Metamorphism is the change of minerals or geologic texture in pre-existing rocks, without the protolith melting into liquid magma. The change occurs due to heat and the introduction of chemically active fluids; the chemical components and crystal structures of the minerals making up the rock may change though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism occurs between diagenesis, melting; the geologists who study metamorphism are known as "metamorphic petrologists." To determine the processes underlying metamorphism, they rely on statistical mechanics and experimental petrology. Three types of metamorphism exist: contact and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism. Metamorphic rocks can change without melting. Heat causes atomic bonds to break, the atoms move and form new bonds with other atoms, creating new minerals with different chemical components or crystalline structures, or enabling recrystallization.
When pressure is applied, somewhat flattened grains that orient in the same direction have a more stable configuration. The temperature lower limit on what is considered to be a metamorphic process is considered to be 100 – 200 °C; the upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is 700 – 900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not segregated from the refractory residue. Since the 1980s it has been recognized that rocks are dry enough and of a refractory enough composition to record without melting "ultra-high" metamorphic temperatures of 900 – 1100 °C; the metamorphic process has to be over pressure of at least 100 mega pascals but below 300 mega pascals, the depth of 100 mega pascals varies depending on what type of rock is applying pressure. Regional or Barrovian metamorphism covers large areas of continental crust associated with mountain ranges those associated with convergent tectonic plates or the roots of eroded mountains.
Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism; these orogenic mountains are eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone produce regional metamorphic effects, characterized by paired metamorphic belts; the techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane. Contact metamorphism occurs around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock; the area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole.
Contact metamorphic rocks are known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are fine-grained. Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact; the size of the aureole depends on the heat of the intrusion, its size, the temperature difference with the wall rocks. Dikes have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have thick and well-developed contact metamorphism; the metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is related to the metamorphic temperatures of pelitic or aluminosilicate rocks and the minerals they form; the metamorphic grades of aureoles are sillimanite hornfels, pyroxene hornfels. Magmatic fluids coming from the intrusive rock may take part in the metamorphic reactions. An extensive addition of magmatic fluids can modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism.
If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest. A special type of contact metamorphism, associated with fossil fuel fires, is known as pyrometamorphism. Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition; the difference in composition between an existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic, circulating ocean water. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas
Mid West (Western Australia)
The Mid West region is one of the nine regions of Western Australia. It is a sparsely populated region extending from the west coast of Western Australia, about 200 kilometres north and south of its administrative centre of Geraldton and inland to 450 kilometres east of Wiluna in the Gibson Desert, it has a total area of 472,336 square kilometres, a permanent population of about 52,000 people, more than half of those in Geraldton. The western portion of this region was known earlier as "The Murchison" based on the river of the same name, the named goldfield; the Mid West region has a diversified economy that varies with the climate. Near the coast, annual rainfall of between 400 and 500 millimetres allows intensive agriculture. Further inland, annual rainfall decreases to less than 250 millimetres, here the economy is dominated by mining of iron ore, gold and other mineral resources. Geraldton is an important hub for the tourism industry; the Mid West has the highest value fishing industry in Western Australia, with Geraldton the hub of the Western Rock Lobster industry.
Western Rock Lobster netted $234.5 million in revenue for WA in the 2012–13 financial year, making it Australia's most valuable single-species wild capture fishery. Gross Regional Product for the Mid West in 2013 was AU$6,000,000,000 On 25 August 2015 The Hon. Terry Redman, MLA, Minister for Regional Development, launched the "Mid West Regional Blueprint"; the Blueprint proposes strategies against five priority pillars to drive or reduce barriers to, regional growth and development in the Mid West. The Blueprint strategies are intended to focus on the region’s key strengths and the identification of regional opportunities, providing a guide for regional development to 2050. Due to its relative isolation from radio-frequency interference, The Mid West region was selected to host one of two primary radiotelescope locations of the Square Kilometre Array project; when operational in 2024, this AU$2bn project will be 50 times more sensitive than any existing radio interferometer instrument. The radiotelescope antennae and the Murchison Radio-astronomy Observatory are located near Boolardy, Western Australia 315 km northeast of Geraldton, with logistics and science support provided by CSIRO from their facilities in Geraldton.
