十勝岳1988〜89年本質噴出物とこれに伴うガラス質岩の岩石学 : 特集: 十勝岳1988〜1989年噴火

概要

論文の詳細を見る
The 1988-89 explosive eruption of Tokachi-dake volcano brought about ash-and block-fall, pyroclastic surge, and small-scale pyroclastic flow deposit. Petrographical, mineralogical, and petrochemical studies were made on the essential ejecta of olivine-bearing orthopyroxene-clinopyroxene mafic andesite and asscciated glassy rocks of peculiar origin. The mafic andesites (SiO_2=52-53 wt.%) have modal, mineralogical, petrochemical, and Sr isotope compositions similar to those of the 1962 mafic andesites of Tokachi-dake. However, the 1988-89 mafic andesites occur mainly as angular fragments which are less porous and possess more crystalline groundmasses than the 1962 mafic andesites. These observations suggest that the viscosity of the 1988-89 magma was higher than that of the 1962 magma. Mineral chemistries of the 1988-89 and 1962 mafic andesites display episodes of magma mixing. The glassy rocks occur as discrete fragments and are also found in ejected blocks of vent breccia. The glassy rocks are composed of deep brown glass, pale brown-colorless glass and dust, showing eutaxitic texture. The rocks also carry preserved clay and andesite shards. Areas which are gray frequently contain high-silica (SiO_2=94%) pseudomorph after plagioclase displaying optical isotropy. The area-perimeter relationship for the deep brown glass is fractal, indicating the glass movement turned to be in a turbulent flow condition. Bulk chemistries of the glassy rocks have a wide compositional range (SiO_2=61-7 9wt.%) and are similar to stoneware clay being extremely depleted in CaO and Na_2O and enriched in Al_2O_3. The Sr isotopic data of the glassy rocks are similar to those of the associated andesites from Tokachi-dake volcano. Variations in Fe_2O_3-SiO_2-Al_2O_3 compositions indicate that the glassy rocks are derived from an andesite suffered from alteration under strongly or weakly acidic condition. It is considered that the glassy rocks were derived from the melting or softening of an altered andesite around the vent. The above evidence suggests that low fluidity of the ascending mafic andesite and glassy product with a turbulent flow mode may have brought about the explosive nature of the 1988-89 eruption.
特定非営利活動法人日本火山学会の論文
1990-07-02

十勝岳1988〜89年本質噴出物とこれに伴うガラス質岩の岩石学 : 特集: 十勝岳1988〜1989年噴火

スポンサーリンク

概要

著者

関連論文

スポンサーリンク