Black Pumice Research Articles

Variety of the drift pumice clasts from the 2021 Fukutoku‐Oka‐no‐Ba eruption, Japan

AbstractPumice rafts that arrived at the Nansei Islands, Japan, provided a unique opportunity to investigate the Fukutoku‐Oka‐no‐Ba (FOB) eruption of August 2021. Despite drifting for 2 months for ~1300 km, the drift pumice raft had a large volume and contained a variety of pumice clasts, some of which were deposited during a high tide in a typhoon, while others were washed up on a sandy beach. Most of the drift pumice clasts are gray in color, vesicular, and have a groundmass containing black enclaves. Rare black pumice and the main gray pumice components have similar trachytic compositions, with SiO2 = 61–62 mass% and total alkalis = 8.6–10 mass% (on an anhydrous basis). Both pumice types contain clinopyroxene, plagioclase, and rare olivine phenocrysts. Thin‐section observations show that the gray pumice has more elongated vesicles as compared with the black pumice that has spherical vesicles, even where the two types of pumice are in the same clast. The glass in the black pumice is transparent and brown in color, while that in the gray pumice is colorless. No micro or nano‐crystals were observed during electron and optical microscopy. Raman spectra of the brown‐colored glass exhibit a clear magnetite peak, suggesting magnetite nanolites cause the brown color. High‐Mg olivine in the black pumice has an equilibrium temperature of c. 1200 °C and a rim diffusion profile indicative of re‐equilibration with the surrounding melt over a period of hours to days. The textural relationships between the gray and black pumice suggest that the black pumice had become black and viscous before the two types of pumice mixed. Therefore, crystallization of magnetite nanolites and a corresponding increase in melt viscosity were important in the eruption preparation process, which then resulted in a large‐scale Plinian eruption.

Surface roughness of acrylic denture base resins polished with different abrasive agents: an in-vitrostudy

Background: Excessive surface roughness on the denture base can adversely impact the oral health of the patient. Therefore, it is necessary to polish the denture before they are delivered to the patient. The abrasive and polishing agents should provide a smoother surface without affecting the physical and mechanical properties of denture bases. Aim: This study aims to examine and understand the potential of different polishing materials on surface roughness of acrylic denture base resins. Materials and methods: A total of 60 Heat-cure acrylic specimens (acrylic bars) were made and grouped into six groups. Control (no abrasive), Pumice (Micro-white), Eggshell powder, Seashell powder, Black sand powder, White sand powder are used as abrasive materials for polishing these specimens. These polished specimens were subjected to profilometer surface roughness analysis. Results: The acrylic specimens polished with eggshell powder on acrylic specimens showed the least surface roughness followed by black sand, white sand, pumice and seashell powders. Tukey HSD showed significant differences (p=0.000) between unpolished and polished specimens. Conclusion: Eggshell powder effectively reduced the surface roughness of denture base resin material. However, the surface roughness demonstrated by all the abrasive materials used was within the threshold limit (2 µm). Therefore, all the materials can be used as abrasives.

Lahar deposits generated after the Millennium eruption of the Changbaishan Tianchi volcano in the Erdaobaihe River system, China

After the Plinian Millennium Eruption (including the ME-I and ME-II eruptive stages) of Changbaishan Tianchi volcano in 969 ± 20 CE, two large-volume lahars were deposited mainly on the northern flank of the Erdaobaihe River, which flows through the only outlet from Tianchi Lake: (1) a lahar deposit containing gray and black pumice (Lahar (G&B) P) and (2) a lahar deposit containing lithic particles (Lahar L). Based on the sedimentary characteristics of lahar outcrops and grain-size and compositional analyses performed using standard dry sieving techniques, examination of thin sections and geochemical analyses, the origins, transport and deposit processes and fluid phase variations of the two lahar deposits are studied. These two lahar deposits are fan-like in the flat medial area (23–60 km from the crater) and fill channels in the distal mountainous area (>60 km from the crater). Lahar L in the medial area represents a complete composite lahar flow deposit with a frontal debris head (very coarse gravel texture, extremely poor sorting, massive and matrix-supported structure) and a hyperconcentrated fluid phase body and tail (coarse sandy texture, very poor sorting, faint horizontal stratification with thin lenses of gravel, clast-supported structure). Lahar L in the distal area represents the material that passed the debris flow head, and the fluid phase changes from debris flow to hyperconcentrated (poor sorting, clear thin horizontal stratification) downcurrent. The fluid phase of Lahar (G&B) P changes from debris flow in the medial area (medium gravel to fine sand, very poor sorting, thick horizontal stratification with floating pumice on each unit surface) to hyperconcentrated flow (poor sorting, thin horizontal stratification, clear normal graded bedding) and to stream flow (moderate sorting, cross-bedding) in the distal area. Lahar (G&B) P was likely formed by the mixing of water from Tianchi Lake and pyroclastic flows of ME-II. Lahar L was likely triggered by a sudden release (dam break) from Tianchi Lake. An assessment of the risk posed to the stability of Tianchi Lake is as important as an assessment of the eruption risk.

