FORMATION OF SUBMARINE VALLEYS IN THE LIGHT OF THE DYNAMICS OF THE EARTH'S AXIS The problem of the formation of submarine valleys has not yet been fully solved. Hypotheses of the subaerial formation of submarine canyons assume improbably large variations in the amount of water in the oceans or similarly large vertical movements of the continent*. Therefore scientists were forced to discard the hypothesis of the subaerial formation of these canyons and to seek an explanation of this phenomenon in under water processes.
Variations in the ocean level (and consequently the possibility of suitable conditions for subaerial formation of submarine valleys) may also occur without any variation in the amount of water in the oceanic depressions.
The hypothesis advanced by Khizanashvili /10/ demonstrates that variations in the ocean level, even of considerable amplitude, may occur on Earth irrespective of any variation in the quantity of oceanic waters. This hypothesis may be briefly summarized as follows: shifts of masses continuously occurring on our planet deflect the rotation axis within the Earth's body; the solid part of the Earth changes its position with respect to its rotation axis; the hydrosphere, being liquid, continued to rotate in such a manner that the minor axis of the oceanic spheroid always coincides with the rotation axis. As a result, the ocean level rises in two opposite quadrants of the Earth's surface while falling in the other two; a rise in level in certain areas is compensated by subsidence in other areas, while the volume of oceanic waters remains unchanged /10, pp. 5-14/.
An explanation of the genesis of submarine valleys based onKhizanashvili's hypothesis suggests that a deflection of the rotation axis within the Earth's body causes a depression in the ocean level in regions approached by the poles. The rivers and other streams follow the retreating seacoast by cutting their valleys through the newly dry land. Subsequently, when the deflection of the rotation axis changes its direction and the poles start retreating from these areas, the ocean level again rises and part of the river valleys become submerged, forming submarine canyons.
Nevertheless, fluctuations in ocean level may still prove inadequate to explain the origin of submarine valleys by erosion. The submarine canyons are not formed unless the retreat of the ocean is followed by vigorous erosion processes which destroy the newly exposed dry land. Consequently, another no less important factor is represented by erosion agents on dry land. One such agent is atmospheric precipitation.
It will be useful to remind the reader of Shepard's view of the lower incidence of submarine canyons in desert regions where the amounts of atmospheric precipitation are insufficient for intensive erosion. Shepard wrote that there are grounds for assuming that canyons should occur less frequently in the zone centered at 30° lat. where the atmospheric precipita tion is light. However, this phenomenon may be partially explained by the insufficient number of soundings in this latitude. Yet, characteristically, not a single large canyon has been discovered in the desert region /12, p.243/. Similarly, the absence of submarine canyons off the Australian shores is related by Shepard to the underdeveloped river system of this continent /9/.
Thus, the following are the two principal prerequisites for the formation of submarine valleys: 1) fluctuations of the ocean level; and 2) adequate atmospheric precipitation to form streams capable of vigorous erosion.
The formation of submarine canyons must undoubtedly be affected by many other factors besides those mentioned. For instance a significant factor may be the relief of the adjacent sea-bottom areas; this factor may assume decisive importance in certain cases. However, the ocean floor is variable everywhere, and while hindering the formation of submarine canyons in one area, it may facilitate their formation in an adjacent area. Thus, the effect of the sea-bottom relief is confined to localized areas.