Title: Continent Content: thumb|right|upright=1.5|A map showing the continents: North America South America Antarctica Africa Europe Asia Oceania A continent is a large area of the land on Earth that is joined together. There are no strict rules for what land is considered a continent, but in general it is agreed there are six or seven continents in the world, including Africa, Antarctica, Asia, Europe, North America, Oceania (or Australasia),Either are accepted terms and South America.Continent (2009). In Encyclopædia Britannica. Retrieved 12 December 2009. ==Demography==  The most populous continent by population is Asia, followed by Africa. The third most populous continent is Europe. The fourth most populous is North America, and then South America. In sub-Saharan Africa, the largest age group are denarians (in their teens). In north Africa, the largest age group are vicenarian (in their twenties). In Europe, most people are tricenarian (in their thirties) or quadragenarian (in their forties). == Continents == Geologists use the term continent to mean continental crust, a platform of metamorphic and igneous rock, largely of granitic composition. Continental crust is less dense and much thicker than oceanic crust, which is why it "floats" higher than oceanic crust on the underlying mantle. This explains why the continents form high platforms surrounded by deep ocean basins. === Australia === Some sources say that Australia is one of the seven continents. Others say that Australia is part of a larger continent, such as Australasia, or Oceania. Oceania is a region which includes Australia, New Zealand and the Pacific Islands. Australasia includes at least all countries on the Australian continental plate. This includes the islands of New Guinea, Tasmania, New Zealand and a number of smaller islands. It is on the south-eastern side of the Wallace Line, with distinct differences in its biology from the Asian side of the line. :"It includes all the islands of the Malay Archipelago... as well as the various groups of islands in the Pacific. The term has been used in very different senses".Wallace, Alfred Russel 1893. Australasia, vol 1. London: Stanford. Chapter 1: Definition. === Zealandia === Zealandia is an almost entirely submerged land mass, and 93% of it still remains under water. Zealandia may have broken off the Australian plate between 85 and 130 million years ago. === North and South America === North America and South America together are often described as one continent, "the Americas", or simply "America". This has the advantage of including Central America and the Caribbean islands. Otherwise, Central America is counted as part of North America. === Eurasia === Eurasia is not really an alternative, rather it is a recognition that the landmasses of Europe and Asia are continuous, and some of its largest countries are in both regions. Russia extends from eastern Europe to the far east of Asia without a break. The Ural Mountains, which run roughly north–south, are the traditional dividing-line between Europe and Asia. For many purposes it is convenient to consider the great landmass as a single continent, Eurasia. When British people talk about "the Continent" (or "Continental" things) they mean the European mainland. This meaning is not used as much as it used to be, but is still seen in phrases like "Continental breakfast" (rolls with cheese, jam etc. as distinct from an "English breakfast" which is a cooked breakfast). Continents not only move but also sometimes move against each other. The Indian subcontinent has been colliding with the Eurasian continent for a while now. As these continents push against each other, they buckle and bend. Because of this, the Himalaya Mountains, with Mount Everest, are still being built up today. === Antarctica === Antarctica is Earth's fifth largest continent. Antarctica, the coldest place on Earth, covers Earth's South Pole. The continent only have two seasons, summer and winter. Antarctica is a cold desert. It does not rain or snow much there. Ever since its discovery in 1812, Antarctica with a surface area of ~13.6 –14 million km2 about 1.4 times the size of Europe, was a great challenge for explorers. Despite being nearly completely covered by a thick layer of ice, Antarctica harbors a wide range of aquatic and terrestrial environement. == Origin of continents == A craton is an old and stable part of the continental lithosphere. It is the Earth's two topmost layers, the crust and the uppermost mantle. There are various hypotheses of how cratons have been formed.. Continents may have been formed by giant meteorite impacts in the first billion years of Earth's existence.Kearey P.; Klepeis K.A.; Vine F.J. 2009. Global tectonics. 3rd ed, Oxford: Wiley-Blackwell ISBN 9781405107778 The question is not yet settled. What is clear is that the cratons are very old, and are the basis for the continents we see today. ==Related pages== * List of countries by continents