A continent is one of several large landmasses of the world. Identified by convention rather than any strict criteria, up to seven regions are regarded as continents. Ordered from largest in area to smallest, they are: Asia, North America, South America, Antarctica and Australia. Geologically, the continents correspond to areas of continental crust that are found on the continental plates. However, some areas of continental crust are regions covered with water not included in the list of continents. Zealandia is one such area. Islands are grouped with a neighbouring continent to divide all the world's land into geopolitical regions. Under this scheme, most of the island countries and territories in the Pacific Ocean are grouped together with the continent of Australia to form a geopolitical region called Oceania. By convention, "continents are understood to be large, discrete masses of land, ideally separated by expanses of water." Several of the seven conventionally recognized continents are not discrete landmasses separated by water.
The criterion "large" leads to arbitrary classification: Greenland, with a surface area of 2,166,086 square kilometres is considered the world's largest island, while Australia, at 7,617,930 square kilometres is deemed the smallest continent. Earth's major landmasses all have coasts on a single, continuous World Ocean, divided into a number of principal oceanic components by the continents and various geographic criteria; the most restricted meaning of continent is that of a continuous area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. In this sense the term continental Europe is used to refer to mainland Europe, excluding islands such as Great Britain, Ireland and Iceland, the term continent of Australia may refer to the mainland of Australia, excluding Tasmania and New Guinea; the continental United States refers to the 48 contiguous states and the District of Columbia in central North America and may include Alaska in the northwest of the continent, while excluding Hawaii, Puerto Rico, Guam in the oceans.
From the perspective of geology or physical geography, continent may be extended beyond the confines of continuous dry land to include the shallow, submerged adjacent area and the islands on the shelf, as they are structurally part of the continent. From this perspective, the edge of the continental shelf is the true edge of the continent, as shorelines vary with changes in sea level. In this sense the islands of Great Britain and Ireland are part of Europe, while Australia and the island of New Guinea together form a continent; as a cultural construct, the concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered Madagascar part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental plate with other islands in the Pacific into one continent called Oceania; this divides the entire land surface of Earth into quasi-continents. The ideal criterion that each continent is a discrete landmass is relaxed due to historical conventions.
Of the seven most globally recognized continents, only Antarctica and Australia are separated from other continents by the ocean. Several continents are defined not as distinct bodies but as "more or less discrete masses of land". Asia and Africa are joined by the Isthmus of Suez, North and South America by the Isthmus of Panama. In both cases, there is no complete separation of these landmasses by water. Both these isthmuses are narrow compared to the bulk of the landmasses they unite. North America and South America are treated as separate continents in the seven-continent model. However, they may be viewed as a single continent known as America or the Americas; this viewpoint was common in the United States until World War II, remains prevalent in some Asian six-continent models. This remains the more common vision in Latin American countries, Portugal, Italy and Hungary where they are taught as a single continent; the criterion of a discrete landmass is disregarded if the continuous landmass of Eurasia is classified as two separate continents: Europe and Asia.
Physiographically and South Asia are peninsulas of the Eurasian landmass. However, Europe is considered a continent with its comparatively large land area of 10,180,000 square kilometres, while South Asia, with less than half that area, is considered a subcontinent; the alternative view—in geology and geography—that Eurasia is a single continent results in a six-continent view of the world. Some view separation of Eurasia into Asia and Europe as a residue of Eurocentrism: "In physical and historical diversity and India are comparable to the entire European landmass, not to a single European country.." However, for historical and cultural reasons, the view of Europe as a separate continent continues in several categorizations. If continents are defined as discrete landmasses, embracing all the contiguous land of a body Africa and Europe form a single continent which may be referred to as Afro-Eurasia; this produces a four-continent model consisting of Afro-Eurasia, America and Australia. When sea levels were lower during the Pleistocene ice ages, gre
Gondwana, was a supercontinent that existed from the Neoproterozoic until the Jurassic. It was formed by the accretion of several cratons. Gondwana became the largest piece of continental crust of the Paleozoic Era, covering an area of about 100,000,000 km2. During the Carboniferous Period, it merged with Laurussia to form a larger supercontinent called Pangaea. Gondwana broke up during the Mesozoic Era; the remnants of Gondwana make up about two thirds of today's continental area, including South America, Antarctica and the Indian Subcontinent. The formation of Gondwana began c. 800 to 650 Ma with the East African Orogeny, the collision of India and Madagascar with East Africa,and was completed c. 600 to 530 Ma with the overlapping Brasiliano and Kuunga orogenies, the collision of South America with Africa and the addition of Australia and Antarctica, respectively. The continent of Gondwana was named by Austrian scientist Eduard Suess, after the Gondwana region of central India, derived from Sanskrit for "forest of the Gonds".
