Continent Europe and Asia together as a single continent, dubbed "Eurasia". In this model, the world is divided into six continents, with North America and South America considered separate continents. Geologists use the term "continent" in a different manner from geographers. In geology a continent is defined by continental crust: a platform of metamorphic and igneous rock, largely of granitic composition. Some geologists restrict the term 'continent' to portions of the crust built around stable Precambrian "shield", typically 1.5 to 3.8 billion years old, called a craton. The craton itself is an accretionary complex of ancient mobile belts (mountain belts) from
Middle-earth primeval epoch. The Valar left Middle-earth and went to the newly formed continent of Aman in the west, where they created their home called Valinor. To discourage Melkor from assailing Aman, they thrust the continent of Middle-earth to the east, thus widening Belegaer at its middle and raising five major mountain ranges in Middle-earth which adopted a relatively symmetrical distribution, namely the Blue, Red, Grey, Yellow Mountains and the Mountains of the Wind. This act, however, ruined the symmetry of the shape of the continents and their intervening seas. Many centuries after Valinor was established, Melkor raised the Misty Mountains