Australia the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It is the world's sixth-largest country by total area; the neighbouring countries are Papua New Guinea and East Timor to the north. The population of 25 million is urbanised and concentrated on the eastern seaboard. Australia's capital is Canberra, its largest city is Sydney; the country's other major metropolitan areas are Melbourne, Brisbane and Adelaide. Australia was inhabited by indigenous Australians for about 60,000 years before the first British settlement in the late 18th century, it is documented. After the European exploration of the continent by Dutch explorers in 1606, who named it New Holland, Australia's eastern half was claimed by Great Britain in 1770 and settled through penal transportation to the colony of New South Wales from 26 January 1788, a date which became Australia's national day; the population grew in subsequent decades, by the 1850s most of the continent had been explored and an additional five self-governing crown colonies established.
On 1 January 1901, the six colonies federated. Australia has since maintained a stable liberal democratic political system that functions as a federal parliamentary constitutional monarchy, comprising six states and ten territories. Being the oldest and driest inhabited continent, with the least fertile soils, Australia has a landmass of 7,617,930 square kilometres. A megadiverse country, its size gives it a wide variety of landscapes, with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east. A gold rush began in Australia in the early 1850s, its population density, 2.8 inhabitants per square kilometre, remains among the lowest in the world. Australia generates its income from various sources including mining-related exports, telecommunications and manufacturing. Indigenous Australian rock art is the oldest and richest in the world, dating as far back as 60,000 years and spread across hundreds of thousands of sites. Australia is a developed country, with the world's 14th-largest economy.
It has a high-income economy, with the world's tenth-highest per capita income. It is a regional power, has the world's 13th-highest military expenditure. Australia has the world's ninth-largest immigrant population, with immigrants accounting for 26% of the population. Having the third-highest human development index and the eighth-highest ranked democracy globally, the country ranks in quality of life, education, economic freedom, civil liberties and political rights, with all its major cities faring well in global comparative livability surveys. Australia is a member of the United Nations, G20, Commonwealth of Nations, ANZUS, Organisation for Economic Co-operation and Development, World Trade Organization, Asia-Pacific Economic Cooperation, Pacific Islands Forum and the ASEAN Plus Six mechanism; the name Australia is derived from the Latin Terra Australis, a name used for a hypothetical continent in the Southern Hemisphere since ancient times. When Europeans first began visiting and mapping Australia in the 17th century, the name Terra Australis was applied to the new territories.
Until the early 19th century, Australia was best known as "New Holland", a name first applied by the Dutch explorer Abel Tasman in 1644 and subsequently anglicised. Terra Australis still saw occasional usage, such as in scientific texts; the name Australia was popularised by the explorer Matthew Flinders, who said it was "more agreeable to the ear, an assimilation to the names of the other great portions of the earth". The first time that Australia appears to have been used was in April 1817, when Governor Lachlan Macquarie acknowledged the receipt of Flinders' charts of Australia from Lord Bathurst. In December 1817, Macquarie recommended to the Colonial Office. In 1824, the Admiralty agreed that the continent should be known by that name; the first official published use of the new name came with the publication in 1830 of The Australia Directory by the Hydrographic Office. Colloquial names for Australia include "Oz" and "the Land Down Under". Other epithets include "the Great Southern Land", "the Lucky Country", "the Sunburnt Country", "the Wide Brown Land".
The latter two both derive from Dorothea Mackellar's 1908 poem "My Country". Human habitation of the Australian continent is estimated to have begun around 65,000 to 70,000 years ago, with the migration of people by land bridges and short sea-crossings from what is now Southeast Asia; these first inhabitants were the ancestors of modern Indigenous Australians. Aboriginal Australian culture is one of the oldest continual civilisations on earth. At the time of first European contact, most Indigenous Australians were hunter-gatherers with complex economies and societies. Recent archaeological finds suggest. Indigenous Australians have an oral culture with spiritual values based on reverence for the land and a belief in the Dreamtime; the Torres Strait Islanders, ethnically Melanesian, obtained their livelihood from seasonal horticulture and the resources of their reefs and seas. The northern coasts and waters of Australia were visited s
Sclerophyll is a type of vegetation that has hard leaves, short internodes and leaf orientation parallel or oblique to direct sunlight. The word comes from the Greek sklēros and phyllon. Sclerophyllous plants occur in many parts of the world, but are most typical in the chaparral biomes, they are prominent throughout western and southern parts of Australia, in the Mediterranean forests and scrub biomes that cover the Mediterranean Basin, Californian chaparral and woodlands, Chilean Matorral, the Cape Province of South Africa. The sclerophyll leaves have three leaf stress traits used to cope with hot and dry summers: the leaves are hard due to lignin, which prevents wilting and allows plants to grow; the term was coined by A. F. W. Schimper in 1898 as a synonym of xeromorph, but was changed. Most areas of the Australian continent able to support woody plants are occupied by sclerophyll communities as forests, savannas or heathlands. Common plants include the Proteaceae, tea-trees, acacias and eucalypts.
