Part of the former giant supercontinent Zealandia has been discovered in New Zealand

In the summer of 2018, when the heat hit California, Rose Turnbull took advantage of the coolness of a windowless basement to sort out grains of fine sand. A geologist based in New Zealand, she was working at the time in a fellow laboratory at California State University, Northridge. She was trying to find tiny crystals of zircon, in an attempt to solve the mysteries of the eighth continent, Zealandia, also known as TeRiu-a-Māui in Maori.

This task took training and a bit of elbow grease, or in this case the nose. Ms. Turnbull gave a small presentation on Zoom. She wore her tweezers on the outside of her nose to collect some fat. This technique prevents sand grains from spreading throughout the room when picked up.

The crystals came from rocks collected on the islands of New Zealand, which are among the few evidence from Zealandia. Submerged continent stretches More than 5 million square kilometers. Zealandia is the thinnest and smallest underwater continent ever discovered. Its existence has only recently been recognized by the scientific community. Mrs. Turnbull He works for GNS Science, a research and advisory group in New Zealand. She and her colleagues wanted to learn more about the processes that shaped this strange stretch of land.

The results they obtained surprised them. Beneath the east coast of the South Island and Stewart Island in New Zealand has hidden a piece of the giant continent that is more than 1 billion years old. This finding suggests that Zealandia may not be as small as they thought. This age difference could support his qualification as a continent.

“Continents are like icebergs types,” he explains Keith Kleibes, study author and a structural geologist at the University of Vermont. “What you see on the surface doesn’t really represent the full extent of the monster.”

this discovery, described in the review geology, could help scientists solve a puzzle that has baffled them for years. Most of the continents have a rocky core called a kraton. Usually, this part of the Earth is billions of years old and serves as a stable base on which the continents are formed. So far, the oldest continental crust discovered in Zealandia is only 500 million years old, a relatively young age in geological terms. If Zealandia is indeed a continent, why does its kraton seem to have disappeared?

The recently discovered ancient rock fragment may be part of the missing piece on the continent. According to Ms. Turnbull, the discovery “ticks the last square”. “We are on a continent.”

Additionally, these observations may provide insight into how Zealandia or any other continental crust formed Joshua Schwartz, study author and granite geologist at California State University at Northridge.

“The layer that lines the top of the Earth, which we call the crust and which is very thin, is where all the activities necessary for life take place. The continental crust is the plateau where we live, we grow our food, we draw water, we extract minerals and so on.” Basically, our whole life is built on This crust. “

In search of the lost continent

Scientists have been tracking Zealandia for decades, but determining what continent really is is no small feat. “The terrible secret of geology is that there is no true, precise and clear definition of a continent,” Schwartz explains.

One of the main elements is rock formation. The ocean floor that surrounds Zealandia is not made of magnesium and iron-rich rocks, which make up the majority of other ocean crusts. In contrast, they are made of silica-rich rocks, such as granite, and are more common in the continental crust. These rocks extend over a vast area, much thicker and higher than the oceanic crust that normally surrounds the continent.

A team of scientists led by Nick Mortimer, of GNS Science, identified these various points, among other things, when they firmly defended Their intent to consider Zealandia as a continent In 2017. Mortimer and his team mentioned one curiosity: the apparent absence of Craton.

“It’s weird,” Kleibes notes. Continental crust is stronger than its oceanic counterpart. It is therefore more resistant to processes that allow surface rock to be recycled back into the mantle. The core formed by the kraton provides the continents with a stable base on which they can form. At the same stage of formation, the slow progression of tectonic plates forms island arcs and other land masses along their coasts.

For example, when Mr. Schwartz was on a family vacation in New Mexico, he said it was a “southern Wyoming Craton”. This stretch of rock that connects each other More than three billion years, is one of the cratons that make up the stable inner surface of the North American continent. Meanwhile, the Santa Fe boulders on which Mr. Schwartz set his feet recently grafted onto the mainland, when a chain of islands smashed into the old coast.

Currently, the oldest Zealandia crust ever discovered is believed to have formed when the continent was at the edge of the supercontinent. Gondwana, 500 million years ago. Evidence of older rocks has been found in Zealandia, including pieces of the ancient mantle 2.7 billion years, but no ancient crust fragments were detected.

The new study focuses on 169 samples from the South Island and Stewart Island in southern New Zealand. Some were collected by Mrs. Turnbull and her team during their multiple trips to the region. Others came from the country’s mineralogy catalog. Thus, the assembly sites represent both islands.

Back in the lab, they crushed the rocks and sorted the grains according to their density and magnetism. They proceeded with this selection until all that was left were fine sand and zircon crystals. Then Mrs. Turnbull took thousands of zircons and placed them on microscopic slides. Then, it was covered with epoxy resin and polished before it could finally do the chemical analysis.

“It’s a complete process,” she asserts.