Soil Classification and Pedogenesis (integrated)

A soil horizon is a layer generally parallel to the soil crust, whose physical characteristics differ from the layers above and beneath. Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material (i.e., "coarser" or "sandier" than the horizons above and below). The differentiation of the soil into distinct horizons is largely the result of influences, such as air, water, solar radiation and plant material, originating at the soil-atmosphere interface. Since the weathering of the soil occurs first at the surface and works its way down, the uppermost layers have been changed the most, while the deepest layers are most similar to the original parent material.

Identification and description of the horizons present at a given site is the first step in soil classification at higher levels, through the use of systems such as the USDA soil taxonomy or the Australian Soil Classification. The World Reference Base for Soil Resources lists 40 diagnostic horizons.^[1] Soil scientists often dig a large hole, called a soil pit (sometimes several meters deep and about a meter wide) to expose soil horizons for study.

The vertical section exposing a set of horizons, from the ground surface to the parent rock, is termed a soil profile. Most soils, especially in temperate climates, conform to a similar general pattern of horizons, often represented as "ideal" soil in diagrams. Each main horizon is denoted by a capital letter, which may then be followed by several alphanumerical modifiers highlighting particular outstanding features of the horizon. While the general O-A-B-C-R sequence seems fairly universal, some variation exists between the classification systems in different parts of the world. In addition, the exact definition of each main horizon may differ slightly – for instance, the US system uses the thickness of a horizon as a distinguishing feature, while the Australian system does not. It should be emphasised that no one system is more correct – as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner. Also, many subtropical and tropical areas have soils such as oxisols or aridisols that have very different horizons from "ideal" soil, or no horizons at all.

O horizon[edit]

Main article: Organic matter

This layer generally forms above the mineral soil or occurs in an organic soil profile. The "O" stands for organic matter. It is a surface layer dominated by the presence of large amounts of organic material derived from dead plant and/or animal residues which is in varying stages of decomposition. The O horizon is generally absent in grassland regions. The O horizon usually occurs in forested areas and is commonly referred to as the forest floor. The O horizon should be considered distinct from the layer of leaf litter covering many heavily vegetated areas, which contains no weathered mineral particles and is not part of the soil itself. O horizons may be divided into O1 and O2 categories, whereby O1 horizons contain decomposed matter whose origin can be spotted on sight (for instance, fragments of rotting leaves), and O2 horizons contain only well-decomposed organic matter, the origin of which is not readily visible. O horizons may also be divided into three subordinate O horizons denoted as: Oi, Oe, and Oa.

P horizon[edit]

These horizons are also heavily organic, but are distinct from O horizons in that they form under waterlogged conditions. The "P" designation comes from their common name, peats. They may be divided into P1 and P2 in the same way as O Horizons. This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.^[4]

A horizon[edit]

Main article: Topsoil

The A horizon is the topmost mineral horizon, often referred to as the 'topsoil'. This layer generally contains enough partially decomposed (humified) organic matter to give the soil a color darker than the lower horizons. The A horizons are often coarser in texture, having lost some of the finer materials by translocation to lower horizons and by erosion. This layer is known as the zone in which the most biological activity occurs. Soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are concentrated here, often in close association with plant roots. Thus the A horizon may be referred to as the biomantle.^[5][6] However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A horizon.

E horizon[edit]

Main article: Eluvium

"E", being short for eluviated, is most commonly used to label a horizon that has been significantly leached of clay, iron, and aluminium oxides, which leaves a concentration of resistant minerals, such as quartz, in the sand and silt sizes. These are present only in older, well-developed soils, and generally occur between the A and B horizons.The E horizon often has a pale color that is generally lighter in color than either the horizon above or below it. E horizons are commonly found in soils developed under forests, but are rare in soils developed under grasslands. In regions where this designation is not employed, leached layers are classified firstly as an A or B according to other characteristics, and then appended with the designation "e" (see the section below on horizon suffixes). In soils that contain gravels, due to animal bioturbation, a stonelayer commonly forms near or at the base of the E horizon.

The above layers may be referred to collectively as the "solum". The layers below have no collective name but are distinct in that they are noticeably less affected by surface soil-forming processes.

B horizon[edit]

Main article: Subsoil

B horizons form below an O, A, or E horizon and they have undergone sufficient changes during soil genesis, such that the properties of their original parent material are no longer discernible. The B horizon is commonly referred to as the "subsoil". In humid regions, B horizons are the layers of maximum accumulation of materials such as silicate clays, iron (Fe) and aluminium (Al) oxides, and organic material. These materials typically accumulate through a process termed illuviation, wherein the materials gradually wash in from the overlying horizons. Accordingly, this layer is also referred to as the "illuviated" horizon or the "zone of accumulation". In addition, it is defined as having a distinctly different structure or consistency than the horizon(s) above and the horizon(s) below. The B horizon may also have stronger colors (higher chroma) than the A horizon. In arid and semiarid regions, calcium carbonate or calcium sulfate may accumulate in the B horizon.

