The Mystery of the Eighth Continent

© getty images

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Learning that we live on a planet that once housed life but is now submerged in the ocean is like seeing a ruin in the wilderness or finding a dead civilized planet in space - both are extremely romantic things, but the degree of feeling they bring is somewhere in between.

We never hesitate to use our most secretive imaginations to imagine what kinds of trees once grew there and what strange creatures once danced there.

The year was 1642, and Abel Tasman was on a mission.

This experienced Dutch navigator had an elaborate moustache and a thick goatee, and was somewhat impulsive and reckless in his actions - he later tried to hang some of the crew members on a whim because he was drunk.

Tasman believed there was a vast continent in the southern hemisphere and vowed to find it.

At the time, the Southern Hemisphere was still largely a mystery to Europeans, but they were convinced that there was a large continent there - some had named it Australasia (meaning "unknown southern continent") - that would match the continents of the Northern Hemisphere. This idea actually dates back to the ancient Romans, but it was not until the 16th and 17th centuries that Europeans were able to test it.

So, on August 14, 1642, Tasman commanded a fleet of two small boats, set out from the company headquarters in Jakarta, Indonesia, sailed west, then south, then east, and finally arrived at the South Island of New Zealand. His first encounter with the local indigenous Maori (according to the current academic view, the Maori had settled in the South Island centuries ago) was not pleasant: the day after Tasman arrived in the South Island, several Maori paddled canoes and rammed a small boat that was delivering news to the Dutch fleet. Four Europeans died in this conflict. The Europeans fired back at 11 Maori canoes - the number of Maori casualties in this conflict is unknown.

This conflict also became the end of Tasman's mission - ironically, they named the place where the bloodshed took place "Murderer's Bay" and set sail for home a few weeks later without even getting off the ship and setting foot on the South Island. Although he believed that he had indeed discovered the southern hemisphere continent, it was obvious that this place was not a commercially developed utopia as expected. After that, Tasman never returned to the island. (In fact, at this time, Europeans already knew about Australia, but they did not think it was the legendary continent they were looking for. Later, Europeans repeatedly searched for the lost continent without success, so they changed their minds and gave Australia the name "Australis Continent".)

What Tasman didn't know was that he had been right all along and there was indeed a lost southern continent.

It can be said that Abel Tasman did discover the lost southern continent - although he didn't realise that 94% of it was underwater. © Alamy

In 2017, a team of geologists announced the discovery of Zealandia, a continent that covered 4.9 million square kilometers, about six times the size of Madagascar.

Before this, encyclopedias, maps and search engines around the world firmly believed that the Earth had only seven continents since a certain historical point, but this team of geologists confidently told the world that this conclusion was wrong. The Earth now has eight continents, and this eighth continent broke many records. It is the smallest, thinnest and youngest.

© TRT World

The key is that 94% of Zealandia is underwater, with only a few islands such as New Zealand extending from the depths of the ocean to the surface. Therefore, this continent can be said to have been hidden under our noses.

"Zealand is an example of something that's obvious but takes a while to discover," said Andy Tulloch, a geologist at the Royal New Zealand Institute of Geological and Nuclear Sciences Ltd., who was part of the team that discovered Zealandia.

However, this is just the beginning. Today, more than four years have passed since Zealandia was discovered, but the continent is still as mysterious as before, and the two-kilometer-deep sea water firmly guards its secrets. What formed Zealandia? What kind of creatures once lived on Zealandia? How long has Zealandia been submerged underwater?

Difficult exploration

In fact, studying Zealandia is a rather daunting task.

More than 100 years after Tasman discovered New Zealand in 1642, British cartographer James Cook set out on a scientific expedition to the Southern Hemisphere. His official mission was to observe the motion of Venus between the Earth and the Sun so that he could calculate our distance from the Sun.

The mysterious kiwi bird's closest relative is native to Madagascar, possibly for geological reasons. © Alamy

However, Cook also carried a sealed envelope, which would not be opened until after his first mission was completed. The contents of the envelope were a top-secret mission to discover a southern continent—in fact, he probably crossed that continent before he even reached New Zealand.

The first strong evidence for Zealandia came from the Scottish naturalist Sir James Hector, who in 1895 took part in a scientific expedition to explore a series of islands off the south coast of New Zealand. Hector studied the geological structure of the islands and concluded that New Zealand was "the remains of a great mountain range, beneath which a vast continent, extending far to the south and east, should have been buried..."

This was an early breakthrough in the search for the lost continent of the Southern Hemisphere, but other than that, information about the possible existence of Zealandia remained sparse until the 1960s. "This field has been developing very slowly," said Nick Mortimer, a geologist at the Institute of Geological and Nuclear Sciences Ltd. who led the research that discovered Zealandia in 2017.

