The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ～ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ～ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions.The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H₂O, several hundred ppm of CO₂, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ～ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H₂O, 8700 ppm Cl, 6000 ppm F, and CO₂ below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ～ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H₂O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.     

Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism

Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.     

New radiometric and petrological constraints on the evolution of the Pichincha volcanic complex (Ecuador)

Fieldwork, radiometric (⁴⁰Ar/³⁹Ar and ¹⁴C) ages and whole-rock geochemistry allow a reconstruction of eruptive stages at the active, mainly dacitic, Pichincha Volcanic Complex (PVC), whose eruptions have repeatedly threatened Quito, most recently from 1999 to 2001. After the emplacement of basal lavas dated at ∼1100 to 900 ka, the eruptive activity of the old Rucu Pichincha volcano lasted from ∼850 ka to ∼150 ka before present (BP) and resulted in a 15 × 20 km-wide edifice, which comprises three main building stages: (1) A lower stratocone (Lower Rucu, ∼160 km³ in volume) developed from ∼850 to 600 ka; (2) This edifice was capped by a steeper-sided and less voluminous cone (the Upper Rucu, 40–50 km³), the history of which started 450–430 ka ago and ended around 250 ka with a sector collapse; (3) A smaller (8–10 km³) but more explosive edifice grew in the avalanche amphitheatre and ended Rucu Pichincha's history about 150 ka ago. The Guagua Pichincha volcano (GGP) was developed from 60 ka on the western flank of Rucu with four growth stages separated by major catastrophic events. (1) From ∼60 to 47 ka, a basal effusive stratocone developed, terminating with a large ash-and-pumice flow event. (2) This basal volcano was followed by a long-lasting dome building stage and related explosive episodes, the latter occurring between 28–30 and 22–23 ka. These first two stages formed the main GGP (∼30 km³), a large part of which was removed by a major collapse 11 ka BP. (3) Sustained explosive activity and viscous lava extrusions gave rise to a new edifice, Toaza (4–5 km³ in volume), which in turn collapsed around 4 ka BP. (4) The ensuing amphitheatre was partly filled by the ∼1-km³ Cristal dome, which is the historically active centre of the Pichincha complex. The average output rate for the whole PVC is 0.29 km³/ka. Nevertheless, the chronostratigraphic resolution we obtained for Lower Rucu Pichincha and for the two main edifices of Guagua Pichincha (main GGP and Toaza), leads to eruptive rates of 0.60–0.65 km³/ka during these construction stages. These output rates are compared to those of other mainly dacitic volcanoes from continental arcs. Our study also supports an overall SiO₂ and large-ion lithophile elements enrichment as the PVC develops. In particular, distinctive geochemical signatures indicate the involvement of a new magma batch at the transition between Rucu and Guagua. At the GGP, the same phenomenon occurs at each major collapse event marking the onset of the ensuing magmatic stage. Since the 11-ka-BP collapse event, this magmatic behaviour has led to increasingly explosive activity. Four explosive cycles of between 100 and 200 years long have taken place at the Cristal dome in the past 3.7 ka, and repose intervals between these cycles have tended to decrease with time. As a consequence, we suggest that the 1999–2001 eruptive period may have initiated a new eruptive cycle that might pose a future hazard to Quito (∼2 million inhabitants).     

The eruptive history of Pantelleria (Sicily channel) in the last 50 ka

Six silicic eruptive cycles have been recognized in the last 50 ka at Pantelleria. The products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte. The relationships between the range of chemical variation, the erupted volume and the time of eruptions, allow us to estimate an average differentiation rate of 5% crystal fractionation per 1000 years and a constant long-term rate of magma discharge of 0.1 km³ per 1000 years. Pressure increase in the magma chamber caused by the addition of new magma, accumulation of highly-differentiated, volatile-rich magma in the roof zone and a concomitant build-up of a vapour phase, is postulated as a possible triggering mechanism for eruptions.     

Reconstructing the largest explosive eruptions of Mt. Ruapehu,New Zealand: lithostratigraphic tools to understand subplinian–plinian eruptions at andesitic volcanoes

We analysed the tephra record of Mt. Ruapehu for the period 27,097 ± 957 to ~10,000 cal. years BP to determine the largest-scale explosive eruptions expected from the most active New Zealand andesitic volcano. From the lithostratigraphic analysis, a systematic change in the explosive behaviour is identified from older deposits suggesting dry magmatic eruptions and steady eruptive columns, characterised by frothy to expanded pumice fabrics, to younger deposits that are products of unsteady conditions and collapsing columns, characterised by microvesicular, fibrous, and colour-banded pumice fabrics. The end-members were separated by eruptions with steady columns linked to water–magma interaction and highly unstable conduit walls. Dry magmatic eruptions producing steady plinian columns were most common between 27,097 ± 957 and shortly after 13,635 + 165 cal. years BP. Following this time, activity continued with eruptions that produced dominantly oscillating unsteady columns, which engendered pyroclastic density currents, until ~10 ka when there was an abrupt transition at Mt. Ruapehu since which eruptions have been an order of magnitude lower in intensity and volume. These data demonstrate long-period transitions in eruption behaviour at an andesitic stratovolcano, which is critical to understand if realistic time-variable hazard forecasts are to be developed.     