As with the A horizon, the B horizon may be divided into B1, B2, and B3 types under the Australian system. B1 is a transitional horizon of the opposite nature to an A3 – dominated by the properties of the B horizons below it, but containing some A-horizon characteristics. B2 horizons have a concentration of clay, minerals, or organics and feature the highest soil development within the profile. B3 horizons are transitional between the overlying B layers and the material beneath it, whether C or D horizon.

The A3, B1, and B3 horizons are not tightly defined, and their use is generally at the discretion of the individual worker.

Plant roots penetrate through this layer, but it has little humus. It is usually brownish or reddish due to residual clay and iron oxides.

C horizon[edit]

Main article: Parent rock

The C horizon (parent material) is below the B Horizon. This layer is barely affected by soil-forming processes and they thus have a lack of pedological development. In other words, the C horizon is the unconsolidated material underlying the solum (A and B horizons). It may or may not be the same as the parent material from which the solum formed. The C horizon forms as the R horizon weathers and rocks break up into smaller particles. The C horizon is below the zones of greatest biological activity and it has not been sufficiently altered by soil genesis to qualify as a B horizon. In dry regions, carbonates and gypsum may be concentrated in the C horizon. While loose enough to be dug with a shovel, C horizon material often retains some of the structural features of the parent rock or geologic deposits from which it formed. The A and B layers usually originated from the C horizon. The upper layers of the C horizon may in time become a part of the solum as weathering and erosion continue. The C Horizon may contain lumps or more likely large shelves of unweathered rock, rather than being made up solely of small fragments as in the solum. It contains rocks with cracks and crevices.

D horizon[edit]

D horizons are not universally distinguished, but in the Australian system are "any soil material below the solum that is unlike the solum in general character, is not C horizon, and cannot be given reliable designation… [it] may be recognized by the contrast in pedologic organization between it and the overlying horizons" (MacDonald et al., 1990, p. 106).

R horizon[edit]

Main article: Bedrock

R horizons are the layers of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons are composed largely of consolidated masses of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer.

L horizon[edit]

L (limnic) horizons or layers indicate mineral or organic material that has been deposited in water by precipitation or through the actions of aquatic organisms. Included are coprogenous earth (sedimentary peat), diatomaceous earth, and marl; and is usually found as a remnant of past bodies of standing water.

Pedogenesis (from the Greek pedo-, or pedon, meaning 'soil, earth,' and genesis, meaning 'origin, birth') (also termed soil development, soil evolution, soil formation, and soil genesis) is the process of soil formation as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.^[1]

⚐ Overview

Soil develops through a series of changes. The starting point is weathering of freshly accumulated parent material. Primitive microbes feed on simple compounds (nutrients) released by weathering, and produce acids which contribute to weathering. They also leave behind organic residues. New soils increase in depth by a combination of weathering, and further deposition. Gradually soil is able to support higher forms of plants and animals, starting with pioneer species, and proceeding to more complex plant and animal communities.

Soils deepen with accumulation of organic matter primarily due to the activities of higher plants. Topsoil deepen through soil mixing. Soils develop layers as organic matter accumulates and leaching takes place. This development of layers is the beginning of the soil profile.

A rate of pedogenesis of 1/10 mm per year is in order of magnitude of the global soil production estimates

⚐ Factors of soil formation

Russian geologist Vasily Dokuchaev determined that soil formation occurs over time under the influence of climate, vegetation, topography, and parent material. He demonstrated this in 1898^[5] using the soil forming equation:

soil = f(cl, o, p) tr

(where cl = climate, o = organisms, p = biological processes) tr = relative time (young, mature, old)

❧ Clorpt

Main article: Clorpt

In 1941, the American soil scientist Hans Jenny published Factors of Soil Formation. His equation of state differs from the Vasily Dokuchaev equation, treating time (t) as a factor, adding topographic relief (r), and pointedly leaving the ellipsis "open" for more factors (state variables) to be added as our understanding becomes more refined:

soil = f(Cl, o, r, p, t, ...)

❧ Climate

Climate regulates soil formation. Soils are more developed in areas with higher rainfall and more warmth. The rate of chemical weathering increases by 2-3 times when the temperature increases by 10 degrees Celsius. Climate also affects which organisms are present, affecting the soil chemically and physically (movement of roots).

