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Zoroaster

Zarathushtra Spitama (Zaraθuštra Spitāma), more commonly known as Zoroaster, or Zarathustra , was an Iranian religious reformer who challenged the tenets of the contemporary Ancient Iranian religion, becoming the spiritual founder of Zoroastrianism (known by its adherents as , meaning "Mazda-worship", and  meaning "good religion"). Variously described as either a poet, prophet, philosopher, sage, and/or a wonderworker; in the oldest Zoroastrian scriptures, the Gathas, which he is believed to have authored, he is described as a preacher and a poet-prophet. He also had an impact on Heraclitus, Plato, Pythagoras, and the Abrahamic religions, including Judaism, Christianity, and Islam. He spoke an Eastern Iranian language, named Avestan by scholars after the corpus of Zoroastrian religious texts written in that language. Based on this, it is tentative to place his homeland somewhere in the eastern regions of Greater Iran (perhaps in modern-day Afghanistan or Tajikistan), but his exact birthplace is uncertain.

His life is traditionally dated to sometime around the 7th and 6th centuries BC, making him a contemporary of Cyrus the Great, though most scholars, using linguistic and socio-cultural evidence, suggest a dating to somewhere in the second millennium BC. Zoroastrianism eventually became Iran's most prominent religion from around the 6th century BC, enjoying official sanction during the time of the Sassanid Empire, until the 7th century AD, when the religion itself began to decline following the Arab-Muslim conquest of Iran. Zoroaster is credited with authorship of the Gathas as well as the, a series of hymns composed in Old Avestan that cover the core of Zoroastrian thinking. Little is known about Zoroaster; most of his life is known only from these scant texts. By any modern standard of historiography, no evidence can place him into a fixed period and the historicization surrounding him may be a part of a trend from before the 10th century AD that historicizes legends and myths.

Name and etymology
Zoroaster's name in his native language, Avestan, was probably. His translated name, "Zoroaster", derives from a later (5th century BC) Greek transcription, (Ζωροάστρης), as used in Xanthus's  (Fragment 32) and in Plato's First Alcibiades (122a1). This form appears subsequently in the Latin Zōroastrēs, and, in later Greek orthographies, as Ζωροάστρις. The Greek form of the name appears to be based on a phonetic transliteration or semantic substitution of Avestan with the Greek ζωρός (literally 'undiluted') and the BMAC substrate  with ἄστρον.

In Avestan, is generally accepted to derive from an Old Iranian ; The element half of the name  is thought to be the Indo-Iranian root for 'camel', with the entire name meaning 'he who can manage camels'. Reconstructions from later Iranian languages—particularly from the Middle Persian (300 BC), which is the form that the name took in the 9th- to 12th-century Zoroastrian texts—suggest that might be a zero-grade form of. Subject then to whether derives from  or from, several interpretations have been proposed.

If is the original form, it may mean 'with old/aging camels', related to Avestic  (cf. Pashto  and Ossetian, 'old'; Middle Persian , 'old'):
 * 'with angry/furious camels': from Avestan, 'angry, furious'.
 * 'who is driving camels' or 'who is fostering/cherishing camels': related to Avestan, 'to drag'.
 * Mayrhofer (1977) proposed an etymology of 'who is desiring camels' or 'longing for camels' and related to Vedic Sanskrit, 'to like', and perhaps (though ambiguous) also to Avestan.
 * 'with yellow camels': parallel to Younger Avestan.

The interpretation of the  in the Avestan  was for a time itself subjected to heated debate because the  is an irregular development: as a rule,  (a first element that ends in a dental consonant) should have Avestan  or  as a development from it. Why this is not so for has not yet been determined. Notwithstanding the phonetic irregularity, that Avestan with its  was linguistically an actual form is shown by later attestations reflecting the same basis. All present-day Iranian-language variants of his name derive from the Middle Iranian variants of, which, in turn, all reflect Avestan's fricative.

In Middle Persian, the name is, in Parthian , in Manichaean Middle Persian , in Early New Persian , and in modern (New Persian), the name is.

The name is attested in Classical Armenian sources as (often with the variant ). The most important of these testimonies were provided by the Armenian authors Eznik of Kolb, Elishe, and Movses Khorenatsi. The spelling was formed through an older form which started with, a fact which the German Iranologist Friedrich Carl Andreas (1846–1930) used as evidence for a Middle Persian spoken form. Based on this assumption, Andreas even went so far to form conclusions from this also for the Avestan form of the name. However, the modern Iranologist Rüdiger Schmitt rejects Andreas's assumption, and states that the older form which started with was just influenced by Armenian  ('wrong, unjust, idle'), which therefore means that "the name must have been reinterpreted in an anti-Zoroastrian sense by the Armenian Christians". Furthermore, Schmitt adds: "it cannot be excluded, that the (Parthian or) Middle Persian form, which the Armenians took over ( or the like), was merely metathesized to pre-Arm. ".

Date
There is no consensus on the dating of Zoroaster. The Avesta gives no direct information about it, while historical sources are conflicting. Some scholars base their date reconstruction on the Proto-Indo-Iranian language and Proto-Indo-Iranian religion, while others use internal evidence. While many scholars today consider a date around 1000 BC to be the most likely, others still consider a range of dates between 1500 and 500 BC to be possible.

Classical scholarship
Classical scholarship in the 6th to 4th century BC believed he existed 6,000 years before Xerxes I's invasion of Greece in 480 BC (Xanthus, Eudoxus, Aristotle, Hermippus), which is a possible misunderstanding of the Zoroastrian four cycles of 3,000 years (i.e. 12,000 years). This belief is recorded by Diogenes Laërtius, and variant readings could place it 600 years before Xerxes I, somewhere before 1000 BC. However, Diogenes also mentions Hermodorus's belief that Zoroaster lived 5,000 years before the Trojan War, which would mean he lived around 6200 BC. The 10th-century Suda provides a date of 500 years before the Trojan War. Pliny the Elder cited Eudoxus which placed his death 6,000 years before Plato, c. 6300 BC. Other pseudo-historical constructions are those of Aristoxenus who recorded Zaratas the Chaldeaean to have taught Pythagoras in Babylon, or lived at the time of mythological Ninus and Semiramis. According to Pliny the Elder, there were two Zoroasters. The first lived thousands of years ago, while the second accompanied Xerxes I in the invasion of Greece in 480 BC. Some scholars propose that the chronological calculation for Zoroaster was developed by Persian magi in the 4th century BC, and as the early Greeks learned about him from the Achaemenids, this indicates they did not regard him as a contemporary of Cyrus the Great, but as a remote figure.

