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Fires can have devastating effects on cultural stone heritage. Micro and macro changes occur in building stones exposed to high temperatures. Micro changes also cause changes in the physical properties of rocks. These changes not only cause instant damage to the monuments, but also make them less resistant to atmospheric processes and increase the rate of deterioration. Modelling these changes in cultural heritage in a laboratory environment can play an important role in ensuring the continuity of monuments. For this purpose, pyroclastic rock taken from the Niğde (Turkey) region was subjected to heat-treatment and the changes in the samples were investigated with XRD, SEM-EDX, TGA, spectrophotometry, porosity, P-wave velocity, capillary water absorption, and heat treatment loss. It was determined that the mineralogical changes in the structure of the rock with the increase in temperature affected various micro and macro properties. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the usability of the Leeb hardness test in determining changes in strength as a result of atmospheric weathering in works of cultural heritage built with low-strength pyroclastic rocks. To this end, the effects of weathering processes on strength properties were investigated in two building stones commonly used in Niğde province located in the Cappadocia (Turkey) region, which contains the most important works of cultural heritage created using low-strength pyroclastic rocks. The index, strength, mineralogical, and petrographic properties of rocks were first investigated. Then, freeze–thaw (F-T) and salt crystallization (SC) tests, the weathering processes of which consisted of six periods, were performed on samples prepared in cubic form. After the F-T and SC processes, the macro change in the samples and changes in weight loss, uniaxial compressive strength (UCS), and Leeb hardness (HL) values were determined. Highly correlated linear relationships were obtained between the SC and F-T cycles of the samples and the UCS and HL values. The HL test was applied to samples for which the UCS test could not be applied due to the loss of sample integrity after the advancing cycles of the accelerated weathering tests. Linear relationships with high correlation were determined between the UCS and HL values obtained from the samples after the accelerated weathering test. This study revealed that the HL approach could be used as an alternative in modeling the strength parameters of the weathering processes of the structures of cultural heritage built using low-strength rocks. [ABSTRACT FROM AUTHOR]

Indigenous oral traditions of the Líl̓wat Nation recount observations of Qw̓elqw̓elústen (Mount Meager), a Garibaldi Volcanic Belt volcano in southwestern British Columbia, Canada; and associated eruptive activity, mass-wasting, and outburst flooding. We present Líl̓wat observations relating to Qw̓elqw̓elústen's ∼2360 cal year B.P. eruption and its aftermath, a devastating outburst flood down the Lillooet valley. The Copper Canoe story correlates with the event sequence of pyroclastic damming of the Lillooet River and an outburst flood traveling far downstream, interrupting salmon runs and displacing people. Other stories suggest an eruptive plume and fumaroles. Recounted valley-floor changes, with proximal scouring and downstream filling of marshes allowing human resettlement, closely parallel and augment geological evidence, showing that oral traditions are equally important in holding landscape history. Oral traditions portray dramatic landscape changes, some by the Transformers, said to have traveled this land to make imperfect things right. Geologically documented debris-flow delta progradation and infill of the upper 50 km of Lillooet Lake since ∼12 000 cal B.P. underscore the land's dynamism and the need for both sources to inform planning for future eruptive, mass-wasting, and flooding events. Traditional landscape knowledge, like Western science, is observational and evidence-based, though interpretations can differ given Indigenous belief in a sentient landscape, capable of acting with intention. Binding of stories to geographical locations has functioned as a powerful mnemonic device to preserve orally transmitted information across many generations. [ABSTRACT FROM AUTHOR]

The remnants of explosive volcanism on Mercury have been observed in the form of vents and pyroclastic deposits, termed faculae, using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the Mercury surface, space environment, geochemistry, and ranging (MESSENGER) spacecraft. Although these features present a wide variety of sizes, shapes, and spectral properties, the large number of observations and the lack of high-resolution hyperspectral images complicates their detailed characterisation. We investigate the application of unsupervised deep learning to explore the diversity and constrain the extent of the Hermean pyroclastic deposits. We use a three-dimensional convolutional autoencoder (3DCAE) to extract the spectral and spatial attributes that characterise these features and to create cluster maps constructing a unique framework to compare different deposits. From the cluster maps we define the boundaries of 55 irregular deposits covering 110 vents and compare the results with previous radius and surface estimates. We find that the network is capable of extracting spatial information such as the border of the faculae, and spectral information to altogether highlight the pyroclastic deposits from the background terrain. Overall, we find the 3DCAE an effective technique to analyse sparse observations in planetary sciences. [ABSTRACT FROM AUTHOR]

In extensional and transtensional basins, the dual action of basin‐forming tectonism and explosive volcanic activity results in deposition of complex volcano‐sedimentary successions. Since both aspects contemporaneously affect sediment‐dispersal systems, it is difficult to distinguish one from the other by analyzing volcano‐sedimentary successions. To discriminate tectonic influence, basin fills of the Miocene, pull‐apart Janggi Basin were investigated. Here, the contemporaneous influence of tectonic subsidence and explosive volcanism formed <966 m thick, conformable volcano‐sedimentary successions composed of laterally extensive, pumice‐bearing, unwelded pyroclastic deposits and nonmarine sedimentary rocks formed under humid climatic conditions. Based on Laser Ablation – Multiple Collector – Inductively Coupled Plasma Mass Spectrometry zircon U–Pb age data, these successions were deposited from 21.1 ± 0.2 to 20.1 ± 0.1 Ma. After deposition of basinwide, tens of metres thick, and unwelded pyroclastic deposits, the directly overlying facies successions (FS‐1 and FS‐2) show contrasting depositional features to the classical model of volcaniclastic sedimentation. Facies succession‐1 is represented by fluvial conglomerates composed mostly of (sub)rounded pebble and cobble sourced from basement. The supply of basement‐derived clasts through fluvial systems resulted from development of physiographic relief during or soon after the eruption by syndepositional tectonic subsidence. Facies succession‐2 occurs directly on pyroclastic deposits that cover FS‐1 and show coarsening‐upward trends, and is composed of basal lacustrine mudstones and overlying resedimented volcaniclastic sandstones showing a progradational geometry, interpreted as a result of progressive filling of the lake by remobilized volcaniclastic sediments. Occurrence of basal lacustrine laminated mudstones indicates that syndepositional creation of the accommodation by tectonic subsidence exceeded forceful input of pyroclastic and remobilized volcaniclastic sediments, resulting in a delayed sedimentary response to explosive volcanic eruptions. This study shows that, despite voluminous production of volcaniclastic sediments, tectonic activity controlled sediment type and stacking patterns. Therefore, these depositional features allow to discriminate tectonic effects from complex volcano‐sedimentary successions, enabling to reconstruct basin evolution. [ABSTRACT FROM AUTHOR]

Recognition of ancient pyroclastic rocks made up of particles of explosive volcanic origin and deposited by primary volcanic processes and/or rapid sedimentation of freshly erupted, texturally unmodified particles vis-a-vis secondary volcaniclastic deposits derived by significant reworking during transport of primary volcanic particles prior to deposition, but long after volcanism (epiclastic) remains a major challenge. A volcanic conglomerate having both rhyolitic clasts and matrix within the ~2.5 Ga Bijli Rhyolite in the Dongargarh large igneous province in the Bastar craton, is earlier interpreted to be of epiclastic origin, primarily because of the presence of large rounded rhyolite clasts imparting conglomeratic appearance to the deposit, and thereby considered representing a significant time break (unconformity) between explosive Bijli volcanism and the deposit. Based on new field and petrography studies, we identified the ~125 m thick volcanic conglomerate as rapidly sedimented texturally unmodified rhyolitic breccia-conglomerate linked to coeval incipiently welded pyroclastic flow that occurred during caldera collapse related to mafic-recharge-mediated Bijli volcanism, without significant time break. We ascribed the rounding of rhyolite clasts to surface tension of hot crystallising molten magma in plastic state and partly to mechanical interactions of particles on steep slopes in such volcanic settings. This study may help clarify origin of similar deposits in deformed metamorphosed provinces elsewhere. [ABSTRACT FROM AUTHOR]

Mount Kelud is located in the eastern part of Java in Indonesia, and an eruption occurred there in 2014. This eruption produced volcanic materials, particularly distributed on the northeast and southern slopes. This Plinian eruption and the new pyroclastic materials could possibly have influenced changes to the soil development and classification but to date there have been no investigations to determine the nature of such changes. The main objectives of this study were to determine these changes in soil characteristics and classification affected by pyroclastic materials which were thicker on the north-east compared to the south slopes. For each slope, north-east (PTL) and south (PS), five slope positions were studied, i.e., top (1), upper slope (2), middle slope (3), lower slope (4) and plains (5). The results show that the thickness of the new pyroclastic material is less than 50 cm (in fact always less than 30 cm occurred in field measurements), but having slight varia- tions in thickness depending on the sequence concerned. Hence the definitional requirement for a buried soil of 50 cm was not found following the 2014 eruption. Compared to the south (PS) slopes where no C horizon was developed, on the northeast (PTL) a varying thickness but less than 30 cm of new materials was found. This new pyroclastic material had a loose, single structure, light soil colour, high bulk density in the top layers but coarse texture, low organic carbon content and was low in exchangeable cations. The andic properties as defined in Soil Taxonomy (Soil Survey Staff, 2014) were not met in these new deposits. However based on this study, classification differences have occurred in the Upper Slope (PTL-2 Vitrandic Humudepts and PS-2 Humic Udivitrands), and in the Middle Slope (PTL-3 Vitrandic Humudepts and PS-3 Typic Udivitrands). The Lower Slope (PTL-4) and Plains (PS-5) were Fluvaquentic Endoaquepts and Fluvaquentic Humaquents respectively. [ABSTRACT FROM AUTHOR]

