Finding Zealandia. Earth’s Eighth Continent
The newly discovered continent once was part of Antarctica
Beyond the beaches of the warm, verdant island nation of New Zealand stretches a collection of vast submerged plateaus. In fact, the portion of that nation above sea level represents only about five percent of a newly revealed sunken continent: Zealandia. In 1995, I named this feature, Earth’s eighth continent.
The revelation sprung from explorations made in Antarctica a few years earlier. My team sought evidence there for the breakup of Gondwana millions of years ago. Our goal was to understand how the New Zealand microcontinent (the term used at the time) was ripped away and drifted north to more hospitable climes.
Pieces of Gondwana
Our research recognized that along with New Zealand other continental crust pieces once attached to Gondwana made the trip north. These included the submerged continental fragments of the Campbell Plateau, Lord Howe Rise, and the Chatham Rise along with other smaller fragments. They all were one mass within the Gondwana supercontinent. Tectonic forces separated and distorted them as they drifted with the new ocean floor created during the breakup and drifting process.
I thought it made sense to name all these distributed pieces — that once were a part of Gondwana, a single name to recognize that fact. Zealand seemed to be a necessary part of any name, and inspired by the Finnish composer Jean Sibelius, I coined the name Zealandia in parallel with the name of his composition Finlandia.
All seemed quiet for about twenty years until a gorgeous book, Zealandia, was published. Two New Zealand geologists expanded on my suggestion and put real depth into it, chronicling the natural history of Zealandia including the country of New Zealand since the Gondwana breakup. But they made the startling claim that Zealandia was in fact its own continent — a notion that was implicit in my study but I did not make explicit because that was not my purpose.
Zealandia came to fame in 2017 when the magazine GSA Today published a definitive argument by Nick Mortimer and colleagues that Zealandia is a continent — the eighth one, previously ignored, mainly because most of it is out of sight beneath the waves. The tiny portion above sea level is the nation of New Zealand.
What is a continent?
Why is Zealandia a continent? After all, only a small part of it pokes above sea level. Geologic factors listed in the GSA Today article explain why. Because it meets the geologic criteria for a continent, namely; (1) high elevation relative to adjacent regions floored by oceanic crust; (2) it’s made of a broad range of silica-rich igneous, metamorphic, and sedimentary rocks, distinct from ocean crust; (3) it has thicker crust and lower seismic velocity (speed of sound in rock) than oceanic crustal regions; and (4), it has well-defined borders around a large enough area to be considered a continent rather than a microcontinent or continental fragment.
How large is it? About the size of India, which is not a continent now but once was before its northward journey caused it to collide and join with Asia between forty and thirty million years ago.
The main problem with the continent proposal is that most of Zealandia is below sea level. We don’t think of continents as underwater — we think of continents as places where people live. The reason much of it is below sea level is that the Gondwana breakup process thinned its crust. The continental crust of Zealandia is thinner than other continents in comparison. However, the fact that much of it is below sea level for 95% of its territory is not a limiting factor. Sea level goes up and down over geologic time. For example, the sea level was 120–130 meters lower about twenty-four thousand years ago when ice sheets covered much of the northern hemisphere. The land area of Zealandia would have been much greater then. Consider if a plug could be pulled and the oceans drained. Zealandia would stand high like other continents if a bit lower.
The important insight from the recognition of Zealandia is that continents are not defined by coastlines, that is current sea level. Rather they are defined by geologic criteria that are independent of sea level. Nowadays, sea level is rising due to global warming. In a few hundred years, most of Florida will be below the surface of the Atlantic Ocean. But Florida will not be gone. It will still be Florida without cities and people. It will still be part of the North American continent but beneath the waves.
Useful reading:
Luyendyk, B.P. 1995. Hypothesis for Cretaceous Rifting of East Gondwana caused by Subducted Slab Capture. Geology, v. 23, pp. 373–376.
Mortimer, Nick, and Hamish Campbell. 2014. Zealandia: Our Continent Revealed. Auckland, NZ: Penguin Group (NZ). 271 pp.
Nick Mortimer and others. 2017. Zealandia: Earth’s Hidden Continent. GSA TODAY. Volume 27 Issue 3. (March/April 2017). pp. 27–35.
Useful Wikipedia articles:
Sea level over time
Future sea level on Earth
