Zealandia: earth’s hidden continent, finds study

A Journal of People report

Earth has a continent hidden in the region of the southwest Pacific Ocean. It is attached to New Zealand. The hidden continent is “Zealandia”. A recently published research report (volume 27, issue 3, March/April 2017) has made the claim.

A team of 11 researchers found that New Zealand and New Caledonia are actually part of a huge 4.9 million sq km single slab of continental crust that is separate from Australia.

The study, published by the Geological Society of America, found that the region is 94 percent submerged, mostly as a result of crustal thinning before the super-continental break-up. The study has used upgraded satellite-based elevation and gravity map technology.

Zealandia’s crust thickness typically ranges from 10 to 30km and is roughly the size of India. It’s believed to have broken off from Antarctica about 100 million years ago, and then again from Australia about 80 million years ago.

Researchers conducting the study are calling Zealandia a “realisation” rather than a “discovery”, as New Zealand has been considered a continent in its own right by some experts in the field for years.

“This is not a sudden discovery but a gradual realisation; as recently as 10 years ago we would not have had the accumulated data or confidence in interpretation to write this paper,” write the scientists.

“Zealandia illustrates that the large and the obvious in natural science can be overlooked.”

“The scientific value of classifying Zealandia as a continent is much more than just an extra name on a list,” the scientists wrote. “That a continent can be so submerged yet unfragmented makes it a useful and thought-provoking geodynamic end member in exploring the cohesion and breakup of continental crust.”

The scientists say: It should be considered a geological continent, rather than the previously-held theory that it was a collection of continental islands and fragments.

“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,” concludes the study.

As geologists count Europe and Asia as one giant continent called ‘Eurasia’, the new addition of Zealandia brings the total number of official geologic continents to seven.

The authors of the scientific study are Nick Mortimer, Hamish J. Campbell, Andy J. Tulloch, Peter R. King, Vaughan M. Stagpoole, Ray A. Wood, Mark S. Rattenbury, Rupert Sutherland, Chris J. Adams, Julien Collot and Maria Seton.

The scientists write:

The region of the southwest Pacific Ocean is made up of continental crust. It has elevated bathymetry relative to surrounding oceanic crust, diverse and silica-rich rocks, and relatively thick and low-velocity crustal structure. Its isolation from Australia and large area support its definition as a continent — Zealandia. The identification of Zealandia as a geological continent, rather than a collection of continental islands, fragments, and slices, more correctly represents the geology of this part of Earth. Zealandia provides a fresh context in which to investigate processes of continental rifting, thinning, and breakup.

They write:

Earth’s surface is divided into two types of crust, continental and oceanic, and into 14 major tectonic plates. In combination, these divisions provide a powerful descriptive framework in which to understand and investigate Earth’s history and processes. In the past 50 years there has been great emphasis and progress in measuring and modeling aspects of plate tectonics at various scales. Simultaneously, there have been advances in our understanding of continental rifting, continent-ocean boundaries (COBs), and the discovery of a number of micro­-continental fragments that were stranded in the ocean basins during supercontinent breakups. Continents are Earth’s largest surficial solid objects, and it seems unlikely that a new one could ever be proposed.

The Glossary of Geology defines a continent as “one of the Earth’s major land masses, including both dry land and continental shelves”. 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 first three points are defining elements of continental crust and are explained in many geoscience textbooks and reviews. To our knowledge, the last point — how “major” a piece of continental crust has to be to be called a continent—is almost never discussed, Cogley (1984) being an exception. Perhaps this is because it is assumed that the names of the six geological continents — Eurasia, Africa, North America, South America, Antarctica, and Australia — suffice to describe all major regions of continental crust.

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 . Implicit in Luyendyk’s paper was that this was a large region of continental crust, although this was only mentioned in passing and he did not characterize and define Zealandia as we do here.

In the research paper the scientists summarize and reassess a variety of geoscience data sets and show that a substantial part of the southwest Pacific Ocean consists of a continuous expanse of continental crust. Further­more, the 4.9 Mkm2 area of continental crust is large and separate enough to be considered not just as a continental fragment or a microcontinent, but as an actual continent—Zealandia. This is not a sudden discovery but a gradual realization; as recently as 10 years ago we would not have had the accumulated data or confidence in interpretation to write this paper.

They write:

However, it is still not well known to the broad international science community. A correct accounting of Earth’s continents is important for multiple fields of natural science; the purpose of this paper is to formally put forth the scientific case for the continent of Zealandia and explain why its identification is important.

They write:

New Zealand and New Caledonia are large, isolated islands in the southwest Pacific Ocean. They have never been regarded as part of the Australian continent, although the geographic term Australasia often is used for the collective land and islands of the southwest Pacific region.

They summarize the four key attributes of continents and assess Zealandia meeting these criteria.

Zealandia is everywhere substantially 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. Zealandia has a modal elevation of ~ − 1100 m and is ~94% submerged below current sea level. The highest point of Zealandia is Aoraki – Mount Cook at 3724 m.

Traditionally, continents have been subdivided into cratons, platforms, Phanerozoic orogenic belts, narrow rifts, and broad extensional provinces. Eurasia, Africa, North America, South America, Antarctica, and Australia all contain Precambrian cratons. The oldest known rocks in Zealandia are Middle Cambrian limestones of the Takaka Terrane and 490–505 Ma granites of the Jacquiery Suite. Precambrian cratonic rocks have not yet been discovered within Zealandia, but their existence has been postulated on the basis of Rodinian to Gondwanan age detrital zircon ratios. Furthermore, some Zealandia mantle xenoliths give Re-Os ages as old as 2.7 Ga. Geologically, Zealandia comprises multiple Phanerozoic orogenic belts on which a broad extensional province and several narrow rift zones have been superimposed.

Atop its geological basement rocks, Zealandia has a drape of at least two dozen spatially separate Late Cretaceous to Holocene sedimentary basins.

The thinnest crust within Zealandia is in the 2200-km-long and 200–300-km-wide New Caledonia Trough, where the water depth varies from 1500 to 3500 m.

Where oceanic crust abuts continental crust, various kinds of continent-ocean boundaries (COBs) define natural edges to continents. The six commonly recognized geological continents (Africa, Eurasia, North America, South America, Antarctica, and Australia) are thus not only large but they are also spatially isolated by geologic and/or bathymetric features.

The edges of Australia and Zealandia continental crust approach to within 25 km across the Cato Trough. The Cato Trough is 3600 m deep and floored by oceanic crust.

The scientists’ conclusion includes:

Zealandia is approximately the area of greater India and, like India, Australia, Antarctica, Africa, and South America, was a former part of the Gondwana supercontinent. As well as being the seventh largest geological continent, Zealandia is the youngest, thinnest, and most submerged. That a continent can be so submerged yet unfragmented makes it a useful and thought-provoking geodynamic end member in exploring the cohesion and breakup of continental crust.

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