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20

THE CLAM-SHELL

EXPANSION

In the foregone images, the continent of Antarctica is just off to the right hand edge of the globes. Because of its position at the maximum opening point of the clam-shell, it is the continent which moves its position the most. Starting off attached to Asia, it is eventually displaced half the circumference of the Earth, as the Pacific Ocean bed forms on either side of a mid-ocean ridge.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The above diagrams show how the crust splits apart by crustal tension due to hydrostatic forces from below; magma extrudes into the fissure. Then the crust stretches apart even more causing more magma to extrude, forming a constantly renewed mid-ocean ridge. As the new ocean beds form, at the rate of about 5cm per year on each side of the ridge, over millions of years the two continents become separated great distances.

 

As magma crystallises to form this new ocean bed crust, the magnetic field is recorded as remanence within the rock. Curiously, this remanence betrays the fact that the Earth’s polarity has reversed quite frequently. The result is that there are symmetrical bands of rock with reversed polarity on either side of the mid-ocean ridge. These bands are useful in indicating the direction of growth of the ocean bed and the age of the beds at different reference points. They confirm that the ocean beds are indeed ‘new‘ structures of a vast size, and covering an area which is consistent with the increase in surface area

required for a much larger Earth.

 

 

The Hinge of the Clam-shell

 

The hinge line of the clam-shell expansion has always been in the Mediterranean area, running west to east, although its angle has rotated slightly - as its area has increased. In a different chapter I will discuss the evolution of the Mediterranean basin and how it has developed in quite recent times. The following diagram, however, shows the drift of the position of the hinge line on a Google image of the Earth.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stretch Marks, Splits and Rucking up

 

While the Earth was growing in diameter, the continental crust had to adapt to a surface with less curvature. The continents had to ‘flatten out’ a little.

 

If this was indeed happening, one would expect the edges of continents to split while the more central areas to ruck up. Possible examples of the former are perhaps the Persian Gulf and Red Sea, where Asia has split away from the Middle East which, in turn, has split away from Africa. Some mountain building may be due to the rucking up phenomenon.

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