Different magma sources and evolutions of white and black pumice from the middle Okinawa Trough: Evidence from major, trace elements and Sr–Nd–Pb isotopes

The Okinawa Trough (OT) is a back‐arc, initial marginal sea basin that is located behind the Ryukyu arc–trench system. Pumice, including both white and black pumice, is the most widely distributed magmatic rock in the OT. Here, the major and trace element concentrations and Sr–Nd–Pb isotopic compositions of the white and black pumice from the middle OT (MOT) are presented and combined with previously published elemental data for volcanic rocks from the MOT. Their different magma sources and magmatic evolutions are discussed, and their different magma chamber structures are provided in this paper. These results demonstrate that the white pumice has two magma sources: One was produced by the crystallization‐based differentiation of basaltic magma, and the other was produced by the assimilation‐fractional crystallization (AFC; with less than 5% contamination) of basaltic magma. The black pumice also includes two magma sources: One is similar to that of the first white pumice, whereas the other was produced by basaltic magma that experienced more than 20% upper crustal contamination. There are two different structure magma chambers for white and black pumice in the MOT crust: The deeper one generated no contaminated pumice, whereas the shallow one generated contaminated black pumice. The contamination of the white pumice (<5%) may have occurred during the eruption of magma on the seafloor, rather than in the magma chamber. Different magma sources and evolutions of white and black pumice imply the complexity of magmatism in the Okinawa Trough.

A complex magmatic system beneath the middle and northern Okinawa Trough: evidence from pyroxene characteristics

The Okinawa Trough is an initial marginal back‐arc basin. In this paper, we present a comparative study of pyroxenes in white and black pumice from the Okinawa Trough to explore the magmatic evolution of the region and the characteristics of the magmatic system. The results show two species of pyroxenes in the white pumice (clinopyroxene and orthopyroxene) but only one species in the black pumice (orthopyroxene). Many isomorphic replacements among the cations occurred during clinopyroxene crystallization. Clinopyroxene‐melt thermobarometry yielded a temperature of 847–988°C and a pressure of 2.73–7.42 kbar, which vary considerably and correspond to a maximum depth of approximately 24 km. All of the orthopyroxenes in both the white and black pumice crystallized at similar pressures ranging from 0.67 to 4.59 kbar, which correspond to a maximum depth of approximately 15 km. The clinopyroxenes crystallized mainly in the lower crust, whereas the orthopyroxenes formed at the boundary depth between the upper crust and the lower crust. The similarities in the pyroxene chemical compositions and the whole‐rock characteristics between the two types of pumice indicate that both types of pumice are linked to the same magmatic system at depth. The pyroxenes crystallized from genetically related melts within a magmatic system that had undergone various degrees of differentiation. The physicochemical characteristics of the pyroxenes show that the magma chamber module changed during the crystallization process from clinopyroxenes to orthopyroxenes. This change most likely resulted from local characteristics of the near‐surface structure. These observations support the model of a ‘two‐layered magma chamber’ in the northern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.

Volcanostratigraphic Sequences of Kebo-Butak Formation at Bayat Geological Field Complex, Central Java Province and Yogyakarta Special Province, Indonesia

DOI:10.17014/ijog.3.2.77-94Bayat Complex is usually used as a work field for students of geology and other geosciences. The study area is located in the southern part of the Bayat Complex. Administratively, it belongs to Central Java Province and Yogyakarta Special Province. The lithology of Bayat is very complex, composed of various kinds of igneous, sedimentary, metamorphic, and volcanic rocks. Most of previous researchers interpreted Bayat as a melange complex constructed within a subduction zone. Kebo-Butak is one of formations that forms the Bayat field complex. The formation is composed of basalt, layers of pumice, tuff, shale, and carbonaceous tuff. Most of them are known as volcanic rocks. These imply that volcanic activities are more probable to construct the geology of Bayat rather than the subducted melange complex. The geological mapping, supported by geomorphology, petrology, stratigraphy, and geological structures, had been conducted in a comprehensive manner using the deduction-induction method. The research encounters basalt, black pumice, tuff with basaltic glasses fragments, zeolite, argilic clay, as well as feldspathic- and pumice tuff. Petrographically, the basalt is composed of labradorite, olivine, clinopyroxene, and volcanic glass. Black pumice and tuff contain prismatic clinopyroxene, granular olivine, and volcanic glasses. Feldspathic tuff and pumice tuff are crystal vitric tuff due to more abundant feldspar, quartz, and amphibole than volcanic glass. Zeolite comprises chlorite and altered glasses as deep sea altered volcanic rocks. The geologic structure is very complex, the major structures are normal faults with pyrite in it. There were two deep submarine paleovolcanoes namely Tegalrejo and Baturagung. The first paleovolcano erupted effusively producing basaltic sequence, while the second one erupted explosively ejecting feldspathic-rich pyroclastic material. The two paleovolcanoes erupted simultaneously and repeatedly.

