Boulders linked to ancient eruptions

Lesley Hema (right) gains a vantage point from some of the large boulders which caused so much damage to property at Matata. Picture / Alan Gibson

Boulders weighing up to eight tonnes that were dumped by floods in Matata could have come from historic volcanic eruptions, says a geologist.

The rocks, some up to 2m long, were dislodged from the hills above the town and washed down the Awatarariki Stream, wiping out houses in their path.

The former director-general of the Department of Science and Industrial Research, David Kear, said the rocks could be from two historic eruptions.

Geologists had not yet been able to visit Matata to determine whether they were ignumbrite or andesite rocks, which would shed light on their origins.

Dr Kear said if they were andesite they were probably created during the Manawahe eruption about 400,000 years ago.

If they were ignumbrite they were likely to have come from the Okataina volcanic centre, which includes Mt Tarawera, probably from an eruption about 280,000 years ago.

"They are very big on either score," he said. "I've seen lots of film of them on TV in the last couple of days."

Boulders like those had been used in construction all over the Bay of Plenty.

Although not historically significant, geologists would be interested in the rocks' origins because "we don't like having boulders when we can't quite say where they are from", he said.

Geologists from the Institute of Geological and Nuclear Sciences had been in touch with Environment Bay of Plenty about the boulders.

Environment BoP spokesman Bruce Fraser said staff had taken helicopter flights above Matata and the boulders would continue to be examined.

He said the sheer amount of rainfall had caused slips that brought the boulders down the Awatarariki Stream, which runs into the township.

The translation of Awatarariki is "stream of the gods", he said.

A number of boulders had been in the town before the floods so it was not the first time this dislodgement had happened.

"I know there were rocks there before. They have been used as part of the landscaping on a reserve.

"It's something we will keep looking at."

The size of the boulders meant a significant event for geologists.

Dr Kear said the boulders might have got their shape through weathering or having run down a river.

Weathering was the most likely option, with the rocks eventually being worn down by water getting into cracks over hundreds of thousands of years and breaking and rounding them.

"They will lie around on the surface of the ground until they are so rounded they will fall down a hillside," he said.

"Then they gather around at the bottom and when a flash flood comes along like this they will get picked up and carried away."