High-capacity optical fibre cables connect the SKA telescope to the MRO support facility in Geraldton and to the Pawsley Supercomputing Centre in Perth, Western Australia. Located at Yatharagga, 40 km from Mingenew, the WA Space Centre is an $8,000,000 114ha satellite park owned and operated by Space Australia, a subsidiary company of the Swedish Space Corporation; the park is located in one of several Radio Quiet Zones in the region, making it an ideal location for radioastronomy. The SSC is the largest commercial operator of satellite tracking ground station facilities; the facilities include compounds operated by NASA, The European Space Agency, CSIRO, MOBLAS and VLBI, amongst others. The local government areas in the Mid West region are Carnamah, Chapman Valley, Cue, Greater Geraldton, Meekatharra, Morawa, Mount Magnet, Northampton, Sandstone, Three Springs and Yalgoo. Regions of Western Australia Mid West Development Commission Yamatji Marlpa Barna Baba Maaja Aboriginal Corporation, the Native Title Representative Body incorporating the Yamatji Land and Sea Council
A river delta is a landform that forms from deposition of sediment, carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, estuary, reservoir, or another river that cannot carry away the supplied sediment; the size and shape of a delta is controlled by the balance between watershed processes that supply sediment, receiving basin processes that redistribute and export that sediment. The size and location of the receiving basin plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers, they can impact drinking water supply. They are ecologically important, with different species' assemblages depending on their landscape position. River deltas form when a river carrying sediment reaches either a body of water, such as a lake, ocean, or reservoir, another river that cannot remove the sediment enough to stop delta formation, or an inland region where the water spreads out and deposits sediments.
The tidal currents cannot be too strong, as sediment would wash out into the water body faster than the river deposits it. The river must carry enough sediment to layer into deltas over time; the river's velocity decreases causing it to deposit the majority, if not all, of its load. This alluvium builds up to form the river delta; when the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment; as a result, sediment drops out of deposits. Over time, this single channel builds a deltaic lobe; as the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation. As the slope of the river channel decreases, it becomes unstable for two reasons. First, gravity makes the water flow in the most direct course down slope. If the river breaches its natural levees, it spills out into a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.
Second, as its slope gets lower, the amount of shear stress on the bed decreases, which results in deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. This makes it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope; when the channel does this, some of its flow remains in the abandoned channel. When these channel-switching events occur, a mature delta develops a distributary network. Another way these distributary networks form is from deposition of mouth bars; when this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth-bar, which splits the river into two distributary channels. A good example of the result of this process is the Wax Lake Delta. In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition.
This results in the smoothing of the planform shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan; the more the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position results in more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's-feet, are examples of rivers that do not avulse enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name and more approximate an ideal fan shape. Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the Mississippi, Amazon, Ganges and Yangtze discharging along passive continental margins; this phenomenon is due to three big factors: topography, basin area, basin elevation. Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate overtime to form large river deltas.
Topography along active margins tend to be steeper and less widespread, which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf. There are many other smaller factors that could explain why the majority of river deltas form along passive margins rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, volcanic activity resulting in delta formation to exist closer to the sediment source; when sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion, delta lobe switching, auto cyclicity. Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.
However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in de
Late Heavy Bombardment
The Late Heavy Bombardment is an event thought to have occurred 4.1 to 3.8 billion years ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. During this interval, a disproportionately large number of asteroids are theorized to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus and Mars; the Late Heavy Bombardment happened after the Earth and other rocky planets had formed and accreted most of their mass, but still quite early in Earth's history. Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses attempt to explain the apparent spike in the flux of impactors in the inner Solar System, but no consensus yet exists; the Nice model, popular among planetary scientists, postulates that the giant planets underwent orbital migration and in doing so, scattered objects in the asteroid and/or Kuiper belts into eccentric orbits, into the path of the terrestrial planets.
Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, that the apparent clustering of impact-melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin. They note that the rate of impact cratering could differ between the outer and inner zones of the Solar System; the main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are believed to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter; the Apollo 15, 16, 17 landing sites were chosen as a result of their proximity to the Imbrium and Serenitatis basins, respectively. The apparent clustering of ages of these impact melts, between about 3.8 and 4.1 Ga, led to postulation that the ages record an intense bombardment of the Moon. They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga.
If these impact melts were derived from these three basins not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds. At the time, the conclusion was considered controversial; as more data has become available from lunar meteorites, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites originated from the lunar far side, impact melts within these have been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga. Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga. Dating of howardite and diogenite meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga.
The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require high inclinations or the large eccentricities of asteroids on planet crossing orbits; such objects are rare in the current asteroid belt but the population would be increased by the sweeping of resonances due to giant planet migration. Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. If the history of decay of late heavy bombardment on Mercury followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins and Imbrium, all of the plains units are older than 3 billion years.
