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83-464: Tharsis can have many meanings depending on historical and scientific context. The name is commonly used in a broad sense to represent a continent -sized region of anomalously elevated terrain centered just south of the equator around longitude 265°E. Called the Tharsis bulge or Tharsis rise, this broad, elevated region dominates the western hemisphere of Mars and is the largest topographic feature on

166-651: A 1.5-bar CO 2 atmosphere and a global layer of water 120 m thick. Martian magmas also likely contain significant amounts of sulfur and chlorine . These elements combine with water to produce acids that can break down primary rocks and minerals. Exhalations from Tharsis and other volcanic centers on the planet are likely responsible for an early period of Martian time (the Theiikian) when sulfuric acid weathering produced abundant hydrated sulfate minerals such as kieserite and gypsum . Two European Space Agency probes have discovered water frost on Tharsis. Previously, it

249-574: A continent as "a large space of dry land comprehending many countries all joined together, without any separation by water. Thus Europe, Asia, and Africa is one great continent, as America is another." However, the old idea of Europe, Asia and Africa as "parts" of the world ultimately persisted with these being regarded as separate continents. By convention, continents "are understood to be large, continuous, discrete masses of land, ideally separated by expanses of water". In modern schemes with five or more recognized continents, at least one pair of continents

332-520: A continent. This divides the entire land surface of Earth into continents, regions, or quasi-continents. The criterion that each continent is a discrete landmass is commonly relaxed due to historical conventions and practical use. Of the seven most globally recognized continents, only Antarctica and Australia are completely separated from other continents by the ocean. Several continents are defined not as absolutely distinct bodies but as " more or less discrete masses of land". Africa and Asia are joined by

415-615: A discrete landmass is completely disregarded if the continuous landmass of Eurasia is classified as two separate continents (Asia and Europe). Physiographically , Europe and the Indian subcontinent are large peninsulas of the Eurasian landmass. However, Europe is considered a continent with its comparatively large land area of 10,180,000 square kilometres (3,930,000 sq mi), while the Indian subcontinent, with less than half that area,

498-425: A geological continent has been disputed by some geologists. However, a study conducted in 2021 found that part of the submerged continent is indeed Precambrian , twice as old as geologists had previously thought, which is further evidence that supports the idea of Zealandia being a geological continent. All seven geological continents are spatially isolated by geologic features. The term "continent" translates

581-550: A nearby continent to divide all the world's land into geographical 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 the geographical region Oceania . In geology , a continent is defined as "one of Earth's major landmasses, including both dry land and continental shelves ". The geological continents correspond to seven large areas of continental crust that are found on

664-468: A part of South America. Apart from the current continents, the scope and meaning of the term continent includes past geological ones. Supercontinents , largely in evidence earlier in the geological record, are landmasses that comprise most of the world's cratons or continental cores. These have included Vaalbara , Kenorland , Columbia , Rodinia , Pannotia , and Pangaea . Over time, these supercontinents broke apart into large landmasses which formed

747-523: A separate land. In 1507 Martin Waldseemüller published a world map, Universalis Cosmographia , which was the first to show North and South America as separate from Asia and surrounded by water. A small inset map above the main map explicitly showed for the first time the Americas being east of Asia and separated from Asia by an ocean, as opposed to just placing the Americas on the left end of

830-738: A single landmass or a part of a very large landmass, as in the case of Asia or Europe. Due to this, the number of continents varies; up to seven or as few as four geographical regions are commonly regarded as continents. Most English-speaking countries recognize seven regions as continents. In order from largest to smallest in area, these seven regions are Asia , Africa , North America , South America , Antarctica , Europe , and Australia . Different variations with fewer continents merge some of these regions; examples of this are merging North America and South America into America , Asia and Europe into Eurasia , and Africa, Asia, and Europe into Afro-Eurasia . Oceanic islands are occasionally grouped with

913-405: A system of immense northwest-oriented valleys up to 200 kilometres (120 mi) wide. These northwestern slope valleys (NSVs) - which debouch into Amazonis Planitia - are separated by a parallel set of gigantic "keel-shaped" promontories. The NSVs may be relics from catastrophic floods of water, similar to the huge outflow channels that empty into Chryse Planitia, east of Tharsis. Central Tharsis

