火山 第 57 巻 ( 2012)
第 4 号 145-158 頁

論 説

安山岩質降下軽石と溶岩流の結晶破砕度
─浅間火山と桜島火山の噴出物の場合─
安 井真也＊（2011 年 11 月 7 日受付，2012 年 9 月 13 日受理）

Degrees of Fragmentation of Crystals Contained in Andesitic Pumice Fall Deposits and Lava
Flows: Case Study of the Eruptive Products of Asama Volcano and Sakurajima Volcano
Maya YASUI＊
Broken crystals are often contained in the eruptive products of explosive and nonexplosive eruptions. Although
little is known of the actual mechanism of crystal fragmentation, the broken crystal itself is useful for understanding
phenomena in the course from magma ascent to eruption. In order to describe the nature of broken crystals, the degreesof fragmentation of plagioclase (referred to as “b/a value”, hereafter) were measured in this study. The b/a values wereobtained by dividing the length of a broken surface (b) by the circumference of the crystal (a) on the thin section.
Plagioclase contained in the following lava flows and pumice fall deposits of Sakurajima Volcano and Asama Volcano
were measured. In the case of the 1783 Asama eruption, the activity culminated to its climactic, pyroclastic eruption thatgenerated Plinian pumice falls and clastogenic lava flows and formed a pyroclastic cone after the intermittent Vulcanianeruptions for about three months. The 1914-1915 Sakurajima eruption progressed from the initial vigorous pyroclasticeruption (Stage 1) via lava flowage with intermittent Vulcanian eruptions (Stage 2) to the long-lasting, gentle outflow oflava (Stage 3). The eruptive style of Stage 1 is similar to that of the climactic stage of the Asama 1783 eruption. The
eruptive style of the 1946 Sakurajima eruption was similar to that of Stage 2. Concerning the pumice fall deposits,
crystals in a single pumice clast and free crystals of various grain sizes were measured. Multiple timings and fields ofcrystal fragmentation are indicated from the following results. In the case of Stage 3 of the Sakurajima 1914-1915
eruption, a small amount of plagioclase shows smaller b/a values suggesting that a small amount of poorly fragmented
crystals was produced prior to the eruption. In this case, crystal breakage related to melt inclusions foaming during thedecompression of ascending magma and to shear-induced fragmentation of ascending magma near the conduit wall are
instanced as the possible mechanism. In the cases of Stage 2 and the 1946 eruption, b/a values are higher than that inStage 3. Crystal fragmentation within the conduit could occur, probably due to shock by repetitive Vulcanianexplosions. In the cases of Stage 1 of the Sakurajima 1914-1915 eruption and the climactic stage of the Asama 1783
eruption, average b/a values increased in the order of crystals in pumice clast, free crystals, and crystals in lava. Free
crystals consisting of broken surfaces without glass coating and crystal faces covered by vesicular glass are dominant. It
is considered that the abundant broken free crystals are produced by magma fragmentation and then by quenching oferupting pyroclastic materials in the eruptive column. Additional fragmentation during flowage can be considered within
the clastogenic lava flows.
Key words : broken crystal, fragmentation, pumice, free crystal, lava

1．は じ め に積物のような著しいマグマの発泡・破砕を伴う火砕噴火

火山噴出物にはしばしば破片状の結晶が含まれる（例の産物に多く含まれるが（安井・菅沼，2003），溶岩にもえば Ross and Smith, 1961）．破片状結晶は，降下軽石堆

様々な含有量で含まれることがわかってきた（安井，＊〒156-8550 東京都世田谷区桜上水 3-25-40
日本大学文理学部地球システム科学科
Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University, 3-25-40, Sakurajosui,

Setagaya-ku, Tokyo 156-8550, Japan
Corresponding author: Maya Yasui
e-mail: yasui.maya@nihon-u.ac.jp