While the cataclysm hypothesis has gained in popularity among dynamicists who have identified possible causes for such a phenomenon, the cataclysm hypothesis is still controversial and based on debatable assumptions. Two criticisms are that the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, that the lack of impact melt rocks older than about 4.1 Ga is related to all such samples having been pulverized, or their ages being reset. The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin; the Imbrium impact basin is the youngest and largest of the multi-ring basins found on the central nearside of the Moon, quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites.
According to this alternative hypothesis, the cluster of impact melt ages near 3.9 Ga reflects material being collected from a single impact event and not several. Additional criticism argues that the age spike at 3.9 Ga identified in 40Ar/39Ar dating could be produced by an episodic early crust formation followed by partial 40Ar losses as the impact
Oldest dated rocks
The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently broken down by erosion or melted, are more than 4 billion years old, formed during the Hadean Eon of Earth's geological history. Such rocks are exposed on the Earth's surface in few places; some of the oldest surface rock can be found in the Canadian Shield, Africa and in a few other old regions around the world. The ages of these felsic rocks are between 2.5 and 3.8 billion years. The approximate ages have a margin of error of millions of years. In 1999, the oldest known rock on Earth was dated to 4.031 ±0.003 billion years, is part of the Acasta Gneiss of the Slave craton in northwestern Canada. Researchers at McGill University found a rock with a old model age for extraction from the mantle in the Nuvvuagittuq greenstone belt on the coast of Hudson Bay, in northern Quebec. Older than these rocks are crystals of the mineral zircon, which can survive the disaggregation of their parent rock and be found and dated in younger rock formations.
In January 2019, NASA scientists reported the discovery of the oldest known Earth rock – on the Moon. Apollo 14 astronauts returned several rocks from the Moon and scientists determined that a fragment from one of the rocks contained "a bit of Earth from about 4 billion years ago." The rock fragment contained quartz and zircon, all common on the Earth, but uncommon on the Moon. The oldest material of terrestrial origin, dated is a zircon mineral of 4.404 ±0.008 Ga enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the Narryer Gneiss Terrane of Western Australia. The 4.404 ±0.008 Ga zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga. This zircon is part of a population of zircons within the metamorphosed conglomerate, believed to have been deposited about 3.060 Ga, the age of the youngest detrital zircon in the rock. Recent developments in atom-probe tomography have led to a further constraint on the age of the oldest continental zircon, with the most recent age quoted as 4.374 ±0.006 Ga.
In January 2019, NASA scientists reported the discovery of the oldest known Earth rock – on the Moon. Apollo 14 astronauts returned several rocks from the Moon and scientists determined that a fragment from one of the rocks contained "a bit of Earth from about 4 billion years ago." The rock fragment contained quartz and zircon, all common on the Earth, but uncommon on the Moon. The oldest rock formation is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane, Nuvvuagittuq greenstone belt, or the Acasta Gneiss; the difficulty in assigning the title to one particular block of gneiss is that the gneisses are all deformed, the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dike. This map, it is thus premature to claim that any of these rocks, or indeed that of other formations of Hadean gneisses, is the oldest formations or rocks on Earth. The oldest cratons on Earth include the Kaapvaal Craton, the Western Gneiss Terrane of the Yilgarn Craton, the Pilbara Craton, portions of the Canadian Shield.
Parts of the poorly studied. The oldest dated rocks of the Baltic Shield are 3.5 Ga old. The Acasta Gneiss in the Canadian Shield in the Northwest Territories, Canada is composed of the Archaean igneous and gneissic cores of ancient mountain chains that have been exposed in a glacial peneplain. Analyses of zircons from a felsic orthogneiss with presumed granitic protolith returned an age of 4.031 ±0.003 Ga. On September 25, 2008, researchers from McGill University, Carnegie Institution for Science and UQAM announced that a rock formation, the Nuvvuagittuq greenstone belt, exposed on the eastern shore of Hudson Bay in northern Quebec had a Sm–Nd model age for extraction from the mantle of 4.28 billion years. However, it is argued that the actual age of formation of this rock, as opposed to the extraction of its magma from the mantle, is closer to 3.8 billion years, according to Simon Wilde of the Institute for Geoscience Research in Australia. The zircons from the Western Australian Jack Hills returned an age of 4.404 billion years, interpreted to be the age of crystallization.