Zoroastrian and Muslim scholarship
Some later pseudo-historical and Zoroastrian sources (the, which references a date "258 years before Alexander") place Zoroaster in the 6th century BC, which coincided with the accounts by Ammianus Marcellinus from the 4th century AD. The traditional Zoroastrian date originates in the period immediately following Alexander the Great's conquest of the Achaemenid Empire in 330 BC. The Seleucid rulers who gained power following Alexander's death instituted an "Age of Alexander" as the new calendrical epoch. This did not appeal to the Zoroastrian priesthood who then attempted to establish an "Age of Zoroaster". To do so, they needed to establish when Zoroaster had lived, which they accomplished by (erroneously, according to Mary Boyce some even identified Cyrus with Vishtaspa) counting back the length of successive generations, until they concluded that Zoroaster must have lived "258 years before Alexander". This estimate then re-appeared in the 9th- to 12th-century Arabic and Pahlavi texts of Zoroastrian tradition, like the 10th century Al-Masudi who cited a prophecy from a lost Avestan book in which Zoroaster foretold the Empire's destruction in 300 years, but the religion would last for 1,000 years.

Modern scholarship
In modern scholarship, two main approaches can be distinguished: a late dating to the 7th and 6th centuries BC, based on the indigenous Zoroastrian tradition, and an early dating, which places his life more generally in the 15th to 9th centuries BC.

Late date
Some scholars propose a period between 7th and 6th century BC, for example, c. 650–600 BC or 559–522 BC. The latest possible date is the mid 6th century BC, at the time of Achaemenid Empire's Darius I, or his predecessor Cyrus the Great. This date gains credence mainly from attempts to connect figures in Zoroastrian texts to historical personages; thus some have postulated that the mythical Vishtaspa who appears in an account of Zoroaster's life was Darius I's father, also named Vishtaspa (or Hystaspes in Greek). However, if this was true, it seems unlikely that the Avesta would not mention that Vishtaspa's son became the ruler of the Persian Empire, or that this key fact about Darius's father would not be mentioned in the Behistun Inscription. It is also possible that Darius I's father was named in honor of the Zoroastrian patron, indicating possible Zoroastrian faith by Arsames.

Early date
Scholars such as Mary Boyce (who dated Zoroaster to somewhere between 1700 and 1000 BC) used linguistic and socio-cultural evidence to place Zoroaster between 1500 and 1000 BC (or 1200 and 900 BC). The basis of this theory is primarily proposed on linguistic similarities between the Old Avestan language of the Zoroastrian Gathas and the Sanskrit of the Rigveda (c. 1700–1100 BC), a collection of early Vedic hymns. Both texts are considered to have a common archaic Indo-Iranian origin. The Gathas portray an ancient Stone-Bronze Age bipartite society of warrior-herdsmen and priests (compared to Bronze tripartite society; some conjecture that it depicts the Yaz culture), and that it is thus implausible that the Gathas and Rigveda could have been composed more than a few centuries apart. These scholars suggest that Zoroaster lived in an isolated tribe or composed the Gathas before the 1200–1000 BC migration by the Iranians from the steppe to the Iranian Plateau. The shortfall of the argument is the vague comparison, and the archaic language of Gathas does not necessarily indicate time difference.

Another possible date from the 9th century BC or before was suggested by Silk Road Seattle, using its own interpretations of Victor H. Mair's writings on the topic. Mair himself guessed that Zoroaster could have been born in the 2nd millennium BC.

Almut Hintze, the British Library, and the European Research Council have dated Zoroaster to roughly 3,500 years ago, in the 2nd millennium BC.

Traditions favoring a late date for Zoroaster's life have fallen out of vogue with some Zoroastian communities, who see the prospect of their faith having more ancient roots than previously thought as a welcome development.

Place
The birthplace of Zoroaster is also unknown, and the language of the Gathas is not similar to the proposed north-western and north-eastern regional dialects of Persia. It is also suggested that he was born in one of the two areas and later lived in the other area.

9 and 17 cite the Ditya River in Airyanem Vaējah (Middle Persian ) as Zoroaster's home and the scene of his first appearance. The Avesta (both Old and Younger portions) does not mention the Achaemenids or of any West Iranian tribes such as the Medes, Persians, or even Parthians. The refers to some Iranian peoples that are unknown in the Greek and Achaemenid sources about the 6th and 5th century BC Eastern Iran. The contain 17 regional names, most of which are located in north-eastern and eastern Iran.

However, in 59.18, the, or supreme head of the Zoroastrian priesthood, is said to reside in 'Ragha' (Badakhshan). In the 9th- to 12th-century Middle Persian texts of Zoroastrian tradition, this 'Ragha' and with many other places appear as locations in Western Iran. While the land of Media does not figure at all in the Avesta (the westernmost location noted in scripture is Arachosia), the, or "Primordial Creation", (20.32 and 24.15) puts Ragha in Media (medieval Rai). However, in Avestan, Ragha is simply a toponym meaning 'plain, hillside.'

Apart from these indications in Middle Persian sources that are open to interpretations, there are a number of other sources. The Greek and Latin sources are divided on the birthplace of Zoroaster. There are many Greek accounts of Zoroaster, referred usually as Persian or Perso-Median Zoroaster; Ctesias located him in Bactria, Diodorus Siculus placed him among Ariaspai (in Sistan), Cephalion and Justin suggest east of greater Iran whereas Pliny and Origen suggest west of Iran as his birthplace. Moreover, they have the suggestion that there has been more than one Zoroaster.

On the other hand, in post-Islamic sources Shahrastani (1086–1153), an Iranian writer originally from Shahristān, in present-day Turkmenistan, proposed that Zoroaster's father was from Atropatene (also in Medea) and his mother was from Rey. Coming from a reputed scholar of religions, this was a serious blow to the various regions which all claimed that Zoroaster originated from homelands, some of which then decided that Zoroaster must then have then been buried in their regions or composed his Gathas there or preached there. Arabic sources of the same period and the same region of historical Persia also consider Azerbaijan as the birthplace of Zarathustra.

By the late 20th century, most scholars had settled on an origin in eastern Greater Iran. Gnoli proposed Sistan, Baluchistan (though in a much wider scope than the present-day province) as the homeland of Zoroastrianism; Frye voted for Bactria and Chorasmia; Khlopin suggests the Tedzen Delta in present-day Turkmenistan. Sarianidi considered the Bactria–Margiana Archaeological Complex region as "the native land of the Zoroastrians and, probably, of Zoroaster himself." Boyce includes the steppes to the west from the Volga. The medieval "from Media" hypothesis is no longer taken seriously, and Zaehner has even suggested that this was a Magi-mediated issue to garner legitimacy, but this has been likewise rejected by Gershevitch and others.

