The Tasman Glacier is the largest of several glaciers which flow south and east towards the Mackenzie Basin from the Southern Alps in New Zealand's South Island. It is New Zealand's longest glacier.
At 27 kilometres (17 mi) in length, Tasman Glacier is New Zealand's longest glacier.It is as much as 4 kilometres (2.5 mi) wide and 600 metres (2,000 ft) thick, and lies entirely within the borders of Aoraki/Mount Cook National Park. The glacier covers an area of 101 square kilometres (39 sq mi) and starts at a height of 3,000 metres (9,800 ft) above sea level. Snowfall during the winter and spring seasons accumulates to 50 metres (160 ft). After the summer melt, 7 metres (23 ft) remains.
The Tasman flows south from the southern slopes of the Minarets peak, along the eastern flank of New Zealand's two highest mountains, Mount Tasman and its higher southern neighbour Aoraki/Mount Cook.Although its upper reaches are snow-covered, rocks carried by the glacier are exposed by ablation along its course, and the lower glacier is entirely rock-covered. It is almost met near its end by the meltwater of the Murchison Glacier, which approaches from the northeast before turning to flow beside the Tasman Glacier outside the moraine wall.
The waters from both these glaciers pool at the end of the glacier in Lake Tasman, before flowing south to join the outflow from the nearby Hooker and Mueller Glaciers in the wide valley of the Tasman River, whose braided streams flow south into Lake Pukaki. They eventually flow into the Waitaki River and to the Pacific Ocean north of Oamaru.
The glacier has retreated about 180 metres (590 ft) a year on average since the 1990s and the glacier is now in a period of faster retreat where the rate of retreat is calculated to be between 477 to 822 metres (1,565 to 2,697 ft) each year. It is estimated that the Tasman Glacier will eventually disappear and the terminal Tasman Lake will reach a maximum size in 10 to 19 years time. In 1973 Tasman Glacier had no terminal lake and by 2008 Tasman Lake was 7 kilometres (4.3 mi) long, 2 kilometres (1.2 mi) wide, and 245 metres (804 ft) deep.
A large calving event was possibly triggered, or at least contributed to, by the 2011 Canterbury earthquake on 22 February 2011. On this day 30,000,000 metric tons (33,000,000 short tons) to 40,000,000 metric tons (44,000,000 short tons) of ice dropped from the terminal face of the Tasman Glacier and fell into the Tasman Lake. Boats were hit with tsunami waves of up to 3.5 metres (11 ft) as the ice fell into the Tasman Lake under the glacier. Similar events in the past have been attributed to buoyancy effects, a result of high basal water pressures and increased lake level following heavy rainfall events.