Volcanic glass textures, shape characteristics and compositions of phreatomagmatic rock units from the Western Hungarian monogenetic volcanic fields and their implications for magma fragmentation

Abstract

Abstract:The majority of the Mio-Pleistocene monogenetic volcanoes in Western Hungary had, at least in their initial eruptive phase, phreatomagmatic eruptions that produced pyroclastic deposits rich in volcanic glass shards. Electron microprobe studies on fresh samples of volcanic glass from the pyroclastic deposits revealed a primarily tephritic composition. A shape analysis of the volcanic glass shards indicated that the fine-ash fractions of the phreatomagmatic material fragmented in a brittle fashion. In general, the glass shards are blocky in shape, low in vesicularity, and have a low-to-moderate microlite content. The glass-shape analysis was supplemented by fractal dimension calculations of the glassy pyroclasts. The fractal dimensions of the glass shards range from 1.06802 to 1.50088, with an average value of 1.237072876, based on fractal dimension tests of 157 individual glass shards. The average and mean fractal-dimension values are similar to the theoretical Kochflake (snowflake) value of 1.262, suggesting that the majority of the glass shards are bulky with complex boundaries. Light-microscopy and backscattered-electron-microscopy images confirm that the glass shards are typically bulky with fractured and complex particle outlines and low vesicularity; features that are observed in glass shards generated in either a laboratory setting or naturally through the interaction of hot melt and external water. Textural features identified in fine-and coarse-ash particles suggest that they were formed by brittle fragmentation both at the hot melt-water interface (forming active particles) as well as in the vicinity of the interaction interface. Brittle fragmentation may have occurred when hot melt rapidly penetrated abundant water-rich zones causing the melt to cool rapidly and rupture explosively.

A Badacsony freatomagmás piroklasztit-sorozata: következtetések a monogenetikus bazaltvulkáni működés folyamataira és formáira

“…Jelen tanulmányban ez utóbbit követjük. A komponensek általános tulajdonsága, hogy a szemcsék a freatomagmás robbanások során képződött kitörési termé -kek esetében szögletesek (NÉMETH 2010). A juvenilis szemcsék a friss, kitörő magmából származó, buboréko so dott és fragmentálódott, majd gyorsan kihűlt piroklasztok (FISHER & SCHMINCKE 1984).…”

“…A juvenilis szemcsék a friss, kitörő magmából származó, buboréko so dott és fragmentálódott, majd gyorsan kihűlt piroklasztok (FISHER & SCHMINCKE 1984). Ezek a teljesen üveges szem csék től a mikrolitos változatokig terjedhetnek (NÉMETH 2010). A juvenilis szemcséken belül kétféle kőzetüveg típus külö nít -hető el: a szideromelán és a tachylit (FISHER & SCHMINCKE 1984).…”

“…Az alábbiakban az egyes juvenilis kőzetszemcsék makroszkópos elkülönítését mutatjuk be a fentebb leírtak tükré -ben. A szideromelán makroszkópos megjelenése: szürke, sárga, sárgásbarna, szögletes, esetenként hólyagos vulkáni üveg, míg a tachylité: sötétszürke-fekete, nagyobb hólyag -üre geket tartalmazó, mikrolitgazdag vulkáni üveg (FISHER & SCHMINCKE 1984, NÉMETH 2010, THORDARSON 2004. A juvenilis szemcsék közé tartozik a magmás robbanásos kitörésekre jellemző salak is, ami több kézipéldányban jelen van.…”

Hungarian national report on IAVCEI 2007–2010

“…In this volume Lexa et al (2010) overviewed the main characteristics of the volcanic forms found in the Carpathian-Pannonian Region that is completed a number of case-studies (Németh 2010, Keresztúri et al 2010 highlighting that this area could be a natural laboratory for volcanologists. …”

Geoheritage values of one of the largest maar craters in the Arabian Peninsula: the Al Wahbah Crater and other volcanoes (Harrat Kishb, Saudi Arabia)

“…The entire succession is rich in exotic rock fragments from underlying country rocks ( Figure 5). At the microscopic level, the rocks contain relatively small proportions (about 20 vol % by visual estimate) of moderately palagonitized, yellow, blocky and low-to-moderately vesicular sideromelane glass shards, indicating that these pyroclasts formed due to sudden chilling consistent with a magma-water interaction driven phreatomagmatic explosive fragmentation [39][40][41][42]. Bedform characteristics of the tephra ring succession are typical for base surge dominated deposition, which is characteristic for lateral moving low particle concentration pyroclastic density currents regardless of the cause of the explosive fragmentation and turbulent current expansion [40,[43][44][45][46].…”