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Palsas are low, often oval, frost heaves occurring in polar and subpolar climates, which contain permanently frozen ice lenses. Like pingos, palsas consist of an ice core with overlying soil, but they are generally smaller than pingos, often occur in groups and may develop from ground water without additional hydrostatic pressure. Palsas are characteristically found in areas with discontinuous permafrost, and in such areas they may be the only reliable surface evidence of permafrost. Palsas need large quantities of water for the formation of their ice lenses, and for this reason they occur particularly in bogs. ==Genesis== Palsas may be initiated in areas of a moor or bog where the winter freezing front penetrates relatively faster than surrounding areas, perhaps due to an unusually thin cover of snow. The lack of thermal insulation provided by thick snow permits much deeper freezing in winter. This ice may then last through the summer with a persistent 'bump' of up to several cm due to frost heave. The elevated surface of a palsa will tend also to have thinner snow cover, allowing greater winter cooling, while in summer the surface material (especially if organic) will dry out and provide thermal insulation. Thus the interior temperature is consistently lower than that of adjacent ground. This contributes to the formation of an ice lens which grows by drawing up surrounding water. The expansion of the ice upon freezing exerts pressure on the surrounding soil, further forcing water out of its pore spaces which then accumulates on and increases the volume of the growing ice lens. A positive feedback loop develops. Changes in surface moisture and vegetation will then be such as to preserve the newly formed permafrost. The overlying soil layer is gradually lifted up by frost heaving.〔 〕 In cross-section, the ice cores of a palsa show layering, which is caused by the successive winter freezing intervals. The pressing out of water from the pores is not crucial, however, since the boggy soil is water-saturated and thus always provides enough water for ice core growth. Palsas appear to go through a developmental cycle that eventually leads to thawing and collapse. Open cracks that commonly accompany palsa growth and the water that tends to accumulate around palsas, probably as a result of their weight depressing the adjacent bog surface, are important factors in this process. The fact that palsas in various stages of growth and decay occur together shows that their collapse is not necessarily indicative of climatic change. All that is usually left after a palsa collapses is a depression surrounded by a rim. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Palsa」の詳細全文を読む スポンサード リンク
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