Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. [1] Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel when dry, and lacks plasticity when wet. Silt can also be felt by the tongue as granular when placed on the front teeth (even when mixed with clay particles).
Silt is a common material, making up 45% of average modern mud. It is found in many river deltas and as wind-deposited accumulations, particularly in central Asia, north China, and North America. It is produced in both very hot climates (through such processes as collisions of quartz grains in dust storms) and very cold climates (through such processes as glacial grinding of quartz grains.)
Loess is soil rich in silt which makes up some of the most fertile agricultural land on Earth. However, silt is very vulnerable to erosion, and it has poor mechanical properties, making construction on silty soil problematic. The failure of the Teton Dam in 1976 has been attributed to the use of unsuitable loess in the dam core, and liquefication of silty soil is a significant earthquake hazard. Windblown and waterborne silt are significant forms of environmental pollution, often exacerbated by poor farming practices.
Silt is detritus (fragments of weathered and eroded rock) with properties intermediate between sand and clay. A more precise definition of silt used by geologists is that it is detrital particles with sizes between 1/256 and 1/16 mm (about 4 to 63 microns). [2] This corresponds to particles between 8 and 4 phi units on the Krumbein phi scale. [3] [4] Other geologists define silt as detrital particles between 2 and 63 microns or 9 to 4 phi units. [5] A third definition is that silt is fine-grained detrital material composed of quartz rather than clay minerals. [6] Since most clay mineral particles are smaller than 2 microns, [7] while most detrital particles between 2 and 63 microns in size are composed of broken quartz grains, there is good agreement between these definitions in practice. [5]
The upper size limit of 1/16 mm or 63 microns corresponds to the smallest particles that can be discerned with the unaided eye. [8] It also corresponds to a Tanner gap in the distribution of particle sizes in sediments: Particles between 120 and 30 microns in size are scarce in most sediments, suggesting that the distinction between sand and silt has physical significance. [5] As noted above, the lower limit of 2 to 4 microns corresponds to the transition from particles that are predominantly broken quartz grains to particles that are predominantly clay mineral particles. [5]
Assallay and coinvestigators further divide silt into three size ranges: C (2–5 microns), which represents post-glacial clays and desert dust; D1 (20–30 microns) representing "traditional" loess; and D2 (60 microns) representing the very coarse North African loess. [5]
Silt can be distinguished from clay in the field by its lack of plasticity or cohesiveness and by its grain size. Silt grains are large enough to give silt a gritty feel, [7] particularly if a sample is placed between the teeth. Clay-size particles feel smooth between the teeth. [9] The proportions of coarse and fine silt in a sediment sample are determined more precisely in the laboratory using the pipette method, which is based on settling rate via Stokes' law and gives the particle size distribution accordingly. [10] The mineral composition of silt particles can be determined with a petrographic microscope for grain sizes as low as 10 microns. [11]
Vadose silt is silt-sized calcite crystals found in pore spaces and vugs in limestone. This is emplaced as sediment is carried through the vadose zone to be deposited in pore space. [12]
ASTM American Standard of Testing Materials: 200 sieve – 0.005 mm.
USDA United States Department of Agriculture 0.05–0.002 mm.
ISSS International Society of Soil Science 0.02–0.002 mm.
Civil engineers in the United States define silt as material made of particles that pass a number 200 sieve (0.074 mm or less) but show little plasticity when wet and little cohesion when air-dried. [13] The International Society of Soil Science (ISSS) defines silt as soil containing 80% or more of particles between 0.002 mm to 0.02 mm in size [14] while the U.S. Department of Agriculture puts the cutoff at 0.05mm. [15] The term silt is also used informally for material containing much sand and clay as well as silt-sized particles, or for mud suspended in water. [8]
Silt is a very common material, and it has been estimated that there are a billion trillion trillion (1033) silt grains worldwide. Silt is abundant in eolian and alluvial deposits, including river deltas, such as the Nile and Niger River deltas. Bangladesh is largely underlain by silt deposits of the Ganges delta. Silt is also abundant in northern China, central Asia, and North America. [5] However, silt is relatively uncommon in the tropical regions of the world. [16]
Silt is commonly found in suspension in river water, and it makes up over 0.2% of river sand. It is abundant in the matrix between the larger sand grains of graywackes. Modern mud has an average silt content of 45%. [17] Silt is often found in mudrock as thin laminae, as clumps, or dispersed throughout the rock. Laminae suggest deposition in a weak current that winnows the silt of clay, while clumps suggest an origin as fecal pellets. Where silt is dispersed throughout the mudrock, it likely was deposited by rapid processes, such as flocculation. [18] Sedimentary rock composed mainly of silt is known as siltstone. [19]
Silt is common throughout the geologic record, but it seems to be particularly common in Quaternary formations. This may be because deposition of silt is favored by the glaciation and arctic conditions characteristic of the Quaternary. [5] Silt is sometimes known as rock flour or glacier meal, especially when produced by glacial action. [20] Silt suspended in water draining from glaciers is sometimes known as rock milk or moonmilk. [21]
A simple explanation for silt formation is that it is a straightforward continuation to a smaller scale of the disintegration of rock into gravel and sand. [22] However, the presence of a Tanner gap between sand and silt (a scarcity of particles with sizes between 30 and 120 microns) suggests that different physical processes produce sand and silt. [23] The mechanisms of silt formation have been studied extensively in the laboratory [24] and compared with field observations. These show that silt formation requires high-energy processes acting over long periods of time, but such processes are present in diverse geologic settings. [5]
