The Himalayas formed by continental-continental collision between the Indian Plate and the Eurasian Plate, beginning ~50 Ma (Eocene). Because both plates are low-density continental crust (which cannot subduct efficiently), the collision caused extreme crustal thickening, uplift, and the formation of the Tibetan Plateau. The Andes formed by oceanic-continental subduction (Pacific plate under South America). The Alps formed by a complex collision involving the African and Eurasian plates.