Natural geophysical mass flows are among the most complex granular systems and their dynamics are often modified by the presence of an interstitial fluid. Prediction of their runout requires the development of models estimating the solid stresses in these hazardous currents wherein excess pore-fluid pressure can develop. We use discrete element modelling (DEM-CFD) with a Coarse-Graining post-processing technique (CG) to investigate the rheology of unsteady gas-particle fluidized to non-fluidized granular beds placed on horizontal and inclined planes. Similar to fluidized beds immersed in viscous fluids, the effective friction coefficient of air-fluidized beds can be defined as a function of the classic μ(I)-rheology and the non-dimensional fluid or solid pressure to explain the failure and dynamics of granular flows with excess pore pressure on inclines. However, dilation imposed by fluid drag and particle collisions in gas-particle fluidized beds can drastically change its effective frictional properties. In contrast with the common assumption in water-particle flows that granular temperature is negligible, in our gas-particle simulations, the contribution of the velocity fluctuations to the stress tensor is significant. Hence, the shear stress is found to be non-zero even when the flow is fully fluidized in the inertial regime. These results suggest the need to better understand velocity fluctuations to predict the effective viscosity of sheared fluidized mixtures and are relevant for many applications. Notably, a unified approach is useful for many geophysical flows that encompass a range of fluidization conditions in a single flow such as pyroclastic density currents and snow avalanches. [ABSTRACT FROM AUTHOR]

Arabia Terra is an ancient, geologically complex region of Mars. It is not typically described as volcanic because it lacks prominent, easily recognizable shield volcanoes. However, Arabia Terra contains flood lavas and widespread layered materials that might represent airfall pyroclastics from unknown sources. Here we explore the geology of some complex topographic depressions in Arabia Terra previously hypothesized to represent collapse features termed "plains style caldera complexes," an enigmatic category of volcanic feature on Mars characterized by surface collapse rather than construction of topographic shields. Geomorphological observations and geologic mapping carried out here support a volcanic origin of these features, providing a possible source for volcaniclastic sediments. The identification of caldera complexes in Arabia Terra raises the questions of how they are linked to the geotectonic setting of the region and whether similar features might be much more abundant in the ancient Martian crust than is currently recognized. Plain Language Summary: The most recognizable volcanoes on Mars correspond to large topographic shields, similar to Hawaiian volcanoes. These are constructional features built by layer upon layer of lava, with a summit caldera. It is possible that Mars contains many other volcanoes that are less recognizable, however, because they consist only of the caldera complex without the topographic rise built by lava. These calderas could signify much more explosive volcanism or collapse due to migration of magma in the subsurface, or both. In this work, we describe more identifications of plains style caldera complexes, which are collapsed calderas without the easily recognizable shield topography. We describe the geology of these calderas in the Arabia Terra region, their possible links to tectonic setting there, and the possibility that similar features are much more common on Mars than is currently understood. Key Points: Collapsed calderas are recognized in eastern Arabia TerraAncient Martian crust may have far more collapsed calderas than commonly recognizedPlains style caldera complexes could be a source of huge volumes of volcaniclastic materials on Mars [ABSTRACT FROM AUTHOR]

Sedimentary structures deposited from dilute pyroclastic currents are largely dominated by the products of metre‐scale dune bedforms containing backset lamination. Since the first seminal works in the 1970s, the vast majority of the literature called these structures antidunes and chutes‐and‐pools. This consensus led to a loose terminology, either as descriptive terms for structures containing backset beds or as an interpretation of hydrodynamic conditions related to Froude‐supercritical flows. Here, the main characteristics of pyroclastic dune bedforms are summarized and classified under four categories. The literature is examined and a discussion is provided regarding a possible interpretation in terms of supercritical‐flow bedforms, including antidunes, chutes‐and‐pools and cyclic steps. The interpretation of pyroclastic dune bedforms as related to supercritical conditions is a possibility among others, yet open questions remain, and under the present state of knowledge alternative interpretations are equally valid. There is consensus that an interpretation of pyroclastic dune bedforms as related to supercritical flows implies deposition from a basal underflow supporting an internal free‐surface formed at a density interface within the overall current structure. Antidunes are likely to be limited to low‐angle, incipient bedforms. Chutes‐and‐pools may occur in the form of series of steep truncations formed by an erosive supercritical 'chute', with a depositional signature occurring exclusively in the subcritical 'pool' of a Froude jump. Rare examples of cyclic steps may be found as large‐scale undulations with superimposed metre‐scale bedforms, or for short‐scale, fully‐aggrading structures in periodic trains. Additionally, some pyroclastic bedforms may be formed by granular flows passing granular Froude jumps or as frozen granular stationary waves. The dominance of chute‐and‐pool structures over bedforms characterized by relatively more stable morphodynamics (antidunes and cyclic steps) could be the likely result of pulsating flow conditions and abrupt high‐rate deposition, impeding stable flow conditions. In a majority of cases, the growth of pyroclastic bedforms appears triggered by the influence of an inherited bed topography and by bed‐flow feedback effects. Apart from a possible interpretation as supercritical bedforms, other specific dynamics of dilute pyroclastic currents may well explain the characteristics of pyroclastic dune bedform structures. In particular, the inherent turbulence fluctuations, internal organization and highly depositional dynamics may be key to the formation of pyroclastic dune bedforms. In this respect, rather than solely focusing on the Froude dimensionless number of the formative current, theoretical considerations accounting for the Reynolds, Richardson and Rouse dimensionless parameters may be also implemented for the understanding of pyroclastic bedforms. [ABSTRACT FROM AUTHOR]

Dense, glassy pyroclasts found in products of explosive eruptions are commonly employed to investigate volcanic conduit processes through measurement of their volatile inventories. This approach rests upon the tacit assumption that the obsidian clasts are juvenile, that is, genetically related to the erupting magma. Pyroclastic deposits within the Yellowstone-Snake River Plain province almost without exception contain dense, glassy clasts, previously interpreted as hyaloclastite, while other lithologies, including crystallised rhyolite, are extremely rare. We investigate the origin of these dense, glassy clasts from a coupled geochemical and textural perspective combining literature data and case studies from Cougar Point Tuff XIII, Wolverine Creek Tuff, and Mesa Falls Tuff spanning 10 My of silicic volcanism. These results indicate that the trace elemental compositions of the dense glasses mostly overlap with the vesiculated component of each deposit, while being distinct from nearby units, thus indicating that dense glasses are juvenile. Textural complexity of the dense clasts varies across our examples. Cougar Point Tuff XIII contains a remarkable diversity of clast appearances with the same glass composition including obsidian-within-obsidian clasts. Mesa Falls Tuff contains clasts with the same glass compositions but with stark variations in phenocryst content (0 to 45%). Cumulatively, our results support a model where most dense, glassy clasts reflect conduit material that passed through multiple cycles of fracturing and sintering with concurrent mixing of glass and various crystal components. This is in contrast to previous interpretations of these clasts as entrained hyaloclastite and relaxes the requirement for water-magma interaction within the eruptive centres of the Yellowstone-Snake River Plain province. [ABSTRACT FROM AUTHOR]

We present a new geological map of Warrumbungle Volcano created from volcanic facies field mapping, new geophysical, geochemical and geochronological data as well as data from previous studies. Field mapping and petrography defined 19 volcanic and two mixed volcanic--sedimentary facies. Facies identification and distribution in conjunction with geochemical analyses indicate an early shield-forming phase of predominantly mafic and intermediate lavas and pyroclastic deposits, and minor felsic lavas deposited on an irregularly eroded basement of Surat and Gunnedah basin rocks. The shield was subsequently intruded by felsic intermediate to felsic magmas that formed dykes and other intrusions, including possible cryptodomes, and also erupted as lava domes and block-and-ash-flow deposits. A radial dyke swarm cross-cuts most units, although it is concentrated within basement sandstone surrounding the central area of the volcano, suggesting late inflation accompanied by dyke emplacement. Geochemistry indicates differentiation of a single although repeatedly recharged alkaline magmatic suite. Fractionation of olivine, Ti-magnetite and clinopyroxene occurred in mafic magmas, and after reaching 62 wt% SiO2, crystallisation of apatite and alkali feldspar took place. A new U–Pb zircon SHRIMP magmatic crystallisation age of 16.25 ± 0.12 Ma on a felsic block-and-ash flow deposit is in agreement with the recalculated 40Ar/39Ar isochron dates of previous workers. Based on our mapping and the use of volcanic facies to define mappable units, we recommend that the previous Warrumbungle Volcanics be elevated from formation to group status and renamed the Warrumbungle Volcanic Complex. New geological map of Warrumbungle Volcano based on volcanic facies mapping. The volcanic facies and their distribution suggest a complex alkaline shield volcano erupted along regional-scale basement lineaments over an intracontinental 'hot-spot' or heat plume, and intruded by lava domes, cryptodomes, dykes and a radial dyke swarm. New and previous geochemical data suggest differentiation of a single, although repeatedly recharged, alkaline magmatic suite. A new U–Pb zircon SHRIMP date of 16.25 ± 0.12 Ma is in agreement with the recalculated 40Ar/39Ar isochron dates of previous workers. [ABSTRACT FROM AUTHOR]

