The theory of biorhexistasy describes climatic conditions necessary for periods of soil formation (
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. 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 under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
induces strong pedogenesis characterized by chemical alteration of
parent material
Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are ...
and intensified
eluviation
In geology, eluvium or eluvial deposits are those geological deposits and soils that are derived by ''in situ'' weathering or weathering plus gravitational movement or accumulation.
The process of removal of materials from geological or soil horiz ...
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. The ...
of soil minerals within the
surface soil and
subsoil layers (the
solum
The solum (plural, sola) in soil science consists of the surface and subsoil layers that have undergone the same soil forming conditions. The base of the solum is the relatively unweathered parent material.
Solum and soils are not synonymous. Som ...
). These processes contribute to the formation of eluvial and
argillic horizons and an increased concentration of iron oxides, aluminum oxides, and other
sesquioxide
A sesquioxide is an oxide of an Chemical element, element (or Radical (chemistry), radical), where the ratio between the number of atoms of that element and the number of atoms of oxygen is 2:3. For example, aluminium oxide and phosphorus(III) oxi ...
s in the
subsoil. 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 ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
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, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
. 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 defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...
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 with water. Silt usually has a floury feel ...
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 ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...
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 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 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, depending ...
s for insight into
paleoclimatic and biopedological conditions at the land surface.
See also
*
Land degradation
Land degradation is a process in which the value of the biophysical environment is affected by a combination of human-induced processes acting upon the land.
It is viewed as any change or disturbance to the land perceived to be deleterious o ...
*
Soil production function
Soil production function refers to the rate of bedrock weathering into soil as a function of soil thickness.
A general model suggested that the rate of physical weathering of bedrock (de/dt) can be represented as an exponential decline with soil ...
References
Further reading
* {{cite book, author=Erhart, H. , year=1951 , title=La genèse des sols en tant que phénomène géologique. Esquisse d'une théorie géologique et géochimique. Biostasie et rhéxistasie. , publisher = Masson, Paris 90 p, autres éditions en 1956 , 1967 (177 p.)
Pedology
Sedimentology