Pumice Grains Research Articles

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

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

Petrographic and geochemical properties of the ignimbrites around Hatunsaray (Konya)

The study area is located around the town of Hatunsaray, approximately 40 km away from the south-west of Konya. In this study, the petrological and geochemical characteristics of the Bulumya ignimbrite, Detse ignimbrite and Sadıklar ignimbrite observed in the region were revealed. The study area contains Upper Miocene - Lower Pliocene aged Güneydere formation and overlying Bulumya ignimbrite, Detse ignimbrite, Sadıklar ignimbrite and Quaternary alluviums. All these units were formed in the Upper Miocene - Lower Pliocene aged fluvial and lake environment and have a lateral vertical transition with carbonate, clastic units. The gray-colored Bulumya ignimbrite contains andesite-dacite rock fragments and large pumice grains. The Detse Ignimbrite is yellow in color and shows a well sorted lapilli tuff composition. The Sadıklar Ignimbrite, on the other hand, contains agglomeratic levels with yellow colored slightly fused lenses and wedge geometry. All ignimbrite samples have porphyric texture and were classified as “crystal-vitric tuff” and “crystal lithic-vitric tuff” in the glass-crystal-rock fragment classification. Petrographic investigations of ignimbrites show that major components are quartz, plagioclase, plagioclase microliths, biotite, amphibole, opaque minerals and rock fragments. Geochemical data shows that all ignimbrite samples are subalkaline, trachy-andesite, andesite-basaltic andesite and calc-alkaline in character. When the main oxide, trace and rare earth elements in ignimbrites are evaluated, fractional crystallization controlled by feldspar minerals is observed. In addition, the high K and Rb content in the spider diagrams indicate crustal contamination. The distribution of the ignimbrite samples in the Rb/Y-Nb/Y diagram suggests that the magma source forming the samples is enriched by subduction and/or crustal components.

Composition, age and origin of Pleistocene turbidite deposits at ODP site 1232, Nazca plate: Implications for volcanism and climate change in central south Chile

A 371 m sequence of Pleistocene age turbidites was recovered at ODP (Ocean Drilling Program) Leg 202, Site 1232 at 39°53.45'on the Nazca Plate, seaward of the Peru-Chile Trench. Coarsest sediments within the turbidites (<300 μm) contain large amounts of fresh, angular volcanic glass shards. We investigated the variation in proportions of coarsest (>149 μm) glass, minerals and polymineralic grains, and microfossils downcore with the goal of discerning temporal changes and their underlying causes. Between 214 and 160 mbsf, the dominant coarse particle type changes from rounded, clay-rich granules, interpreted as altered pumice, upward to fresh, angular volcanic glass. This change correlates with an upward increase in turbidite thickness noted by Leg 202 scientists. Major and trace element analyses of individual glass shards show a geochemical signature consistent with a source within the adjacent Andean Southern Volcanic Zone. New calcareous nannofossil biostratigraphy brackets the sediment ages of the upper 288 m of the section to between 12.47 kyr and 275 kyr, which is consistent with published ages for the upper 97 m of the section and estimated maximum ages.Based on our dating of the section, the observed change in dominant coarse sediment type occurs between 225 and 185 kyr and correlates with a climatic transition from interglacial to glacial conditions. A plausible connection with climate is that the altered pumice grains formed by chemical weathering and fluvial transport under warm surface conditions, whereas the glass shards were transported by aeolian and glacial processes, leading to a lack of weathering and an angular texture. However, as a climate-related temporal change, this scenario conflicts with the absence of a similar change in sediment type during the subsequent Valdivian and Llanquihue interglacial-glacial transition starting at about 125kyr. Therefore, our conclusion is that, although the two coarse sediment types formed and were transported under different climate conditions, the temporal change is related to a shift in sediment source, supply or transportation pathway during the 185–225 kyr interval. A related possibility is that the onset of angular glass involvement in turbidity flows, during a glacial stage, marks the eruption of a new volcanic source or sources. Resolving these possibilities could be addressed with more detailed geochemical analysis.

