On the Earth's surface, the World Ocean forms the major part of the hydrosphere which is among the most ancient external shells of our planet.
The volume of the hydrosphere was mainly formed through smelting and degassing of mantle matter and was governed by in-depth geophysical processes. The degassing is a result of the active gravitational differentiation of mantle matter near the Earth nucleus, which caused the convective circulation in the mantle with the period corresponding to the global tectonic cycles.
Recent geological studies indicate that the oceans existed on the Earth practically in all geological epochs. This is supported by the presence of the most ancient sedimentary rocks aged c. 3.76 ± 0.07 x 109yr BP, which were found in southwestern Greenland.
Formation of oceans as huge reservoirs of surface waters began simultaneously with that of the oceanic crust, the surface of which is now at the average depth of 4.42 km. The oceanic crust was formed in close relation to volcanism, which is particularly active at present within the global system of the mid-ocean ridges. In these zones, there is a permanent input of basalt matter and juvenile waters and at the same time the oceanic crust is formed.
At present, the total length of the mid-ocean ridges is about 60 000 km, and the average rate offloor spreading is 5 cm yr~ . The oceanic crust (without sedimentary layer) is 6.5 km thick and its average density is 2.88 gcm~3. Annually about 56 x 101 g of basalt is generated as oceanic crust and the same amount of matter (plus sediments) sinks in subduction zones, thus providing for the balance of matter at the bottom of the ocean. Probable variations in spreading rates could result in different juvenile water inputs to the ocean through time.
The study of global evolution of the Earth with due consideration for the processes of gravitational differentiation of matter and mantle degassing suggests that there was a gradual acceleration of the hydrosphere formation and accumulation of the oceanic water with the maximum probably dating back to the Late Riphean (Mezoproterozoic), about 1.5 x 109yr BP. Simultaneously the Earth's crust developed through the growth of geosynclinals and mountain-building and weathering processes. The cores of future continents were formed and gradually expanded, and reliefofthe Earth's surface became more and more contrasting. The Earth's crust was differentiated into oceanic and continental.
By considering the Earth crust evolution and the increase in the total volume of the hydrosphere, models have been developed for the description of water volume formation and changes on the Earth's surface:
where c0 is concentration of water in primary matter and the mantle, mg is mass of the Earth, a is parameter of mobility of H2O component in the active layer of upper mantle, and »(1) is the normalization coefficient (»(1) = »(t) if t =1).
Fluctuations of the sea level during the course of geological time could be partially caused by the changes in the size of oceanic depressions accompanied by the increase of the total amount of water and steady deepening of the ocean. Various correlations of these factors governing the tectonic and sedimentation processes resulted in global transgressions and regressions.
It is necessary to consider the process of ocean volume formation in view of the changing relief of the Earth's surface. This could be illustrated by a specially developed dynamic model of hypsographic curve (Figure 1). This curve can be represented as an integral equation accounting for varying frequency distribution (a), mean square deviation (5), and altitude of the surface (hm).
Figure 1 Hypsographic curve showing change in ocean volume with respect to change in relief of the earth's surface.
The hypsographic curve can be described as the sum of three integral functions:
In short, it can be represented as
The analysis of hydrosphere evolution at the early stages of the Earth's geological history on the basis of the developed model allows calculating the changes of the total area of the oceans and the trends of the gradual rise of sea level in the geological past (Table 1; Figure 2).
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