In the 1960s, geologists finally agreed on a definition of a continent—a broad definition of a continent is a geological region of high ground, rich rock variety, and thick crust. Of course, a continent has to be large. “It can’t be just a little piece of land,” Mortimer said. This gave geologists a way to continue their work—if they could gather evidence that met the definition of a continent, then they would have proof that the eighth continent really existed.

In reality, though, progress on the mission remained stagnant—discovering a continent was a huge undertaking, expensive, and, as Mortimer points out, without any urgency. Then, in 1995, American geophysicist Bruce Luyendyk once again described New Zealand and nearby areas as a continent, suggesting the name Zealandia. Since then, Tulloch says, the search for an eighth continent has grown exponentially.

Tasman's fleet left New Zealand after a bloody conflict with the Maori - but he believed he had discovered the fabled southern continent. © Alamy

Around the same time, the United Nations Convention on the Law of the Sea came into force, which finally gave the geological community some impetus to seriously search for the eighth continent. According to the United Nations Convention on the Law of the Sea, the legal territorial scope of a country can extend beyond its exclusive economic zone - the sea area within 370 kilometers of the coastline - to the "extended continental shelf area", and the mineral and oil resources in the sea area naturally belong to the country to which it belongs.

If New Zealand could prove that it was part of a single continent, the country would expand sixfold. Suddenly, funding for surveys of the region began to mount, and the evidence began to pile up. Every rock sample collected was a step closer to proving Zealandia existed.

The final big chunk of data comes from satellites, which allow geologists to map the ocean floor by tracking tiny variations in gravity between different regions of the Earth’s crust. This technology made it clear that Zealandia was an irregular continent almost as large as Australia.

Satellite data shows what Zealandia might look like - the light blue inverted triangle east of Australia. © GNS Science

When the continent was finally unveiled to the world, it became the largest ocean territory in the world. “It’s kind of cool,” Mortimer said. “If you think about it, every continent on the planet has a lot of countries, but Zealandia only has three.”

Besides New Zealand, Zealandia also includes New Caledonia—a French colony known for its stunning lagoon—and two tiny Australian territories, Lord Howe Island and Ball's Pyramid, the latter of which an 18th-century explorer described as looking "no bigger than a small boat."

Mysterious stretch

Zealandia was originally part of the ancient supercontinent Gondwana, which formed about 550 million years ago and was essentially all of Earth's southern hemisphere landmass. Zealandia was located on the eastern tip of Gondwana and bordered several other landmasses, including half of West Antarctica and all of eastern Australia.

Then, about 105 million years ago, "due to a process that we don't fully understand yet, Zealandia began to break away from the supercontinent Gondwana," Tulloch said.

Continental crust is usually about 40 kilometers deep—much deeper than oceanic crust, which is only about 10 kilometers deep. As a result of the compression, Zealandia was stretched so much that its crust was reduced to its current thickness of about 20 kilometers. As a result, this extremely thin continent—but still much thicker than normal oceanic crust—sank and disappeared into the ocean.

Although Zealandia thinned and eventually sank, geologists were able to infer that it was a continent based on the variety of rocks found in the area. Continental crust is usually made up of igneous, metamorphic, and sedimentary rocks—like granite, shale, and limestone—while oceanic crust is usually made up of only igneous rocks, such as basalt.

After the ancient supercontinent Gondwana broke apart, its fragments drifted around the globe. Many of the ancient plants that once inhabited that area still live in Australia's Dorrigo Forest. © Getty Images

However, many unknown questions remain unanswered. The unusual origin of the eighth continent has intrigued and puzzled geologists. For example, it is still unclear how Zealandia managed to remain intact and not break into smaller microcontinents after becoming so thin.

Another question is when did Zealandia completely sink underwater? And was it ever completely above water? The part of Zealandia that is now above sea level is a ridge formed by the collision and compression of the Pacific Plate and the Australian Plate. Tullock said that there are two main views in the academic community on this issue. One is that Zealandia has always been underwater, with only some small islands above sea level, and the other is that Zealandia was once completely above sea level.

This question also raises another question: What kind of creatures originally lived on Zealandia?

With its mild climate and vast expanse of land, Gondwana itself was home to a wide variety of plants and animals, including the earliest four-limbed land animals and the largest animals ever to appear on Earth, the titanosaurs. So could the rocks of Zealandia be rife with the remains of these creatures?

