General Characteristics
Boris Gradusov

Lithology refers to hard and loose rocks and deposits of various stages of continental lithogenesis, which form the surface layer involved in active daily and annual thermo-hydrologic cycles. These rocks and deposits contribute matter to the turnover of biogeochemical processes and soil formation, and deliver substances transported by runoff.

The lithogenic basis constitutes parent rocks from which soils arise. These rocks form a layer of the geological hard and/or loose rocks, which is the underlying soil profile and is not influenced by soil-forming processes. Many studies have been carried out to distinguish the characteristics of parent rocks from those of soils. Thus the chemical and mineralogical composition of parent rocks, which plays an important role in ecosystems functioning, is well-known.

Most geographical publications, however, consider genetic types of rocks, ignoring their mineralogy, texture, and chemical composition. Yet, these are crucial for understanding the evolution and functioning of the ecosystem structure. On the basis of geotectonic factors guiding ecosystem formation, two main groups of lithogenic basis have been distinguished for the territory of Russia: mountain-geosynclines and plain-platform (see Relief section). Further classification distinguishes lithological genetic types associated with the deposits origin and lithological species, which are characterized by sediments composition and quality, e.g. carbonate content, petrography, texture, etc. (Table 1).

There is a lack of correlation between genetic types of rocks and their petrographic-mineralogical composition. A single genetic type of rock might contain different petrographic-mineralogical associations and, vice versa, the same petrographic-mineralogical associations may be found in different genetic types of rocks. Loess-like loams and loess, the mineralogical composition of which is relatively homogeneous, are the exception from this regularity. However, even in this genetic type, the petrographic-mineralogical composition might vary from hydromica-smectite in the Russian Plain to hydromica in Siberia.

Two fundamentally different types of the earth's crust determine the main patterns of geographical distribution of the lithological basis, such as oceanic and continental, as well as the  major features of spatial distribution of petrographical-mineralogical associations. Minerals and mixed-layer aluminosilicates have been formed by the present-day and past processes of diagenesis, catagenesis, and metamorphism in the zone of tectonic activity. Further, these minerals (including clayey mixed-layer silicates) have been eroded and redeposited, serving as a source for all sedimentary rocks including parent materials and soils derived from them. Thus, the lithogenic basis reflects a long-term and complicated evolution of the earth's crust. It embraces both endogenous and exogenous processes, including denudation and sedimentation, evolution of climate, biosphere etc.

The lithogenic basis is a component of ecosystems that takes a long time to develop and evolve. Changes in oxide-silicate minerals are irreversible, and thus guide ecosystem evolution in a certain way. Human induced impacts tend to increase a role of the lithogenic basis for structure, functioning, and evolution of the ecosystems.

Table 1. Genetic types and lithological species of parent and underlying rocks.

Bibliography

Vernadskiy V.I. 1992. Selected Publications on Biogeochemistry and Geochemistry of Soils. Nauka, Moscow, 435 pp.[In Russian]

Gradusov B.P. 1976. Clay minerals in parent rocks and soils. Weathering Crust 15:131–148. [In Russian]

Pedro G. 1987. Current aspects of mineralogy of clays and soils. Proceedings of the 20^th Colloquium of International Potash Institute. Bern, Switzerland, pp. 11–45.