A D MACKAY © All Rights Reserved

PREVIOUS
NEXT
CONTENTS
31

Crust Types

 

1. Archaean Crust Type - Fennoscandian Craton

 

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.

 

ASIA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

THE EVOLUTION OF THE EARTH’S CRUST

Page 31
Page 5
Page 32