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The Southern Alps is a mountain range extending along much of the length of New Zealand's South Island, reaching its greatest elevations near the island's western side. The term "Southern Alps" generally refers to the entire range, although separate names are given to many of the smaller ranges that form part of it.
The range includes the South Island's Main Divide, which separates the water catchments of the more heavily populated eastern side of the island from those on the west coast. Politically, the Main Divide forms the boundary between the Canterbury and West Coast Regions.
The Southern Alps run 450 km north to south. The tallest peak is Aoraki / Mount Cook, the highest point in New Zealand at 3,754 metres (12,316 ft) and there are sixteen other points in the range that exceed 3,000 metres (9,800 ft) in height. The mountains are cut through with glacial valleys and lakes. According to an inventory conducted in the late 1970s, the Southern Alps contained over 3000 glaciers larger than a hectare, the longest of which – the Tasman Glacier – is 29 kilometres in length down towards Lake Pukaki. A chain of glacial lakes are found on the eastern side of the ridge from Lake Coleridge in the north to Lake Wakatipu in Otago in the south.
The Southern Alps were named by Captain Cook on March 23, 1770, who described their "prodigious height". They had previously been noted by Abel Tasman in 1642, whose description of the South Island's west coast is often translated as "a land uplifted high".
The Southern Alps lie along a geological plate boundary, part of the Pacific Ring of Fire, with the Pacific Plate to the southeast pushing westward and colliding with the northward-moving Indo-Australian Plate to the northwest. Over the last 45 million years, the collision has pushed up a 20 km thickness of rocks on the Pacific Plate to form the Alps, although much of this has been eroded away. Uplift has been fastest during the last 5 million years, and the mountains continue to be raised today by tectonic pressure, causing earthquakes on the Alpine Fault. Despite the substantial uplift, most of the relative motion along the Alpine Fault is sideways, not vertical.
The climate is cold with snow and ice year-round at the highest points.
Because of its orientation perpendicular to the prevailing westerly winds, the range creates excellent wave soaring conditions for glider pilots. The town of Omarama, in the lee of the mountains, has gained an international reputation for its gliding conditions. The prevailing westerlies also create a weather pattern known as the Nor'west arch, in which moist air is pushed up over the mountains, forming an arch of cloud in an otherwise blue sky. This weather pattern is frequently visible in summer across Canterbury and North Otago. The 'Nor'wester' is a foehn wind similar to the Chinook of Canada, where mountain ranges in the path of prevailing moisture laden winds force air upwards, thus cooling the air and condensing the moisture to rain, producing hot dry winds in the descending air lee of the mountains.
The mountains are rich in flora with about 25% of the country's plant species being found above the treeline in alpine plant habitats and grassland with mountain beech forest at lower elevations (of the eastern side but not in Westland). The cold windswept slopes above the treeline are covered with areas of fellfield. To the east the Alps descend to the high grasslands of Canterbury and Otago. Plants adapted to the alpine conditions include woody shrubs like Hebe, Dracophyllum, and Coprosma, the conifer Snow Totara (Podocarpus nivalis) and Carex sedge grasses.
Wildlife of the mountains includes the endemic Rock Wren (Xenicus gilviventris) and the kea, a large parrot that was once hunted as a pest. There are also a number of endemic insects adapted to these high altitudes especially flies, moths, beetles, and bees. The beech forests of the lower elevations are important habitat for two birds; the Great Spotted Kiwi (Apteryx haastii) and the South Island kaka (Nestor meridionalis meridionalis).
The mountains are inaccessible and retain their natural vegetation. A large proportion of the range is well protected as part of various national parks, notably the Westland Tai Poutini National Park, Mount Aspiring National Park, and Aoraki/Mount Cook National Park or protected areas such as Lake Sumner Forest Park. Indigenous plant life is affected by introduced animals such as red deer (Cervus elaphus), chamois (Rupicapra rupicapra), and Himalayan tahr (Hemitragus jemlahicus) all of which have at times been targeted for culling, while the birds and reptiles are vulnerable to introduced predators.