Les Éruptions Historiques de L’Etna: Chronologie et Localisation

A careful examination of historical documents pertaining to the long (over three thousand years) history of Mount Etna has been carried out. Despite the abundance of details on eruptions, sometimes very ancient (e.g. that of 479/8 B.C. described by PINDAR), it is shown that the reports are often too imprecise for identifying with certainty the type of eruptive activity and the spatial extension of lava flows — or even the lava flows and eruptive centres themselves —, many of these having been buried by subsequent volcanic activity. Conversely, it can be shown that well preserved cones and flows were apparently produced by eruptions that went unoticed by historians. These considerations are supported by previous paleomagnetic work : lava flows and eruptive systems taken as belonging to eruptions of the years 1651 (Scorciavacca), 1595 (Gallo Bianco), 1566 (Linguaglossa), 1536 (Mt Pomiciaro NW, and a small flow W of Mt Vetore), 1494 (Mt Frumento Supino), 1408 (Trecastagni), 1381 (N of Catania), 1329 (Linera, Mt Ilice), 1284 (N of Zafferana), 1169 or 812 (Mt Sona) ..., have paleomagnetic directions inconsistent with that of the geomagnetic field at these respective dates. The reinterpretation of ancient documents by recent authors is often misleading, and some major errors have been corrected. Obviously, many gaps or uncertainties remain in the summary of eruptive sequences, except perhaps for the last two or three centuries. Eruptive models that are based on the use of historical documents should be examined in the view of these uncertainties with great care.     

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.     

Regimes of magma recharge and their control on the eruptive behaviour during the period 2001–2005 at Mt. Etna volcano

Mount Etna volcano (Italy) during the period 2001–2005 has undergone a period of intense eruptive activity marked by three large eruptions (2001, 2002–2003 and 2004–2005). These eruptions encompassed diverse eruptive styles and regimes: from intensely explosive, during 2001 and 2002–2003 eruptions, to exclusively effusive in the 2004–2005 event. In this work, we put forward the idea that these three eruptions are the response of the progressive arrival into the uppermost segment of the open-conduit system of a new magma, which was geochemically distinct in terms of trace element and Sr–Nd–Pb isotope signature from the products previously emitted by the Etnean volcano. The magma migrated upwards mainly through a peripheral tectonic system, which can be considered as eccentric in spite of its relative proximity to the main system. The ingress of the new magma and its gradual displacement from the eccentric system into the uppermost sector of the open-conduit gave rise to different eruptive behaviours. At the beginning, the ascent of the undegassed magma, able to exsolve a gas phase at depth, and its interaction with closed-system magma reservoirs less than 10 km deep gave rise to the explosive events of 2001 and 2002–2003. Later, when the same magma entered into the open-conduit system, it took part in the steady-state degassing and partially lost its volatile load, leading to a totally effusive eruption during the 2004–2005 event. One further consideration highlighted here is that in 2001–2005, migration of the feeding axis from an eccentric and peripheral position towards the main open-conduit has led to the development of a new vent (South East Crater 2) located at the eastern base of the South East Crater through which most of the subsequent Etnean activity occurred.     

Secular geomagnetic variations and volcanic pulses in the Permian-Triassic traps of the Norilsk and Maimecha-Kotui provinces

Detailed paleomagnetic studies have shown that the effusive Permian-Triassic traps in the Kotui River valley were formed as the result of volcanic activity, which occurred in the form of volcanic pulses and individual eruptions with net duration of at most 7000–8000 years, excluding the periods of volcanic quiescence. According to the analysis of the paleomagnetic data earlier obtained by Heunemann and his coauthors [2004b] on the Abagalakh and Listvyanka sections in the Norilsk region, those geological units were formed during 25 volcanic pulses and separate eruptions, which all lasted up to 8000 years altogether, whereas the total time of formation (including the periods of volcanic quiescence) exceeded 10000–100000 years for the Norilsk section and was probably a bit shorter for the Kotui section. Comparison of the positions of virtual geomagnetic poles calculated for the Norilsk and the Kotui sections provides no grounds to suggest that these sections were formed at different geological times. The scatter in the positions of the virtual geomagnetic poles (VGP) for the directional groups and individual directions (58 altogether) jointly for the two sections (more than 160 lava flows) indicates that the secular geomagnetic variations at the Permian-Triassic boundary had similar amplitudes to those that occurred in the past 5 Ma.     

Ar/Ar and C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District,Central Italy

New ⁴⁰Ar/³⁹Ar and ¹⁴C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The ⁴⁰Ar/³⁹Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined ⁴⁰Ar/³⁹Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence.     