The 2005 Encyclopedia Iranica article on the history of Zoroastrianism summarizes the issue with "while there is general agreement that he did not live in western Iran, attempts to locate him in specific regions of eastern Iran, including Central Asia, remain tentative".

Life

Zoroaster is recorded as the son of Pourushaspa of the Spitama family, and Dugdōw, while his great-grandfather was Haēčataspa. All the names appear appropriate to the nomadic tradition. His father's name means 'possessing gray horses' (with the word meaning 'horse'), while his mother's means 'milkmaid'. According to the tradition, he had four brothers, two older and two younger, whose names are given in much later Pahlavi work.

Zoroaster's training for priesthood probably started very early around seven years of age. He became a priest probably around the age of 15, and according to Gathas, gaining knowledge from other teachers and personal experience from traveling when he left his parents at age 20. By the age of 30, Zoroaster experienced a revelation during a spring festival; on the river bank he saw a shining being, who revealed himself as (Good Purpose) and taught him about  (Wise Lord) and five other radiant figures. Zoroaster soon became aware of the existence of two primal spirits, the second being (Destructive Spirit), with opposing concepts of  (order) and  (deception). Thus he decided to spend his life teaching people to seek. He received further revelations and saw a vision of the seven, and his teachings were collected in the and the.

Eventually, at the age of about 42, Zoroaster received the patronage of queen Hutaosa and a ruler named Vishtaspa, an early adherent of Zoroastrianism (possibly from Bactria according to the Shahnameh).

According to the tradition, he lived for many years after Vishtaspa's conversion, managed to establish a faithful community, and married three times. His first two wives bore him three sons, Isat Vâstra, Urvatat Nara, and Hvare Chithra, and three daughters, Freni, Thriti, and Pouruchista. His third wife, Hvōvi, was childless. Zoroaster died when he was 77 years and 40 days old. There are conflicting traditions on Zoroaster's manner of death. The most common is that he was murdered by a (priest of the old religion) named Brādrēs, while performing at an altar. The Dēnkart, and the epic, ascribe his death to a Turanian soldier named Baraturish, potentially a spin on the same figure, while other traditions combine both accounts or hold that he died of old age.

Cypress of Kashmar
The Cypress of Kashmar is a mythical cypress tree of legendary beauty and gargantuan dimensions. It is said to have sprung from a branch brought by Zoroaster from Paradise and to have stood in today's Kashmar in northeastern Iran and to have been planted by Zoroaster in honor of the conversion of King Vishtaspa to Zoroastrianism. According to the Iranian physicist and historian Zakariya al-Qazwini, King Vishtaspa had been a patron of Zoroaster who planted the tree himself. In his ('The Wonders of Creatures and the Marvels of Creation'), he further describes how the Al-Mutawakkil in 247 AH (861 AD) caused the mighty cypress to be felled, and then transported it across Iran, to be used for beams in his new palace at Samarra. Before, he wanted the tree to be reconstructed before his eyes. This was done in spite of protests by the Iranians, who offered a very great sum of money to save the tree. Al-Mutawakkil never saw the cypress, because he was murdered by a Turkic soldier (possibly in the employ of his son) on the night when it arrived on the banks of the Tigris.

In Christianity
Athanasius Kircher identified Zoroaster with Ham. The French figurist Jesuit missionary to China Joachim Bouvet thought that Zoroaster, the Chinese cultural hero Fuxi and Hermes Trismegistus were actually the Biblical patriarch Enoch.

In Islam
The Encyclopædia Iranica indicates that the stories of Zoroaster's life were distorted by quoting stories from Christianity and Judaism and attributing them to Zoroaster, but the most quotations were from Islam after the entry of Muslims into Persia, as it was a means for the Zoroastrian clergy to strengthen their religion.

The orientalist Arthur Christensen in his book  Iran During The Sassanid Era  , mentioned that the sources dating back to the era of the Sasanian state in ancient Persian that refer to the Zoroastrian doctrine do not match the sources that appeared after the collapse of the state, such as the Pahlavi source and others. The reason is that because of the fall of the Sasanian state, the Zoroastrian clerics tried to save their religion from extinction through modifying it to resemble the religion of Muslims to retain followers in the Zoroastrian religion.

A number of parallels have been drawn between Zoroastrian teachings and Islam. Such parallels include the evident similarities between Amesha Spenta and the archangel Gabriel, praying five times a day, covering one's head during prayer, and the mention of Thamud and Iram of the Pillars in the Quran. These may also indicate the influence of the Achaemenid Empire on the development of either religion.

The Sabians, who believed in free will coincident with Zoroastrians, are also mentioned in the Quran 22:17.

Muslim scholastic views
Like the Greeks of classical antiquity, Islamic tradition understands Zoroaster to be the founding prophet of the Magians (via Aramaic, Arabic, collective ). The 11th-century Cordoban Ibn Hazm (Zahiri school) contends that "of the Book" cannot apply in light of the Zoroastrian assertion that their books were destroyed by Alexander. Citing the authority of the 8th-century al-Kalbi, the 9th- and 10th-century Sunni historian al-Tabari (I, 648) reports that Zaradusht bin Isfiman (an Arabic adaptation of "Zarathushtra Spitama") was an inhabitant of Israel and a servant of one of the disciples of the prophet Jeremiah. According to this tale, Zaradusht defrauded his master, who cursed him, causing him to become leprous (cf. Elisha's servant Gehazi in Jewish scripture).

The apostate Zaradusht then eventually made his way to Balkh (present day Afghanistan) where he converted Bishtasb (i.e. Vishtaspa), who in turn compelled his subjects to adopt the religion of the Magians. Recalling other traditions, al-Tabari (I, 681–683) recounts that Zaradusht accompanied a Jewish prophet to Bishtasb/Vishtaspa. Upon their arrival, Zaradusht translated the sage's Hebrew teachings for the king and so convinced him to convert (Tabari also notes that they had previously been ) to the Magian religion.

