Continents



Geology - Earth - Landmass - Continents

A Continent is one of several large landmasses on Earth. In geography, they are identified by convention rather than any strict criterion, with seven regions commonly regarded as continents—they are (from largest to smallest): Asia, Africa, North America, South America, Antarctica, Europe, and Australia.

Definitions and application
Conventionally, "Continents are understood to be large, continuous, discrete masses of land, ideally separated by expanses of water." Many of the seven most commonly recognized continents identified by convention are not discrete landmasses separated by water. The criterion 'large' leads to arbitrary classification: Greenland, with a surface area of 2,166,086 km2 is considered the world's largest island, while Australia, at 7,617,930 km2 is deemed to be a continent. Likewise, the ideal criterion that each be a continuous landmass is often disregarded by the inclusion of the continental shelf and oceanic islands, and contradicted by classifying North and South America and Asia and Africa as continents, with no natural separation by water in either case. This anomaly reaches its extreme if the continuous land mass of Europe and Asia is considered to constitute two continents. The Earth's major landmasses are washed upon by a single, continuous World Ocean, which is divided into a number of principal oceanic components by the continents and various geographic criteria.

Separation
The ideal criterion that each continent be a discrete landmass is commonly disregarded in favor of more arbitrary, historical conventions. Of the seven most commonly recognized continents, only Antarctica and Australia are distinctly separated from other continents.

Several continents are defined not as absolutely distinct bodies but as "more or less discrete masses of land". Asia and Africa are joined by the Isthmus of Suez, and North and South America by the Isthmus of Panama. Both these isthmuses are very narrow in comparison with the bulk of the landmasses they join, and both are transected by artificial canals (the Suez Canal and Panama Canal, respectively) which effectively separate these landmasses.

Other divisions
Aside from the conventionally known continents, the scope and meaning of the term 'continent' may vary. Supercontinents, largely in evidence earlier in the geological record, are landmasses which comprise more than one craton or continental core. These have included Laurasia, Gondwana, Vaalbara, Kenorland, Columbia, Rodinia, and Pangaea; arguably, Eurasia is a contemporary supercontinent.

Certain parts of continents are recognized as subcontinents, particularly those on different tectonic plates to the rest of the continent. The most notable examples are the Indian subcontinent and the Arabian Peninsula. Greenland, generally reckoned as the world's largest island on the northeastern periphery of the North American Plate, is sometimes referred to as a subcontinent. Where the Americas are viewed as a single continent (America), it is divided into two subcontinents (North America and South America) or various regions.

Some areas of continental crust are largely covered by the sea and may be considered submerged continents. Notable examples are Zealandia, emerging from the sea primarily in New Zealand and New Caledonia, and the almost completely submerged Kerguelen continent in the southern Indian Ocean.

Some islands lie on sections of continental crust that have rifted and drifted apart from a main continental landmass. While not considered continents because of their relatively small size, they may be considered microcontinents. Madagascar, the largest example, is usually considered an island of Africa but has been referred to as "the eighth continent".

In addition, a number of mythical continents exist: perhaps the most notable is Atlantis, and also Hyperborea, Thule, and Lemuria.

Terra Australis
Terra Australis Ignota (Latin, "the unknown land of the south") was a hypothetical continent appearing on European maps from the 15th to the 18th centuries. Other names for the continent include Terra Australis incognita (the unknown land of the South), Magallanica or Magellanica (the land of Magellan), La Australia del Espiritu Santo (the southern land of the Holy Spirit), and La grande isle de Java (the great island of Java).

The notion of Terra Australis was introduced by Aristotle. His ideas were later expanded by Ptolemy (1st century AD), who believed that the Indian Ocean was enclosed on the south by land, and that the lands of the Northern Hemisphere should be balanced by land in the south.

Antarctica was finally sighted in the hypothetical area of Terra Australis on January 27, 1820 by Russian Fabian Gottlieb von Bellingshausen, the first confirmed sighting.

Pangaea
Pangaea, Pangæa, or Pangea (pronounced /pænˈdʒiːə/[1], from Ancient Greek πᾶν pan "entire", and Γαῖα Gaia "Earth", Latinized as Gæa) was the supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration.[2] Some people know Pangea as the "Supercontinent".

The name was coined in the scientific discussion of Alfred Wegener's theory of continental drift. In his book "The Origin of Continents and Oceans" (Die Entstehung der Kontinente und Ozeane) he postulated that all the continents had at one time formed a single supercontinent which he called the "Urkontinent", before later breaking up and drifting to their present locations. The term Pangaea appeared in 1928 during a symposium to discuss Wegener's theory. [3]

The single enormous ocean which surrounded Pangaea was accordingly named Panthalassa.

