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→Rafting versus ridging: Explanations added
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{{short description|Compression overlapping of floating ice cover in alternating overthrusts and underthrusts}} |
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[[File:Nilas sea ice 2.jpg|thumb|400px|Aerial photograph showing two thin ice sheets (made of [[Sea ice|nilas]]) that are moving toward each other. Along the length of the contact between the two sheets, segments where one sheet climbs onto the other alternate with others where it is the other way around. The lighter areas are where the ice thickness has doubled due to the overlapping process. This pattern is known as ''finger rafting''. In many cases, it is highly systematic.]] |
[[File:Nilas sea ice 2.jpg|thumb|400px|Aerial photograph showing two thin ice sheets (made of [[Sea ice|nilas]]) that are moving toward each other. Along the length of the contact between the two sheets, segments where one sheet climbs onto the other alternate with others where it is the other way around. The lighter areas are where the ice thickness has doubled due to the overlapping process. This pattern is known as ''finger rafting''. In many cases, it is highly systematic.]] |
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[[File:Finger rafting Weddell Sea.jpeg|thumb|400px|Finger rafting in the [[Weddell Sea]], [[Operation IceBridge]] photo, 2017.]] |
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| ⚫ | [[File:Drawing of rafted ice.svg|thumb|Idealized three-dimensional representation of finger rafting, It occurs when two thin ice sheets converge toward each other.]] |
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[[File:Finger rafting (sea ice) with real hands.JPG|thumb|Finger rafting derives its name from its resemblance to the interlocking of fingers. Note that the fingers shown here are not interlaced normally (with all ten fingers showing from the top), but interlaced as in finger rafting (with only five fingers visible from either the top or bottom). <ref group="note" name="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.</ref>]] |
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[[File: |
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[[File:Rafted sea ice.jpg|thumb|Rafted ice observed during [[MOSAiC Expedition]] in June 2020.]] |
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[[File:Finger rafting (sea ice) with real hands.JPG|thumb|400px|Finger rafting derives its name from its resemblance to the interlocking of fingers.]] |
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'''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.<ref name="WMO">http://www.aari.nw.ru/gdsidb/XML/volume1.php?lang1=0&lang2=1&arrange=1</ref><ref name="EC Glossary">http://www.ec.gc.ca/glaces-ice/default.asp?lang=En&n=501D72C1-1</ref><ref name="NSIDC">http://nsidc.org/cryosphere/seaice/index.html |
'''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.<ref name="WMO">[http://www.aari.nw.ru/gdsidb/XML/volume1.php?lang1=0&lang2=1&arrange=1 Wmo Sea-Ice Nomenclature • Terminology<!-- Bot generated title -->]</ref><ref name="EC Glossary">[http://www.ec.gc.ca/glaces-ice/default.asp?lang=En&n=501D72C1-1 Environment Canada – Weather and Meteorology – Ice Glossary<!-- Bot generated title -->]</ref><ref name="NSIDC">[http://nsidc.org/cryosphere/seaice/index.html All About Sea Ice, Introduction :: National Snow and Ice Data Center<!-- Bot generated title -->]</ref> Such a pattern derives its name from its resemblance to the interlocking of fingers.<ref group="note" name="fingers"/> |
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== The process == |
== The process == |
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''Rafting'', also called ''telescoped ice'',<ref name="NSIDC"/> is most noticeable when it involves |
''Rafting'', also called ''telescoped ice'',<ref name="NSIDC"/> is most noticeable when it involves new and young ice, but also occurs in ice of all thicknesses.<ref name="Weeks">Weeks, W. F. (2010) ''On sea ice'' (Chap. 12). University of Alaska Press, Fairbanks, 664 p.</ref> The process of finger rafting as such is commonly observed inside a [[Lead (sea ice)|lead]], once a thin layer of ice (at the [[Sea ice#New ice, nilas and young ice|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 [[Sea ice growth processes|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 [[Sea ice|nilas]] is already slippery, account for overthrust distances in excess of {{convert|100|m|ft}} (a length-to-thickness ratio of 1000 to one).<ref name="Weeks"/><ref group="note">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 {{convert|2|cm|in}}.</ref> |
