On 14 February 2017, the Geological Society of America published a feature by a team of New Zealand geologists in the journal GSA Today about the classification of a block continental rock largely submerged under water. In that study, lead author Nick Mortimer and colleagues argued that the region is not a collection of continental fragments, but a “new” single continent called Zealandia (after the one portion of land above water):
Zealandia illustrates that the large and the obvious in natural science can be overlooked. Based on various lines of geological and geophysical evidence, particularly those accumulated in the last two decades, we argue that Zealandia is not a collection of partly submerged continental fragments but is a coherent 4.9 Mkm2 continent. Currently used conventions and definitions of continental crust, continents, and microcontinents require no modification to accommodate Zealandia.
Although many news outlets ran this story with some variation of the headline “Scientists discover new continent”, this story represents neither a discovery nor a “new” portion of Earth’s crust. Instead, this paper, titled “Zealandia: Earth’s Hidden Continent,” reviews a decades-long debate regarding the classification of continental crust that makes up New Zealand and the underwater mounds that surround it. The team ultimately concluded that geologic convention requires Zealandia to be considered the eighth continent:
The progressive accumulation of bathymetric, geological, and geophysical data since the nineteenth century has led many authors to apply the adjective continental to New Zealand and some of its nearby submarine plateaus and rises. “New Zealand” was listed as a continent by Cogley (1984), but he noted that its continental limits were very sparsely mapped. The name Zealandia was first proposed by Luyendyk (1995) as a collective name for New Zealand, the Chatham Rise, Campbell Plateau, and Lord Howe Rise (Fig. 2). Implicit in Luyendyk’s paper was that this was a large region of continental crust […].
Although there is no official body that would “ratify” the decision to name this region a continent, the authors argue that it already meets the definition already widely accepted by the geologic community:
It is generally agreed that continents have all the following attributes: (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.
The scientists then provide arguments for each one of these points. Zealandia, they aver, is indeed elevated above the surrounding oceanic crust, “The main difference with other continents is that it has much wider and deeper continental shelves than is usually the case.”
From a geological perspective, the scientists argue that the rock that makes up this raised land had to be different from the more basaltic oceanic crust, but also plausibly similar to other suites of old continental material it may once have been connected to. Zealandia’s geology, they argue, indicates it broke off from the east Australian continental basins around 84 million years ago, and is similar in composition in many parts.
While Zealandia’s crust is not as thick as other continental crusts, the authors maintain that it is significantly different enough from the surrounding coastal crust to be clearly defined:
Whereas most of Zealandia’s crust is thinner than the 30–46 km that is typical of most continents, the above studies show that it is everywhere thicker than the ~7-km-thick crust of the ocean basins. This result is visible in the global CRUST1.0 model of Laske et al. (2013) […]. Collectively, the crustal structure results show that the rock samples [across Zealandia] are not from separate continental fragments or blocks now separated by oceanic crust, but are from a single continental mass.
Finally, they declare that Zealandia’s size, defined by the information referenced above, would make it substantially larger than anything currently defined as a microcontinent, making continent be a more reasonable classification:
Being >1 million km2 in area, and bounded by well-defined geologic and geographic limits, Zealandia is, by our definition, large enough to be termed a continent. At 4.9 Mkm2, Zealandia is substantially bigger than any features termed microcontinents and continental fragments, ~12× the area of Mauritia and ~6× the area of Madagascar (Fig. 4). It is also substantially larger than the area of the largest intraoceanic large igneous province, the Ontong Java Plateau (1.9 Mkm2). Zealandia is about the same area as greater India (Figs. 1 and 4). Figure 4 makes a case for a natural twofold grouping of continents and microcontinents.
While it presents no new data, this paper synthesizes decades of research to make an argument to the geologic community, one that Mortimer said it will be “nothing new” to those who study the region.
As there is no single group responsible for ratifying such a classification, it remains to be seen if idea of a Zealandia continent takes off. Speaking to the Guardian, University of Melbourne professor of earth sciences Barry Kohn said that:
There was a “fair consensus in the scientific community” in favour of [Zealandia’s] existence. “It’s pretty clear that that whole area is not part of the ocean. It’s got all the hallmarks of a continent.”
Mortimer told the Guardian that the best validation of this work wouldn’t be some sort of official declaration, just that the term becomes commonly used and accepted. “If Zealandia makes its way into popular culture and onto maps, that’s all the validation that we’ll seek.”