maximum size in 10 to 19 years time. In 1973 Tasman Glacier had no terminal lake and by 2008 Tasman Lake was long, wide, and deep. A large calving event was possibly triggered, or at least contributed to, by the 2011 Christchurch earthquake on 22 February 2011. On this day 30 to 40 million metric tons (33 to 44 million 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 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. Access and tourism Tasman Glacier has a long history of tourism, with heli skiing tours offered on the upper glacier since the early 1970s. The tributary Ball Glacier was also popular for skiing, with national championships being held there in the 1930s. It has since diminished too far to be safely accessed. Similarly, Tasman Glacier's significant ice loss over the past decades has impacted tourism, with an increasing number of crevasses being exposed and not filled in by snow any more, requiring guided tours to avoid these areas, and restricting the heli skiing season to July, August and September. The proglacial Tasman Lake is a popular destination for boat tours among the icebergs often floating in the lake. Boats are not allowed closer than to the tall terminal face of Tasman Glacier for safety reasons. The Ball Shelter Track, part of the Ball Hut Route, leads along the western side of the glacier, separated from Tasman Lake by a tall moraine wall until about 6 kilometres in, where it climbs high enough for the view to open up. At that point, the rock-covered lower Tasman Glacier in the valley between the moraine walls is wide. See also Glaciers of New Zealand List of glaciers Retreat of glaciers since 1850 – Oceania References Further reading Satellite Image Atlas of Glaciers of the World, Glaciers of New Zealand, Chinn, T.H., USGS Professional Paper 1386, 1988. External links Department of Conservation – Tasman Valley walking tracks Landforms of Canterbury, New Zealand Glaciers of New Zealand Southern Alps