Zealandia
Zealandia (pronounced /ziːˈlændiə/), also known as Tasmantis or the New Zealand continent, is a nearly submerged continent or microcontinent that sank after breaking away from Antarctica between 85 and 130 million years ago, and then from Australia 60-85 million years ago. It may have been completely submerged about 23 million years ago, and most of it (93%) remains submerged beneath the Pacific Ocean.

Zealandia is 3,500,000 km² in area, larger than Greenland or India, and almost half the size of Australia. It is unusually long and narrow, stretching from New Caledonia in the north to beyond New Zealand's sub-Antarctic islands in the south (from latitude 19° south to 56° south, analogous to ranging from Haiti to Hudson Bay or from Sudan to Sweden in the northern hemisphere). New Zealand is the largest part of Zealandia that is above sea level, followed by New Caledonia.

Pangaea Ultima
Pangaea Ultima (also called Pangaea Proxima, Neopangaea, and Pangaea II) is a possible future supercontinent configuration and an alternative to the Amasia supercontinent. Consistent with the supercontinent cycle, Pangaea Ultima could occur within the next 250 million years. This potential configuration, hypothesized by Christopher Scotese, earned its name from its similarity to the previous Pangaea supercontinent.

Supercontinents describe the merger of all, or nearly all, of the Earth's landmass into a single continuous continent. In the Pangaea Ultima scenario, subduction at the western Atlantic, east of the Americas, leads to the subduction of the Atlantic mid-ocean ridge followed by subduction destroying the Atlantic oceanic basin, causing the Atlantic Ocean to close, bringing the Americas back together with Africa and Europe. As with most supercontinents, the interior of Pangaea Ultima would probably become a semi-arid desert prone to temperature extremes.

Europe
Europe (/ˈjʊərəp/) is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains (or the Kuma-Manych Depression),[1] and the Black Sea to the southeast.[2] Europe is bordered by the Arctic Ocean and other bodies of water to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and the Black Sea and connected waterways to the southeast. Yet the borders for Europe—a concept dating back to classical antiquity—are somewhat arbitrary, as the term continent can refer to a cultural and political distinction or a physiographic one.

Europe is the world's second-smallest continent by surface area, covering about 10,180,000 square kilometres (3,930,000 sq mi) or 2% of the Earth's surface and about 6.8% of its land area. Of Europe's approximately 50 states, Russia is the largest by both area and population, while the Vatican City is the smallest.

Asia
Asia is the world's largest and most populous continent, located in the eastern and northern hemispheres. It covers 8.6% of the earth's total surface area (or 29.9% of its land area) and with approximately 4 billion people, it hosts 60% of the world's current human population.

Asia is traditionally defined as part of the landmass of Eurasia — with the western portion of the latter occupied by Europe — located to the east of the Suez Canal, east of the Ural Mountains and south of the Caucasus Mountains (or the Kuma-Manych Depression) and the Caspian and Black Seas.[3] It is bounded on the east by the Pacific Ocean, on the south by the Indian Ocean and on the north by the Arctic Ocean. Given its size and diversity, Asia — a toponym dating back to classical antiquity — is more a cultural concept incorporating a number of regions and peoples than a homogeneous physical entity.

Africa
Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² (11.7 million sq mi) including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the World's human population. The continent is surrounded by the Mediterranean Sea to the north, both the Suez Canal and the Red Sea along the Sinai Peninsula to the northeast, the Indian Ocean to the southeast, and the Atlantic Ocean to the west. Not counting the disputed territory of Western Sahara, there are 53 countries, including Madagascar and various island groups, associated with the continent.

North America
North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific Ocean; South America lies to the southeast. North America covers an area of about 24,709,000 square kilometers (9,540,000 square miles), about 4.8% of the planet's surface or about 16.5% of its land area. As of July 2008, its population was estimated at nearly 529 million people. It is the third-largest continent in area, following Asia and Africa, and the fourth in population after Asia, Africa, and Europe.

South America
South America is the southern continent of America, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. It is bordered on the west by the Pacific Ocean and on the north and east by the Atlantic Ocean; North America and the Caribbean Sea lie to the northwest.

Austrailia
Australia is the smallest of the geographic continents, though not of geological continents. There is no universally accepted definition of the word "continent"; the lay definition is "One of the main continuous bodies of land on the earth's surface." By that definition, the continent of Australia includes only the Australian mainland, and not nearby islands such as Tasmania or New Guinea. From the perspective of geology or physical geography, however, a "continent" may be understood to include the continental shelf (the submerged adjacent area) and the islands on the shelf, which are taken to be structurally part of the continent. By that definition Tasmania, New Guinea and other nearby islands such as (Aru Islands and Raja Ampat Islands) are part of the Australian continent, also known as Sahul, since they are part of the same geological landmass. These islands are separated by seas overlying the continental shelf — the Arafura Sea and Torres Strait between Australia and New Guinea, and Bass Strait between mainland Australia and Tasmania.

