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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]

The eruption of the Cumbre Vieja volcano on 19 September 2021, resulted in the deposition of large quantities of volcanic ash (VA), causing a great impact on the citizens. This work aims to study the properties of this volcanic ash as pozzolanic raw materials to establish their potential use in the development of sustainable cement. Results of chemical and technical characterization are presented. To achieve this goal, Ordinary Portland Cement (OPC) was replaced with standardized percentages of OPC/VA: 10, 25, and 40%. Characterization studies were carried out using chemical analysis of X-ray fluorescence (XRF), chemical quality analysis (QCA), pozzolanicity test (PT) at 8 and 15 days, as well as determining the mechanical strength (MS) at 7, 28, and 90 days. The results obtained by XRF and QCA established that the chemical composition of the VA corresponds to that of the natural pozzolan typical of pyroclastic genesis. The PT test showed that the analyzed samples have a marked pozzolanic nature, both at 8 and 15 days, showing a significant increase in their hydraulic reactivity. Likewise, the MS tests confirmed a continuous increase in mechanical compressive strength, which increased significantly from 7 to 90 days of curing, reaching more than 58 MPa. On the other hand, mechanical tests showed that the three types of dosages used OPC/VA: 10, 25, and 40% were equally effective, with OPC/VA formulations: 10–25% being the most effective. The results obtained in this research could be used by local industries as a guide for the correct use of the volcanic materials of this island, both for the manufacture of construction materials, such as aggregates, and to produce pozzolanic cement with low CO2 emissions, thus having a positive impact on the environment. Finally, the great natural availability of natural VAs in the surrounding areas of La Palma could cover part of the needs for materials used in the construction and restoration of houses and infrastructures damaged during the volcanic eruption. [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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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: The rheological behavior of pyroclastic deposits during welding is incompletely understood and is based on a surprisingly small number of experimental studies. Here we present results from a new experimental apparatus comprising an automated uniaxial compression load frame that can run constant load (up to 1150 kg) or constant displacement rate (10−6 to 0.25 cm/s) tests at elevated temperatures (≤1100 °C). Deformation experiments were performed on pre-fabricated cylinders (4.5 cm diameter, ∼6 cm length) of soda lime silica glass beads (N=32), sintered rhyolite ash (N=7) and cores of pumiceous rhyodacite (N=6). Experimental runs used strain rates from 10−5 to 10−3 s−1 and stresses of ∼0 to 5.24 MPa. Temperatures varied from 535 to 650 °C for experiments on soda lime silica glass beads and 825 to 950 °C for natural materials. In all cases experimental cores showed a strain-dependent rheology that is more strongly affected by temperature than by load or strain rate. Results from these experiments are used to develop a relationship in which the effective viscosity (η e) of the experimental cores is predicted by:where η 0 is melt viscosity and ϕ f is sample porosity. This rheological model provides a means for exploring the relative roles of emplacement temperature, load and glass transition temperature in welding of pyroclastic deposits. [Copyright &y& Elsevier]

