Editing Silt (section)

== Sources ==
[[File:Sunlight on silty stream at Myrstigen 5.jpg|thumb|A stream carrying silt from fields in [[Brastad]], Sweden]]
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.<ref>{{cite journal |last1=Schubert |first1=C. |year=1964 |title=Size-frequency distribution of sand-sized grains in an abrasion mill |journal=Sedimentology |volume=3 |number=4 |pages=288–295|doi=10.1111/j.1365-3091.1964.tb00643.x |bibcode=1964Sedim...3..288S }}</ref> 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.<ref>{{cite journal |last1=Rogers |first1=J.J.W. |last2=Krueger |first2=W.C. |last3=Krog |first3=M. |title=Sizes of Naturally Abraded Materials |journal=SEPM Journal of Sedimentary Research |date=1963 |volume=33 |pages=628–632 |doi=10.1306/74D70ED9-2B21-11D7-8648000102C1865D}}</ref> The mechanisms of silt formation have been studied extensively in the laboratory<ref name=WrightEtal1998>{{cite journal |last1=Wright |first1=J. |last2=Smith |first2=B. |last3=Whalley |first3=B. |title=Mechanisms of loess-sized quartz silt production and their relative effectiveness: laboratory simulations |journal=[[Geomorphology (journal)|Geomorphology]] |date=May 1998 |volume=23 |issue=1 |pages=15–34 |doi=10.1016/S0169-555X(97)00084-6 |bibcode=1998Geomo..23...15W}}</ref> 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.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Quartz silt grains are usually found to have a platy or bladed shape.<ref>{{cite journal |last1=Krinsley |first1=D. H. |last2=Smalley |first2=I. J. |title=Shape and Nature of Small Sedimentary Quartz Particles |journal=Science |date=22 June 1973 |volume=180 |issue=4092 |pages=1277–1279 |doi=10.1126/science.180.4092.1277|pmid=17759122 |bibcode=1973Sci...180.1277K |s2cid=11606901 }}</ref> 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]].<ref>{{cite journal |last1=Blatt |first1=H. |title=Perspectives; Oxygen isotopes and the origin of quartz |journal=Journal of Sedimentary Research |date=1 March 1987 |volume=57 |issue=2 |pages=373–377 |doi=10.1306/212F8B34-2B24-11D7-8648000102C1865D|bibcode=1987JSedR..57..373B }}</ref> Theoretically, particles formed by random fracturing of an isotropic material, such as quartz, naturally tend to be blade-shaped.<ref>{{cite journal |last1=Rogers |first1=C.F. |last2=Smalley |first2=I.J. |year=1993 |title=The shape of loess particles |journal=Naturwissenschaften |volume=80 |number=10 |pages=461–462|doi=10.1007/BF01136036 |bibcode=1993NW.....80..461R |s2cid=44606484 }}</ref> 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.''<ref>{{cite journal |last1=Moss |first1=A. J. |last2=Green |first2=Patricia |title=Sand and silt grains: Predetermination of their formation and properties by microfractures in quartz |journal=[[Journal of the Geological Society of Australia]] |date=1975 |volume=22 |issue=4 |pages=485–495 |doi=10.1080/00167617508728913 |bibcode=1975AuJES..22..485M}}</ref> 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 {{convert|573|C||sp=us}},<ref>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |page=68}}</ref> which creates strain and crystal defects in the quartz grains in a cooling body of granite.<ref>{{cite journal |last1=Smalley |first1=I.J. |year=1966 |title=Formation of quartz sand |journal=Nature |volume=211 |number=5048 |pages=476–479|doi=10.1038/211476a0 |bibcode=1966Natur.211..476S |s2cid=4258725 }}</ref>

Mechanisms for silt production include:{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}
* Erosion of initially silt-sized grains from low-grade metamorphic rock.
* Production of silt-sized grains from fracture of larger grains during initial rock weathering and [[soil formation]],<ref>{{cite journal |last1=Nahon |first1=D. |last2=Trompette |first2=R. |title=Origin of siltstones: glacial grinding versus weathering |journal=[[Sedimentology (journal)|Sedimentology]] |date=February 1982 |volume=29 |issue=1 |pages=25–35 |doi=10.1111/j.1365-3091.1982.tb01706.x |bibcode=1982Sedim..29...25N}}</ref> through processes such as [[frost shattering]]<ref>{{cite journal |last1=Lautridou |first1=J. P. |last2=Ozouf |first2=J. C. |title=Experimental frost shattering |journal=[[Progress in Physical Geography]] |date=19 August 2016 |volume=6 |issue=2 |pages=215–232 |doi=10.1177/030913338200600202|s2cid=140197148 }}</ref> and [[haloclasty]].<ref>{{cite journal |last1=Goudie |first1=A. S. |last2=Watson |first2=A. |title=Rock block monitoring of rapid salt weathering in southern Tunisia |journal=[[Earth Surface Processes and Landforms]] |date=January 1984 |volume=9 |issue=1 |pages=95–98 |doi=10.1002/esp.3290090112 |bibcode=1984ESPL....9...95G}}</ref> This produces silt particles whose size of 10–30 microns is determined by Moss defects.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}
* Production of silt-sized grains from grain-to-grain impact during transport of coarser sediments.
* Formation of authigenic quartz during weathering to clay.
* Crystallization of the tests of siliceous organisms deposited in mudrock.

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.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} However, the main process is likely [[Abrasion (geology)|abrasion]] through transport, including [[fluvial]] [[comminution]], [[Aeolian processes|aeolian]] [[Attrition (weathering)|attrition]] and [[glacial]] grinding.<ref name=WrightEtal1998/>

Because silt deposits (such as ''[[loess]]'', a soil composed mostly of silt<ref name="Frechen 2011">{{cite journal | last1 = Frechen | first1 = M | year = 2011 | title = Loess in Europe: Guest Editorial| journal = E&G Quaternary Science Journal| volume = 60 | issue = 1| pages = 3–5 | doi = 10.3285/eg.60.1.00 | doi-access = free }}</ref>) 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.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} Loess has long been thought to be absent or rare in deserts lacking nearby mountains (Sahara, Australia).<ref>{{cite journal |last1=Smalley |first1=Ian J. |last2=Krinsley |first2=David H. |title=Loess deposits associated with deserts |journal=CATENA |date=April 1978 |volume=5 |issue=1 |pages=53–66 |doi=10.1016/S0341-8162(78)80006-X|bibcode=1978Caten...5...53S }}</ref> However, laboratory experiments show eolian and fluvial processes can be quite efficient at producing silt,<ref name=WrightEtal1998/> as can weathering in tropical climates.<ref>{{cite journal |last1=Pye |first1=Kenneth |title=Formation of quartz silt during humid tropical weathering of dune sands |journal=Sedimentary Geology |date=April 1983 |volume=34 |issue=4 |pages=267–282 |doi=10.1016/0037-0738(83)90050-7|bibcode=1983SedG...34..267P }}</ref> 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.<ref>{{cite journal |last1=Wright |first1=Janet S |title="Desert" loess versus "glacial" loess: quartz silt formation, source areas and sediment pathways in the formation of loess deposits |journal=Geomorphology |date=February 2001 |volume=36 |issue=3–4 |pages=231–256 |doi=10.1016/S0169-555X(00)00060-X|bibcode=2001Geomo..36..231W }}</ref>

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.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}