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996-821: A third part of the world. Division into three parts eventually came to predominate. From the Greek viewpoint, the Aegean Sea was the center of the world; Asia lay to the east, Europe to the north and west, and Africa to the south. The boundaries between the continents were not fixed. Early on, the Europe–Asia boundary was taken to run from the Black Sea along the Rioni River (known then as the Phasis ) in Georgia . Later it

1079-502: A very large peninsula on the fringe of a continent. Where the Americas are viewed as a single continent (America), it is divided into two subcontinents ( North America and South America ) or three ( Central America being the third). When Eurasia is regarded as a single continent, Asia and Europe are treated as subcontinents. Some areas of continental crust are largely covered by the ocean and may be considered submerged continents . Notable examples are Zealandia , emerging from

1162-535: A volcano grows in size and weight, the stress field underneath the volcano changes from compressional to extensional. A subterranean rift may develop at the base of the volcano where the crust is wrenched apart. This volcanic spreading may initiate further structural deformation in the form of thrust faults along the volcano's distal flanks, pervasive grabens and normal faults across the edifice, and catastrophic flank failure (sector collapse). Mathematical analysis shows that volcanic spreading operates on volcanoes at

1245-515: A warm and wet early history that has long since dried up, these lakes existed in the Hesperian Epoch, a much later period. Using detailed images from NASA's Mars Reconnaissance Orbiter , the researchers speculate that there may have been increased volcanic activity, meteorite impacts or shifts in Mars' orbit during this period to warm Mars' atmosphere enough to melt the abundant ice present in

1328-487: A wide range of scales and is theoretically similar to the larger-scale rifting that occurs at mid-ocean ridges ( divergent plate boundaries ). Thus, in this view, the distinction between tectonic plate , spreading volcano, and rift is nebulous, all being part of the same geodynamic system. According to Borgia and Murray, Mount Etna in Sicily is a good terrestrial analogue for the much larger Tharsis bulge, which to them

1411-707: Is a chasma in the Lunae Planum high plateau north of the Valles Marineris canyon system of Mars . It is in the Coprates quadrangle . Clay has been found within it, meaning that water once sat there for a time. It may have been one of the many lakes that have been advanced for the Martian past. Research from 2010 suggests that Mars had lakes along parts of the equator, which is where Echus Chasma lies. Although earlier research had showed that Mars had

1494-466: Is approximately 3,500 kilometres (2,200 mi) long and includes most of the region covered by the Tharsis quadrangle and the northwestern portion of the adjoining Phoenicis Lacus quadrangle to the south. Olympus Mons and its associated lava flows and aureole deposits form another distinct subprovince of the Tharsis region. This subregion is about 1,600 kilometres (990 mi) across. It lies off

1577-493: Is bounded to the west by a highly elevated zone of fractures ( Claritas Fossae ) and mountains (the Thaumasia Highlands) that curves south then east to northeast in a wide arc that has been compared to the shape of a scorpion’s tail. The plateau province is bounded to the north by Noctis Labyrinthus and the western three-quarters of Valles Marineris . It is bounded to the east by a north-south oriented ridge called

1660-451: 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 the separation of Eurasia into Asia and Europe as a residue of Eurocentrism : "In physical, cultural and historical diversity, China and India are comparable to the entire European landmass, not to a single European country. [...]." However, for historical and cultural reasons,

1743-419: Is defined by the three massive Tharsis Montes volcanoes ( Arsia Mons , Pavonis Mons , and Ascraeus Mons ), a number of smaller volcanic edifices, and adjacent plains consisting of young (mid to late Amazonian) lava flows. The lava plains slope gently to the east where they overlap and embay the older (Hesperian-aged) terrain of Echus Chasma and western Tempe Terra . To the west, the lava plains slope toward

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1826-435: Is divided into several principal oceanic components by the continents and various geographic criteria. The geological definition of a continent has four criteria: high elevation relative to the ocean floor; a wide range of igneous, metamorphic and sedimentary rocks rich in silica; a crust thicker than the surrounding oceanic crust; and well-defined limits around a large enough area. The most restricted meaning of continent

1909-469: Is divided into two broad rises: a northern and a larger southern rise. The northern rise partially overlies sparsely cratered, lowland plains north of the dichotomy boundary. This region is dominated by Alba Mons and its extensive volcanic flows. Alba Mons is a vast, low-lying volcanic construct that is unique to Mars. Alba Mons is so large and topographically distinct that it can almost be treated as an entire volcanic province unto itself. The oldest part of

