volcanic deposits and landforms (<50,000 yrs old) remains a challenge due to the limitations inherent to the different isotopic chronometers used. The Trans-Mexican Volcanic Belt is one of the most active and populated continental arcs worldwide, yet its temporal pattern of activity is poorly constrained. Such deficiency is particularly problematic for the Sierra Chichinautzin Volcanic Field (SCVF) that is located at the doorstep of Mexico City and Cuernavaca and is hence a major source of risk for these cities. Existing ages for this area derive mostly from either radiocarbon on charcoal, which is rare and may be contaminated, or 40 Ar/ 39 Ar on rock matrix, which is poorly precise for this time period and rock type. Here, we focus on the Pelado monogenetic volcano, which is located in the central part of the SCVF and erupted both explosively and effusively, producing a large lava shield and a widespread tephra blanket. This unique eruptive event was previously dated at ∼12 calibrated (cal) kyrs BP, using radiocarbon dating on charcoal from deposits related to the eruption. To test alternative dating approaches and confirm the age of this significant eruption, we applied two less conventional techniques, radiocarbon dating of bulk paleosol samples collected below the complete tephra sequence at nine sites around the shield, and in-situ 36 Cl exposure dating of two samples of an aphyric lava from the base of the shield. Radiocarbon paleosol ages span a continuous time interval from 13.2 to 20.2 cal kyrs BP (2σ), except for one anomalously young sample. This wide age spread, along with the low organic contents of the paleosols, may be due to erosive conditions, related to the sloping topography of the sampling sites and the cool and relatively dry climate of the Younger Dryas (11.7–12.9 ka), during which the Pelado eruption probably occurred. The two 36 Cl-dated lava samples have consistent ages at 1σ analytical errors of 15.5 ± 1.4 ka and 13.2 ± 1.2 ka, respectively, yielding an average age of 14.3 ± 1.6 ka for this lava flow. The high full uncertainty in 36 Cl ages (24%) is due to high rock Cl content. We conclude that paleosol radiocarbon dating is useful if numerous samples are analyzed and climatic and relief conditions at the time of the eruption and at the sites of tephra deposition are considered. The 36 Cl dating technique is an alternative method to date volcanic eruptions, as it gave consistent results, but in the specific case of Pelado volcano, the high Cl content in the analyzed rocks increases the age uncertainties. • Catalogue of recent eruptions in Trans-Mexican Volcanic Belt is incomplete. • 14 C on paleosol and 36 Cl dating are used on a previously dated monogenetic volcano. • New ages are coherent with previous 14 C charcoal age. • Spread in paleosol ages likely due to erosive conditions at time of eruption. • 36 Cl ages are statistically consistent but poorly precise, due to high Cl in rock.

Four widespread East Asian tephra marker horizons during early MIS 3: ∼60–50 ka tephrostratigraphy of Huguangyan Maar Lake southern China

Tephra layers from explosive eruptions can provide valuable isochronous marker horizons for paleoenvironmental studies across large regions. East Asia and Southeast Asia are host to more than half of the Earth's subaerial volcanoes, and tephras from these volcanoes offer significant numbers of widely distributed marker layers. However, the integrated tephrostratigraphic framework of this region is less refined because only limited (crypto-) tephra sequences have been established. In this study, we present a cryptotephra sequence covering the period ∼60–50 ka in East Asia from a lacustrine sediment from Huguangyan Maar Lake, southern China. Four cryptotephra layers were identified, one of which can be correlated with the ∼53.8 ka Unnan eruption (SUn) of Sambe volcano, Japan. This tephra provides a critical isochronous marker horizon for linking paleoenvironmental records from central Japan to southern China, illustrating a consistent environmental shift during early MIS 3 across this region. The three other tephras cannot be correlated with known eruptions, but glass geochemical compositions suggest that they may be from Japan or surrounding areas, implying that they too may be widely distributed around East Asia. Importantly, two of them feature at the start of MIS 3, and thus can be potentially be used to establish the synchroneity of the onset of MIS 3 around East Asia, overcoming challenges that presently hinder the dating of early MIS 3 in this region.

