Defining Loess
Loess refers to an aeolian landform that originates from the sedimentary deposition of silty and loamy substances. Some notable instances of this landform include:
- The banks of the Yellow River in China
- The peripheries of Inner Asia's continental deserts
- The extensive Great Plains in North America
- The Snake River Plain and the Palouse region in the United States' Pacific Northwest
- The eastern regions of the Caspian Sea
- Large expanses of central Asia, including the Tien Shan peninsula, Kazakhstan, and Uzbekistan
- The middle region between the Lena River and Lake Baikal
- The Pampas region of Argentina
- The southern catchment areas of the Yenisey and Ob rivers
- Parts of Ukraine in Europe
- The Danube Basin of the Rhine River
- The Paris Basin
- The Canterbury Plains
Characteristics of Loess
- Loess is a homogeneous, pale or light yellowish formation with high porosity and friability. It is typically non-stratified and composed of clay, silt, and sand, but it can also be calcareous.
- In terms of the geological timescale, Loess is a relatively recent formation compared to other sedimentary bodies.
- Most Loess dunes align with wind flows from the last ice age.
- These formations, known as "greda ridges" in Europe and "paha ridges" in America, are believed to be the result of wind and tundra climate interactions.
Physical Properties of Loess
- Grain-size Fraction: Medium to coarse grain with a fraction ranging from 0.02 mm to 0.05 mm and clay particles with a fraction below 0.005 mm.
- Composition: Comprises 50% grain particles and 5% to 10% clay.
- Porosity: Ranges from 50% to 55% (or 34% to 45% for clay-rich loess).
- Moisture Retention: Exhibits low porosity of 10% to 15%, which may increase as porosity decreases.
Chemical Properties of Loess
- Silica: 50% to 60%
- Alumina: 8% to 12%
- Iron oxide (Fe2O3): 2% to 4%
- Iron oxide (FeO): 0.8% to 1.1%
- Manganese and Titanium dioxide: 0.5%
- Lime: 4% to 16%
- Magnesium oxide: 2% to 6%
Mineral Composition of Loess
- Quartz: 60% to 70%, but can vary between 40% and 80%
- Micas and Feldspars: 10% to 20%
- Carbonates: 5% to 35%
- Silt: 2% to 5%
Formation of Loess
According to Pye, the formation of Loess requires four fundamental factors:
- A source of dust particles
- Adequate arid power for weathering and transportation of mass
- An appropriate area of deposition
- Time
These factors result in two types of formations: periglacial and non-glacial.
Periglacial Loess
Periglacial loess forms from the accumulation of debris on the floodplains of glacial braided rivers. These rivers often deposit a vast volume of sediments carried by meltwater from mountain ice caps and continental glaciers.
During autumn and winter, the flow of water in these rivers decreases due to a pause in glacier melting. This exposes large areas of the floodplains to wind activities, which picks up the silt and clay from the plain and deposits them in the direction of the wind.
An excellent example of such a landmass can be found on the banks of the Mississippi River Alluvial Valley.
Non-glacial Loess
Non-glacial Loess typically originates from the sedimentary accumulation of sand from deserts, volcanic ash, playa lakes, and dune fields. Strong wind forces transport weathered particles from vast arid and semi-arid regions exposed to aeolian attrition.
Subtypes of this landform include desert, tropical, volcanic, trade wind, gypsum, and anticyclonic Loess. Volcanic variations can be found in Argentina and Ecuador, gypsum in Spain, and tropical in Brazil, Argentina, and Uruguay.
Furthermore, the formation of this landscape is quite common due to natural denudation processes. Wind, being a potent weathering agent, erodes and transports dust particles from arid areas to moisture-laden regions. Here, the moisture and natural vegetation accumulate the particles, forming loess.
This comprehensive guide on loess for the UPSC exam should prove beneficial for aspirants. It is also recommended that they refer to more UPSC Notes on this topic to further enrich their understanding.
