a while back i asked my college geology professor if the existence of Pangea could have been caused by a massive impact with another asteroid/planetoid, shearing one half of our planet away. how else could all of that landmass exist on one side of a sphere? he suggested that perhaps plate tectonics was a cyclical system. Pangea was the last time the prominent landmasses crashed together, in contrast to our present "spreading" pattern. eventually the Pacific will close and western America will crash into Asia. can anyone shed any light on this discussion?