1992-507: Is inhabited by more numerous people and animals than our Europe, or Asia or Africa", the first known explicit identification of part of the Americas as a continent like the other three. Within a few years, the name "New World" began appearing as a name for South America on world maps, such as the Oliveriana (Pesaro) map of around 1504–1505. Maps of this time, though, still showed North America connected to Asia and showed South America as

2075-430: Is joined by land in some fashion. The criterion "large" leads to arbitrary classification: Greenland , with a surface area of 2,166,086 square kilometres (836,330 sq mi), is only considered the world's largest island, while Australia , at 7,617,930 square kilometres (2,941,300 sq mi), is deemed the smallest continent. Earth's major landmasses all have coasts on a single, continuous World Ocean , which

2158-399: Is one immense volcano they call Tharsis Rise. Mount Etna is a complex spreading volcano that is characterized by three main structural features: a volcanic rift system that crosses the summit in a north-northeast direction; a peripheral compression belt (thrust front) surrounding the base of the volcano; and an east-northeast trending system of transtensional (oblique normal) faults that connect

2241-612: 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 (sometimes referred to in Britain as "the Continent") is used to refer to mainland Europe, excluding islands such as Great Britain , Iceland , Ireland , and Malta , while the term continent of Australia may refer to the mainland of Australia , excluding New Guinea , Tasmania , and other nearby islands. Similarly,

2324-407: Is usually considered an island of Africa, but its divergent evolution has caused it to be referred to as "the eighth continent" from a biological perspective. Geologists use four key attributes to define a continent: With the addition of Zealandia in 2017, Earth currently has seven recognized geological continents: Due to a seeming lack of Precambrian cratonic rocks, Zealandia's status as

2407-458: The continental United States refers to "the 49 States (including Alaska but excluding Hawaii ) located on the continent of North America, and the District of Columbia ." 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 (the continental shelf ) and the islands on

2490-666: The Black Sea . The names were first applied just to lands near the coast and only later extended to include the hinterlands . But the division was only carried through to the end of navigable waterways and "... beyond that point the Hellenic geographers never succeeded in laying their finger on any inland feature in the physical landscape that could offer any convincing line for partitioning an indivisible Eurasia ..." Ancient Greek thinkers subsequently debated whether Africa (then called Libya ) should be considered part of Asia or

2573-613: The Isthmus of Suez , and North America and South America by the Isthmus of Panama . In both cases, there is no complete separation of these landmasses by water (disregarding the Suez Canal and the Panama Canal , which are both narrow and shallow, as well as human-made). Both of these isthmuses are very narrow compared to the bulk of the landmasses they unite. North America and South America are treated as separate continents in

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2656-456: The Pacific Ocean , as well as the continent of Australia . Zealandia (a submerged continent ) has been called the eighth continent. The following table provides areas given by Encyclopædia Britannica for each continent in accordance with the seven-continent model, including Australasia along with Melanesia , Micronesia , and Polynesia as parts of Oceania. It also provides populations of continents according to 2021 estimates by

2739-584: The Pleistocene ice ages , greater areas of the continental shelf were exposed as dry land, forming land bridges between Tasmania and the Australian mainland . At those times, Australia and New Guinea were a single, continuous continent known as Sahul . Likewise, Afro-Eurasia and the Americas were joined by the Bering Land Bridge . Other islands, such as Great Britain , were joined to

2822-782: The United Nations Statistics Division based on the United Nations geoscheme , which includes all of Egypt (including the Isthmus of Suez and the Sinai Peninsula ) as a part of Africa, all of Armenia, Azerbaijan, Cyprus, Georgia, Indonesia, Kazakhstan, and Turkey (including East Thrace ) as parts of Asia, all of Russia (including Siberia ) as a part of Europe, all of Panama and the United States (including Hawaii ) as parts of North America, and all of Chile (including Easter Island ) as

2905-422: The tectonic plates , but exclude small continental fragments such as Madagascar that are generally referred to as microcontinents . Continental crust is only known to exist on Earth. The idea of continental drift gained recognition in the 20th century. It postulates that the current continents formed from the breaking up of a supercontinent ( Pangaea ) that formed hundreds of millions of years ago. From