❧ Organisms

The organisms living in and on the soil form distinct soil types. Coniferous forests have acidic leaf litter that form soils classed as inceptisols. Mixed or deciduous forests leave a larger layer of humus, changing the elements that are either leeched or accumulated in the soil, and thereby forming soils classed as alfisols. Prairies have very high humus accumulation, creating a dark, thick A horizon characteristic of mollisols.

For example, three species of land snails in the genus Euchondrus in the Negev desert are noted for eating lichens growing under the surface limestone rocks and slabs (endolithic lichens).^[6] They disrupt and eat the limestone.^[6] Their grazing results in the weathering of the stones, and the subsequent formation of soil.^[6] They have a significant effect on the region: the total population of snails is estimated to process between 0.7 and 1.1 metric ton per hectare per year of limestone in the Negev desert.^[6]

❧ Parent material

The rock from which soil is formed is called parent material. The main types are: aeolian sediments, glacial till, glacial outwash, alluvium, lacustrine sediments and residual parent material (coral or bedrock).

Pedologists see soil formation or soil properties as a function of regional climate, biota, topography, parent material, time and other variables.^[7]

Soils develop from parent material by various weathering processes. Organic matter accumulation, decomposition, and humification are as critically important to soil formation as weathering. The zone of humification and weathering is termed the solum.

Soil acidification resulting from soil respiration supports chemical weathering. Plants contribute to chemical weathering through root exudates.

Soils can be enriched by deposition of sediments on floodplains and alluvial fans, and by wind-borne deposits.

Soil mixing (pedoturbation) is often an important factor in soil formation. Pedoturbation includes churning clays, cryoturbation, and bioturbation. Types of bioturbation include faunal pedoturbation (animal burrowing), floral pedoturbation (root growth, tree-uprootings), and fungal pedoturbation (mycelia growth). Pedoturbation transforms soils through destratification, mixing, and sorting, as well as creating preferential flow paths for soil gas and infiltrating water. The zone of active bioturbation is termed the soil biomantle.

Soil moisture content and water flow through the soil profile support leaching of soluble constituents, and eluviation. Eluviation is the translocation of colloid material, such as organic matter, clay and other mineral compounds. Transported constituents are deposited due to differences in soil moisture and soil chemistry, especially soil pH and redox potential. The interplay of removal and deposition results in contrasting soil horizons.

⚐ Key Soil Formation Processes

Key soil forming processes especially important to macro-scale patterns of soil formation are:^[8]

• Laterization
• Podsolization
• Calcification
• Salinization
• Gleization

Note: These are soil forming processes, which are the mechanisms; soil type is oxisol, mollisol, aridisol, podzol etc. They occur due to leaching.

❧ Background Knowledge: Leaching

In pedology, leaching is the loss of mineral and organic solutes due to very heavy rainfall, high temperature and percolation. It is a mechanism of soil formation distinct from the soil forming process of eluviation, which is the loss of mineral and organic colloids. Leached and elluviated materials tend to be lost from topsoil and deposited in subsoil. A soil horizon accumulating leached and eluviated materials is referred to as a zone of illuviation.

Laterite soil, which develops in regions with high temperature and heavy rainfall, is an example of this process in action.

Mechanical illuviation[edit]

When percolating rain water reaches a drier soil horizon, water from the suspension is removed by capillary action of microchannels, leaving fine deposits (cutans) oriented along percolation macrochannels.

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 process of deposition of illuvium is termed illuviation.^[1] It is a water-assisted transport in a basically vertical direction, as compared to alluviation, the horizontal running water transfer. The resulting deposits are called illuvial deposits.

Illuvial deposits of clays, oxides, and organics accumulate in subsoil as distinctive soil horizons classified as "B horizons" or "zones of illuviation".

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 horizons is called eluviation or leaching. There is a difference in the usage of this term in geology and soil science. In soil science, eluviation is the transport of soil material from upper layers of soil to lower levels by downward precipitation of water across soil horizons, and accumulation of this material (illuvial deposit) in lower levels is called illuviation.^[1][2] In geology, the removed material is irrelevant, and the deposit (eluvial deposit) is the remaining material. Eluviation occurs when precipitation exceeds evaporation.

A soil horizon formed due to eluviation is an eluvial zone or eluvial horizon. In a typical soil profile, the eluvial horizon refers to a light-colored zone located (depending on context and literature) either at the lower part of the A horizon (symbol: Ae) or within a distinct horizon (E horizon) below the A, where the process is most intense and rapid. Yet some sources consider the eluvial zone to be the A horizon plus the (distinct) E horizon, as eluviation technically occurs in both.