The Cretaceous age Fort à la Corne (FALC) kimberlite province comprises at least 70 bodies, which were emplaced near the edge of the Western Canadian Interior Seaway during cycles of marine transgression and regression. Many of the bodies were formed during a marine regression by a two-stage process, firstly the excavation of shallow, but wide, craters and then subsequent infilling by xenolith-poor, crater-facies, subaerial, primary pyroclastic kimberlite. The bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin, but high volume, pyroclastic kimberlite deposits. Each body is composed of contrasting types of kimberlite reflecting different volcanic histories and, therefore, are considered separately.The 140/141 kimberlite is the largest delineated body in the province, estimated to have an areal extent below glacial Quaternary sediments in excess of 200 ha. The infilling of the 140/141 crater is complex, resulting from multiple phases of kimberlite. The central part of the infill is dominated by several contrasting phases of kimberlite. One of these phases is a primary pyroclastic airfall mega-graded bed up to 130 m in thickness. The constituents grade in size from very fine to coarse macrocrystic kimberlite, through to a basal breccia. The mega-graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe. A second different phase of kimberlite is interpreted as representing a younger nested crater within the mega-graded bed. Centrally located thicker intersections (>450 m) of this younger kimberlite may indicate a vent for the kimberlite crater. The thickness of the mega-graded bed increases with proximity to the younger kimberlite in the study area.Macrodiamond minibulk sample grades from the mega-graded bed have been obtained from nine large diameter drill holes, located within the northwest part of the body from an area of ∼20 ha, which represents approximately 10% of the currently modeled kimberlite outline. Diamond grade increases with depth within the mega-graded bed and also increases, within the same unit, towards the centrally positioned younger kimberlite. Macrodiamond sample grades vary from low at the top of the mega-graded bed, to considerably higher grades near the base. Total sample grade per drill hole varies from moderate near the vent feature to lower grades 200–300 m from the vent feature. Macrodiamond stone frequency measured in stones per tonne shows a pronounced relationship with depth and proximity to the vent feature within the mega-graded bed. There is a strong correlation between depth and increased stones per tonne, and a similar correlation between stones per tonne and proximity to the vent feature. The data supports the emplacement model of the mega-graded bed and, in turn, this information is useful in understanding the macrodiamond distribution within this bed. [Copyright &y& Elsevier]

The pipe shapes, infill and emplacement processes of the Attawapiskat kimberlites, including Victor, contrast with most of the southern African kimberlite pipes. The Attawapiskat kimberlite pipes are formed by an overall two-stage process of (1) pipe excavation without the development of a diatreme (sensu stricto) and (2) subsequent pipe infilling. The Victor kimberlite comprises two adjacent but separate pipes, Victor South and Victor North. The pipes are infilled with two contrasting textural types of kimberlite: pyroclastic and hypabyssal-like kimberlite. Victor South and much of Victor North are composed of pyroclastic spinel carbonate kimberlites, the main features of which are similar: clast-supported, discrete macrocrystal and phenocrystal olivine grains, pyroclastic juvenile lapilli, mantle-derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices. These partly bedded, juvenile lapilli-bearing olivine tuffs appear to have been formed by subaerial fire-fountaining airfall processes.The Victor South pipe has a simple bowl-like shape that flares from just below the basal sandstone of the sediments that overlie the basement. The sandstone is a known aquifer, suggesting that the crater excavation process was possibly phreatomagmatic. In contrast, the pipe shape and internal geology of Victor North are more complex. The northwestern part of the pipe is dominated by dark competent rocks, which resemble fresh hypabyssal kimberlite, but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias±volcaniclastic kimberlite. Current evidence suggests that the hypabyssal-like kimberlite is, in fact, not intrusive and that the northwestern part of Victor North represents an early-formed crater infilled with contrasting extrusive kimberlites and associated breccias. The remaining, main part of Victor North consists of two macroscopically similar, but petrographically distinct, pyroclastic kimberlites that have contrasting macrodiamond sample grades. The juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically. The nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different, indicating that they derive from different magma pulses, or phases of kimberlite, and thus represent separate eruptions. The initial excavation of a crater cross-cutting the earlier northwestern crater was followed by emplacement of phase (i), a low-grade olivine phenocryst-rich pyroclastic kimberlite, and the subsequent eruption of phase (ii), a high-grade olivine phenocryst-poor pyroclastic kimberlite, as two separate vents nested within the original phase (i) crater. The second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade. Thus, the final pyroclastic pipe infill of the main part of the Victor North pipe appears to consist of at least three geological/macrodiamond grade zones.In conclusion, the Victor kimberlite was formed by several eruptive events resulting in adjacent and cross-cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal-like kimberlite, which is now interpreted to be of probable extrusive origin. Within the pyroclastic kimberlites of Victor North, there are two nested vents, a feature seldom documented in kimberlites elsewhere. This study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism. [Copyright &y& Elsevier]

The Mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in-fill, albeit without the tuff ring.The geology of Mwadui can be subdivided into five geological units, viz. the primary pyroclastic kimberlite (PK), re-sedimented volcaniclastic kimberlite deposits (RVK), granite breccias (subdivided into two units), the turbidite deposits, and the yellow shales listed in approximate order of formation. The PK can be further subdivided into two units—lithic-rich ash and lapilli tuffs which dominate the succession, and lithic-poor juvenile-rich ash and lapilli tuffs. The lower crater is well bedded down to at least 684 m from present surface (extent of current drill data). The bedding is defined by the presence of juvenile-rich lapilli tuffs vs. lithic-rich lapilli tuffs, and the systematic variation in granite content and clast size within much of the lithic-rich lapilli tuffs. Four distinct types of bedding have been identified in the pyroclastic deposits. Diffuse zones characterised by increased granite abundance and size, and upward-fining units, represent the dominant types throughout the deposit.Lateral heterogeneity was observed, in addition to the vertical changes, suggesting that the eruption was quite heterogeneous, or that more than one vent may have been present. The continuous nature of the bedding in the pyroclastic material and the lack of ash-partings suggest deposition from a high concentration (ejecta), sustained eruption column at times, e.g. the massive, very diffusely stratified deposits. The paucity of tractional bed forms suggest near vertical particle trajectories, i.e. a clear air-fall component, but the poorly sorted, matrix-supported nature of the deposits suggest that pyroclastic flow and/or surge processes may also have been active during the eruption.Available diamond sampling data were examined and correlated with the geology. Data derive from the old 120 (37 m), 200 (61 m), 300 (92 m) and 1200 ft (366 m) levels, pits sunk during historical mining operations, drill logs, as well as more recent bench mapping. Correlating macro-diamond sample data and geology shows a clear relationship between diamond grade and lithology. Localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post-eruptive sedimentary in-fill processes. Clear distinction can be drawn between upper (re-sedimented) and lower (pyroclastic) crater deposits at Mwadui, both from a geological and diamond grade perspective.Finally, an emplacement model for the Mwadui kimberlite is proposed. Geological evidence suggests that little or no sedimentary cover existed at the time of emplacement. The nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous, but that the eruption column may have been heterogeneous, both petrologically as well as geometrically. Volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re-sedimented volcaniclastic material derived from the tuff ring.The Mwadui kimberlite exhibits marked similarities compared to the Orapa kimberlite in Botswana. [Copyright &y& Elsevier]

Submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur. Consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows, however, that significant pyroclastic activity can occur even at depths in excess of 3000 m. In order to document and illustrate the full range of submarine eruption styles, we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions: (1) no gas exsolution; (2) gas exsolution but no magma disruption; (3) gas exsolution, magma disruption, and hawaiian-style fountaining; (4) volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse; (5) magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity; and (6) volatile content builds up in the top of a dike leading to vulcanian eruptions. We also examine the role of bulk-interaction steam explosivity and contact-surface steam explosivity as processes contributing to volcaniclastic formation in these environments. We concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities, the most likely circumstance is the quiet effusion of magma with minor gas exsolution, and the production of somewhat vesicular pillow lavas or sheet flows, depending on effusion rate. The amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption. The most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity, due to the localized concentration of volatiles in magma that has a low rise rate; magmatic gas collects by bubble coalescence, and ascends in large isolated bubbles which disrupt the magma surface in the vent, producing localized blocks, bombs, and pyroclastic deposits. Another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions; these deposits, being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater, should be easily distinguishable from strombolian and hawaiian eruptions. However, we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles. In this case, a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian-style activity; the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile-induced hawaiian eruption. Specifically, typical pyroclast sizes might be smaller; fountain heights may exceed those expected for the purely magmatic hawaiian case; cooling of descending pyroclasts would be more efficient, leading to different types of proximal deposits; and runout distances for density flows would be greater, potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer. In addition, flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles, and thus extremely poor in vesicles relative to typical flows associated with hawaiian-style eruptions in the primary magmatic gas case. We examine several cases of reported submarine volcaniclastic deposits found at depths as great as ∼3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid-ocean ridges. Furthermore, the latter stages of development of volcanic edifices (seamounts) formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth, but also to: (1) the presence of a summit magma reservoir, which favors the buildup of magmatic foams (enhancing hawaiian-style activity) and episodic dike emplacement (which favors strombolian-style eruptions); and (2) the common occurrence of alkalic basalts, the CO2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts. These models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles. [Copyright &y& Elsevier]