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== Rafting versus ridging == |
== Rafting versus ridging == |
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Rafting and ridging are two possible responses expected from the interaction between two converging ice sheets or floes.<ref name="Weeks"/><ref name="Lepp">Leppäranta, M. (2005). The Drift of Sea Ice. Springer-Verlag, New York, 266 p.</ref> The term |
Rafting and [[pressure ridge (ice)|ridging]] are two possible responses expected from the interaction between two converging ice sheets or floes.<ref name="Weeks"/><ref name="Lepp">Leppäranta, M. (2005). The Drift of Sea Ice. Springer-Verlag, New York, 266 p.</ref> The term 'ridging' refers to the process of [[Pressure ridge (ice)|ridge formation]], involving the breaking up of the ice sheet into distinct blocks (which does not happen during rafting).<ref name="WMO"/><ref name="EC Glossary"/><ref name="NSIDC"/> The reason why breaking happens is that, as the ice thickness increases, the bending moment exerted on the upper surface of the ice exceeds its tensile strength.<ref name="Lepp"/> In other words, the ice is no longer flexible enough to withstand the overthrust event without breaking. |
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== Maximum thickness for rafting== |
== Maximum thickness for rafting == |
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A theoretical formula has been used to estimate the maximum thickness an ice sheet can have in order to be able to raft.<ref name="Lepp"/> This thickness (<math>h_{rf}</math>) is |
A theoretical formula has been used to estimate the maximum thickness an ice sheet can have in order to be able to raft.<ref name="Lepp"/> This thickness (<math>h_{rf}</math>) is |
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:<math> |
:<math> |
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h_{rf}=\frac{14.2(1-\nu^2)}{\rho_wg}\frac{\sigma_t^2}{Y} |
h_{rf}=\frac{14.2(1-\nu^2)}{\rho_wg}\frac{\sigma_t^2}{Y} |
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</math> |
</math> |
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where <math>\nu</math> is [[Poisson's ratio]], <math>\sigma_t</math> is the tensile strength of the ice, <math>\rho_w</math> is water [[density]], ''Y'' is the [[Young's modulus]] of [[sea ice]] and ''g'' is the [[gravitational acceleration]]. What this equation shows is that, assuming a representative tensile strength of 0.65 MPa, the maximum thickness is in the range of {{convert|0.2|m|in}}.<ref name="Lepp"/> |
where <math>\nu</math> is [[Poisson's ratio]], <math>\sigma_t</math> is the tensile strength of the ice, <math>\rho_w</math> is water [[density]], ''Y'' is the [[Young's modulus]] of [[sea ice]] and ''g'' is the [[gravitational acceleration]]. What this equation shows is that, assuming a representative tensile strength of 0.65 MPa, the maximum thickness for rafting to occur is in the range of {{convert|0.2|m|in}}.<ref name="Lepp"/> |
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== See also == |
== See also == |
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{{Reflist}} |
{{Reflist}} |
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[[Category:Sea ice]] |
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{{Uncategorized|date=December 2012}} |
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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.[1][2][3] Such a pattern derives its name from its resemblance to the interlocking of fingers.[note 1]
Rafting, also called telescoped ice,[3] is most noticeable when it involves new and young ice, but also occurs in ice of all thicknesses.[4] 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 100 metres (330 ft) (a length-to-thickness ratio of 1000 to one).[4][note 2]
Rafting and ridging are two possible responses expected from the interaction between two converging ice sheets or floes.[4][5] The term 'ridging' refers to the process of ridge formation, involving the breaking up of the ice sheet into distinct blocks (which does not happen during rafting).[1][2][3] The reason why breaking happens is that, as the ice thickness increases, the bending moment exerted on the upper surface of the ice exceeds its tensile strength.[5] In other words, the ice is no longer flexible enough to withstand the overthrust event without breaking.
A theoretical formula has been used to estimate the maximum thickness an ice sheet can have in order to be able to raft.[5] This thickness (
where 
