Why is Greenland an island but Australia a continent? Mostly convention
Greenland, at about 2.17 million square kilometres, is the world's largest island. Australia, roughly three and a half times bigger, is the smallest continent. No strict rule separates the two. Depending on who is counting, Earth has seven continents, four, or by one geological argument just three.
Continents are big land regions picked out mostly by convention rather than firm criteria. A continent might be one landmass, a landmass plus nearby islands, or a slice of something larger, as Europe and Asia are of Eurasia. Most English-speaking countries teach seven, ranked by size as Asia, Africa, then North and South America, Antarctica, Europe and finally Australia. Other schemes merge Eurasia, the Americas or even Afro-Eurasia, dropping the count to as few as four.
The English noun grew in the 16th century from continent land, meaning connected land, a rendering of Latin terra continens. At first it could describe any mainland, and 17th-century writers spoke of the continents of the Isle of Man, Ireland and Wales; Sumatra got the label in 1745. Geographers kept echoing Herodotus in asking why one mass should be carved up. Peter Heylin in the mid-1600s, Ephraim Chambers in 1727 and Emanuel Bowen in 1752 all treated Europe, Asia and Africa as a single great continent, yet the older idea of three separate parts won out.
Geologists define a continent by four traits: it rides high above the sea floor, contains a varied mix of silica-rich rocks, has crust thicker than oceanic crust, and occupies a large, clearly bounded area. That yields seven blocks of continental crust, with fragments like Madagascar demoted to microcontinents, and continental crust is known only on Earth. Counting continental shelves, Britain and Ireland belong to Europe and New Guinea joins Australia; pushed to the limit, a shelf linking Afro-Eurasia and America near the Bering Sea leaves only three continents. Continental drift, accepted in the 20th century, traces today's landmasses to the break-up of the supercontinent Pangaea.
Only Antarctica and Australia are wholly cut off by ocean. Africa meets Asia at the Isthmus of Suez and the Americas meet at Panama, and the Americas were widely treated as one continent in the United States until World War II, as they still are in most Latin American models. Europe, at about 10.18 million square kilometres, counts as a continent while the Indian subcontinent, less than half that size, does not, a split some critics see as a leftover of Eurocentrism.
Source: Continent
