Pumice Flow Research Articles

Crystal-mush reactivation by magma recharge: Evidence from the Campanian Ignimbrite activity, Campi Flegrei volcanic field, Italy

Processes of crystal-mush remobilization by mafic magma recharges are often related to the outpouring of large volumes of silicic melt during caldera-forming eruptions. This occurred for the Campanian Ignimbrite (CI) eruption (Campi Flegrei, Italy), which produced a voluminous trachy-phonolitic ignimbrite in southern-central Italy about 40 ka ago. We focussed on the proximal-CI deposits at San Martino that are composed of a main sequence of early-erupted, crystal-poor units and a late-erupted (post-caldera collapse) crystal-rich Upper Pumice Flow Unit (UPFU). Detailed micro-analytical geochemical data were performed on glasses and crystals of pyroclasts from these deposits and coupled with Sr-Nd isotopic measurements on glasses. Results show that the CI eruption was fed by two distinct melts for the early-erupted units and the late UPFU, respectively. The glasses of the early-erupted units have negative Eu anomalies and show more evolved compositions and higher Nd isotope ratios than those of the UPFU, which have positive Eu/Eu*. The magmas of the early units formed the main volume of eruptible melt of the CI reservoir, and are interpreted as having been extracted from cumulate crystal-mush without a vertical geochemical gradient within the magma reservoir. The data indicate that the generation of the distinctive UPFU melts involved the injection of a new batch of mafic magma into the base of the CI reservoir. The mafic magma allowed heating and reactivation of the CI crystal-mush by melting of low-Or sanidines (+/− low-An plagioclases), leaving high-An plagioclases and high-Mg# clinopyroxenes as residual phases and a crystal-mush melt, made of 20% of the initial mush interstitial melt (with a composition similar to the early-erupted units) and 80% of sanidine melt. When the mush crystallinity was sufficiently reduced, the mafic magma was able to penetrate into the reactivated crystal-mush, mixing with variable proportions of crystal-mush melt and generating cooler hybrid melts, which underwent further crystallization of high-Or sanidine at variable degrees (10–25%). Finally, possibly a short time before the eruption, the UPFU magmas were able to mix and mingle with the crystal-poor eruptible melts still persisting in the CI reservoir at the time of UPFU emission. We suggest that the complex mechanisms described for the magma evolution feeding the CI eruption may occur whenever a crystal-mush is reactivated by new mafic magma inputs

Geomorphology and tephrochronology review of the Hasandağ volcano (southern Cappadocia, Turkey)

This research proposes a suite of volcanic events that took part in the edification of the double-peaked Hasandag stratovolcano in southern Cappadocia. Inter-correlations of sections dispatched along geographic transects across the volcano evidence continuities/discontinuities and stratigraphic relationships using key layers identified through this process which is, later, framed by a radiometric dating control of some of these formations. The main goal is to provide some chronological markers of the geomorphological evolution of the volcano. The stratigraphy, lithology and facies, the landform definitions and new dates provide information about eruption types and their role in shaping the morphologies of the volcano through time. Recent ages from literature and seven new K/Ar dates contribute in enriching the story of the volcanic activity that built the Hasandag stratovolcano landforms. The part of the story exposed in this article starts mainly c. 700/650 ka ago with the construction of a Mid-Pleistocene volcano. Later, between 220 and 120 ka ago, main events occurred in the NE part of the volcano. After an initial Plinian eruption, a caldera collapse is recorded by pumice flows. Close to the emission point, a small collapse structure is today filled with a much younger dacite flow. After the Plinian eruption, the partial destruction of a volcano caused one or two avalanches containing several meters-thick distinct blocks that flew north c. 16–18 km over the roof of the Cappadocian Miocene ignimbrites. Remains of the destructed volcano flanks are not visible. Either they are buried below younger lava flows forming the Kucuk Hasandag cone, or a seism in the Tuz Golu Fault Zone during the avalanche may have resulted in an explosion. This event was followed by extrusion of rhyolitic domes positioned on the caldera rim, emitting pumice falls now filling-in the Guvercin valley stream down to Ihlara village. During Late Pleistocene emission of andesite and dacite flows and domes, accompanied by several pyroclastic flows formed today’s terminal cones of the stratovolcano.