These zircons show another interesting feature. The importance and accuracy of these interpretations is the subject of scientific debate, it may be that the oxygen isotopes and other compositional features record more recent hydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization. In a paper published in the journal Earth and Planetary Science Letters, a team of scientists suggest that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate. Using an ion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in Western Australia, comparing these chemical fingerprints to lithium c
Murchison River (Western Australia)
The Murchison River is the second longest river in Western Australia. It flows for about 820 km from the southern edge of the Robinson Ranges to the Indian Ocean at Kalbarri; the Murchison-Yalgar-Hope river system is the longest river system in Western Australia. It has a mean annual flow of 208 gigalitres, although in 2006, the peak year on record since 1967, flow was 1,806 gigalitres; the Murchison River basin covers an area of about 82,000 square kilometres in the Mid West region of Western Australia. It extends about 550 km inland from the Indian Ocean, onto the Yilgarn Craton east of Meekatharra and north of Sandstone. Rain falls in the upper basin during summer cyclones, so for much of the year the Murchison River does not flow, leaving a dry sandy river bed and intermittent permanent pools; the eastern reaches of the basin contain large chains of salt lakes, which flow only following rainfall. The drainage lines from these lakes merge to form the Murchison River about 90 km north-northeast of Meekatharra.
From here the river flows west southwest west to the Indian Ocean. The Murchison River rises on the southern slopes of the Robinson Ranges, about 75 km north of Meekatharra in central Western Australia. From there it flows in a westerly direction for about 130 km to its junction with its largest tributary, the Yalgar River west for another 100 km before turning south-southwest for 120 km, at which point it is joined by the Roderick River, about 30 km east of Murchison Settlement. Another 70 km to the south-southwest it joins the Sanford River. Over the next 100 km it makes a number of sharp turns, it flows to the southwest, flowing under the North West Coastal Highway at the Galena Bridge. Entering the Kalbarri National Park, it flows first to the northwest and to the north, flowing through the Murchison Gorge, passing through a number of tight bends known as the Z Bend and The Loop respectively, it turns to the southwest, passing through one more dogleg before discharging into the Indian Ocean at Kalbarri, the only town on the river.
Murchison Gorge is a deep gorge in near pristine condition. It is popular with tourists, there are a number of tourist lookouts, it is of geological interest, as it exposes a section through the Tumblagooda Sandstone, a geological sequence rich in Ordovician trace fossils. The final 18 kilometres of the Murchison River, from the Murchison House ford to the mouth, are estuarine, consist of a sequence of long sandbars and shallow pools less than a metre deep; the estuary is permanently open to the sea, so is affected by tides and the inflow of saline sea water. When river flow is low, the estuary accumulates sediment from the ocean, narrowing the river channel; because of the high sediment load, continual stirring by wind and river flow, the water is turbid. The mouth of the estuary is a small delta, closed by a sandbar except for a narrow channel. Although this channel is permanently open, it is very narrow and shallow, so is now dredged every year to allow passage by western rock lobster fishing boats.
The Murchison River was named by the explorer George Grey, whose boats were wrecked at its mouth on 1 April 1839, during his second disastrous exploratory expedition. Murchison's advocacy had been essential in securing official support for Grey's Western Australian expeditions; the estuary and river mouth was used as a holiday destination by families from the Galena mines in the 1920s and 1930s, a military holiday camp was built there during World War II. In 1951 the town of Kalbarri was gazetted at the river mouth, by the end of the 1990s the population was about 2,000. In 1963 the Kalbarri National Park was gazetted, formally protecting the lower reaches of the river, including the gorge; the Galena Bridge, carrying North West Coastal Highway over the river at Galena, was opened by the Main Roads Department in December 1983. Flooding occurred in 1866 resulting in the Geraldine Mine being drowned, more flooding occurred in 1882; the southern branch flooded out to a distance of 6 miles from the river bank in 1884 and the main homestead at Moorarie Station was washed away with about 3,000 ewes and lambs.
The river was once again flooded in 1900 following heavy rains with the river estimated to be running 8 miles wide, road to Cue and Peak Hill were submerged under 10 feet of water. Roads were cut for up to a fortnight resulting in food shortages in many isolated towns. Ernest Lee Steere of Belele Station reported that over 5 inches of rain fell in less than a fortnight. Further downstream the river was reported to be running 15 miles wide and at depths of up to 70 feet. Heavy flooding occurred along parts of the river in March 1926 following heavy rains. 15 to 20 men had to be rescued by dingy. Flooding again occurred in 1939 and once more following another significant rain event in February 1945 that resulted in flooding and the old Galena Bridge being swept away stranding the citizens of Carnarvon. A ferry service was established using a fishing boat. Bananas were the main item. Following Cyclone Emma in 2006, much of the catchment area received 100 millimetres of rainfall; the river swelled, reachin
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the