Volcanic tuffs are important in determining the stratigraphic age of sedimentary sequences, which is closely related to the tectonic, sedimentological, geomorphological, palaeoclimatic, and palaeo-ecological evolution of the sequences. However, it may be difficult practically to identify tuffs in stratigraphic sections, especially when they are altered after deposition. In this study, a series of petrographic, mineralogical, and geochemical analyses was deployed in the tuff layers cropping out in the Lunpola Basin of the central Tibetan Plateau, which is a crucial site for studying the dynamics and processes of the Cenozoic uplift of the Tibetan Plateau and its environmental impacts. In these pyroclastic layers, authigenic analcime is the main volcanic glass alteration product. The analcime-bearing samples are Na-enriched, in contrast to the K-abundant magmatic rocks in the central Tibetan Plateau. The distribution patterns of the rare earth elements (REEs) of the bulk analcime-bearing samples are similar to those of the magmatic rocks but different from those of the analcime-poor sediments and analogues of well-mixed upper continental material (e.g. the upper continental crust and post-Archaean Australian average shales). The distinct distributions of Na and REEs among analcime-bearing samples, analcime-poor sediments, and volcanic rocks reveal that analcime formed both from alteration of volcanic material and from mixing processes with non-volcanic sediments. It is proposed, therefore, that the discrimination plot of Na2O/Al2O3-(La/Yb)N may be used to distinguish the analcime-related pyroclastic rocks from the basin sedimentary sequence, and thus it may provide a means of discriminating between various volcanic material and of supporting age dating by tephrochronology in the central Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Lunar mare regolith is traditionally thought to have formed by impact bombardment of newly emplaced coherent solidified basaltic lava. We use new models for initial emplacement of basalt magma to predict and map out thicknesses, surface topographies and internal structures of the fresh lava flows, and pyroclastic deposits that form the lunar mare regolith parent rock, or protolith. The range of basaltic eruption types produce widely varying initial conditions for regolith protolith, including (1) autoregolith, a fragmental meter‐thick surface deposit that forms upon eruption and mimics impact‐generated regolith in physical properties, (2) lava flows with significant near‐surface vesicularity and macroporosity, (3) magmatic foams, and (4) dense, vesicle‐poor flows. Each protolith has important implications for the subsequent growth, maturation, and regional variability of regolith deposits, suggesting wide spatial variations in the properties and thickness of regolith of similar age. Regolith may thus provide key insights into mare basalt protolith and its mode of emplacement. Plain Language Summary: Following recent studies of how lava eruptions are emplaced on the lunar surface, we show that solid basalt is only one of a wide range of starting conditions in the process of forming lunar soil (regolith). Gas present in the lavas during eruption also produced bubbles, foams, and explosive products, disrupting the lava and forming other starting conditions for mare soil parent material. Key Points: New basalt magma emplacement models explore fresh lava flows that form the lunar mare regolith parent rock, or protolithSome conditions predict immediate formation of a fragmental meter‐thick "autoregolith" that mimics impact‐generated regolithLava flows with significant near‐surface vesicularity, macroporosity, and magmatic foams will create nontraditional regolith growth rates [ABSTRACT FROM AUTHOR]

Sabalan volcano is located in the northwest part of Iran, 25km south of Meshkinshahr in the west Alborz-Azerbaijan structural zone. The eastern and the northern slopes of Sabalan are predominantly covered with significant amounts of white-gray pyroclastic deposits indicating a very intense explosive activity at the last stage of this huge volcano activity. After conducting field studies and analyzing the results obtained using the mathematical calculations, a Plinian eruption was detected, and the extrapolated volume of the latest pyroclastic fall eruption was estimated 3.24 km3, and the eruption of Sabalan rated a VEI of 5. Also, due to the shape of the isopach curve, the role of wind is significant in dispersing these deposits from the west to the east. The overall geochemical features of the Sabalan’s ignimbrite including the rhyodacitic nature with calc-alkaline signatures as well as the high ratios of Ba/Nb>28 and Ba/Ta>450 are consistent with subduction related zone. [ABSTRACT FROM AUTHOR]

In this study, the effects of drying on Atterberg limits is investigated using 30 soil samples taken from 0.2 m to 4 m depths, from nine different landslide sites in pyroclastic deposits in Northern Turkey. Samples for Atterberg limits are prepared by (i) starting from in-situ moisture content without any drying, (ii) drying at 60 °C and (iii) drying at 110 °C. The range of liquid limit values is 35%–103% and plasticity index is 0%–37% when samples are prepared from their in-situ moisture content. Liquid limits of the samples, when they are prepared by drying at 110 °C oven, are 1%–30% lower than the liquid limits prepared from moist condition. The mineralogy of the samples is investigated via X-ray diffraction analyses and Scanning Electron Microscopy photographs, indicating existence of halloysite mineral. Organic contents of the samples, determined by loss on ignition test, is in the range of 4.3%–12.1%. It is concluded that drying significantly influences the results of Atterberg limits if the sample contains some organic material and/or halloysite mineral. Both liquid limit and plasticity index values of the soils decreased as the sample preparation method changed from moist preparation to drying at 110°C. Existing soil classification methods may fail to represent the influence of organic matter on soil behaviour for soils containing <15% organic material. [ABSTRACT FROM AUTHOR]

Abstract: Pyroclastic density currents (PDCs) are a significant volcanic hazard. However, their dominant transport mechanisms remain poorly understood, in part because of the large variability of PDC types and deposits. Here we combine field data with experimental and numerical simulations to illuminate the twofold fate of particles settling from an ash cloud to form the dense PDC basal flow. At solid fractions >1 vol %, heterogeneous drag leads to formation of mesoscale particle clusters that favor rapid particle settling and result in a mobile dense layer with significant bed weight support. Conversely, at lower concentrations the absence of particle clusters typically leads to formation of poorly mobile dense beds that deposit massive layers. Based on this transport dichotomy, we present a numerical dense‐dilute parameter that allows a PDC's dominant transport mechanism to be determined directly from the deposit geometry and grainsize characteristics. [ABSTRACT FROM AUTHOR]

To better understand the alteration of volcanic ash in different depositional environments, we measured the clay mineralogy, major and trace element geochemistry, and Sr and Nd isotopic composition of altered ashes in two Permian-Triassic boundary (PTB) successions in southern China. The Pengda and Xinmin sections, in Guizhou Province, represent different depositional settings, allowing us to investigate the role of the early burial environment on authigenic clay formation. The PTB ash beds in both sections consist predominantly of mixed-layer illite-smectite clays. Altered ashes in the relatively shallow Pengda section contain predominantly R2 and R3 I/S clays, with 70% to 85% illite layers, whereas their stratigraphic equivalents in the deeper Xinmin section contain only R3 I/S clays, which are of two types, with 88% and 99% illite layers, respectively. The two studied ash beds can be stratigraphically correlated between the Pengda and Xinmin sections, and show similar 143 Nd/ 144 Nd ratios (0.511991 for PD-1 and 0.511983 for XM-1; 0.512073 for PD-2 and 0.512074 for XM-2). REE distributions and immobile element cross plots are consistent in suggesting that the altered ashes may have originated via felsic volcanism. Altered ashes in the deep sea Xinmin section have notably high 87 Sr/ 86 Sr ratios (0.764209 and 0.795921) relative to those from equivalent horizons in shallower environments (0.728455 and 0.749953). This difference in 87 Sr/ 86 Sr ratios is likely attributable to different degrees of chemical weathering. Elemental ratios such as SiO 2 /Al 2 O 3 , K 2 O/Al 2 O 3 , Y/Sr and Zr/Sr all support more intensive chemical leaching in deep-water environments. The altered ashes at Xinmin contain substantially more K 2 O than those at Pengda, suggesting greater incorporation of K + into clay minerals due to lower pH during early diagenesis. Clay mineral assemblages and stacking structures of I/S clays in the altered ashes are consistent with a general sequence of smectite illitization during diagenesis and burial metamorphism in both sections. [ABSTRACT FROM AUTHOR]

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 (> 0.5 cm) 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 ≥ 15 cm. 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. [ABSTRACT FROM AUTHOR]

Nearly forty years after the return of the first lunar samples to Earth, improvements in laboratory detection limits made possible the first definitive discovery of magmatic water in lunar volcanic samples. The intervening decade has seen an exponential increase in the amount of data on the abundance of magmatic water, and its hydrogen isotope composition, in various rock types recovered from the Moon. Here we review these data and describe how the abundance of water in the lunar interior places important constraints on models for the high-temperature origin and evolution of the Moon. [ABSTRACT FROM AUTHOR]

