Flow behavior of large-scale pyroclastic flows — Evidence obtained from petrofabric analysis |
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| Authors: | Tadahide Ui Keiko Suzuki-Kamata Rumi Matsusue Kei Fujita Hideya Metsugi Mami Araki |
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| Institution: | (1) Department of Earth Sciences, Faculty of Science, Kobe University, 1-1, Rokkodai, Nada-ku, 657 Kobe, Japan;(2) Present address: Faculty of Education, Kobe University, 3-11, Tsurukabuto, Nada-ku, 657 Kobe, Japan;(3) Present address: Nomura Securities Co. Ltd., 1-9-1 Nihonbashi, Chuo-ku, 103 Tokyo, Japan;(4) Present address: Metal Mining Agency, 1-24-14, Toranomon, Minato-ku, 105 Tokyo, Japan |
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| Abstract: | The grain orientations within the matrix of two large-scale welded, two small-scale nonwelded and two nonwelded low-aspect ratio pyroclastic flow deposits are measured to analyze flow behavior. Preferred grain alignments are especially apparent in the middle part of layer 2 of each deposit. Preferred grain alignments do not vary laterally within a 10 m interval. The grain alignments obtained are radial from the source caldera, especially in proximal to medial and plateau-forming facies of pyroclastic flow deposits. Grain alignments are controlled by valley-channel directions for the valley-ponded facies of pyroclastic flow deposits, especially at medial to distal locations. Such local topographic factors strongly affect the data for high-aspect ratio and smallscale deposits, and weakly affect the data for widespread low-aspect ratio pyroclastic flow deposits. This work suggests that grain alignment analysis should be used with care when attempting to determine the location of an unknown source. |
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