The 12th-century heresiographer al-Shahrastani describes the Majusiya into three sects, the (an otherwise undocumented sect that – per Sharastani – seems to have had a stronger doctrine of Ahriman's "non-reality"), the  and the, among which Al-Shahrastani asserts that only the last of the three were properly followers of Zoroaster. As regards the recognition of a prophet, Zoroaster has said: "They ask you as to how should they recognize a prophet and believe him to be true in what he says; tell them what he knows the others do not, and he shall tell you even what lies hidden in your nature; he shall be able to tell you whatever you ask him and he shall perform such things which others cannot perform." (Namah Shat Vakhshur Zartust, .5–7. 50–54)

Ahmadiyya view
The Ahmadiyya Community views Zoroaster as a Prophet of Allah and describe the expressions of the all-good Ahura Mazda and evil Ahriman as merely referring to the coexistence of forces of good and evil enabling humans to exercise free will.

In Manichaeism
Manichaeism considered Zoroaster to be a figure in a line of prophets of which Mani (216–276) was the culmination. Zoroaster's ethical dualism is—to an extent—incorporated in Manichaeism's doctrine which, unlike Mani's thoughts, viewed the world as being locked in an epic battle between opposing forces of good and evil. Manicheanism also incorporated other elements of Zoroastrian tradition, particularly the names of supernatural beings; however, many of these other Zoroastrian elements are either not part of Zoroaster's own teachings or are used quite differently from how they are used in Zoroastrianism.

In the Bahá'í Faith
Zoroaster appears in the Bahá'í Faith as a "Manifestation of God", one of a line of prophets who have progressively revealed the Word of God to a gradually maturing humanity. Zoroaster thus shares an exalted station with Abraham, Moses, Krishna, Jesus, Muhammad, the Báb, and the founder of the Bahá'í Faith, Bahá'u'lláh. Shoghi Effendi, the head of the Bahá'í Faith in the first half of the 20th century, saw Bahá'u'lláh as the fulfillment of a post-Sassanid Zoroastrian prophecy that saw a return of Sassanid emperor Bahram; Effendi also stated that Zoroaster lived roughly 1000 years before Jesus.

Philosophy

In the, Zoroaster sees the human condition as the mental struggle between and. The cardinal concept of —which is highly nuanced and difficult to translate—is at the foundation of all Zoroastrian doctrine, including that of (who is ), creation (that is ), existence (that is ), and as the condition for free will.

The purpose of humankind, like that of all other creation, is to sustain and align itself to. For humankind, this occurs through active ethical participation in life, ritual, and the exercise of constructive/good thoughts, words, and deeds.

Elements of Zoroastrian philosophy entered the West through their influence on Judaism and Platonism and have been identified as one of the key early events in the development of philosophy. Among the classic Greek philosophers, Heraclitus is often referred to as inspired by Zoroaster's thinking.

In 2005, the Oxford Dictionary of Philosophy ranked Zoroaster as first in the chronology of philosophers. Zoroaster's impact lingers today due in part to the system of religious ethics he founded called. The word is Avestan and is translated as 'Worship of Wisdom/Mazda' in English. The encyclopedia Natural History (Pliny) claims that Zoroastrians later educated the Greeks who, starting with Pythagoras, used a similar term, philosophy, or "love of wisdom" to describe the search for ultimate truth.

Zoroaster emphasized the freedom of the individual to choose right or wrong and individual responsibility for one's deeds. This personal choice to accept and shun  is one's own decision and not a dictate of. For Zoroaster, by thinking good thoughts, saying good words, and doing good deeds (e.g. assisting the needy, doing good works, or conducting good rituals) one increases in the world and in themselves, celebrating the divine order, and coming a step closer on the everlasting road to. Thus, mankind are not the slaves or servants of, but can make a personal choice to be co-workers, thereby perfecting the world as saoshyants ("world-perfecters") and eventually achieving the status of an ("master of ").

Iconography

Although a few recent depictions of Zoroaster show him performing some deed of legend, in general the portrayals merely present him in white vestments (which are also worn by present-day Zoroastrian priests). He often is seen holding a collection of unbound rods or twigs, known as a (Avestan; Middle Persian ), which is generally considered to be another symbol of priesthood, or with a book in hand, which may be interpreted to be the Avesta. Alternatively, he appears with a mace, the —usually stylized as a steel rod crowned by a bull's head—that priests carry in their installation ceremony. In other depictions he appears with a raised hand and thoughtfully lifted finger, as if to make a point. Alternatively, this could be an Islamic influence, drawing parallels between both religions' conception of the oneness of God..

Zoroaster is rarely depicted as looking directly at the viewer; instead, he appears to be looking slightly upwards, as if beseeching. Zoroaster is almost always depicted with a beard along with other factors bearing similarities to 19th-century portraits of Jesus.

Many modern depictions of Zoroaster derive from a Sassanid-era rock-face carving at Taq-e Bostan. In this depiction, a figure is seen to preside over the coronation of either Ardashir I or II. The figure is standing on a lotus, with a in hand and with a gloriole around his head. Until the 1920s, this figure was commonly thought to be a depiction of Zoroaster, but in recent years is more commonly interpreted to be a depiction of Mithra.

In classical antiquity
The Greeks—in the Hellenistic sense of the term—had an understanding of Zoroaster as expressed by Plutarch, Diogenes Laertius, and Agathias that saw him, at the core, to be the "prophet and founder of the religion of the Iranian peoples," Beck notes that "the rest was mostly fantasy". Zoroaster was set in the ancient past, six to seven millennia before the Common Era, and was described as a king of Bactria or a Babylonian (or teacher of Babylonians), and with a biography typical of a Neopythagorean sage, i.e. having a mission preceded by ascetic withdrawal and enlightenment. However, at first mentioned in the context of dualism, in Moralia, Plutarch presents Zoroaster as "Zaratras," not realizing the two to be the same, and he is described as a "teacher of Pythagoras".

Zoroaster has also been described as a sorcerer-astrologer – the creator of both magic and astrology. Deriving from that image, and reinforcing it, was a "mass of literature" attributed to him and that circulated the Mediterranean world from the 3rd century BC to the end of antiquity and beyond.

The language of that literature was predominantly Greek, though at one stage or another various parts of it passed through Aramaic, Syriac, Coptic, or Latin. Its ethos and cultural matrix was likewise Hellenistic, and "the ascription of literature to sources beyond that political, cultural and temporal framework represents a bid for authority and a fount of legitimizing "alien wisdom". Zoroaster and the magi did not compose it, but their names sanctioned it." The attributions to "exotic" names (not restricted to magians) conferred an "authority of a remote and revelatory wisdom."