In sedimentology, winnowing is the natural removal of fine material from a coarser sediment by wind or flowing water. Once a sediment has been deposited, subsequent changes in the speed or direction of wind or water flowing over it can agitate the grains in the sediment and allow the preferential removal of the finer grains. This action can improve the sorting and increase the mean grain size of a sediment after it has been deposited.^[1]

❧ Laterization*

Laterization is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics of Cancer and Capricorn.

Laterite is a soil and rock type rich in iron and aluminium, and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and long-lasting weathering of the underlying parent rock.

Laterites are formed from the leaching of parent sedimentary rocks (sandstones, clays, limestones); metamorphic rocks (schists, gneisses, migmatites); igneous rocks (granites, basalts, gabbros, peridotites); and mineralised proto-ores;^[3]:5 which leaves the more insoluble ions, predominantly iron and aluminium. The mechanism of leaching involves acid dissolving the host mineral lattice, followed by hydrolysis and precipitation of insoluble oxides and sulfates of iron, aluminium and silica under the high temperature conditions^[8] of a humid sub-tropical monsoon climate.^[9]

An essential feature for the formation of laterite is the repetition of wet and dry seasons.^[10] Rocks are leached by percolating rain water during the wet season; the resulting solution containing the leached ions is brought to the surface by capillary action during the dry season.^[10] These ions form soluble salt compounds which dry on the surface; these salts are washed away during the next wet season.^[10]Laterite formation is favoured in low topographical reliefs of gentle crests and plateaus which prevents erosion of the surface cover.^[5]:4The reaction zone where rocks are in contact with water—from the lowest to highest water table levels—is progressively depleted of the easily leached ions of sodium, potassium, calcium and magnesium.^[10] A solution of these ions can have the correct pH to preferentially dissolve silicon oxide rather than the aluminium oxides and iron oxides.^[10]

The mineralogical and chemical compositions of laterites are dependent on their parent rocks.^[3]:6 Laterites consist mainly of quartz, zircon, and oxides of titanium, iron, tin, aluminium and manganese, which remain during the course of weathering.^[3]:7 Quartz is the most abundant relic mineral from the parent rock.^[3]:7

Laterites vary significantly according to their location, climate and depth.^[8] The main host minerals for nickel and cobalt can be either iron oxides, clay minerals or manganese oxides.^[8] Iron oxides are derived from mafic igneous rocks and other iron-rich rocks; bauxites are derived from granitic igneous rock and other iron-poor rocks.^[10] Nickel laterites occur in zones of the earth which experienced prolonged tropical weathering of ultramafic rocks containing the ferro-magnesian minerals olivine, pyroxene, and amphibole.^[5]

^{Oxisols are an order in USDA soil taxonomy, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator.}

^{The main processes of soil formation of oxisols are weathering, humification and pedoturbation due to animals. These processes produce the characteristic soil profile. They are defined as soils containing at all depths no more than 10 percent weatherable minerals, and low cation exchange capacity. Oxisols are always a red or yellowish color, due to the high concentration of iron(III) and aluminium oxides and hydroxides. In addition they also contain quartz and kaolin, plus small amounts of other clay minerals and organic matter.}

<sup>Scientists originally thought that the heavy vegetation of tropical rain forests would provide rich nutrients, but as rainfall passes through the litter on the forest floor the rain is acidified and leaches minerals from the above soil layers. This forces plants to get their nutrition from decaying litter as oxisols are quite infertile due to the lack of organic matter and the almost complete absence of soluble minerals leached by the wet and humid climate.

Laterization is a pedogenic process common to soils found in tropical and subtropical environments.  Movements of large amounts of water through the soil cause eluviation and leaching to occur. Almost all of the by products of weathering, very simple small compounds or nutrient ions, are translocated out of the soil profile by leaching if not taken up by plants for nutrition. The two exceptions to this process are iron and aluminum compounds. Iron oxides give tropical soils their unique reddish coloring. Heavy leaching also causes these soils to have an acidic pH because of the net loss of base cations.</sup>

❧ Calcification*

Calcification occurs when evapotranspiration exceeds precipitation causing the upward movement of dissolved alkaline salts from the groundwater. At the same time, the movement of rain water causes a downward movement of the salts. The net result is the deposition of the translocated cations in the B horizon. In some cases, these deposits can form a hard layer called caliche. The most common substance involved in this process is calcium carbonate. Calcification is common in the prairie grasslands.

