New light shed on world’s most unusual continent
The Pacific “Ring of Fire”, rather than Australia, might be responsible for the evolution of recently discovered continent Zealandia.
A new theory about Zealandia, the seventh and smallest of the world’s continents, is being proposed by Te Herenga Waka—Victoria University of Wellington Professor Rupert Sutherland.
Professor Sutherland, of the School of Geography, Environment and Earth Sciences, was co-lead scientist on a multi-million-dollar international drilling expedition in the Tasman Sea to unlock the secrets of the largely submerged continent.
He says the theory came from observations on new geophysical data.
The ring of fire – the plate boundaries around the Pacific on which volcanoes and large earthquakes occur – had permanent and transient effects on Zealandia.
“Things that were in 1000 metres of water came up to sea level and then subsided down to be more than 1000m deep again. The permanent effects included the New Caledonia Trough that comes all the way down to Taranaki.”
Zealandia’s existence was thrust into the limelight early in 2017 when seminal research by New Zealand geologists appeared in the journal of the Geological Society of America and generated huge public interest.
The 4.9 million square kilometre continent is 94 percent submerged, with New Zealand and New Caledonia the largest parts of the continent above water.
Zealandia once made up about 5 percent of Gondwana.
Its continental crust is mostly between 10km and 30km thick, which makes it thinner than the 30km to 45km thickness of the other six continents, although thicker than oceanic crust, which is about 7km.
Professor Sutherland’s theory is based on exhaustive work from the 27 July to 26 September 2017 International Ocean Discovery Programme Expedition 371, in which six boreholes were made by a 300-tonne drill up to 900m below the sea floor at sites on the Norfolk Ridge, New Caledonia Trough, Lord Howe Rise and Tasman abyssal plain.
Rock and sediment cores from those bores were then analysed for clues about the timing and length of uplift of Zealandia.
The ship had about 120 on board, including 32 scientists from New Zealand, the United States, Italy, Spain, New Caledonia, China, the Netherlands, Germany, Brazil, Japan, the United Kingdom and South Korea. The other co-lead scientist was Professor Gerald Dickens from Rice University in Texas in the US.
Professor Sutherland says that in 1972, three holes were drilled “pretty much randomly” in northern Zealandia.
“That was the first time anyone had really properly sampled offshore there. And that was the last time pretty much, although in 1986 they went back and re-drilled some of the shallow parts but didn’t learn anything fundamentally new.
“The question is – how did Zealandia form? Six of the continents on Earth look incredibly similar in terms of continental slope and mountains, but Zealandia is really, really different, so it is a really unusual continent.
“That very first expedition in 1972 led to the hypothesis that it was underwater because it was ripped away from Australia and Antarctica in the Cretaceous, about 85 million years ago. In the process of doing that, the crust was stretched and made thinner.
“But there was a whole bunch of observations I was making on the geophysical data that just weren’t adding up with that idea.
“The hypothesis we were proposing was that a key factor in the geography of Zealandia was the formation of the Pacific Ring of Fire. It wasn’t just the ripping away from Australia and Antarctica.
“What we discovered is the formation of the Pacific Ring of Fire subduction margin had a really big impact on the geography. And the Lord Howe Rise actually came up and was above sea level when this process was going on, about 50 million years ago.
“Zealandia has formed at that subduction margin. Our hypothesis is that these margins can die and can come back to life in subduction resurrection.
“All the Zealandia continent formed over 500 or 600 million years by stuff being plastered on to the edge of subduction margins. So it has been at a proto-Pacific Ring of Fire for hundreds of millions of years, in fact its whole life. It was born in that environment, but it hasn’t been in that environment continuously.
“So why did that subduction die and then come back to life and die again and be resurrected? From a geologist’s point of view that is a very interesting thing.
“The formation of the Pacific Ring of Fire is quite a puzzle, because that actually caused everything on the planet to change. It was a really significant global event with far-reaching implications for natural resources, climate and global geochemical cycles.”
All six sites provided useful new stratigraphic and paleogeographic information.
“We were out in the Tasman Sea for 58 days. These are huge expeditions, giant experiments, with huge budgets, tens of millions of dollars. They are once-in-several decades type opportunities.
“It is really, really hard work. It is absolutely shattering. You’re working 14 hours a day, starting at 11pm and finishing at three in the afternoon and do that every single day for 58 days in a row.”
Professor Sutherland and Professor Dickens will be giving a free 30-minute public talk on the researchers’ findings, followed by an optional 90-minute first cut of filmmaker Adam Kurtz’s documentary about the expedition, at Government Buildings Lecture Theatre 2 on University’s Pipitea campus on Wednesday 12 February at 5.30 pm.
Scientists involved in the project will be at the University from 12–14 February to synthesise their findings and ideas.
The findings are published in the Geological Society of America’s journal Geology.
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