The theory of biorhexistasy describes climatic conditions necessary for periods of

soil formation Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order ( anisotropy) within soils. These alteration ...

(

pedogenesis Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations ...

) separated by periods of

soil erosion Soil erosion is the denudation or wearing away of the Topsoil, upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, Atmosphere of Ea ...

. Proposed by pedologist Henry Erhart in 1951, the theory defines two climatic phases: biostasy and rhexistasy.

Biostasy

During biostasy, abundant and regular

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

induces strong pedogenesis characterized by chemical alteration of

parent material Parent material is the underlying geological material (generally bedrock or a superficial deposits, superficial or drift (geology), drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from th ...

and intensified eluviation and

illuviation Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. ...

of soil minerals within the surface soil and

subsoil Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus. The su ...

layers (the solum). These processes contribute to the formation of eluvial and argillic horizons and an increased concentration of iron oxides, aluminum oxides, and other sesquioxides in the

. Climatic conditions favor a vegetative cover which protects the soil from physical erosion but abundant rainfall results in the loss of mineral ions and increased concentration of those minerals in receiving bodies of water. Abundant marine calcium results in

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

formation.

Rhexistasy

During rhexistasy (from rhexein, to break) the protective vegetative cover is reduced or eliminated as a result of a drier climate. Rainfall intensity is higher. The drier climate slows pedogenesis and soils no longer contribute the limestone building mineral components that characterize biostasy. Unprotected by thick vegetation or deep soils, wind acts to expose subsoil to erosion and rock to physical

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

. Freeze-thaw acts to increase the production of coarse detrital materials. The intensity of punctuating rainfall events during rhexistasy results in erosion, and the accumulation of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

and

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

as sedimentary layers. During rhexistasy, the dominance of chemical weathering that characterizes biostasy is replaced by the dominance of physical weathering. During the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

epoch, the periods of

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...

are considered to be periods of rhexistasy and the

interglacial An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene i ...

are considered periods of biostasy.

Current use

The theory of biorhexistasy is used in various capacities: * to discuss the potential for man to affect either rhexistasy-like or biostasy-like environments, and what to expect from those environments, * to explain the role of extreme events in erosion at a site recovering from disturbance, and * to evaluate

speleothem A speleothem (; ) is a geological formation made by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depen ...

s for insight into paleoclimatic and biopedological conditions at the land surface.

Biostasy

Rhexistasy

Current use

See also

References

Further reading