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Distribution of Oceans and Continents: Post - Drift Theories

Introduction


The idea of continental drift offered by Wegener was not widely accepted. It was questioned by academics, resulting in several research attempting to explain the other causative processes. It is worthwhile to note that the majority of evidence for continental drift was obtained from continental regions in the form of the spread of flora and fauna or deposits such as tillite.


A number of post-World War II discoveries contributed to the expansion of geological knowledge. Specifically, the data collected from ocean floor mapping added new dimensions to the study of ocean and continent distribution.


Convectional Current Theory - Arthur Holmes


Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion. These currents are produced by radioactive elements that cause heat variations in the mantle. Holmes claimed that such currents persist across the whole mantle portion. This was an effort to explain the question of force, based on which modern scientists rejected the notion of continental drift.



Mapping of the Ocean Floor


Extensive research into the ocean's topography found that the ocean bottom is not a flat plain, but is instead riddled with relief. In the post-World War II era, expeditions to map the ocean bottom produced a thorough image of the ocean topography and revealed the presence of submerged mountain ranges and deep trenches, most of which are found around the continent boundaries. Volcanic eruptions were determined to be more frequent around the mid-ocean ridges. The dating of oceanic crust rocks shows that they are much younger than continental regions. It was discovered that rocks on each side of the crest of oceanic ridges and located at equal distances from the crest have striking similarities in terms of their composition and age.



Sea Floor Spreading - Harry Hess


As stated above, the post-drift research offered significant information that was unavailable when Wegener proposed his theory of continental drift. Specifically, ocean bottom mapping and palaeomagnetic analyses of rocks from marine locations showed the following facts:


Along the mid-oceanic ridges, it was discovered that volcanic eruptions are frequent and send vast quantities of lava to the surface.


The equidistant rocks on each side of the crest of mid-ocean ridges exhibit striking similarities in terms of formation time, chemical composition, and magnetic characteristics. Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest.


Ocean crust rocks are much younger than continental rocks. There are no rocks in the marine crust older than two hundred million years. Some continental rock formations are over 3,200 million years old.


Unexpectedly, the sediments on the ocean bottom are rather thin. If the ocean bottoms were as ancient as the continent, scientists expected to find a continuous sequence of sediments from a considerably longer time span. However, nowhere was a sediment column older than 200 million years discovered.


In locations of the mid-oceanic ridge, earthquake foci have shallow depths, while in areas with deep trenches, earthquake foci have deep depths.


These facts and a detailed analysis of magnetic properties of the rocks on either sides of the mid-oceanic ridge led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”. Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. Thus, the ocean bottom expands. The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not cause the shrinking of the other, made Hess think about the consumption of the oceanic crust. He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed.



Plate Tectonics- Mcenzy and Parker


There are three types of plate boundaries:


Divergent Boundaries

As the tectonic plates move apart, a new crust is formed there. The locations where the plates move apart from one another are known as spreading sites. The best-known example of divergent boundaries is the Mid-Atlantic Ridge. At this, the American Plate(s) is/are separated from the Eurasian and African Plates.


Convergent Boundaries

Where the crust is destroyed because one plate has submerged under another. A subduction zone is the area where the subduction of a plate occurs. There are three possible convergence scenarios. The first is between an oceanic and continental plate, the second is between two oceanic plates, and the third is between two continental plates.


Ocean-Ocean Converge - Trenches

Continenent-Continent Convergence- Fold mountains

Ocean- Continent Convergence- Island arc



Transform Boundaries

Where the crust is neither created nor destroyed as the plates slide past one another horizontally. Transform faults are the planes of separation generally perpendicular to the mid- oceanic ridges. As the eruptions do not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth. Also, the rotation of the earth has its effect on the separated blocks of the plate portions.



Conclusion

The fact that the plates move is now well acknowledged. The mobile rock beneath the rigid plates is believed to be moving in a circular manner. The heated material rises to the surface, spreads, and starts to cool before sinking again to greater depths. This cycle is repeated several times to produce a convection cell or convective flow, as defined by scientists. There are two primary sources of heat inside the earth: radioactive decay and residual heat.

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