The aim of this work was to evaluate the toxicity of volcanic ash from the volcanic complex Puyehue- Caulle Range on the grasshopper, Dichroplus vittigerum, and compare the results with the effect of diatomaceous earth, an organic pesticide. To test this, we carried out 2 experiments: (i) an acute toxicity test, which consisted of exposing adults of D. vittigerum differentially to volcanic ash and diatomaceous earth, and (ii) a study to determine the survival time of the adult stage by separately exposing the insects to diatomaceous earth and volcanic ash, and by simulating with fans the wind gusts that occur naturally in the area of influence of the volcano. The laboratory data showed that constant exposure to volcanic ash and diatomaceous earth induces mortality in adult grasshoppers. Interestingly, the toxicities of the 2 products were very similar. These results suggest that the onset of the volcanic eruption with the consequent suspension of the pyroclastic material in the air may have had a negative impact on D. vittigerum populations through the mortality of individual adults. In summary, volcanic ash has an insecticidal effect similar to that of diatomaceous earth. [ABSTRACT FROM AUTHOR]

• The Paleoproterozoic Stroud Breccia is a result of subaqueous pyroclastic volcanism. • It represents continued early rifting of the primitive arc at Snow Lake. • It marks the transition to the overlying mature arc at Snow Lake. • It delineates synvolcanic faults and associated subsidence structures. • These structures controlled ore-formation in the Anderson sequence below. The Stroud Breccia is a ≤400 m thick package of dominantly felsic volcaniclastic rocks within the Paleoproterozoic Snow Lake arc assemblage. It stratigraphically overlies the ore-interval for the Anderson, Stall and Rod volcanogenic massive sulfide (VMS) deposits at Snow Lake, Manitoba, Canada and occupies the stratigraphic interval between the underlying primitive arc Anderson sequence and the overlying mature arc Chisel sequence. The Stroud Breccia is interpreted to be a product of subaqueous pyroclastic and effusive, dominantly rhyolitic, volcanism, and concomitant subsidence. It consists of felsic tuff to lapilli-tuff, crystal-rich felsic tuff, felsic lapillistone, felsic tuff breccia, mafic tuff to lapilli-tuff, heterolithic tuff breccia to breccia and rhyolite. The finer felsic volcaniclastic units are characterised by angular or broken quartz crystals, angular lithic rhyolite lapilli, and pumice. Fine ash, broken crystals and pumice suggest they were deposited by eruption-fed density currents associated with a phreatomagmatic eruption. The coarser felsic volcaniclastic units are characterized by blocks of strongly quartz-feldspar porphyritic rhyolite and pumice and are interpreted to have been deposited during the reworking of pyroclastic debris flow deposits on the seafloor during mass wasting events at fault scarps. Planar and cross laminations, graded beds and bomb sag structures suggest the mafic tuff to lapilli-tuff unit was deposited by a combination of subaqueous eruption-fed density currents and eruption driven ballistically emplaced ejecta, likely through subaqueous tephra jetting. Lithofacies analyses, and geochemistry has allowed comparison between the felsic volcanic rocks of the Stroud Breccia and those that host the Anderson, Stall and Rod VMS deposits. Detailed mapping has led to the identification of synvolcanic faults, defining basins and volcanic vents, that indicate that the process of incipient rifting of the Anderson sequence primitive arc continued during emplacement of the Stroud Breccia. These structures also define magma and fluid pathways critical in controlling the formation of the Anderson VMS deposits, meaning they represent deep-seated and long-lived structures that can be used as vectors to mineralization at depth. [ABSTRACT FROM AUTHOR]