2988-492: The "subcontinents" concerned are on different tectonic plates from the rest of the continent, providing a geological justification for the terminology. Greenland , generally considered the world's largest island on the northeastern periphery of the North American Plate , is sometimes referred to as a subcontinent. This is a significant departure from the more conventional view of a subcontinent as comprising

3071-638: The 16th century the English noun continent was derived from the term continent land , meaning continuous or connected land and translated from the Latin terra continens . The noun was used to mean "a connected or continuous tract of land" or mainland . It was not applied only to very large areas of land—in the 17th century, references were made to the continents (or mainlands) of the Isle of Man , Ireland and Wales and in 1745 to Sumatra . The word continent

3154-533: The Americas, Columbus never believed he had reached a new continent—he always thought it was part of Asia. In 1501, Amerigo Vespucci and Gonçalo Coelho attempted to sail around what they considered the southern end of the Asian mainland into the Indian Ocean , passing through Fernando de Noronha . After reaching the coast of Brazil , they sailed along the coast of South America much farther south than Asia

3237-697: The Black Sea and the Caspian Sea , and the border between Asia and Africa at the isthmus between the Red Sea and the mouth of Lake Bardawil on the Mediterranean Sea . The Roman author Pliny the Elder , writing in the 1st century CE, stated that "The whole globe is divided into three parts, Europe, Asia, and Africa", adding: I shall first then speak of Europe, the foster-mother of that people which has conquered all other nations, and itself by far

3320-691: The Catholic church, began to associate itself with the concept of Europe. Through the Roman period and the Middle Ages , a few writers took the Isthmus of Suez as the boundary between Asia and Africa, but most writers continued to consider it the Nile or the western border of Egypt (Gibbon). In the Middle Ages, the world was usually portrayed on T and O maps , with the T representing the waters dividing

3403-557: The Coprates rise. These boundaries enclose a broad high plateau and shallow interior basin that include Syria , Sinai, and Solis Plana (see list of plains on Mars ). The highest plateau elevations on the Tharsis bulge occur in northern Syria Planum , western Noctis Labyrinthus , and the plains east of Arsia Mons . Between the northern and southern portions of the Tharsis bulge lies a relatively narrow, northeast-trending region that may be considered Tharsis proper or central Tharsis. It

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3486-498: The Earth, the text claims that the Earth has seven continents and Lord Vishnu Measured the entire universe from his first foot from the land of Earth which has 7 continents. ato devā avantu no yato viṣṇurvicakrame | pṛthivyāḥ saptadhāmabhiḥ || idaṃ viṣṇurvi cakrame tredhā ni dadhe padam | samūḷhamasya pāṃsure || trīṇi padā vi cakrame viṣṇurghopā adābhyaḥ | ato dharmāṇi dhārayan || The Gods be gracious unto us even from

3569-619: The Greek word ἤπειρος , meaning "landmass, terra firma", the proper name of Epirus and later especially used for Asia (i.e. Asia Minor ). The first distinction between continents was made by ancient Greek mariners who gave the names Europe and Asia to the lands on either side of the waterways of the Aegean Sea , the Dardanelles strait, the Sea of Marmara , the Bosporus strait and

3652-409: The Tharsis region may be a single giant volcano. This is the thesis of geologists Andrea Borgia and John Murray in a Geological Society of America special paper published in 2010. The key to understanding how a vast igneous province like Tharsis can itself be a volcano is to re-think the notion of volcano from one of simple conical edifice to that of an environment or " holistic " system. According to

3735-480: The center of the bulge that stretches halfway across the planet. Geologic evidence, such as the flow direction of ancient valley networks around Tharsis, indicates that the bulge was largely in place by the end of the Noachian Period, some 3.7 billion years ago. Although the bulge itself is ancient, volcanic eruptions in the region continued throughout Martian history and probably played a significant role in

3818-477: The concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered a part of Europe, and Madagascar a part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental landmass with other islands in the Pacific Ocean into Oceania , which is usually considered a region rather than

3901-509: The conventional view in geology, volcanoes passively build up from lava and ash erupted above fissures or rifts in the crust. The rifts are produced through regional tectonic forces operating in the crust and underlying mantle. Traditionally, the volcano and its magmatic plumbing have been studied by volcanologists and igneous petrologists , while the tectonic features are the subject for structural geologists and geophysicists . However, recent work on large terrestrial volcanoes indicates that

