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THE EVOLUTION OF THE EARTH’S CRUST

Summary

 

It is clearly evident, from the pattern of Moho contours on the Earth’s continental crust, that sequential stretching has come about - and this can only have been caused by successive regimes of hydrostatic pressure building up within the mantle below. These continually rising pressures also power the volcanic activity of mid-ocean ridges and the associated development of new crust material.

 

It can be seen that metamorphism of igneous rocks prior to 330 million years ago was largely caused by conditions of strain. Since that period a pressured system from ocean-bed formation has come into play and has been responsible for the majority of mountain building and associated compression metamorphism.

 

There is symmetry between previous continental neighbours, both in their geological formations and topography. Originally contiguous sedimentary slabs have fractured apart and each separating continent has carried its own section with it, often to a great distance.

 

The stretching of the Earth’s crust has brought about the many examples of uncomformity within geological sequences. Often gaps (troughs) have been created at the edges of sedimentary basin deposits as the basin itself has been extended by further strain on the underlying platform rock. In some areas these troughs have been filled by vegetation which has been washed down from upland areas. Many of the British coal-measures were formed in this way. The coal measures of western Germany and northern France were similarly deposited in the equivalent trough at the other end of what was once a continuous limestone slab but it is now separated several hundred kilometres apart. In this way Carboniferous deposits exist below the level of upper Cambrian deposits on one side, and Ordovician limestones on the other.

 

Crustal stretching also explains the Ordovician mass extinction event. The Cambrian sedimentary structures were established on a small Earth, entirely covered by ocean water. As the Earth’s crust expanded, these sedimentary structures were fractured in to separate slabs; basins between them were formed. These basins widened as the Earth grew further in size. The water which once covered the Cambrian slabs now drained down into the basins between them - eventually exposing, for the first time, dry land. As these highlands dried out, the environment was no longer suitable for many of the species which had evolved for the oceans of the Cambrian Period. As a consequence many forms of life went into extinction.

 

The same processes of planetary expansion have clearly taken place on Mars and progressive thinning of the crust has led to the development of the northern lowlands.

 

 

References

 

Kukkonen, I.T., Kusisto, M., Lehtonen, M., and Peltonen, P, Technophysics, Vol 457, Issues 3-4, Oct. 2008, p 111-127: Delamination of the eclogitized lower crust 2008 Geological Survey of Finland.

Cann, 1979

Smith D E, Zuber M T, The Crustal Thickness of Mars: Accuracy and Resolution. Lunar & Planetary Science XXXIII 2002

Robinson C A, The crustal dichotomy of Mars, Physics and Astronomy, Vol 69, Number 3, 249-269

 

 

 

 

 

 

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