Reconstructing the middle to late Pleistocene explosive eruption histories of Popocatépetl, Iztaccíhuatl and Tláloc-Telapón volcanoes in Central México

The Sierra Nevada Volcanic Range (SNVR), which includes Popocatépetl, Iztaccíhuatl and Tláloc-Telapón volcanoes, has been the source of multiple large explosive eruptions that have dispersed tephra across central México. Several eruptions since 40 ka have previously been described, particularly from Popocatépetl, the southernmost volcano of the range. However, the longer-term eruption history of the SNVR is poorly understood, due to challenges with correlating limited exposures of older pyroclastic sequences, and in discriminating between tephras derived from different sources. Here we describe two extensive exposures located between Popocatépetl and Iztaccíhuatl volcanoes, which provide a more complete and longer-term explosive eruption record of the SNVR: the Nepopualco and Xalitzintla tephra sequences. A detailed tephrostratigraphic survey, together with new 40Ar/39Ar geochronological analyses and glass geochemistry, has permitted the characterization of identified eruption units further leading to the determination of geochemical fields for each volcano and the subsequent discernment of volcanic sources. Our results show that, since the collapse of the Los Pies Cone, which destroyed the Paleo-Iztaccíhuatl edifice at 631 ± 44 ka (2σ), Iztaccíhuatl has produced at least 6 explosive rhyolitic eruptions. After coeval activity with Popocatépetl, between ~600 and ~500 ka, Iztaccíhuatl's explosive activity ceased while Popocatépetl's continued until present day. Popocatépetl has produced at least 27 medium to large explosive eruptions (inferred VEI 4–6), commonly of andesitic to dacitic compositions. Some of these eruptions deposited pumice fallout of >1 m thick in both the Nepopualco and Xalitzintla sequences (e.g. the 339 ± 16 ka [2σ] NT-23/WRT-7 eruption), suggesting that Popocatépetl has produced several eruptions similar in magnitude to the well-known ~14 ka Tutti Frutti Pumice (a VEI 6 eruption with a ~5 km3 tephra volume). The Popocatépetl and Iztaccíhuatl tephras are interbedded with deposits from more distal volcanoes, including some mafic to intermediate products of unknown sources (possibly from nearby monogenetic cones) and tephras related to the late Pleistocene eruptions of Tláloc-Telapón (including the tephra layer produced by the San Valentin Ignimbrite, recently 40Ar/39Ar dated in this study at ~102 ka; 2σ). Our new chemical, stratigraphic and geochronologic investigations of these pyroclastic deposits, predominantly from Popocatépetl and Iztaccíhuatl, provide information on the scale and frequency of medium to large magnitude explosive eruptions over a longer-time period than currently known and that have had potential to disperse tephra across central México since the middle to late Pleistocene. This new data can be used to determine the source of further unknown tephras in the region as well as to better assess the volcanic hazard to the densely populated megalopolis of México City.

Syn‐ and post‐eruptive gully formation near the Laacher See volcano

AbstractThe Laacher See volcano (LSV) is located at the western margin of the Neuwied Basin, the central part of the Middle Rhine Basin of Germany. Its paroxysmal Plinian eruption c. 13 ka ago (Laacher See event; LSE) deposited a complex tephra sequence in the Neuwied Basin, whilst the distal ashes became one of the most important chronostratigraphic markers in Central Europe. However, some other impacts on landscape formation have thus far been largely neglected, such as buried gully structures in the proximity of the LSV. In this contribution, we map and discuss the spatial extent of these landforms at the site Lungenkärchen c. 4 km south of the LSV based on geophysical prospection as well as contrasting pedo‐sedimentary characteristics of the gully infill (particle‐size distribution, bulk‐sediment density, thin‐section analysis, saturated hydraulic conductivity) and the surrounding soils and tephra layers. These data are combined with a luminescence‐ and carbon‐14 (14C)‐based age model that relates them to the LSE. It is demonstrated how these gullies seem to have been formed and rapidly infilled by rainfall and surface discharge both during and subsequent to the eruptive phase, with modern analog processes documented for the 1980 Mount St Helens eruption (Washington State, USA). Given the density of the gullies at the site and their deviating pedo‐sedimentary properties compared to the surrounding soils, we propose a significant influence on agricultural production in the proximity of the LSV, which remains to be tested in future studies. Finally, in contrast, gullies of similar lateral and vertical dimensions identified in post‐LSE reworked loess and tephra deposits of the Wingertsbergwand (close to the main study site and proximal to the LSV) have shown to be unrelated to the LSE and can either be attributed to periglacial processes at the Younger Dryas‐Preboreal transition or to linear incision during the early Holocene.

