The Spectacular and Dynamic Glaciers of New Zealand

Aug 3, 2010
Brian Cross

Tasman Glacier Lake - Image by Picasaweb user Raymond Lee

There are many significant glaciers in New Zealand's Southern Alps. Tasman Glacier, the largest of them all, is about to move spectacularly.

News on 3 August 2010 that the terminal face of the Tasman Glacier has risen 20 metres, that an iceberg calving is imminent and that warnings have been issued about the Tasman Lake tsunami that will result, has thrown the spotlight on the massive rivers of ice that exist on New Zealand’s South Island. While the very accessible Franz Josef and Fox glaciers of the west coast are more widely known, Aoraki/Mt Cook's Tasman is New Zealand’s largest glacier.

Tasman Glacier Set to Calve

The Timaru Herald has reported Aoraki/Mt Cook Alpine Village tourism general manager Denis Callesen as saying that the recent change in the glacier face has been amazing. "We've never ever seen the face rise up like that. The whole process that's going on at the moment is incredible,” said Mr Calleson. "In the coming weeks it's inevitable there's going to be a massive calving."

The glacier’s sudden change of aspect follows significant rain in the lake’s mountainous catchment and an increase in water volume under the ice. Glaciologist Trevor Chinn explained in the Timaru Herald report that "... the central ice is held down by the sides and if the ice on the bottom gets water under it then it floats."

Mr Chinn suggested that the part that has risen out of the water might already have separated from the glacier. "It's dreadfully unstable because it's probably leaning back against the glacier…when it's floating, there's nine times more underneath."

The Tasman Glacier is nearly 30km long, 600m deep in places and up to 1.6km wide. Fox and Franz Josef glaciers by comparison are both around 12km long. Tasman Lake has formed over the last 25 years as a result of the glacier’s steady retreat, and continues to increase in size. It is now 7km long and is expected to more than double in length over the next two decades. Tasman Lake drains through the Tasman River into Lake Pukaki.

Glacier Formation and Action

All three of New Zealand’s most famous glaciers lie within the Te Wahipounamu World Heritage Area, which covers 10% of New Zealand. The region’s Ice Age remnants were one important reason for its qualification as a world heritage site, along with its natural beauty, wilderness status and general geological formation.

The last Ice Age glaciation left its indelible mark as it ground its way through the landscape, forming deep U-shaped valleys, many of which eventually filled with running water to create lakes such as Whakatipu and Te Anau. When it ended in New Zealand, the Ice Age left behind the glaciers that are known there today, over 3,000 measuring a hectare or more. All are in the South Island, except for 18 on the slopes of Mt Ruapehu.

Glaciers form in the higher mountain altitudes as the permanent snows compact over time and are seasonally added to. As gravity does its work and forces the glacial formation downhill into higher temperature zones, melting occurs at the leading face. Global warming is blamed for a recent increase in the melt rate, causing in turn a faster break-up and retreat of the terminal face. NIWA’s annual glacier surveys, the last done in November 2009, show a pattern of reducing glacier mass.

Waiting for the Tasman Iceberg

As the scientists and the small permanent population of Aoraki/Mt Cook monitor the Tasman Glacier for the release of a great (in on-land terms) iceberg into the lake, tourists are being warned to keep to high vantage points. Mr Callesen told the Timaru Herald that tourist vessels would not be allowed to go within 1.5 kilometres of the terminal face. That's an extension of the normal 800 metre limit.

A three to four metre swell is expected when the glacier calves, but Mr Callesen said the tourist vessel crews are well trained to handle it. "It's not a breaking wave and the boats are relatively safe."

In February 2009, a block of ice of around 4.6 million tonnes was calved from the Tasman Glacier. It was the largest recorded, but the next one is expected to be significantly bigger.