大雪火山群の構造土

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  • THE PATTERNED GROUNDS ON THE DAISETSU VOLCANIC GROUP, CENTRAL HOKKAIDÔ
  • オオユキ カザングン ノ コウゾウド

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The Daisetsu volcanic group has been formed during the period from early Pleistocene up to Holocene. No records are known of their activities in historic time, though there are fumaroles still emitting sulphurous gases at the bottom of two craters. In the area surveyed, timber line and the limit of pinus pumila are found at the height of 1, 200-1, 800 m. and of 1, 650-2, 100 m. above sea level respectively. They are higher on lee-ward slopes than on wind-ward slopes. Above the limit of pinus pumila, there develop patterned grounds in alpine meadows, fell-fields and barren-fields (Fig. 1).<br> The writer has obsrved patterned grounds at 123 localities in the area from 1957 to 1964, and the results of his investigations are summarized as follows:<br> (1) Among the observed patterned grounds, more than 80% of them develop on wind-ward crest-slopes, about 15% in depressions such as craters, nivation hollows and others, and less than 5 % on flat surfaces. Their lower limits coincide with the limit of pinus pumila at respective localities.<br> (2) Nonsorted patterns are associated with some specific vegetations (Figs. 3 and 6), as physical conditions (thickness of snow cover, micro-climate, soil, groundwater, etc.) control the kind of vegetation and the development of patterns.<br> (3) Processes of the formation of nonsorted polygons or circles are considered to be as follows: (a) During freezing periods, polygonal meshes are at first formed by the net-work of frost cracks due to the contraction of freezing ground surface, then the surface of each mesh is differentially heaved by a growing frost layer in the ground so as to swell up a flat dome. (b) Vegetation takes root first along the margin of earch mesh adjacent to the crack that has suffered least heaving. (c) The fringing vegetation invades gradually from periphery toward the center of each mesh. It is at this stage that frost boils or frost scars are recognizable in polygonal meshes (Fig. 7). (d) As the contraction of freezing ground and the differential heaving of meshed surfaces continue rather rapidly in a short period and contrarily frozen ground thaws very slowly, the ground now thatched with vegetation having intermingled roots is deformed only in the freezing period, thus the cracks widen unilaterally and the swelling up of meshed surfaces is non-reversibly intensified. (e) As these processes progress, the polygonal meshes (nonsorted polygons) transform themselves into frost hummocks (nonsorted nets or circles) entirely covered with dense vegetation. At the same time, the frost cracks grow up as “intervals” between hummocks. The nonsorted patterns as mentioned above develop only on the ground of fine textured soils with little contents of pebbles and gravels.<br> (4) Sorted patterns on gravelly barren fields are formed through the processes as described below. (a) The pebbles and gravels in the soil are brought up to the surface by frost heaving, while the formation of frost cracks and the swelling up of meshed ground surface advance as mentioned above. (b) The pebbles and gravels on the ground surface creep to and fall into frost cracks, and the initial pattern of the sorted polygons and nets, etc., is formed. (c) As thawing of the frozen and already patternrd ground progresses from three sides: that is, from the surface, from the bottom of frozen layer and from the marginal cracks, a differential thaw-down of the meshed ground surface occurs. This causes an increase in the gradient from center to margin, and facilitates the movement of gravels toward the cracks. (d) The cracks widen their spans (as mentioned in 3 a) and are filled with pebbles and gravels by repetition of these processes, and finally well-developed sorted patterns.<br> (5) In thawing season, a slow flowage of water-saturated top soil occurs on the slopes which makes a convex front at its terminal (comparable to the front of a lava flow).

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  • 地理学評論

    地理学評論 38 (3), 179-199, 1965

    公益社団法人 日本地理学会

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