What went on before the break-up of Pangaea?
Plate-tectonic movements since the break-up of the supercontinent Pangaea are now fairly well understood. Most scientists believe that similar processes must also have occurred earlier. However, the pre-Pangaea history of plate tectonics is very difficult to decipher, because nearly all of the evidence has been obscured by later geologic and plate-tectonic processes, including the subduction of older oceanic crust, which carried with it the record of magnetic reversals and hotspot traces. The clues to past plate tectonics can only be found on the present-day continents-in rocks, fossils, and structures older than about 200 million years. This is because the average age of the present-day oceanic crust is about 55 million years; the oldest parts are about 180 million years old, indicating that oceanic crust is entirely recycled every 150 million years or so. By contrast, the average age of the present-day continental crust is about 2.3 billion years, with the oldest known rocks (other t