Ferrallitisation is the process in which rock

Rock music

Rock music is a genre of popular music that developed during and after the 1960s, particularly in the United Kingdom and the United States. It has its roots in 1940s and 1950s rock and roll, itself heavily influenced by rhythm and blues and country music...

 is changed into a soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 consisting of clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 (kaolinite

Kaolinite

Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...

) and 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 form of hydrated oxides of iron and aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. In humid tropical areas, with consistently high tempetures and rainfall for all or most of the year, chemical weathering rapidly breaks down the rock. This at first produces clays which later also break down to form silica. The silica is removed by leaching

Leaching (pedology)

In pedology, leaching is the loss of mineral and organic solutes due to percolation. It is a mechanism of soil formation. It is 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...

 and the sesquioxides of iron and aluminium remin, giving the charartistic red colour of many tropical soils. Ferrallitisation is the reverse of podsolisation

Podsolisation

Podsolisation is an extreme form of leaching which causes the eluviation of iron and aluminium sesquioxides.The process generally occurs in areas where precipitation is greater than evapotranspiration. The minerals are removed by a process known as leaching.When organic material is broken down...

, where silica remains and the iron and aluminum are removed. In tropical rain forests with rain throuhtout the year, ferrallitic soils develop. In savanna

Savanna

A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...

 areas, with altering dry and wet climates, ferruginous

Ferruginous

Ferruginous can refer to:* Iron oxide* Ferruginous bodies* Ferruginous Hawk* Ferruginous Duck...

soil occur.