Often you will hear Australia referred to as the world’s largest island, or conversely, its smallest continent, or sometimes, the island continent. Which is it? Most people have no idea what the difference is.
One person whose ideas have been spread around asserts the following:
- Areas of geologically stable continental crust, or cratons, tectonically independent from other continents
- Biological distinctiveness, with unique animal and plant life
- Cultural uniqueness
- Local opinion that they are a separate continent1
These are a bit on the silly side, because there is no definition of any of these terms. How stable is stable continental crust? Presumably everybody would agree that Australia is a stable continent, as it has very few major earthquakes, and it is exceptionally rare for these to reach a magnitude of 6.0. Is Asia stable? Parts of it certainly are, but devastating earthquakes commonly occur in China, Nepal, Tibet, Pakistan and Iran. Is South America stable? The most powerful earthquake ever recorded (at magnitude 9.5), struck Chile on May 22nd, 1960. Is North America stable? The second most powerful earthquake ever recorded hit Prince William Sound in Alaska on March 28th, 19642,3. From this, it is meaningless to call a continent stable when parts of supposed continents are subjected to the most violent of earthquakes.
The phrase ‘tectonically independent from other continents” is nonsensical, because all continents to varying degrees interact with other continents in their vicinity because all are moving and it is that motion which causes the buildup of stress, which eventually leads to the most violent earthquakes. It was the northward convergence of the Australian plate (on which Australia sits) with the Eurasian plate4, which caused the massive earthquake off Sumatra which, in turn, caused the Boxing Day Tsunami in 2004, killing nearly a quarter of a million people.
‘Biological distinctiveness, with unique animal and plant life’ is also nonsensical. All parts of the world have unique plants and animals and the number of these varies depending on the geological and climatic history of the island or continent. For instance, the tree genus Araucaria (which includes the Bunya, Norfolk Island and Hoop Pine) is found in Southern South America, eastern Australia, New Caledonia, New Zealand, Norfolk Island and New Guinea5. The genus used to live in the northern hemisphere as well, but died out at the end of the Cretaceous. The flora and fauna of Western Australia differs considerably from that of the eastern Australian states, again because of geological history and climatic history and the intervening aridity of South Australia and the Northern Territory
Cultural uniqueness is another furphy. Nobody could concede that the culture of Africa is ‘unique’, given that it is so varied. This assertion reeks of people looking from afar and unable to see any difference at such a distance when compared to our European lifestyle. Not only this, but the genetic diversity of the human population of Africa is greater than that of the rest of the world combined6.
Local opinion is probably the weirdest criterion that you could possibly imagine. If I and a large proportion of my compatriots suddenly decided that Australia was an island, then it would become one. That is a silly proposition.
Now for some basic geology: There are two types of crust on the planet, continental and oceanic. Continental crust tends to be quite thick (25-70 km) and comprises igneous, metamorphic and sedimentary rocks. It is often termed ‘sial’ because it is relatively rich in silicon (Si) and Aluminium (Al). It forms continents and continental shelves. Oceanic crust is quite thin at generally less than 10 km and it sits directly on top of the mantle. It mostly consists of basalt erupted along the mid oceanic ridges, and is often termed ‘sima’ because it is relatively rich in silicon (Si) and Magnesium (ma). It forms the crust in all of the large ocean basins. It is unclear whether oceanic crust underlies all continental crust.
There are two basic types of island7. The first is a continental island, which is simply a piece of a continent separated by sea from the mainland. An example of this is Tasmania. Bass Strait, which separates Tasmania from mainland Australia is relatively shallow, with an average depth of only about 60 metres and is underlain by continental crust8. Similarly, the sea separating New Guinea from northern Australia is only about 60 metres deep and is also underlain by continental crust. Tasmania and New Guinea were part of the Australian mainland up until about 12,000 years ago. With the end of the most recent glacial event of the Pleistocene Ice Age, and the melting of the icecaps, sea level rose by about 100 metres flooding the intervening lowlands and cutting off New Guinea and Tasmania9.
The second type of island is what is termed an oceanic island. These rise from the ocean basins, where the water is 3-6 km deep, as volcanoes. An example of this is the Hawaiian Islands. They consist almost entirely of basalt and sometimes are surrounded by fringing coral reefs and their associated sediments.
So, arguing about whether a tract of dry land is an island or a continent is a pointless exercise, especially when most of the arguments centre on those tracts underlain by continental crust.