❧ Salinization*

Salinization is a process that functions in the similar way to calcification. It differs from calcification in that the salt deposits occur at or very near the soil surface. Salinization also takes place in much drier climates.

❧ Podsolization

Podzolization is associated with humid cold mid-latitude climates and coniferous vegetation. Decomposition of coniferous litter and heavy summer precipitation create a soil solution that is strongly acidic. This acidic soil solution enhances the processes of eluviation and leaching causing the removal of soluble base cations and aluminum and iron compounds from the A horizon. This process creates a sub-layer in the A horizon that is white to gray in color and composed of silica sand.

❧ Gleization

Gleization is a pedogenic process associated with poor drainage. This process involves the accumulations of organic matter in the upper layers of the soil. In lower horizons, mineral layers are stained blue-gray because of the chemical reduction of iron.

⚐ Examples

A variety of mechanisms contribute to soil formation, including siltation, erosion, overpressure and lake bed succession. A specific example of the evolution of soils in prehistoric lake beds is in the Makgadikgadi Pans of the Kalahari Desert, where change in an ancient river course led to millennia of salinity buildup and formation of calcretes and silcretes.^[9]

⚐ 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 thickness:

${\displaystyle de/dt=P_{0}\exp {[-kh]}\ }$

where h is soil thickness [m], P₀ [mm/year] is the potential (or maximum) weathering rate of bedrock and k [m⁻¹] is an empirical constant.

The reduction of weathering rate with thickening of soil is related to the exponential decrease of temperature amplitude with increasing depth below the soil surface, and also the exponential decrease in average water penetration (for freely-drained soils).

Karst Landscapes

https://courses.washington.edu/tesc243/jamaica/index_files/page0004.htm

https://en.wikipedia.org/wiki/Karst

Mass Movement (General, Incomplete) (integrated)

Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, sand, regolith, and rock move downslope typically as a mass, largely under the force of gravity, but frequently affected by water and water content as in submarine environments and mudflows.^[1] Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place over timescales from seconds to years. Mass wasting occurs on both terrestrial and submarine slopes

When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass wasting) occurs. The slope material's strength and cohesion and the amount of internal friction between material help maintain the slope's stability and are known collectively as the slope's shear strength. The steepest angle that a cohesionless slope can maintain without losing its stability is known as its angle of repose. When a slope made of loose material possesses this angle, its shear strength perfectly counterbalances the force of gravity acting upon it.

Mass wasting may occur at a very slow rate, particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilized the surface. It may also occur at very high speed, such as in rockslides or landslides, with disastrous consequences, both immediate and delayed, e.g., resulting from the formation of landslide dams.

Factors that change the potential of mass wasting include: change in slope angle, weakening of material by weathering, increased water content; changes in vegetation cover, and overloading.

Types of mass movement[edit]

Types of mass movement are distinguished based on how the soil, regolith or rock moves downslope as a whole.

Creeps[edit]

Soil creep is a long term process. The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope. The steeper the slope, the faster the creep. The creep makes trees and shrubs curve to maintain their perpendicularity, and they can trigger landslides if they lose their root footing. The surface soil can migrate under the influence of cycles of freezing and thawing, or hot and cold temperatures, inching its way towards the bottom of the slope forming terracettes. This happens at a rate that is not noticeable to the naked eye.

Landslides[edit]

A landslide, also called a landslip, is a rapid movement of a large mass of earth and rocks down a hill or a mountainside. Little or no flowage of the materials occurs on a given slope until heavy rain and resultant lubrication by the same rainwater facilitate the movement of the materials, causing a landslide to occur. The common forms of landslides are slump, debris slide, rock slide, rock fall, debris fall and avalanche.

Flows[edit]

Movement of soil and regolith that more resembles fluid behavior is called a flow. These include avalanches, mudflows, debris flows, earth flow, lahars and sturzstroms. Water, air and ice are often involved in enabling fluidlike motion of the material.

Slump[edit]

A slipping of coherent rock material along the curved surface of a decline. Slumps involve a mass of soil or other material sliding along a curved surface (shaped like a spoon). It forms a small, crescent-shaped cliff, or abrupt scarp at the top end of the slope. There can be more than one scarp down the slope.

Falls[edit]

A fall, including rockfall, occurs where regolith cascades down a slope, but is not of sufficient volume or viscosity to behave as a flow. Falls are promoted in rocks which are characterised by presence of vertical cracks. Falls can result from undercutting by running water as well as from undercutting by waves. They usually occur at very steep slopes such as a cliff face. The rock material may be loosened by earthquakes, rain, plant-root wedging, and expanding ice, among other things. The accumulation of rock material that has fallen and resides at the base of the structure is known as talus.