The Lechmine N'kettane is a Quaternary volcano, located within the Middle Atlas Volcanic Field (MAVF) in central Morocco. It is built on the faulted contact between Liassic limestone and Plio-Quaternary fluvio-lacustrine deposits. In map-view it consists of an elliptical maar crater, surrounded by a tephra ring, within which a scoria cone is nested in its northern crater zone. The Lechmine N'Kettane volcano is monogenetic in the sense of its small eruptive product volume and lack of evidence of significant time breakthrough it grows. The volcano formed from an eruptive locus that migrated laterally and vertically within the short duration of the eruption in a zigzagging pattern, along a complex set of generally NE-SW and NW-SE-trending faults. It represents a perfect example of how a volcano form and evolve under the influence of a combination of specific factors such as the lithological characteristics of the substrate, its hydrogeological parameters, magma flux and the local structural framework of the country rocks. The petrographic, granulometric and morphological (including terrain modelling) analyses of the Lechmine N'kettane pyroclastic deposits show that it was constructed in four eruptive phases with variable eruptive styles. The first, relatively dry, phreatomagmatic phase, that took place on a NE-SW fault in the northeastern part of the crater, was generated by the interaction between the ascending basaltic magma with meteoric water in the karst aquifer hosted by the Liassic limestone. The second phase is represented by a magmatic scoria fallout deposit whose explosion locus moved westward, along the same NE-SW fault. During the third phase, the explosion center migrated southward, along a NNW-SSE fault, and produced the last phreatomagmatic event by interaction of magma and water-saturated Plio-Quaternary sediment. The fourth eruptive phase is a purely scoria event, corresponding to the construction of the nephelinitic scoria cone in the northwestern part of the tephra ring. Between eruptive products formed in respective eruptive phases no evidence was recognized to establish significant time gaps between their formation. • A detailed study of a complex monogenetic volcano in a karst setting. • The examination of the interaction between magma and water-saturated Plio-Quaternary sediment. • The observation of lateral and vertical migration during the eruption despite its short duration. • The identification of four distinct eruptive phases with varying styles, ranging from phreatomagmatic to magmatic. [ABSTRACT FROM AUTHOR]

Either directly or indirectly, building stone is exposed to diverse atmospheric interactions depending on the seasonal conditions. Due to those interactions, objects of historic and cultural heritage, as well as modern buildings, partially or completely deteriorate. Among processes involved in rock deterioration, the freeze–thaw (F–T) cycle is one of the most important. Even though pyroclastic rocks have been used as building stone worldwide due to their easy workability, they are the building stone most affected by the F-T cycle. A historical region in Central Anatolia, Turkey, Cappadoia encompasses exceptional natural wonders characterized by fairy chimneys and unique historical and cultural heritage. Human-created caves, places of worship and houses have been dug into the pyroclastic rocks, which have in turn been used in architectural construction as building stone. Using 10 pyroclastic rock samples collected from Cappadocia, we determined the rock’s index-mechanical properties to develop a statistical model for estimating percentage loss of uniaxial compressive strength a critical parameter of F-T cycle’s important value. We used dry density (ρ d ), ultrasonic velocity (V p ), point load strengths (I S(50) ), and slake-durability test indexes (I d4 ) values of unweathered rocks in our model, which is highly reliable ( R 2 = 0.84 ) for predetermination of percentage loss of uniaxial compressive strengths of pyroclastic rocks without requiring any F-T tests. [ABSTRACT FROM AUTHOR]

Hydrated halloysite was discovered in books, a morphology previously associated exclusively with kaolinite. From ∼1.5 to ∼1500 μm in length, the books showed significantly greater mean Fe contents (Fe2O3 = 5.2 wt.%) than tubes (Fe2O3 = 3.2 wt.%), and expanded rapidly with formamide. They occurred, along with halloysite tubes, spheroids and plates, in highly porous yet poorly permeable, silt-dominated, Si-rich, pumiceous rhyolitic tephra deposits aged ∼0.93 Ma (Te Puna tephra) and ∼0.27 Ma (Te Ranga tephra) at three sites ∼10-20 m stratigraphically below the modern landsurface in the Tauranga area, eastern North Island, New Zealand. The book-bearing tephras were at or near saturation, but have experienced intermittent partial drying, favouring the proposed changes: solubilized volcanic glass + plagioclase→halloysite spheroids→halloysite tubes→halloysite plates→ halloysite books. Unlike parallel studies elsewhere involving both halloysite and kaolinite, kaolinite has not formed in Tauranga presumably because the low permeability ensures that the sites largely remain locally wet so that the halloysite books are metastable. An implication of the discovery is that some halloysite books in similar settings may have been misidentified previously as kaolinite. [ABSTRACT FROM AUTHOR]

Ulaş district of the Sivas province is Yeşildiyar (İtkıran) of the village, 1, 5 km southwest of the village, there are two closed depressions. These depressions are aligned northeast-southwest direction which located in watershed between the Kızılırmak River Basin and Karasu (Fırat) River Basin. Tepegöl I is in the northeast depression, 900 m mouth diameter, bottom diameter 400 metres and has a depth of approximately 130 m. The lake called Tepegöl is located at the base of the depression. The rest of the base is covered with alluvial deposit. Floor level elevation of Tepegöl II is 1851 m and a base diameter of 450 metres and the diameter of the mouth portion 250 meters. The depth of this depression having a circular shape is about 85 meters. It is 60 meters higher than the bottom of the depression in the northeast. These depressions have typical features of maar structures. Pyroclastics are disseminated due to severe gas pressure and deposited as concentric layers with slopes toward outside. Pyroclastics has a thickness of more than 30 metres. Pyroclastic material was partially destroyed as a result of erosive activities. Ophiolitic formation of the base of the rock fragments is observed in layered pyroclastics. Maars have probably occurred in the Plio-Quaternary. Depressions are developed in control of the left lateral strike-slip fault. The Tepegöl Lake, 1791 meters high above sea level, is typically a maar lake. [ABSTRACT FROM AUTHOR]

Maar–diatreme volcanoes in their simplest form are thought to result primarily from discrete, subsurface phreatomagmatic explosions. Tephra ring deposits around maar craters are commonly interpreted in terms of a framework wherein explosions begin at shallow levels and migrate downward, ejecting progressively deeper-seated lithic clasts that contribute to the tephra rings. Additionally, variations in grain size of the deposits are sometimes inferred to record different degrees of fragmentation, which are in turn related to variations in magma–water ratios at the explosion sites. Recent detailed studies of diatremes and of maar tephra rings suggest a different conceptual model wherein explosions can happen at various vertical and lateral locations within a diatreme during its eruptive lifetime, rather than being limited to simple downward migration. Experiments, numerical modeling, and field data indicate that most explosions deeper than about 200–250 m will not vent to the surface, but instead contribute to mixing within the diatremes. Arrival of deep-seated lithics at the surface is related to this mixing process. Experiments also indicate that explosion phenomena can result in a range of emplacement mechanisms and grain sizes in tephra rings, even in the absence of variations in explosion mechanisms (fragmentation efficiency). Here we document the tephra ring of Stracciacappa maar (central Italy), and interpret its eruption and emplacement history in terms of evolving conceptual models along with a comparison to more traditional interpretations. Because most elements of both conceptual models are viable, the work points to an important need to develop criteria to distinguish between the various inferred mechanisms. [ABSTRACT FROM AUTHOR]

Kandovan is one of the geo-tourism attractions in the East Azarbaijan Province of Iran, where rural houses were carved within the cone-shaped pyroclastic rocks several hundred years ago. The present paper mainly discusses the effects of climate condition and engineering geology properties on the destruction of these cone-shaped rocky houses. The annual averages of wetting-drying and freezing-thawing cycles are 28 and 14 times, respectively. The greatest impact of these cycles is found to be on physical weathering, which has low resistance to weathering and erosion. Such a low resistance is because of the components of Kandovan rock mass that are largely plagioclase and pumice fragments with low resistance, low welding and sorting, high sphericity, and rounding. Although the weathering and erosion along the existing discontinuities and joints is the most important causative agent of cone-shaped forms, there is a possibility of further damage of the houses due to continuous weathering and erosion potential processes. High porosity, high water absorption, high sensitivity to expansion, contraction by freezing-thawing and wetting-drying cycles; and low strength of Kandovan pyroclastic rocks lead to weathering and disintegration due to environmental impacts. The correlation of physical and mechanical properties of Kandovan pyroclastic rocks and Cappadocian tuffs has shown that the unit weights of rocks are low for both of them. The total porosity of Kandovan rocks is greater than for Cappadocian rocks, while water absorption percent is greater for Cappadocian rocks than for Kandovan rocks. The strength of Kandovan rocks is generally greater than that of Cappadocian rocks. [ABSTRACT FROM AUTHOR]

We analyse a seemingly-unique landform on Mercury: a conical structure, encircled by a trough, and surrounded by a 23,000 km 2 relatively bright and red anomaly of a type interpreted elsewhere on the planet as a pyroclastic deposit. At first glance, this could be interpreted as a volcanically-constructed cone, but if so, it would be the only example of such a landform on Mercury. We make and test the alternative hypothesis that the cone is the intrinsic central peak of an impact crater, the rim crest of which is visible beyond the cone-encircling trough, and that the trough is a vent formed through explosive volcanism that also produced the surrounding bright, red spectral anomaly. We test this hypothesis by comparing the morphology of the cone and the associated landform assemblage with morphologically-fresh impact craters of the same diameter as the putative host crater, and additionally, by modelling the original morphology of such a crater using a hydrocode model. We show that the present topography can be explained by formation of a vent completely encircling the crater’s central peak and also make the observation that explosive volcanic vents frequently occur circumferential to the central peaks of impact craters on Mercury. This indicates that, although this cone initially appears unique, it is in fact an unusually well-developed example of a common process by which impact-related faults localize magma ascent near the centre of impact craters on Mercury, and represents an extreme end-member of the resulting landforms. [ABSTRACT FROM AUTHOR]

