Tourists warned of lake 'tsunami'

Heavy rainfall in the Mt Cook area has caused the Tasman Glacier's terminal face to rise more than 20m. Photo / Supplied

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 a tsunami-like surge of water 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 more than 20m.

"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."

"If the whole lot goes together, upwards of 10 million tonnes 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 for the season in a few weeks and Mr Callesen said they wouldn't be allowed to go within 1.5km of the terminal face.

In February last year, 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 - 730m above sea level - that it melts rapidly.

MELTDOWN

* Heavy rain could lead to a huge iceberg breaking off at the Tasman Glacier.
* The iceshelf at the end of the glacier has risen off the bottom of the lake.
* If the whole lot goes together, more than 10 million tonnes of ice could break off in one hit.
* This will cause a huge surge of water.

- NZPA