Zealandia : Earth’s Hidden Continent

  Scientists discovered Earth’s missing and hidden 8th continent  

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 modelling 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 which were stranded in the ocean basins during supercontinent breakups. But what about the major continents? 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; 

(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. The last point, how “major” a piece of continental crust has to be to be called a continent, is almost never discussed. 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. “By dating these rocks and studying the magnetic anomalies they presented,” the researchers said, “we were able to map the major geological units across North Zealandia.” Beneath the turquoise waters of the South Pacific hides a massive secret, Zealandia, a sunken landmass stretching nearly two million square miles. Though mostly underwater, this geological giant has sparked debate as a possible eighth continent. Just 5% of its surface peeks above sea level, making it one of Earth’s most elusive landforms.

Here we 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 this stage. Since it was first proposed by Luyendyk (1995), the use of the name Zealandia for a southwest Pacific continent has had moderate uptake. 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 is to formally put forth the scientific case for the continent of Zealandia and explain why its identification is important. 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. 

Zealandia wasn’t always underwater. Its story begins over 100 million years ago when it was part of the vast southern supercontinent, Gondwana. The ancient land once held what would become Africa, South America, Antarctica, Australia and the Indian subcontinent. As Gondwana slowly broke apart, Zealandia’s own journey began. Roughly 85 million years ago, the southern section split from what is now West Antarctica. About 25 million years later, the northern section detached from Australia. These shifts pushed Zealandia away from its neighbors, setting the stage for its quiet descent beneath the ocean’s surface. Unlike nearby continents, Zealandia didn’t stay afloat. During the Paleogene period, its crust thinned and cooled. This shift triggered its gradual sinking. Today, only New Zealand and New Caledonia remain above water, offering hints of the massive landmass which lies beneath. A tectonic boundary now divides Zealandia into northern and southern sections, tracing along the Pacific and Australian plates. Continents and their continental shelves vary in height but are always elevated relative to oceanic crust. The elevation is a function of many features, fundamentally lithosphere density and thickness, as well as plate tectonics. The accuracy and precision of seafloor mapping have improved greatly over the past decades and a deliberately chosen colour ramp on a satellite gravity-derived bathymetry map provides an excellent visualization of the extent of continental crust. The approximate edge of Zealandia can be placed where the oceanic abyssal plains meet the base of the continental slope, at water depths between 2500 and 4000 m below sea level. The precise position of the foot of the continental slope around Zealandia was established during numerous surveys in support of New Zealand’s Law of the Sea submission. 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.

Although the idea of an underwater continent first surfaced decades ago, it struggled to gain wide scientific support. But recent findings have changed that. With new data and sharper tools, researchers are reevaluating Zealandia’s geological identity, and taking it seriously. One breakthrough came from a study led by geologist Nick Mortimer and a team from GNS Science. Their work offers strong evidence for Zealandia’s continental status, pushing this sunken world into the scientific spotlight. The team conducted geological surveys across the northern stretches of Zealandia, employing advanced dredging techniques. They collected a range of rock samples from the Fairway Ridge to the Coral Sea, including sandstone, mudstone, limestone and basaltic lava. These samples provided crucial insights into Zealandia's geological timeline. “By dating these rocks and analysing their magnetic anomalies, we mapped the major geological units of North Zealandia,” the team explained. This marked the completion of the first comprehensive offshore geological mapping of the Zealandia continent.