Detailed examination of volcaniclastic and sedimentary rocks collected from the Taney (30 Ma), President Jackson (4 Ma), Vance (3 Ma) and Alarcon (2 Ma) near-ridge seamount chains of the North Pacific reveals seven clastic lithofacies that record various modes of eruption, magma fragmentation, and particle dispersal. Lithofacies are distinguished by differences in lithology, bedding habit, compositional heterogeneity, and relationship to volcanic landforms. Breccia lithofacies were produced through mechanical fragmentation during caldera collapse (polymict) or effusive eruptions onto steep slopes (monomict). Rare globular lapilli mudstone lithofacies contain clasts with morphologies formed by magma-sediment mingling processes (peperite). Seamount summit pyroclastic deposits include proximal lapilli tuff with vesicular pyroclasts, and more distal limu o Pele tuff lithofacies. Much finer-grained hydrothermal mudstone/tuff lithofacies occurs around caldera rims and contains greenschist minerals, hydrothermal clays and basaltic ash that record subsurface phreatomagmatic fragmentation processes. Very fine-grained ash is transported to distal regions by oceanic currents and hydrothermal plumes, and is a component of the regional pelagic sediment budget. Pyroclastic lithofacies only occur on seamount summits suggesting formation during the latter stages of seamount evolution. As a seamount drifts away from an adjacent ridge axis and associated heat source, its magma supply is reduced allowing for magmatic gas buildup and mild explosive eruptions. During this stage, the diminished melt supply under the seamount is unable to fully compensate for extension along the ridge axis and vertical seamount growth. Lateral intrusion into spreading-related structures in this stage causes magma withdrawal and caldera formation. Formation of caldera ring faults also promotes seawater ingress into subseafloor hydrothermal cells, which interact with magma conduits causing phreatomagmatic fragmentation. [ABSTRACT FROM AUTHOR]

Ulleung Island is a Quaternary volcanic island located in the mid-western part of the East Sea (Sea of Japan) back-arc basin, which has erupted from the Pliocene until the late Holocene. This study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent, trachytic/phonolitic pyroclastic sequence, named the Nari Tephra Formation. This formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the Nari Caldera. The embayment acted as a topographic trap for proximal pyroclastic deposits, and contains a complete record of the past 19,000 years of eruption history. The formation includes evidence for five separate eruptive episodes (Member N-1 to N-5), with intervening weathered and/or soil horizons indicating hundreds to thousands of years of repose between each eruption. Eruption styles and depositional mechanisms varied between and during individual episodes, reflecting changing dynamics of the magma plumbing system, magmatic gas coupling, and a variable role of external water. Extra-caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents (PDCs) large enough to overtop the caldera wall. Thus tephra sequences outside the caldera provide an underestimate of eruption frequency, and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records. In addition, topographic effects of caldera structures should be considered for the assessment of PDC-related hazards in such moderately sized pyroclastic eruptions. [ABSTRACT FROM AUTHOR]

Abstract: The Upper Oligocene Smrekovec Volcanic Complex (SVC) represents the remains of a submarine stratovolcano which underwent Early Miocene tectonic dissection and large-scale displacement along the Periadriatic Line. According to the lithofacies architecture, abundance and spatial distribution, central, medial and distal zones of the former volcano edifice have been recognized. The zones extend at a distance of 0–2km, 2–5km and 5–20km south of the Periadriatic Line, respectively. Lavas, shallow intrusive bodies, autobreccia, peperite and hyaloclastite deposits occur mainly in the central zone, which is extensively altered under hydrothermal conditions, characterised by the development of zeolites (mainly laumontite), prehnite, chlorite, albite and epidote. Explosive volcanic activity intensified with the evolution of magma from basaltic andesitic to dacitic. Gas- and water-supported eruption-fed density flows are assumed. Their deposits show lithofacies organisation in pyroclastic depositional units (PDUs) similar to either volcaniclastic turbidites (Type 1) or thin subaqueous ignimbrites (Type 2). Volcaniclastic debris flow deposits are developed as volcaniclastic breccia and sandy debris flow deposits; the former occurs in the central and medial zones, and the latter occurs in the medial zone only. The most widespread lithofacies in the SVC are volcaniclastic turbidite deposits. Three types of sedimentary unit (TSU) have been distinguished. Type 1 is 2–5m thick and dominated by the massive division. It mainly occurs in the central zone. Type 2 is 0.7–3.5m thick and occurs in the central and medial zones. It comprises the massive division, bedded fining- and thinning-upward division, and a horizontally, wavy and cross-laminated division, topped by massive fine-grained tuff. Type 3 is 0.1–1.5m thick and mainly occurs in the distal zone. The massive division is absent, and thinly-bedded and laminated divisions are topped by massive fine-grained tuff. Volcaniclastic turbidites were settled from low-density turbidity currents and related suspension clouds. Mixed volcaniclastic–siliciclastic deposits are subordinate in occurrence and restricted to the distal zone. They are commonly reworked by bottom currents. [Copyright &y& Elsevier]

Abstract: This work postulates that highly polydisperse materials have an effective size distribution that controls permeability. Existence of such effective distribution implies that not all clasts participate to the permeable network resisting to gas flow and that clasts smaller than the minimal effective size are elutriated. When this concept is coupled to a generalized Blake–Kozeny equation, the resulting semi-empirical law links permeability to material properties only (bed void fraction, clast sizes and densities). After calibration of an experimental constant, it is able to replicate within ±0.6log unit experimentally measured permeabilities of both loosely packed and expanded beds made of highly polydisperse (from 1μm to 4mm) pyroclastic deposits that were resampled so as to ensure homogeneous fluidization. The presence of an experimentally calibrated constant and the necessary absence of segregation during fluidization limit the extrapolation of the proposed law to any pyroclastic bed. Satisfactory fitting of the experimental values, however, confirms that the permeability of homogeneously fluidized beds is controlled by a balance between settling and elutriation. This balance suggests a first-order link between permeability and bed expansion, which has implications on the kinetics of dense pyroclastic flows. [Copyright &y& Elsevier]

Energy cone is a unique but characteristic slope that describes the extent of deposits left around a volcano by various flowage phenomena, usually regarded as the boundary of pyroclastic and the source region of lahar. The energy cone value is determined as 0.07 by the energy-line model combined with the parameters of plume height and gradient, and the energy cone spread extent is defined by the numerical simulation method LAHARZ to simulate with this value based on the 1: 50 000 digital elevation model of Tianchi (天池) Volcano, and the source region profiles in the north and south slope can prove the correctness of this threshold. This energy cone threshold and extent can be used as the reference of pyroclastic flow and lahar simulation. [ABSTRACT FROM AUTHOR]

Abstract: The Voorspoed Group II kimberlite pipe is atypical in terms of South African kimberlite pipes. Located in the central region of the Kaapvaal Craton (South Africa), the Voorspoed pipe is one of six pipes comprising the Kroonstad Kimberlite Cluster. Reconstruction of the palaeo-stratigraphy at the time of emplacement, of the Kroonstad kimberlites, indicates that significant post-emplacement erosion has occurred and the pipes are currently exposed at ~1600m depth from the original land surface. The volcaniclastic kimberlite (VK) infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other South African type kimberlite pipes. Three textural volcaniclastic kimberlite varieties are observed: massive volcaniclastic kimberlite (MVK), bedded volcaniclastic kimberlite (BVK) and fine-grained volcaniclastic kimberlite (fgVK) layers. The BVK and fgVK are volumetrically insignificant and the bulk of the pipe is infilled with MVK. The MVK can be further sub-divided into seven varieties. The two dominant varieties are described in detail, which include: main MVK (mMVK) and the juvenile-rich MVK (jrMVK). The MVKs are typically massive, clastic, poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud-rich matrix. Magmaclasts within the MVKs are spherical, crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed. BVK units are composed of several normally graded beds of volcaniclastic material. fgVK layers are well sorted relative to the MVKs and all constituents are typically <2mm in size. The BVK and fgVK can be deposited only within an open vent in order to account for the sorting of the components. A distinct basalt and sandstone rich unit (bsBreccia), which is essentially devoid of kimberlite material, is also observed. Three volcaniclastic zones are identified based on consistent vertical variation in the internal stratigraphy of the infill: outer east/west zone with a sequence bsBreccia–BVK–mMVK/softMVK; a central zone with a sequence mMVK–BVK–jrMVK–mMVK and an outer north zone with a uniform mMVK sequence. The evidence suggests that the volcaniclastic infill has formed through complex processes involving re-working of extra-crater tephra back into the vent and coeval pyroclastic deposition. It is also likely that there is significant mixing of the resedimented and pyroclastic deposits. Distinction between deposits formed through re-working of material through repeated explosive eruptions within the vent and that formed through varying degrees of re-working extra-crater tephra from the surface (with possible coeval pyroclastic eruption) is probably impossible. For this reason we interpret the mMVK as being deposited by dominantly resedimentation of tephra whereas the jrMVK has been deposited dominantly by pyroclastic processes. The presence of BVK and fgVK deposited deep within the pipe through slumping of the crater margins at the surface suggests that a crater ~2000m in depth was periodically open. The identification of the complex, layered, variable internal structure of the volcaniclastic infill at the Voorspoed has significant implications for diamond ore evaluation in South Africa. Typical South Africa kimberlite pipes are infilled with tuffisitic kimberlite, which are characterised by their homogenous nature over large vertical intervals (>1000m). This is not the case at the Voorspoed pipe where MVK layers are variable in thickness (<10m up to 230m). [Copyright &y& Elsevier]