Effect of pumice stone and pearlite abrasives characteristics on denim abrasion

The manufacture of abrasives based on waste grains of pumice stones and pearlites represents the aim of this work is to minimize both the cost of the product and to recover industrial waste that harms the environment. We also used a non-woven based on cellulosic fibers as reinforcement and three types of resins (polyurethane, acrylic, and polyester). Obtaining these products is designed by two processes (by spraying and by coating). The effect of the manufacturing process, the type and size of the grains on the abrasive wear of denim fabrics, the material removal rate (MRR), and the surface morphologies obtained were quantitatively evaluated. A chemical analysis (FTIR) and a thermal analysis were carried out (ATG/DSC). Scanning electron microscopy (SEM) was performed to see the appearance of these grains and measure their diameter. Inductively Coupled Plasma Mass Spectrometry (ICP/MS) analyses show that pumice grains contain more calcium atoms than pearlite grains. The different abrasion grains were compared according to their polishing effect. We note that the abrasive manufacturing process, type, and size of abrasive particles affect the MRR. Pumice grits allow for deeper polishing compared to other grits.

Geological history controlling the debris avalanches of pyroclastic fall deposits induced by the 2009 Padang earthquake, Indonesia: The sequential influences of pumice fall, weathering, and slope undercut

Rapid-moving landslides occurred in many locations during the 2009 Padang earthquake, Sumatra. We interpreted satellite images, performed field surveys, in situ dynamic cone penetration tests, and laboratory analyses for mineralogy and physical and mechanical properties. We found that landslides occurred at 159 locations in an area of 64 km2, and these areas had pumice fall deposits overlying paleosols which were heavily weathered debris flow deposits. These landslides in the investigated area occurred in areas with pumice fall deposits thicker than 350 cm; this depth was probably vital in inducing mechanical instability. The areas had sliding surfaces at the base of the pumice fall deposits, where pumice grains were mixed with the underlying paleosol and had been heavily weathered into halloysite-rich clayey materials by interaction with the percolating water from the ground surface. After the deposition of this pumice fall with mantle bedding, the beds were undercut by subsequent river erosion that loosen their downslope lateral support. This geologic history of pumice fall, weathering, and undercutting is not confined to the affected area but it is common to many volcanic areas, creating the opportunity to predict areas susceptible to earthquake-induced catastrophic debris avalanches.

Separation of pumice from soil mixtures

Pumice-rich deposits are found in a number of locations around the world, and in particular across large areas of the North Island of New Zealand. Pumice grains are commonly described as being lightweight, highly crushable, and vesicular in nature. These characteristics give rise to a unique set of behaviours under loading, and pumice-rich soils are highly problematic in terms of in situ characterisation in large part due to their crushability. The presence of pumice within a soil mixture has the potential to completely alter the stress–strain behaviour of these soils as well as require a different interpretation of results from commonly used site characterisation technique. It is therefore important to be able to determine quantitatively the percentage of pumice within a given soil deposit. This paper proposes a methodology based on a gravity separation of pumice-bearing mixtures with a heavy fluid. The application of the method to artificial mixtures of fine pumice and non-pumiceous sands is shown to be sufficiently accurate for engineering purposes.

Paleocene and Early Eocene volcanic ash layers in the Schlieren Flysch, Switzerland: U–Pb dating and Hf-isotopes of zircons, pumice geochemistry and origin