AMS 14C age of the sandy gravel bed layer covering Shikotsu air fall pumice deposit and Shikotsu pumice flow deposit, distributed around Fumonkegawa River in Abira Town, Hokkaido

AMS ¹⁴C age of the sandy gravel bed layer covering Shikotsu air fall pumice deposit and Shikotsu pumice flow deposit, distributed around Fumonkegawa River in Abira Town, Hokkaido

LA-ICP-MS U/Pb zircon timescale constraints of the Pleistocene latest magmatic activity in the Sancy stratovolcano (French Massif Central)

It has been demonstrated for several years that the UPb zircon system can provide useful dating information for understanding the time-evolution of the volcanic systems. In addition, in situ techniques were successfully used for determining the formation age of young Pliocene and Pleistocene volcanic rocks and to record the spatial and temporal distribution and the lifetime evolution of the magma chambers, generally in the case of large volcanic domains.Here we show that it is possible to obtain a precision at the percent level on zircon from a short-size and late volcanic event from the Sancy stratovolcano (French Massif Central). LA-ICP-MS dating of Pleistocene magmatic zircons allow to distinguish the timescale of these late magmatic events within the complex volcanic activity of this large stratovolcano. A syenite xenolith enclosed into a trachyte pumice flow were dated at 277.6 ± 3.2 ka and 260.1 ± 3.6 ka, respectively, whereas an earlier trachyandesite dome was emplaced at 284.4 ± 4.2 ka. These different ages allow to propose a minimum evolution time of about 24 ± 8 kyrs for a differentiated magma chamber, which corresponds to a short lifetime associated with low magma volumes. The duration of this discrete volcanic event should be compared to the hundreds of thousands of years reported for the large volcanic systems. Interestingly, the zircon crystals from the pumices were embedded into a thin layer of glass of trachytic composition, demonstrating that these zircons crystallized into the melt only a short time before eruption.

Magmatic evolution of the Boku Volcanic Complex, Main Ethiopian Rift

The Boku volcanic complex is a Quaternary center situated on the axial segment of the Main Ethiopian Rift (MER), located 92 km South-east from Addis Ababa. The main objective of this study is to understand the magmatic evolution of the volcanic complex and to develop a model to answer some outstanding questions related to bimodal products of rift related volcanism, using geological mapping, petrographic and geochemical approaches. The Boku complex is characterized by two main phases of activity: pre-caldera/caldera forming eruptive activity and post-caldera eruptive activity. The volcanic stratigraphy consists from bottom to top of a sequence of rhyolitic lava flows, pumice flows, welded ignimbrite, pumice fall, rhyolitic lava dome, obsidian flow, lower basaltic lava flow, ash flow, basaltic scoria and upper basaltic lava flows. The lithologic varieties together with the geochemical results indicate that the Boku eruptive products are bimodal in composition; no intermediate compositions are found. The mafic rocks are transitional to weakly subalkaline basalts while the silicic rocks are predominantly peralkaline rhyolites. These two groups of rocks are co-genetic and related to each other by fractional crystallization processes starting from mantle-derived basaltic magma with a small component of crustal contamination. The available geophysical, geochemical and field data suggest that the evolution of the evolved silicic center which hosts a bimodal rock distribution can be explained as a result of prolonged stagnation of transitional basaltic melt (sourced from the mantle) at relatively high pressures, where these evolve to intermediate compositions. The transitional basaltic melts can occasionally erupt to the surface along weakness lines such as faults that infrequently cut the lower part of the shallow reservoir, generating basalts from the intra-caldera and lateral eruptive centers. We consider the intermediate magma is mechanically trapped at mid-crustal depths because of its higher crystal load (ca. 50%). Silicic magma is formed at shallow depth by prolonged fractional crystallization of the intermediate magma and minor assimilation of crustal material. These silicic magmas generate both effusive and explosive eruption products in the overall stratigraphy of the volcanic complex.

70.渡島駒ケ岳1929年軽石流の地形 : 日本火山学会1978年春季大会

70.渡島駒ケ岳1929年軽石流の地形 : 日本火山学会1978年春季大会

鹿児島県安楽川ダムサイトにみられる大隅軽石流(高溶結部)の14C年代

鹿児島県安楽川ダムサイトにみられる大隅軽石流(高溶結部)の14C年代

The emplacement dynamics of pumice lobes ascertained from morphology and granulometry: Examples from the 1993 deposits at Lascar Volcano, Chile