Among the named works attributed to "Zoroaster" is a treatise On Nature, which appears to have originally constituted four volumes (i.e. papyrus rolls). The framework is a retelling of Plato's Myth of Er, with Zoroaster taking the place of the original hero. While Porphyry imagined Pythagoras listening to Zoroaster's discourse, On Nature has the Sun in middle position, which was how it was understood in the 3rd century. In contrast, Plato's 4th-century BC version had the Sun in second place above the Moon. Colotes accused Plato of plagiarizing Zoroaster, and Heraclides Ponticus wrote a text titled Zoroaster based on his perception of "Zoroastrian" philosophy, in order to express his disagreement with Plato on natural philosophy. With respect to substance and content in On Nature only two facts are known: that it was crammed with astrological speculations, and that Necessity (Ananké) was mentioned by name and that she was in the air.

Pliny the Elder names Zoroaster as the inventor of magic (Natural History 30.2.3). "However, a principle of the division of labor appears to have spared Zoroaster most of the responsibility for introducing the dark arts to the Greek and Roman worlds." That "dubious honor" went to the "fabulous magus, Ostanes, to whom most of the pseudepigraphic magical literature was attributed." Although Pliny calls him the inventor of magic, the Roman does not provide a "magician's persona" for him. Moreover, the little "magical" teaching that is ascribed to Zoroaster is actually very late, with the very earliest example being from the 14th century.

Association with astrology according to Roger Beck, were based on his Babylonian origin, and Zoroaster's Greek name was identified at first with star-worshiping (, 'star sacrificer") and, with the, even as the 'living' star. Later, an even more elaborate mythoetymology evolved: Zoroaster died by the living  flux  of fire from the star  which he himself had invoked, and even, that the stars killed him in revenge for having been restrained by him.

The alternate Greek name for Zoroaster was Zaratras or Zaratas/Zaradas/Zaratos. Pythagoreans considered the mathematicians to have studied with Zoroaster in Babylonia. Lydus, in On the Months, attributes the creation of the seven-day week to "the Babylonians in the circle of Zoroaster and Hystaspes," and who did so because there were seven planets. Lucian of Samosata, in Mennipus 6, reports deciding to journey to Babylon "to ask one of the magi, Zoroaster's disciples and successors," for their opinion.

While the division along the lines of Zoroaster/astrology and Ostanes/magic is an "oversimplification, the descriptions do at least indicate what the works are "; they were not expressions of Zoroastrian doctrine, they were not even expressions of what the Greeks and Romans " the doctrines of Zoroastrianism to have been". The assembled fragments do not even show noticeable commonality of outlook and teaching among the several authors who wrote under each name.

Almost all Zoroastrian pseudepigrapha is now lost, and of the attested texts—with only one exception—only fragments have survived. Pliny's 2nd- or 3rd-century attribution of "two million lines" to Zoroaster suggest that (even if exaggeration and duplicates are taken into consideration) a formidable pseudepigraphic corpus once existed at the Library of Alexandria. This corpus can safely be assumed to be pseudepigrapha because no one before Pliny refers to literature by "Zoroaster", and on the authority of the 2nd-century Galen of Pergamon and from a 6th-century commentator on Aristotle it is known that the acquisition policies of well-endowed royal libraries created a market for fabricating manuscripts of famous and ancient authors.

The exception to the fragmentary evidence (i.e. reiteration of passages in works of other authors) is a complete Coptic tractate titled (after the first-person narrator) discovered in the Nag Hammadi library in 1945. A three-line cryptogram in the colophones following the 131-page treatise identify the work as "words of truth of Zostrianos. God of Truth []. Words of Zoroaster." Invoking a "God of Truth" might seem Zoroastrian, but there is otherwise "nothing noticeably Zoroastrian" about the text and "in content, style, ethos and intention, its affinities are entirely with the congeners among the Gnostic tractates."

Another work circulating under the name of "Zoroaster" was the (or ), and which ran to five volumes (i.e. papyrus rolls). The title and fragments suggest that it was an astrological handbook, "albeit a very varied one, for the making of predictions." A third text attributed to Zoroaster is On Virtue of Stones, of which nothing is known other than its extent (one volume) and that pseudo-Zoroaster 'sang' it (from which Cumont and Bidez conclude that it was in verse). Numerous other fragments preserved in the works of other authors are attributed to "Zoroaster", but the titles of those books are not mentioned.

These pseudepigraphic texts aside, some authors did draw on a few genuinely Zoroastrian ideas. The Oracles of Hystaspes, by "Hystaspes", another prominent magian pseudo-author, is a set of prophecies distinguished from other Zoroastrian pseudepigrapha in that it draws on real Zoroastrian sources. Some allusions are more difficult to assess: in the same text that attributes the invention of magic to Zoroaster, Pliny states that Zoroaster laughed on the day of his birth, although in an earlier place, Pliny had sworn in the name of Hercules that no child had ever done so before the 40th day from his birth. This notion of Zoroaster's laughter also appears in the 9th– to 11th-century texts of genuine Zoroastrian tradition, and for a time it was assumed that the origin of those myths lay with indigenous sources. Pliny also records that Zoroaster's head had pulsated so strongly that it repelled the hand when laid upon it, a presage of his future wisdom. The Iranians were however just as familiar with the Greek writers, and the provenance of other descriptions are clear. For instance, Plutarch's description of its dualistic theologies reads thus: "Others call the better of these a god and his rival a daemon, as, for example, Zoroaster the Magus, who lived, so they record, five thousand years before the siege of Troy. He used to call the one Horomazes and the other Areimanius".

In the modern era
An early reference to Zoroaster in English literature occur in the writings of the physician-philosopher Sir Thomas Browne who asserted in his Religio Medici (1643):

"I believe, besides Zoroaster, there were divers that writ before Moses, who notwithstanding have suffered the common fate of time."

In E. T. A. Hoffmann's novel Klein Zaches, genannt Zinnober (1819), the mage Prosper Alpanus states that Professor Zoroaster was his teacher.

In his seminal work Thus Spoke Zarathustra (1885), the philosopher Friedrich Nietzsche uses the native Iranian name Zarathustra, which has a significant meaning as he had used the familiar Greek-Latin name in his earlier works. It is believed that Nietzsche invents a characterization of Zarathustra as the mouthpiece for Nietzsche's own ideas about morality. By choosing the name of 'Zarathustra' as prophet of his philosophy, as he has expressed clearly, he followed the paradoxical aim of paying homage to the original Iranian prophet and reversing his teachings at the same time. The original Zoroastrian world view interprets being essentially on a moralistic basis and depicts the world as an arena for the struggle of the two fundamentals of being, Good and Evil, represented in two antagonistic divine figures. On the contrary, Nietzsche wants his philosophy to be Beyond Good and Evil.

