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Finger rafting
Finger rafting develops in an ice cover as a result of a compression regime established within the plane of the ice. As two expanses of sea ice converge toward another, one of them slides smoothly on top of the other (it is overthrusted) along a given distance, resulting in a local increase in ice thickness. The term ''finger rafting'' refers to the systematic alternation of interlocking overthrusts and underthrusts involved in this process.〔(Wmo Sea-Ice Nomenclature • Terminology )〕〔(Environment Canada - Weather and Meteorology - Ice Glossary )〕〔(All About Sea Ice, Introduction :: National Snow and Ice Data Center )〕 Such a pattern derives its name from its resemblance to the interlocking of fingers.〔Whereby the first finger in one hand ''overlies'' the same finger in the other hand, the second finger in one hand ''underlies'' the second finger in the other hand, the third fingers do as the first fingers, etc.〕
== The process ==
''Rafting'', also called ''telescoped ice'',〔 is most noticeable when it involves new and young ice, but also occurs in ice of all thicknesses.〔Weeks, W. F. (2010) ''On sea ice'' (Chap. 12). University of Alaska Press, Fairbanks, 664 p.〕 The process of finger rafting as such is commonly observed inside a lead, once a thin layer of ice (at the nilas stage) has formed. Although this ice is typically very weak (it is unable to support its own weight outside the water), it contains a lot of brine and is also relatively ''warm'', since being that thin, its temperature is near that of the water. Rafting is accompanied with rapid draining of the brine inside the overlying ice sheet. This brine acts as a lubricant, significantly reducing the friction between the two sheets during overthrusting. Such a mechanism, and the fact that the upper surface of nilas is already slippery, account for overthrust distances in excess of (a length-to-thickness ratio of 1000 to one).〔〔According to this source, finger rafting is also possible in lakes and rivers even though this ice is non saline, but only if it is very thin, i.e. less than .〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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