Geographic Shell

(also landscape shells, epigeo-sphere), the earth’s shell, where the lithosphere, hydrosphere, atmosphere, and biosphere are contiguous and interact. The composition and structure of the geographic envelope are complex. Its upper boundary is generally drawn at the stratopause, since it is to this point that the thermal influence of the earth’s surface affects atmospheric processes. The boundary of the geographic shell in the lithosphere is often regarded as coinciding with the lower limits of the region of hypergenesis. (Sometimes the lower boundary is considered to be the foot of the stratisphere, the average depth of seismic or volcanic focuses, the base of the earth’s crust, or the level of zero annual temperature amplitude.) Thus, the geographic shell completely encompasses the hydrosphere, reaching a depth of 10-11 km below the surface of the earth in oceans, the upper zone of the earth’s crust, and the lower part of the atmosphere (to a level of 25-30 km). The maximum thickness of the geographic shell is close to 40 km.

The geographic shell may be distinguished from other shells of the earth in a number of ways. It is formed by the action of terrestrial as well as cosmic processes, and it is exceptionally rich in various forms of free energy. Matter is present there in all states of aggregation, and the degree of aggregateness of matter varies greatly, ranging from free elementary particles through atoms, ions, and molecules to chemical compounds and the most complex biological organisms. The shell may also be distinguished by its concentration of heat flowing from the sun and by the presence of human society.

The basic material components of the geographic shell are the rocks making up the earth’s crust (with their formations, or relief), air masses, bodies of water, the mantle of soil, and biocenosis. The ice accumulations at the polar latitudes and in high mountains also play an important role. The basic energy components are gravitational energy, heat from the interior of the planet, radiant energy from the sun, and the energy from cosmic rays. Although the component elements are limited, their combinations can be highly diverse. This depends both on the number of components involved and on their intrinsic variations (since each component is itself a very complex natural aggregate). Mainly, however, this diversity depends on the way in which the components interact and interrelate, that is, on their geographic structures.

The geographic shell is characterized by a number of important features—integrity, a cycle of matter, rhythm, and continuous development. Its integrity results from a continuous exchange of matter and energy among its component parts, since the interaction of all the components binds them into a single material system in which a change in even one link involves a corresponding change in all the others.

The geographic shell is characterized by the presence of matter cycles (and of related energy cycles), which ensure the recurrence of the same processes and phenomena and their maximum effectiveness even under the conditions of the limited volume of the original matter taking part in these processes. The complexity of cycles varies. Some involve mechanical motion, such as atmospheric circulation and the system of marine surface currents. Others are accompanied by changes in the aggregate composition of matter (the hydrologic cycle on earth). In yet other cycles there is a chemical transformation of the matter as well (the biological cycle). Cycles, however, are not closed, and differences between their initial and final stages testify to the development of the whole system.

Another feature of the shell is its rhythm, that is, the recurrence in time of various processes and phenomena. It is caused for the most part by astronomic and geologic forces. There are rhythms occurring over the period of a 24-hour day (the alternation of day and night), annual rhythms (the alternation of the seasons of the year), rhythms occurring within a century (for example, the 25-30 year cycles in which are observed variations in climate, glaciers, the levels of lakes, and the volumes of rivers), rhythms occurring over centuries (for example, the alternations of a phase of cool-humid climate with a phase of hot, dry climate, occurring every 1,800-1,900 years), geologic rhythms (the Caledonian, Hercynian, and Alpine cycles, each lasting from 200 to 240 million years), and so on. Rhythms, like cycles, are not closed —the conditions found in the initial stage of the rhythmic cycle do not recur at the end.

Continuity of development is an inseparable feature of the geographic shell as an integral system under the influence of the conflicting interaction of exogenous and endogenous forces. There are several results and characteristics of this continuous development. (1) There is a territorial differentiation of continents, oceans, and the ocean bottom according to internal features and external appearance such as landscapes and geocomplexes. This differentiation is determined by spatial changes in geographic structures. Special forms of territorial differentiation are geographic zonality and altitude zonality. (2) There is polar asymmetry, that is, considerable differences in nature in the geographic shell in the northern and in the southern hemispheres. This manifests itself, for example, in the distribution of continents and oceans—the overwhelming proportion of dry land is found in the northern hemisphere—in the climate, in the composition of animal and plant life, and in the character of landscape zones. (3) There is also heterochronism (according to K. K. Markov, meta-chronism) of the development of the geographic shell, caused by the spatial heterogeneity of nature on earth. This results in different regions at the same time either experiencing different phases of an identically directed evolutionary process or being differentiated from one another by the direction of development. For example, the ancient Ice Age in various parts of the earth began and ended at different times. In some geographic zones the climate is becoming drier, while in others at the same time it is becoming more humid.

The geographic shell is the object of study in physical geography.

P. I. Brounov (1910) and R. I. Abolin (1914) first developed the concept of the geographic shell. A. A. Grigor’ev (1932) introduced and substantiated the term itself. Concepts analogous to the concept of geographic shell also exist in foreign geographic literature, for example, A. Hettner’s and R. Hartshorne’s concept of the earth’s shell and H. Carol’s concept of the geosphere. However, in these concepts the geographic shell is usually viewed not as a natural system but as an aggregate of natural and social phenomena.