The work presented here focuses on lobe shapes and clast populations within lobate termini of the 1993 pumice flow deposits at Lascar Volcano, Chile. A new method to analyze a coarse-tail grain size population with field photographs is presented. Using this method, >33,000 (>0.5cm) clasts from the pumice lobes of the 1993 pumice flow deposits were measured at 36 sites, and the resultant grain size distributions were then related to lobe morphology. Lobe margins (i.e., levees, clefts, and snouts) were found to contain significantly larger pumice clasts and be more poorly sorted than lobe central channels (i.e., locations away from the margins). Previous laboratory experiments suggest lobe margins form by the floatation and deflection of larger clasts to the margins of an advancing flow lobe. Results here indicate that the same sorting process efficiently segregates clasts into two flow regimes: 1) a mobile central channel depleted in coarse clasts, and 2) friction-dominated margins enriched in clasts ≥15cm. The lobe margins, 60% enriched in larger particles with matrix <20%, slow and frictionally freeze from the base up and before the material in the central channel stops flowing. The advancing pumice lobes finally stop when the margins reach ~12 clasts thick and the central channel has insufficient mass flux or momentum to break through or over-top the static margins. These processes form a unique lobe and channel morphology deposit that is diagnostic of granular flow and typical of small to intermediate volume pumice flow emplacement.

Geochemistry of Mesoproterozoic Deonar Pyroclastics from Vindhyan Supergroup of Central India: Evidences of Felsic Magmatism in the Son Valley

ABSTRACT Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intra-basinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin.

H31-11 箱根火山新期軽石流期の TPf1(東京軽石流)について

H31-11 箱根火山新期軽石流期の TPf1(東京軽石流)について

Tephra dispersal during the Campanian Ignimbrite (Italy) eruption: implications for ultra-distal ash transport during the large caldera-forming eruption

The Campanian Ignimbrite eruption dispersed ash over much of the central eastern Mediterranean Sea and eastern Europe. The eruption started with a Plinian phase that was followed by a series of pyroclastic density currents (PDCs) associated with the collapse of the Plinian column and the caldera. The glass compositions of the deposits span a wide geochemical range, but the Plinian fallout and PDCs associated with column collapse, the Lower Pumice Flow, only erupted the most evolved compositions. The later PDCs, the Breccia Museo and Upper Pumice Flow, erupted during and after caldera collapse, tap a less evolved component, and intermediate compositions that represent mixing between the end-members. The range of glass compositions in the Campanian Ignimbrite deposits from sites across the central and eastern Mediterranean Sea allow us to trace the dispersal of the different phases of this caldera-forming eruption. We map the fallout from the Plinian column and the plumes of fine material associated with the PDCs (co-PDCs) across the entire dispersal area. This cannot be done using the usual grain-size methods as deposits in these distal regions do not retain characteristics that allow attribution to either the Plinian or co-PDC phases. The glass compositions of the tephra at ultra-distal sites (>1500 km from the vent) match those of the uppermost PDC units, suggesting that most of the ultra-distal dispersal was associated with the late co-PDC plume that was generated during caldera collapse.

Evolution of the 120 ka caldera-forming eruption of Kutcharo volcano, eastern Hokkaido, Japan: Geologic and petrologic evidence for multiple vent systems and rapid generation of pyroclastic flow

We investigated the eruptive sequence and temporal evolution of juvenile materials during the 120ka Kutcharo pumice flow IV (Kp IV) eruption, which was the most voluminous (175km3: bulk volume) caldera-forming eruption of Kutcharo volcano. The eruptive deposits are divided into four units in ascending order. Unit 1 is widely dispersed and consists of silt-sized, cohesive ash. Unit 2 is a thin, moderately sorted pumice fall deposit with a restricted distribution and small volume (<0.2km3). Unit 3, consisting of widely distributed ignimbrite, is the most voluminous. Unit 4 is also composed of pyroclastic flow deposits, but its distribution is limited to the northwest side of the caldera. Juvenile materials consist mainly of rhyolite pumice (74%–78% SiO2) associated with a minor amount of scoria (52%–73% SiO2) that are found only northwest of the caldera in Unit 3 and Unit 4. These scoriae can be classified on the basis of the P2O5 contents of their matrix glass into low-P, medium-P, and high-P types, which are almost entirely restricted to the lower part of Unit 3, Unit 4, and the upper part of Unit 3, respectively. These three types display distinct mixing trends with the rhyolitic compositions in SiO2–P2O5 variation diagrams. This evidence indicates that three distinct mafic magmas were independently and intermittently injected into the main body of silicic magma to erupt from the northwestern part of the magma system. Mafic injections did not occur in the southern part of the magma system. This petrologic evidence implies that the northwestern and southeastern flows of Unit 3 are heterotopic, contemporaneous products derived from multiple vent systems. Although Unit 2 was derived from an eruptive column, its volume is very small compared to Plinian fall deposits of typical caldera-forming eruptions. In our interpretation, the activity of the Kp IV eruption reached its climax rapidly, depositing Unit 3, without first producing a stable Plinian column. The presence of multiple vent systems could have allowed the system to bypass an initial eruptive stage with a stable Plinian column and begin its climactic stage, represented by Unit 3, rapidly. Multiple vents could have been the result of sequential injections of mafic magma in the early stages of the Kp IV eruption.

