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A 2.7 billion-year-old peridotite rock found on the shores of Lake Wanaka is New Zealand’s oldest known rock.
New Zealand’s oldest rock has existed for 2.7 billion years, but it has spent the last five years in a drawer at the University of Otago.
Associate professor of geology James Scott made the find while collecting peridotite rock from known sites around New Zealand.
The goal was to better understand the composition of the mantle beneath the crust of the continent of Zealandia by collecting ancient peridotite rocks.
“I knew these rocks were on the shore of Lake Wānaka, but no one had ever dated them.”
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However, he was surprised with the result after he underwent isotope dating analysis in Canada in 2015 and his age was revealed.
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University of Otago associate professor of geology James Scott has found the oldest rock in New Zealand.
Previously, the oldest known rocks in New Zealand were found in Nelson and were about 500 million years old.
The discovery led to a change in thinking about New Zealand in geological terms, Scott said.
The continent of Zealandia was geologically believed to be young compared to other parts of the world, but this rock was comparable to the oldest rocks in the world, found in parts of Africa and Canada.
The rock is normally located about 30 kilometers below the earth’s crust.
It is believed that it came to the surface through a series of extinct volcanoes below Lake Wānaka, which would have blasted the rock out about 23 million years ago.
Samples of the rock had been observed along the shoreline near the Mt Albert station in the 1980s, but no one had investigated further, Scott said.
Simon Maude / Stuff.co.nz
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“I assumed it was still there, so we went and took a look.”
It was “incredibly fortuitous” to find the magnesium-rich green rock.
Most of the sample he took was ground into powder to allow age testing to be performed and only a fragment about 2 cm by 2 cm remained.
It had been on his desk until he recently shared it with a curious journalist, sparking a surge of interest in the media.
It was significant, but Scott didn’t think it had any monetary value.
It could end up in a museum, he said.
“To be honest, I hadn’t thought about it that much until today and yesterday.”
Amateur geologists could search for more rock, but it was not abundant and it was necessary to know what he was looking for, Scott said.