The name had been used in a geological context, first by H. B. Medlicott in 1872, from which the Gondwana sedimentary sequences are described; the term "Gondwanaland" is preferred by some scientists in order to make a clear distinction between the region and the supercontinent. The assembly of Gondwana was a protracted process during the Neoproterozoic and Paleozoic, which however remains incompletely understood because of the lack of paleo-magnetic data. Several orogenies, collectively known as the Pan-African orogeny, led to the amalgamation of most of the continental fragments of a much older supercontinent, Rodinia. One of those orogenic belts, the Mozambique Belt, formed 800 to 650 Ma and was interpreted as the suture between East and West Gondwana. Three orogenies were recognized during the 1990s: the East African Orogeny and Kuunga orogeny, the collision between East Gondwana and East Africa in two steps, the Brasiliano orogeny, the successive collision between South American and African cratons.
The final stages of Gondwanan assembly overlapped with the opening of the Iapetus Ocean between Laurentia and western Gondwana. During this interval, the Cambrian explosion occurred. Laurentia was docked against the western shores of a united Gondwana for a short period near the Precambrian/Cambrian boundary, forming the short-lived and still disputed supercontinent Pannotia; the Mozambique Ocean separated the Congo–Tanzania–Bangweulu Block of central Africa from Neoproterozoic India. The Azania continent was an island in the Mozambique Ocean; the Australia/Mawson continent was still separated from India, eastern Africa, Kalahari by c. 600 Ma, when most of western Gondwana had been amalgamated. By c. 550 Ma, India had reached its Gondwanan position. Meanwhile, on the other side of the newly-forming Africa, Kalahari collided with Congo and Rio de la Plata which closed the Adamastor Ocean. C. 540–530 Ma, the closure of the Mozambique Ocean brought India next to Australia–East Antarctica, both North and South China were located in proximity to Australia.
As the rest of Gondwana formed, a complex series of orogenic events assembled the eastern parts of Gondwana c. 750 to 530 Ma. First the Arabian-Nubian Shield collided with eastern Africa in the East African Orogeny c.750 to 620 Ma. Australia and East Antarctica were merged with the remaining Gondwana c. 570 to 530 Ma in the Kuunga Orogeny. The Malagasy orogeny at about 550–515 Mya affected Madagascar, eastern East Africa and southern India. In it, Neoproterozoic India collided with the combined Azania and Congo–Tanzania–Bangweulu Block, suturing along the Mozambique Belt; the 18,000 km -long Terra Australis Orogen developed along Gondwana's western and eastern margins. Proto-Gondwanan Cambrian arc belts from this margin have been found in eastern Australia, New Zealand, Antarctica. Though these belts formed a continuous arc chain, the direction of subduction was different between the Australian-Tasmanian and New Zealand-Antarctica arc segments. A large number of terranes were accreted to Eurasia during Gondwana's existence but the Cambrian or Precambrian origin of many of these terranes remains uncertain.
