Europe is traditionally reckoned as one of seven continents. Physiographically, however, it is the northwestern peninsula of the larger landmass known as Eurasia (or Afro-Eurasia): Asia occupies the eastern bulk of this continuous landmass (save the Suez Canal separating Asia and Africa) and all share a common continental shelf. Europe's eastern frontier is delineated by the Ural Mountains in Russia. The south-east boundary with Asia is not universally defined. Most commonly the Ural or, alternatively, the Emba River serve as possible boundaries. The boundary continues to the Caspian Sea, the crest of the Caucasus Mountains or, alternatively, the Kura River in the Caucasus, and on to the Black Sea; the Bosporus, the Sea of Marmara, and the Dardanelles conclude the Asian boundary. The Mediterranean Sea to the south separates Europe from Africa. The western boundary is the Atlantic Ocean; Iceland, though on the Mid-Atlantic Ridge and nearer to Greenland (North America) than mainland Europe, is generally included in Europe for cultural reasons. There is ongoing debate on where the geographical centre of Europe is.