Abstract: We present a global survey of candidate pyroclastic deposits on Mercury, derived from images obtained during MESSENGER flybys 1–3 that provided near-global coverage at resolutions between 5 and 0.5km/pixel. Thirty-five deposits were identified and characterized and are located principally on the floors of craters, along rims of craters, and along the edge of the Caloris basin. Deposits are commonly centered on rimless, often irregularly shaped pits, mostly between 5 and 45km in diameter. The deposits identified are generally similar in morphology and absolute reflectance to lunar pyroclastic deposits. Spectrally the deposits appear brighter and redder than background Mercury terrain. On the basis of the available coverage, the candidate pyroclastic deposits appear to be essentially globally distributed. The diameters of the deposits, when mapped to lunar gravity conditions, are larger than their lunar counterparts, implying that more abundant volatiles were present during the typical eruptive process than on the Moon. These observations indicate that if these deposits resulted from hawaiian-style eruptions, the volatile contents required would be between ∼1600 and 16,000ppm CO or an equivalent value of H2O, CO2, SO2, or H2S (for a more oxidizing interior), or N2, S2, CS2, S2Cl, Cl, Cl2, or COS (for a more reducing interior). These abundances are much greater than those predicted by existing models for Mercury''s formation. An apparent lack of small deposits, compared with the Moon, may be due to resolution effects, a topic that can be further assessed during the orbital phase of the MESSENGER mission. These results provide a framework within which orbital observations by MESSENGER and the future BepiColombo mission can be analyzed. [Copyright &y& Elsevier]

Abstract: Taipei City, with a population of around 8million, as well as two nuclear power plants is located in close proximity to the Quaternary, dominantly andesitic Tatun Volcanic Group (TVG) of Northern Taiwan. We have investigated the stratigraphy of the youngest volcaniclastic deposits, as well as the morphology of lava flows and domes of the TVG in order to reconstruct the character and timing of the most recent eruptions and related hazardous events in the area. Our data indicate that recent eruptions of the group were dominated by long-term, voluminous extrusions of crystal-rich, very viscous lavas. These eruptions formed closely spaced monogenetic domes and lava flows. Based on morphological parameters of the lava flows (thicknesses 80–150m, lengths up to 5.6km, and volumes up to 0.6km3), average rates of magma effusion ranged from 1 to 10m3/s, eruption durations from 500 to 1800days, and lava front speeds from 0.5 to 6m/h. Explosive activity of TVG was diverse, ranging from weak phreatic to highly explosive (VEI 4) Plinian eruptions; vulcanian activity with deposition of lithic ashes was most common. Interaction of rising magma with ground water frequently occurred during the eruptions. This study presents the first radiocarbon dates of various volcaniclastic deposits of the TVG, which indicate that Cising, Siaoguanyin, and possibly Huangzuei volcanoes had magmatic eruptions in the period 13,000–23,000years ago. In addition, Mt. Cising had a phreatic eruption 6000years ago, and possibly an effusive eruption just before that. Gravitational collapses of volcanic edifices with volumes 0.01–0.1km3 and H/L 0.16–0.25 were also common. They occurred on intersections with tectonic faults and may have been triggered by seismic activity. The youngest collapses occurred at Mt. Siaoguanyin (23,000 BP) and Mt. Cising (6000 BP). It is concluded that the TVG should be considered volcanically active. The results of this study provide a basis for volcanic hazard assessment and mitigation in the area. [Copyright &y& Elsevier]

Post-collisional potassic-rich volcanism of Golcuk Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte-trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite-phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Golcuk flows and pyroclastics are mainly characterized by strong incompatible element enrichment in large ion lithophile elements (LILEs; e.g. Cs, Ba, U, and Th) relative to K, Rb, and high-field strength elements (e.g. Nb, Ta, and Ti). We conclude that the Golcuk lavas were derived from a metasomatized lithospheric mantle source containing phlogopite-amphibole garnet peridotite; the latter resulted from metasomatism by a hydrous fluid phase related to subducted sediments and oceanic crust. The parental magma for the extracaldera volcanics was lamprophyric, and that for the intracaldera volcanics was basanitic. All the geological and geochemical data show that the alkaline Golcuk lavas display a gradual decrease in silica content with decreasing eruption age, indicating that, in the Isparta volcanic province, the asthenospheric melt component became more important over time. In the extracaldera volcanics, 87Sr/86Sr isotope ratios of the evolved trachyte-trachyandesites range between 0.70366 and 0.70504, whereas these ratios are lower in the less evolved basaltic trachyandesite and lamprophyres, varying in narrow ranges around 0.70365 and between 0.70374 and 0.70453, respectively. The 143Nd/144Nd values lie between 0.51264 and 0.51273 in trachyandesites, 0.51267 and 0.51273 in basaltic trachyandesites, and 0.51270 and 0.51274 in lamprophyres. In the intracaldera lavas, the 87Sr/86Sr isotope ratio is 0.70361 in the tephriphonolite and 0.70388 in the intracaldera trachytic lava dome. The 143Nd/144Nd isotope ratio is 0.51274 in the analysed tephriphonolitic flow and 0.51271 in the intracaldera trachytic lava dome; these values are higher than that of trachyte-trachyandesites of the extracaldera volcanics. The Sr-Pb isotopic signatures indicate that crustal contamination was significant for the evolved extracaldera volcanics, but was negligible for the intracaldera volcanics. The εNd values of the Golcuk volcanics range between 0 and 2.0. The low Sr isotope ratios and positive εNd values are characteristic features of a depleted mantle source. The isotopically depleted and incompatible enriched nature of the Golcuk lavas point to recent enrichment processes prior to partial melting of the mantle source. Conversely, their radiogenic lead isotope compositions (206Pb/204Pb = 19.19-19.54, 207Pb/204Pb = 15.64-15.67, 208Pb/204Pb = 39.12-39.49) indicate an enriched mantle source region. [ABSTRACT FROM AUTHOR]

Abstract: The extent and timing of vesiculation and microlite formation in ascending basaltic melts are relatively well understood for subaerial eruptions, but they have not been studied thoroughly for submarine eruptions. This is largely due to a lack of samples of appropriate volume and grain size. We present a quantitative textural evaluation of coarse lapilli from two cone-forming deposits at ∼1km depth on Lō`ihi Seamount, Hawai`i, collected during a sampling-intensive submersible dive series in 2006. Lapilli from the deposits, named the northern and southern cones, have modal bulk vesicularities of 55 and 42%, respectively, and vesicle populations dominated by sub-spherical, non-interconnected vesicles, with number densities on the order of 104 to 105 cm−3. Clasts of modal vesicularity have margin-parallel zonation, with seawater-quenched rims interpreted to preserve “syn-fragmentation” magmatic textures in microlite-free sideromelane glass enclosing “post-fragmentation” tachylitic interiors with vesicle and microlite textures that progressively coarsen from rim-to-core. Degassing scenarios are linked to syn-fragmentation vesicle textures to demonstrate that both magmas degassed in dominantly closed systems. Diffusion-controlled cooling rates of basaltic pyroclasts in contact with water are linked to post-fragmentation evolution of vesicle and microlite textures to make inferences about transportation and dispersal of the pyroclasts in low submarine jets. The textural analysis shows that the northern and southern cone eruptions were not strictly analogous to any classical subaerial eruption type. They were uniquely submarine, with ambient seawater playing a crucial role in the processes of degassing, vesiculation, crystallization, and dispersal. [Copyright &y& Elsevier]

