In the centres of the continents there
are large stable shield areas of rock, the oldest rock on the planet
surface. They are the thickest parts of the Earth's crust and date
from the Pre-Cambrian period, between 600 million and 4.6 billion
years ago ( referred to here as 'original shields' ).
These shields are permanently magnetically striped ( a process
which occurs when fresh rock emerges and cools, it's orientation
in relation to the magnetic poles recorded in its magnetism: also
called paleo-magnetism ).
How these shields move in relation to each other is shown by the
positions of the fault lines ( the boundaries between the plates
on which these shields lie ) and whether the fault lines are extensional
( creating new crust where the seafloor spreads ) or compressional
( destroying old crust at plate boundaries and subsuming it back
into the mantle: also called subduction )
Note: see Plate-Tectonic sub-section ( ptss
) for 'Pre-Cambrian', 'Paleo-Magnetism' and 'Subduction'.