Determining the boundaries between the continents is generally a matter of geographical convention and consensus.
Several slightly different conventions are in use. The number of continents is most commonly considered seven (in English-speaking countries) but may range as low as four when Afro-Eurasia and the Americas are both considered as single continents.
An island can be considered to be associated with a given continent by either lying on the continent's adjacent continental shelf (e.g. Singapore, the British Isles) or being a part of a microcontinent on the same principal tectonic plate (e.g. Madagascar and Seychelles). An island can also be entirely oceanic while still being associated with a continent by geology (e.g. Bermuda, the Australian Indian Ocean Territories) or by common geopolitical convention (e.g. Ascension Island, the South Sandwich Islands). Another example is the grouping into Oceania of the Pacific Islands with Australia and Zealandia.
There are three overland boundaries subject to definition: between Africa and Asia (dividing Afro-Eurasia into Africa and Eurasia): at the Isthmus of Suez; between Asia and Europe (dividing Eurasia): along the Turkish Straits, the Caucasus, and the Urals and the Ural River (historically also north of the Caucasus, along the Kuma–Manych Depression or along the Don River); between North America and South America (dividing the Americas): at some point on the Isthmus of Panama, with the most common demarcation in atlases and other sources following the Darién Mountains watershed along the Colombia–Panama border where the isthmus meets the South American continent (see Darién Gap).
While today the isthmus between Asia and Africa is navigable via the Suez Canal, and that between North and South America via the Panama Canal, these artificial channels are not generally accepted as continent-defining boundaries in themselves. The Suez Canal happens to traverse the Isthmus of Suez between the Mediterranean Sea and the Red Sea, dividing Africa and Asia. The continental boundaries are considered to be within the very narrow land connections joining the continents.
The remaining boundaries concern the association of islands and archipelagos with specific continents, notably: the delineation between Africa, Asia, and Europe in the Mediterranean Sea; the delineation between Asia and Europe in the Arctic Ocean; the delineation between Europe and North America in the North Atlantic Ocean; the delineation between North and South America in the Caribbean Sea; the delineation of Antarctica from Africa, Australia, and South America in the Indian, South Pacific, and South Atlantic oceans, respectively (referred to collectively by some geographers as the Southern Ocean or the Antarctic Ocean); the delineation of Asia from Australia in the Ceram Sea, Arafura Sea, Timor Sea, Halmahera Sea, and the Wallacean region of the Indonesian Archipelago the delineation of Asia from North America in the North Pacific Ocean.
Africa and Asia
Historically in Greco-Roman geography, "Africa" meant ancient Libya, and its eastern extent was taken to be around Marmarica, at the Catabathmus Magnus. This was not considered a continent. As wider knowledge of geography developed, the shape of the African landmass (and Egypt's "natural" inclusion in that landmass) became apparent. In 1806, William George Browne still titled his travelogue Travels in Africa, Egypt, and Syria. Similarly, James Bruce in 1835 published Travels through part of Africa, Syria, Egypt, and Arabia. On the other hand, as early as 1670 John Ogilby under the title Africa published "an accurate Description of the Regions of Egypt, Barbary, Libya, and Billedulgerid, the Land of Negroes, Guinea, Æthiopia, and the Abyssines, with all the adjacent Islands, either in the Mediterranean, Atlantic, Southern, or Oriental Seas, belonging thereunto".
The usual line taken to divide Africa from Asia today is at the Isthmus of Suez, the narrowest gap between the Mediterranean Sea and the Gulf of Suez, the route today followed by the Suez Canal. This makes the Sinai Peninsula geographically Asian, and Egypt a transcontinental country. Less than 2% of the Egyptian population live on the Sinai Peninsula, and hence Egypt, even though technically transcontinental, is usually considered an African country entirely and not partly Asian. But when discussing the geopolitical region of the Middle East and North Africa, Egypt is usually grouped with the Western Asian countries as part of the Middle East, while Egypt's western neighbor Libya is grouped with the remaining North African countries as the Maghreb. Both are members of the African Union. However, the geological boundary is located further to the east, along the Gulf of Aqaba and the Dead Sea Transform, with the Sinai Peninsula, Israel, Palestine, much of Lebanon, and the coastal governorates of Syria being located on the African Plate.
The Comoros, Madagascar, Mauritius, and Seychelles are island countries in the Indian Ocean which are nominally associated with Africa. The Socotra Archipelago may be considered African as it lies on its continental shelf; however, it is politically a part of Yemen, an Asian country.