Notable influence on modern Western culture
The German composer Richard Strauss's large-scale tone-poem Also sprach Zarathustra (1896) was inspired by Nietzsche's book.

A sculpture of Zoroaster by Edward Clark Potter, representing ancient Persian judicial wisdom and dating to 1896, towers over the Appellate Division Courthouse of New York State at East 25th Street and Madison Avenue in Manhattan. A sculpture of Zoroaster appears with other prominent religious figures on the south side of the exterior of Rockefeller Memorial Chapel on the campus of the University of Chicago.

title: Zoroaster


text:
Microphone array

A microphone array is any number of microphones operating in tandem. There are many applications:

 * Systems for extracting voice input from ambient noise (notably telephones, speech recognition systems, hearing aids)
 * Surround sound and related technologies
 * Binaural recording
 * Locating objects by sound: acoustic source localization, e.g., military use to locate the source(s) of artillery fire. Aircraft location and tracking.
 * High fidelity original recordings
 * Environmental noise monitoring
 * Robotic navigation (acoustic SLAM)

Typically, an array is made up of omnidirectional microphones, directional microphones, or a mix of omnidirectional and directional microphones distributed about the perimeter of a space, linked to a computer that records and interprets the results into a coherent form. Arrays may also be formed using numbers of very closely spaced microphones. Given a fixed physical relationship in space between the different individual microphone transducer array elements, simultaneous DSP (digital signal processor) processing of the signals from each of the individual microphone array elements can create one or more "virtual" microphones. Different algorithms permit the creation of virtual microphones with extremely complex virtual polar patterns and even the possibility to steer the individual lobes of the virtual microphones patterns so as to home-in-on, or to reject, particular sources of sound. The application of these algorithms can produce varying levels of accuracy when calculating source level and location, and as such, care should be taken when deciding how the individual lobes of the virtual microphones are derived.

In case the array consists of omnidirectional microphones they accept sound from all directions, so electrical signals of the microphones contain the information about the sounds coming from all directions. Joint processing of these sounds allows selecting the sound signal coming from the given direction. An array of 1020 microphones, the largest in the world until August 21, 2014, was built by researchers at the MIT Computer Science and Artificial Intelligence Laboratory.

Currently the largest microphone array in the world was constructed by Sorama, a Netherlands-based sound engineering firm, in August 2014. Their array consists of 4096 microphones.

Soundfield microphone
The soundfield microphone system is a well-established example of the use of a microphone array in professional sound recording.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Microphone array


text:
Conjunctivitis

Conjunctivitis, also known as pink eye, is inflammation of the outermost layer of the white part of the eye and the inner surface of the eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness, or itchiness may occur. The affected eye may have increased tears or be "stuck shut" in the morning. Swelling of the white part of the eye may also occur. Itching is more common in cases due to allergies. Conjunctivitis can affect one or both eyes.

The most common infectious causes in adults are viral, whereas in children bacterial causes predominate. The viral infection may occur along with other symptoms of a common cold. Both viral and bacterial cases are easily spread between people. Allergies to pollen or animal hair are also a common cause. Diagnosis is often based on signs and symptoms. Occasionally, a sample of the discharge is sent for culture.

Prevention is partly by handwashing. Treatment depends on the underlying cause. In the majority of viral cases, there is no specific treatment. Most cases due to a bacterial infection also resolve without treatment; however, antibiotics can shorten the illness. People who wear contact lenses and those whose infection is caused by gonorrhea or chlamydia should be treated. Allergic cases can be treated with antihistamines or mast cell inhibitor drops.

About 3 to 6 million people get acute conjunctivitis each year in the United States. Typically, people get better in one or two weeks. If visual loss, significant pain, sensitivity to light or signs of herpes occur, or if symptoms do not improve after a week, further diagnosis and treatment may be required. Conjunctivitis in a newborn, known as neonatal conjunctivitis, may also require specific treatment.

Signs and symptoms

Red eye, swelling of the conjunctiva, and watering of the eyes are symptoms common to all forms of conjunctivitis. However, the pupils should be normally reactive, and the visual acuity normal.

Conjunctivitis is identified by inflammation of the conjunctiva, manifested by irritation and redness. Examination using a slit lamp (biomicroscope) may improve diagnostic accuracy. Examination of the palpebral conjunctiva, that overlying the inner aspects of the eyelids, is usually more diagnostic than examination of the bulbal conjunctiva, that overlying the sclera.

Viral
Approximately 80% of cases of conjunctivitis in adults and less than 20% in children are due to viruses, with 65% to 90% of these cases being attributed to adenoviruses. Viral conjunctivitis is often associated with an infection of the upper respiratory tract, a common cold, or a sore throat. Other associated signs may include pre-auricular lymph node swelling and contact with another person with a red eye. Eye pain may be present if the cornea is also involved. Its symptoms include excessive watering and itching. The discharge in viral conjunctivitis is usually (but not always) watery in nature. The infection usually begins in one eye but may spread easily to the other eye.

Viral conjunctivitis manifests as a fine, diffuse pinkness of the conjunctiva which may be mistaken for iritis, but corroborative signs on microscopy, particularly numerous lymphoid follicles on the tarsal conjunctiva, and sometimes a punctate keratitis are seen.

Allergic
Allergic conjunctivitis is inflammation of the conjunctiva due to allergy. The specific allergens may differ among patients. Symptoms result from the release of histamine and other active substances by mast cells, and consist of redness (mainly due to vasodilation of the peripheral small blood vessels), swelling of the conjunctiva, itching, and increased production of tears.

Bacterial
Bacteria are responsible for approximately 70% of conjunctivitis in children and less than 20% of cases in adults. Common bacteria responsible for bacterial conjunctivitis are Staphylococcus including Staph aureus, Streptococcus such as strep pneumoniae, Haemophilus species and Moraxella catarrhalis. Less commonly, Chlamydia spp. and Niesseria species (Neisseria gonorrhoeae and Neisseria meningitidis) may be the cause. Infection with Escherichia coli may also cause conjunctivitis, particularly in the neonatal subtype ophthalmia neonatorum. Bacterial conjunctivitis usually causes a rapid onset of conjunctival redness, swelling of the eyelid, and a sticky discharge. Typically, symptoms develop first in one eye, but may spread to the other eye within 2–5 days. Conjunctivitis due to common pus-producing bacteria causes marked grittiness or irritation and a stringy, opaque, greyish or yellowish discharge that may cause the lids to stick together, especially after sleep. Severe crusting of the infected eye and the surrounding skin may also occur. The gritty or scratchy feeling is sometimes localized enough that patients may insist that they have a foreign body in the eye.

