Biorhexistasy
Encyclopedia
The Theory of Biorhexistasy describes climatic conditions necessary for periods of soil formation (pedogenesis
) separated by periods of soil erosion. Proposed by pedologist
H. Erhart in 1951, the theory defines two climatic phases: biostasy and rhexistasy.
induces strong pedogenesis characterized by chemical alteration of parent material
and intensified eluviation and illuviation of soil minerals within the surface soil
and subsoil
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 sesquioxide
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
formation.
. 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
and silt
as sedimentary layers
. During rhexistasy, the dominance of chemical weathering that characterizes biostasy is replaced by the dominance of physical weathering.
During the Pleistocene
epoch, the periods of glaciation are considered to be periods of rhexistasy and the interglacial
are considered periods of biostasy.
Pedogenesis
Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
) separated by periods of soil erosion. Proposed by pedologist
Pedology (soil study)
Pedology is the study of soils in their natural environment. It is one of two main branches of soil science, the other being edaphology...
H. Erhart in 1951, the theory defines two climatic phases: biostasy and rhexistasy.
Biostasy
During biostasy, abundant and regular precipitationPrecipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
induces strong pedogenesis characterized by chemical alteration of parent material
Parent material
In soil science, parent material is the underlying geological material in which soil horizons form...
and intensified eluviation and illuviation of soil minerals within the surface soil
Topsoil
Topsoil is the upper, outermost layer of soil, usually the top to . It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.-Importance:...
and subsoil
Subsoil
Subsoil, or substrata, is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and/or sand that has only been partially broken down by air, sunlight, water, wind etc., to produce true soil...
layers (the solum
Solum
The solum 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, termed substratum....
). These processes contribute to the formation of eluvial and argillic horizons and an increased concentration of iron oxides, aluminum oxides, and other sesquioxide
Sesquioxide
A sesquioxide is an oxide containing three atoms of oxygen with two atoms of another element. For example, aluminium oxide is a sesquioxide.Many sesquioxides contain the metal in the +3 oxidation state and the oxide ion, e.g., Al2O3, La2O3...
s in the subsoil
Subsoil
Subsoil, or substrata, is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and/or sand that has only been partially broken down by air, sunlight, water, wind etc., to produce true soil...
. 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
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
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 weatheringWeathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
. 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
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
as sedimentary layers
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. During rhexistasy, the dominance of chemical weathering that characterizes biostasy is replaced by the dominance of physical weathering.
During the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
epoch, the periods of glaciation are considered to be periods of rhexistasy and the interglacial
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
are considered periods of biostasy.
Current use
The theory of biorhexistasy is used in various capacities:- to discuss the potential for man to effect 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 speleothemSpeleothemA speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...
s for insight into paleoclimatic and biopedological conditions at the land surface.