The most common sclerophyll communities in Australia are savannas dominated by grasses with an overstorey of eucalypts and acacias. Acacia shrublands cover extensive areas. All the dominant overstorey acacia species and a majority of the understorey acacias have a scleromorphic adaptation in which the leaves have been reduced to phyllodes consisting of the petiole. Many plants of the sclerophyllous woodlands and shrublands produce leaves unpalatable to herbivores by the inclusion of toxic and indigestible compounds which assure survival of these long-lived leaves; this trait is noticeable in the eucalypt and Melaleuca species which possess oil glands within their leaves that produce a pungent volatile oil that makes them unpalatable to most browsers. These traits make the majority of woody plants in these woodlands unpalatable to domestic livestock, it is therefore important from a grazing perspective that these woodlands support a more or less continuous layer of herbaceous ground cover dominated by grasses.
Sclerophyll forests cover a much smaller area of the continent, being restricted to high rainfall locations. They have a eucalyptus overstory with the understory being hard-leaved. Dry sclerophyll forests are the most common forest type on the continent, although it may seem barren dry sclerophyll forest is diverse. For example, a study of sclerophyll vegetation in Seal Creek, found 138 species. Less extensive are wet sclerophyll forests, they have a taller eucalyptus overstory than dry sclerophyll forests, 30 metres or more, a soft-leaved dense understory. They require ample rainfall — at least 1000mm. Sclerophyllous plants are anything but newcomers. By the time of European settlement, sclerophyll forest accounted for the vast bulk of the forested areas. Most of the wooded parts of present-day Australia have become sclerophyll dominated as a result of the extreme age of the continent combined with Aboriginal fire use. Deep weathering of the crust over many millions of years leached chemicals out of the rock, leaving Australian soils deficient in nutrients phosphorus.
Such nutrient deficient soils support non-sclerophyllous plant communities elsewhere in the world and did so over most of Australia prior to European arrival. However such deficient soils cannot support the nutrient losses associated with frequent fires and are replaced with sclerophyllous species under traditional Aboriginal burning regimens. With the cessation of traditional burning non-sclerophyllous species have re-colonised sclerophyll habitat in many parts of Australia; the presence of toxic compounds combined with a high carbon: nitrogen ratio make the leaves and branches of scleromorphic species long-lived in the litter, can lead to a large build-up of litter in woodlands. The toxic compounds of many species, notably Eucalyptus species, are volatile and flammable and the presence of large amounts of flammable litter, coupled with an herbaceous understorey, encourages fire. All the Australian sclerophyllous communities are liable to be burnt with varying frequencies and many of the woody plants of these woodlands have developed adaptations to survive and minimise the effects of fire.
Sclerophyllous plants resist dry conditions well, making them successful in areas of seasonally variable rainfall. In Australia, they evolved in response to the low level of phosphorus in the soil — indeed, many native Australian plants cannot tolerate higher levels of phosphorus and will die if fertilised incorrectly; the leaves are hard due to lignin, which prevents wilting and allows plants to grow when there isn't enough phosphorus for substantial new cell growth. Mediterranean forests and scrub Chaparral California chaparral and woodlands Chilean Matorral Tropical and subtropical coniferous forest Fynbos Garrigue Kwongan Mallee Woodlands and Shrublands Maquis shrubland Matorral
Government of Queensland
The Government of Queensland referred to as the Queensland Government, is the Australian state democratic administrative authority of Queensland. The Government of Queensland, a parliamentary constitutional monarchy, was formed in 1859 as prescribed in its Constitution, as amended from time to time. Since the Federation of Australia in 1901, Queensland has been a state of the Commonwealth of Australia, the Constitution of Australia regulates its relationship with the Commonwealth. Under the Australian Constitution, Queensland ceded legislative and judicial supremacy to the Commonwealth, but retained powers in all matters not in conflict with the Commonwealth. Key state government offices are located at 1 William Street in the Brisbane central business district; the Government of Queensland operates under the Westminster system, a form of parliamentary government based on the model of the United Kingdom. The Governor of Queensland, as the representative of Elizabeth II, Queen of Australia, holds nominal power, although in practice only performs ceremonial duties.