Abstract: The Saptarshi Field comprises a suite of at least eight known diamondiferous pipes and dykes within Rio Tinto''s Bunder Project on the Bundelkhand Craton of Madhya Pradesh, India. The discovery, made in 2004, followed regional scale area selection, gravel sampling, and subsequent ground geophysics, soil geochemistry and prospecting. The geology and petrography of the largest and most economically prospective pipe, Atri, suggests that near the surface it forms a large vent comprised of two distinct lobes filled predominantly with primary pyroclastic rocks. Atri is composed of relict olivine, phlogopite, apatite, minor spinel and Ti-oxides. The rock is altered; no primary olivine and little phlogopite remains, instead much of the rock is now composed of serpentine–chlorite–carbonate. Other occurrences within the Saptarshi Field include small pipes and altered dykes. Atri is one example of the diverse assemblage of potassic rocks found on the Indian subcontinent: many parallels may be drawn with the nearby diamondiferous Majhgawan pipe, including mineral chemistry and petrography. Atri has many characteristics in common with olivine lamproite, but many subtle deviations from type-lamproite are observed. We suggest that Atri represents a local expression of potassic magmatism unique to the Bundelkhand Craton of India and is the product of partial melting of modally diverse veins in ancient metasomatized lithospheric mantle. These rocks are not readily classified according to existing schemes devised for potassic rocks known from other petrological provinces. A practical and workable scheme has not been devised that recognizes the unique characteristics of the diversity of lamproite-like rocks. The authors tentatively classify Atri as a diamondiferous ultramafic intrusion with lamproitic affinities, recognizing its (obscured) primary mineralogy, its petrographic and textural characteristics and the important implications this classification has with respect to exploration. The ≈1100 Ma age of the Saptarshi intrusions imply that they are part of a larger igneous province, which includes the nearby Majhgawan and Hinota diamondiferous pipes. This age temporally relates to the lamproites of Western Australia. Rio Tinto completed an Order of Magnitude (OoM) scoping study comprising large diameter and slim-line delineation drilling, and surface bulk sampling on the Project in 2008. [Copyright &y& Elsevier]

Abstract: This contribution reviews the evolution of geological models for the DO-27 kimberlite, located in the Lac de Gras kimberlite province, Northwest Territories, Canada. The DO-27 kimberlite was discovered as a geophysical anomaly in 1992, and early models of the kimberlite (Models 1–2) were based largely on geophysical surveys, with early core drilling confirming the presence of kimberlite. Core drilling completed in 1993 was used to create the first geological model, Model 3. Model 3 interpreted the dominant pipe infill to be ‘diatreme-facies’ kimberlite. Influencing the interpretation of Model 3 was the understanding of kimberlites at the time, which assumed most kimberlites were comparable to southern African-style kimberlites; these kimberlites were typically large, steep-sided bodies infilled by ‘diatreme-facies’ kimberlite. However, this was inconsistent with the much smaller kimberlites being discovered in the early 1990s in the Lac de Gras area. Bulk sample evaluation in 1994 was conducted based on the interpretation in Model 3, targeting the ‘diatreme-facies’ kimberlite. The resulting diamond grades were below expectations and the project was not evaluated further. However, several partners in the project questioned the validity of Model 3 and acquired a re-assessment of the same 1993 drill cores. The resulting geological model, Model 4, interpreted ‘pyroclastic kimberlite’ to be the dominant pipe infill; this pipe was located south of where the 1994 evaluation was focused. The ‘diatreme-facies’ kimberlite of Model 3 was reinterpreted to be low-volume intrusive sills, not infilling a pipe-like body. Based on this re-interpretation, DO-27 was re-evaluated from 2005–2007 by Peregrine Diamonds Ltd. through bulk sampling and core drilling. These new data were used to create Model 5, which builds on the interpretations of Model 4. Diamond grades obtained from the 2005–2007 bulk sampling were substantially higher than those from the 1994 evaluation, which has significant implications for the economic viability of the DO-27 kimberlite. The progression in understanding and geological modeling of DO-27 demonstrates the importance of reliable and accurate geological models in the economic assessment of kimberlite pipes. [Copyright &y& Elsevier]

Abstract: Images obtained by the MESSENGER spacecraft have revealed evidence for pyroclastic volcanism on Mercury. Because of the importance of this inference for understanding the interior volatile inventory of Mercury, we focus on one of the best examples determined to date: a shield-volcano-like feature just inside the southwestern rim of the Caloris impact basin characterized by a near-central, irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin. This candidate pyroclastic deposit has a mean radius of ~24 km, greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions. From the extent of the candidate pyroclastic deposit, we characterize the eruption parameters of the event that emplaced it, including vent speed and candidate volatile content. The minimum vent speed is ~300 m/s, and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million (ppm) of the volatiles typically associated with pyroclastic eruptions on other bodies (e.g., CO, CO2, H2O, SO2, H2S). For comparison, measurements of the exsolution of volatiles (H2O, CO2, S) from basaltic eruptive episodes at Kilauea volcano, Hawaii, indicate values of ~1300–6500 ppm for the terrestrial mantle source. Evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of Mercury is an unexpected result and provides a new constraint on models for the planet''s formation and early evolution. [Copyright &y& Elsevier]

Abstract: The western Snake River Plain volcanic field in SW Idaho contains up to 400 basaltic vents and centers that produced lava shields, pahoehoe lava fields, scoria cones, and a great variety of phreatomagmatic volcanoes between late Miocene and middle Pleistocene time. Tephra deposits produced by phreatomagmatic eruptions are particularly well exposed in the walls of the Snake River canyon, where thick accumulations of pyroclastic rocks indicate widespread phreatomagmatic eruptive events throughout most of the volcanic history of the region. Previously, many of the phreatomagmatic deposits were considered to be the products of subaqueous eruptions that took place on the floor of one or more large freshwater intra-continental lakes. Recent field based observations confirm the presence of widespread phreatomagmatic pyroclastic rocks; however, some that had been interpreted as being subaqueous exhibit textural features that are more consistent with subaerial depositional environments. Intrusive and extrusive magmatic bodies with features associated with peperite formation have also been identified. Most of these peperites can be attributed to magma–sediment mixing in intra-crater/conduit or vent settings, and therefore they can only be used as widespread paleoenvironmental indicators with limitations to demonstrate magma and surface water (e.g. lake) non-explosive interaction. One of the studied sites (“71 Gulch Volcano”) was previously used to indicate the presence of a shallow lake. At this site there is clear field evidence that peperitic feeder dykes contacted muddy, sandy siliciclastic sediments forming globular peperite. The peperitic feeder dykes transition to pillowed, ponded lava up section. The ponded lavas are partially surrounded by a ~5-m-thick unit composed of gently dipping, dune bedded, volcanic glass shard-rich, unsorted, tuff and lapilli tuff containing abundant impact sags caused by volcanic lithics. We suggest that the 3D architecture of the erosional remnant of “71 Gulch Volcano” does not require the presence of a lake at the time of its formation; it is equally possible that that it represents a subaerial phreatomagmatic upper conduit — crater filling succession. This interpretation opens up many questions about the Mio/Pliocene evolution of SW Idaho, the timing of the volcanism, and its association with the evolution of the lacustrine systems in the region. In addition, re-evaluations of the volcanic features in SW Idaho have some general implications for the usage of phreatomagmatic pyroclastic rocks for paleoenvironmental reconstruction. [Copyright &y& Elsevier]

Abstract: Processes responsible for the formation of crater-facies kimberlite remain poorly understood because there are few occurrences of well-preserved deposits. New open-pit exposures of the A154N kimberlite pipe at Diavik in the Lac de Gras region of the Northwest Territories, Canada reveal well-preserved crater-facies deposits. Here, we describe the geometries, structures, and componentry of deposits from the uppermost 130 m of the A154N pipe. We then interpret the processes responsible for their emplacement on the basis of these characteristics. The base of the section features a poorly-sorted, primary massive volcaniclastic kimberlite deposit (MVK1) grading upward into a better-sorted and weakly-stratified volcaniclastic kimberlite deposit (MVK2); which is, itself, overlain by ~45 m of stratified volcaniclastic kimberlite (SVK). The MVK–SVK contact is sharp and marks an abrupt transition from sustained eruption of pyroclastic kimberlite to episodic sedimentation from collapse of an unstable volcanic edifice into the open crater. The SVK deposit is overlain dis-conformably by a poorly to well-sorted, diffusely-bedded and graded pyroclastic kimberlite (PK1 and PK2) that erupted from another, younger kimberlite volcano. The PK deposits show a variety of textures supporting a pyroclastic origin, including vesiculated juvenile pyroclasts, a grain and clast-supported fabric, and the absence of mud-size particles in the matrix. Vertical clast-size grading over 40–50 m and the absence of bedding in the PK2 suggest hydraulic sorting during en-masse deposition of pyroclastic kimberlite into a deep water column. Granulometric analysis shows the PK2 deposit was originally transported as a pyroclastic density current that was sourced from one of the many adjacent pipes in the Diavik area and inundated the A154N pipe. These observations have implications for kimberlite volcanic processes and grade distribution. [Copyright &y& Elsevier]

Abstract: In Canada more than 770 kimberlites have erupted in diverse tectonic settings over a period of 1000 Ma and over an area at least 5000 km across. The kimberlites represent at least 30 comagmatic fields. One pipe type dominates each of the different kimberlite fields. Even with the discovery of a significant number of new kimberlite fields, this review of recent data substantiates the previous proposal that there are at least three distinct classes of kimberlite pipes which show a correlation with the nature of the country rocks into which they were emplaced. New data show that a variation on the Prairies pipe type occurs when kimberlites were emplaced into competent Paleozoic sediments resulting in steeper-sided pipes infilled with similar pyroclastic kimberlite. New data for each pipe type are summarised and the contrasting characteristics of the three pipe types are considered to reflect fundamentally different styles of eruption and deposition. Apparently uniform, near-surface, olivine- and volatile-rich kimberlite magmas were modified differently during the contrasting emplacement processes resulting in distinct textural rock types, pyroclastic kimberlite (PK), tuffisitic kimberlite (TK) and associated hypabyssal kimberlite (HK) or resedimented kimberlite (RVK) dominating each type of pipe. The substantiated correlation of pipe type with country rock geology could indicate variable constraints on volatile exsolution which affected the nature of the magmatic eruptions in different settings. [Copyright &y& Elsevier]

