<P> Paleomagnetic study of apparent polar wandering paths also support the theory of a supercontinent . Geologists can determine the movement of continental plates by examining the orientation of magnetic minerals in rocks; when rocks are formed, they take on the magnetic properties of the Earth and indicate in which direction the poles lie relative to the rock . Since the magnetic poles drift about the rotational pole with a period of only a few thousand years, measurements from numerous lavas spanning several thousand years are averaged to give an apparent mean polar position . Samples of sedimentary rock and intrusive igneous rock have magnetic orientations that are typically an average of the "secular variation" in the orientation of magnetic north because their remanent magnetizations are not acquired instantaneously . Magnetic differences between sample groups whose age varies by millions of years is due to a combination of true polar wander and the drifting of continents . The true polar wander component is identical for all samples, and can be removed, leaving geologists with the portion of this motion that shows continental drift and can be used to help reconstruct earlier continental positions . </P> <P> The continuity of mountain chains provides further evidence for Pangaea . One example of this is the Appalachian Mountains chain, which extends from the southeastern United States to the Caledonides of Ireland, Britain, Greenland, and Scandinavia . </P> <P> There were three major phases in the break - up of Pangaea . The first phase began in the Early - Middle Jurassic (about 175 Ma), when Pangaea began to rift from the Tethys Ocean in the east to the Pacific in the west . The rifting that took place between North America and Africa produced multiple failed rifts . One rift resulted in a new ocean, the North Atlantic Ocean . </P> <P> The Atlantic Ocean did not open uniformly; rifting began in the north - central Atlantic . The South Atlantic did not open until the Cretaceous when Laurasia started to rotate clockwise and moved northward with North America to the north, and Eurasia to the south . The clockwise motion of Laurasia led much later to the closing of the Tethys Ocean and the widening of the "Sinus Borealis", which later became the Arctic Ocean . Meanwhile, on the other side of Africa and along the adjacent margins of east Africa, Antarctica and Madagascar, new rifts were forming that would lead to the formation of the southwestern Indian Ocean that would open up in the Cretaceous . </P>

What two continents formed when pangaea began breaking up