Tephrostratigraphy of the eastern Lake Victoria Basin including the Nyanza Rift, Kenya: Building a stratigraphic and chronological framework for modern human evolution

Modern human evolution in Africa over the last ∼300 kyr is complex, with a variety of behavioral and biological changes appearing at different times and places. Explaining this pattern, as well as its relationship to paleoenvironmental circumstances, requires chronological and stratigraphic control of the paleoanthropological record. This study employs tephrostratigraphy, the chemical correlation of volcanic ashes (tephras), to provide chronostratigraphic context for modern human evolution in the Middle Pleistocene–Holocene of equatorial East Africa. This work geographically expands the eastern Lake Victoria Basin tephrostratigraphic framework over an area >16,000 km2. Geochemical comparisons show that most eastern Lake Victoria Basin tephras are derived from the volcanoes of the Central Kenyan Rift. New tephra correlations in the eastern Lake Victoria Basin incorporate 11 terrestrial localities throughout the Nyanza Rift as well as the V95–1P sediment core from the Ugandan waters of Lake Victoria. The spatial expansion of the eastern Lake Victoria Basin tephrostratigraphy also extends the previously documented ∼94–36 ka chronology of the tephra sequence to encompass most of the last ∼240 kyr. Tephra correlations presented here provide new ages for previously excavated, but undated, archaeological sites including Songhor (GqJe-1) = >94 ka, Simbi (GrJe-2) = 45–36 ka, and Muguruk (GqJc-1) where the correlation of a ∼49–36 ka tuff may help constrain the age for some or all of the site’s multiple archaeological levels. The tephrostratigraphy presented here thus constitutes a necessary step in expanding a Middle Pleistocene–Holocene chronostratigraphic framework across equatorial East Africa, which is fundamental to future studies on modern human evolution and behavioral change through time.

Chapter 6.2 Englacial tephras of East Antarctica

Abstract Driven by successful achievements in recovering high-resolution ice records of climate and atmospheric composition through the Late Quaternary, new ice–tephra sequences from various sites of the East Antarctic Ice Sheet (EAIS) have been studied in the last two decades spanning an age range of a few centuries to 800 kyr. The tephrostratigraphic framework for the inner EAIS, based on ash occurrence in three multi-kilometre-deep ice cores, shows that the South Sandwich Islands represent a major source for tephra, highlighting the major role in the ash dispersal played by clockwise circum-Antarctic atmospheric circulation penetrating the Antarctic continent. Tephra records from the eastern periphery of the EAIS, however, are obviously influenced by explosive activity sourced in nearby Antarctic rift provinces. These tephra inventories have provided a fundamental complement to the near-vent volcanic record, in terms of both frequency/chronology of explosive volcanism and of magma chemical evolution through time. Despite recent progress, current data are still sparse. There is a need for further tephra studies to collect data from unexplored EAIS sectors, along with extending the tephra inventory back in time. Ongoing international palaeoclimatic initiatives of ice-core drilling could represent a significant motivation for the tephra community and for Quaternary Antarctic volcanologists.

Sub-millennial eruptive recurrence in the silicic Mangaone Subgroup tephra sequence, New Zealand, from Bayesian modelling of zircon double-dating and radiocarbon ages

Accurate dating of young (<1 Ma) volcanic eruptions has long been a challenge for modern geochronology given the scarcity of datable mineral phases and low quantities of radiogenic daughter products. Combined U–Th–Pb and (U–Th)/He dating of zircon (i.e., zircon double-dating, ZDD) is a relatively new dating approach that offers a viable option for dating zircon-bearing volcanic and pyroclastic deposits as young as ca. 3 ka, and has a great potential for application in many fields within the Quaternary sciences, including volcanology, palaeoclimatology, and archaeology. In our study, a stratigraphically and spatially well-defined sequence of 13 rhyodacitic to rhyolitic tephra beds – the Mangaone Subgroup (MSg) – erupted from the Okataina Volcanic Centre (OVC), is used as a natural laboratory to conduct a cross-validation experiment in which the ZDD eruption ages are compared with published and new radiocarbon (14C) eruption ages. These ZDD and 14C ages are then used together to underpin a Bayesian age model developed (using ChronoModel) to provide new ages for the entire MSg sequence. New ZDD eruption ages of 36.1 ± 4.4, 31.5 ± 5.2, 30.9 ± 5.6, 31.2 ± 4.4 ka BP for four MSg tephras (Units D, I, J, and K, respectively) are statistically indistinguishable from 14C-based eruption ages. These results validate the feasibility of ZDD to date late Quaternary eruptions accurately. The Bayesian age sequence model provides provides an eruptive geochronology eruptive geochronology for all 13 MSg tephra beds for the first time (and for the stratigraphically-interbedded Taupo-volcano-derived Tahuna tephra, 38.4−1.4+1.7 ka BP), and constrains the beginning of the MSg eruption period to 42.7−3.5+3.7 ka BP (Unit A) and the end to 30.6−1.5+0.6 ka BP (Unit L). Thus, the entire MSg sequence was emplaced in ∼12,100 years, representing an eruption frequency of one event per ∼930 years on average. Our study demonstrates the efficacy of ZDD to yield accurate eruption ages on pyroclastic deposits, highlighting its potential for dating young (<1 Ma) magmatic and eruption events that are difficult to date by other geochronological methods, and also shows that ZDD dates can be integrated with 14C ages using Bayesian modelling to develop new age models for long sequences of tephra beds, in this case those of the MSg tephras that were deposited during MIS 3. In addition, the U–Th zircon crystallization data revealed distinct U–Th model age spectra for older and younger MSg tephras, providing geochronological evidence for a decreasing degree of interconnectedness within the OVC magma reservoir during the MSg eruption period that followed caldera collapse associated with the pre-MSg Rotoiti (Rotoehu) eruption at ca. 45 ka BP.

