著者

横山 勝三 Shozo YOKOYAMA

出版者

日本地形学連合

雑誌

地形 = Transactions, Japanese Geomorphological Union (ISSN:03891755)

巻号頁・発行日

vol.34, no.4, pp.303-311, 2013-10-25

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阿蘇カルデラの外輪山には多数の安山岩質岩塔(トア)が見られる.その多くは,きわめて強く帯磁しており,その強い磁性はクリノメーターを使って容易に捉えられる.この強く帯磁した岩石すなわち磁石石は,落雷の影響で生じたと思われる.これらの磁石石(トア)の上面には風化穴(weathering pit,ナマ)がみられるものがある.一方,落雷の近年の報道には,花崗岩やコンクリートなどの建造物に損傷を与えた事例がある.また,いくつかの空港の滑走路上に落雷で小孔が生じた事例も報告されている.このような落雷の破壊力から判断すると,落雷で岩石表面に孔が生じる可能性はきわめて高い.これらのことから,磁石石上の風化穴は,落雷による最初の小孔(initial depression)の形成から始まったと考えられる.Numerous tors of andesite are found on the somma of Aso caldera, Kyushu, Japan. Many of them are very strongly magnetized as can be detected easily by a geologic compass. Such strongly magnetized rocks, namely lodestones, are presumed to have been formed by the effect of lightning strike. Weathering pits are found on some of the top surfaces of the lodestone tors. According to the recent reports, there are examples of lightning strikes which damaged constructions of granite and concrete. Also, pits on runways formed by lightning strikes have been reported from some airports. Judging from the destructive power of lightning strikes, it is highly probable that pits are formed on rock surfaces by lightning strikes. Thus, it is interpreted, from these, that the weathering pits on the lodestones started from initial depressions formed by lightning strikes.