By Matt Kaplan
Our planet is divided into
seven continents total.
And if you follow along, we’re
about to compile a list,
From largest to smallest …
Your children might return home with tunes like this catchy song, or something similar. But do we truly have seven continents?
Anyone who looks at a map knows that Asia and Europe are joined. They are frequently referred to as Eurasia for this reason. The distinction is rather arbitrary, more so defined by culture than science. Therefore, can we assert that we only have six continents?
This may just be the initial dubious step on a well-greased slope. What about North America and Asia?
They are connected via the Bering Sea Shelf, which was formerly dry land traversed by humans and has been submerged only in the not-so-distant geological past. From a technical perspective, Asia, North America, and Europe could be regarded as a single continent. Does that imply there are merely five?
Some specialists argue that five, six, and seven are incorrect, advocating instead for eight continents. There are even those who claim that there exist only two.
Embedded within the simplicity of the song is a deceptive sense of consensus regarding the number of continents.
The contention arises partly because two distinct types of continents exist: those acknowledged by cultures globally and those recognized by geologists. Cultures may define a continent as they wish, while geologists must adhere to a definition. Recent geological studies have complicated the delineation of continental boundaries as researchers uncover unexpected continental materials.
“This sparks considerable interest because there are profound implications for our comprehension of the mechanisms behind continent separation, ocean formation, and plate tectonics,” stated Valentin Rime, a geologist from the University of Fribourg in Switzerland. He further mentioned, “But following the excitement comes meticulous verification and discussion to ensure the evidence is robust.”
From a geological standpoint, for a piece of the Earth to qualify as a continent, it must possess four characteristics:
— A high elevation relative to the ocean floor.
— A diverse array of igneous, metamorphic, and sedimentary rocks that are rich in silica.
— A crust thicker than the surrounding oceanic crust.
— Clearly defined boundaries encompassing a sufficiently large area.
The first three criteria are commonly found in almost every geology textbook. However, the fourth is not so clearly defined. What constitutes “large enough,” or how “well-defined” the limits of a potential continent need to be, are subjects that receive less attention, unless a geologist is evaluating parts of the Earth that are on the verge of being classified as continental.
“Anything large enough to alter the world map is significant,” remarked Nick Mortimer, a geologist associated with the government-funded GNS Science research institute in New Zealand. “Identifying and classifying a portion of the Earth as a continent, even a small, thin, and submerged one, is more informative than leaving a map blank.”
This introduces complications into how we count continents.
Take Iceland, which lies above a rift that circles the globe, the Atlantic segment of the mid-oceanic ridge. Volcanic activity there gradually pulls apart the tectonic plates that support North America and Europe. Most of the ridge remains deep underwater. Yet in Iceland, it rises above sea level.
Another puzzle is that the volcanoes there frequently emit lava composed of molten continental crust, even though Iceland is situated thousands of miles away from any continents. Some geologists believe that Iceland is not merely an isolated island in the sea, but rather a segment of a continent (although determining which one can become complex).
This theory is supported off the eastern coast of Africa.
A mid-oceanic ridge in the Red Sea is splitting Africa from Asia. This gradual separation occurs at a pace comparable to the growth of fingernails. Along most of the ridge, the separation is straightforward. However, it becomes far more complicated where the Red Sea meets the Gulf of Aden. Instead of a clear location of thinning where ocean crust is developing, the continental crust between Africa and Asia is fracturing into numerous fragments. There is no evident point where Africa concludes and Asia begins in that area.
“It resembles very thick and strong toffee being stretched out, but not breaking,” explained Gillian Foulger, a geologist at Durham University in England.
Rime and colleagues recently published findings in the journal Geology indicating that beneath the surrounding seas, Iceland too exhibits heavily extended toffee-like features. Instead of a distinct boundary separating North America from Europe, there appears to be a complicated mixture of magma and continental crust fragments dispersed along a corridor between the two landmasses that traverses right through Iceland. Similar to the juncture where the Red Sea meets the Gulf of Aden, there is no clear demarcation where North America ends and Europe begins.
Then there’s New Zealand, which truly complicates matters for our children.
Even though New Zealand and Australia are often classified together, they are not on the same continent. While Australia is widely accepted as a distinct continent, the suggestion that New Zealand constitutes its own continent, Zealandia, is a more recent proposition.
Submerged shelves that rise significantly above the ocean floor extend for miles beyond this small island nation. At the peripheries of these shelves, there are deep waters and oceanic crust that is thinner than the crust beneath the shelves. Sample drill cores, seabed dredged materials, and rocks collected from the area indicate that the massive structure that New Zealand rests upon comprises igneous, metamorphic, and sedimentary rocks rich in silica, similar to other continents.
While few people consider Zealandia a continent in cultural terms, “it is increasingly being recognized as one geologically,” Mortimer remarked.
However, not everyone is in accord, pointing to that troublesome fourth criterion often overlooked in most texts. Zealandia’s crust is between 10 and 30 kilometers thick, which surpasses the 7 kilometers typical of most oceanic crust. Yet, it does not reach the thickness of other continent’s crust, which generally ranges from 30 to 46 kilometers. This raises issues regarding the clarity of the boundaries between Zealandia and the ocean, rendering them more challenging to define. Size is also a concern. At 4.9 million square kilometers, the proposed Zealandia is significantly smaller than Australia, which measures 7.7 million square kilometers.
Additionally, Zealandia remains predominantly submerged. While being above water is not part of the geological definition of a continent, it seems to hold cultural significance, as people tend to envision continents as dry surfaces.
Geologists continue to debate the implications these discoveries about continental and oceanic crust have on the count of continents. What is evident is that the research exposes multiple mechanisms through which continents can separate, and that this split is not always clean or entirely definitive.
“Essentially, there are only two major continents,” Rime argues. “Antarctica and everything else, since South America connects to North America via Panama, North America links to Asia through the Bering Strait, and Asia connects to Europe, Africa, and Australia through the Urals, the Sinai, and Indonesia, respectively.”
Mortimer does not concur.
“Zealandia is separated from Australia by a 25-kilometer-wide, 3,600-meter-deep ocean trench,” he stated. “Following Valentin’s logic, that implies there are actually three continents.”
Nevertheless, he acknowledged some ambiguity, adding, “Unless the trench is proven to be a very deep continental crust, similar to Iceland’s scenario, in which case Zealandia would be considered part of Australia.”
And there is also the possibility that Iceland rests atop its own large fragment of floating crust, potentially representing continent No. 9.