For example, some Palaeozoic terranes and microcontinents that now make up Central Asia called the "Kazakh" and "Mongolian terranes", were progressively amalgamated into the continent Kazakhstania in the Late Silurian. Whether these blocks originated on the shores of Gondwana is not known. In the Early Palaeozoic the Armorican terrane, which today form large parts of France, was part of either Peri-Gondwana or core Gondwana. Precambrian rocks from the Iberian Peninsula suggest it too formed part of core Gondwana before its detachment as an orocline in the Variscan orogeny close to the Carboniferous–Permian boundary. South-east Asia is made of Gondwanan and Cathaysian continental fragments that were assembled during the Mid-Palaeozoic and Cenozoic; this p
The Americas comprise the totality of the continents of North and South America. Together, they comprise the New World. Along with their associated islands, they cover 8% of Earth's total surface area and 28.4% of its land area. The topography is dominated by the American Cordillera, a long chain of mountains that runs the length of the west coast; the flatter eastern side of the Americas is dominated by large river basins, such as the Amazon, St. Lawrence River / Great Lakes basin, La Plata. Since the Americas extend 14,000 km from north to south, the climate and ecology vary from the arctic tundra of Northern Canada and Alaska, to the tropical rain forests in Central America and South America. Humans first settled the Americas from Asia between 17,000 years ago. A second migration of Na-Dene speakers followed from Asia; the subsequent migration of the Inuit into the neoarctic around 3500 BCE completed what is regarded as the settlement by the indigenous peoples of the Americas. The first known European settlement in the Americas was by the Norse explorer Leif Erikson.
However, the colonization never became permanent and was abandoned. The Spanish voyages of Christopher Columbus from 1492 to 1502 resulted in permanent contact with European powers, which led to the Columbian exchange and inaugurated a period of exploration and colonization whose effects and consequences persist to the present. Diseases introduced from Europe and West Africa devastated the indigenous peoples, the European powers colonized the Americas. Mass emigration from Europe, including large numbers of indentured servants, importation of African slaves replaced the indigenous peoples. Decolonization of the Americas began with the American Revolution in the 1770s and ended with the Spanish–American War in the late 1890s. All of the population of the Americas resides in independent countries; the Americas are home to over a billion inhabitants, two-thirds of which reside in the United States, Brazil, or Mexico. It is home to eight megacities: New York City, Mexico City, São Paulo, Los Angeles, Buenos Aires, Rio de Janeiro, Bogotá, Lima.
The name America was first recorded in 1507. Christie's auction house says a two-dimensional globe created by Martin Waldseemüller was the earliest recorded use of the term; the name was used in the Cosmographiae Introductio written by Matthias Ringmann, in reference to South America. It was applied to both North and South America by Gerardus Mercator in 1538. America derives from the Latin version of Italian explorer Amerigo Vespucci's first name; the feminine form America accorded with the feminine names of Asia and Europa. In modern English and South America are considered separate continents, taken together are called America or the Americas in the plural; when conceived as a unitary continent, the form is the continent of America in the singular. However, without a clarifying context, singular America in English refers to the United States of America. In the English-speaking world, the term America used to refer to a single continent until the 1950s: According to historians Kären Wigen and Martin W. Lewis, While it might seem surprising to find North and South America still joined into a single continent in a book published in the United States in 1937, such a notion remained common until World War II.
By the 1950s, however all American geographers had come to insist that the visually distinct landmasses of North and South America deserved separate designations. This shift did not seem to happen in Romance-speaking countries, where America is still considered a continent encompassing the North America and South America subcontinents, as well as Central America; the first inhabitants migrated into the Americas from Asia. Habitation sites are known in Alaska and the Yukon from at least 20,000 years ago, with suggested ages of up to 40,000 years. Beyond that, the specifics of the Paleo-Indian migration to and throughout the Americas, including the dates and routes traveled, are subject to ongoing research and discussion. Widespread habitation of the Americas occurred during the late glacial maximum, from 16,000 to 13,000 years ago; the traditional theory has been that these early migrants moved into the Beringia land bridge between eastern Siberia and present-day Alaska around 40,000–17,000 years ago, when sea levels were lowered during the Quaternary glaciation.
