Oceans Subduct and Continents Rift

Why Pangaea formed and subsequently dispersed is not known with certainly. It has been suggested that these events were due to changes in the circulation pattern within Earth’s mantle. At the time of Pangaea, 300 million years ago, three-quarters of the globe was covered by a vast ocean. If massive subduction of oceanic plates brought Pangaea together — as India has recently been brought together with Asia — then mid-ocean ridges must have been a dominant feature on the floor of the ocean at that time. As the continents locked into place, the buoyancy of the continental lithospheres resisted subduction, and the process ceased at those points. New subduction zones formed in the oldest oceanic lithosphere; subsequently, subduction in the ocean became an important plate force, which, along with plumes, began the job of tearing the supercontinent apart.

When a plate is pulled by oceanic lithosphere sinking into a subduction zone, it will be stressed and may break, or rift, along the weakest part of the plate. If continental lithosphere forms part of the plate, the rift will form there because it is weakest.

From the reconstruction of the plates of Pangaea, scientists have concluded that most oceanic rifts actually started as continental rifts. We can think of this behavior as “oceans subduct, continents rift.” Oceanic rifts provide a line of weakness about which new ocean floor forms, but their origins can ultimately be traced back to the splitting of a continent. Once formed and cooled, a new oceanic plate tends not to rift but to sink back into Earth at subduction zones. Oceanic rifts propagate into new areas of oceanic lithosphere, and the ridge system is an ever-changing pattern, but older ocean floor appears to resist rifting. Evidence from rock record of the continents indicates that ancient rifting and continental collisions have taken place for at least the past 1000 million years, and possibly longer.