A continent is one of the very large landmasses of Earth. People have traditionally treated each one as a distinct region. Encyclopædia Britannica describes a continent as a "large continuous mass of land conventionally regarded as a collective region". That wording makes the point that the idea depends as much on convention as on science. No single authority decides how many continents there are. School systems and scholarly traditions have used schemes with as few as four and as many as seven. Geologists use a different approach. For them, a continent is a body of thick, buoyant continental crust, whether or not it rises above sea level. Under that definition the count can change. The best-known example is the mostly submerged landmass Zealandia.
Etymology
The English word came into use in the 1550s and meant "continuous tract of land". It translates Medieval Latin terra continens "continuous land," from Latin continens "continuous," present participle of continere "to hold together, enclose,". It has been used as "one of the large land masses of the globe" from 1610s. In British usage, "the Continent" has also meant "the mainland of Europe" (from the point of view of Britain), from c. 1600.
Historical development of the concept
The scholars of ancient Greece were the first known to divide the known world into continents. They named three parts, Europe, Asia and Libya (Africa). They placed the boundaries along waterways such as the Nile, the Don and the straits connecting the Mediterranean to the Black Sea. Greek writers, Herodotus among them, already noticed that these divisions were arbitrary, because the three parts formed one connected landmass. The tripartite scheme lasted through the Middle Ages and shaped medieval world maps.
The Age of Exploration in the 15th and 16th centuries expanded European knowledge of Earth's geography, leading to the identification of the Americas and later, Australia and Antarctica. The Americas were treated as a "New World" separate from the Old World of Afro-Eurasia. Australia and Antarctica were added to the list after European navigators reached them in the 17th and 19th centuries. The modern conventional boundary between Europe and Asia was set in the 18th and 19th centuries. It usually runs along the Ural Mountains, the Ural River and the Caucasus. This boundary reflects cultural and political history more than any clear physical divide.
Continent models
Several counting schemes are in use. The seven-continent model recognizes Africa, Antarctica, Asia, Australia, Europe, North America, and South America as separate continents. It is the continent model most commonly taught in countries such as the United States, Canada, Australia, and much of Western Europe. It is also the standard model in China. Six-continent models either combine Europe and Asia into Eurasia or treat the Americas as one continent. The second version is common in Latin America and parts of Europe. The five-continent model counts only the inhabited continents: Africa, the Americas, Europe, Asia and Oceania. It is the model represented by the five rings of the Olympic Games. A four-continent model treats Afro-Eurasia, America, Antarctica and Australia as the only landmasses that are physically separate from one another.
Many smaller islands are assigned to nearby continents. In everyday geography, islands near a continent's shallow continental shelf are often grouped with that continent (for example, Great Britain with Europe). Remote Pacific islands are usually grouped into the broader region of Oceania rather than a continent in the strict sense. The continents differ greatly in size, with Asia being the largest at approximately 44.5 million square kilometers and Australia the smallest at about 7.7 million square kilometers. Together they make up a little under a third of Earth's surface. The world ocean covers the rest.
Geological definition
Geologists define a continent as a major land mass, including both dry land and the continental shelves that lie off the coast. A continent is made of continental crust – a base of igneous, metamorphic and sedimentary rocks. Oceanic crust is made up of basalt and is thinner and denser than continental crust. Because continental crust is lighter, it floats higher on the mantle. It forms high-standing regions with mountain belts and ancient stable cores called cratons. Continental crust is only known to exist on Earth.
In geology, the continents correspond to seven large areas of continental crust that are found on the tectonic plates, but exclude small continental fragments such as Madagascar that are generally referred to as microcontinents. In this scheme Europe and Asia form a single continent, Eurasia. Zealandia, centred on New Zealand and New Caledonia, is added. A 2017 paper in GSA Today argued for Zealandia's status as a continent. The paper described it as the seventh largest geological continent and the youngest, thinnest, and most submerged. Zealandia covers approximately 4.9 million square kilometers, but about 94% of it lies under water. Like India, Australia, Antarctica, Africa and South America, it was a former part of the Gondwana supercontinent. The authors argued that recognising it "more correctly represents the geology of this part of Earth".
Continental drift and plate tectonics
The continents have not always been where they are today. Alfred Wegener proposed the hypothesis of continental drift in 1912. He set it out more fully in his 1915 book The Origin of Continents and Oceans. As evidence, he pointed to the fit of coastlines, similar fossils, signs of past glaciation and matching rock formations on continents that are now far apart. Wegener called the former single landmass the "primal continent". In the 1920 edition of his book he named it "Pangäa," derived from the ancient Greek word meaning "all lands" or "all earth". Pangaea existed approximately between 270 and 200 million years ago. It later split into Laurasia in the north and Gondwana in the south. These two masses in turn broke up into today's continents.
The weakest part of Wegener's idea was that it gave no workable mechanism for the movement. It was not until the development of the theory of plate tectonics in the 1960s that a sufficient geological explanation of that movement was found. In plate tectonics, the continents ride on lithospheric plates that move a few centimetres a year. Over long periods they collide and build mountain ranges such as the Himalayas, then rift apart again. Even before the formation of Pangaea, the continents were already drifting; there have been a number of cycles of continental break-up, drift, and collision, each lasting a few hundred million years. Earlier supercontinents, such as Rodinia, came before Pangaea. Models predict that a new one will form in the distant future.
Significance
The positions of the continents affect global climate, ocean circulation and the courses of the great rivers. Their separation has also shaped how life is distributed. When populations were isolated on different landmasses, speciation followed and distinctive regional faunas developed. Alfred Russel Wallace used such patterns to found the field of biogeography, and they help explain the distribution of biodiversity across the globe. Continental geography has also influenced human evolution and migration out of Africa. It affected how crops and livestock spread, and it shaped the history of trade routes such as the Silk Road, which crossed Eurasia from east to west. In politics and culture, continents still serve as frames for international organisations, statistics and regional identities, although their boundaries remain conventional.
References
- Continents of the World: Definition + 7, 6 & 5 Modelsgeographypin.com
- Continent - Etymology, Origin & Meaningetymonline.com
- Continent (Geography) - Overviewstudyguides.com
- Geography Facts about the World's Continents - Geography Realmgeographyrealm.com
- Zealandiasciencelearn.org.nz
- Continenten.wikipedia.org
- Zealandia: Earth's Hidden Continentgeosociety.org
- How Many Continents Are There? The 7 Continents Explainedskysonar.com
- Found: A Possible Eighth Continentatlasobscura.com
- A Drifting World: The Supercontinent of Pangaeablogs.loc.gov
- Pangaeaebsco.com
- 2.2.2%3A Continental 'drift'geo.libretexts.org