Triggers of mass wasting[edit]

Soil and regolith remain on a hillslope only while the gravitational forces are unable to overcome the frictional forces keeping the material in place (see slope stability). Factors that reduce the frictional resistance relative to the downslope forces, and thus initiate slope movement, can include:

• earthquakes
• increased overburden from structures
• increased soil moisture
• reduction of roots holding the soil to bedrock
• undercutting of the slope by excavation or erosion
• weathering by frost heave
• bioturbation
• volcanic edifice over-steepening

The importance of water in mass wasting[edit]

Water can increase or decrease the stability of a slope depending on the amount present. Small amounts of water can strengthen soils because the surface tension of water increases soil cohesion. This allows the soil to resist erosion better than if it were dry. If too much water is present the water may act to increase the pore pressure, reducing friction, and accelerating the erosion process and resulting in different types of mass wasting (i.e. mudflows, landslides, etc.). A good example of this is to think of a sand castle. Water must be mixed with sand in order for the castle to keep its shape. If too much water is added the sand washes away, if not enough water is added the sand falls and cannot keep its shape. Water also increases the mass of the soil, this is important because an increase in mass means that there will be an increase in velocity if mass wasting is triggered.

Additional Material:

Detailed Document (48 pages) on Mass Movement:

https://www.google.com.sg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwji0qqilu_SAhVFwI8KHddoALoQFggeMAE&url=http%3A%2F%2Fgis.ess.washington.edu%2Fgrg%2Fcourses10_11%2Fess326%2Ftextbook%2FChapter_5_Revised.doc&usg=AFQjCNHkPjkrqaYAZtLOmzrQzpdW7AqMNg&sig2=ATfkpeelnAJfNJN8Gs-nZw

(Unformatted) El Nino Phenomenon: Walker Circulation & El Nino Southern Oscillation (integrated)

Concept:
At the end of every year, the cold, northward strong Peruvian current is replaced by a warm, southward weak countercurrent. This occurrence was orginally named 'El Nino', after Christ as it usually appeared during the Christmas season. The warm weak southward countercurrents typically last for a few weeks before it is replaced by the cold strong northward Peruvian current. However, this weak warm southward countercurrent becomes stronger every 3 to 7 years, and along with this current is the accumulation of warm surafce water in the Eastern Pacific. Scientists these days use the term 'El Nino' specifically referring to the episodes of ocean warming that occurs every few years only.

The first observation of something related would be by Sir Gilbert Walker, who noticed an irregular oscillation between atomspheric pressure in Darwin, northern Australia, and Tahiti, a central Pacific equatorial Island.  Tahiti usually has higher atmospheric pressure than Darwin, but every time, towards the end of the year, where El Nino occurs, the barometric pressure drops over large portions of the South eastern Pacific, whereas in the Western Pacific, near Indonesia and Northern Australia, the pressure increases. Then, when the currents change, the pressure difference between the two regions change back to normal, with the Eastern Pacific having higher pressure while the Western Pacific turned to having lower pressure.

From Wikipedia:
El Niño /ɛl ˈniːnjoʊ/ (Spanish pronunciation: [el ˈniɲo]) is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. El Niño Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature, SST, of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called "La Niña" with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, cause global changes of both temperatures and rainfall.^[2][3]

Originally the term El Niño applied to an annual weak warm ocean current that ran southwards along the coast of Peru and Ecuador at about Christmas time.^[7] However, over time the term has evolved and now refers to the warm and negative phase of the El Niño Southern Oscillation and is the warming of the ocean surface or above-average sea surface temperatures in either the central and eastern tropical Pacific Ocean.^[8][9] This warming causes a shift in the atmospheric circulation with rainfall becoming reduced over Indonesia and Australia, while rainfall and tropical cyclone formation increases over the tropical Pacific Ocean.^[10] The low-level surface trade winds, which normally blow from east to west along the equator, either weaken or start blowing from the other direction.^[9]

Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years.^[24] The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration is longer, it is classified as an El Niño "episode".^[25]

Effects on the global climate[edit]

El Nino affects the global climate and disrupts normal weather patterns, which as a result can lead to intense storms in some places and droughts in others.^[60][61]

Tropical cyclones[edit]

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.^[62] Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago.^[63]