The interpretative value of transformed tephra sequences

ABSTRACTWe explore developments in tephra science that consider more than chronology, using case studies of morphological transformations of tephra deposits. Volcanic processes and prevailing weather conditions determine the distribution of tephra deposits immediately after an eruption, but as these freshly fallen tephra become part of the stratigraphic record, the thickness, morphology and definition of the layers they form changes, reflecting the interplay of the tephra, climate, Earth surface processes, topography and vegetation structure, plus direct or indirect modification caused by people and animals. Once part of the stratigraphic record, there can be further diagnostic changes to the morphology of tephra layers, such as the creation of over folds by cryoturbation. Thus, tephra layers may contain proxy evidence of both past surface environments and subsurface processes. Transformations of tephra deposits can complicate the reconstruction of past volcanic processes and make the application of classical tephrochronology as pioneered by Thorarinsson (Sigurður Þórarinsson in Icelandic) challenging. However, as Thorarinsson also noted, novel sources of environmental data can exist within transformed tephra sequences that include the spread or removal of tephra, variations in layer thickness and internal structures, the nature of contact surfaces and the orientation of layers.

Middle to late Pleistocene record of explosive volcanic eruptions in marine sediments offshore Kamchatka (Meiji Rise, NW Pacific)

ABSTRACTThis paper presents the first detailed study of a late Pleistocene marine tephra sequence from the NW Pacific, downwind from the Kamchatka volcanic arc. Sediment core SO201‐2‐40, located on the Meiji Rise ~400 km offshore the peninsula, includes 25 tephras deposited within the last 215 ka. Volcanic glass from the tephras was characterized using single‐shard electron microprobe analysis and laser ablation inductively coupled mass spectrometry. The age of tephras was derived from a new age model based on paleomagnetic and paleoclimate studies. Geochemical correlation of distal tephras to Kamchatkan pyroclastic deposits allowed the identification of tephras from the Karymsky, Gorely, Opala and Shiveluch eruptive centers. Three of these tephras were also correlated to other marine and terrestrial sites and hence are identified as the best markers for the north‐west Pacific region. These are an early Holocene tephra from the Karymsky caldera (~8.7 ka) and two tephras falling into the Marine Isotope Stage (MIS) 6 glacial time: an MIS 6.4 tephra from Shiveluch (~141 ka) and the MIS 6.5 Rauchua tephra (~175 ka) from Karymsky. The data presented in this study can be used in paleovolcanological and paleoceanographic reconstructions.

Tephrochronological dating of paleoearthquakes in active volcanic arcs: A case of the Eastern Volcanic Front on the Kamchatka Peninsula (northwest Pacific)

AbstractInvestigation of active faults is crucial for the seismic hazard assessment and, in the case of volcanic belts, it provides a deeper understanding of the interactions between volcanism and tectonic faulting. In this study, we report the results of the first paleoseismological and tephrochronological investigation undertaken on Holocene faulting in Kamchatka's volcanic belts. The studied trenches and additional excavations are located along the axial fault zone of the Eastern Volcanic Front, where the earlier dated tephra layers provide a robust age control of the faulting events. Electron microprobe analysis of glass from 22 tephra samples permitted correlations among the disparate tephra profiles for constructing a summary tephra sequence. The latter, together with published geochronological data, allowed the construction of a Bayesian age model. Detailed examination of the tephra layers deformed by faulting allowed us to reconstruct and date six faulting events with the offsets of 1 to 20 cm indicating paleoearthquakes with magnitudes of Mw&lt; 5.4. Holocene crustal seismicity of the Eastern Volcanic Front manifests temporal clustering rather than a uniform flux of events. However, no correlation between dated seismic events and the largest Holocene eruptions of proximal volcanoes was observed.

