Structural soil

Microtopography is a symmetrical, geometric pattern found on the earth's surface. It is classified into circular, polygonal (tortoiseshell), rectangular, net, striped, and stepped soils based on the planar shape, and into gravelly (selected) and unselected soils based on the composition of the material. Most are periglacial landforms, formed by the movement of soil material due to repeated freezing and thawing, and by the formation of cracks in the surface due to freezing. Large polygonal soils with widths of tens of meters are formed in permafrost zones. In Japan, they are found in alpine zones above the tree line, and many of the structured soils that are still forming today are several centimeters to around one meter in diameter. In arid regions, structured soils called gilgai are formed by the shrinkage and expansion of soil due to repeated drying and wetting.

[Yugo Ono]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

地表面にみられる対称的、幾何学的な模様の微地形。平面形態によって円形土、多角形土（ポリゴンpolygon―亀甲土(きっこうど)）、方形土、網状土、条線土、階状土に区分され、構成物質によって礫(れき)質（淘汰(とうた)）と土質（不淘汰）とに分けられる。大部分は周氷河地形であり、凍結・融解の繰り返しによる土壌物質の移動や、凍結による表層部での割れ目の形成によって生じる。永久凍土帯では幅が数十メートルにも及ぶ大規模な多角形土がつくられる。日本では森林限界以上の高山帯にみられ、現在でも形成中の構造土は直径数センチメートルから1メートル内外のものが多い。乾燥地域では、乾燥や湿潤の繰り返しに基づく土壌の収縮や膨張によって、ギルガイgilgaiとよばれる構造土がつくられる。

［小野有五］

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