Ah…Antarctica. The seventh continent? Or is it? When you get down to it, pinning down the exact number of continents is like trying to touch a mirage: you can see it, but is it really there? Unlike the latter-day effort in astronomy to define what a “planet” is, there is not even a serious attempt to define what is or is not a “continent”. By convention they’re a rather arbitrary collection of geographical regions, but as the fact it’s an etymological doublet of “continuous” suggests (they both descend from the Latin word “contineo” (in continuous’s case via the later Latin word “continuus”)), the salient characteristic of a “continent” is some degree of contiguity and discreteness of landmass.
What are the Continents?
In the case of Australia and Antarctica, continentality is straightforward; they’re large contiguous mainlands surrounded by ocean water. All models that I’m aware of agree that these two landmasses are continents. Beyond that it starts getting steadily more slippery.
The next-largest contiguous landmass that’s surrounded by ocean water is America, “America” as in from Argentina to Alaska, that America. So America should be a continent, right? But no! America is typically divided into two “continents”, a South America and a North America. Somewhat defensibly, considering the only land connection between them is the Isthmus of Panama, far narrower than either of the lobes it connects.
For similar reasons Africa is usually considered a separate continent, despite not being surrounded by water; the only land bridge is the Isthmus of Suez, just 78 miles wide.
Europe and Asia are the odd men out in the conventional seven-continent model; Europe is neither surrounded by ocean nor is it linked to other landmasses only by narrow isthmuses. Rather, physically Europe is a great peninsula. A distinct region, to be sure, but if Europe can be a continent why can’t India? India is a region that’s even more distinct geographically, being surrounded by mountains, including some of the highest in the world. Yet it only gets to be a “subcontinent”? Hmm.
Prisoners of History
Of course there’s a historical reason for this. Our concept of the continents ultimately descends from the ancient Greeks, and from the point of view of Greece Europe and Asia really do look like quite distinct landmasses! The Bosporus is the narrowest sea channel between them, but otherwise the Mediterranean pretty well separates them, along with the Black Sea; even where they connect, there’s only a relatively narrow land bridge between the Black Sea and the Caspian Sea to the east, and even that is cut in two by a formidable range of high mountains. If the furthest you’d gone on this track was the Caspian, even if you’d ventured up the Volga a fair bit, you would still be left with a picture where Europe and Asia were regions mostly separated by water, with even their land bridges being relatively narrow and/or separated by high mountains.
If we were apprehending these landmasses now, there’s no way Europe would be considered a separate continent, at least not without also considering India a full-fledged continent. For this reason a popular view, especially in the Russian world, has been to combine these two regions into “Eurasia”. Which yields a six-continent model, all six being discrete landmasses almost, if not entirely, surrounded by ocean.
However, even these divisions are ultimately rather arbitrary. The links might be narrow, but nevertheless these landmasses are connected. For this reason there are models with still fewer continents. Often, as mentioned earlier the Americas are combined into one continent called “America”. Less commonly, Africa is considered part of the same continent as Eurasia, sometimes called “Afro-Eurasia”.
If one of these combinations is accepted, this yields a five-continent model. If you accept both of them, you are left with a four-continent model: Afro-Eurasia, America, Australia, and Antarctica. That should do it, right, since all these four landmasses are contiguous and completely surrounded by ocean?
Scientific Continenthood?
Maybe not! Although these facts were largely unknown to the ancient Greeks, there are geological considerations. Continents and oceans are discrete divisions of the Earth’s crust, geologically; there are masses of “continental crust” on one hand and “oceanic crust” on the other hand. Continental crust is not all dry land; much of it is inundated by the ocean, comprising the world’s shallow seas. But despite being inundated by saltwater it’s fundamentally the same types of rock we see on dry land. In plate tectonics, they move with the continents: they’re part of them!
Where it gets even fancier is that the degree of inundation is subject to variation across geologic time: just thousands of years ago, a blink of an eye in terms of the sort of timescales involved with the drift of the continents and the building and erosion of the mountains, the climate was much colder, with much more of Earth’s water frozen in ice sheets. With less liquid water, the sea level drew down to be lower, exposing significantly more continental crust as dry land than exists today…including the Bering Strait that separates America from Asia.
Yes, that’s all continental crust: viewed geologically, America and Asia connect. The mass of continental crust bridging the two landmasses is a thousand miles wide at its narrowest point, for god’s sake! And it’s not just hypothetical; during glacial periods, which account for the vast majority of the time in Earth’s current climate regime (we’re in an interglacial, which is a geologically short period of warmer weather), most of this connection was exposed.
So would we say, if we used the criterion of being surrounded by water, that America and Asia are continents now but during glacial periods they’re not? It seems much more elegant to use connections of continental crust as the criterion, in which case you are left with a three-continent model: Afro-Eurasia-America, Australia, and Antarctica.
Antarctica is far away from any other substantial landmasses, and is surrounded by oceanic crust. Australia is closer to Asia’s continental crust, but is separated by several straits of deep water. This geological history is not just an abstract intellectual exploration: it’s decisively shaped the current environment of these regions, as revealed by the fauna. The “Wallace Line” (along with the similar but less famous “Weber Line” and “Lydecker Line”) tracks the oceanic-crustal divisions between these two contiguous masses of continental crust!
One Door for Continents closes, another Door opens?
The geological perspective also sheds light on what landmasses qualify as continents and which don’t. Madagascar, for instance, has been described as “Earth’s eighth continent” from a biodiversity point of view; it’s very much a distinct landmass and separated from Africa by deep water that lies over oceanic crust, but geologically it’s a wayward fragment of Africa’s continental shelf.
New Zealand might have a stronger case for continenthood; geologically is part of a vast separate and distinct mass of continental crust, known as “Zealandia”, that’s about the same size as Australia…only difference is it’s almost completely submerged and has been for a very long time, with the only substantial bits of land above sea level (in or out of a glacial period) being the two main islands of New Zealand, which were built up volcanically in relatively recent geological history as the remains of the continent drifted over a tectonic plate boundary. Should New Zealand, then, be recognized as a continent? Maybe.
And of course, there’s the Kerguelen Plateau, which has been dubbed a “microcontinent” and were it all above water would be comparable in size to New Zealand or even Madagascar. Makes you think, doesn’t it?
Conclusion
Personally, I cast my vote in favor of the three-continent model; when the Bering Strait is linked by a tunnel, let alone when the ice sheets return and the sea levels draw down again, the fundamental contiguity of America and Asia will become much more obvious, in a way that will never be possible for the other landmasses that are currently surrounded by water, but perhaps I’m just strange.
Then again, perhaps I’m just thinking ahead of my time. The world’s land is more interconnected than you think…