History of scoria‐cone eruptions on the eastern shoulder of the Kenya–Tanzania Rift revealed in the 250‐ka sediment record of Lake Chala near Mount Kilimanjaro

ABSTRACTReconstructions of the timing and frequency of past eruptions are important to assess the propensity for future volcanic activity, yet in volcanic areas such as the East African Rift only piecemeal eruption histories exist. Understanding the volcanic history of scoria‐cone fields, where eruptions are often infrequent and deposits strongly weathered, is particularly challenging. Here we reconstruct a history of volcanism from scoria cones situated along the eastern shoulders of the Kenya–Tanzania Rift, using a sequence of tephra (volcanic ash) layers preserved in the ~250‐ka sediment record of Lake Chala near Mount Kilimanjaro. Seven visible and two non‐visible (crypto‐) tephra layers in the Lake Chala sequence are attributed to activity from the Mt Kilimanjaro (northern Tanzania) and the Chyulu Hills (southern Kenya) volcanic fields, on the basis of their glass chemistry, textural characteristics and known eruption chronology. The Lake Chala record of eruptions from scoria cones in the Chyulu Hills volcanic field confirms geological and historical evidence of its recent activity, and provides first‐order age estimates for seven previously unknown eruptions. Long and well‐resolved sedimentary records such as that of Lake Chala have significant potential for resolving regional eruption chronologies spanning hundreds of thousands of years.

The Talili Pyroclastics eruption sequence: VEI 5 precursor to the seventh century CE caldera-forming event at Rabaul, Papua New Guinea

Studies of the products of the activity precursory to caldera-forming events are essential in revealing how caldera systems evolve in the prelude to climactic eruptions. Here, we investigate the deposits formed by the Talili Pyroclastics eruption sequence, which preceded the youngest major eruptive event, the seventh century CE (1400 BP) Rabaul Pyroclastics VEI 6 caldera-forming eruption, at Rabaul in northeastern Papua New Guinea. The Talili Pyroclastics deposits are essentially a bimodal (dacite-basalt) tephra sequence that was emplaced during the 2.8 ky leading to the Rabaul Pyroclastics eruption. The dominant activity during the Talili period, starting at 4200 BP, involved strong (up to VEI 5), water-modified explosive eruptions of dacite from intra-caldera vents. This activity generated > 4.4 km3 of unstratified, bedded and laminated fine-grained fall deposits, pyroclastic surges and a small ignimbrite. In addition, all three of the young stratovolcanoes on the eastern and northeastern fringes of Rabaul Caldera were active during the Talili era, producing > 0.7 km3 of basaltic scoria fall and flow deposits. The main phase of Talili era basalt production occurred at 4100 BP and involved sequential eruptions from two of the stratovolcanoes, Palangiangia and Kabiu. The sequence of dacitic Talili eruptives shows a progressive decline then late-stage increase in SiO2 contents, with corresponding changes in Fe2O3, MgO and CaO. Early and late products have compositions similar to those of the most-evolved products of the major eruptions at Rabaul (intermediate to high-SiO2 dacites), while middle-stage products are similar to those of the historical eruptions (lower-SiO2 dacites and andesites). The basaltic eruptives have three different compositions, attributed to three different (stratovolcano) sources. There is no evidence of mixing or mingling of basalt and dacite. However, andesitic scoria inclusions in the early-erupted dacitic ignimbrite represent mingling of andesitic and dacitic magmas. As with the Talili Pyroclastics, at least two other earlier tephra sequences at Rabaul probably resulted from phases of VEI 5 volcanism and directly underlie the deposits of major eruptions. This leads to the inference that cycles of escalating eruptive activity have prevailed prior to some of the major eruptions, and suggests that the strength of volcanism at Rabaul may rise from the lower levels (maximum of VEI 4) evident since the Rabaul Pyroclastics event as the system moves towards the next major eruption.