These people are believed to have followed herds of now-extinct pleistocene megafauna along ice-free corridors that stretched between the Laurentide and Cordilleran ice sheets. Another route proposed is that, either on foot or using primitive boats, they migrated down the Pacific coast to South America. Evidence of the latter would since have been covered by a sea level rise of hundreds of meters following the last ice age. Both routes may have
Asia is Earth's largest and most populous continent, located in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa. Asia covers an area of 44,579,000 square kilometres, about 30% of Earth's total land area and 8.7% of the Earth's total surface area. The continent, which has long been home to the majority of the human population, was the site of many of the first civilizations. Asia is notable for not only its overall large size and population, but dense and large settlements, as well as vast populated regions, its 4.5 billion people constitute 60% of the world's population. In general terms, Asia is bounded on the east by the Pacific Ocean, on the south by the Indian Ocean, on the north by the Arctic Ocean; the border of Asia with Europe is a historical and cultural construct, as there is no clear physical and geographical separation between them. It has moved since its first conception in classical antiquity.
The division of Eurasia into two continents reflects East–West cultural and ethnic differences, some of which vary on a spectrum rather than with a sharp dividing line. The most accepted boundaries place Asia to the east of the Suez Canal separating it from Africa. China and India alternated in being the largest economies in the world from 1 to 1800 CE. China was a major economic power and attracted many to the east, for many the legendary wealth and prosperity of the ancient culture of India personified Asia, attracting European commerce and colonialism; the accidental discovery of a trans-Atlantic route from Europe to America by Columbus while in search for a route to India demonstrates this deep fascination. The Silk Road became the main east–west trading route in the Asian hinterlands while the Straits of Malacca stood as a major sea route. Asia has exhibited economic dynamism as well as robust population growth during the 20th century, but overall population growth has since fallen. Asia was the birthplace of most of the world's mainstream religions including Hinduism, Judaism, Buddhism, Taoism, Islam, Sikhism, as well as many other religions.
Given its size and diversity, the concept of Asia—a name dating back to classical antiquity—may have more to do with human geography than physical geography. Asia varies across and within its regions with regard to ethnic groups, environments, historical ties and government systems, it has a mix of many different climates ranging from the equatorial south via the hot desert in the Middle East, temperate areas in the east and the continental centre to vast subarctic and polar areas in Siberia. The boundary between Asia and Africa is the Red Sea, the Gulf of Suez, the Suez Canal; this makes Egypt a transcontinental country, with the Sinai peninsula in Asia and the remainder of the country in Africa. The border between Asia and Europe was defined by European academics; the Don River became unsatisfactory to northern Europeans when Peter the Great, king of the Tsardom of Russia, defeating rival claims of Sweden and the Ottoman Empire to the eastern lands, armed resistance by the tribes of Siberia, synthesized a new Russian Empire extending to the Ural Mountains and beyond, founded in 1721.
The major geographical theorist of the empire was a former Swedish prisoner-of-war, taken at the Battle of Poltava in 1709 and assigned to Tobolsk, where he associated with Peter's Siberian official, Vasily Tatishchev, was allowed freedom to conduct geographical and anthropological studies in preparation for a future book. In Sweden, five years after Peter's death, in 1730 Philip Johan von Strahlenberg published a new atlas proposing the Urals as the border of Asia. Tatishchev announced; the latter had suggested the Emba River as the lower boundary. Over the next century various proposals were made until the Ural River prevailed in the mid-19th century; the border had been moved perforce from the Black Sea to the Caspian Sea into which the Ural River projects. The border between the Black Sea and the Caspian is placed along the crest of the Caucasus Mountains, although it is sometimes placed further north; the border between Asia and the region of Oceania is placed somewhere in the Malay Archipelago.
