Mudcracks are sedimentary structures

Sedimentary structures

Sedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional...

 formed as muddy sediment dries

Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

 and contracts.

Formation

Naturally forming mudcracks start as wet, muddy sediment desiccates

Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

, causing contraction via a decrease in tensile strength

Tensile strength

Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 . Individual cracks join up forming a polygonal, interconnected network. These cracks may later be filled with sediment and form casts

Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...

 on the base of the overlying bed.

Syneresis cracks

Syneresis cracks

Syneresis cracks are a sedimentary structure developed by the shrinkage of sediment without desiccation. Syneresis is the expulsion of a liquid from a gel-like substance. Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a...

 are broadly similar features that form from subaqueous shrinkage of muddy sediment caused by differences in salinity or chemical conditions, rather than subaerial exposure and desiccation. Syneresis cracks can be distinguished from mudcracks because they tend to be discontinuous, sinuous

Sinuous

Sinuous is a fictional snake from the third season of the animated children's television series The Animals of Farthing Wood based upon the books of the same name by author Colin Dann....

, and trilete or spindle-shaped.

Morphology and classification

Mudcracks are generally polygonal in plan view 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 mud cracks based on their completeness, orientation, shape, and type of infill.

Completeness

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.
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

Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

. 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 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 mud cracks form in sediment that was once saturated with water. Abandoned river channels, floodplain

Floodplain

A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...

 muds, and dried ponds are localities that form mudcracks.
Mud cracks 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 mud cracks, while closer spaced more regular mud cracks indicates a shady formation environment.
Polygonal crack networks similar to mudcracks can form in man-made materials such as ceramic glazes, paint film, and poorly made concrete. Similar features also occur in frozen ground, lava flows (as columnar basalt), and igneous dykes and sill

Sill (geology)

In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

s.

Preservation

Mud cracks 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 were 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 mud cracks that have already formed and are completely dried are covered with fresh wet sediment and buried. Through burial and pressure the new wet sediment is further pushed into the cracks where it dries and hardens. The mud cracked rock is then later exposed to erosional factors. In these cases, the original mud cracks will erode faster than the newer material filling in the spaces. These types of mud cracks are useful for geologists

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 to determine the vertical orientation of rock samples that have been altered through fold

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

ing or faulting

Fault mechanics

Fault mechanics is a field of study that investigates the behavior of geologic faults.Behind every good earthquake is some weak rock. Whether the rock remains weak becomes an important point in determining the potential for bigger earthquakes....

.