Glass shards are typically remnants of tiny gas bubbles that developed and grew in size during the final ascent of magma toward the surface; such shards may consist of many gas bubbles or only a portion of a single gas bubble. During eruption, the expanding gas broke the bubbles and surrounding glass into shards of various sizes and shapes. Shards formed by phreatomagmatic eruptions see eruption style often have a particularly angular shape resulting from the violent explosive interaction between magma and water.
Glass is relatively hard 5 on Moh's scale , and the more angular the glass shards the more abrasive the ash. Rising magma will incorporate pieces of different types of rocks often called lithics through which it moves, including rocks located deep beneath a volcano and within the volcano itself. The rapid ascent of magma during explosive eruption will often rip fragments from the walls of the magma conduit, which are ejected and fragmented further by the explosive expansion of volcanic gases.
These non-magmatic rock fragments are found in varying abundances within ash deposits and often have a shape and texture distinctly different than glass shards. It may create a thick layer of dust-like material on surfaces for miles around the original eruption. Unlike the ash produced by burning wood and other organic materials, volcanic ash can be dangerous. Its particles are very hard and usually have jagged edges. As a result, it can cause eye, nose, and lung irritation, as well as breathing problems.
While in the air, ash can cause problems for jet engines, forcing airlines to cancel flights through the affected area.
Earth, Planets and Space volume 68 , Article number: 67 Cite this article. Metrics details. The volcanic ash of the eruption on September 27, at Ontake volcano consists mostly of altered rock fragments.
The ash contains partly altered volcanic rock fragments consisting of primary igneous minerals plagioclase, orthopyroxene, titanomagnetite, and feldspars and volcanic glass accompanied by alteration minerals to some extents, and contains no juvenile fragments.
These features indicate that the eruption was a non-juvenile hydrothermal eruption that was derived from the hydrothermal system developed under the crater. The major minerals derived from hydrothermal alteration zones are silica mineral, kaolin-group mineral, smectite, pyrophyllite, muscovite, alunite, anhydrite, gypsum, pyrite, K-feldspar, albite, and rutile. Minor chlorite, biotite, and garnet are accompanied. The associations indicate development of advanced argillic, sericite, and potassic alteration zones under the crater.
Occurrence of anhydrite veinlet and the set of alteration zones indicate hydrothermal alteration zones similar to late-stage porphyry copper systems. The zonation of alteration minerals produced in a hydrothermal system reflects spatial variations in hydrothermal processes and hydrothermal fluid chemistry e. The relation between the distribution of hydrothermal fluid and consequent mineralogical zonation has been intensively studied through geological studies on geothermal fields as well as ore deposits.
Some of the fields are considered to be the interior of ancient stratovolcanoes, and their structural models have been well established e. Although the depths of hydrothermal systems are largely inaccessible under active volcanoes, we are able to obtain minerals precipitated at the depths when a hydrothermal eruption supplies them from the depths to the earth surface Hedenquist and Henley Volcanic ash from the eruption at Ontake volcano on September 27, consists abundantly of hydrothermally altered minerals Miyagi et al.
The minerals were derived from the subvolcanic hydrothermal system under the volcano, and therefore, the properties of hydrothermal alteration such as temperature, acidity, and locus can be estimated from the mineralogy of the volcanic ash.
This study aims to estimate the chemical conditions and locus of the source fluid by correlating the mineralogy of the volcanic ash to proposed models of mineral zonation in well-studied ancient hydrothermal systems beneath eroded old volcanoes. The estimation on the locus will be compared with previous geophysical estimations Kato et al.
Ontake volcano MASL located in central Japan is a stratovolcano consisting of basalt, andesite, and dacite. The basement of Ontake volcano is largely composed of Jurassic to Paleogene rhyolite to rhyodacite and marine sediments Yamada and Kobayashi ; Takeuchi et al.
Volcanic activity at Ontake volcano commenced — ka with effusion of basaltic to andesitic lava flows with minor dacite to form the ancestral volcanic edifice Yamada and Kobayashi The younger activity that formed the current edifice commenced at 80 ka. The activity is subdivided into two stages: the early explosive stage that produced rhyolitic to dacitic pyroclasts and the later lava stage. The lava is composed of andesite containing phenocrysts of plagioclase, orthopyroxene, clinopyroxene, and magnetite.
The summit area at Kengamine is composed of andesite lava Yamada and Kobayashi Phreatic or hydrothermal eruptions frequently occur at Ontake volcano. Before the eruption, at least three phreatic eruptions were documented in , , and Oikawa et al. Oikawa et al. Surface hydrothermal manifestations have been continuously observed on the southwestern flank of Kengamine cone for last years Oikawa Hydrothermally altered rocks had been exposed on the southwestern flank of Kengamine before the eruption.
The eruption took place on September 27th on the southwestern flank of Kengamine. The eruption ejected approximately a million tons of volcanic ash, similar to the volume of the eruption Takarada et al. The volcanic ash was distributed in the summit area and on the eastern flank of the volcano Fig. Miyagi et al. Some geophysical studies have reported analyses on seismicity linked to the eruption, investigating pre- and post-eruptive processes e.
