Tourists at the Tasman Glacier near Mt Cook have been warned that heavy rain in the area is likely to lead to a huge iceberg in its terminal lake “calving” – breaking off explosively and sending water surging down the lake.
The iceshelf at the end of the glacier has risen off the bottom of the lake.
“At some stage it’s going to break off and we’re going to have some of the most significant iceberg calvings on the lake since it was formed,” said Department of Conservation manager Richard McNamara.
The area has had 250mm of rain over 72 hours, and the rising water level could trigger the dumping of millions of tonnes of ice into the lake.
Aoraki Mt Cook Alpine Village Ltd tourism general manager Denis Callesen said the face of the glacier had risen over 20 metres.
“It’s incredible to see this very distinct line of ice that’s risen out of the water, we’ve never seen that happen before across the whole face,” he said.
“There’s going to be a massive calving,” he said.
“If the whole lot goes together, upwards of 10 million tons of ice could break off in one hit. When that happens we’ll have one or more icebergs launched into the lake, creating a huge surge of water. It will be a truly impressive sight.”
Tourist boat trips on the lake are due to start foer the season in a few weeks and Callesen said they won’t be allowed to go within 1.5km of the terminal face.
In February 2009 the largest single iceberg to date – estimated to be 250m long by 250m wide and 80m high – plunged from the terminal face into the lake, and another significant one calved in January 2008.
Scientists have predicted climate change will see most of the Tasman Glacier melt over the next 20 years.
The glacier is the largest in the Southern Alps and at 29km is noted as one of the longest in the world’s temperate zones, but it is at such a low altitude – 730 metres above sea level – that it melts rapidly.
In 1973 there was no lake in front of the glacier face, but its formation was a “tipping point” according to a glaciologist, Trevor Chinn: “No amount of snow at the head of the glacier, the neve, can compensate melting triggered by the lake.”
The lake is expected to eventually double in length, to a maximum of about 16km.