The Geologic Time Scale divides Earth's geological history into eons, eras, and periods. The absolute age of the boundaries between the named intervals are shown in millions of years. The Earth formed in the Hadean eon 4.6 billion years ago. The assembly of Pangea was complete by the Permian period of the Paleozoic era. Pangea began to break apart during the Triassic period of the Mesozoic era. Modified from Marshak (2001).