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| N. Mortimer; Geol. Soc. America |
Twenty-two years ago, I published a study of the breakup of the eastern parts of the Gondwana supercontinent 1. My 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. The study was related to my research in Marie Byrd Land, the region in West Antarctica from where New Zealand broke away.
My 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 Gondwana.
Tectonic forces separated and distorted them as they drifted with the new ocean
floor created during the breakup process.
I thought it made sense to name all these distributed pieces—that
once were one, 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 calm for about twenty years until a gorgeous book
Zealandia,2 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 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 last month when GSA Today published a definitive argument by Nick Mortimer and colleagues
3 that Zealandia is a continent—the eighth one, previously ignored, mainly
because 95% of it is out of sight beneath the waves. The 5% above sea level is
the nation of New Zealand. Does New Zealand then "own" Zealandia? The United Nations Law of the Sea offers guidance.
Why is it a continent? The authors argue because it meets
the geologic criteria, namely; (1) high elevation relative to regions floored
by oceanic crust; (2) a broad range of siliceous igneous, metamorphic, and
sedimentary rocks; (3) thicker crust and lower seismic velocity structure than
oceanic crustal regions; and (4) well-defined limits 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
but once was before its northward journey caused it to collide and join with Asia
between forty and thirty million years ago.
Publication of the GSA
Today article (online) started a global media frenzy. I was contacted and spoke
to reporters from around the world, and appeared on live TV news programs (see Business Insider). For Mortimer,
he tells me it was overwhelming. Just imagine why this news was so shocking. A continent
was found that we didn’t know about.
The main problematic issue is that most of Zealandia is
below sea level. We don’t think of continents as under water—we think of
continents as places where people live. The reason much of it is below sea
level is because the breakup process thinned its crust. The continental crust
of Zealandia is thinner that other continents in comparison. However, the fact
that much of it remains below sea level for 95% of its territory is not a
limiting factor. Sea level goes up and down over time. For example, sea level
was 120-130 meters lower about twenty-four thousand years ago.
The land area of Zealandia (now mostly the nation of New Zealand) would have
been much greater then. Consider a plug could be pulled and the oceans drained.
Zealandia would stand high like other continents if a bit lower.
The main insight from the “discovery” 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. My recent blogs point
to the fact that 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. It will still be part of the North American
continent but beneath the waves.
1. Luyendyk, B.P. 1995. Hypothesis for Cretaceous Rifting of
East Gondwana caused by Subducted Slab Capture. Geology, v. 23, pp. 373-376.
2. Mortimer, Nick, and Hamish Campbell. 2014. Zealandia:
Our Continent Revealed. Auckland, NZ: Penguin Group (NZ). 271 pp.
3. Nick Mortimer and others. 2017. Zealandia: Earth’s Hidden
Continent. GSA TODAY. Volume 27 Issue 3. (March/April 2017). pp. 27–35.
