ORIGINS OF THE OCEANS AND CONTINENTS
At the early stages of formation, the Earth appeared as a cold body in space, containing all of the chemical elements known in Nature. The atmosphere and hydrosphere did not yet exist; the surface of the planet was completely lifeless. But gradually, due to gravitational forces, energy released by the breakdown of radioactive elements and lunar tides deep inside the core of the Earth began to heat up. When temperatures near the core of the Earth reached that level where the melting of iron oxides and other compounds could occur, the active processes began for the formation of a nucleus and the main environment of the planet.

The general process of formation of the Earth's environment, according to a hypothesis of the Academician A.P. Vinogradov, was through zoned melting in the mantle, situated around a nucleus. Thus the dense, and heaviest sank toward the centre, increasing the size of the nucleus, and less dense and lighter elements rose to the surface, forming the lithosphere, the top-most part of which is the Earth's crust. These processes caused the onset of great volcanic activity over all areas on the surface of the Earth and produced great and extensive outpourings of basaltic lava, releasing gases and water vapour. Gravity forces kept the gases and water vapour in the near-earth proximity, and these formed a primitive, proto-atmosphere, but deprived of oxygen.

By radiating heat into Space, the Earth's surface gradually cooled. The water vapours (gas) condensed, and became liquid water. Active elements and compounds, discharged from still more volcanic activity, interacted with the water, forming acids and salts.

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It is probable that such processes occurred on the Earth's surface and deep within the core of the Earth as well, between about 4 billion years ago (top drawing) to 3 billion years ago (bottom).

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Deep tectonic processes and the heterogeneous nature of the earth's crust are not uniform. Therefore, raised areas with thickened but chemically lighter crust have developed into land masses, and sunken areas with heavier crust have become the sea floor, covered by water. The availability of an atmosphere, a planetary water-cycle, and seasonal and daily changes of temperature above the surface of the sea promoted the development of processes of weathering, erosion and mass-wasting. The products of mass-wasting and erosion accumulated at the bottoms of basins and reservoirs, forming the layers (strata) of sedimentary rocks. A continuous cycle of formation and destruction of parts of the earth's surface occurred, causing a constant change in the morphology of the mountains, plains and seas.

Redistribution of mantle materials within the Earth and their rise to the surface and above caused the development of the atmosphere and hydrosphere. The duration of these processes was all throughout the geological history of the planet and is still going on today. The weight of the water on the surface and in the oceans gradually increased, having formed a World Ocean, and the modern salt structure of the seas was established many hundreds of millions of years ago. In the atmosphere, with occurrence and evolution of vegetable (plant) organisms, oxygen was released by the normal processes of plant growth and living. The oxygen accumulated, lowering the ratio of carbon dioxide to oxygen. The Earth's crust was divided into continental and oceanic areas, sharply distinguished by their structure and thickness. The crust under the oceans has a high density, with an average thickness of 5-6 km; continental (subaerial and coastal) density is lower and the average thickness of the land crust is between 30-40 km. Although the thicknesses are very different, the balance is maintained because of the densities of the crust, keeping the planet rotating smoothly on its axis.

There are various points of view regarding the origin and development of continents and oceans, some of which sometimes contradict each another:

According to one of these points of view, the bottom of the oceans represents primary basalt from the earth's crust, and the continents were formed later as a result of the accumulation of very great thicknesses of sedimentary rocks which were deposited in great, shallow ocean basins, which, because of the great weight, became compressed into folds, forming folded mountain systems.

Another hypothesis assumes that modern ocean basins (or their parts) occurred in areas where earlier, huge continents partially collapsed, compressing these materials into the basins and transforming them from "continental" into "oceanic" deposits, during a cycle called "oceanisation."

There is another hypothesis which shows that the Earth has expanded over long periods, during which time, the area of the bottom of the oceans between the continents began moving apart. The distance between these continents increased, and the water, earlier only the lower parts of continents, completely flowed into the ocean basins.

The hypothesis of horizontal movements of lithospheric plates is the most widely accepted today. According to this hypothesis, the uppermost part of the Earth - the lithosphere - is joined by a number of adjacent, rigid plates, under which the effects of convection currents in the mantle cause these plates to move relative to one-another.

Where the plates diverge due to spreading, there is an upwelling of mantle material into the crust in the rifted area. As this hot mantle material (magma) rises, it cools and crystallises. As more magma upwells into the rift zone, it pushes the crystallised material upward and away from the rift zones, causing mid-ocean ridges to form. Moreover, the iron minerals in the magma take the exact properties of the Earth's magnetic field at the time that these minerals crystallise, and are sometimes used in geophysical measurements. You will learn more about this process later.

Where these plates converge upon each other, one of the plates is pushed downward beneath the other plate. This is called "subduction" and occurs at all convergent plate boundaries. As the one subducting plate is being pushed down, the other plate rides up and over the downward moving plate. Very deep trenches are formed in these subduction zones (for example, the Kurile Islands Trench), and just behind them, island arcs (such as the Kurile Islands and Japan), or mountain systems (such as the Andes and Rocky Mountains in the western hemisphere.

Along the edges and borders of plate divergence and convergence, because of the active tectonic processes beneath them, high seismicity and intensive volcanic activity occurs. The movement of the plates have resulted in continental drift, closing and opening of oceans, but the water of the ocean remained, flowing from one depression to another.

What is the practical value of the study of these geological processes? The knowledge of the history of the formation of the Earth permits us to understand the formation of useful minerals which have come to us from deep within the core of the Earth. By tracing the different continents and oceans, geological structures and borders of lithosphere plates, scientists can reconstruct their previously adjoining areas, and predict the formation of petroleum and other exploitable minerals and ores. They can also try to predict disastrous earthquakes and other natural hazardous phenomena.
 
 

Main lithosphere plates

1.Direction of plate movement

Intensity of plate movement:
2.At spreading
3.At subduction

INTERNAL MECHANICS OF LITHOSPHERIC PLATE MOVEMENT

 
 

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The constant rise of mantle material (magma) along an axis of a mid - oceanic ridge causes divergence of lithosphere plates.
 
 

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Collision of oceanic and continental plates. The subducting of an oceanic plate under a continental plate creates a deep trench adjacent to the continental coast and a great mountain range on land.
 
 

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Down-drag of an oceanic plate in the mantle results in the formation of an volcanic island-arc system in the ocean and deep trench seaward of the island-arc.



 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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1 Core

2 Earth's crust 

3 Mid-ocean ridge 

4 Convection currents 

5 Island-arc 

6 Trench 

The arrows show the directions of movement of the lithosphere plates moved by convection currents in the mantle.

Information provided by HDNO:  http://www.oceansatlas.com/unatlas/about/ContactInfoForHDNO.html