Akettle (also known as akettle hole,kettlehole,pothole, or, in parts of the United Kingdom,pingo[1][2]) is a depression or hole in anoutwash plain formed by retreatingglaciers or drainingfloodwaters. The kettles are formed as a result of blocks ofdead ice left behind by retreating glaciers, which become surrounded bysediment deposited bymeltwater streams as there is increased friction.[3] The ice becomes buried in the sediment and when the ice melts, a depression is left called a kettle hole, creating a dimpled appearance on the outwash plain.Lakes often fill these kettles; these are called kettle hole lakes. Another source is the sudden drainage of an ice-dammed lake and when the block melts, the hole it leaves behind is a kettle. As the ice melts, ramparts can form around the edge of the kettle hole. The lakes that fill these holes are seldom more than 10 m (33 ft) deep and eventually fill with sediment. In acidic conditions, a kettlebog may form but in alkaline conditions, it will bekettle peatland.[clarification needed]
Kettles arefluvioglacial landforms occurring as the result of blocks ofice calving from the front of a recedingglacier and becoming partially to wholly buried by glacial outwash.Glacial outwash is generated when streams ofmeltwater flow away from the glacier and deposit sediment to form broad outwash plains calledsandurs. When the ice blocks melt, kettle holes are left in the sandur. When the development of numerous kettle holes disrupt sandur surfaces, a jumbled array of ridges and mounds form, resemblingkame and kettle topography.[4] Kettle holes can also occur in ridge shaped deposits of loose rock fragments calledtill.[5]
Kettle holes can form as the result of floods caused by the sudden drainage of an ice-dammed lake. These floods, calledjökulhlaups, often rapidly deposit large quantities of sediment onto the sandur surface. The kettle holes are formed by the melting blocks of sediment-rich ice that were transported and consequently buried by the jökulhlaups. It was found in field observations and laboratory simulations done by Maizels in 1992 thatramparts form around the edge of kettle holes generated by jökulhlaups. The development of distinct types of ramparts depends on the concentration of rock fragments contained in the melted ice block and on how deeply the block was buried by sediment.[6]
Most kettle holes are less than two kilometres in diameter, although some in the U.S. Midwest exceed ten kilometres.Puslinch Lake in Ontario, Canada, is the largest kettle lake in Canada spanning 160 hectares (400 acres). Fish Lake in the north-central Cascade Mountains of the U.S. state of Washington is 200 hectares (490 acres).[7]
The depth of most kettles is less than ten meters.[5] In most cases, kettle holes eventually fill with water, sediment, or vegetation. If the kettle is fed by surface or underground rivers or streams, it becomes akettle lake. If the kettle receives its water fromprecipitation, the groundwater table, or a combination of the two, it is termed akettle pond orkettle wetland, if vegetated. Kettle ponds that are not affected by the groundwater table will usually become dry during the warm summer months, in which case they are deemedephemeral.[8]
If water in a kettle becomesacidic due to decomposingorganic plant matter, it becomes a kettlebog; or, if underlying soils arelime-based andneutralize the acidic conditions somewhat, it becomes a kettlepeatland. Kettle bogs areclosed ecosystems because they have no water source other than precipitation. Acidic kettle bogs and fresh water kettles are important ecological niches for some symbiotic species of flora and fauna.[9]
TheKettle Moraine, a region ofWisconsin covering an area fromGreen Bay to south-central Wisconsin, has numerous kettles,moraines and other glacial features. It has many kettle lakes, some of which are 100 to 200 feet (61 m) deep.
ThePrairie Pothole Region extends from northernAlberta, Canada toIowa, United States and includes thousands of smallsloughs and lakes.
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