KEY POINTS:
A party of four top scientists from Geological and Nuclear Sciences and the Department of Conservation visited the Mt Ruapehu crater rim in near-blizzard conditions yesterday.
But the trip was well worth the trouble, said Harry Keys of DoC, because it confirmed a textbook result for those who had spent nearly 10 years preparing for the lahar.
"It's fantastic," Dr Keys said, "There's been a huge breach in the tephra dam. Most of it has gone. There are only small boulders on either side. Water is still coming out of the lake at ... about one cubic metre a second".
The breach was about 40m wide at the base of the dam and 60m wide at what had been the top, he said.
The Crater Lake's rim was now down to hard rock, rather than the permeable layer of ash, scoria and sand that had existed previously.
"It's very satisfying because we had estimated it [the lake level] to go down to 2529.3m, plus or minus 0.3m. Our prediction was most successful, because it's now at 2529.4m."
One geophone detector, which measures vibrations at the crater rim, was found precariously perched on the edge of the remnant of the tephra dam. "It was frozen-in so we spent some time digging it out and moving it."
Dr Keys estimated 1.29 million cubic metres (1.29 billion litres) of sulphur and water had gone down the Whangaehu Valley, leaving 8.9 million cubic meters inside the crater.
The lahar warning system, installed at a cost of $375,000, was again fully operational.
