Zealandia: The Drowned Continent of the South

The topography of Zealandia.
The topography of Zealandia shows it underlying the islands of New Zealand, the South Pacific, New Caledonia and other islands. It was officially named a continent in 2017. World Data Center for Geophysics and Marine Geology, National Geophysical Data Center, NOAA

Earth has seven continents. That's something we all learn in school, as quickly as we learn their names: Europe, Asia (really Eurasia), Africa, North America, South America, Australia, and Antarctica.  But these aren't the only ones our planet has hosted since it formed. As it turns out, there's an eighth continent —the drowned continent of Zealandia. It can't be seen from the surface of Earth, but satellites can spot it, and geologists know about it. They confirmed its existence early in 2017, after years of mystery about just what was going on deep beneath the waves of the South Pacific near New Zealand.

Uncovering the Mystery of Zealandia

The clues to this lost continent have been tantalizing: continental rocks where none should exist, and gravity anomalies surrounding a large chunk of underwater territory. The culprit in the mystery? Huge slabs of rock buried deep beneath the continents. These huge conveyor-belt-like subsurface chunks of rock are called tectonic plates. The motions of those plates have substantially changed all the continents and their positions since the time Earth was born, some 4.5 billion years ago. Now it turns out they also caused a continent to disappear. It seems incredible, but Earth is a "living" planet, changing constantly through the motions of tectonics.

That's the story geologists are uncovering with the revelation that New Zealand and New Caledonia in the South Pacific are really the highest points of the long-lost continent Zealandia. It's a tale of long, slow motions over millions of years that sent much of Zealandia plummeting below the waves, and the continent wasn't even suspected to exist until the twentieth century.

The Story of Zealandia

So, what's the scoop about Zealandia? This long-lost continent, sometimes also called Tasmantis, formed very early in Earth's history. It was part of Gondwana, a huge supercontinent that existed as early as 600 million years ago. Earth's very early history was dominated by large single continents that eventually broke up as the slow motions of plates moved land masses around.

As it, too, was carried by tectonic plates, Zealandia eventually merged with another primordial continent called Laurasia to form an even larger supercontinent called Pangaea. Zealandia's watery fate was sealed by the motions of two tectonic plates that lay beneath it: the southernmost Pacific Plate and its northern neighbor, the Indo-Australian plate. They were slipping past each other a few millimeters at a time each year, and that action slowly pulled Zealandia away from Antarctica and Australia beginning some 85 million years ago. The slow motions caused Zealandia to sink, and by the late Cretaceous period (some 66 million years ago) much of it was underwater. Only New Zealand, New Caledonia and a scattering of smaller islands remained above sea level.

Zealandia's Geology

The motions of the plates that caused Zealandia to sink continue to shape the underwater geology of the region into sunken regions called grabens and basins. Volcanic activity also occurs throughout the areas where one plate is subducting (diving under) another. Where the plates compress against each other, the Southern Alps exist where uplifting motion has sent the continent upward. This is similar to the formation of the Himalaya Mountains where the Indian Subcontinent meets the Eurasian plate.

Zealandia's oldest rocks date back to the Middle Cambrian period (about 500 million years ago). These are mainly limestones, sedimentary rocks made of the shells and skeletons of marine organisms. There is also some granite, an igneous rock made up of feldspar, biotite, and other minerals, that dates back to about the same time. Geologists continue to study rock cores in the hunt for older materials and to relate Zealandia's rocks with its former neighbors Antartica and Australia. The older rocks found so far are underneath layers of other sedimentary rocks that show evidence of the breakup that began to sink Zealandia millions of years ago. In the regions above water, volcanic rocks and features are evident throughout New Zealand and some of the remaining islands.

How Did Geologists Find Zealandia?

The story of Zealandia's discovery is a sort of geological puzzle, with the pieces coming together over many decades. Scientists knew of the submerged areas of the region for many years, dating back to the early part of the 20th century, but it was only about twenty years ago that they began to consider the possibility of a lost continent. Detailed studies of the ocean surface in the region showed that the crust was different from other ocean crust. Not only was it thicker than oceanic crust, but the rocks also brought up from the ocean bottom and drilling cores were not from the oceanic crust. They were the continental type. How could this be, unless there was actually a continent hidden beneath the waves?

Then, in 2002, a map taken using satellite measurements of the gravity of the region revealed the rough structure of the continent. Essentially, the gravity of oceanic crust is different from that of continental crust and that can be measured by satellite. The map showed a definite difference between the regions of deep-ocean bottom and Zealandia. That was when geologists began to think that a missing continent had been found. Further measurements of rock cores, subsurface studies by marine geologists, and more satellite mapping influenced geologists to consider that Zealandia actually is a continent. The discovery, which took decades to confirm, was made public in 2017 when a team of geologists announced that Zealandia was officially a continent.

What's Next for Zealandia?

The continent is rich with natural resources, making the land of special interest to international governments and corporations. But it is also home to unique biological populations, as well as mineral deposits that are actively under development. For geologists and planetary scientists, the area holds many clues to the past of our own planet and may help scientists understand landforms seen on other worlds in the solar system.

Key Takeaways

  • Zealandia is a lost continent beneath the waves of the South Pacific Ocean. It was discovered using satellite mapping.
  • Geologists found rocks in the region that were continental-type rocks, not oceanic rocks. That led them to suspect a drowned continent.
  • Zealandia contains rich plant and animal populations, as well as minerals and other natural resources.