Bacteria such as Chlamydia trachomatis or Moraxella spp. can cause a nonexudative but persistent conjunctivitis without much redness. Bacterial conjunctivitis may cause the production of membranes or pseudomembranes that cover the conjunctiva. Pseudomembranes consist of a combination of inflammatory cells and exudates and adhere loosely to the conjunctiva, while true membranes are more tightly adherent and cannot be easily peeled away. Cases of bacterial conjunctivitis that involve the production of membranes or pseudomembranes are associated with Neisseria gonorrhoeae, β-hemolytic streptococci, and Corynebacterium diphtheriae. C. diphtheriae causes membrane formation in conjunctiva of unimmunized children.

Chemical
Chemical eye injury may result when an acidic or alkaline substance gets in the eye. Alkali burns are typically worse than acidic burns. Mild burns produce conjunctivitis, while more severe burns may cause the cornea to turn white. Litmus paper may be used to test for chemical causes. When a chemical cause has been confirmed, the eye or eyes should be flushed until the pH is in the range 6–8. Anaesthetic eye drops can be used to decrease the pain.

Irritant or toxic conjunctivitis is primarily marked by redness. If due to a chemical splash, it is often present in only the lower conjunctival sac. With some chemicals, above all with caustic alkalis such as sodium hydroxide, necrosis of the conjunctiva marked by a deceptively white eye due to vascular closure may occur, followed by sloughing off of the dead epithelium. A slit lamp examination is likely to show evidence of anterior uveitis.

Biomarkers
Omics technologies have been used to identify biomarkers that inform on the emergence and progression of conjunctivitis. For example, in chronic inflammatory cicatrizing conjunctivitis, active oxylipins, lysophospholipids, fatty acids, and endocannabinoids alterations, from which potential biomarkers linked to inflammatory processes were identified.

Other
Inclusion conjunctivitis of the newborn is a conjunctivitis that may be caused by the bacterium Chlamydia trachomatis, and may lead to acute, purulent conjunctivitis. However, it is usually self-healing.

Causes
Infective conjunctivitis is most commonly caused by a virus. Bacterial infections, allergies, other irritants, and dryness are also common causes. Both bacterial and viral infections are contagious, passing from person to person or spread through contaminated objects or water. Contact with contaminated fingers is a common cause of conjunctivitis. Bacteria may also reach the conjunctiva from the edges of the eyelids and the surrounding skin, from the nasopharynx, from infected eye drops or contact lenses, from the genitals or the bloodstream. Infection by human adenovirus accounts for 65% to 90% of cases of viral conjunctivitis.

Viral
Adenoviruses are the most common cause of viral conjunctivitis (adenoviral keratoconjunctivitis). Herpetic keratoconjunctivitis, caused by herpes simplex viruses, can be serious and requires treatment with aciclovir. Acute hemorrhagic conjunctivitis is a highly contagious disease caused by one of two enteroviruses, enterovirus 70 and coxsackievirus A24. These were first identified in an outbreak in Ghana in 1969, and have spread worldwide since then, causing several epidemics.

Bacterial
The most common causes of acute bacterial conjunctivitis are Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae. Though very rare, hyperacute cases are usually caused by Neisseria gonorrhoeae or Neisseria meningitidis. Chronic cases of bacterial conjunctivitis are those lasting longer than 3 weeks, and are typically caused by S. aureus, Moraxella lacunata, or Gram-negative enteric flora.

Allergic
Conjunctivitis may also be caused by allergens such as pollen, perfumes, cosmetics, smoke, dust mites, Balsam of Peru, or eye drops. The most frequent cause of conjunctivitis is allergic conjunctivitis and it affects 15% to 40% of the population. Allergic conjunctivitis accounts for 15% of eye related primary care consultations; most including seasonal exposures in the spring and summer or perpetual conditions.

Other

 * Computer vision syndrome
 * Dry eye syndrome
 * Reactive arthritis: Conjunctivitis is part of the triad of reactive arthritis, which is thought to be caused by autoimmune cross-reactivity following certain bacterial infections. Reactive arthritis is highly associated with HLA-B27. Conjunctivitis is associated with the autoimmune disease relapsing polychondritis.

Diagnosis

Cultures are not often taken or needed as most cases resolve either with time or typical antibiotics. If bacterial conjunctivitis is suspected, but no response to topical antibiotics is seen, swabs for bacterial culture should be taken and tested. Viral culture may be appropriate in epidemic case clusters.

A patch test is used to identify the causative allergen in allergic conjunctivitis.

Although conjunctival scrapes for cytology can be useful in detecting chlamydial and fungal infections, allergies, and dysplasia, they are rarely done because of the cost and the general dearth of laboratory staff experienced in handling ocular specimens. Conjunctival incisional biopsy is occasionally done when granulomatous diseases (e.g., sarcoidosis) or dysplasia are suspected.

Classification
Conjunctivitis may be classified either by cause or by extent of the inflamed area.

Causes

 * Allergy
 * Bacteria
 * Viruses
 * Chemicals
 * Autoimmune

Neonatal conjunctivitis is often grouped separately from bacterial conjunctivitis because it is caused by different bacteria than the more common cases of bacterial conjunctivitis.

By extent of involvement
Blepharoconjunctivitis is the dual combination of conjunctivitis with blepharitis (inflammation of the eyelids).

Keratoconjunctivitis is the combination of conjunctivitis and keratitis (corneal inflammation).

Blepharokeratoconjunctivitis is the combination of conjunctivitis with blepharitis and keratitis. It is clinically defined by changes of the lid margin, meibomian gland dysfunction, redness of the eye, conjunctival chemosis and inflammation of the cornea.

Differential diagnosis
Some more serious conditions can present with a red eye, such as infectious keratitis, angle-closure glaucoma, or iritis. These conditions require the urgent attention of an ophthalmologist. Signs of such conditions include decreased vision, significantly increased sensitivity to light, inability to keep the eye open, a pupil that does not respond to light, or a severe headache with nausea. Fluctuating blurring is common, due to tearing and mucoid discharge. Mild photophobia is common. However, if any of these symptoms is prominent, considering other diseases such as glaucoma, uveitis, keratitis, and even meningitis or carotico-cavernous fistula is important.

