The 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 material from the mantle. These can be seen around Syrtis Major and on either side of Olympus Mons and Valles Marineris.

( The surface magnetic field, or crustal magnetism' in this section is based on the World Magnetic Declination Field and the Mars Crustal Magnetism chart from the Mars Global Surveyor )