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loess
loess Loess is a terrestrial sediment formed by the accumulation of wind-blown dust. It is mainly composed of silt-size particles, most being in the size range 0.01–0.06 mm. Loess has been accumulating on the land surface for several million years, but accumulation increased notably at the beginning of the Quaternary period (i. e. since about 2.6 million years ago), the greatest accumulation rates coinciding with the extremely cold and arid conditions during the glaciations. Loess is found in many parts of the globe, but the thickest and most extensive cover occurs in northern China, middle Asia (Tajikistan, Uzbekistan, Kazakhstan), central and western Europe, central North America, the pampa and sub-Andean basins of South America, and New Zealand. The thickness of the loess and its average grain size decrease with distance from the source of the dust. Both display a close relationship to prevailing wind directions, reflecting both present and former atmospheric circulation patterns.
The mineralogy of loess is rather simple: quartz, often reaching 60 per cent by weight, is the principal mineral in most deposits, with feldspars and micas as important ancillary minerals. Also commonly present is calcium carbonate, which, because of its solubility, varies considerably with variations in climate; for example, 16–30 per cent by weight in the humid south-eastern parts of the North China loess belt, falling to as little as 7 per cent on the margins of the sand deserts to the west. The mineralogy of loess commonly differs considerably from that of the underlying bedrock, further demonstrating its derivation from a distant source. As an air-fall deposit, loess is massive in structure, any evidence of bedding, and the inclusion of coarser grains such as pebbles, being indicative of post-depositional reworking. Such reworked loess is referred to as a loess-like or loessic deposit; for example, loessic alluvium (when reworked by water). An additional characteristic of loess, also arising from its wind-lain origin, is its low bulk density (high porosity). The generally low moisture contents in loess in the continental interiors keep any intergranular water at high tensions. This, together with the cementing action of the scattered calcium carbonate, is a source of some strength and characteristic sub-vertical joint systems so that, despite its low mechanical strength, loess commonly maintains steep, cliff-like slopes. For many centuries, the Chinese have taken advantage of such ‘dry-strength’ by excavating cave houses that are warm in winter and cool in summer. These same properties make loess a friable and so a readily cultivated soil capable of high crop yields: in fact, loess is a major component of the soils of the world's ‘breadbasket’ regions. At the same time, the characteristic high porosity and weak cementation render loess subject to instantaneous collapse (hydrocollapsibility) when groundwater conditions approach saturation. Large-scale landslides (specifically flowslides) and mass flowage of loess are frequently triggered by heavy, seasonal rainstorms (including the summer monsoonal rains) in the drylands of Asia and elsewhere. This gives rise to some of the highest known rates of soil erosion (around 20 000 tonnes km−2 year−1 in parts of the Loess Plateau of north-central China). Essentially dry loess may also suffer collapse as a result of earthquake shock, a common phenomenon in central and eastern Asia. Such failures are a major geological hazard with a costly annual toll in lives, livelihood, and infrastructure, as well as in delivering large pulses of sediments into some of the world's greatest rivers and so directly affecting their flow regimes and flood behaviour. Edward Derbyshire Bibliography Pye, K. (1987) Aeolian dust and dust deposits. Academic Press, London. |
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PAUL HANCOCK and BRIAN J. SKINNER. "loess." The Oxford Companion to the Earth. 2000. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. PAUL HANCOCK and BRIAN J. SKINNER. "loess." The Oxford Companion to the Earth. 2000. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O112-loess.html PAUL HANCOCK and BRIAN J. SKINNER. "loess." The Oxford Companion to the Earth. 2000. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O112-loess.html |
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loess
lo·ess / les; ləs; ˈlōˌes/ • n. Geol. a loosely compacted yellowish-gray deposit of windblown sediment of which extensive deposits occur, e.g., in eastern China and the American Midwest. DERIVATIVES: lo·ess·i·al / ˈlesēəl; ˈlə-; lōˈes-/ adj. lo·ess·ic / ˈlesik; ˈlə-; lōˈes-/ adj. |
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"loess." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. "loess." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O999-loess.html "loess." The Oxford Pocket Dictionary of Current English. 2009. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-loess.html |
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loess
loess Unconsolidated, wind-deposited sediment composed largely of silt-sized quartz particles (0.015–0.05 mm diameter) and showing little or no stratification. It occurs widely in the central USA, northern Europe, Russia, China, and Argentina. It can give rise to a rugged topography with steep slopes (up to 70°). The soils derived from loess are of a very high quality and support excellent crop yields.
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MICHAEL ALLABY. "loess." A Dictionary of Ecology. 2004. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "loess." A Dictionary of Ecology. 2004. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O14-loess.html MICHAEL ALLABY. "loess." A Dictionary of Ecology. 2004. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O14-loess.html |
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loess
loess Unconsolidated, wind-deposited sediment composed largely of silt-sized quartz particles (0.015–0.05 mm diameter) and showing little or no stratification. It occurs widely in the central USA, northern Europe, Russia, China, and Argentina. It can give rise to a rugged topography with steep slopes (up to 70°). The soils derived from loess are of a very high quality and support excellent crop yields.
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MICHAEL ALLABY. "loess." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "loess." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O7-loess.html MICHAEL ALLABY. "loess." A Dictionary of Plant Sciences. 1998. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O7-loess.html |
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loess
loess Fine-grained clay, a sedimentary material made up of rock fragments. It is earthy, porous and crumbly, and usually yellowish or brown in colour. Loess consists of mainly quartz and calcite from glaciated areas, blown by the wind and often built up into thick layers. The largest expanse is in the Huang He valley, n China.
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"loess." World Encyclopedia. 2005. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. "loess." World Encyclopedia. 2005. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O142-loess.html "loess." World Encyclopedia. 2005. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-loess.html |
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loess
loess Unconsolidated, wind-deposited sediment composed largely of silt-sized quartz particles (0.015–0.05 mm diameter) and showing little or no stratification. It occurs widely in the central USA, northern Europe, Russia, China, and Argentina. It can give rise to a rugged topography with steep slopes (up to 70°).
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Cite this article
AILSA ALLABY and MICHAEL ALLABY. "loess." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "loess." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O13-loess.html AILSA ALLABY and MICHAEL ALLABY. "loess." A Dictionary of Earth Sciences. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-loess.html |
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loess
loess (geol.) deposit of loam. XIX. — G. löss, f. Swiss G. lösch ‘loose’.
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T. F. HOAD. "loess." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. T. F. HOAD. "loess." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O27-loess.html T. F. HOAD. "loess." The Concise Oxford Dictionary of English Etymology. 1996. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O27-loess.html |
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loess
loess •abyss, amiss, bis, bliss, Chris, Diss, hiss, kiss, Majlis, miss, piss, reminisce, sis, Swiss, this, vis
•dais
•Powys, prowess
•loess, Lois
•Lewes, lewis
•abbess • ibis
•Anubis, pubis
•cannabis • arabis • duchess • purchase
•caddis, Gladys
•Candice
•Sardis, Tardis
•vendace • Charybdis
•bodice, goddess
•demigoddess • Aldiss • jaundice
•de profundis • prejudice • hendiadys
•cowardice • stewardess • preface
•Memphis • aphis • edifice • benefice
•orifice • artifice • office
•surface, surface-to-surface
•undersurface • haggis • aegis
•burgess
•clerkess, Theodorákis
•Colchis
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"loess." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. "loess." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O233-loess.html "loess." Oxford Dictionary of Rhymes. 2007. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O233-loess.html |
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