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.
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.
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