|MadSci Network: Science History|
Unfortunately, there really isn't a good definition of a continent. The Oxford English Dictionary gives a pretty standard definition: "One of the main continuous bodies of land on the earth's surface." Even geology and geography books aren't much more precise. A definition this vague means that the number of continents will vary depending on what criteria you consider.
There are two main ideas that go into what makes a continent. The first is the "large continuous land mass" idea, and the second deals with continental plates. If you look at continuous land masses, then Europe and Asia should definitely be a single continent. But what exactly does "continuous" mean in regard to continents? Africa is definitely connected to Asia, but people usually think of it as being a separate continent. The same goes for North and South America. If you're thinking that any connection is enough to make two land masses continuous, then there are four continents: Antarctica, North and South America, Australia, and Europe/Asia/Africa. Size is also an issue. There's no strict lower limit to how big a piece of land has to be to be a continent, but Greenland, for example, is considered an island, not a continent.
Continental plates may seem to be a good criterion for dividing up continents, as they are fairly separate and unambiguous. Unfortunately, there are lots of them, and they divide up land masses in some pretty odd ways. Here is a good map of the continental plates. As you can see, India would be a separate continent, as would Asia Minor and Central America. One could decide that they are too small to be continents on their own. Making things more confusing, a good chunk of western Siberia would be included with North America!
To summarize, the definition of continent is vague enough that what is and is not considered to be a continent is largely determined by history and custom. It ends up being a mix of continuous land masses, continental plates, and size, so whether there are five or seven continents just depends on how you want to divide up the Earth. As an aside, note that the division between Europe and Asia can't be justified by either the continuous land mass or the continental plates idea; it's just a cultural consensus.
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