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Mudcracks (also known as mud cracks, desiccation cracks or cracked mud) are
sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition (geology), deposition. Sediments and sedimentary rocks are characterized by bed (geology), bedding, which occurs when layer ...
formed as muddy sediment dries and contracts.Jackson, J.A., 1997, ''Glossary of Geology'' (4th ed.), American Geological Institute, Alexandria, VA, 769 p.Stow, D.A., 2005, ''Sedimentary Rocks in the Field'', Academic Press, London, 320 p. Crack formation also occurs in clay-bearing soils as a result of a reduction in water content.


Formation of mudcrack

Naturally forming mudcracks start as wet, muddy sediment dries up and contracts. A strain is developed because the top layer shrinks while the material below stays the same size. When this strain becomes large enough, channel cracks form in the dried-up surface to relieve the strain. Individual cracks spread and join up, forming a polygonal, interconnected network. These cracks may later be filled with sediment and form
casts
casts
over the base.
Syneresis cracks Syneresis cracks (also known as subaqueous shrinkage cracks) are a sedimentary structures, sedimentary structure developed by the shrinkage of sediment without desiccation – not to be confused with mudcracks, desiccation cracks. Syneresis (chem ...
are broadly similar features that form from underwater shrinkage of muddy sediment caused by differences in salinity or chemical conditions, rather than aerial exposure and desiccation. Syneresis cracks can be distinguished from mudcracks because they tend to be discontinuous,
sinuous Image:rio-cauto-cuba.JPG, The meandering ''Cauto River, Rio Cauto'' at Guamo Embarcadero, Cuba, is not taking the shortest path downslope. Therefore, its sinuosity index is > 1. Sinuosity, sinuosity index, or sinuosity coefficient of a ...
, and trilete or spindle-shaped.Allen, J.R.L., 1982, ''Sedimentary Structures: Their Character and Physical Basis'' (v. 2): Elsevier, Oxford, 593 p.


Morphology and classification of mudcrack

Mudcracks are generally polygonal when seen from above and v-shaped in cross section. The "v" opens towards the top of the bed and the crack tapers downward. Allen (1982) proposed a classification scheme for mudcracks based on their completeness, orientation, shape, and type of infill.


Completeness of mudcrack

''Complete mudcracks'' form an interconnected network. The connection of cracks often occurs when individual cracks join together forming a larger continuous crack. ''Incomplete mudcracks'' are not connected to each other but still form in the same region or location as the other cracks.


Plan-view geometry

''Orthogonal'' intersections can have a preferred orientation or may be random. In oriented orthogonal cracks, the cracks are usually complete and bond to one another forming irregular polygonal shapes and often rows of irregular polygons. In random orthogonal cracks, the cracks are incomplete and unoriented therefore they do not connect or make any general shapes. Although they do not make general shapes they are not perfectly geometric.Linholm, R., 1987, ''A Practical Approach to Sedimentology'', Allen and Unwin, London, 276 p. ''Non-orthogonal'' mudcracks have a geometric pattern. In uncompleted non-orthogonal cracks they form as a single three-point star shape that is composed of three cracks. They could also form with more than three cracks but three cracks in commonly considered the minimum. In completed non-orthogonal cracks, they form a very geometric pattern. The pattern resembles small polygonal shaped tiles in a repetitive pattern.


Mud curls

Mud curls form during one of the final stages in
desiccation in sludge Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah. A dinosaur footprint is at center. Desiccation (from Latin de- "thoroughly" + siccare "to dry") is the state of extreme dryness, or the ...
. Mud curls commonly occur on the exposed top layer of very thinly bedded mud rocks. When mud curls form, the water that is inside the sediment begins to evaporate causing the
stratified Stratification may refer to: In mathematics: * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Stratified sampling , Data stratification in statistics In earth sciences: * Stable and unstable stratificati ...
layers to separate. The individual top layer is much weaker than multiple layers and is therefore able to contract and form curls as desiccation occurs. If transported by later currents, mud curls may be preserved as mud-chip rip-up clasts.


