‘Ring of fire’ changed continent of Zealandia

Scientists have shed fresh light on the sunken continent our country sits on, finding it was once transformed by a process they’ve likened to a “massive, super-slow earthquake”.

Zealandia has been known to geologists for decades — but it gained public recognition only in recent years, after scientists said it should count as the seventh, and smallest, continent. New Zealand to the south and New Caledonia to the north are the only major land masses of the otherwise mostly underwater Zealandia, which, at 4.9 million sq km, is about two-thirds the size of Australia.

The first scientific drilling expedition to Zealandia in 1972 hypothesised it was underwater after its crust was stretched, thinned and ripped away from the ancient supercontinent of Gondwana about 85 million years ago.

But new samples collected and tested by Victoria University’s Professor Rupert Sutherland and his colleagues via boreholes up to 900m below the sea floor, point to a fresh factor — Zealandia’s dramatic change during the formation of the Pacific Ring of Fire about 50 million years ago. The ring is a zone of volcanoes and earthquakes that result from subduction, where a tectonic plate sinks back deep into the Earth.

The process by which the zone formed has been a mystery. “We propose that a ‘subduction rupture event’ propagated around the whole of the western Pacific at that time,” Sutherland said. “We suggest the process was similar to a massive super-slow earthquake that resurrected ancient subduction faults that had lain dormant for many millions of years.

“This concept of ‘subduction resurrection’ is a new idea and may help explain a range of different geological observations.”

Sutherland’s team, involved in a nine-week voyage as part of the International Ocean Discovery Programme, included more than 30 scientists. He co-led it with Gerald Dickens of Texas.

The findings have been published in the Geological Society of America’s journal, Geology.