Africa and Europe
The African and European mainlands are non-contiguous, and the delineation between these continents is thus merely a question of which islands are to be associated with which continent.
The Portuguese Atlantic island possession of the Azores is from Europe and from Africa, and is sometimes grouped with Europe. By contrast, the Canary Islands and the Madeira islands (the latter of which also include the Desertas Islands and the Savage Islands) off the Atlantic coast of Morocco are much closer to, and usually grouped with, Africa; the Canary Islands are only from the African mainland at their closest point but from the European mainland, while Madeira is from the African mainland and from the European mainland.; In his 1895 book Africa: Volume 2, author Augustus Henry Keane remarked, "In the North Atlantic there are four insular groups — Madeira, the Canaries, Cape Verde, and the Azores — which are usually assigned to Africa, although the Canaries and Cape Verde alone belong geographically to that continent, the Azores are lost, so to say, in mid-Atlantic."
The Mediterranean island country of Malta was considered a part of Africa for centuries.Falconer, William; Falconer, Thomas. Dissertation on St. Paul's Voyage , BiblioLife (BiblioBazaar), 1872. (1817.), p. 50, These islands Pliny, as well as Strabo and Ptolemy, included in the African sea However, it is much closer to the coast of Sicily (a distance of about ) than it is to the African mainland (roughly away). The nearby Italian island of Lampedusa (principal island of the Pelagian Islands) is from Sicily while just from the African mainland; similarly, Pantelleria is from Sicily and just from the African mainland. All of these Mediterranean islands, including Malta and Sicily, are actually located on the African Plate, and could be considered part of the African continent geologically. However, for political and cultural reasons, maps generally display them as part of Europe instead.
A single Spanish islet, known as Alboran Island, is also debatably located on either the African Plate or the Eurasian Plate. This island is located in the Alboran Sea, 50 km (31 mi) north of the Moroccan coast and 90 km (56 mi) south of Spain.
European-governed territories in Africa There are six definitive occurrences of territories unequivocally being a part of the African continent, but legally being administered by a European state. Three of these are administered by France, and the other three by Spain. The existence of these territories further blurs the line between the borders of Africa and Europe, in particular with regard to the Spanish territories which lie immediately adjacent to and/or connected to the African mainland. The French territories lie within the Indian Ocean, and, consequently, are more dislocated from the principal boundary between the two continents.
The uninhabited Spanish unincorporated overseas minor territories, known as the plazas de soberanía ("Localities of Sovereignty"), are small islands that lie immediately adjacent to the North African coastline, with the exception of Peñón de Vélez de la Gomera, which was originally an island like the other 'localities' but has subsequently become directly connected to the Moroccan (African) mainland. The two other Spanish territories are the exclaves of Ceuta and Melilla, which are two populated coastal cities located directly on the African mainland, both bordering only Morocco.
Two of the French territories are the inhabited overseas departments and regions of Mayotte and Réunion. Mayotte is an island territory located west of the island country of Madagascar within the Mozambique Channel. Réunion is an island territory located near the island country of Mauritius and to the east of Madagascar (both nominally considered part of the African continent). The final territory is the Scattered Islands in the Indian Ocean, administratively a part of the French Southern and Antarctic Lands. This French territory consists of a range of minor uninhabited atolls in the Indian Ocean, located in the deep sea surrounding Madagascar.
Antarctica Antarctica along with its outlying islands have no permanent population. All land claims south of 60°S latitude are held in abeyance by the Antarctic Treaty System.
Australia's Heard Island and McDonald Islands (an external territory) and the French Kerguelen Islands are located on the Kerguelen Plateau, on the Antarctic continental plate. Both are still within the bounds of the Indian Ocean. The United Nations categorizes Heard Island and McDonald Islands, which are politically affiliated with Australia, as being part of Oceania. The islands are 4,000 km from Perth in Australia, and have never been inhabited by indigenous peoples of Oceania or any other humans. Heard Island has only been visited 240 times throughout its entire history, and the McDonald Islands have only ever been visited twice, in 1971 and 1980. The World Factbook categorizes these islands as part of Antarctica rather than Oceania.
The French Crozet Islands, Île Amsterdam, Île Saint-Paul, and the Norwegian Bouvet Island are also located on the Antarctic continental plate, and are not often associated with other continents. The United Nations categorize Bouvet Island as part of South America, while the World Factbook categorizes it as part of Antarctica.