The GEOLOGY of GÖKÇEADA (ÇANAKKALE)

The geology and especially the magmatic rocks of Gokceada, which is the biggest island of Turkey and located at 20 km’s west of Biga Peninsula, constitute the subject of this study. Late Ediacaran/Early Paleozoic aged Camlica metamorphics which crop out with a tectonic uplift in a narrow area in northwest of Gokceada are the oldest rocks of the island. Early Eocene aged Karaagac Formation which is formed by submarine fan deposits unconformably overlies Camlica metamorphics. As for the Dagicitepe volcanic member which is formed by rhyolitic lavas, tuff and tuffites emplaced into Karaagac Formation cutting Camlica metamorphics is the oldest volcanic unit of the study area. On Karaagac Formation, Koyunbaba Formation has unconformably been deposited which consists of Middle Eocene shallow marine sediments. Then it has conformably been overlain by Sogucak Formation which consists of SE-NW ex- tending reefal limestone. Middle-Upper Eocene aged Ceylan Formation which conformably overlies the Sogucak Formation and the early Oligocene aged Mezardere Formation which conformably overlies Ceylan Formation have been deposited due to turbiditic currents in deep marine environment. Late Eocene(?) – Oligocene aged subvolcanics which cut Me- sozoic and Eocene units and emplaced into Eocene aged sedimentary units in the form of crypto dome and dome form the recent rigid topography of the study area and are the second magmatic phase called the “Gokceada Domes”. Diorite-monzonites porphyry which crys- tallized in lower zones of subvolcanics on the other hand constitutes Mutludere intrusion. In eastern and southern parts of Gokceada, Late Oligocene Gokceada ignimbrites are located which are observed in the form of pumice flows on Mezardere Formation. These ignimbrites are then overlain by Early Miocene aged Kesmekaya volcanics which are formed by blocky ash flows. Middle Miocene aged Eselek volcanics consisting of lava and pyroclastics with composition basaltic andesite and andesite are observed on a large area in east of Gokceada. Upper Miocene aged Canakkale Formation which is generally formed by the intercalation of poor consolidated conglomerate, sandstone, siltstone and marl crops out in narrow regions at east, southeast and south of Gokceada. Quaternary alluvial deposits and debris composed of loose, unconsolidated sand, silt and other sediments unconformably overlie all previous units and complete the succession. Main tectonic structures of Gokceada are formed by right lateral oblique faults which developed in Neo-tectonic period

GÖKÇEADA’NIN (ÇANAKKALE) JEOLOJİSİ

The geology and especially the magmatic rocks of Gokceada, which is the biggest island of Turkey and located at 20 km’s west of Biga Peninsula, constitute the subject of this study. Late Ediacaran/Early Paleozoic aged Camlica metamorphics which crop out with a tectonic uplift in a narrow area in northwest of Gokceada are the oldest rocks of the island. Early Eocene aged Karaagac Formation which is formed by submarine fan deposits unconformably overlies Camlica metamorphics. As for the Dagicitepe volcanic member which is formed by rhyolitic lavas, tuff and tuffites emplaced into Karaagac Formation cutting Camlica metamorphics is the oldest volcanic unit of the study area. On Karaagac Formation, Koyunbaba Formation has unconformably been deposited which consists of Middle Eocene shallow marine sediments. Then it has conformably been overlain by Sogucak Formation which consists of SE-NW extending reefal limestone. Middle-Upper Eocene aged Ceylan Formation which conformably overlies the Sogucak Formation and the early Oligocene aged Mezardere Formation which conformably overlies Ceylan Formation have been deposited due to turbiditic currents in deep marine environment. Late Eocene(?) – Oligocene aged subvolcanics which cut Mesozoic and Eocene units and emplaced into Eocene aged sedimentary units in the form of crypto dome and dome form the recent rigid topography of the study area and are the second magmatic phase called the “Gokceada Domes”. Diorite-monzonites porphyry which crystallized in lower zones of subvolcanics on the other hand constitutes Mutludere intrusion. In eastern and southern parts of Gokceada, Late Oligocene Gokceada ignimbrites are located which are observed in the form of pumice flows on Mezardere Formation. These ignimbrites are then overlain by Early Miocene aged Kesmekaya volcanics which are formed by blocky ash flows. Middle Miocene aged Eselek volcanics consisting of lava and pyroclastics with composition basaltic andesite and andesite are observed on a large area in east of Gokceada. Upper Miocene aged Canakkale Formation which is generally formed by the intercalation of poor consolidated conglomerate, sandstone, siltstone and marl crops out in narrow regions at east, southeast and south of Gokceada. Quaternary alluvial deposits and debris composed of loose, unconsolidated sand, silt and other sediments unconformably overlie all previous units and complete the succession. Main tectonic structures of Gokceada are formed by right lateral oblique faults which developed in Neo-tectonic period

