The consequence of plate divergence is the opening of new oceanic crust at the expense of older oceans. The animation on the right shows the breakup of Pangea, and the opening of the Indian ocean at the expenses of Thetys over the past 160 Myr. (Pangea=Gondwana+Laurasia)
As volcanic rocks cool at mid-oceanic ridges they record the polarity of the Earth's magnetic field (alignment of magnetite grains with the magnetic field). These magnetic stripes of positive and negative polarity are symmetric across MOR which allows for accurate plates reconstruction over the past 250 Myr.
These stripes are now well dated using K-Ar and Ar-Ar radiometric techniques on amphiboles so we now have accurate age maps of the ocean floor such those shown below.