울릉도 부석 기질의 암석.광물학적 특성

Mineralogical and petrological characteristics were investigated on matrix of dense gray, vesiculate gray, brown and black pumice in Ulleung Island by using XRD, FT-IR, XRF, SEM and thermal analysis. According to the analysis, most of pumice matrix are amorphous and include very small amount of sanidine and anorthoclase. Since the adsorption moistures, which commonly observed as O-H peak in FT-IR spectrum, are not identified in thermal analysis, it seems reasonable to conclude that content of the adsorption moisture has very low level. Although pumice has a large specific surface area, with long time elapsed after eruption, pumice matrix shows very low degree of hydration alteration due to the low level of water content. In SEM images, most surfaces of pumice show morphological characteristics such as various shapes of vesicle with wrinkled and thin walls resulted from ductile coalescence. Dense gray pumice formed in the initial stage includes small vesicles less than in size with subangular to angular shapes, free of ovoid vesicle. These characteristics are interpreted to have related to the hydrous environment derived from phreato-plinian eruption. Submicron particles observed as amorphous alumina silicate assemblages in vesicle surface are considered as particles sticked to the matrix surface through rapidly cooling process during ascent of alkali phonolitic magma. It indicates that these particles coexisted partly with crystallized alkali feldspar.

Mafic enclaves in the rhyolitic products of Lipari historical eruptions; relationships with the coeval Vulcano magmas (Aeolian Islands, Italy)

The recent finding of mafic enclaves in the Rocche Rosse (RR) lava flow, the last magmatic product on Lipari (Aeolian Islands, Italy) (AD 1230 ± 40), opens the possibility to investigate in detail the most recent magmatic system of the island, an important issue for the volcanic hazard assessment of the area. The RR lava flow is an aphyric rhyolitic coulee consisting of grey and black pumice and black and grey obsidian. Enclaves have ellipsoidal to spheroidal shape and vary from mm-sized in the central portion of the flow, to cm-sized, at the top and in the flow front, where they are also more abundant. Enclaves are shoshonitic-latitic (group A) and trachytic (group B) in composition. The mineralogy of group A consists of dominant clinopyroxene crystals with minor abundance of feldspar (plagioclase > K-feldspar), olivine and biotite, while group B is composed of feldspar (K-feldspar > plagioclase) with minor clinopyroxene, olivine and biotite. Geochemical modeling suggests that the host rhyolitic rocks could be the product of AFC (Assimilation plus Fractional Crystallization) of a magma compositionally similar to the associated shoshonitic-latitic enclaves, which, in turn, could be obtained, through an AFC process, from the primitive melts erupted as olivine hosted melt inclusions during the last 15 ka at Vulcano. The already-known last 42 ka relationship between Lipari and Vulcano Islands is here reinforced until historical time, especially for the last 1 ka. The geochemical and petrological overlap between Lipari and Vulcano is interpreted to reflect the existence of a similar magmatic system underneath the two islands. The nearly aphyric RR rhyolites are interpreted to be the products of a superheated (temperature far above the liquidus) and initially water-undersaturated magma that underwent degassing close to the surface inhibiting microlite crystallization.

The 322 ka Tiribí Tuff: stratigraphy, geochronology and mechanisms of deposition of the largest and most recent ignimbrite in the Valle Central, Costa Rica

The Tiribi Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibas Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribi Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tarcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10−3) covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 (15 km3 DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new 40Ar/39Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribi Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments.