Magmatic reactivation of the Campi Flegrei volcanic system: insights from the Baia–Fondi di Baia eruption

The Baia–Fondi di Baia was a multi-stage, small-scale eruption which occurred in the western part of the Campi Flegrei caldera at 9525–9696 BP, marking the onset of Epoch 2 of post-Neapolitan Yellow Tuff volcanism. The eruption was characterized by a complex series of events related to two distinct eruptive episodes (Baia and Fondi di Baia) separated by a short time interval, and each characterized by several eruptive phases. Mineralogical, geochemical (major, and trace elements on whole rocks, major and volatile elements on matrix glasses, and melt inclusions), and Sr isotope characterization of the tephra material sampled along the entire sequence was carried out in order to constrain magmatic evolution and dynamics of the feeding system. Three main compositional groups were identified in matrix glasses and interpreted as representative of different magma bodies: (i) a trachyte (SiO2 60.3–64.7 wt.%), which is volumetrically predominant; (ii) a tephriphonolite-latite (SiO2: 55.1–57.9 wt.%); and (iii) an intermediate magma group between phonolite and trachyte compositions. This wide compositional heterogeneity contrasts with the narrow variability recognized in the bulk-rock compositions, which are all trachytic. Mineral, melt inclusions, and Sr isotope data suggest that the trachytic magma possibly derived from the Campanian Ignimbrite reservoir located at 6–9 km depth. Volatile content in matrix glass indicates a storage depth of at least 6 km for the tephriphonolite-latitic magma. The intermediate magma is interpreted as being derived from a remelting and assimilation process of a partially crystallized trachytic body (crystal mush) by the hotter tephriphonolite-latitic magma. As the tephriphonolite-latite was erupted together with the trachyte from the beginning of the eruption, we suggest that the ascent of this magma played a fundamental role in triggering the eruption. Upwards through the tephra sequence, we observed a progressive increase of the tephriphonolite-latitic and intermediate phonolite-trachytic components. The presence of banded clasts characterized by different compositions is also indicative of syn-eruptive mingling during the final phases of the eruption.

The 1914 Taisho eruption of Sakurajima volcano: stratigraphy and dynamics of the largest explosive event in Japan during the twentieth century

The 1914 Taisho eruption of Sakurijima volcano was Japan’s highest intensity and magnitude eruption of the twentieth century. After a 35-year period of quiescence, the volcano suddenly rewoke a few days before the eruption, when earthquakes began to be felt on Sakurajima Island. The eruption began on January 12, 1914, from two fissures located on opposite sides of the volcano, and was characterized by a complex time evolution and changes in eruptive styles. The eruption began with a subPlinian explosive phase in which two convective columns rose from the two fissures. Both plumes were sustained for at least 2 days. This resulted in deposition of a widely dispersed tephra sequence. After this phase, the eruption evolved to a final, waning phase, shifting toward effusive activity that lasted until April 1914. During the first weeks, effusive activity was also accompanied by ash emission. The complex sequence of events, characterized by contemporaneous explosive and effusive activity, is typical of several recently observed mid-intensity eruptions, such as during the 2011 eruption of Cordón Caulle, Chile. The stratigraphic sequence of the eruptive deposits from the Taisho eruption comprises alternating coarse-to-fine lapilli beds with ash beds dispersed toward the ESE and SE. These deposits can be subdivided into three lapilli-bearing units (Units T1, T2 and T3, which correspond to the subPlinian phase) and one ash-bearing unit (Unit T4, which corresponds to the final ash venting, accompanying the first day/weeks of lava flow activity). Grain size analyses from each unit reveal a marked polymodal distribution generally described by the sum of two or three Gaussian subpopulations. Both the modes and the relative amounts of the coarse subpopulations vary with distance from vent, with those of the fine subpopulation remaining nearly constant. Within the vertical sequence, component analysis shows a progressive increase in lithic fragments, suggesting that conduit enlargement continued until the final stages of the eruption. The estimated volume of the tephra deposit of the subPlinian phase of the eruption is 0.33 ± 0.11 km3 (dense rock equivalent (DRE) volume = 0.09 ± 0.03 km3). The height of the eruption column was also assessed by using four different isopleth maps compiled based on different strategies for the characterization of the largest clasts. The maximum height attained by the eruption column is estimated at 15.0 ± 1.2 km above the vent, resulting in a maximum mass discharge rate of 3.6 ± 1.2 × 107 kg s−1 (calculated taking into account the strong effect of wind advection). Finally, different classification schemes were applied to classify the eruption, which generally straddles the fields between Plinian and subPlinian.