Quartz silt grains are usually found to have a platy or bladed shape. [25] This may be characteristic of how larger grains abrade, or reflect the shape of small quartz grains in foliated metamorphic rock, or arise from authigenic growth of quartz grains parallel to bedding in sedimentary rock. [26] Theoretically, particles formed by random fracturing of an isotropic material, such as quartz, naturally tend to be blade-shaped. [27] The size of silt grains produced by abrasion or shattering of larger grains may reflect defects in the crystal structure of the quartz, known as Moss defects. [28] Such defects are produced by tectonic deformation of the parent rock, and also arise from the high-low transition of quartz: Quartz experiences a sharp decrease in volume when it cools below a temperature of about 573 °C (1,063 °F), [29] which creates strain and crystal defects in the quartz grains in a cooling body of granite. [30]
Mechanisms for silt production include: [5]
Laboratory experiments have produced contradictory results regarding the effectiveness of various silt production mechanisms. This may be due to the use of vein or pegmatite quartz in some of the experiments. Both materials form under conditions promoting ideal crystal growth, and may lack the Moss defects of quartz grains in granites. Thus production of silt from vein quartz is very difficult by any mechanism, whereas production of silt from granite quartz proceeds readily by any of a number of mechanisms. [5] However, the main process is likely abrasion through transport, including fluvial comminution, aeolian attrition and glacial grinding. [24]
Because silt deposits (such as loess , a soil composed mostly of silt [34] ) seem to be associated with glaciated or mountainous regions in Asia and North America, much emphasis has been placed on glacial grinding as a source of silt. High Asia has been identified as a major generator of silt, which accumulated to form the fertile soils of north India and Bangladesh, and the loess of central Asia and north China. [5] Loess has long been thought to be absent or rare in deserts lacking nearby mountains (Sahara, Australia). [35] However, laboratory experiments show eolian and fluvial processes can be quite efficient at producing silt, [24] as can weathering in tropical climates. [36] Silt seems to be produced in great quantities in dust storms, and silt deposits found in Israel, Tunisia, Nigeria, and Saudi Arabia cannot be attributed to glaciation. Furthermore, desert source areas in Asia may be more important for loess formation than previously thought. Part of the problem may be the conflation of high rates of production with environments conducive to deposition and preservation, which favors glacial climates more than deserts. [37]
Loess associated with glaciation and cold weathering may be distinguishable from loess associated with hot regions by the size distribution. Glacial loess has a typical particle size of about 25 microns. Desert loess contains either larger or smaller particles, with the fine silt produced in dust storms and the coarse silt fraction possibly representing the fine particle tail of sand production. [5]
Loess underlies some of the most productive agricultural land worldwide. However, it is very susceptible to erosion. [5] The quartz particles in silt do not themselves provide nutrients, but they promote excellent soil structure, and silt-sized particles of other minerals, present in smaller amounts, provide the necessary nutrients. [16] Silt, deposited by annual floods along the Nile River, created the rich, fertile soil that sustained the Ancient Egyptian civilization. The closure of the Aswan High Dam has cut off this source of silt, and the fertility of the Nile delta is deteriorating. [38]
Loess tends to lose strength when wetted, and this can lead to failure of building foundations. [5] The silty material has an open structure that collapses when wet. [39] Quick clay (a combination of very fine silt and clay-sized particles from glacial grinding) is a particular challenge for civil engineering. [40]
The failure of the Teton Dam has been attributed to the use of loess from the Snake River floodplain in the core of the dam. [41] Loess lacks the necessary plasticity for use in a dam core, but its properties were poorly understood, even by the U.S. Bureau of Reclamation, with its wealth of experience building earthen dams. [5]
Silt is susceptible to liquefaction during strong earthquakes due to its lack of plasticity. This has raised concerns about the earthquake damage potential in the silty soil of the central United States in the event of a major earthquake in the New Madrid Seismic Zone. [42] [43]
Silt is easily transported in water [44] and is fine enough to be carried long distances by air in the form of dust. [45] While the coarsest silt particles (60 micron) settle out of a meter of still water in just five minutes, the finest silt grains (2 microns) can take several days to settle out of still water. [46] When silt appears as a pollutant in water the phenomenon is known as siltation. [47]
Silt deposited by the Mississippi River throughout the 20th century has decreased due to a system of levees, contributing to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans. [48]
In southeast Bangladesh, in the Noakhali district, cross dams were built in the 1960s whereby silt gradually started forming new land called "chars". The district of Noakhali has gained more than 73 square kilometres (28 sq mi) of land in the past 50 years. With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, and the effort has since become a multi-agency operation building roads, culverts, embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some 100 square kilometres (20,000 acres) to 21,000 families. [49]
A main source of silt in urban rivers is disturbance of soil by construction activity. [50] A main source in rural rivers is erosion from plowing of farm fields, [51] clearcutting or slash and burn treatment of forests. [52]
The fertile black silt of the Nile river's banks is a symbol of rebirth, associated with the Egyptian god Anubis. [53] [54]
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Till or glacial till is unsorted glacial sediment.
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.
Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.
Gravel is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentary and erosive geological processes; it is also produced in large quantities commercially as crushed stone.
A loess is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loesses or similar deposits.
Deposition is the geological process in which sediments, soil and rocks are added to a landform or landmass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.
Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility.
Parent material is the underlying geological material in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering and the mode by which the materials were most recently transported.
Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than 1⁄16 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.
In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases, the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.
A subaqueous fan is a fan-shaped deposit formed beneath water, that is commonly related to glaciers and crater lakes.
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.