2.3 WEATHERING
27
Fig. 2.5. Diagrammatic profile of the modern hemisphere showing the relationship between climate, soil, and
weathering. (For sources, see Strakhov, 1962, and Lisitzin, 1972.)
2.3.1.1 Modern Soils
In polar climates true soil profiles do not develop due to the absence of organisms. A
weathering mantle may be present, but this is frozen for much, if not all of the year. This
is called permafrost. In temperate climates leaching plays a dominant role. The A zone
is intensely weathered, though it may support an upper peaty zone of plant material.
The high pH of such soil inhibits or delays bacterial decay. The B zone may be deep but
is typically well developed as a limonitic or calcareous hard-pan that inhibits drainage.
Soils of this general type include the "podsols" of cool temperate climates and the humus rich "tchernozems," or "black earths," of warm temperate zones.
In arid climates, by contrast, the downward percolation of chemicals by leaching is
offset by the upward movement of moisture by capillary attraction. Precipitation of
solute occurs, therefore, at or close to the land surface. Organic content is very low. By
this means are formed the "duricrusts" that are found at or close to the surface in many
modern deserts (Woolnough, 1927). These hard crusts commonly show mottled, nodular, pisolitic, and concretionary structures as the minerals are precipitated in colloform
habits (see Plate 5A). Sometimes cylindrical and anastomosing tubular concretions occur in ancient soil profiles. These are attributed to the precipitation of minerals around
plant root systems that have subsequently dissolved. Such structures are referred to as
"rhizoconcretions" or "dikaka" (Glennie and Evamy, 1968). Modern rhizoconcretions
occur around the roots of palm trees in modern desert oases. They are composed of the
27
Fig. 2.5. Diagrammatic profile of the modern hemisphere showing the relationship between climate, soil, and
weathering. (For sources, see Strakhov, 1962, and Lisitzin, 1972.)
2.3.1.1 Modern Soils
In polar climates true soil profiles do not develop due to the absence of organisms. A
weathering mantle may be present, but this is frozen for much, if not all of the year. This
is called permafrost. In temperate climates leaching plays a dominant role. The A zone
is intensely weathered, though it may support an upper peaty zone of plant material.
The high pH of such soil inhibits or delays bacterial decay. The B zone may be deep but
is typically well developed as a limonitic or calcareous hard-pan that inhibits drainage.
Soils of this general type include the "podsols" of cool temperate climates and the humus rich "tchernozems," or "black earths," of warm temperate zones.
In arid climates, by contrast, the downward percolation of chemicals by leaching is
offset by the upward movement of moisture by capillary attraction. Precipitation of
solute occurs, therefore, at or close to the land surface. Organic content is very low. By
this means are formed the "duricrusts" that are found at or close to the surface in many
modern deserts (Woolnough, 1927). These hard crusts commonly show mottled, nodular, pisolitic, and concretionary structures as the minerals are precipitated in colloform
habits (see Plate 5A). Sometimes cylindrical and anastomosing tubular concretions occur in ancient soil profiles. These are attributed to the precipitation of minerals around
plant root systems that have subsequently dissolved. Such structures are referred to as
"rhizoconcretions" or "dikaka" (Glennie and Evamy, 1968). Modern rhizoconcretions
occur around the roots of palm trees in modern desert oases. They are composed of the