Within the Atlantic Ocean vertical wind shear is increased, which inhibits tropical cyclone genesis and intensification, by causing the westerly winds in the atmosphere to be stronger.^[64] The atmosphere over the Atlantic Ocean can also be drier and more stable during El Niño events, which can also inhibit tropical cyclone genesis and intensification.^[64]  (Recall: Air is drier means unsaturated and thus conditional stability due to environmental lapse rate being between the dry adiabatic lapse rate and the moist adiabatic lapse rate. Additionally, air may also be absolutely stable because the environmental lapse rate is below the moist adiabatic lapse rate.) Within the Eastern Pacific basin: El Niño events contribute to decreased easterly vertical wind shear and favours above-normal hurricane activity.^[65] However, the impacts of the ENSO state in this region can vary and are strongly influenced by background climate patterns.^[65] The Western Pacific basin experiences a change in the location of where tropical cyclones form during El Niño events, without a major change in how many develop each year.^[64]As a result of this change Micronesia is more likely to be affected by several tropical cyclones, while China has a decreased risk of being affected by several tropical cyclones.^{[citation needed]} A change in the location of where tropical cyclones form also occurs within the Southern Pacific Ocean between 135°E and 120°W, with tropical cyclones more likely to occur within the Southern Pacific basin than the Australian region.^[10][64] As a result of this change tropical cyclones are 50% less likely to make landfall on Queensland, while the risk of a tropical cyclone is elevated for island nations like Niue, French Polynesia, Tonga, Tuvalu and the Cook Islands.^[10][66][67]

Remote influence on tropical Atlantic Ocean[edit]

A study of climate records has shown that El Niño events in the equatorial Pacific are generally associated with a warm tropical North Atlantic in the following spring and summer.^[68] About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool to become unusually large in summer.^[69] Occasionally, El Niño's effect on the Atlantic Walker circulation over South America strengthens the easterly trade winds in the western equatorial Atlantic region. As a result, an unusual cooling may occur in the eastern equatorial Atlantic in spring and summer following El Niño peaks in winter.^[70] Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.^[22]

Antarctica[edit]

Many ENSO linkages exist in the high southern latitudes around Antarctica.^[71] Specifically, El Niño conditions result in high-pressure anomalies over the Amundsen and Bellingshausen Seas, causing reduced sea ice and increased poleward heat fluxes in these sectors, as well as the Ross Sea. The Weddell Sea, conversely, tends to become colder with more sea ice during El Niño. The exact opposite heating and atmospheric pressure anomalies occur during La Niña.^[72] This pattern of variability is known as the Antarctic dipole mode, although the Antarctic response to ENSO forcing is not ubiquitous.^[72]

Regional impacts[edit]

Observations of El Niño events since 1950, show that impacts associated with El Niño events depend on what season it is.^[73] However, while certain events and impacts are expected to occur during events, it is not certain or guaranteed that they will occur.^[73] The impacts that generally do occur during most El Niño events include below-average rainfall over Indonesia and northern South America, while above average rainfall occurs in southeastern South America, eastern equatorial Africa, and the southern United States.^[73]

Australia and the Southern Pacific[edit]

During El Niño events, the shift in rainfall away from the Western Pacific, can mean that rainfall across Australia is reduced.^[10] Over the southern part of the continent, warmer than average temperatures can be recorded as weather systems are more mobile and fewer blocking areas of high pressure occur.^[10] The onset of the Indo-Australian Monsoon in tropical Australia is delayed by two to six weeks, which as a consequence means that rainfall is reduced over the northern tropics.^[10] The risk of a significant bushfire season in south-eastern Australia is higher following an El Niño event, especially when it is combined with a positive Indian Ocean Dipole event.^[10] During an El Niño event, New Zealand tends to experience stronger or more frequent westerly winds during their summer, which leads to an elevated risk of drier than normal conditions along the east coast.^[74] There is more rain than usual though on New Zealand's West Coast, because of the barrier effect of the North Island mountain ranges and the Southern Alps.^[74]

Fiji generally experiences drier than normal conditions during an El Niño, which can lead to drought becoming established over the Islands.^[75] However, the main impacts on the island nation is felt about a year after the event becomes established.^[75] Within the Samoan Islands, below average rainfall and higher than normal temperatures are recorded during El Niño events, which can lead to droughts and forest fires on the islands.^[76] Other impacts include a decrease in the sea level, possibility of coral bleaching in the marine environment and an increased risk of a tropical cyclone affecting Samoa.^[76]

Africa[edit]

In Africa, East Africa — including Kenya, Tanzania, and the White Nile basin — experiences, in the long rains from March to May, wetter-than-normal conditions. Conditions are also drier than normal from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique, and Botswana.