Abstract: The 2360 years B.P. eruption of Mount Meager, British Columbia, produced a succession of rarely observed, welded block and ash flow deposits and non-welded equivalents. We report on these rhyodacitic block and ash flow deposits by describing the deposits, documenting the nature of the welding process, and establishing the origins of these pyroclastic deposits. Variations in welding intensity are tracked by image analysis of field texture maps, petrographic study of shard morphology, and measurements of sample density and porosity. Pyroclast oblateness from image analysis of texture maps provides a qualitative record of welding intensity, but underestimates the amount of compactional (e.g., volume) strain when compared to estimates from physical property measurements (e.g., density, porosity). Unconsolidated deposits have an average matrix porosity of ∼ 41% (38–45%) and clasts have an average porosity of ∼ 32% (24–40%). In unconsolidated deposits, isolated porosity is pervasive (< 8%) in juvenile clasts, but is near absent in samples of matrix (< 1%). Welding and compaction cause a reduction in both connected and isolated porosity. Clasts and matrix record equivalent amounts of strain (∼ 38%) and, thus, are fully coupled during the welding and compaction process. The average integrated strain for vertical sections of the deposit is 31% implying > 50 m shortening of the thickest deposit (from 162 m to the current 112 m). The pyroclastic flow deposits result from accumulation in a narrow, confined river valley; the accumulation was sufficiently rapid to keep the deposits above their glass transition temperature thereby allowing the succession to weld as a single cooling unit. The nature and distribution of isolated porosity in the juvenile clasts of rhyodacite suggest an explosive (rather than effusive) origin for these block and ash flow deposits that may be analogous to some Vulcanian-eruption-triggered dome-collapse processes observed at Soufrière Hills Volcano, Montserrat (rather than gravitational collapse). [Copyright &y& Elsevier]

Lunar pyroclastic deposits reflect an explosive stage of the basaltic volcanism that filled impact basins across the nearside. These fine-grained mantling layers are of interest for their association with early mare volcanic processes, and as possible sources of volatiles and other species for lunar outposts. We present Earth-based radar images, at 12.6 and 70 cm wavelengths, of the pyroclastic deposit that blankets the Aristarchus Plateau. The 70 cm data reveal the outlines of a lava-flow complex that covers a significant portion of the plateau and appears to have formed by spillover of magma from the large sinuous rille Vallis Schröteri. The pyroclastics mantling these flows are heavily contaminated with rocks 10 cm and larger in diameter. The 12.6 cm data confirm that other areas are mantled by 20 m or less of material, and that there are numerous patches of 2 cm and larger rocks associated with ejecta from Aristarchus crater. Some of the radar-detected rocky debris is within the mantling material and is not evident in visible-wavelength images. The radar data identify thick, rock-poor areas of the pyroclastic deposit best suited for resource exploitation. [ABSTRACT FROM AUTHOR]

Abstract: Neogene alkaline basaltic volcanic fields in the western Pannonian Basin, Hungary, including the Bakony–Balaton Highland and the Little Hungarian Plain volcanic fields are the erosional remnants of clusters of small-volume, possibly monogenetic volcanoes. Moderately to strongly eroded maars, tuff rings, scoria cones, and associated lava flows span an age range of ca. 6 Myr as previously determined by the K/Ar method. High resolution 40Ar/39Ar plateau ages on 18 samples have been obtained to determine the age range for the western Pannonian Basin Neogene intracontinental volcanic province. The new 40Ar/39Ar age determinations confirm the previously obtained K/Ar ages in the sense that no systematic biases were found between the two data sets. However, our study also serves to illustrate the inherent advantages of the 40Ar/39Ar technique: greater analytical precision, and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field. Periods of increased activity with multiple eruptions occurred at ca. 7.95 Ma, 4.10 Ma, 3.80 Ma and 3.00 Ma. These new results more precisely date remnants of lava lakes or flows that define geomorphological marker horizons, for which the age is significant for interpreting the erosion history of the landscape. The results also demonstrate that during short periods of more intense activity not only were new centers formed but pre-existing centers were rejuvenated. [Copyright &y& Elsevier]

We propose a classification scheme that unifies terminology for all primary volcaniclastic deposits, assigns initial depositional mechanism as the basis for classifying them, and refines the grain-size classes used to pigeonhole samples. By primary volcaniclastic deposits and rocks, we mean the entire range of fragmental products deposited directly by explosive or effusive eruption. This definition thus focuses on the primary transport and deposition of particles, rather than those processes by which the particles form or the nature of the fluid in which they are carried. We favor this approach for all primary volcaniclastic deposits because they typically contain assemblages of clasts formed by different processes and/or at different times that are subsequently brought together during eruption. [ABSTRACT FROM AUTHOR]

Abstract: 49 known extrusive carbonatite occurrences are listed with brief details of their tectonic setting, structure, lithologies, associated silicate rocks, chemistry and presence or absence of included mantle materials. Half the occurrences appear to be related to tephra cones, tuff rings, diatremes and maars and the rest occur within strato-volcanoes. Pyroclastic carbonatitic rocks are present at all the localities, with carbonatite lava flows occurring at only 14 of them. The pyroclastic rocks, which include fallout tephra and deposits from pyroclastic surges and flows and products of phreatomagmatic eruptions, vary from rocks composed principally of carbonate to varieties with as little as 20% igneous carbonate. The most abundant silicate rocks associated with extrusive carbonatites are melilite-bearing rocks, nephelinite and/or ijolite, and phonolite and/or nepheline syenite; seven occurrences have no associated silicate rocks. 16 occurrences, most of them associated with small extrusive centres, contain mantle xenoliths or megacrysts, details of which are tabulated, with spinel lherzolite the most abundant rock type, but amphibole, phlogopite and garnet are also recorded. The lack of such materials in intrusive carbonatites may reflect their less energetic environment of emplacement. It is proposed that carbonatites are essentially of two types: (a) those rising energetically and rapidly from the mantle, which form small explosion craters, ash or tuff cones, or diatremes, have only low-volume associated silicate rocks, and entrain mantle debris, and (b) those which occur in strato-volcanoes, are associated with large volumes of silicate rocks and follow a more complex genesis, probably involving ponding and differentiation (separation from carbonate-bearing silicate magma) at higher levels in the mantle and/or crust. Most of the classic intrusive carbonatite complexes probably fall into the second category. [Copyright &y& Elsevier]

Abstract: Flood volcanic provinces are assumed generally to consist exclusively of thick lavas and shallow intrusive rocks (mostly sills), with any pyroclastic rocks limited to silicic compositions. However, mafic volcaniclastic deposits (MVDs) exist in many provinces, and the eruptions that formed such deposits are potentially meaningful in terms of potential atmospheric impacts and links with mass extinctions. The province where MVDs are the most voluminous—the Siberian Traps—is also the one temporally associated with the greatest Phanerozoic mass extinction. A lot remains to be learned about these deposits and eruptions before a convincing genetic link can be established, but as a first step, this contribution reviews in some detail the current knowledge on MVDs for the provinces in which they are better known, i.e. the North Atlantic Igneous Province (including Greenland, the Faeroe Islands, the British Isles, and tephra layers in the North Sea basin and vicinity), the Ontong Java plateau, the Ferrar, and the Karoo. We also provide a brief overview of what is known about MVDs in other provinces such as the Columbia River Basalts, the Afro-Arabian province, the Deccan Traps, the Siberian Traps, the Emeishan, and an Archean example from Australia. The thickest accumulations of MVDs occur in flood basalt provinces where they underlie the lava pile (Faeroes: >1 km, Ferrar province: ≥400 m, Siberian Traps: 700 m). In the Faeroes case, the great thickness of MVDs can be attributed to accumulation in a local sedimentary basin, but in the Ferrar and Siberian provinces the deposits are widespread (>3×105 km2 for the latter). On the Ontong Java plateau over 300 m of MVDs occur in one drill hole without any overlying lavas. Where the volcaniclastic deposits are sandwiched between lavas, their thickness is much less. In most of the cases reviewed, primary MVDs are predominantly of phreatomagmatic origin, as indicated by the clast assemblage generally consisting of basaltic clasts of variable vesicularity (dominantly non- to poorly-vesicular) mixed with abundant country rock debris. The accidental lithic components often include loose quartz particles derived from poorly consolidated sandstones in underlying sedimentary basins (East Greenland, Ferrar, Karoo). These underlying sediments or sedimentary rocks were not only a source for debris but also aquifers that supplied water to fuel phreatomagmatic activity. In the Paraná–Etendeka, by contrast, the climate was apparently very dry when the lavas were emplaced (aeolian sand dunes) and no MVDs are reported. Volcanic vents filled with mafic volcaniclastic material, a few tens of metres to about 5 km across, are documented in several provinces (Deccan, North Atlantic, Ferrar, Karoo); they are thought to have been excavated in relatively soft country rocks (rarely in flood lavas) by phreatomagmatic activity in a manner analogous to diatreme formation. [Copyright &y& Elsevier]