Because geography is defined by local convention, there are several conceptions as to which landmasses qualify as continents. There are names for six, but America is often divided, and Europe is often united with Asia. Ignoring cases where Antarctica is omitted, there are half a dozen lists.
|7 continents:||Antarctica||South America||North America||Europe||Asia||Africa||Australia.|
|6 continents:||Antarctica||South America||North America||<center>Eurasia||Africa||Australia.|
|5 continents:||Antarctica||South America||<center>Laurasia||Africa||Australia.|
The 7-continent model is usually taught in Western Europe, the United States, Australia, and much of Asia. In Canada, the government-approved Atlas of Canada names 7 continents and teaches Oceania instead of Australia, however most schools view North and South America as subcontinents, as well as Europe and Asia, hence most Canadians consider there to be 5 continents. The 6-continent combined-America model is taught in Japan, Iran, and Latin America. The 6-continent Eurasia model is preferred by the scientific community, and as such is commonly found in all parts of the world, but is especially used in Russia and other countries of Eastern Europe. Historians may use the 5-continent Laurasian model (Jared Diamond) or the 4-continent Afrasian model (Andre Gunder Frank).
In its original sense, "continent" meant (and still means) mainland. In the Greco-Roman world, there was but one known, the Continent, which we today call the Old World. In the mid 1600s Peter Heylin wrote in his Cosmographie that "A Continent is a great quantity of Land, not separated by any Sea from the rest of the World, as the whole Continent of Europe, Asia, Africa." As late as 1727 Ephraim Chambers wrote in his Cyclopædia, "The world is ordinarily divided into two grand continents: the old and the new." However, since Classical times this Continent was divided into "peninsulas" which also came to be called continents, since they were great land masses themselves. Through the Middle Ages, there were three such continents in the Western conception: Europe, Africa, and Asia. The European discovery of America in 1492 made four; and Australia in 1606 would make five, though not right away: as late as 1813 geographers wrote of Australia as "New Holland, an immense Island, which some geographers dignify with the appellation of another continent". However, dividing America in two was commonplace by this time, and would also produce a fifth continent. The idea of the Five Continents is still strong in Europe and Asia, and is represented by the five rings on the Olympic flag, though it is now considered somewhat archaic.
Antarctica was sighted in 1820, for the sixth and last continent to be given a separate name, though a great "antarctic" (antipodean) landmass had been anticipated for millennia. Dividing the Americas (which is as reasonable geographically as dividing Africa from Eurasia) now made seven continents, nicely symmetrical with the magical number of the Seven Seas, Seven Heavens, and the seven heavenly bodies that gave their names to the seven days of the week. However, this division never appealed to Latin America, which saw itself spanning America as a single landmass, and there the conception of six continents remains, as it does in scattered other countries such as Japan. From a modern perspective, the continent with the least reason for separate recognition is Europe, and in scientific circles people generally prefer to subsume Europe and Asia into Eurasia. This appealed to Russia, which spans Eurasia, and in Russia and (at least formerly) in Eastern Europe, Eurasia is or was taught as being one of six continents.
Geographers and historians often find it useful to define larger land masses connected by land bridges:
- Afrasia (or Eurafrasia): the combined land mass of Africa and Eurasia;
- America (or the Americas): the combined land mass of North America and South America;
- Laurasia: the combined land mass of Eurasia and North America, which were connected by Beringia during the Ice Age;
- Sahul: the combined land mass of Australia, New Guinea, and Tasmania during the Ice Age.
That is, during the last Ice Age, there were three large landmasses: Afrasia+America (which has no name), Sahul, and Antarctica. These larger land masses are usually considered supercontinents rather than continents, however.
In the last century it has also become conventional to subdivide Eurasia into the regions of Europe, Asia, and the Middle East. America is often divided into the regions of North America, Central America, and South America. Continents are also sometimes subdivided into subcontinents that are isolated by geological features. The prototype of this is the Indian subcontinent.
Islands are usually considered to belong geographically to the continent they are closest to. The Coral Sea and South Pacific islands may be associated with Australia/Australasia to form the "continent" of Oceania (though the Pacific islands without Australia are also called Oceania). The British Isles have always been considered part of Europe, and Greenland is considered part of North America.
When the Continent is referred to without clarification by a speaker of British English, it is usually presumed to mean Continental Europe, that is Europe, explicitly excluding Great Britain and Ireland. The Continental United States excludes Hawaii. Contiguous or Co(n)terminous United States means the United States without Alaska or Hawaii (the "Lower 48"), but it is very common for people to say continental for contiguous.
Geologically, the surface of Earth consists of many tectonic plates, consisting of rigid lithospheric mantle and crust moving together over the much less viscous asthenosphere. Continental crust is primarily granitic in composition, overlain by sedimentary and metamorphic rocks. Much of the continental crust extends above sea level as dry land. Oceanic crust is basaltic in composition, and much thinner than continental crust, thus generally lying below sea level.
Although from a human perspective shallow inland seas such as the Bering Sea appear to divide up land masses into continents, such ephemeral features do not define continents geologically. For instance, many times over the past few million years, the continents of Eurasia and America were connected by dry land. A geologic continent, therefore, is a continuous piece of continental crust, whether wet or dry at a particular time. As such, Laurasia and Africa-Arabia are one continent, which for the past three million years has also been linked to South America. This world-spanning land mass has no name except for the Classical meaning of "The Continent". The other large geologic continents are Sahul and Antarctica, but there are many so-called microcontinents as well: Madagascar, the Seychelles (the northern Mascarene Plateau), New Zealand, New Caledonia, etc., which are splinters of the ancient supercontinent of Gondwana. Note that volcanic Iceland is an exposed bit of oceanic crust at the mid-ocean ridge, and therefore not a microcontinent. Likewise, the British Isles, Sri Lanka, Borneo, and Newfoundland are integral parts of the Laurasian continent which happen to be separated by shallow (and temporary) inland seas flooding its margins.
During the 20th century, it became accepted by geologists that continents move location on the face of the planet over the geologic timescale, a process known as continental drift, explained by the theory of plate tectonics. It is the tectonic plates that have drifted, broken apart and joined together over time to give rise to the continents we now recognize. Consequently, in the geological past and prior to the present continents, other continents existed - see Category:Historical continents.
Sometimes the continents are considered to be divided by tectonic plates, so that Arabia on the Arabian plate, India on the Indian plate, Central America on the Caribbean plate, and California on the Pacific plate might be considered continents.
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