In February 2021, I wrote a blog about the “discovery” of Zealandia, the eighth continent that many had predicted to exist, but nobody had found. The revelation that it did actually exist profoundly changed the reality of the earth’s geology, even though 95% of Zealandia was under water, and had been for 25 million years. Research reported this year reflects a much more detailed description of the continent, and a more accurate definition of its size; it’s bigger that scientists first thought, as you can see from the picture above.
The last “new” continent discovered was found, in 1820, by a Russian ship packed with sailors and, oddly, penguins (destined for the men’s dinner). They spotted a towering shore of ice on the horizon. This was the first ever sighting of the Fimbul Ice Shelf, and it marked the official discovery of a new continent: Antarctica. It also cemented the modern idea, upheld by most maps across the English-speaking world, that there are seven major landmasses. Zealandia has proved that modern idea to be wrong. The seven-continent model has been a mistake all along.
This month, an international team of researchers released the most detailed maps of Zealandia to date – incorporating all five million square kilometres (two million square miles) of this underwater region and its geology. In the process, they have uncovered hints as to how this mysterious continent formed – and why it has been obscured beneath the waves for the last 25 million years.
Zealandia is thought to have formed around 83 million years ago, during the Late Cretaceous period. However, its journey began up to 100 million years before that, when the supercontinent of Gondwana – which contained much of today’s land in one giant lump – started to break up. As Gondwana disintegrated, the world’s smallest, thinnest and youngest continent struck out on its own, while the regions of Gondwana that had once lain directly to its north west and south west became Australia and Antarctica, respectively. It’s thought that all, or part, of Zealandia may have existed as an island for a while, but then, around 25 million years ago, it disappeared beneath the ocean.
The first real clues that New Zealand might represent just a tiny, visible portion of a vast incognito landmass came in 2002, when scientists used bathymetry – the study of the depth of bodies of water – to analyse the region. The ocean above what we now call Zealandia is considerably shallower than the ocean surrounding it, suggesting that the area was not underlain by an oceanic tectonic plate – like most of the world’s oceans – but a continental one. The clincher came in 2017, when scientists put several lines of evidence together to propose that this is indeed a new continent. Zealandia is not a mere continental fragment or microcontinent, as had previously been proposed, but the real deal, 95% of which is submerged underwater.
In 2019, an international team of scientists mapped the geology of South Zealandia. Their research revealed that at some point, Zealandia had been stretched – pulled apart by tectonic forces, which reduced the thickness of the continent compared to regular continental plates. In the process, it became twisted and this made reconstructing its history much more challenging. The researchers’ analysis of rocks from the lost continent revealed that the stretching happened in two stages. The first began around 89-101 million years ago, which led to a rip which became the Tasman Sea in between Australia and New Zealand. The second phase started 80-90 million years ago, and led to Zealandia splitting off from West Antarctica.
For the latest study, another research group – involving many of the same geologists as before – charted North Zealandia. This time, they analysed rocks that had been dredged up from the Fairway Ridge, a region of the South Pacific off the coast of Australia, which forms the northernmost tip of Zealandia. These ancient remnants, which have not had a dry day for 25 million years, included a mixture of igneous rocks – those formed by volcanic processes – and sedimentary ones made in shallow basins just off the coast of Zealandia. By analysing their chemistry, and the radioactive isotopes deep within, the scientists could estimate their age and origins. The oldest were pebbles dating to the Early Cretaceous (around 130-110 million years old), followed by sandstone from the Late Cretaceous (around 95 million years old) and relatively young basalts from the Eocene (around 40 million years old).
The resulting maps of Zealandia transform it from a featureless mass into a place with many bands of distinctive geology running along its length from northwest to southeast. These fit together with the geology of West Antarctica like a jigsaw puzzle, confirming that this region and Zealandia once slotted together.
The next stage of the investigation looked at measurements of magnetic anomalies in the ocean floor around Zealandia. These variations in the strength of Earth’s magnetic field form an invisible record of how tectonic plates have moved around over time. They have revealed more about the continent’s ancient stretching, which continued for millions of years and even changed direction – leading to an ultra-thin continent that eventually sank from a lack on underlying support.
Zealandia is slowly giving up its secrets. However, with the vast majority of its landmass under kilometres of water, it’s going to take a while to reveal them all.
