The Tasman Glacier, in the shadow of Aoraki/Mt Cook, is by far New Zealand’s largest and longest glacier – up to 600 metres thick and 29 kilometres long. Some 22,000 to 16,000 years ago, it was even more imposing. Joined by ice from the Murchison, Hooker and Mueller glaciers, the Tasman stretched for 115 kilometres. It gouged out the trough filled by Lake Pūkaki, which lies behind hills of moraine (rocks, soil and clay carried by glaciers).
The present outline of the Tasman Glacier has changed little over the last 2,000 years. Insulated by a thick cover of rock debris over its slow-moving lower reaches, it has responded to shifts in climate with changes in thickness rather than in the position of its terminus (front edge). A century ago travellers clambered 10 metres up from moraines alongside to reach the Tasman’s surface. The ice level has now fallen 130 metres below the moraines.
The ice has thinned so much that in the mid-1980s, ponds at the Tasman’s terminus joined to form a lake. The lakes are increasing in size as icebergs calve (break off) into the lake.
Franz Josef and Fox glaciers
Two spectacular Westland glaciers, Franz Josef and Fox, are New Zealand icons, and a seeming paradox – ice that descends into rainforest. Of the many glaciers in the Southern Alps, why do only these two reach so close to sea level?
Franz Josef Glacier has a high-altitude snowfield of 20 square kilometres, and Fox Glacier 32 square kilometres. These funnel vast amounts of ice into narrow valleys. The effect is like pressing on a tube of toothpaste. The ice is pushed right down the steep valleys to the coast, at speeds of several metres per day. The fast-moving ice does not melt until it is near sea level, where there is warmer air and frequent heavy rain.
Frozen with grief
The Māori name for the Fox Glacier is Te Moeka o Tuawe. The ancestor Tuawe fell to his death while exploring the area, and the bed of the glacier became his final resting place (moeka). His lover Hine Hukatere wept, and her everlasting tears formed the Franz Josef Glacier – referred to in the name, Kā Roimata o Hine Hukatere.
Advance and retreat
Both the Franz Josef and the Fox glaciers are very sensitive to variations in climate. Even small changes in snowfall can result in substantial changes in the position of the terminus. There is a 5–6-year delay between a change in snowfall in their upper basins, and an advance or retreat of their terminus.
When first visited by geologist and explorer Julius Haast in 1864, the front edge of the Franz Josef stood near Sentinel Rock, several kilometres further downvalley than today. Franz Josef began a rapid retreat in the mid-1930s. A lake formed in front of the glacier between 1939 and 1949, but rapidly filled with rock debris. Since then, the Franz Josef and the Fox have been retreating, with occasional brief advances. The Franz Josef was at its smallest extent in 1982, but since then its front has once again advanced about a kilometre.
Part of the terminus of the Fox Glacier is hidden. Large masses of stagnant ice have been buried for over half a century under the gravel riverbed in front of the glacier. As the underlying ice melts, the gravel collapses, forming ponds.
In recent years both the Franz Josef and Fox glaciers have been the scene of ‘jökulhlaups’ or outbreak floods. Water from rainstorms and melting ice sometimes gets trapped in tunnels inside the glaciers. When the ice around the tunnels eventually gives way, the water, along with chunks of ice and rock debris, floods the valley downstream. One such flood in 1989 destroyed a bridge on the access route to the Franz Josef Glacier.