Laterite formation and evolution

In tectonically quiescent, flat areas located in tropical regions, the earth surface undergone very strong weathering condition associated with tropical climate and high temperature and precipitation. With time, the rock present at the surface is partially modified by dissolution and new mineral precipitation, producing a geological formation called laterite given a strong reddish color to tropical area. In humid tropical areas, laterites occupy 30% of the continental surface, mainly in the tropics, and form the substrate for the intensive farming sustaining a large part of the population living in these regions.
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Laterites are strongly depleted in nutriment element that are washout during weathering, and only adapted vegetation can be developed over the laterite. The name laterite comes from later in Latin referring to brick, that was firstly described in India in the early 17th century by H. Buchanan. It refers to reddish materials used to construct houses and the broad meaning of the term laterite designates all materials, loose or indurated, rich in iron or aluminum oxides and hydroxides, constituting soils, superficial horizons, deep horizons of weathering profile.
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The term laterite has slightly evolved though time depending on the community and countries. The international present definition of laterite refers as the weathering part from the fresh host rock to the organic soil sensus stricto. Laterite can be from few meters up to hundreds of meters deep. Laterite is formed from the bottom to the top by hard saprolite, saprolite rich in clays, a duricrust rich in iron and/or aluminum oxides and hydroxides, and a soil formed with the disablement of the laterite named latosol, oxisol or ferralite. An organic soil s.s. is present at the top of the laterite.
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Laterite formation model

Laterites and their components (primary minerals, secondary minerals, water, plants…) are a tropical expression of the Critical Zone "CZ". The formation of laterites, like that of all soils, results from the transfer of chemical elements between different compartments of the CZ. The "rockderived" soluble elements such as Na, Ca, Mg, K, Sr, or U are released by these weathering reactions. Some of these elements are nutrients (Ca, Mg, Si, K), and their dynamics also yield information on the role of biological cycling on soil formation. The release of chemical elements from the parent rock will lead to the precipitation of secondary phases such as clays (e.g., kaolinite) and oxides (e.g., iron oxides, aluminum oxides and manganese oxides) hosting the less soluble elements such as Al, Fe, or Ti.
The formation of these secondary phases entails a fractionation of the different isotopes of some elements involved into these reactions (major elements such as Si or Fe, or trace elements such as Li). With time and favorable hydrological condition, the laterite profile will be deeper and show mineralogical compartmentalization. Such deep weathering profiles can accumulate only through the combination of intense chemical weathering and slow physical erosion.
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