When sea levels were lower during the Pleistocene ice age, including the last glacial maximum about 18,000 BC, the lands formed a single, continuous landmass. During the past ten thousand years, rising sea levels overflowed the lowlands and separated the continent into today's low-lying semi-arid mainland and the two mountainous islands of New Guinea and Tasmania.

Antarctica
Antarctica is Earth's southernmost continent, underlying the South Pole. It is situated in the Antarctic region of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14.0 million km2 (5.4 million sq mi), it is the fifth-largest continent in area after Asia, Africa, North America, and South America. About 98% of Antarctica is covered by ice, which averages at least 1.6 kilometres (1.0 mi) in thickness.

Antarctica, on average, is the coldest, driest, and windiest continent, and has the highest average elevation of all the continents. Antarctica is considered a desert, with annual precipitation of only 200 mm (8 inches) along the coast and far less inland. There are no permanent human residents but anywhere from 1,000 to 5,000 people reside throughout the year at the research stations scattered across the continent. Only cold-adapted plants and animals survive there, including penguins, seals, many types of algae, and tundra vegetation.

Although myths and speculation about a Terra Australis ("Southern Land") date back to antiquity, the first confirmed sighting of the continent is commonly accepted to have occurred in 1820 by the Russian expedition of Mikhail Lazarev and Fabian Gottlieb von Bellingshausen. The continent, however, remained largely neglected for the rest of the 19th century because of its hostile environment, lack of resources, and isolation. The first formal use of the name "Antarctica" as a continental name in the 1890s is attributed to the Scottish cartographer John George Bartholomew.

Continental drift
thumb|306px|leftContinental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912. However, 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.

Evidence
The notion that continents have not always been at their present positions was suggested as early as 1596 by the Dutch map maker Abraham Ortelius in the third edition of his work Thesaurus Geographicus. Ortelius suggested that the Americas, Eurasia and Africa were once joined and have since drifted apart "by earthquakes and floods", creating the modern Atlantic Ocean. For evidence, he wrote: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three continents." Francis Bacon commented on Ortelius' idea in 1620, as did Benjamin Franklin and Alexander von Humboldt in later centuries.

Evidence for continental drift is now extensive. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica. There is also living evidence — the same animals being found on two continents. An example of this is a particular earthworm found in South America and South Africa.

The complementary arrangement of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, seafloor spreading, continental drift, and other forces of tectonophysics will further separate and rotate those two continents. It was this temporary feature which inspired Wegener to study what he defined as continental drift, although he did not live to see his hypothesis become generally accepted.

Widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. The continuity of glaciers, inferred from oriented glacial striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift. Striations indicated glacial flow away from the equator and toward the poles, in modern coordinates, and supported the idea that the southern continents had previously been in dramatically different locations, as well as contiguous with each other.

Volcanoes
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano island off Sicily which in turn, was named after Vulcan, the Roman god of fire.

Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.

Volcanoes can be caused by mantle plumes. These so-called hotspots, for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.

Fold Mountains
Fold mountains are mountains formed mainly by the effects of folding on layers within the upper part of the Earth's crust. In the time before either Plate tectonic theory developed, or the internal architecture of thrust belts became well understood, the term was used for most mountain belts, such as the Himalayas. The term is still fairly common in physical geography literature but has otherwise generally fallen out of use except as described below. The forces responsible for formation of the fold mountains are called Orogenic movements. The term orogenic is derived from a Greek word meaning mountain building. These forces act at tangent to the surface of the earth and are primarily involved in plate tectonics. Fold mountains are generally tourist hotspots.

examples:
 * Himalaya - India and Asia
 * Andes - Pacific Ocean and South America
 * Rockies - Pacific Ocean and North America
 * Alps - Europe and Italy

Fault-Block Mountain
Fault-block landforms (mountains, hills, ridges, etc.) are formed when large areas of bedrock are widely broken up by faults creating large vertical displacements of continental crust.

Vertical motion of the resulting blocks, sometimes accompanied by tilting, can then lead to high escarpments. These mountains are formed by the Earth's crust being stretched and extended by tensional forces. Fault block mountains commonly accompany rifting, another indicator of tensional tectonic forces.

Rift Valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion. When the tensional forces are strong enough to cause the plate to split apart it will do so such that a center block will drop down relative to its flanking blocks. This creates the nearly parallel steeply dipping walls. This feature is the beginning of the rift valley. As this process continues the valley gets wider and wider until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. One of the better long term examples of this process is the Basin and Range province in Nevada and Utah. Rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges. They can occur in continental crust or in oceanic crust. Rift valleys are often associated with a number of adjoining subsidiary or co-extensive valleys which are typically considered geologically part of the principal rift valley.

Mid-Ocean Ridge
A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading. The uplifted seafloor results from convection currents which rise in the mantle as magma at a linear weakness in the oceanic crust, and emerge as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long and the total length of the system is 80,000 km (49,700 mi)