In 2017, an eighth continent was confirmed. Zealandia encompasses the islands of New Zealand and New Caledonia, but in total 94 percent of the continent is submerged. New research suggests that the forces that carved Zealandia into its present form, between 50 and 35 million years ago, were also responsible for the formation of the infamous Ring of Fire – a 40,000-kilometer (24,855 miles) horseshoe region around the Pacific Ocean home to over 450 volcanoes.
Published in Geology, the study analyzes fossils found in sediment cores that had been collected in 2017 by International Ocean Discovery Program Expedition 371. The tiny fossils indicated the ecosystems in which they had lived.
Three of the sites where the samples were collected in northern Zealandia showed that they had become a lot shallower, potentially even with land areas, between 50 and 35 million years ago. At a similar time, single-celled plankton species, which live in deeper waters, were found in samples taken near New Caledonia, suggesting subsidence in that part of the continent. After the to-ing and fro-ing, the whole continent sank another kilometer underwater.
This discovery offers up new evidence for the low profile of Zealandia. The prevailing wisdom since the 1970s was that when the crust separated from the supercontinent Gondwana 85 million years ago, it just thinned out. This model has Zealandia “doing nothing but gently cooling and subsiding,” Rupert Sutherland, the paper’s lead author and geophysicist at Victoria University of Wellington, New Zealand, said in a statement. However, their new research suggests that the 4.9 million-square-kilometers (1.9 million square miles) continent actually underwent a lot of dynamic movement.
But the story doesn’t end there. The topological changes to Zealandia occurred at the same time as a global reorganization of tectonic plates evidenced in part by the formation of underwater volcanoes encircling the western Pacific – the Ring of Fire. Movement of tectonic plates is driven by subduction, where at the boundaries between plates one tectonic plate slides over another, and forces it to sink into the Earth’s mantle.
However, how this process is started is not well understood, let alone in the case of the Ring of Fire. “One of the amazing things about our observations, is that they reveal the early signs of the Ring of Fire were almost simultaneous throughout the western Pacific,” explained Sutherland.
Sutherland and his co-authors offered up a new suggestion of what caused such a dramatic onset of subduction across a large area and in a short space of time. They theorize that there was a “subduction rupture event”, which not only triggered the formation of the Ring of Fire, but also caused the dramatic deformation of Zealandia.
Described by the researchers as a process similar to a massive slow-moving earthquake, the event potentially took more than a million years to spread across the whole of the western Pacific. In doing so, old subduction faults became resurrected, ready to move again.
“We don’t know where or why,” said Sutherland, “but something happened that locally induced movement, and when the fault started to slip, like in an earthquake the motion rapidly spread sideways onto adjacent parts of the fault system and then around the western Pacific.”
The researchers say that this event has no modern analog and possibly happened fewer than 100 times in Earth’s history. However, looking into Zealandia’s past will help advance future understanding of subduction rupture events. “Ultimately, Zealandia’s sedimentary record should help us determine how and why this event happened and what the consequences were for animals, plants, and global climate,” concluded Sutherland.