3984-418: The crust horizontally as large tabular bodies, such as sills and laccoliths , that can cause a general doming and fracturing of the overlying crust. Thus, the bulk of Tharsis is probably made of these intrusive complexes in addition to lava flows at the surface. One key question about the nature of Tharsis has been whether the bulge is mainly the product of active crustal uplifting from buoyancy provided by

4067-465: The distinction between volcanic and tectonic processes is quite blurry, with significant interplay between the two. Many volcanoes produce deformational structures as they grow. The flanks of volcanoes commonly exhibit shallow gravity slumps, faults and associated folds . Large volcanoes grow not only by adding erupted material to their flanks, but also by spreading laterally at their bases, particularly if they rest on weak or ductile materials. As

4150-405: The division into three of what is really a single landmass, a debate that continues nearly two and a half millennia later. Herodotus believed Europe to be larger (at least in width) than the other two continents: I wonder, then, at those who have mapped out and divided the world into Libya, Asia, and Europe; for the difference between them is great, seeing that in length Europe stretches along both

4233-580: The earth'. A. Glucklich adds that 'In the Matsya Purana , for instance, there is a seven-part map of the world ... [it has] one centre, where an immense mountain – Mount Meru (or Maha Meru, Great Meru) – stands ... The continents encircle the mountain in seven concentric circles ... It seems clear that the Himalayas were the approximate location of Mt. Meru and the text is clear that the earth has seven continents. Echus Chasma Echus Chasma

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4316-689: The east. The bulge is slightly elongated in the north-south direction, running from the northern flanks of Alba Mons (about 55°N) to the southern base of the Thaumasia highlands (about 43°S). Depending on how the region is defined, Tharsis covers 10–30 million square kilometres (4–10 million square miles), or up to 25% of Mars’ surface area. The greater Tharsis region consists of several geologically distinct subprovinces with different ages and volcano-tectonic histories. The subdivisions given here are informal and may rise all or parts of other formally named physiographic features and regions. Tharsis

4399-508: The equator between 4.2 and 3.9 billion years ago. Such shifts, known as true polar wander , would have caused dramatic climate changes over vast areas of the planet. A more recent study reported in Nature agreed with the polar wander, but the authors thought the eruptions at Tharsis happened at a slightly different time. Spacecraft exploration over the last two decades has shown that volcanoes on other planets can take many unexpected forms. Over

4482-497: The ground. Volcanoes would have released gases that thickened the atmosphere for a temporary period, trapping more sunlight and making it warm enough for liquid water to exist. In this study, channels were discovered that connected lake basins near Ares Vallis . When one lake filled up, its waters overflowed the banks and carved the channels to a lower area where another lake would form. These dry lakes would be targets to look for evidence ( biosignatures ) of past life. Echus Chasma

4565-403: The magma produced in a large igneous province erupts at the surface as lava. Much of it stalls in the crust where it slowly cools and solidifies to produce large intrusive complexes ( plutons ). If the magma migrates through vertical fractures it produces swarms of dikes that may be expressed at the surface as long, linear cracks ( fossae ) and crater chains (catenae). Magma may also intrude

4648-412: The main topographic bulge, but is related to the volcanic processes that formed Tharsis. Olympus Mons is the youngest of the large Tharsis volcanoes. Tharsis is commonly called a volcano-tectonic province, meaning that it is the product of volcanism and associated tectonic processes that have caused extensive crustal deformation. According to the standard view, Tharsis overlies a hot spot , similar to

4731-490: The mainlands of their continents. At that time, there were just three discrete landmasses in the world: Africa - Eurasia - America , Antarctica , and Australia - New Guinea ( Sahul ). There are several ways of distinguishing the continents: In the English-speaking countries, geographers often use the term Oceania to denote a geographical region which includes most of the island countries and territories in

4814-429: The map and Asia on the right end. In the accompanying book Cosmographiae Introductio , Waldseemüller noted that the earth is divided into four parts, Europe, Asia, Africa, and the fourth part, which he named "America" after Amerigo Vespucci's first name. On the map, the word "America" was placed on part of South America. The Sanskrit text Rig Veda often dated 1500 BCE has the earliest mention of seven continents in

