Earth's 8th Continent
Introduction
Continents are the largest solid objects on the Earth’s surface. Given the millennia of exploration by humans, it would seem unlikely that a new continent would ever be discovered or proposed in addition to the familiar seven: Europe, Asia, Africa, North and South America, Australia and Antarctica. However, a 2017 scientific paper by New Zealand, New Caledonian and Australian geologists summarised the scientific case for another continent – Zealandia.
Continents defined
What is a continent? In Samuel Johnson's 1755 dictionary, a continent is defined as ‘land not disjoined by the sea from other lands.’ Most modern dictionaries state it more directly as ‘one of the Earth’s large continuous landmasses.’ This sounds reasonable enough. But how large is large? And how permanent is a landmass?
In Fig 1a, Old World and New World are the two largest continuous landmasses on Earth, and Australia and Antarctica could just be large islands. But most people don't think we have two or four continents but seven (Fig. 1b). This larger number arises from cultural prejudices being superimposed on the continuous land definition. The maps of Fig. 1a and b suffer from the weakness of being modern anthropocentric snapshots. In the last Ice Age 15,000 years ago, sea level was about 125 m lower and at other times in the past, sea level has been up to 200 m higher than it is today. So, with definitions based on coastlines and culture, the number, size, and shape of continents vary with climate, time and opinion. Is there a better way of defining continents?
The Glossary of Geology states that a continent is ‘one of the Earth's major land masses, including both dry land and continental shelves’. Because all continents have submarine continental shelves, this definition results in expanded continental areas (Fig. 1c). The advantage of including submarine continental shelves is that the definition of a continent is scientifically based, and is independent of climate-related sea level change and cultural biases.
Geologists identify two kinds of crust on Earth: continental and oceanic. Continental crust underlies mountains, plains and submarine shelves, oceanic crust underlies the deep ocean plains. Continental crust has diverse geology, and relatively thick, low density, low seismic velocity crust, compared to oceanic crust regions (Figs. 2, 3). As explained in many geoscience textbooks, elevation, geology and crustal physical properties are the essential features of continental crust.
Te Riu-a-Maui / Zealandia as a continent
Elevation
Is Zealandia high enough to be a continent? Yes. Continents, including their continental shelves always stand higher than the seabed of oceanic crust (Fig. 1c, 2, 3, 4). The existence of submarine plateaus and ridges around New Zealand and New Caledonia has been known for more than a century. Modern bathymetry maps of the SW Pacific region (e.g. Fig. 4 and the bathymetry webmap) show the close match between high elevation (oranges and yellows) and continental crust. When you know what to look for, Zealandia’s ghostly outline is also visible on Google Earth maps.
The edge of Zealandia is placed at the foot of the continental slope, the edge of the deep oceanic plains. This is typically 2.5 to 4 km below sea level. Whereas most continents have large expanses of land, and small submarine continental shelves, Zealandia has small land area, and large continental shelves (Fig. 3). About 95% of Zealandia is submerged below current sea level; Zealandia deserves its name 'the hidden continent'.
Geology
Does Zealandia have the right sort of rocks to be a continent? Yes. Oceanic crust is made from the dense black igneous rocks basalt and gabbro. In contrast, continents have a whole range of igneous, metamorphic and sedimentary rocks including granite, gneiss, schist, rhyolite, quartz-rich sandstone and limestone (Fig. 5).
All these continental rocks are found in New Zealand and New Caledonia. Geological ground truth for the other 95% of Zealandia is provided by about three dozen island outcrops, drill core and seabed dredge samples of continental rocks (red dots in Fig. 3). Many drill and dredge samples have been obtained from expeditions in the last 20 years.
Crustal structure
Does Zealandia have the right thickness, density and velocity of crust to be a continent? Yes. Geophysicists build pictures of the deep Earth by making measurements at the Earth’s surface. They calculate rock density from microgravity surveys, and seismic velocity (speed of sound in rock) along crustal ray paths from earthquakes. From these we know that Zealandia's deep crust properties match those of other continents (Fig. 2). Zealandia's crust is between 10 and 40 km thick. This compares with 30-46 km for most continents, and is everywhere thicker than c. 7 km ocean crust.
Size
Is Zealandia big enough to be called a continent? Yes.
At about 5.0 million sq. km, Zealandia is the smallest modern-day continent, but is still a large area compared with other countries (Fig. 6) and is big enough to show up on a world map (Fig 1c). Continental crust does come in still-smaller chunks which are called microcontinents. Madagascar (0.8 million sq. km) is the biggest of these.
Summary
A large region of the southwest Pacific Ocean around New Zealand and New Caledonia is a 95% submerged continent, Te Riu-a-Maui / Zealandia.
Like all continents, Zealandia has high elevation compared with surrounding oceanic crust, rocks such as granite, schist and greywacke, relatively thick and low-velocity crust, and large size. Earth's eighth and newest continent (Fig. 7) provides a fresh way to look at the natural history of the SW Pacific. Seabed surveys are becoming more thorough and probably only microcontinents, rather than continents, remain to be discovered. Present day Te Riu-a-Māui / Zealandia developed from the supercontinent Gondwana.
Written by Nick Mortimer, GNS Science, Dunedin
May 2020