Thin mm to cm thick bentonite layers of Paleocene to Early Eocene age in the Tonsteinschichten of the Schlieren Flysch represent volcanic ash layers. Heavy mineral analysis of the layers indicates basic to acidic volcanic sources. U/Pb dating of single zircon crystals of a Paleocene layer (WW1948) by LA-ICP-MS points to an eruption at 59.87±0.41Ma, whereas ID-TIMS shows an eruption age of 60.96±0.07Ma. Taking into account the external precision of LA-ICP-MS analyses of 1–2% both ages are overlapping and indicate an apparent minimal durations of zircon crystallization of 350ka. Hf-isotope analysis of the same zircon crystals reveals the hybrid character of the source magma. The geochemical composition of the pumice grains of all bentonite layers is strongly affected by alteration. Nevertheless, the original character of the volcanic source can be evaluated. The Paleocene ashes (Lower Tonsteinschichten, LT) show a more fractionated multi-element pattern than the ashes of Early Eocene (Upper Tonsteinschichten, UT). The LT ash series are of rhyodacite to dacite character whereas the UT ashes fall in the field of alkali basalts. Both ash series seem to originate from a within-plate volcanic setting according to their trace element concentrations. Geochemical and temporary counterparts can be found in ash layers from Anthering (Austria) and the Danish Basin. As proposed for those ashes, volcanism connected to the opening of the North Atlantic might be the source as well for the ashes in the Schlieren Flysch. By comparison of the composition of rocks from the British Paleogene Igneous Province BPIP and the Schlieren Flysch ashes many correlations can be drawn which supports the suggestion of a North Atlantic origin of the Alpine ashes.

Millennial-scale reworking of tephra in alluvial to shallow marine settings: Distinguishing pseudo-isochrons from genuine ones

Primary tephra layers can provide isochrons in a stratigraphic sequence. Such isochrons are powerful tools in terms of correlation and chronology of strata, and they constrain a chronological framework for geology, geomorphology, archaeology and palaeo-environmental studies in the Neogene to Quaternary. However, reworking processes can re-distribute tephra materials in both time and space, so that a given tephra can form a new discrete layer later in the depositional sequence, or it can be distributed in low concentrations (no visible layers, i.e., crypto-tephras) over a range of different stratigraphic horizons. The present study, based on sedimentological, petrographical, and geochemical analyses of reworked tephras in Japan, evaluates the duration and persistence of reworking and the tephra mobilisation and delivery to watersheds. Furthermore, it discusses how a unique isochron can be constructed from tephra distribution in a sedimentary sequence. Examples presented here are from fluvial and deltaic sequences in the Japanese Quaternary, which contain several visible tephra layers and a certain amount of tephra materials from multiple sources at different stratigraphical horizons. In the Niigata Plain, 5.4ka Numazawako tephra material continued to be reworked for more than 4000 years after the eruption of Numazawa volcano, and was re-deposited as reworked pumice grains consisting of a “visible” tephra layer in delta front deposits. Mixed tephra materials originating from an earlier eruption of Numazawa volcano and eruptions from other calderas were also identified. In the Tsugaru Plain, post-2.5ka subsurface deposits have >10% glass shards content throughout the sequence. Glass shards were derived from several different tephra materials, such as those from the most recent eruption of Towada volcano (AD 915), and earlier caldera forming eruptions (30 ka, 15ka). Some of the glass shards probably originated from erosion of exposed Pliocene pyroclastic bedrock. Thus, tephra reworking processes in alluvial to shallow marine settings can continue long after an eruption has ended, and can persist even during inter-eruptive (i.e., background sedimentation) periods. As tephra reworking involves complicated processes, careful observation and interpretations are necessary to evaluate whether tephra materials can truly provide an isochron for the strata in which they are found to exist.

Improving clay content measurement in oxidic and volcanic ash soils of Hawaii by increasing dispersant concentration and ultrasonic energy levels

Quantifying clay content is a fundamental step in predicting and managing soil behaviors such as nutrient and water retention. However, clay measurements are underestimated when using standard methods of dispersion in soils rich in oxides and volcanic ash-derived non-crystalline minerals. Increasing levels of the chemical dispersant and ultrasonic energy are two simple techniques found to increase dispersion and clay measurements in temperate soils, but their effects are less known for oxidic and volcanic ash soils. In this study we investigated the effects of increasing dispersion concentration and ultrasonic energy on clay measurements for a range of oxidic and volcanic ash soils from Hawaii. While doubling and tripling the standard sodium hexametaphosphate concentration of 0.441gL−1 did not increase estimates of clay content, increasing levels of ultrasonic energy up to 1600JmL−1 significantly increased measured clay content for all oxide and volcanic ash soils. The response to ultrasonication was dependent on soil carbon, oxide content, and surface charge, with more energy needed to disperse soils higher in carbon, oxides, and positive charge. Scanning electron microscopy revealed damage to the sand fraction in some soils when ultrasonicated, but the extent of this damage was viewed as negligible. Porous sand-sized particles resembling pumice grains were also observed in some soils, suggesting that conventional particle size analysis and clay interpretations may not adequately describe surface related behaviors.

