Intact remains of ancient crust can be found in Finland and constitutes the Fennoscandian
Craton. Its thickness has been measured by the Geological Survey of Finland (T Kukkonen
et al) and reaches a depth of 63 km on the eastern side, reducing to 50 km on the
western side. This is the thickest craton in the World and has been dated as being
formed between 1.87 and 1.91 billion years ago.
Earth at its very small Rodinian stage would have had a uniform thickness of crust
of a least 70 km.
2. Primary Crust
This crust type forms in areas where the original thick crust has undergone some
degree of stretching and igneous intrusion. It has a Moho depth of 40-50 km and is
usually described as a platform. Dykes caused by igneous intrusion in these areas
can be dated and this gives us an indication when this initial stretching took place
and its direction.
When Earth was at its Pannotian stage (35% current diameter) its crust will have
comprised of these first two types.
3. Secondary Crust
This type of crust forms much of western Europe. These areas are where some of the
peripheral Primary Crust has undergone further stretching and basins were formed.
Often these basins became filled with water and sedimentary deposits accumulated
in them. By dating the lowest deposits we can determine when the secondary crust
had formed. This crust is between 30 km and 40 km thick and is generally metamorphosed.
4. Tertiary Crust and Marginal Areas
These are areas where subsequent tension on the peripheral secondary crust areas
has caused further stretching, just prior to failure. Their Mohos are between 20
and 30 km. Many rocks are highly metamorphosed. These are weak, fatigued areas of
crust which are susceptible to forces of compression from subsequent ocean-bed crust
formation, and are zones where mountains are easily formed.
Pangaean Earth will have featured the four types of crust described so far.
5. Ruptured Margins
These are areas near to where the continental crust has reached failure point and
ruptured apart. Magma issues from this rupture line and the process of ocean-bed
formation begins. These marginal areas have a crustal thickness of 20 km down to
5 km and generally form shelves around continents.
6. Ocean-bed Crust
The ocean bed crust develops on both sides of a ridge of volcanoes which form along
the rupture line between continents. New ocean-bed crust develops at a rate of about
3-5 cm per year on each side of the ridge, providing an increasing distance of separation
between two continents. Oceanic crust is usually 7 km thick.