NZ’s oldest rock found atWa¯naka
New Zealand’s oldest rock has existed for 2.7 billion years – but has spent the past five years sitting in a drawer at the University of Otago.
Associate professor of geology James Scott made the find while gathering peridotite rock from known sites around New Zealand. The goal was to better understand the composition of the mantle below the crust of the continent of Zealandia by collecting ancient peridotite rock. ‘‘I knew that these rocks were there on the shore of Lake Wa¯naka, but nobody had ever done the dating of them.’’
However, he was surprised after it went through isotope dating analyses in Canada in 2015 and its age was revealed.
Previously, the oldest known rocks in New Zealand were found in Nelson and were about 500million years old. The discovery led to a shift in thinking about New Zealand in geological terms, he said.
The Zealandia continent was geologically believed to be young in comparison to other parts of the world but this rock was comparable to the world’s oldest rocks, found in parts of Africa and Canada.
The rock normally sits about 30 kilometres under the Earth’s crust. It is believed to have come to the surface via extinct volcanoes underneath Lake Wa¯naka, which would have thrown out the rock in an explosion about 23 million years ago.
Samples of the rock had been noted along the shoreline, near Mt Albert Station, in the 1980s, but nobody had looked into it any further, Scott said. ‘‘I assumed it was still there, so we went and had a poke around.’’
It was ‘‘incredibly fortuitous’’ to find the green, magnesium-rich rock.
It had been sitting in his desk until he shared it recently with a journalist. It was significant, but Scott did not think it had any monetary value. It could end up in a museum, he said.