The large late-glacial Ho eruption of the Hudson volcano, southern Chile

Lakes formed in the Aysen region of southern Chile after the retreat of mountain glaciers, established by ~17,900 calendar years before present (cal years BP) or earlier, contain numerous late-glacial and Holocene tephra layers derived from >70 eruptions of the volcanoes in the region, including Hudson, the southernmost in the Andean Southern Volcanic Zone (SVZ). Sediment cores from seven of these lakes contain an unusually thick late-glacial age tephra layer, which based on its distribution and bulk trace-element composition was derived from a large explosive eruption of Hudson volcano between 17,300 and 17,440 cal years BP and is termed Ho. In 13 cores from six of these lakes, each located ~100 km generally northeast of Hudson, the Ho tephra layer ranges between 50 and 88 cm in thickness, and contains pumice grains up to 2 cm in maximum diameter. Comparison with three previously documented large explosive Holocene Hudson eruptions (H1 at 7,750 cal years BP, H2 at 3,920 cal years BP, and H3 in 1991 AD) suggests that Ho was larger, with an estimated tephra volume of >20 km3, the largest post-glacial eruption documented for any volcano in the southern Andes and most likely responsible for the formation of the Hudson caldera. In total, Hudson has erupted ≥45 km3 of pyroclastic material in the last ~17,500 years, making it the most productive volcano in the southern Andes in terms of the total volume erupted since the beginning of deglaciation in the region. Chemical stratification is not seen in the waterlain Ho tephra, but these deposits are bi-modal, consisting of a much greater proportion of dark glassy basaltic-trachyandesite dense fragments and pumice, with glasses which range between 55 and 59 wt.% SiO2, along with volumetrically less-significant lighter-colored trachydacite pumice, with glass of 66 wt.% SiO2. In contrast, H1 products are trachyandesitic in composition, H2 ones are more felsic than H1, being composed essentially of trachydacite, and although H3 1991 AD again produced tephra of bi-modal compositions, it erupted a much smaller proportion of mafic compared to felsic material than did Ho. Thus, the repetitive large explosive eruptions of Hudson volcano have evolved to progressively less-mafic overall compositions from late-glacial to historic times, and their volumes have decreased. Sr-isotopic composition of bulk samples of the most mafic dense glass and most felsic pumice components of the Ho tephra, as well as samples from other Hudson eruptions, which overall range from 51 to 66 wt.% SiO2, with 525 to 227 ppm Sr, are all similar (0.70444 ± 0.00007), indicating that crystal-liquid fractionation rather than crustal assimilation was the main process responsible for these chemical variations.

