黒富士火砕流中の炭化埋木と '煙の化石'

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A well-preserved and nearly rooted charred tree trunk with "fossil smoke" was found from the Kuro-fuji pyroclastic-flow deposit (Fig. 1). A flow-unit enclosing the trunk is 120 cm thick and consists of poorly vesiculated essential fragments and volcanic ash. Within the bed, the constituent fragments characteristically decrease their sizes downward (Fig. 3), and at the bottom a seam of volcanic ash covers directly the lower unit. The tree trunk is 7.5 m long and 16 cm across at most and lies nearly flat (Fig. 2). No branches are observed to be left intact. The lower end connects with a block of earth. Below this block of earth is a hole corresponding to it in size and shape, apparently indicating the former position of the root. The "fossil smoke", here called, is a portion of the deposit which is characterized by brownish pigmentation and lack of finer, ashy fraction. It extends from the trunk upwards branching off many vertical pipes which are truncated at the suaface of the flow unit by overlying another unit. The fossil trunk located at 4.5 km NW from the eruption of outflow of the pyroclastic flow, and lies in a direction about 70 degrees deflected from that which joins this locality and the eruption center. The facts mentioned above suggest us that the trunk was pushed down, buried and charred by the pyroclastic flow at the site where it grew. The Infrared absorption analysis was applied to six samples collected from various parts of the trunk and each temperature of charring was estimated (Fig. 4). The outer parts of the trunk show temperatures between 350° and 400℃, whereas the inner part a rather lower temperature close to 350℃. The process of the pyroclastic-flow emplacement may be as follows (Fig. 6) : Although the pyroclastic-flow cloud started to rush down toward NW, the lower part of the cloud was deflected its course to SW along the valley, separated from the higher part which was going straight over the topographic relief. The first blast of the pyroclastic flow passed slashing the branches of the standing tree, and uninterruptedly the main part of the pyroclastic flow reached and pushed over the tree trunk. The trunk was buried in situ as the basal part of the pyroclastic flow was deposited, and was overlain by the upper part of the pyroclastic flow which immediately followed. Though the pyroclastic flow was deposited within a very short span of time, in more detail it was deposited layer by layer from base to top.
特定非営利活動法人日本火山学会の論文
1975-08-01

黒富士火砕流中の炭化埋木と '煙の化石'

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