Location of sampling site left and volcanic ash from the eruption right. Volcanic ash was sampled along the road. The samples were collected from clean, flat, and hard surface of the road and also from clean leaves. The volcanic ash distribution is drawn based on Takarada et al. The volcanic ash from the September 27, eruption was sampled along a road 8 km northeast of the vent Fig.
Bulk ash samples were also analyzed by XRD. A randomly oriented sample and an oriented sample were prepared for each fraction. Ethylene glycol and HCl treatments were applied to all fractions to determine clay mineral species. Semiquantitative analysis was carried out at an acceleration voltage of 15 kV, a probe current of 2. Minerals identified in the ash are listed in Table 1.
Feldspar and anhydrite only occur in the course and the medium fractions. The intensities of X-ray peaks of the sheet silicate minerals are higher in the fine fraction than in other fraction. SEM—EDS microprobe analysis was carried out to identify mineral species in individual grains of the medium and coarse fractions. There is no mineralogical difference between the coarse and medium fractions except for the presence of calcium sulfate minerals in the medium fraction.
Each ash grain consists mostly of multiple mineral crystals ranging from submicron to millimeter in size. The EDS analysis on very fine crystals sometimes yields confusing results because the induced X-rays from adjacent crystals affect those of the target crystal e. To reduce this effect, only crystals that are compositionally and texturally homogeneous were analyzed. Some grains exhibit apparently homogeneous textures, although these consist of very fine crystals smaller than the resolution of the image.
The compositional homogeneity was examined by repeating the analysis on multiple positions in the grain. If a grain that is even apparently homogeneous is a mixture of different fine crystals, elemental proportion arbitrarily changes by position. Fine alunite crystals interweaved with silica mineral show spectra imposed by Si peaks, which can be still identified as the mixture of these minerals based on their distinct elemental proportions.
Similarly, rutile that occurs as tiny crystals mixed with silica minerals is identified from distinct titanium spectra accompanied by silica peaks with arbitrarily changing intensity. Feldspar, pyroxene, and garnet were identified with stoichiometric proximities between the ideal formulae and the analytical results.
Results for the very fine feldspar yielded X-ray spectra influenced by juxtaposed crystals, resulting in discrepancy from the ideal formula, and were discarded. Spectra with SiO 2 and Al 2 O 3 indicate the presence of hydrous aluminous silicate mineral either pyrophyllite or kaolin-group minerals. Kaolin-group mineral is characterized by the chemical formula wherein the atomic concentrations of Si and Al are the same.
Kaolin frequently occurs as very fine crystals inwrought with silica mineral, resulting in a deceptive spectrum with higher silica concentration than kaolin. Chlorite was identified from spectra comprising distinct Si, Al, Mg, Fe peaks and negligible peaks of Na, K, Ca if the total of cations is 20 when the number of oxygen is assumed to be Mica minerals are characterized by an abundance of SiO 2 , Al 2 O 3 , and K 2 O and can be distinguished from potassium feldspar by their stoichiometry.
Mica was identified when the mineral has two potassium when oxygen number is assumed to be Muscovite is the most common mica mineral in the sample. The simple chemical formula of muscovite is KAl 2 Si 3 Al O 10 OH 2 , in which potassium can be partly replaced by sodium and calcium, and the octahedral aluminum by iron, manganese, magnesium, and titanium. Mica mineral rich in Fe and Mg is regarded as biotite. Ash grains in the coarse fraction have various appearances under a binocular microscope: opaque white-colored grains, opaque gray-colored grains, translucent brownish gray-colored grains, translucent white-colored grains, transparent grains, and free crystals Fig.
The complete table of the mineral assemblages is included in the supplemental material Additional file 1. Photographs of ash particles from the September 27, eruption. Partly altered volcanic fragments are common in the ash, occurring as angular to subangular blocky fragments of partly altered volcanic rocks and minerals comprised of plagioclase, orthopyroxene, and magnetite.
The partly altered rock fragments preserve the texture of volcanic rocks containing volcanic glass. Isolated crystals of plagioclase and orthopyroxene are inferred to be derived from volcanic rocks because they contain glass inclusions. Volcanic rocks of the cone are the source of the minerals, which contain them as phenocrysts Yamada and Kobayashi Two types of groundmass-derived fragments were identified: one consisting of abundant volcanic glass and minor microlites of plagioclase and pyroxene that form hyalo-ophitic texture, and the other one consisting of microlites of plagioclase, anorthoclase, and sanidine, and minor interstitial glass that form hyalopilitic texture Fig.
The original igneous minerals and glass are partly replaced by pyrite, silica mineral, kaolin-group mineral, and muscovite. BEI images of volcanic ash particles. Some of plagioclase microlites and glass were replaced by pyrite, silica mineral, kaolin-group mineral, and muscovite. Pyroxene phenocryst is replaced by silica mineral, and pyrite crystals fill the cleavage of the pseudomorphs.
Narrow veinlets of vug and pyrite crystals are recognized in the deformed part arrow. Original crystals were replaced by silica mineral or kaolin darker or became void. The colloform texture consists of K-feldspar-rich bands brighter and a fine-grained mixture of silica mineral and K-feldspar darker. Anhydrite crystals bright are attached conjugate with the grain. This association consists dominantly of silica mineral, subordinate pyrite, and minor rutile.
Grains of this association are abundant in the ash. They appear opaque white under a binocular microscope.
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