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, 4.6 billion years ago ( referred to here as 'original shields' ).

These shields are permanently magnetically striped ( a process which ocurs 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 ( pt ) for 'Pre-Cambrian', 'Paleo-Magnetism' and 'Subduction'.