Measuring sheet erosion using synthetic color-contrast aggregates

The bulk of eroded soils measured at the outlets of plots, slopes and watersheds are suspended sediments, splash-induced sheet erosion. It is depending on rainfall intensity and antecedent soil moisture contents and contributes to a significant proportion of soil loss that usually is ignored in soil erosion and sediment studies. A digital image processing method for tracing and measuring non-suspended soil particles detached/transported by splash/runoff was therefore used in the present study. Accordingly, fine mineral pumice grains aggregated with white cement and coloured with yellow pigment powder, with the same size, shape and specific gravity as those of natural soil aggregates, called synthetic color-contrast aggregates, were used as tracers for detecting soil particle movement. Subsequently, the amount of non-suspended soil particles detached and moved downward the slope was inferred with the help of digital image processing techniques using MATLAB R2010B software (Mathworks, Natick, Massachusetts, USA). The present study was conducted under laboratory conditions with four simulated rainfall intensities between 30–90 mm h-1, five antecedent soil moisture contents between 12–44 % v v-1 and a slope of 30%, using sandy loam soils taken from a summer rangeland in the Alborz Mountains, Northern Iran. A range of total transported soil between 90.34 and 1360.93 g m-2 and net splash erosion between 36.82 and 295.78 g m-2were observed. The results also showed the sediment redeposition ratio ranging from 87.27% [sediment delivery ratio (SDR) = 12.73%] to 96.39% (SDR = 3.61%) in various antecedent soil moisture contents of rainfall intensity of 30 mm h-1 and from 80.55% (SDR = 19.45%) to 89.42% (SDR = 10.58%) in rainfall intensity of 90 mm h-1. Copyright © 2013 John Wiley & Sons, Ltd.

Reworked pyroclastic beds in the early Miocene of Patagonia: Reaction in response to high sediment supply during explosive volcanic events

Two meter-scale pyroclastic levels are interbedded within the early Miocene succession of the Estancia 25 de Mayo (Patagoniense transgression) and Santa Cruz formations in the foreland Austral (or Magallanes) Basin, Argentina. The Lower Pyroclastic Level (LPL) is a tabular body interbedded within offshore marine deposits, laterally continuous for 30km and varying in thickness from few centimeters to around 4m. Grain-size grades from coarse to extremely fine ash with upward-fining along with a northeastern-fining trends. Structureless fine to very fine tuffs dominate and rare parallel laminations are the only tractive sedimentary structures. The Upper Pyroclastic Level (UPL) lies within low energy fluvial deposits and is laterally discontinuous, and it is composed by lenticular bodies reaching a maximum of 15m thick and 100m wide, with a concave-up base and a plane top. Grain-size range is similar to the LPL but it coarsens upward. The lower portion of the UPL shows parallel lamination, current ripple lamination and mud drapes with large pumice lapilli and plant debris, whereas the upper portion shows parallel lamination and trough cross-stratification. Both pyroclastic levels are composed mainly of pumice grains and glass shards with minor proportions of quartz and plagioclase crystals and lithic fragments. The LPL shows no mixing with epiclastic material whereas the UPL shows an upward increase in epiclastic material, and an upward increment in the scale of cross-bedding.The large thickness in relation to the possible emission center and the content of plant debris of the LPL does not suggest a direct, submarine, ash-fallout origin. The LPL is interpreted as a deposit of hyperpycnal-flows generated at the coastal zone when tephra-laden rivers plunged into the ocean. Large amounts of well preserved plant debris support the hypothesis of a terrestrial source of the sediments. The UPL is entirely composed of tractive deposits, so an ash fallout origin is disregarded. This, together with the lenticular shape and the alluvial plain origin of the encasing sediments, suggests accumulation within fluvial channels. Cycles of upper-flow-regime parallel lamination, current-ripple lamination and mud drapes at the lower portion, suggest short-lived turbulent flows that initially filled semi-abandoned channels. They were followed by sheet floods and channel reactivation, expressed by large-scale cross-bedding. The low degree of particle mixing observed in both levels is explained by the inability of streams to erode the substrate as they are suddenly over-saturated with pyroclastic sediments during and after the eruption. The grain-size distribution of the LPL and geochemical data indicate a contemporaneous volcanic source located to the west/southwest in the Andean ranges, where the South Patagonian Batholith is presently located.

