Plate 105

Desiccation structures: mud cracks, polygons, and curls


It is possible to see, in this close-up view, the relations between the sizes of desiccation cracks and polygons. The edges of the thinner superficial film have been curled; the process goes on until dry mud rolls and curls are completely detached from underlying, wetter mud. Curling is just starting at the edges of larger polygons.

In conclusion, it can be seen that the ultimate result of this deformative mechanism is similar to that of intraformational erosion:  it produces mud clasts, which can be picked up by transporting agents and redeposited somewhere else.


Sediments can be fissured by processes other than desiccation. Cracks can form, for example, by dilation rather than contraction. This is the case of ice wedge cracks:  in periglacial and circumpolar areas, the soil is frozen most of the year. Thawing can occur in superficial layers during the warmer season. It is well known that water increases its volume when freezing; as a result of this expansion, water pipes break in cities; in the field, ice layers form in surface water of ponds and lakes (see color photo 28) or in saturated topsoil, where ice wedges can penetrate to a certain depth. Upon seasonal melting, the ice leaves open fissures, and soil debris and stones can fill them. The cracks can thus fossilize and testify to cold conditions of the past; they are utilized as paleoclimatic indicators.  Other kinds of deformation occur in glaciated areas; I skip these peculiar structures, just mentioning the term (cryoturbation ) that encompasses all of them.

Cracks can form in subaqueous mud, too; the mechanism is one of shrinkage, as in the case of desiccation, but the process is clearly different. It is called syneresis, and is well described by the A.G.I. Glossary: "the spontaneous separation or throwing-off of a liquid from or by a gel  or flocculated colloidal suspension during aging, resulting in shrinkage." 1 Actually, the "spontaneous" separation of water from mud is induced by mechanical disturbances or chemical changes, such as variations of salinity (contents of dissolved salts), etc. Fissures due to this process are called syneresis cracks.

Glossary

Note 1: R. L. Bates and J. A. Jackson, eds. 1987. Glossary of Geology.  3d ed. Alexandria, Va.: American Geological Institute. Back.


Sedimentographica