The Parliament of Queensland holds legislative power, while executive power lies with the Premier and Cabinet, judicial power is exercised by a system of courts and tribunals. The Parliament of Queensland is the state's legislature, it consists of Her Majesty The Queen, a single chamber. Queensland is the only Australian state with a unicameral parliament after a second chamber, the Legislative Council, was abolished in 1922; the Legislative Assembly has 93 members. Elections for the Legislative Assembly are held every four years; the Cabinet of Queensland is the government's chief policy-making organ, consists of the Premier and all ministers. The Queensland Government delivers services, determines policy and regulations, including legal interpretation, by a number of agencies grouped under areas of portfolio responsibility; each portfolio is led by a government minister, a member of the Parliament. As of April 2016 there were nineteen lead agencies, called government departments, that consist of: Department of the Premier and Cabinet Department of Aboriginal and Torres Strait Islander Partnerships Department of Agriculture and Fisheries Department of Communities, Child Safety and Disability Services Department of Education and Training Department of Energy and Water Supply Department of Environment and Heritage Protection Queensland Health Department of Housing and Public Works Department of Infrastructure, Local Government and Planning Department of Justice and Attorney-General Department of National Parks and Racing Department of Natural Resources and Mines Queensland Police Service and Queensland Fire and Emergency Services Department of Science, Information Technology and Innovation Department of State Development Department of Transport and Main Roads Queensland Treasury Department of Tourism, Major Events, Small Business and the Commonwealth GamesA range of other agencies support the functions of these departments.
The judiciary of Queensland consists of the Magistrates Court, the District Court, the Supreme Court, as well as a number of smaller courts and tribunals. The Chief Justice of Queensland is the state's most senior judicial officer; the Magistrates Court is the lowest tier of the judicial hierarchy of Queensland. The court's criminal jurisdiction covers summary offences, indictable offences which may be heard summarily, but all criminal proceedings in Queensland begin in the Magistrates Court if they are not within this jurisdiction. For charges beyond its jurisdiction, the court conducts committal hearings in which the presiding magistrate decides, based on the strength of the evidence, whether to refer the matter to a higher court or dismiss it; the court's civil jurisdiction covers matters in which the amount in dispute is less than or equal to $150,000. Appeals against decisions by the Magistrates Court are heard by the District Court; the District Court is the middle tier of the judicial hierarchy of Queensland.
The court has jurisdiction to hear all appeals from decisions made in the Magistrates Court. Its criminal jurisdiction covers serious indictable offences; the court's civil jurisdiction covers matters in which the amount in dispute is more than $150,000 but less than or equal to $750,000. Appeals against decisions by the District Court are heard by the Court of Appeal, a division of the Supreme Court; the Supreme Court is the highest tier of the judicial hierarchy Queensland. The court has two divisions; the Trial Division's jurisdiction covers serious criminal offences, civil matters involving claims of more than $750,000. The Court of Appeal's jurisdiction allows it to hear cases on appeal from the Trial Division, the District Court, a number of other judicial tribunals in Queensland. Appeals against decisions by the Court of Appeal are heard by the High Court of Australia. There are several factors; the legislature has no upper house. For a large portion of its history, the state was under a gerrymander that favoured rural electorates.