4897-549: The mid-17th century, Peter Heylin wrote in his Cosmographie that "A Continent is a great quantity of Land, not separated by any Sea from the rest of the World, as the whole Continent of Europe, Asia, Africa." In 1727, Ephraim Chambers wrote in his Cyclopædia, "The world is ordinarily divided into two grand continents: the Old and the New ." And in his 1752 atlas, Emanuel Bowen defined

4980-543: The most beauteous portion of the earth. Indeed, many persons have, not without reason, considered it, not as a third part only of the earth, but as equal to all the rest, looking upon the whole of our globe as divided into two parts only, by a line drawn from the river Tanais to the Straits of Gades. Following the fall of the Western Roman Empire , the culture that developed in its place , linked to Latin and

5063-629: The northern rise consists of a broad topographic ridge that corresponds to the highly fractured terrain of Ceraunius Fossae . The ridge is oriented north-south and forms part of the Noachian-aged basement on which Alba Mons sits. Also located in the northern rise are lava flows of the Ceraunius Fossae Formation, which are somewhat older than the Amazonian-aged flows that make up much of the central Tharsis region to

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5146-475: The ocean primarily in New Zealand and New Caledonia , and the almost completely submerged Kerguelen Plateau 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,

5229-517: The one thought to underlie the island of Hawaii . The hot spot is caused by one or more massive columns of hot, low-density material (a superplume ) rising through the mantle. The hot spot produces voluminous quantities of magma in the lower crust that is released to the surface as highly fluid, basaltic lava . Because Mars lacks plate tectonics , the lava is able to build up in one region for billions of years to produce enormous volcanic constructs. On Earth (and presumably Mars as well), not all of

5312-455: The others together, and it appears to me to be wider beyond all comparison. Eratosthenes , in the 3rd century BCE, noted that some geographers divided the continents by rivers (the Nile and the Don), thus considering them "islands". Others divided the continents by isthmuses , calling the continents "peninsulas". These latter geographers set the border between Europe and Asia at the isthmus between

5395-479: The place whence Vishnu strode Through the seven regions of the earth! Through all this world strode Vishnu; thrice his foot he planted, and the whole Was gathered in his footstep's dust. Vishnu, the Guardian, he whom none deceiveth , made three steps; thenceforth Establishing his high decrees. In regard to the above-quoted verses, it is commonly accepted that there are Seven Continents or 'regions of

5478-418: The planet, after the global dichotomy . Tharsis has no formally defined boundaries, so precise dimensions for the region are difficult to give. In general, the bulge is about 5,000 kilometres (3,100 mi) across and up to 7 kilometres (4.3 mi) high (excluding the volcanoes, which have much higher elevations). It roughly extends from Amazonis Planitia (215°E) in the west to Chryse Planitia (300°E) in

5561-558: The present continents. Certain parts of continents are recognized as subcontinents, especially the large peninsulas separated from the main continental landmass by geographical features. The most widely recognized example is the Indian subcontinent . The Arabian Peninsula , Southern Africa , the Southern Cone of South America, and Alaska in North America might be considered further examples. In many of these cases,

5644-481: The production of the planet's atmosphere and the weathering of rocks on the planet's surface. By one estimate, the Tharsis bulge contains around 300 million km of igneous material. Assuming the magma that formed Tharsis contained carbon dioxide (CO 2 ) and water vapor in percentages comparable to that observed in Hawaiian basaltic lava, then the total amount of gases released from Tharsis magmas could have produced

5727-462: The same time period, geologists were discovering that volcanoes on Earth are more structurally complex and dynamic than previously thought. Recent work has attempted to refine the definition of a volcano to incorporate geologic features of widely different shapes, sizes, and compositions throughout the Solar System. One surprising and controversial conclusion from this synthesis of ideas is that

5810-552: The seven-continent model. However, they may also be viewed as a single continent known as America . This viewpoint was common in the United States until World War II , and remains prevalent in some Asian six-continent models. The single American continent model remains a common view in European countries like France, Greece, Hungary, Italy, Malta, Portugal, Spain, Latin American countries and some Asian countries. The criterion of

5893-415: The shelf ( continental islands ), 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. Taken to its limit, this view could support

