loess
-
What is loess?
Loess is a buff or yellowish-brown unstratified sedimentary deposit of silty or loamy material, mainly deposited by wind. The word loess, originating from the German term for "loose," was first used to describe Rhine River valley loess around 1821.
Composed mainly of silt-sized grains loosely cemented by calcium carbonate, loess is typically homogenous, highly porous, and marked by vertical capillaries that cause the sediment to fracture and form vertical bluffs. Fertile soil usually covers loess, making it suitable for intensive agriculture. Loess can maintain vertical walls, which is evident on the Loess Plateau in China, where loess bluffs can reach 150 meters in height.
Loess covers approximately 10% of the Earth's land surface and is widespread in the temperate and semiarid zones of Asia, Europe, and North America.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does loess form?
Loess, a sedimentary deposit, primarily forms through wind deposition of silt-sized grains, often originating from weathering in semiarid and arid regions. Mineral constituents like quartz and feldspar are reduced to minute particles, which the wind then transports and deposits. Glacial activity can also contribute silt, with ice sheets grinding rock into fine sediment that winds carry away.
The accumulated dust undergoes diagenesis, involving the weathering of fine grains and cementation by calcium carbonate and iron oxides, essentially turning the sediment into loosely cemented siltstone. This process is akin to soil formation, with optimum conditions occurring in cold steppes and wooded areas bordering continental deserts.
1 Britannica SourceCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What does loess look like and what is it made of?
Loess is an unstratified sedimentary deposit consisting mainly of silt-sized grains loosely cemented by calcium carbonate. It generally has a buff to yellowish-brown color.
Loess consists of roughly 50% silt-sized grains (0.02-0.05 mm), 5-10% clay-sized particles (less than 0.005 mm), 60-70% quartz, 10-20% feldspars and micas, and 5-35% carbonates. Heavy minerals such as amphiboles, apatite, and zircon make up about 2-5%.
It is homogeneous, highly porous, and traversed by vertical capillaries that allow the sediment to fracture and form vertical bluffs. Fertile soil usually covers the surface, which is conducive to intensive agriculture.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Where in the world can large deposits of loess be found?
Large deposits of loess can be found in various locations around the world, typically between latitudes 55° and 24° N. These areas include China, particularly on the banks of the Huang He, and the margins of the continental deserts of Inner Asia. Loess deposits are also significant in Central Asia, such as in Kazakhstan and Uzbekistan, as well as in Siberia along Lake Baikal and the Lena River.
In Europe, extensive loess covers exist in the South Russian Plain, the Danube Basin, along the Rhine, and in the Paris Basin. North America features loess plains along the Platte, Missouri, Mississippi, and Ohio rivers, as well as on the Columbia Plateau. In the Southern Hemisphere, notable loess regions lie between latitudes 30° and 40° S, including the "pampas loesses" of Uruguay and Argentina, and parts of New Zealand.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why is loess important for farming and soil fertility?
Loess is vital for farming and soil fertility because it forms a fertile soil cover ideal for intensive agriculture. Its silty or loamy makeup, coupled with a loose cementation of calcium carbonate, creates a porous structure that retains moisture effectively. The mineralogical composition of loess includes essential elements like quartz, feldspars, and carbonates, contributing to its fertility. Additionally, the vertical capillaries in loess facilitate the sediment to fracture, which is conducive to root development and overall soil health.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does loess affect landscapes and human activities?
Loess, a wind-deposited sediment of silt-sized grains, significantly shapes landscapes and influences human activities. Its ability to form vertical bluffs is evident on China's Loess Plateau, where inhabitants have carved dwellings into the high loess walls.
Loess typically has a fertile soil surface that is conducive to intensive agriculture. However, when soaked, loess is prone to collapse and slumping, leading to the formation of depressions and ravines, which can be accelerated by gully erosion. People have also used loess-based adobe to make houses and fortresslike closed edifices.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
loess, an unstratified, geologically recent deposit of silty or loamy material that is usually buff or yellowish brown in colour and is chiefly deposited by the wind. Loess is a sedimentary deposit composed largely of silt-size grains that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs. The word loess, with connotations of origin by wind-deposited accumulation, is of German origin and means “loose.” It was first applied to Rhine River valley loess about 1821.