A historical pyroclastic flow emplaced within a pre-existing Pleistocene lava tube: Silidong, Tianchi Volcano, Changbaishan, northeastern China

Pyroclastic flow deposits recently found within a pre-existing lava tube at Tianchi Volcano represent, to the best of our knowledge, the only such reported occurrence worldwide. In this case, pyroclastic flow of Tianchi’s “Millennium eruption” (~1 ka) traveled about 18 km from the summit eruptive source and poured successively into the ~560-m-long accessible segment of Silidong lava tube. Mapping of tube morphology, combined with detailed characterization of the features associated with the pumice flow deposits (e.g., adhering of pyroclastic materials on tube walls, fumarole pipes, rootless vents, and flow fronts of the deposit surface) has enabled plausible inferences regarding the original within-tube conditions and dynamic flow regime during emplacement. We propose a model of an aggrading pyroclastic flow which locally varies its sedimentation rate. The pyroclastic deposit is thicker in locations of reduced flow mobility, and the resultant variations in deposit thickness appear to control the distribution of fumarole pipes and rootless vents.

An inclined Vulcanian explosion and associated products

Vulcanian explosions generate some of the most hazardous types of volcanic phenomena, including pyroclastic density currents. Non-vertical directionality of an explosion promotes asymmetrical distribution of proximal hazards around the volcano. Although critical, such behaviour is relatively uncommon and has been seldom documented. Here we present, for the first time, evidence both from geophysical monitoring and field survey data that records the occurrence of such an event. Thermal imagery captures a Vulcanian explosion at Soufrière Hills Volcano, Montserrat, which occurred during a large partial lava dome collapse in February 2010, and was inclined at about 25° from the vertical in a northerly direction. Pyroclastic products were preferentially distributed to the north and included: an unusual pumice boulder deposit that we propose was formed by a dilute pyroclastic density current; pumice flow deposits; and a proximal lapilli and block fallout lobe. The inclined nature of the explosion is attributed to the asymmetric geometry around the vent. The explosion-derived pyroclastic density currents had notably lower velocities than those associated with lateral blasts, which, we suggest, result from a separate and distinct mechanism. These inclined explosions present an additional mechanism that is able to generate directed pyroclastic density currents, with consequent implications for hazard assessment.

Pumice Flow Deposit of Hakone-Tokyo Tephra Found in a Cellar at Ayase and Air-raid Shelters at Zama, Kanagawa, Japan

Pumice Flow Deposit of Hakone-Tokyo Tephra Found in a Cellar at Ayase and Air-raid Shelters at Zama, Kanagawa, Japan

Age Determination of the Kawagodaira Volcanic Eruption in Japan by 14C Wiggle-Matching

The Kawagodaira Volcano is located on the northwest slope of Amagi Volcano on the Izu Peninsula of central Japan. The fine pumice that erupted from Kawagodaira Volcano is widely distributed in the central and western parts of the Japanese mainland. Here, we report an accurate eruption age determined by radiocarbon wiggle-matching of a Japanese cedar timber excavated from the pyroclastic deposit at the pumice flow terminal of Kawagodaira Volcano. The 14C wiggle-match estimate for the Kawagodaira Volcano eruption is 1210–1187 cal BC (95.4% confidence level).