5976-567: The south. The larger southern portion of Tharsis (pictured right) lies on old cratered highland terrain. Its western boundary is roughly defined by the high lava plains of Daedalia Planum , which slope gently to the southwest into the Memnonia and Terra Sirenum regions. To the east, the southern Tharsis bulge consists of the Thaumasia Plateau , an extensive stretch of volcanic plains about 3,000 km wide. The Thaumasia Plateau

6059-492: The spreading has produced a rift through the summit of the rise and a system of radial tear faults that connect the rift to a basal compression belt. The tear-fault system on Tharsis is represented by the radial fossae , of which Valles Marineris is the largest example. The thrust front is visible as the Thaumasia Highlands. Unlike on Earth, where the rifting of plates produces a corresponding subduction zone ,

6142-434: The summit rift to the peripheral thrust front. The volcano's peak contains an array of steep summit cones, which are frequently active. The entire edifice is also peppered with a large number of small parasitic cones. The structural similarities of Mount Etna to Tharsis Rise are striking, even though the latter is some 200 times larger. In Borgia and Murray's view, Tharsis resembles a very large spreading volcano. As with Etna,

6225-597: The thick lithosphere of Mars is unable to descend into the mantle. Instead, the compressed zone is scrunched up and sheared laterally into mountain ranges, in a process called obduction . To complete the analogy, the huge Olympus Mons and the Tharsis Montes are merely summit cones or parasitic cones on a much larger volcanic edifice. Continent A continent is any of several large geographical regions. Continents are generally identified by convention rather than any strict criteria. A continent could be

6308-651: The three continents. By the middle of the 18th century, "the fashion of dividing Asia and Africa at the Nile, or at the Great Catabathmus [the boundary between Egypt and Libya ] farther west, had even then scarcely passed away". Christopher Columbus sailed across the Atlantic Ocean to the Caribbean in 1492, sparking a period of European exploration of the Americas . But despite four voyages to

6391-431: The underlying mantle plume or whether it is merely a large, static mass of igneous material supported by the underlying lithosphere . Theoretical analysis of gravity data and the pattern of faults surrounding Tharsis suggest the latter is more likely. The enormous sagging weight of Tharsis has generated tremendous stresses in the crust, producing a broad trough around the region and an array of radial fractures emanating from

6474-479: The view of Europe as a separate continent continues in almost all categorizations. If continents are defined strictly as discrete landmasses, embracing all the contiguous land of a body, then Africa, Asia, and Europe form a single continent which may be referred to as Afro-Eurasia . Combined with the consolidation of the Americas, this would produce a four-continent model consisting of Afro-Eurasia, America, Antarctica, and Australia. When sea levels were lower during

6557-471: The view that there are only three continents: Antarctica, Australia-New Guinea, and a single mega-continent which joins Afro-Eurasia and America via the contiguous continental shelf in and around the Bering Sea. The vast size of the latter compared to the first two might even lead some to say it is the only continent, the others being more comparable to Greenland or New Zealand. As a cultural construct ,

6640-438: Was known to extend, confirming that this was a land of continental proportions. On return to Europe, an account of the voyage, called Mundus Novus ("New World"), was published under Vespucci's name in 1502 or 1503, although it seems that it had additions or alterations by another writer. Regardless of who penned the words, Mundus Novus credited Vespucci with saying, "I have discovered a continent in those southern regions that

6723-468: Was thought that water frost on Mars was impossible. The total mass of the Tharsis bulge is approximately 10 kg, about the same as the dwarf planet Ceres . Tharsis is so large and massive that it has likely affected the planet's moment of inertia , possibly causing a change in the orientation of the planet's crust with respect to its rotational axis over time. According to one recent study, Tharsis originally formed at about 50°N latitude and migrated toward

6806-417: Was used in translating Greek and Latin writings about the three "parts" of the world, although in the original languages no word of exactly the same meaning as continent was used. While continent was used on the one hand for relatively small areas of continuous land, on the other hand geographers again raised Herodotus's query about why a single large landmass should be divided into separate continents. In

6889-685: Was viewed as running from the Black Sea through Kerch Strait , the Sea of Azov and along the Don River (known then as the Tanais ) in Russia . The boundary between Asia and Africa was generally taken to be the Nile River. Herodotus in the 5th century BCE objected to the whole of Egypt being split between Asia and Africa ("Libya") and took the boundary to lie along the western border of Egypt, regarding Egypt as part of Asia. He also questioned

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