Environments and substrates

Naturally occurring mudcracks form in sediment that was once saturated with water. Abandoned river channels,
floodplain File:Flood plain 7991.JPG, Riparian vegetation on the floodplain of the Lynches River, close to Johnsonville, South Carolina. These tupelo and Taxodium, cypress trees show the ordinary high water mark, high-water mark of flooding. A floodpl ...
muds, and dried ponds are localities that form mudcracks. Mudcracks can also be indicative of a predominately sunny or shady environment of formation. Rapid drying, which occurs in sunny environments, results in widely spaced, irregular mudcracks, while closer spaced, more regular mudcracks indicate that they were formed in a shady place. Similar features also occur in frozen ground,
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lava flow
s (as
columnar basalt Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...

columnar basalt
), and igneous dykes and sills.


In technology

Polygonal crack networks similar to mudcracks can form in man-made materials such as
ceramic glazes A ceramic is any of the various hard, brittle, heat-resistant and materials made by shaping and then firing an inorganic, nonmetallic material, such as , at a high temperature. Common examples are , , and . The earliest ceramics made by huma ...
, paint film, and poorly made concrete. Mudcrack patterning at smaller scales can also be observed studied using technological
thin film A thin film is a layer of material ranging from fractions of a nanometer Image:Chiraltube.png, one nanometric carbon nano tube, photographed with Scanning Tunneling Microscope file:EM Spectrum Properties edit.svg, 330px, Different lengths as in ...
s deposited using
micro Micro may refer to: Measurement * micro- (μ), a prefix in the SI and other systems of units denoting a factor of 10−6 (one millionth) Places * Micro, North Carolina, town in U.S. People * DJ Micro, (born Michael Marsicano) an American trance ...
and
nanotechnologies Nanotechnology, also shortened to nanotech, is the use of matter on an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having ...

nanotechnologies
.R. Seghir and S. Arscott "Controlled mud-crack patterning and self-organized cracking of polydimethylsiloxane elastomer surfaces" Sci. Rep. (Nature) 5, 14787 (2015). .


Preservation of mudcrack

Mudcracks can be preserved as v-shaped cracks on the top of a bed of muddy sediment or as casts on the base of the overlying bed. When they are preserved on the top of a bed, the cracks look as they did at the time of formation. When they are preserved on the bottom of the bedrock, the cracks are filled in with younger, overlying sediment. In most bottom-of-bed examples, the cracks are the part that sticks out most. Bottom-of-bed preservation occurs when mudcracks that have already formed and are completely dried are covered with fresh, wet sediment and are buried. Through burial and pressure, the new wet sediment is further pushed into the cracks, where it dries and hardens. The mudcracked rock is then later exposed to
erosion In , erosion is the action of surface processes (such as or ) that removes , , or dissolved material from one location on the , and then it to another location. Erosion is distinct from which involves no movement. Removal of rock or soil as ...

erosion
. In these cases, the original mud cracks will erode faster than the newer material that fills the spaces. This type of mudcrack is used by geologists to determine the vertical orientation of rock samples that have been altered through
fold Fold or folding may refer to: Arts, entertainment, and media *Fold (album), ''Fold'' (album), the debut release by Australian rock band Epicure *Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot *Ab ...
ing or
faulting In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...
.


Gallery

Image:mudcracks_roundtop_hill_MD.jpg, Bedding plane view of ancient mudcracks File:Mudcracks mcr3.jpg, Cross-sectional view of ancient mudcrack File:Carmel Formation mudcracks Jurassic.jpg, Mudcracks in the
Carmel Formation . The unconformably overlying dark unit is the Upper Cretaceous Iron Springs Formation. The Carmel Formation is a geologic formation in the San Rafael Group that is spread across the U.S. states of Wyoming, Utah, Colorado, north east Arizona and N ...

Carmel Formation
(Middle Jurassic) near Gunlock, Utah.


See also

*
Ground fissureA ground fissure or Earth fissure is a narrow linear opening in the Earth's surface. It may be caused by an earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden ...
*
Syneresis crack Syneresis cracks (also known as subaqueous shrinkage cracks) are a sedimentary structures, sedimentary structure developed by the shrinkage of sediment without desiccation – not to be confused with mudcracks, desiccation cracks. Syneresis (chem ...
, which occur in muds and can look very similar, without requiring subaerial exposure


References


External links

* {{Commons category-inline, Mudcracks
NEFI: A software that can be used to extract networks from images of mudcracks.
Sedimentary structures Patterned grounds Desiccation