New Zealand to the south and New Caledonia to the north are the only major land masses of the otherwise mostly underwater Zealandia. Image / NASA
New Zealand to the south and New Caledonia to the north are the only major land masses of the otherwise mostly underwater Zealandia. Image / NASA
Scientists have shed fresh light upon the sunken continent that our country sits upon, 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's only been in recent years that it gained public recognition, after scientists announced that it should count as the planet's seventh – albeit 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.
Its continental crust is mostly between 10 and 30km thick, which is thinner than the 30 to 45km of the six other continents, yet thicker than oceanic crust, which is about 7km thick.
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.
Although evidence remained compelling that this was at least part of the case, new samples collected and tested by Victoria University's Professor Rupert Sutherland and his colleagues have pointed to a fresh factor.
That was a dramatic change Zealandia underwent during the formation of the Pacific Ring of Fire about 50 million years ago.
The Pacific Ring of Fire is a zone of volcanoes and earthquakes that result from the geological process of subduction, where a tectonic plate sinks back deep into the Earth.
The process by which the zone formed has always 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."
As a result of the Pacific Ring of Fire, "things that were in 1000 metres of water came up to sea level and then subsided down to be more than 1000m deep again", he said.
"The permanent effects included the New Caledonia Trough that comes all the way to Taranaki."
Sutherland's team, which took part in a nine-week voyage as part of the International Ocean Discovery Programme, included more than 30 scientists from New Zealand, the United States, Italy, Spain, New Caledonia, China, the Netherlands, Germany, Brazil, Japan, the United Kingdom and South Korea.
Source / GNS Science
He co-led it with Professor Gerald Dickens from Rice University in Texas in the United States.
They used a 300-tonne drill to make six boreholes up to 900m below the sea floor, from which rock and sediment cores were collected and analysed for clues about the timing and length of Zealandia's uplift.
"We used fossils from three of the sites to show that northern Zealandia became much shallower and likely even had land areas between 50 and 35 million years ago," Sutherland said.
"At about the same time, two other sites subsided into deeper water, and then the whole region subsided by at least a kilometre to its present depth."
The team's evidence shows that events such as the creation of the Pacific Ring of Fire "can dramatically alter the geography of continents, and the sedimentary record preserved on Zealandia will help us figure out in more detail how and why it happened, and what the consequences were for plants, animals, and regional climate".
The findings have been published in the Geological Society of America's journal Geology.