Isotopic feature and uranium dating of the volcanic rocks in the Okinawa Trough

Volcanic rocks from the northern and middle Okinawa Trough were dated by uranium-series dating method. Differential fractions using magnetic procedure were designed to separate samples. New report on the ages and isotopic data of rocks in the northern trough (especially black pumice) was discussed. Based on the uranium dates and Sr-Nd isotopic ratio, magmatic evolution process of the Okinawa Trough was noted. Firstly, there have been wide silicic volcanic activities in the Okinawa Trough from late Pleistocene to present, and the volcanic rocks can be divided into three subgroups. Secondly, magma generally came from PREMA source area under the Okinawa Trough. Magmatic evolution in the northern trough was similar to the middle, but different to the south. Finally, volcanic activities indicated that opening of the southern Okinawa Trough did not happen due to the collision between Luson Arc and Eurasian Plate until the early Pleistocene.

Geochemical and isotope data from the Acatlán Volcanic Field, western Trans-Mexican Volcanic Belt: Origin and evolution

The Quaternary Acatlán Volcanic Field (AVF) is located at the western edge of the Trans-Mexican Volcanic Belt (TMVB). This region is related to the subduction of the Pacific Cocos and Rivera plates beneath the North American plate since the late Miocene. AVF rocks are products of Pleistocene volcanic activity and include lava flows, domes, erupted basaltic andesite, trachyandesite, trachydacite, and rhyolite of calc–alkaline affinity. Most rocks show depletion in high field-strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction-related volcanic arcs. 87Sr/ 86Sr values range from 0.70361 to 0.70412, while ɛNd values vary from +2.3 to +5.2. Sr–Nd isotopic data plot along the mantle array. On the other hand, lead isotope compositions ( 206Pb/ 204Pb=18.62–18.75, 207Pb/ 204Pb=15.57–15.64, and 208Pb/ 204Pb=38.37–38.67) give evidence for combined influences of the upper mantle, fluxes derived from subducted sediments, and the upper continental crust involved in magma genesis at AVF. Additionally δ 18O whole rock analyses range from +6.35‰ in black pumice to +10.9‰ in white pumice of the Acatlán Ignimbrite. A fairly good correlation is displayed between Sr as well as O isotopes and SiO 2 emphasizing the effects of crustal contamination. Compositional and isotopic data suggest that the different AVF series derived from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source.

The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan

This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the pumice clasts are related to the size and distribution of Fe3+-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.

Sedimentology, Paleoflow Dynamics and Flood History of JoKulhlaup Deposits: Paleohydrology of Holocene Sediment Sequences in Southern Iceland Sandur Deposits

ABSTRACT This paper examines the sedimentology of a series of deposits associated with catastrophic floods caused by subglacial volcanic eruptions in southern Iceland. A simple model is proposed for the interpretation of the sediments in terms of changes in flow characteristics of the fluid-sediment mix. A range of sediment types characteristic of these flood flows is identified, the predominant one being thick, massive, homogeneous, poorly structured, black pumice, fine gravels, with 65 p rcent silt + clay (Type E); and single lithology boulder deposits (Type F). These flood deposits alternate with, or are channeled into, more 'normal' sandur deposits, comprising heterogeneous, poorly sorted, clast-supported, imbricated, rounded cobble and pebble gravels (Type G). The dominant sediment sequences are interpreted as representing hyperconcentrated fluid-sediment mixtures, with dispersive stresses acting to create massive, poorly sorted, nongraded or inversely graded sediments, the latter characterized by large surface boulders. The vertical changes in sediment type reflect significant changes in flow conditions during transport and subsequent deposition, with an initial massive flood surge followed by flow deceleration, sediment 'freezing' and dewatering, producing a more fluid series of flows. The more fluid phases were associated with extensive erosion and channelization, cutting deep scour channels marked by residual streamlined hummocks, boulder lags, and fields of 'megaripples', leading downstream to a 'washed' sandur plain. The paper examines some of the problems associated with the application of paleoflow models for determining former flow dynamics. The models adopted suggest that most of the flows occurred as relatively mobile, low viscosity flows, with little internal resistance to shear. While sediment concentrations are estimated to have reached about 50 percent by weight, with maximum yield strengths between 2.9 103 and 4.2 105 dn cm -2, maximum viscosities ranged between only about 2 and 8 102 poises. A range of paleovelocity models applied to the flood sediment indicates flow rates of between abo t 4 and 13 ms-1, with peak discharges reaching a maximum of 3.3 105 m3 s -1. Dating of the deposits by 14C dating and tephrochronology indicates that at least 8 major sediment-producing jokulhlaups occurred over a 4,000-year period, between c 4500 yr BP and 1357 AD. Hence, although relatively rare, the effects of these floods still dominate the proglacial landscape, probably producing > 85 percent of the total thickness of proglacial sandur sediments.