Wind erosion of volcanic materials in the Hekla area, South Iceland

Abstract Iceland has extensive areas with intense aeolian processes, in spite of humid climate. We measured wind erosion in a 110 km2 heterogeneous sandy area near the Mt. Hekla volcano in South Iceland. The area is sparsely vegetated and covered by volcanic materials. Measurements were made during two summer seasons in 2008–2009 with dust traps at 25 locations employing a ‘single dust trap method’ after characterizing sediment height profiles. Sets of electronic equipment that measured wind erosion and several weather parameters were placed at two locations. The results show a large variation in sediment transport with maximum transport in storms ranging from 0 to 1788 kg m−1 at each site and maximum average transport during storm reaching 244 kg m−1 h−1. The aeolian transport each summer ranged from 1 to 2981 kg m−1. Amount of loose sandy sediments on the surface, sediment texture and proximity to water channels are important factors explaining site differences. Wind erosion was most intense in the north-eastern part of the area, with >80% loose sand on the surface but less intense closest to the volcano where coarse pumice characterizes the surface. The research shows a pathway of sediment transport on a landscape scale with north-easterly winds, into the Thjorsa river, enhanced by landscape characteristics and seasonally active water channel. Pumice grains >8 mm in diameter were transported by saltation. Grains >1 mm were commonly >20% of materials collected at 30 cm height, which is explained by low density of the volcanic materials and high wind velocities.

Robust date for the Bronze Age Avellino eruption (Somma-Vesuvius): 3945 ± 10 calBP (1995 ± 10 calBC)

We found Bronze Age lake sediments from the Agro Pontino graben (Central Italy) to contain a thin (2–3 cm) continuous tephra layer composed of lithics, crystals and minor volcanic glass. Tephrochronological and compositional constraints strongly suggest that this layer represents the Avellino pumice eruption, which has also been identified in Central Italian lake cores. Its provenance is corroborated by electron-microprobe analyses performed on juvenile pumice grains, showing that the tephra layer is probably the distal equivalent of the EU2 event of the Avellino eruption. We used multiple 14C age estimations of two lacustrine sequences with intercalated tephra layer, from the western border zone (Migliara 44.5) and the centre of the former lake (Campo Inferiore), for in tandem dating of this eruption, employing the OxCal code, which yielded a robust age of 3945 ± 10 calBP (1995 ± 10 calBC). To date, this is the only study providing both a terminus post and terminus ante quem of this precision, also demonstrating the advantage of dating distal tephra layers in a clear stratigraphic context over proximal deposits in sequences with major stratigraphic hiatuses. Our new results underscore the importance of the Avellino tephra layer as a precise time marker for studies on the Early Bronze Age of Central Italy.

Involutions resulting from annual freeze–thaw cycles: a laboratory simulation based on observations in northeastern Japan

AbstractA pilot laboratory experiment using a reversed two‐layer soil model simulated small‐scale involutions formed in a seasonal frost environment during the last glacial period. At the modelled site, the interface between the upper aeolian sandy loam and the lower volcanic pumice constitutes small‐scale involutions that display upward‐extending tapered projections and downward‐extending round hollows. Two scale‐reduced laboratory models were subjected to three accelerated annual freeze–thaw cycles with monitoring of frost heave, soil temperature, moisture and pressure. Ice segregation near the layer interface induces upheaving of coarse pumice grains on freezing and earlier settlement of mobilised loam on thawing, resulting in deformation of the interface. A reconstructed 3‐D interface displays mounds and depressions with a diameter of 15–20 cm and a height increasing with freeze– thaw alternations. The experimental results imply that the repetition of differential heave and soft‐loam settlement promotes decimetre‐scale involutions in near‐saturated soils subject to deep seasonal frost penetration. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Microbial communities associated with geological horizons in coastal subseafloor sediments from the sea of okhotsk.