The Maluku Islands in Indonesia are considered to lie on the border of southeast Asia, with New Guinea, to the east of the islands, being wholly part of Oceania. The terms Southeast Asia and Oceania, devised in the 19th century, have had several vastly different geographic meanings since their inception; the chief factor in determining which islands of the Malay Archipelago are Asian has been the location of the colonial possessions of the various empires there. Lewis and Wigen assert, "The narrowing of'Southeast Asia' to its present boundaries was thus a gradual process." Geographical Asia is a cultural artifact of European conceptions of the world, beginning with the Ancient Greeks, being imposed onto other cultures, an imprecise concept causing endemic contention about what it means. Asia does not correspond to the cultural borders of its various types of constituents. From the time of Herodotus a minority of geographers have rejected the three-continent system on the grounds that there is no substantial physical separation between
South America is a continent in the Western Hemisphere in the Southern Hemisphere, with a small portion in the Northern Hemisphere. It may be considered a subcontinent of the Americas, how it is viewed in the Spanish and Portuguese-speaking regions of the Americas; the reference to South America instead of other regions has increased in the last decades due to changing geopolitical dynamics. It is bordered on the west on the north and east by the Atlantic Ocean, it includes twelve sovereign states, a part of France, a non-sovereign area. In addition to this, the ABC islands of the Kingdom of the Netherlands and Tobago, Panama may be considered part of South America. South America has an area of 17,840,000 square kilometers, its population as of 2016 has been estimated at more than 420 million. South America ranks fourth in fifth in population. Brazil is by far the most populous South American country, with more than half of the continent's population, followed by Colombia, Argentina and Peru. In recent decades Brazil has concentrated half of the region's GDP and has become a first regional power.
Most of the population lives near the continent's western or eastern coasts while the interior and the far south are sparsely populated. The geography of western South America is dominated by the Andes mountains. Most of the continent lies in the tropics; the continent's cultural and ethnic outlook has its origin with the interaction of indigenous peoples with European conquerors and immigrants and, more locally, with African slaves. Given a long history of colonialism, the overwhelming majority of South Americans speak Portuguese or Spanish, societies and states reflect Western traditions. South America occupies the southern portion of the Americas; the continent is delimited on the northwest by the Darién watershed along the Colombia–Panama border, although some may consider the border instead to be the Panama Canal. Geopolitically and geographically all of Panama – including the segment east of the Panama Canal in the isthmus – is included in North America alone and among the countries of Central America.
All of mainland South America sits on the South American Plate. South America is home to Angel Falls in Venezuela. South America's major mineral resources are gold, copper, iron ore and petroleum; these resources found in South America have brought high income to its countries in times of war or of rapid economic growth by industrialized countries elsewhere. However, the concentration in producing one major export commodity has hindered the development of diversified economies; the fluctuation in the price of commodities in the international markets has led to major highs and lows in the economies of South American states causing extreme political instability. This is leading to efforts to diversify production to drive away from staying as economies dedicated to one major export. South America is one of the most biodiverse continents on earth. South America is home to many interesting and unique species of animals including the llama, piranha, vicuña, tapir; the Amazon rainforests possess high biodiversity, containing a major proportion of the Earth's species.
Brazil is the largest country in South America, encompassing around half of the continent's land area and population. The remaining countries and territories are divided among three regions: The Andean States, the Guianas and the Southern Cone. Traditionally, South America includes some of the nearby islands. Aruba, Curaçao, Trinidad and the federal dependencies of Venezuela sit on the northerly South American continental shelf and are considered part of the continent. Geo-politically, the island states and overseas territories of the Caribbean are grouped as a part or subregion of North America, since they are more distant on the Caribbean Plate though San Andres and Providencia are politically part of Colombia and Aves Island is controlled by Venezuela. Other islands that are included with South America are the Galápagos Islands that belong to Ecuador and Easter Island, Robinson Crusoe Island, Chiloé and Tierra del Fuego. In the Atlantic, Brazil owns Fernando de Noronha and Martim Vaz, the Saint Peter and Saint Paul Archipelago, while the Falkland Islands are governed by the United Kingdom, whose sovereignty over the islands is disputed by Argentina.
