THE EARTH IS AN ocean-dominated planet. Only one-third of the planet’s surface is dry land. The Earth’s land areas include innumerable islands and several larger expanses of land termed continents. A continent is defined as a mass of land significantly larger than an island, completely or nearly surrounded by ocean water. By common agreement among geographers, the Earth’s smallest continent is Australia while the Earth’s largest island is Greenland. Australia’s area is about 2,966,000 square mile (7,682,000 square kilometer), while the area of Greenland is approximately 836,000 square mile (2,165,000 square kilometer). The outermost rock layer of the earth is termed the crust. The oceanic crust is composed almost entirely of basalt, a dense igneous rock. The earth’s continental crust, in contrast, is composed largely of lighter granitic rock. The continental crust varies greatly in thickness but averages about 35 mile (56 kilometer) thick. The oceanic crust, in contrast, averages only about 4 mile (6 kilometer) thick. Because of the imprecision of the definition, geographers do not agree on the number of the earth’s continents. Geographers in the United States commonly recognize 7 continents: Asia, Africa, , , Antarctica, , and Australia. The ancient Greeks recognized 3 continents bordering and surrounding the Mediterranean Sea (the “sea in the middle of the lands”). Africa was almost completely separated from Asia by the Red Sea, while Asia was separated from Europe by the Aegean Sea, the Dardanelles, the Sea of Marmara, the Bosporus, and the Black Sea. In later centuries, when exploration revealed that Europe was in reality not a separate landmass, Europe was redefined to include extensive land boundaries—in violation of the generally accepted definition of a continent. The boundary between Europe and Asia is now generally accepted to begin at the Aegean and to continue through the Dardanelles, the Black Sea, the Caucasus Mountains, the Caspian Sea, the Ural River, and the Ural Mountains. The Ural Mountains thus separate European Russia from Siberia.