Tourists on a boat trip in Mount Cook National Park were subject to a rare treat yesterday, when a section of densely compressed blue ice at the base of an iceberg was briefly visible.

Denis Callesen, General Manager Tourism for Aoraki Mount Cook Alpine Village Ltd, said an iceberg which calved off the face of the Tasman Glacier last year rolled over as it floated on the glacier's terminal lake yesterday morning.

In doing so, it exposed the clear blue ice crystals, which form when ice becomes so highly-compressed that all the air bubbles are squeezed out and the crystal expands.

"This ice is so dense and compressed; it's five times harder than the ice you might have in your gin and tonic," said Callesen.


"Each ice crystal is the size of a coffee cup, so when it's finally exposed to air, within two hours the air seeps into the weak spots around each crystal and it turns white."

Several boatloads of tourists taking a Glacier Explorers trip happened to be on the terminal lake during that two-hour window and were able to witness the phenomenon before the ice clouded over.

Although the iceberg — which measures more than 100m across — had been floating in the terminal lake since last winter, the blue ice had been protected from oxidisation while it sat beneath the lake's surface.

Callesen said the warmer summer weather meant the icebergs were beginning to melt and become less stable.

"This is the second significant iceberg rollover we've had this week... on Monday another one million tonnes of ice calved off the terminal face," he said.

While blue icebergs are not uncommon — Callesen predicts two or three more rollovers on the scale of yesterday's could occur this summer — it is unusual for tourists to be on the lake to see them.

"To get it that it happens during the day, not at night; on a fine day, not a cloudy day; on a calm day, not a windy day... all the gods lined up yesterday," he said.

"In Disneyland they spend millions of dollars creating 'shows' like this, but here this is simply nature at work.


"We're so very lucky that it's just given to us."