Seven years ago, Zealandia was confirmed as Earth’s eighth continent — a slab of continental crust stretching from New Zealand to New Caledonia, 95% underwater. In 2023, it was fully mapped for the first time.

Now scientists have brought together decades of work by hundreds of researchers to consistently characterise and map all of New Zealand’s coastal sedimentary basins, providing clues to how Zealandia evolved.

“A sedimentary basin is any depression on the Earth’s surface,” explains Lorna Strachan, a geologist at the University of Auckland. “It can be anything from a puddle in your driveway to the Pacific Ocean.”

Sediments accumulate over time in these basins—layer upon layer of sand, mud, rocks, shells, pollen, and bones that tell untold stories about past climates, ocean currents, plate tectonics, the birth of mountain ranges, ecology, and evolution.

“I’m biased, I’m a sedimentologist,” Strachan said. “But to me they’re a really rich archive of the past — I would say the most important.”

In a new paper, scientists from GNS Science Te Pū Ao (Institute of Geological and Nuclear Sciences Limited, New Zealand Crown Research Institute) have collected decades of data to determine the distribution, extent and sediment thickness of 25 major marine basins around New Zealand. The work is part of a much larger, likely decades-long project to unravel the geological history of Zealandia.

Lead author Kyle Bland likened the endeavor to the Maori concept of whakapapa—tracing the ancestors or genealogy of Zealandia. “Why is it shaped like that? When did it break away from Gondwana? Why do we have these central volcanoes on the North Island? Why do we have the Southern Alps? If you understand how it all started, you can explain—at least at a high level—why we have the structure of New Zealand that we have.”

Fragments and holes

One key source of data for the researchers was a collection of rocks drilled from the deep sea floor. The sediment cores were collected by oil companies exploring for deposits, as well as by scientists participating in the International Ocean Discovery Program.

There are millions of years of history hidden in these 10-centimeter-diameter cylinders, Bland said. Most of Zealandia’s basins formed during the mid- to late Cretaceous period (about 100 million to 66 million years ago), although younger basins date back as little as 3 million years ago.

He held up a 3-D-printed shell-shaped object, a model of a type of foraminifera, a family of single-celled organisms that inhabit the oceans. It’s not to scale: “This thing would be about the size of a human hair in terms of thickness,” Bland said. Small, yes, but mighty: Scientists are “reconstructing the continent as we think it looked for 100 million years … mostly with these little things.”

Because foraminifera evolved quite quickly, diversifying into new species that are relatively easy to identify, their fossils can help geologists date the sedimentary rocks in which they were found. “We have scientists sitting at a microscope with a really fine brush, picking and choosing,” Bland said. “And looking at the species we find, we can say, ‘Oh, this rock must be 50 million years old.’”

By applying the same dating methodology consistently to samples from all 25 basins, Bland’s team was able to determine when Zealandia began to separate from the rest of Gondwana—about 105 million years ago. “In all the basins… the number is the same. You have to go back and look at everything together to get a sense of the pattern.”

The sediments also reveal what happened above the waves. Scientists have been able to identify when New Zealand’s Southern Alps began to rise from the spine of the South Island 10 to 12 million years ago, for example as a result of the collision of the Australian tectonic plate with the Pacific.

The cores reveal “an incredible amount of sediment being deposited in areas surrounding the South Island and as far north as Taranaki,” Bland said, referring to the western region of the North Island.

Similarly, about 100 million years ago, sediments in basins in Zealand were full of thick riverbed conglomerates, he said. The pattern suggests high fault activity, hill growth and a type of landscape called horst and graben.

In addition to the cores themselves, Bland’s team compiled seismic images to compare sediment thicknesses across Zealandia’s basins. The deepest — up to 11 kilometres thick — are the Taranaki, Raukumara, East Coast and Pegasus basins, the ones closest to the modern Australia-Pacific plate boundary. Knowing the depth of the harder bedrock will be key to future work calculating seismic hazard risk at different locations, Bland said.

Small Climate Index

In a separate but related paper published in 2022, Bland and other colleagues, including James Crampton, a paleontologist at Te Herenga Waka Victoria University of Wellington, used analyses of data from the same sedimentary basins to support and test the computer model. They then used the model to create a series of paleogeographic maps showing our best current understanding of the evolution of Zealandia from the mid-Cretaceous to the present.

Inorganic deposits found in the cores have helped solidify these interpretations. For example, fragments of volcanic rock and ash contain iron particles that align with magnetic north. That alignment can help identify where on Earth’s surface the rock was formed, informing maps. “They’re like little ancient compasses,” Bland said.

Together, the papers provide a great starting point and a springboard for future work, said Strachan, who was not involved in either study. There are still big questions about Zealandia’s tectonic evolution, she said, especially over the past 30 million years.

The deep-sea sediment cores we have are largely concentrated around areas that are considered potential for oil drilling, leaving vast swaths of the undersea continent essentially unexplored. “We’re trying to reconstruct an area the size of Wales or Switzerland from a single 10-centimetre hole,” Bland admitted.

“I’m not a big puzzle enthusiast,” Strachan said. “But I compare it to putting together a 10,000-piece puzzle and you only have five pieces — and the pictures have come together. It’s hard. There’s a lot of stuff missing. And so you put together the most logical interpretation based on what you have.”

Scientists have a site that sheds light on an important corner of Earth’s surface. “We sit right on all these boundaries of latitude, oceanography, climate and atmosphere. New Zealand was a little climate temperature marker that sat in wonderful isolation in the southwestern Pacific, recording everything for us — and that record is in sedimentary basins,” Bland said.

This article was originally published in Eos magazine.