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Zealandia: what is a continent?

Simplified map of Earth’s tectonic plates and continents, including Zealandia Simplified map of Earth’s tectonic plates and continents, including Zealandia Mortimer et al.
22 Feb
2017
How does the proposed allocation of ‘Zealandia’ as an independent continent affect our understanding of the planet’s continental organisation?

A new report published by the journal of the Geological Society of America recommends recognising the 4,900,000km2 sunken land mass of Zealandia – of which the islands of New Zealand and New Caledonia are the principle exposed features – as a fully independent continent, separate from Australia, Antarctica or any other neighbours.

Roughly 94 per cent underwater, Zealandia has been found to be a piece of continental crust which once formed as much as five per cent of the super-continent of Gondwana, but has failed to gain widespread recognition alongside the larger, more obvious continents of Africa and the Americas.

So, how do we actually determine what is and isn’t a continent? As the report notes, the Glossary of Geology describes a continent as being:

‘one of the Earth’s major land masses, including both dry land and continental shelves.’

However, which terra exactly constitute those ‘major land masses’ is harder to determine than the six/seven we may have learnt about in school, or the five which make up the iconic Olympic rings (Europe and Asia being, clearly, not geologically separate in any way, and therefore defined on a more cultural basis than anything else).

While the name ‘Zealandia’ wasn’t coined until 1995, the idea of it was first noted by J Graham Cogley in 1984 in a Reviews of Geophysics report, when he calculated the total area of the 14 land masses he deemed as continents to be 210,400,000km² – roughly 41 per cent of the Earth’s surface. Explaining the larger-than-expected number of continents, he wrote:

‘Although Eurasia is still the largest continent, it does not include the separate continents India and Arabia and surrenders northeastern Siberia to North America. Central America and New Zealand are recognised as continents, the latter being nine-tenths submerged, and there are four microcontinents: Rockall, Seychelles, Agulhas, and Jan Mayen.’

zealandia 2Spatial limits of Zealandia. NC–New Caledonia; WTP–West Torres Plateau; CT–Cato Trough; Cf Chesterfield Islands; L–Lord Howe Island; N–Norfolk Island; K–Kermadec Islands; Ch–Chatham Islands; B–Bounty Islands; An–Antipodes Islands; Au–Auckland Islands; Ca–Campbell Island (Image: Mortimer et al.)

Therefore, how do we define a separate continent, as opposed to being part of a larger neighbour, or merely a microcontinent? Nick Mortimer, a geologist with GNS Science in New Zealand, and his fellow authors of the most recent report, have a definitive answer:

‘Following Cogley and the vagaries of general conventional usage, we propose that the name continent be applied to regions of continental crust that are >1 Mkm2 [1,000,000km2] in area and are bounded by well-defined geologic limits. By this definition India, prior to its collision with Eurasia, would be termed a continent.’

In these terms, Zealandia emerges as nearly five times larger than necessary to constitute a separate geological continent, making it the seventh largest, as well as the ‘youngest, thinnest, and most submerged’.

However, size and defined boundaries are only one of four requirements generally understood as being necessary to fulfil before making the jump to becoming a fully-fledged, independent continent. Thankfully for Zealandia, the other three criteria – a significantly high elevation relative to the surrounding oceanic crust, a range of igneous, metamorphic, and sedimentary rocks, and a satisfactorily thick underlying continental crust – also produce results which the authors believe pass the necessary tests:

‘We argue that Zealandia is not a collection of partly submerged continental fragments but is a coherent 4.9Mkm2 continent. Currently used conventions and definitions of continental crust, continents and microcontinents require no modification to accommodate Zealandia.’

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