three extensional plates of the seabeds of Central America, having
become an epicentre for increased earthquake and volcanic activity,
continue to drive the overall pattern of the shield and plate movement.
As this tectonic activity slows and eventually ceases, these fault
lines and resulting areas of volcanic activity, come to rest at
the positioning of the three large shield volcanoes of Tharsis,
on the far side of Olympus Mons. The continuing spread of the Pacific
seabeds becomes the Utopia Planitia of the Northern Plains. This
movement can also be seen in the crustal magnetism.
The combined spreading of the
seabeds and the reduction in the size of the sphere lead to an increasingly
parallel crustal magnetic field, with warped and circular areas
where the continents and the now shredded original shields are pushed
together and swamped by new volcanic material (
note - terminology ). These can be seen around Syrtis Major
and on either side of Olympus Mons and Valles Marineris.