Geoscience Reference
In-Depth Information
This is the horizon of concretionary pisolithic ironstones
(A/B) and may be between 1·0 m and 1·5 m thick. Plates
18.21a and 18.21b s how a lateritic podzolic soil from the
Eastern Darling ranges, Western Australia. The pale
surface eluvial layer is up to 20 cm thick in this case, and
overlies the red-brown pisolithic horizon from 20-80 cm
deep. Below this is about 10 m of weathered granite.
As this is at present a semi-arid region, the main soil
formation probably took place in wetter Tertiary times.
Plate 18.18b shows a close-up of the structure of the
pisolithic concretions which reach a maximum of 2 cm
diameter.
In humid tropical regions the main laterite ( plinthite )
horizon with an accumulation of iron and other
sesquioxides occurs below the pisolite. This material is
used extensively as a building material in India and
Thailand; it is dug out of the ground, shaped into bricks
and allowed to dry irreversibly in the sun. This has given
laterite its name (Latin later , 'brick'). This horizon is about
1·0 m to 1·5 m thick and is the horizon of cemented
ironstone sesquioxides (Box). Deeper down the soil
becomes paler with distinctive red mottles. This is the
Tropical soil formation: katamorphism
and laterization
Tropical regions are well known for the speed and
intensity of processes of weathering and soil formation.
Where abundant moisture is available to match the
prevailing high temperatures the soils are the product
of intense tropical weathering which removes all the
geochemically unstable elements (potassium, sodium,
magnesium and calcium) from the soil and concentrates
sesquioxides (oxides of iron, aluminium and manganese)
and silicon. The rapid breakdown of rock minerals and
the thorough leaching of base elements in tropical
environments is termed katamorphism .
The resulting soil profile is shown in Figure 18.15 . At
the surface, organic matter is rapidly decomposed so that
the Ah humus horizon is very thin. Surface eluviation
may produce a thin, eluvial layer (Ea) but this is usually
masked by the red iron colours. The surface horizons are
commonly high in concretionary iron particles about the
size of a large pea. They thus appear gravelly, but the
gravels are pedologically formed, not alluvially formed.
metres
HORIZONS
PROCESSES
0
Ah
Humification
Eluviation
Ae
Oxidation and
dehydration
A/B
1
PLINTHITE
(LATERITE)
Fe and
Al
Box
Wet season
water table
2
Alternate
oxidation and
reduction
MOTTLED
HORIZON
3
Bgox
Fe and
Al
Some
silica
4
Dry season
water table
PALLID
HORIZON
Silicification
Reduction
5
Cg
Loss of
bases and
some
silica
R
Weathering
6
Figure 18.15 Process of laterization (ferralitization).
 
 
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