As just about any high school student will tell you, Earth has seven continents. But researchers from Australia and New Zealand are challenging this fundamental notion by proposing an eighth continent in the oceans east of Australia. Dubbed Zealandia, 94 percent of its landmass is currently underwater, with New Zealand's Aoraki (Mount Cook) its highest point. If ultimately accepted, it might call for a, um, new atlas.

Covering some 1.9 million square miles (4.9 million sq km), Zealandia is larger than the Indian subcontinent, but less than two-thirds the size of the Australian continent, which covers 3.3 million sq mi (8.5 million sq km). It's only land portions are the islands of New Zealand and New Caledonia.

Zealandia is not a new discovery – in fact, the idea of an eighth continent was first proposed in 1995 – but scientists have typically put it in one of two categories. One view is that it's made up of fragments of continental crust, while others believe it to be a microcontinent, since the generally well-defined landmass includes a continental crust that is geologically isolated from other continents.

But the proponents of Zealandia-as-continent say it's much larger than other microcontinents, such as Madagascar (Zealandia is six times the size), believing that any landmass larger than 386,000 sq mi (1 million sq km) should be considered a continent. However, the defining minimum size of a continent has never been established. They also point to Antarctica, where much of its western half would be submerged without its surface ice.

Map of Earth's continents, with Zealandia shown in gray(Credit: N. Mortimer et al./GSA Today)

Zealandia also satisfies other criteria of a continent. It's substantially elevated above the surrounding oceanic crust, even though its continental shelf is mostly submerged. Geologically, Zealandia is made up of a diverse range of igneous, metamorphic, and sedimentary rocks, another qualifier for a continent. A third condition is in the thickness of its crust, 6 to 25 mi (10 to 40 km), throughout a single continental mass, which is thicker than the average oceanic crust of 4 mi (7 km), though less than the 19 to 29 mi (30-46 km) thickness typical of continents.

A sticking point with Zealandia skeptics is the basic idea that continents are landmasses surrounded by water that people live on. It can be continental without being a continent, much like Pluto is not quite a planet anymore – which also illustrates such definitions aren't set in stone.

Zealandia made up about 5 percent of the supercontinent Gondwana, which included what is now Antarctica, Australia, South America and Africa. As Gondwana was breaking up 100 million years ago, Zealandia was being stretched thin, causing it to sink underwater and deeper down into the mantle below. The fact that it wasn't shredded into microcontinents is significant.

If Zealandia does eventually gain acceptance as a continent, it would likely be a long, gradual process.

Source: GSA Today

View gallery - 4 images