Thick loess blankets are composed of loess packets 1 to 5 m (about 3 to 16.5 feet) thick, each of which contains intercalated strata of loessial and loesslike sediments, paleosols (ancient soils), sand layers, and similar material. The totality of these constitutes the loess complex. There are several regional variants of loess that, together with true loess, constitute a loess series, including loessial sand, sandy loess, loess loam, and clayey loess. The individual elements of the loess series are hard to distinguish, and the several sediment types composing it are interpreted differently by workers in different regions or countries.
Loess covers extensive areas in Asia, Europe, and North America. Most widespread in today’s temperate zones and in the marginal semiarid zones of the deserts, loess covers about 10 percent of the land surface of the Earth. Loess usually exhibits a surface cover of fertile soil that is conducive to intensive agriculture. The capacity of loess to retain vertical or even overhanging walls is especially evident on the Loess Plateau in China, where some loess bluffs stand 150 m high and contain innumerable cellarlike dwellings excavated by the local inhabitants. In semiarid regions people such as the Pueblo Indians made houses and fortresslike closed edifices from loess-based adobe.
" height="20" alt="HEW Logo">
Physical and chemical properties.
The dominant grain-size fraction of loess, called the loess fraction, ranges from 0.02 to 0.05 mm (0.0008 to 0.002 inch) and includes grains of coarse and medium-grained dust. Grain-size analyses by various methods indicate that the abundance of this fraction is about 50 weight percent. Clay-sized particles (less than 0.005 mm [0.0002 inch]) make up another 5 to 10 percent. In some loess regions, the grain-size distribution shifts toward finer grains with increasing distance from the source of dust (e.g., eastward from Sand Hills, Neb.).
Loess typically exhibits a low moisture content of 10 to 15 percent that increases as porosity decreases. Its porosity is 50 to 55 percent, decreasing slightly downward to a depth of about 10 m (33 feet). Below this depth, porosity varies as a function of the grain-size distribution. If the loess is enriched with clay, then the porosity may decrease to 34 to 45 percent. The porosity of sandy loess is about 60 percent.
Loess contains 60 to 70 percent quartz with extremes of 40 and 80 percent. Feldspars and micas make up 10 to 20 percent and carbonates 5 to 35 percent. About 2 to 5 percent of the silt is composed of such heavy minerals as amphiboles, apatite, biotite, chlorite, disthene (cyanite), epidote, garnet, glauconite, pyroxenes, rutile, sillimanite, staurolite, tourmaline, and zircon. Grains are typically slightly weathered. In the finest grain-size fractions (below 0.002 mm [0.00008 inch]), such clay minerals as montmorillonite, illite, and kaolinite predominate over the detrital (fragmental) constituents. Clay minerals may be formed by various processes during and after the accumulation of loess.
The mineralogical composition of loess is fairly uniform, but there are some local deviations due to differences in grain size and area of origin. The area of origin of the dust fraction is revealed by the heavy mineral assemblage, and research has shown that the dust sources may be local, neighbouring, or distant.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
The chemical composition of loess most often falls within the following percentage ranges: silica, SiO2, 50 to 60; alumina, Al2O3, 8 to 12; iron oxide as Fe2O3, 2 to 4; iron oxide as FeO, 0.8 to 1.1; titanium dioxide, TiO2, and manganese oxide, MnO, about 0.5; lime, CaO, 4 to 16; and magnesium oxide, MgO, 2 to 6.
The characteristic carbonate content of loess depends on the nature of the dust source, on geochemical and biological processes that occur during and after deposition, and on precipitation and leaching by groundwater. Carbonates are present in loess in a variety of forms, primarily as incrustations on quartz grains and clay-particle aggregates and as small granules and shell fragments. Secondary concentrations include concretions of nodules (Loess-doll) and layers of lime accumulation (caliche). Lime forms frequent tubular incrustations along decayed plant roots, fissure fillings, and similar avenues in loess.
Loess is a rather ill-consolidated sediment of low compressive strength. It is stable, however, as long as it remains dry. Parting surfaces are vertical because capillary incrustations of lime, developed around the roots of a grassy plant cover, lend a vertical texture to loess.
Soaked and loaded loess, however, is liable to collapse and slumping. Wetting decreases cohesion between grains by two-thirds, and the angle of internal friction also decreases (e.g., from 32° to 20°). Groundwater flow in loess will carry away fine, insoluble mineral particles, and this mechanical separation in loess can be accompanied by solution of mineral particles. This process gives rise to depressions, sinkholes, loess wells, and collapse ravines and is much accelerated by gully erosion.