The Dinosaur Debate

Land animal fossils are rare in the Southern Hemisphere, but in the 1990s, some animal remains were unearthed in New Zealand, including a large dinosaur with a long tail and long neck (sauropod), a herbivorous dinosaur with a beak-shaped mouth (hypsilophodontidae), and the ribs of an armored dinosaur (ankylosaur). Then in 2006, the foot bones of a large carnivore (possibly an Allosaurus) were found in the Chatham Islands, about 800 kilometers east of New Zealand's South Island. Most importantly, all of these fossils were formed after Zealandia separated from the ancient supercontinent Gondwana.

Fragments of the eggshells of elephant birds, which were 10 feet tall, are still scattered on the beach. © Alamy

However, this doesn't necessarily mean that most of Zealandia was inhabited by dinosaurs - the islands where the fossils have been found may have been refuges, and much of Zealandia could still be submerged in the sea, as it is today. "It's been a long-standing debate whether land animals could have survived long term in an environment without entire continents," said Rupert Sutherland, professor of geophysics and plate tectonics at Victoria University of Wellington.

Adding to the mystery is the presence of New Zealand's strangest but most beloved local inhabitant, the kiwi, a squat, flightless bird with a beard and hair-like feathers. Curiously, the kiwi's closest relative is not the moa, a ratite that also lived in the area (and went extinct 500 years ago), but the much larger elephant bird, a bird that roamed the forests of Madagascar 800 years ago.

The moa was thought to have become extinct around the 16th century, although there are some reports suggesting that it survived in some remote corners of New Zealand until the 18th and even 19th centuries. This is a reconstruction of a moa hunt from the early 20th century. © wikimedia

The discovery leads scientists to believe that the kiwi and elephant birds diverged from the same ancestor that lived on the ancient supercontinent Gondwana, which took 130 million years to break apart. After the breakup, the continental fragments were scattered around the world to form South America, Africa, Madagascar, Antarctica, Australia, the Arabian Peninsula, the Indian subcontinent and Zealandia.

This, in turn, suggests that at least part of Zealandia, now submerged in water, was once completely above sea level. However, researchers think that the entire continent, including New Zealand, may have gone underwater about 25 million years ago. "All the plants and animals, according to current thinking, appeared on the continent only after that," Sutherland said. So what happened?

After being pushed up by plate tectonic movement, New Zealand has become one of the highest points in Zealandia. © Alamy

Although it's impossible for scientists to collect fossils directly from Zealandia, which lies on the ocean floor, they have been drilling to measure its depth. "The most helpful and distinctive fossils are actually from very shallow waters," Sutherland said. "Because they leave all kinds of records - lots of small, distinctive fossils."

In 2017, a team of researchers conducted the most in-depth survey of the region to date, drilling 1,250 meters below the sea floor at six different locations. The rock samples they collected contained a variety of biological information, including pollen from land plants and spores and shells of organisms that live in warm, shallow seas.

"If you find water but it's only about 10 meters deep, then there's probably land nearby," Sutherland explained. Therefore, the pollen and spores also mean that Zealandia may not be completely submerged in sea water as previously speculated.

Strange bends

Another question that has long puzzled the geological community is the shape of Zealandia.

“If you look at a geological map of New Zealand, two things really stand out,” Sutherland said. One is the Alpine Fault, a plate boundary region that runs along the South Island and is so obvious that it can be seen from space.

The red band of rock – the Central Bedrock Belt – should have run diagonally down Zealandia, but it has deformed and changed its course. © GNS Science Ltd

The second is that New Zealand (and the wider Zealandia region) has a strange bend in its geology. Both New Zealand and Zealandia are bisected by a horizontal line - the point where the Pacific and Australian plates meet.

This area looks like someone deliberately took away the lower half and twisted it, causing the rock belt that should have stretched down not only not to be arranged in a straight line, but even to intersect at an angle close to a right angle.

One possible explanation is that the plates moved and somehow altered the topography of the region, but the exact process and timing of this event remain completely unknown.

"There are various theories to explain this phenomenon, but they all largely involve a lot of unknown information," Tulloch said.

Sutherland explained that Zealandia is unlikely to reveal all its secrets to us any time soon. "Everything is buried 2 kilometers below the surface of the water, and the rock layer that needs to be sampled is 500 meters below the seabed, which greatly increases the difficulty of making major discoveries," he said. "Exploring such a continent is really challenging. Therefore, a lot of time, money and energy are essential, so that we can go out to sea frequently and investigate this area."

But even without new favorable conditions, one thing is certain: Zealandia, the world's eighth continent - nearly 400 years after Tasman first searched for it - still has plenty of mysteries to unravel.

By Zaria Gorvett

Translated by Qiao Qi

Proofreading/Rabbit's Light Footsteps

Original article/www.bbc.com/future/article/20210205-the-last-secrets-of-the-worlds-lost-continent

This article is based on the Creative Commons License (BY-NC) and is published by Qiao Qi on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan

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Source: Leviathan