| 概要 |
In my previous papers, I inferred that the "Shirasu" proper is a deposit formed by a succession of abnormal explosions, perhaps of the nueé ardente or pumice flow type, from the "Aira caldera" (S. TAN...EDA, 1954, 1957, 1971, etc.). I pointed out that the distinction between normal and abnormal type of volcanic activity is reflected usually in the field relationships and mechanical analyses of the erupted matters. In the course of research of the "Shirasu", I measured a, b, c, l and s for ten largest pumice fragments and ten largest lithic fragments per square meter (1m×1m) at each outcrop. Here a, b and c are long diameter (length), intermediate diameter (breadth), and short diameter (thickness) (ZINGG, 1935), respectively. The arithmetic means of each set of ten measurements were calculated, which are called the average of a, b, c, l and s respectively, and marked by a^<av>_P, b^<av>_P, c^<av>_P, l_<P1>, l^<av>_P, s^<av>_P, for pumice fragments and by a^<av>_L, b^<av>_L, c^<av>_L, l_<L1>, l^<av>_L, s^<av>_L for lithic fragments. The l_<P1>, l^<av>_P and l_<L1>, l^<av>_L values are called the apparent maximum size of pumice and lithic fragments. The proportion of pumice and lithic fragments of the "Shirasu" pumice flow deposists (NPF, NLF, NPC and NLC), and median diameter (Md) are also inspected. The areal variation patterns are obtained by the following methods. The southern part of Kyushu was divided into nearly 600 squares by drawing on the map north-south and east-west lines at every 5 km. In each square covered by the pumice flow deposits "Shirasu", one or more (up to several) outcrops of the deposits were selected for the grain-size, median diameter and shape measurements, and for the proportion of pumice and lithic fragments of the "Shirasu". The data obtained are plotted on the map at the outcrop location, and the arithmetic mean of data is calculated for every square. In order to reduce the effect of local irregularities of the data variation, the overlapping means are used. As to the kinds of areal variation patterns, are constructed such six kinds of diagrams as a "Data map", b "Hatching diagram", c "Hatching diagram" using the overlapping means, d "Contour lines", e "Contour lines" using the overlapping means, and f "Ridge and valley lines" using the overlapping means. In this report the areal variations with respect to the distance from their source for maximum grain-size and shape, median diameter and proportion of pumice and lithic fragments of the "Shirasu" pumice flow deposits (l_<P1>, l^<av>_P, l_<L1>, l^<av>_L, l^<av>_P/s^<av>_P, l^<av>_L/s^<av>_L, l^<av>_P/l^<av>_L, Md, NPF. NLF, NPC. NLC) are discussed. Plots of these data seem to give qualitative information on the variables involved in the ejection, transportation and deposition of pyroclastic fragments. The areal variation patterns show the concentric and radial characters suggesting the source locality (the Aira caldera). One of the most significant conclusion is that the increasing of grain-sizes of the "Shirasu" along with the increasing distance is caused by the emission effect of gas included in ejecta, because by the mechanism of the gas-emission hypothesis large blocks should travel farther than small ones. Comparing with the pumice fragments, the lithic fragments begin to settle rather early from the flow according to their grainsize, because of their larger density than that of the matrix.続きを見る
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Mendeley出力
24_02_p109