Origins and energetics of maar volcanoes: examples from the ultrapotassic Sabatini Volcanic District (Roman Province,Central Italy)

Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water. Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions, eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from either monogenetic or polygenetic maars ranged 0.004–0.07 km³ during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 10¹⁵–4 × 10¹⁷ J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation.     

The rhyolitic–andesitic eruptive history of Cotopaxi volcano,Ecuador

At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70–75% SiO₂) and andesitic (56–62% SiO₂) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.1 km³) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra falls with DRE volumes of 0.2–3.6 km³. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode, andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows, infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km³ (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km³ (DRE)/ka for the past 2,100 years, resulting in the present large stratocone.     

Eruptive history and tectonic setting of Medicine Lake Volcano,a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km² of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km³, giving an overall effusion rate of ∼ 1.2 km³ per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H₂O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.     

Fine ash content of explosive eruptions

In explosive eruptions, the mass proportion of ash that is aerodynamically fine enough to cause problems with jet aircraft or human lungs (< 30 to 60 μm in diameter) is in the range of a few percent to more than 50%. The proportions are higher for silicic explosive eruptions, probably because vesicle size in the pre-eruptive magma is smaller than those in mafic magmas. There is good evidence that pyroclastic flows produce high proportions of fine ash by communition and it is likely that this process also occurs inside volcanic conduits and would be most efficient when the magma fragmentation surface is well below the summit crater. Reconstructed total grain size distributions for several recent explosive eruptions indicate that basaltic eruptions have small proportions of very fine ash (~ 1 to 4%) while tephra generated during silicic eruptions contains large proportions (30 to > 50%).     

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

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

Reconstruction of the eruptive activity on the NE sector of Stromboli volcano: timing of flank eruptions since 15 ka

A multidisciplinary geological and compositional investigation allowed us to reconstruct the occurrence of flank eruptions on the lower NE flank of Stromboli volcano since 15 ka. The oldest flank eruption recognised is Roisa, which occurred at ~15 ka during the Vancori period, and has transitional compositional characteristics between the Vancori and Neostromboli phases. Roisa was followed by the San Vincenzo eruption that took place at ~12 ka during the early stage of Neostromboli period. The eruptive fissure of San Vincenzo gave rise to a large scoria cone located below the village of Stromboli, and generated a lava flow, most of which lies below sea level. Most of the flank eruptions outside the barren Sciara del Fuoco occurred in a short time, between ~9 and 7 ka during the Neostromboli period, when six eruptive events produced scoria cones, spatter ramparts and lava flows. The Neostromboli products belong to a potassic series (KS), and cluster in two differently evolved groups. After an eruptive pause of ~5,000 years, the most recent flank eruption involving the NE sector of the island occurred during the Recent Stromboli period with the formation of the large, highly K calc-alkaline lava flow field, named San Bartolo. The trend of eruptive fissures since 15 ka ranges from N30°E to N55°E, and corresponds to the magma intrusions radiating from the main feeding system of the volcano.     

Volcanic stratigraphy of large-volume silicic pyroclastic eruptions during Oligocene Afro-Arabian flood volcanism in Yemen

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km³. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×10⁷ km² and estimated maximum total volumes of up to 5,000 km³, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km³) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.Electronic Supplementary Material Supplementary material is available for this article at     

Frequent eruptions of Mount Rainier over the last ∼2,600 years

Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions were penecontemporaneous with far-traveled lahars, probably created from newly erupted materials melting snow and glacial ice. The most recent magmatic eruption of Mount Rainier, documented geochemically, was the 1,000 cal year BP event. Products from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974)) are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. T. W. Sisson and J. W. Vallance contributed equally to this study.     

A remarkable pulse of large-scale volcanism on New Britain Island,Papua New Guinea

New studies of the deposits from the latest caldera-forming eruption (the “Dk” event) at Dakataua Volcano, New Britain Island, Papua New Guinea, help identify an intense space-time concentration of large-scale volcanism during the 7th century AD on New Britain. Radiocarbon dating of charcoal from the Dk deposits yields an age of 1,383 ± 28 BP. Calibration of this result gives an age in the range AD 635–670 (at 1 s. d.). At about the same time, two other volcanoes on New Britain, Rabaul and Witori, also produced very large eruptions. Very high acidity levels in ice cores from Antarctica and Greenland at AD 639 and AD 640 respectively may be linked to either or both of the Dakataua and Rabaul eruptions. Another ice core high acidity level, at AD 692, may be associated with the Witori eruption. Significant volcanic risk within the New Britain region is indicated by its Late Cenozoic history of relatively frequent large-scale eruptions from as many as 8 caldera systems within an arc-parallel zone about 380 km long. Over the last 20 ka the return period for major (VEI 5+) eruptions in this region was about 1.0 ka and individually high frequencies of major eruptive activity were experienced at Witori and Rabaul. The relatively short return period for major eruptions in the region would tend to increase the chance that such events could cluster in time.