LiDAR derived morphology of the 1993 Lascar pyroclastic flow deposits, and implication for flow dynamics and rheology

Abstract Pumice flows are potentially destructive volcanic events that derive from eruption column collapse and whose dynamics are poorly understood. The challenges in studying these flows include the lack of constraints on the dynamics, kinematics and initial conditions that control their emplacement. We present a morphological study of the distal deposits (lobes) of the pumice flows resulting from the 1993 eruption at Lascar, Chile. The surface geometry of the lobes was measured in detail using a LiDAR device, which allowed for detailed characterisation of their morphology, consisting of central channel and lateral levees, and terminal frontal snout. In particular we find that the ratio of channel/levee height as a function of the ratio of the distance between levees/total width of the lobe has a characteristic curve for these pumice flow lobes. Our analysis of several of the Lascar pumice lobe deposits (south east sector) identified several dimensionless groups of the available parameters which, when compared against published results from both experimental and numerical investigations, allowed us to constrain crucial kinematic and dynamic information on the terminal phase of the pumice flows. Notably, we estimate the velocity of the terminal phase of pumice flows to be 5–10 m/s. Froude numbers of 1.5–2 are comparable with values found for experimental granular flows. Height–width aspect ratios for the levee–channel section of the pumice lobes are similar to those for experimental flows although these same aspect ratios for the snout are much larger for the natural deposits than their small-scale analogues. Finally, we discuss the possible emplacement dynamics of the terminal Lascar 1993 pumice flows. A pseudo-Reynolds number based on the velocity estimation is found to be up to 100 times larger for the pumice flows than experimental-scale flows. This suggests that the flow-retarding frictional forces for large-scale flows are relatively unimportant compared to flows at smaller scales. Mechanical effects such as fluidisation, mobilisation of material lying on the slopes over which they propagate and lubrication due to polydispersivity could provide an explanation for their ability to propagate on shallow slopes (6–11°).

Pyroclastic chronology of the Sancy stratovolcano (Mont-Dore, French Massif Central): New high-precision 40Ar/39Ar constraints

The Sancy (16km2) is the youngest of the two stratovolcanoes that constitute the Mont-Dore Massif (Massif Central, France). The restricted number of high precision radio-isotopic ages currently limits our knowledge of the pyroclastic chronology of this edifice which is the source of many tephra layers detected in middle Pleistocene sequences in southeast Europe. To improve our knowledge of the building phases of this stratovolcano, we collected thirteen pyroclastic units covering the entire proximal record. We present 40Ar/39Ar single grain laser dating performed in the facility hosted at the LSCE (Gif-sur-Yvette, France). The 40Ar/39Ar ages range from 1101±11ka to 392±7ka (1σ external). Four pyroclastic cycles lasting on average 100ka were identified (C. I to C. IV). C. I corresponds to the earlier explosive phase between 1101ka and 1000ka and starts about 100ka earlier than previously thought. The second pyroclastic cycle (C. II) is the main pyroclastic episode spanning from 818 to 685ka. This cycle is constituted of a minimum of 8 major pyroclastic eruptions and includes a major event that corresponds to a large plinian eruption at 719±10ka (1σ external) and recorded as a 1.4m thick layer 60km south-east of the Sancy volcano. The link between this large eruption and formation of a caldera stays however, hypothetical. The third pyroclastic cycle (C. III) found in the northeastern part of the Sancy (Mont-Dore valley) spanned from 642 to 537ka. Finally, the youngest pyroclastic cycle (C. IV) starts at 392ka and probably ends around 280ka. The age versus geographic location of each pyroclastic cycle indicates three preferential directions of channeling of the pyroclastic events and/or collapse of the volcanic edifice: northwest to west (C. I), southeast (C. II) and finally north to northeast (C. III and IV). The new high precision 40Ar/39Ar age for the Queureuilh bas pyroclastic unit (642±9ka) is identical within error with the U/Pb age obtained by Cocherie et al. (2009) [Geochimica et Cosmochimica Acta, 73, 1095–1108] and suggests a short residence time of the magma in a shallow, short-lived, small magmatic chamber. Finally, the source of the t21d tephra layer found in the Piànico Séllere varved sequence (Northern Italy) is not the Rivaux pumice flow as proposed by Brauer et al. (2007) [Journal of Quaternary Science 22, 85–96] and neither one of the C. II pyroclastic units as suggested by Roulleau et al. (2009) [Quaternary International 204, 31–43]. Accordingly, the source for the t21d layer has yet to be found at Sancy or elsewhere.