Thus, with a decrease in the polar flattening (which is tantamount to a decrease in the Earth's rotational velocity) the Earth's crust in the equatorial region undergoes deformations of latitudinal and meridional contraction (the latter occurring predominantly in the latitudinal zone near the critical parallels), accompanied with radial subsidences; the Earth's crust in the polar regions undergoes latitudinal and meridional tensile deformations accompanied with radial upliftings. An increase of the polar flattening (and rotational velocity) of the Earth causes tensile latitudinal and meridional deformations of the Earth's crust in the equato rial region accompanied with radial upliftings, while the crust in the polar regions undergoes compressive latitudinal and meridional deformations accompanied with radial subsidences.
By the properties of ellipsoids the crust undergoes larger deformations in the latitudinal direction, since a variation of the polar flattening (i. e., its decrease or increase) causes a considerably larger variation in the length of the parallels than in the length of the meridional arcs between them. Therefore the meridional compressive deformations of the crust must be the more pronounced in the case of increased velocity of the Earth's rotation in the polar regions, and in the case of its decrease in the equa torial region. In both cases folding deformations must occur near the critical parallels, and therefore folded structures of latitudinal strike near the critical parallels will be polygenetic and highly complex.
There is no doubt that an increase in the Earth's rotational velocity in the polar regions or its decrease in the equatorial region may also produce folded structures fringing the stable crustal fragments that have escaped compressive deformations (in both latitudinal and meridional directions). Wherever variations in the Earth's rotation during the given period generate tensile stresses accompanied with radial upliftings (due to the influx of subcrustal plastic masses), tensile deformations of the crust occur, together with large-scale fissure extrusions of magma. Belts and zones of compressive and tensile deformations are produced in the less stable, weakened areas of the Earth's crust, with nonuniform composition and structure.
The above estimates of the strength of the Earth's shell make it possible to assess the minimum variation in the Earth's rotational velocity that is necessary for the compressive and tensile stresses in the outer portion of the shell (the exosphere) to exceed the shell's elastic limit (with respect to its compressive and tensile strength). According to calculations, this
minimum variation in the Earth's rotation must be approximately 0.06-0.1 On reaching this magnitude, variations in the Earth's rotation will generate irreversible compressive and tensile deformations of the Earth's crust and a transformation of the Earth's figure. Such variations in the Earth's rotation will be the combined result of all the partial variations, and they will be of at least a certain minimum duration. Apparently there have been prolonged increases in the Earth's rotational velocity alternating with prolonged decelerations.
In all probability, large-scale folding of the crust in the polar region (such as the Urals, the Appalachians, the folded structures in north eastern USSR, the Rocky Mountains, the Cordilleras, and others) was preceded by a period of prevailing increase in the Earth's rotational velocity. On the other hand, tensile formations of the Earth's crust (occurring on a very large scale in the polar regions), accompanied with faults, non uniform radial uplifts of individual fragments, subsidences and swellings, as well as with injection and profuse eruptions of magma into and through the fault fissures (mainly basic magma) were preceded by periods of prevailing deceleration of the Earth's rotation. Obviously, these tensile and compressive crustal deformations generated by significant variations in the Earth's rotation played an important part in the transformation of the outer portion of the Earth's shell, even when being greatly complicated by simultaneous crustal deformations resulting from physicochemical variations of the matter inside the Earth. Therefore traces of these deformations manifest themselves fairly distinctly on the surface.
The Devonian geological period was probably characterized by prevailing deceleration of the Earth's rotation. Possibly such deceleration had begun as early as the Silurian and terminated in the Upper Carboniferous. This period of the assumed deceleration of the Earth's rotation occupies an intermediate position between the Caledonian and the Urals (Hercynian) folding in the polar regions. This hypothesis is based on the absence of any appreciable Upper Silurian—Lower Carboniferous folded structures of submeridional strike't in the polar region, and on the presence of an extensive fault zone on the eastern slope of the Urals manifesting itself in numerous porphyrite dikes as well as in large meridional intrustions of peridotites, pyroxenites, and gabbros of the Northern and Central Urals and in other regions.