Asia[edit]

As warm water spreads from the west Pacific and the Indian Ocean to the east Pacific, it takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific. Singapore experienced the driest February in 2014 since records began in 1869, with only 6.3 mm of rain falling in the month and temperatures hitting as high as 35 °C on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.^[77]

Europe[edit]

El Niño's effects on Europe are controversial, complex and difficult to analyse, as it is one of several factors that influence the weather over the continent and other factors can overwhelm the signal.^[78][79]

North America[edit]

See also: Effects of the El Niño–Southern Oscillation in the United States

Over North America, the main temperature and precipitation impacts of El Niño, generally occur in the six months between October and March.^[80][81] In particular the majority of Canada generally has a milder than normal winters and springs, with the exception of eastern Canada where no significant impacts occur.^[82] Within the United States, the impacts generally observed during the six-month period include; wetter-than-average conditions along the Gulf Coast between Texas and Florida, while drier conditions are observed in Hawaii, the Ohio Valley, Pacific Northwest and the Rocky Mountains.^[80] Over California and the South-Western United States, there is a weak relationship between El Nino and above-average precipitation, as it strongly depends on the strength of the El Niño event amongst other factors.^[80]

The synoptic condition for the Tehuantepecer is associated with high-pressure system forming in Sierra Madre of Mexico in the wake of an advancing cold front, which causes winds to accelerate through the Isthmus of Tehuantepec. Tehuantepecers primarily occur during the cold season months for the region in the wake of cold fronts, between October and February, with a summer maximum in July caused by the westward extension of the Azores High. Wind magnitude is greater during El Niño years than during La Niña years, due to the more frequent cold frontal incursions during El Niño winters.^[83] Its effects can last from a few hours to six days.^[84]

South America[edit]

Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean, including several portions of the South American west coast. The effects of El Niño in South America are direct and stronger than in North America. An El Niño is associated with warm and very wet weather months in April–October along the coasts of northern Peru and Ecuador, causing major flooding whenever the event is strong or extreme.^[85] The effects during the months of February, March, and April may become critical. Along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, which in turn sustain abundant sea birds, whose droppings support the fertilizer industry. The reduction in upwelling leads to fish kills off the shore of Peru.^[86]

The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. The world's largest fishery collapsed due to overfishing during the 1972 El Niño Peruvian anchoveta reduction. During the 1982–83 event, jack mackerel and anchoveta populations were reduced, scallops increased in warmer water, but hake followed cooler water down the continental slope, while shrimp and sardines moved southward, so some catches decreased while others increased.^[87] Horse mackerel have increased in the region during warm events. Shifting locations and types of fish due to changing conditions provide challenges for fishing industries. Peruvian sardines have moved during El Niño events to Chilean areas. Other conditions provide further complications, such as the government of Chile in 1991 creating restrictions on the fishing areas for self-employed fishermen and industrial fleets.^{[citation needed]}

The ENSO variability may contribute to the great success of small, fast-growing species along the Peruvian coast, as periods of low population removes predators in the area. Similar effects benefit migratory birds that travel each spring from predator-rich tropical areas to distant winter-stressed nesting areas.^{[citation needed]}

Southern Brazil and northern Argentina also experience wetter than normal conditions, but mainly during the spring and early summer. Central Chile receives a mild winter with large rainfall, and the Peruvian-Bolivian Altiplano is sometimes exposed to unusual winter snowfall events. Drier and hotter weather occurs in parts of the Amazon River Basin, Colombia, and Central America.^{[citation needed]}

Tropical Cyclone Genesis, Maintenance, & intensification (integrated)

Tropical Cyclones
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere.^[1] Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.^[2] 

⭄ There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear.^[3]

Atmospheric Circulation (integrated)

Global Atmospheric Circulation
Atmospheric circulation is the large-scale movement of air, and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth.

⇰ The Earth's weather is a consequence of its illumination by the Sun, and the laws of thermodynamics. There are differences in shortwave solar radiation received and longwave terrestrial radiation emitted for the earth at the equatorial and the polar regions (specifically, the torrid zones and the friggid zones).

⇰ The atmospheric circulation can be viewed as a heat engine driven by the Sun's energy, and whose energy sink, ultimately, is the blackness of space. The work produced by that engine causes the motion of the masses of air and in that process it redistributes the energy absorbed by the Earth's surface near the tropics to space and incidentally to the latitudes nearer the poles.

Tropical Climatic Zones (integrated)

Climate Classification
The variations in interactions between planet earth and the spheres (atmosphere, biosphere, hydrosphere, geosphere) give each location certain distinct climatic characteristics. In order to conveniently analyse them, there is a need to classify them. This would simplify all the climatic types into a few groups, each with their own unique climatic character.