South Georgia and the South Sandwich Islands may be associate
In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, many earth scientists use a different definition: "a clustering of nearly all continents", which leaves room for interpretation and is easier to apply to Precambrian times. Supercontinents dispersed multiple times in the geologic past. According to the modern definitions, a supercontinent does not exist today; the supercontinent Pangaea is the collective name describing all of the continental landmasses when they were most near to one another. The positions of continents have been determined back to the early Jurassic, shortly before the breakup of Pangaea; the earlier continent Gondwana is not considered a supercontinent under the first definition, since the landmasses of Baltica and Siberia were separate at the time. The following table displays historical supercontinents. There are two contrasting models for supercontinent evolution through geological time.
The first model theorizes that at least two separate supercontinents existed comprising Vaalbara and Kenorland. The Neoarchean supercontinent consisted of Sclavia; these parts of Neoarchean age broke off at ~2480 and 2312 Ma and portions of them collided to form Nuna. Nuna continued to develop during the Mesoproterozoic by lateral accretion of juvenile arcs, in ~1000 Ma Nuna collided with other land masses, forming Rodinia. Between ~825 and 750 Ma Rodinia broke apart. However, before breaking up, some fragments of Rodinia had come together to form Gondwana by ~608 Ma. Pangaea formed by ~336 Ma through the collision of Gondwana and Siberia; the second model is based on both palaeomagnetic and geological evidence and proposes that the continental crust comprised a single supercontinent from ~2.72 Ga until break-up during the Ediacaran Period after ~0.573 Ga. The reconstruction is derived from the observation that palaeomagnetic poles converge to quasi-static positions for long intervals between ~2.72–2.115, 1.35–1.13, 0.75–0.573 Ga with only small peripheral modifications to the reconstruction.
During the intervening periods, the poles conform to a unified apparent polar wander path. Because this model shows that exceptional demands on the paleomagnetic data are satisfied by prolonged quasi-integrity, it must be regarded as superseding the first model proposing multiple diverse continents, although the first phase incorporates Vaalbara and Kenorland of the first model; the explanation for the prolonged duration of the Protopangea-Paleopangea supercontinent appears to be that lid tectonics prevailed during Precambrian times. Plate tectonics as seen on the contemporary Earth became dominant only during the latter part of geological times; the Phanerozoic supercontinent Pangaea is still doing so today. Because Pangaea is the most recent of Earth's supercontinents, it is the most well known and understood. Contributing to Pangaea's popularity in the classroom is the fact that its reconstruction is as simple as fitting the present continents bordering the Atlantic-type oceans like puzzle pieces.
A supercontinent cycle is the break-up of one supercontinent and the development of another, which takes place on a global scale. Supercontinent cycles are not the same as the Wilson cycle, the opening and closing of an individual oceanic basin; the Wilson cycle synchronizes with the timing of a supercontinent cycle. However, supercontinent cycles and Wilson cycles were both involved in the creation of Pangaea and Rodinia. Secular trends such as carbonatites, granulites and greenstone belt deformation events are all possible indicators of Precambrian supercontinent cyclicity, although the Protopangea-Paleopangea solution implies that Phanerozoic style of supercontinent cycles did not operate during these times. There are instances where these secular trends have a weak, uneven or lack of imprint on the supercontinent cycle; the causes of supercontinent assembly and dispersal are thought to be driven by convection processes in the Earth's mantle. 660 km into the mantle, a discontinuity occurs, affecting the surface crust through processes like plumes and "superplumes".
When a slab of subducted crust is denser than the surrounding mantle, it sinks to the discontinuity. Once the slabs build up, they will sink through to the lower mantle in what is known as a "slab avalanche"; this displacement at the discontinuity will cause the lower mantle to rise elsewhere. The rising mantle can form a superplume. Besides having compositional effects on the upper mantle by replenishing the large-ion lithophile elements, volcanism affects plate movement; the plates will be moved towards a geoidal low where the slab avalanche occurred and pushed away from the geoidal high that can be caused by the plumes or superplumes. This causes the continents to push together to form supercontinents and was evidently the process that operated to cause the early continental crust to aggregate into Protopangea. Dispersal of supercontinents is caused by the accumulation of heat underneath the crust due to the rising of large convection cells or plumes, a massive heat release resulted in the final break-up of Paleopangea.
Accretion occurs over geo
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics