Why Australia Is A Continent But Greenland Is An Island

Australia is about three and a half times the size of Greenland, and if you have heard an explanation for this it was probably that one. It is the wrong explanation. Size is where the line gets drawn, not why. The actual reason is several kilometers underneath both of them: Australia rides its own tectonic plate and sits on its own continental shelf, while Greenland rides the North American Plate and sits on North America’s shelf, moving in the same direction at the same speed as Canada. Greenland is not a small continent. It is a large piece of North America with some water in the way.

Ayers Rock (Uluru) as seen from helicopter in the Australian Northern Territory. Continents generally sit on their own major tectonic plate, which is a distinct and very large segment of the Earth’s outer shell that moves as a single unit.

Australia does exactly that. It occupies the Australian Plate, sometimes described as part of the wider Indo-Australian Plate, which is a primary tectonic unit in its own right and carries the landmass as an independent piece of crust.

Greenland does not. There is no Greenland Plate, there never was, and without one there is no mechanism by which Greenland is geologically separate from the continent next door. It is a passenger rather than a driver.

Hans Island is a disputed island, with no inhabitants, located between Canada and Greenland (Denmark) The clearest way to see the distinction is to ignore the shoreline entirely and look at what lies under the shallow water around each landmass.

Greenland is joined to North America by continental shelf, the submerged shallow margin of a continent. The water in between is shallow because the crust under it is continental rather than oceanic. Drain those seas and Greenland stops being an island and becomes a peninsula, which is a fairly damning thing to be able to say about a landmass.

Australia has its own shelf, bounded by deep ocean on every side. Nothing shallow connects it to Asia or Antarctica. It is a fully separate block of continental crust rather than an outlying piece of somebody else’s.

Much of Greenland’s landscape is dominated by ice and snow for much of the year, leaving little opportunity for growing crops. Strip away the ice sheet and the argument gets stronger rather than weaker. Greenland’s bedrock is part of the Laurentian Shield, the ancient Precambrian core around which the North American continent was built. It is the same deep crustal foundation that underlies much of Canada, not a separate one that happens to be nearby.

The ice cap, for all its scale, is a surface feature sitting on crust already bolted to North America. Remove the ice and you have not uncovered a continent. You have uncovered more of one that was already there.

A female kangaroo (“jill”) caring for her child (a “joey”). Biology is the third criterion, and it follows from the geology rather than being a separate coincidence. Australia spent tens of millions of years isolated behind deep ocean, and evolution ran an entirely separate experiment there. Marsupials and monotremes took the roles placental mammals filled everywhere else, which is how a continent ends up with animals that carry their young in a pocket and a mammal that lays eggs and has a bill.

Greenland has nothing of the sort. Its plants and animals are broadly those of northern Canada, because there was never a deep-water barrier wide enough to keep the two apart. Nothing novel evolved there because nothing was ever properly stuck there.

The Quokka is a small species of marsupial mammals in the Family Macropodidae, which also includes kangaroos and wallabies. The size point is worth stating properly rather than dismissing, because it does real work in the convention. Somewhere a line has to be drawn between the smallest continent and the largest island, and traditionally it sits in the gap between these two. That gap is enormous: Australia exceeds Greenland by about 5.5 million square kilometers, which is itself larger than India.

But size is the consequence of the classification rather than its cause. Shrink Australia to Greenland’s dimensions and it would still have its own plate. Double Greenland and it would still be welded to North America, just more conspicuously.

The island of Greenland is the world’s least densely populated place. In 2017 a group of geologists made the case that Zealandia, almost entirely submerged beneath the southwest Pacific with New Zealand as its visible fragment, should be recognized as a continent. Making that argument required spelling out what a continent actually is.

The criteria they set out were elevation high above the surrounding oceanic crust, distinctive geology, well-defined limits, and crust substantially thicker than ocean floor. Zealandia meets all four despite being mostly underwater.

Run Greenland through the same four and it fails on well-defined limits, because there is no boundary in the crust between Greenland and North America, only some shallow water. A largely submerged continent nobody can visit has a better claim than the world’s largest island, which should tell you how little the island’s size counts for.

This statue of Leif Erikson stands in Greenland, the land where many of his adolescent years were spent. The word continental is doing technical work in all of this, and it refers to something measurable. Continental crust runs roughly 30 to 50 kilometers thick and is made of lighter rocks like granite, which is why it floats higher on the mantle and forms land. Oceanic crust is thinner, denser and basaltic, which is why it sits lower and holds oceans.

Both Greenland and Australia are continental crust. The difference is that Greenland’s is continuous with North America’s and Australia’s is not.

The long tail of the tree-kangaroo allows it to jump between trees with ease. Honesty requires one caveat, because the tidy criteria above are considerably tidier than the field they come from. There is no agreed scientific definition of a continent, and the seven-continent model is a convention rather than a measurement. Different countries teach four, five, six or seven, splitting or merging Europe, Asia as well as the Americas by local tradition. And Europe is the glaring embarrassment here, because it sits on the same plate and the same continuous landmass as Asia with no geological boundary of any kind. Europe fails every test Greenland fails and then several more, and it got continent status anyway, largely by being the place where the people drawing the maps happened to live.

So the honest framing is that Australia passes every test anybody applies, Greenland fails most of them, and the whole system has never quite agreed on what the tests are.

Wedge-tailed Eagle, Western Australia outback Australia is a continent because it occupies its own tectonic plate, sits on its own continental shelf surrounded by deep ocean, and evolved a biota found nowhere else. Greenland is an island because it rides the North American Plate, rests on North America’s shelf, and is built on the same Precambrian shield as Canada.

Australia being three and a half times larger is where textbooks draw the line, but it is not the reason. Greenland could grow a great deal and remain an island, because what disqualifies it is not its dimensions. It is its address.