Exploring the Lost Continent: Zealandia’s Formation and Submergence

2023-10-03 15:09:11

Photo credit, Alamy

Image caption,

Zealandia was completely submerged around 25 million years ago

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New research into the geology of Zealandia reveals how it formed and why it was submerged.

In 1820, a Russian ship filled with sailors and, curiously, penguins – intended for the men’s dinner – saw an imposing icy shore on the horizon. This was the first observation of the Fimbul Ice Shelf and the official discovery of a new continent: Antarctica. It was also on this occasion that the modern idea was born, defended by most maps of the English-speaking world, according to which there are seven large continental masses.

Today, schoolchildren, explorers and politicians generally accept the neat division of the earth’s land into these simple units, which include Europe, Asia, Africa, North America, South America, Australia and Antarctica.

But in 2017, the story took an unexpected turn: the seven continents model was a mistake all along. This is Zealandia, a long-lost land in the southeast of Australia, also known as the eighth forgotten continent of the planet.

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Scientists had long predicted the existence of this bonus southern landmass, but it remained undiscovered for 375 years, largely because it is almost entirely submerged under 1 to 2 km of water. Today, they are beginning to unravel its secrets.

This month, an international team of researchers published the most detailed maps of Zealandia to date. They integrate the five million square kilometers of this underwater region and its geology. In doing so, they discovered clues about how this mysterious continent formed and why it remained hidden beneath the waves for the last 25 million years.

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An ancient beginning

Zealandia is thought to have formed around 83 million years ago, during the Late Cretaceous Period. However, its journey began 100 million years ago, when the supercontinent Gondwana – which brought together much of today’s land into one giant mass – began to disintegrate.

As it disintegrated, the world’s smallest, thinnest and youngest continent became isolated, while the regions of Gondwana that lay directly to the northwest and southwest of it became respectively Australia and Antarctica.

It is believed that all or part of Zealandia may have existed as an island for some time. But about 25 million years ago, it disappeared under the ocean.

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Image caption,

Mapping Zealandia was not easy because 95% of the land is under deep water

The first real clues that New Zealand might be just a tiny visible part of a vast, unknown landmass came in 2002, when scientists used bathymetry – the study of the depths of land masses. water – to analyze the area. The ocean above what we today call Zealandia is considerably shallower than that surrounding it, suggesting that the area is not covered by an oceanic tectonic plate – like most of the world’s oceans. – but by a continental plate.

The turning point came in 2017, when scientists brought together several lines of evidence, including data on the types of rocks it contains and its relative thickness – ocean plates tend to be thinner – to propose that it is indeed a new continent. It is not a simple fragment of continent or a microcontinent, as had been previously proposed, but a real continent, 95% of which is submerged underwater.

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However, despite the excitement over the discovery of a new continent and more than a decade of intensive research, many details about the early formation of Zealandia have remained elusive. This is partly due to a strange event that occurred when the continent separated from Gondwana.

A delicate outcome

In 2019, an international team of scientists mapped the geology of southern Zealandia. Their research revealed that at some point, Zealandia was stretched – pushed apart by tectonic forces, thinning the continent relative to regular continental plates and creating breaks that later became oceanic crust. During this process, the continent twisted, making it much more difficult to reconstruct its history to determine its original shape.

The researchers’ analysis of rocks from the lost continent revealed that the stretching occurred in two stages. The first began about 89 to 101 million years ago and resulted in a rift that became the Tasman Sea, between Australia and New Zealand. The second phase began 80-90 million years ago and led to the separation of Zealandia from West Antarctica and the creation of the Pacific Ocean. But many mysteries still remain unexplained, and the other half of this continent has not yet been studied in detail.

Photo credit, Alamy

Image caption,

The world’s seventh continent, Antarctica, did not appear on some world maps until more than a century after its discovery.

A gradual subsidence

For this latest study, another research group, made up of largely the same geologists as before, looked at northern Zealandia.

This time, they analyzed rocks taken from the Fairway Ridge, a region of the South Pacific off the coast of Australia that forms the northernmost tip of Zealandia. These ancient remains, which have not seen a dry day in 25 million years, included a mixture of igneous rocks – formed by volcanic processes – and sedimentary rocks formed in shallow basins just off the coast from Zealandia.

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By analyzing their chemistry and the radioactive isotopes they contain, scientists have estimated their age and origin. The oldest were pebbles dating from the Lower Cretaceous (around 130-110 million years), followed by Upper Cretaceous sandstones (around 95 million years) and relatively young Eocene basalts (around 40 million years). years).

The resulting maps of Zealandia transform this featureless mass into a place with many distinctive geological bands that run along its entire length, from northwest to southeast. These bands fit into the geology of West Antarctica like a puzzle, confirming that this region and Zealandia were once nested within each other.

The next step in the study was to measure magnetic anomalies on the ocean floor around Zealandia. These variations in the intensity of the Earth’s magnetic field constitute an invisible record of the movement of tectonic plates over time. They allowed us to learn more about the ancient stretching of the continent, which continued for millions of years and even changed direction, resulting in an ultra-thin continent that eventually sank.

Zealandia is finally beginning to reveal its secrets. But since most of this enigmatic landmass lies beneath miles of water, it will be some time before it reveals them all.

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