Tasman Glacier retreat, size and the science behind it

How thick is the Tasman Glacier? DOC says the ice is about 600 metres deep near the Hochstetter Icefall.³ That icefall itself is a 1,000-metre-high cascade of broken ice, which Alpine Guides' heli hike passes on its return flight.¹⁸ Depth varies along the length of any glacier, and the figure DOC gives refers to the area near the icefall, where tributary ice pours into the main trunk.

Lower down, the glacier hides under rock. DOC notes that it is covered with rock material in its lower reaches.¹⁹ NIWA wrote in 2008 that the lower trunk of a long, low-gradient glacier such as the Tasman behaves as a creeping body of ice under a thick covering of insulating debris.²⁰ NASA's Earth Observatory explains the effect: if the layer of dirt and debris is thick enough, it insulates the ice and slows melting.²¹

People who expect white ice are often surprised at the lookout: the grey hills beyond the lake are glacier under rubble. NIWA tried a thermal camera on its 2018 survey flight to learn more about the debris-covered ice.²² DOC's avalanche guidance draws the same line on the ground, rating the valley floor up to the start of the white ice as simple terrain and the upper glacier from the white ice as challenging.^23,24

A visible sign of retreat is the water. There was no Tasman Lake before 1974, according to DOC.¹³ By 2017, NASA reported, it had swollen to 7 kilometers long.²⁷ NIWA's 2008 assessment was that the lake would need to nearly double before the glacier could settle into balance with the climate.⁴² Each metre the lake gains along the valley is a metre the glacier has lost.

Sometimes the retreat happens in one go. On February 22, 2011, NASA reports, the magnitude 6.3 Christchurch earthquake shook a 30 million-ton slab of ice off the glacier's front.¹⁶ In NASA's view the quake simply finished off ice that was already on the point of breaking away.⁴³ The lake had already undercut the front; the shaking only chose the day.

The retreat is not confined to the snout. NIWA's 2021 survey briefing described a temporary lake that kept filling and draining where the Hochstetter meets the Tasman, a possible trigger for more calving.⁴⁴ The two glaciers still join above Tasman Lake, but NIWA expects cumulative ice loss to separate them eventually.⁴⁵ Our page on the terminal lake covers the icebergs, the boats and the shoreline walks.

DOC writes the glacier's name as Haupapa/Tasman Glacier, with the Maori name first.²⁸ You will see the dual name on DOC signs, maps and track pages across the park, as with Aoraki/Mount Cook itself. The guiding company notes that the English name honours the Dutch explorer Abel Tasman, the first European to sight New Zealand, in 1642.⁵² We have not found an official explanation of the name Haupapa that we can quote, and we do not offer one.

The mountains around the glacier carry deep meaning for Ngai Tahu, the iwi of the region. DOC explains that Ngai Tahu regard Aoraki as their most sacred ancestor.⁵³ It also notes that, for Ngai Tahu, waste left in the park is unacceptable, since the waters flowing from Aoraki are sacred.⁴⁰ Visitors honour that by carrying out every scrap of rubbish and by treating the valley as a place of significance, not only of scenery.

The landscape also carries international recognition. DOC notes that the park forms part of Te Waipounamu, the South Westland World Heritage Area.⁵⁴ Its glaciers have shaped the region for a very long time: Alpine Guides describes the Tasman advancing and retreating several times, leaving great moraine deposits and carving out Lake Pukaki.⁵⁵ The present retreat is the latest chapter of a long story, but a fast one.