This, combined with the decentralised nature of Queensland, meant that politics has been dominated by regional interests. Queensland, along with New South Wales operated a balloting system known as Optional Preferential Voting for state elections; this is different from the predominant Australian electoral system, the instant-runoff voting system, in practice is closer to a first past the post ballot, which some say is to the
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
A cultural landscape, as defined by the World Heritage Committee, is the "cultural properties represent the combined works of nature and of man". "a landscape designed and created intentionally by man" an "organically evolved landscape" which may be a "relict landscape" or a "continuing landscape" an "associative cultural landscape" which may be valued because of the "religious, artistic or cultural associations of the natural element." The concept of'cultural landscapes' can be found in the European tradition of landscape painting. From the 16th century onwards, many European artists painted landscapes in favor of people, diminishing the people in their paintings to figures subsumed within broader, regionally specific landscapes; the word "landscape" itself combines "land" with a verb of Germanic origin, "scapjan/schaffen" to mean "shaped lands". Lands were considered shaped by natural forces, the unique details of such landshaffen became themselves the subject of'landscape' paintings; the geographer Otto Schlüter is credited with having first formally used “cultural landscape” as an academic term in the early 20th century.
In 1908, Schlüter argued that by defining geography as a Landschaftskunde this would give geography a logical subject matter shared by no other discipline. He defined two forms of landscape: the Urlandschaft or landscape that existed before major human induced changes and the Kulturlandschaft a landscape created by human culture; the major task of geography was to trace the changes in these two landscapes. It was Carl O. Sauer, a human geographer, the most influential in promoting and developing the idea of cultural landscapes. Sauer was determined to stress the agency of culture as a force in shaping the visible features of the Earth’s surface in delimited areas. Within his definition, the physical environment retains a central significance, as the medium with and through which human cultures act, his classic definition of a'cultural landscape' reads as follows: "The cultural landscape is fashioned from a natural landscape by a cultural group. Culture is the agent, the natural area is the medium, the cultural landscape is the result" Since Schlüter's first formal use of the term, Sauer's effective promotion of the idea, the concept of'cultural landscapes has been variously used, debated and refined within academia.
In the 1950s, for instance, J. B. Jackson and his publication'Landscape' influenced a generation of American scholars, including architectural historians Denise Scott Brown, Gwendolyn Wright. By 1992, the World Heritage Committee elected to convene a meeting of the'specialists' to advise and assist redraft the Committee's Operational Guidelines to include'cultural landscapes' as an option for heritage listing properties that were neither purely natural nor purely cultural in form; the World Heritage Committee's adoption and use of the concept of'cultural landscapes' has seen multiple specialists around the world, many nations identifying'cultural landscapes', assessing'cultural landscapes', heritage listing'cultural landscapes', managing'cultural landscapes', making'cultural landscapes' known and visible to the world, with practical ramifications and challenges. A 2006 academic review of the combined efforts of the World Heritage Committee, multiple specialists around the world, nations to apply the concept of'cultural landscapes', observed and concluded that: "Although the concept of landscape has been unhooked for some time from its original art associations...
There is still a dominant view of landscapes as an inscribed surface, akin to a map or a text, from which cultural meaning and social forms can be read." Within academia, any system of interaction between human activity and natural habitat is regarded as a cultural landscape. In a sense this understanding is broader than the definition applied within UNESCO, including, as it does the whole of the world's occupied surface, plus all the uses, interactions, beliefs and traditions of people living within cultural landscapes. Following on this, geographer Xoán Paredes defines cultural landscape as: "... the environment modified by the human being in the course of time, the long-term combination between anthropic action on this environment and the physical constraints limiting or conditioning human activity. It is a geographical area – including natural and cultural resources – associated to historical evolution, which gives way to a recognizable landscape for a particular human group, up to the point of being identifiable as such by others."
Some Universities now offer specialist degrees in the study of cultural landscapes, for instance, the Universities of Naples, St.-Étienne, Stuttgart who offer a Master of Cultural Landscapes diploma. The World Heritage Committee has identified and listed a number of areas or properties as cultural landscapes of universal value to humankind, including the following: "In 1993 Tongariro National Park, became the first property to be inscribed on the World Heritage List under the revised criteria describing cultural landscapes; the mountains at the heart of the park have cultural and religious significance for the Maori people and symbolize the spiritual links between this community and its environment. The park has active and extinct volcanoes, a diverse range of ecosystems and some spectacular landscapes." "This park called Uluru National Park, features spectacular geological formations that dominate the vast red sandy plain of central Australia. Uluru, an immense monolith, Kata Tjuta, the rock domes l
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock is subjected to pressure, causing profound physical or chemical change; the protolith may be igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface, they are classified by chemical and mineral assemblage. They may be formed by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it, they can form from tectonic processes such as continental collisions, which cause horizontal pressure and distortion. They are formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior; the study of metamorphic rocks provides information about the temperatures and pressures that occur at great depths within the Earth's crust. Some examples of metamorphic rocks are gneiss, marble and quartzite.
Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, staurolite and some garnet. Other minerals, such as olivines, amphiboles, micas and quartz, may be found in metamorphic rocks, but are not the result of the process of metamorphism; these minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they formed; the change in the particle size of the rock during the process of metamorphism is called recrystallization. For instance, the small calcite crystals in the sedimentary rock limestone and chalk change into larger crystals in the metamorphic rock marble. Both high temperatures and pressures contribute to recrystallization.
High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact. The layering within metamorphic rocks is called foliation, it occurs when a rock is being shortened along one axis during recrystallization; this causes the platy or elongated crystals of minerals, such as mica and chlorite, to become rotated such that their long axes are perpendicular to the orientation of shortening. This results in a banded, or foliated rock, with the bands showing the colors of the minerals that formed them. Textures are separated into non-foliated categories. Foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage. For example, slate is a foliated metamorphic rock. Non-foliated rock does not have planar patterns of strain. Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated.
Where a rock has been subject to differential stress, the type of foliation that develops depends on the metamorphic grade. For instance, starting with a mudstone, the following sequence develops with increasing temperature: slate is a fine-grained, foliated metamorphic rock, characteristic of low grade metamorphism, while phyllite is fine-grained and found in areas of low grade metamorphism, schist is medium to coarse-grained and found in areas of medium grade metamorphism, gneiss coarse to coarse-grained, found in areas of high-grade metamorphism. Marble is not foliated, which allows its use as a material for sculpture and architecture. Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting. In the process atoms are exchanged between the minerals, thus new minerals are formed. Many complex high-temperature reactions may take place, each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism.
Metasomatism is the drastic change in the bulk chemical composition of a rock that occurs during the processes of metamorphism. It is due to the introduction of chemicals from other surrounding rocks. Water may transport these chemicals over great distances; because of the role played by water, metamorphic rocks contain many elements absent from the original rock, lack some that were present. Still, the introduction of new chemicals is not necessary for recrystallization to occur. Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock; the changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole. Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed country rock some distance away.
The formation of important ore minerals may o
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more a half-graben with normal faulting and rift-flank uplifts on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake; the axis of the rift area may contain volcanic rocks, active volcanism is a part of many, but not all active rift systems. Major rifts occur along the central axis of most mid-ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates. Failed rifts are the result of continental rifting; the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third fails, becoming an aulacogen. Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts.
The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, in some cases the polarity, of the main rift bounding fault changes from segment to segment. Segment boundaries have a more complex structure and cross the rift axis at a high angle; these segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity where the segments have opposite polarity. Accommodation zones may be located. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift.
Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are about 70 km wide. Contrary to what was thought, elevated passive continental margins such as the Brazilian Highlands, the Scandinavian Mountains and India's Western Ghats, are not rift shoulders. At the onset of rifting, the upper part of the lithosphere starts to extend on a series of unconnected normal faults, leading to the development of isolated basins. In subaerial rifts, drainage at this stage is internal, with no element of through drainage; as the rift evolves, some of the individual fault segments grow becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults; the longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage influencing drainage and sedimentation in the rift basins. During the climax of lithospheric rifting, as the crust is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised.
At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting causing extreme metamorphism at high thermal gradients of greater than 30 °C; the metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to a kind of orogeneses in extensional settings, referred as to rifting orogeny. Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence; the amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor, but is affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water.
The simple'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence. This has been replaced by the'flexural cantilever model', which takes into account the geometry of the rift faults and the flexural isostasy of the upper part of the crust; some rifts show a complex and prolonged history with several distinct phases. The North Sea rift shows evidence of several separate rift phases from the Permian through to the Earliest Cretaceous, a period of over 100 million years. Many rifts are the sites of at least minor magmatic activity in the early stages of rifting. Alkali basalts and bimodal volcanism are common products of rift-related magmatism. Recent studies indicate that post-collisional granites in collisional orogens are the product of rifting magmatism at converged plate margins; the sedimentary rocks associated with continental rifts host important deposits of both minerals and hydrocarbons.
SedEx mineral deposits are found in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated