|MadSci Network: Earth Sciences|
You have asked a great question that gets at one of the two basic things scientists do -- take measurements and make definitions. In this case, you identified a problem with a definition: What is an island and what is a continent. Before I researched your question, I thought I knew a good answer to this question, but it turns out there is more to this than first meets the eye.
What we should do is take a look at some definitions, and you can decide for yourself the answer (don't worry, I'll give you my answer too).
An island is a body of land surrounded on all sides by water.
A continent is a body of land surrounded on all sides by water.
A continent is a very, very large body of land, while an island is not. Ok, that helps, but maybe we can find some other clues to help us out.
Islands tend to have less variation (differences) in their terrain. They tend to be just deserts, forests, mountains, etc.
Continents have many types of terrain, including mountain ranges, major rivers, and large plains.
Here's a good one. With islands having only one type of major terrain, we can narrow the field down. This means Greenland is an island, but Australia is not, as it has a couple of tiny mountain ranges, and a few kinda big rivers. But if you were to compare Australia with other continents, it's mountains are really tiny, and it's rivers are pretty small too. Antarctica doesn't have any real rivers and it's terrain is all covered in ice.
Islands tend to be isolated biologically. They have plants and animals found nowhere else or only on nearby islands. Also, many islands have different plants and animals from nearby islands. Continents have large groups of animals of many types, and can share these types with other continents.
Ok, now we have a problem. Australia fits this definition of an island perfectly. You can definitely say Antarctica fits this definition too! It looks like we have a contradiction here. Let's explore further.
Here's the kicker, and the real reason they are continents: Antarctica and Australia have types of rocks in them that are "continental rocks." Islands have rocks that are very different -- normally these island rocks are made of similar stuff as ocean rocks. So here's where we can definitely call them continents. They have drifted around for a long, long time (even before the dinosaurs), and have rocks pretty much similar to how they were back when the dinosaurs roamed the earth. Islands normally are a lot "younger," and they tend to not hang around for super long times like continents.
So here's my answer. Both Australia and Antarctica are "island continents." Not because they are islands in the rock sense, but because they are islands in the plants and animals sense. They are rocky continents, and animal-and-planty islands.
One more thing - the reason Australia is always called an "island continent" has nothing to do with my answer. It has everything to do with tradition. It's been called an island continent for a long time by explorers and early settlers, and the name has stuck. These traditions can be pretty hard to go against. My Microsoft Encarta calls Australia an "island continent" in the first sentence. Antarctica is not called much of anything, mostly because it's so new to us humans. We have only really known it exists since the late 1800s and we haven't really started exploring it until the 1940s. Not much time to make a name for itself. Now that I think of it, I have always heard it called the "Mystery Continent." Maybe in a hundred years someone will ask why some people call it that...
My sources were my college Physical Geography textbook (Introducing Physical Geography by Strahler and Strahler) and Microsoft Encarta.
Take Care and Be Safe,
Rock Star and Science Demonstrator
Pacific Science Center, Seattle, Washington
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