Microbial communities from a subseafloor sediment core from the southwestern Sea of Okhotsk were evaluated by performing both cultivation-dependent and cultivation-independent (molecular) analyses. The core, which extended 58.1 m below the seafloor, was composed of pelagic clays with several volcanic ash layers containing fine pumice grains. Direct cell counting and quantitative PCR analysis of archaeal and bacterial 16S rRNA gene fragments indicated that the bacterial populations in the ash layers were approximately 2 to 10 times larger than those in the clays. Partial sequences of 1,210 rRNA gene clones revealed that there were qualitative differences in the microbial communities from the two different types of layers. Two phylogenetically distinct archaeal assemblages in the Crenarchaeota, the miscellaneous crenarchaeotic group and the deep-sea archaeal group, were the most predominant archaeal 16S rRNA gene components in the ash layers and the pelagic clays, respectively. Clones of 16S rRNA gene sequences from members of the gamma subclass of the class Proteobacteria dominated the ash layers, whereas sequences from members of the candidate division OP9 and the green nonsulfur bacteria dominated the pelagic clay environments. Molecular (16S rRNA gene sequence) analysis of 181 isolated colonies revealed that there was regional proliferation of viable heterotrophic mesophiles in the volcanic ash layers, along with some gram-positive bacteria and actinobacteria. The porous ash layers, which ranged in age from tens of thousands of years to hundreds of thousands of years, thus appear to be discrete microbial habitats within the coastal subseafloor clay sediment, which are capable of harboring microbial communities that are very distinct from the communities in the more abundant pelagic clays.

Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.

Origin of the giant eruption cloud of Pinatubo, June 15, 1991

The series of eruptions of June 15, 1991 at Mt. Pinatubo, Philippines were observed hourly by satellite. A giant discshaped cloud covering an area of 60,000 km2 appeared in the satellite images at 14:40, Philippine time. The cloud expanded radially against wind of 20 m/s and spread to an area of more than 120,000 km2 within an hour. According to eyewitness accounts there was heavy fine-ash fall after 14:00, intermittent lapilli fall started at about 14:20, and heavy and continuous lapilli fall widely started at about 15:00. The occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall. The air-fall deposits of the major eruption are widely distributed, including upwind from the vent. They are composed of 3 units; a silt-size fine-ash layer (Layer B), a lapilli layer commonly including pumice grains of > 1 cm in diameter (Layer C), a lapilli bearing volcanic sand layer (Layer D). Judging from its wide distribution and depletion of coarse, grains, most of the fine ash of Layer B is not distal deposits of a small eruption, but is originated from a large co-ignimbrite cloud. It is suggested that the major eruption started with the generation of a pyroclastic flow, which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall. The results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 109 kg/s.

Use of natural products for zeolite synthesis. VI. Zeolite microcrystals from rhyolitic pumice grains

In the frame of the researches on the utilization of natural glasses for the synthesis of zeolites with unconventional techniques, a method for the production of zeolite microcrystals employing directly rhyolitic pumice grains for the hydrothermal zeolitization treatment is proposed, in order to avoid the problems connected with the pumice grinding process. The advantage offered by the limited amounts of chemicals required for the glass to zeolite conversion and the possibility of reusing the mother liquor rich in silica obtained during the process, are also discussed.

Mineralogy of some Italian andosols with special reference to the origin of the clay fraction

The mineralogy of four Italian Andosols - derived from volcanic material either oversaturated or undersaturated with respect to silica - has been investigated by XRD, EM and IR. The crystalline clay minerals in all four profiles are essentially similar, consisting of abundant halloysite with moderate illite and 14A intergrade material, minor kaolinite and occasional gibbsite. The soils also contain large amounts of imogolite and proto-imogolite allophane. With the exception of illite all these clay minerals are believed to be of pedogenic origin. Halloysite occurs in the dehydrated form in the surface horizons but becomes progressively more hydrated with depth. At depths of > 1.4 m the clay fraction consists almost entirely of fully hydrated halloysite, supporting the suggestion that halloysite forms best in a stagnant moisture regime where there is a depositional overburden acting as a silica source. EM observations show that the halloysite may have spherical morphology and may be intimately associated with gas vesicles in pumice grains where it probably forms by the transformation of allophanic material. It seems likely that dehydrated halloysite slowly converts to poorly crystallized kaolinite in the upper horizons of these profiles. The origin of the 2/1 minerals is more problematical. Illite is probably inherited from mica in the parent material but the 14A intergrade material is so poorly ordered that a pedogenic origin seems more likely than formation by inheritance or by transformation of pre-existing 2/1 silicates.