A plethora of in-situ bulk chemical and mineralogical analyses, remote sensing data, and geochemical models suggest that the Martian crust is sulfur(S)-rich, exceeding the S content of terrestrial basalts by almost an order of magnitude. The main source of crustal S on Mars is the volcanic exhalation of SO 2 and H 2 S gases. Volcanic ash, expelled along with gases, may be a sink for up to 30% of the exhaled S gas species. We analyze the elemental composition of the Martian shallow sub-surface using the Mars Odyssey Gamma Ray Spectrometer to find areas that are simultaneously enriched in constituents of major volcanic gases such as S, Cl, and H 2 O. A large sedimentary deposit of likely pyroclastic origin called the Medusae Fossae Formation (MFF) is located within this region of possibly extensive alteration by volcanic gases. Based on reasonable terrestrial analog estimates that the MFF scavenged, at maximum, 30% of the exhaled S gas species, we find that a significant amount of S (>1017 kg) would have been delivered to the atmosphere over the time it took for the MFF to be deposited on Mars. The mass of the S emitted from the MFF-forming event(s) is up to 8 orders of magnitude higher than the mass of S emitted from the largest Quaternary volcanic eruption on Earth. Thus, volcanic degassing from the MFF forming events could have significantly affected surface geological processes, climate evolution, and habitability of Mars. • The Medusae Fossae Formation (MFF) is simultaneously enriched in S, Cl, and H2O. • The enrichment of S, Cl, and H2O at the MFF could be due to volcanic degassing. • The S/Cl molar ratio of the MFF corroborates enrichment by volcanic degassing. • The MFF forming events could have supplied a huge amount of atmospheric S. [ABSTRACT FROM AUTHOR]

Caldera-forming volcanic eruptions are among the most dangerous, and can generate extensive pyroclastic deposits and deliver ash into global atmospheric circulation systems. As calderas collapse, the eruptions can deposit thick proximal ignimbrite sequences and thinner ignimbrites more distally. However, the proximal record of caldera collapse is often obscured by later intrusions, volcanism, faults, alteration, water and sediments, which significantly limits our understanding of these eruptions. A Palaeogene caldera system in central Arran, western Scotland, preserves a rare proximal caldera-fill succession, the Arran Volcanic Formation. This caldera largely comprises highly heterogeneous ignimbrites and minor intra-caldera sedimentary rocks. The current level of erosion, and the general absence of faults, intrusions and sediments, allows a complex stratigraphy and collapse history to be determined, which can be linked to changing eruptive styles at a constantly evolving volcano. The first recorded phase was eruption of a homogeneous rhyolitic lava-like tuff, deposited from high temperature, high mass-flux pyroclastic density currents generated from low fountaining columns that retained heat. A succeeding phase of highly explosive Plinian eruptions, marked by a thick blanket of massive lapilli tuffs, was then followed by piston-like caldera collapse and erosion of steep caldera walls. Volcanism then became generally less explosive, with predominantly lava-like and eutaxitic tuffs and cognate spatter-rich agglomerates interbedded with non-homogenous lapilli tuffs. High topographic relief between distinct units indicate long periods of volcanic quiescence, during which erosive processes dominated. These periods are, in several places, marked by sedimentary rocks and evidence for surface water, which includes a localised basaltic-andesitic phreatomagmatic tuff. The caldera-forming eruptions recorded by the Arran Volcanic Formation provide an important insight into caldera collapse processes and proximal ignimbrite successions. The lack of thick autobreccias and lithic-rich lapilli- and block-layers indicates that subsidence was relatively gradual and incremental in this caldera, and not accompanied by catastrophic wall collapse during eruption. The relatively horizontal nature of the caldera-fill units and paucity of intra-caldera faulting indicate that piston subsidence was the dominant method of collapse, with a relatively coherent caldera floor bounded by a steeply dipping ring fault. Possible resurgence may have caused later doming of the floor and radial distribution of subsequent ignimbrites and sedimentary rocks. Our work emphasises the continued need for field studies of caldera volcanoes. [ABSTRACT FROM AUTHOR]