a mountain range forming the elevated western rim of the Andes mountain system in South America (from Chile and Bolivia to Colombia) and separated in places by tectonic basins from the Coastal Cordillera. The designation Cordillera Occidental came to refer to segments of the western border range in the Andes of Colombia and Ecuador and in the Central Andes (up to 22° S lat.). In the southern part of the Central Andes the Cordillera Occidental is also known as the Main Cordillera, and in some areas it is called by other names.

The Cordillera Occidental is composed of a Mesozoic sedimentary rock mass with thick intrusions of granitoids and effusive rock. Active volcanism and earthquakes continue to the present time. The Cordillera Occidental of Colombia (elevations up to 4,400 m), bounding the Cauca River Valley from the west, has the sharpest contours. At the mountain junction along the border with Ecuador appear the first volcanoes, which continue in the Cordillera Occidental of Ecuador as far as 2° S lat. (including the extinct Chimborazo, 6,262 m). Between 3° and 7° S lat. the Cordillera Occidental descends to 2,500 m. Here it is intensively dissected by rivers and is not clearly pronounced. The rivers of the Pacific Ocean basin also dissect the Cordillera Occidental of Peru south of 8°S lat.; here elevations in certain areas exceed 5,000 m. South of 14° S lat. there again appear numerous huge extinct volcanoes (Coropuna, 6,425 m; Ampato, 6,310 m) as well as active ones (Misti, 5,821 m; Huallatira, 6,060 m; San Pedro, 6,165 m). In the Cordillera Occidental there are large deposits of copper (Chuquicamata, Toquepala), vanadium (Mina Ragra), gold, and poly metals.

In the north the Cordillera Occidental extends over regions with subequatorial and equatorial climates. Its steep western slopes receive much precipitation and are covered by damp evergreen forests with pronounced altitude zonation. The eastern slopes, facing inner valleys and basins, are more arid; here, above altitudes of 3,400 m shrubs and xerophytic alpine meadows—paramos—predominate. South of 7° S lat. the Cordillera Occidental lies within a region of tropical desert climate and scant vegetation. The summits of only the tallest volcanoes are permanently covered with snow.