By itself, relatively high elevation is not enough to establish that a piece of crust is continental. Essential geological ground truth for Zealandia is provided by the many island outcrop, drill core, xenolith and seabed dredge samples of Paleozoic and Mesozoic greywacke, schist, granite and other siliceous continental rocks which have been found within its limits. Many of these have been obtained from expeditions in the past 20 years. Orogenic belts, of which the Median Batholith and Haast Schist are parts, can be tracked through onland New Zealand and across Zealandia. Thus, there is a predictable regional coherency and continuity to the offshore basement geology. 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. 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. 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. These typically contain 2–10-km-thick sequences of terrigenous and calcareous strata and include a widespread continental breakup. The Zealandia Megasequence provides a Zealandia-wide stratigraphic record of continental rifting, and marine transgression events, similar to that seen in formerly conjugate east Australian basins. The retrieved rocks tell a captivating story. Some sandstone samples date back approximately 95 million years to the Late Cretaceous period. Granite and volcanic pebbles from as far as 130 million years ago reveal Zealandia's Early Cretaceous past. Basalt samples, meanwhile, represent more recent history, originating from the Eocene epoch around 40 million years ago. These findings challenge previous assumptions about Zealandia’s formation. Conventional theories suggested a strike-slip breakup, where tectonic plates slide past each other horizontally. However, Mortimer’s team proposed a different scenario. They believe plate stretching created subduction-like fractures, leading to the formation of the Tasman Sea. Subsequent tectonic activity further thinned Zealandia’s crust, relegating it to an underwater existence.

Continental crust varies considerably in thickness and physical properties. Most of Zealandia’s crust is thinner than the 30–46 km which is typical of most continents, studies show that it is everywhere thicker than the ~7-km-thick crust of the ocean basins. Collectively, the crustal structure results show that the rock samples are not from separate continental fragments or blocks now separated by oceanic crust, but are from a single continental mass. 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. This raises the question as to whether the trough is floored by oceanic crust or is a failed continental rift. Two wide-angle seismic profiles across the trough near New Caledonia both show ~2–5 km of sedimentary cover over 8.5 km of crustal basement which has a velocity of ~7 km−1 throughout much of its thickness. These profiles as atypical of normal oceanic crust. The implications of Zealandia’s geological evolution extend beyond academic interest. Understanding its unique features, such as crustal thinning up to 65%, sheds light on broader tectonic processes. These insights also highlight Zealandia’s role in shaping the Pacific region's dynamic geology.  According to Science researcher, “Zealandia’s underwater status in no way diminishes its geological significance.” Its vast expanse and diverse rock formations make it a valuable natural laboratory for studying Earth's tectonic history.

Where oceanic crust abuts continental crust, various kinds of continent-ocean boundaries (COBs) define natural edges to continents. Tectonic plate boundaries, with or without intervening oceanic crust, provide the basis for continent-continent boundaries between Africa and Eurasia, and North and South America. 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 Australian and Zealandian COBs here coincide with, and have been created by, the Cato Fracture Zone along which there has been ~150 km of dextral strike slip movement, linking Paleogene spreading centres in the Tasman and Coral seas. This spatial and tectonic separation, along with intervening oceanic crust, means that the Zealandia continental crust is physically separate from that of Australia. If the Cato Trough did not exist, and the Australian continent was 4.9 Mkm2 larger than previously thought. Being >1 Mkm2 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. 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.

Zealandia remains an active area of research. Its submerged status poses challenges, but advances in technology, such as deep-sea dredging and seismic imaging, continue to unlock its secrets. Ongoing studies aim to refine our understanding of this hidden continent, offering a more comprehensive picture of Earth's geological past. As scientists continue to explore Zealandia, they not only reveal the continent's mysteries but also enhance our understanding of the planet’s ever-evolving story. The importance of Zealandia is not so much that there is now a case for a formerly little-known continent, but that, by virtue of its being thinned and submerged, but not shredded into microcontinents, it is a new and useful continental end member. Zealandia started to separate from Gondwana in the Late Cretaceous as an ~4000-km-long ribbon continent but has since undergone substantial intra­continental deformation, to end up in its present shape and position. To date, Zealandia is little-mentioned and/or entirely overlooked in comparative studies of continental rifting and of COBs. By including Zealandia in investigations, we can discover more about the rheology, cohesion and extensional deformation of continental crust and lithosphere.

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. The scientific value of classifying Zealandia as a continent is much more than just an extra name on a list. 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.