A more comprehensive differential diagnosis for the red or painful eye includes:
 * Corneal abrasion
 * Subconjunctival hemorrhage
 * Pinguecula
 * Blepharitis
 * Dacryocystitis
 * Keratoconjunctivitis sicca (dry eye)
 * Keratitis
 * Herpes simplex
 * Herpes zoster
 * Episcleritis - an inflammatory condition that produces a similar appearance to conjunctivitis, but without discharge or tearing
 * Uveitis
 * Acute angle-closure glaucoma
 * Endophthalmitis
 * Orbital cellulitis

Prevention
The most effective prevention is good hygiene, especially avoiding rubbing the eyes with infected hands. Vaccination against some of the causative pathogens such as Haemophilus influenzae, pneumococcus, and Neisseria meningitidis is also effective.

Povidone-iodine eye solution has been found to prevent neonatal conjunctivitis. It is becoming more commonly used globally because of its low cost.

Management
Conjunctivitis resolves in 65% of cases without treatment, within 2–5 days. The prescription of antibiotics is not necessary in most cases.

Viral
Viral conjunctivitis usually resolves on its own and does not require any specific treatment. Antihistamines (e.g., diphenhydramine) or mast cell stabilizers (e.g., cromolyn) may be used to help with the symptoms. Povidone-iodine has been suggested as a treatment, but as of 2008, evidence to support it was poor.

Allergic
For allergic conjunctivitis, cool water poured over the face with the head inclined downward constricts capillaries, and artificial tears sometimes relieve discomfort in mild cases. In more severe cases, nonsteroidal anti-inflammatory medications and antihistamines may be prescribed. Persistent allergic conjunctivitis may also require topical steroid drops.

Bacterial
Bacterial conjunctivitis usually resolves without treatment. Topical antibiotics may be needed only if no improvement is observed after 3 days. No serious effects were noted either with or without treatment. Because antibiotics do speed healing in bacterial conjunctivitis, their use may be considered. Antibiotics are also recommended for those who wear contact lenses, are immunocompromised, have disease which is thought to be due to chlamydia or gonorrhea, have a fair bit of pain, or have copious discharge. Gonorrheal or chlamydial infections require both oral and topical antibiotics.

The choice of antibiotic varies based on the strain or suspected strain of bacteria causing the infection. Fluoroquinolones, sodium sulfacetamide, or trimethoprim/polymyxin may be used, typically for 7–10 days. Cases of meningococcal conjunctivitis can also be treated with systemic penicillin, as long as the strain is sensitive to penicillin.

When investigated as a treatment, povidone-iodine ophthalmic solution has also been observed to have some effectiveness against bacterial and chlamydial conjunctivitis, with a possible role suggested in locations where topical antibiotics are unavailable or costly.

Chemical
Conjunctivitis due to chemicals is treated via irrigation with Ringer's lactate or saline solution. Chemical injuries, particularly alkali burns, are medical emergencies, as they can lead to severe scarring and intraocular damage. People with chemically induced conjunctivitis should not touch their eyes to avoid spreading the chemical.

Epidemiology
Conjunctivitis is the most common eye disease. Rates of disease is related to the underlying cause which varies by the age as well as the time of year. Acute conjunctivitis is most frequently found in infants, school-age children and the elderly. The most common cause of infectious conjunctivitis is viral conjunctivitis.

It is estimated that acute conjunctivitis affects 6 million people annually in the United States.

Some seasonal trends have been observed for the occurrence of different forms of conjunctivitis. In the northern hemisphere, the occurrence of bacterial conjunctivitis peaks from December to April, viral conjunctivitis peaks in the summer months and allergic conjunctivitis is more prevalent throughout the spring and summer.

History
An adenovirus was first isolated by Rowe et al. in 1953. Two years later, Jawetz et al. published on epidemic keratoconjunctivitis. "Madras eye" is a colloquial term that has been used in India for the disease.

Outbreak in Pakistan
In September 2023, a significant outbreak of conjunctivitis occurred in Pakistan. The outbreak began in Karachi and quickly spread to Lahore, Rawalpindi, and Islamabad. By the end of the month, over 86,133 cases had been reported in Punjab alone. The rapid spread of the disease led to the temporary closure of schools in the region. This event marked one of the largest outbreaks of Pink Eye in the country's recent history.

Society and culture
Conjunctivitis imposes economic and social burdens. The cost of treating bacterial conjunctivitis in the United States was estimated to be $377 million to $857 million per year. Approximately 1% of all primary care office visits in the United States are related to conjunctivitis. Approximately 70% of all people with acute conjunctivitis present to primary care and urgent care.

title: Conjunctivitis


text:
Sedimentary rock

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus (organic matter). The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (marine snow). Sedimentation may also occur as dissolved minerals precipitate from water solution.

The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but sedimentary rock is estimated to be only 8% of the volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. Sedimentary rocks are often deposited in large structures called sedimentary basins. Sedimentary rocks have also been found on Mars.

The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other structures. Sedimentary rocks are also important sources of natural resources including coal, fossil fuels, drinking water and ores.

The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography, paleoclimatology and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry and structural geology.

Classification based on origin

Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, volcanism, and other minor processes.

Clastic sedimentary rocks

Clastic sedimentary rocks are composed of rock fragments (clasts) that have been cemented together. The clasts are commonly individual grains of quartz, feldspar, clay minerals, or mica. However, any type of mineral may be present. Clasts may also be lithic fragments composed of more than one mineral.

Clastic sedimentary rocks are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (1/16 to 2 mm diameter), and mud (<1/16 mm diameter). Mud is further divided into silt (1/16 to 1/256 mm diameter) and clay (<1/256 mm diameter). The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature.

The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks).

Conglomerates and breccias
Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel.

Sandstones

Sandstone classification schemes vary widely, but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains.

 * Composition of framework grains
 * The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance.

 * Quartz sandstones have >90% quartz grains
 * Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains
 * Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grains

 * Abundance of muddy matrix material between sand grains
 * When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud (silt and/or clay sized particle).
 * "Clean" sandstones with open pore space (that may later be filled with matrix material) are called arenites.
 * Muddy sandstones with abundant (>10%) muddy matrix are called wackes.

Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.

Although the Dott classification scheme is widely used by sedimentologists, common names like greywacke, arkose, and quartz sandstone are still widely used by non-specialists and in popular literature.

Mudrocks
Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as the flow calms and the particles settle out of suspension.

Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles. Most authors use "shale" as a term for a fissile mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock.

Biochemical sedimentary rocks
Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include:
 * Most types of limestone are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera.
 * Coal, formed from plants that have removed carbon from the atmosphere and combined it with other elements to build their tissue.
 * Deposits of chert formed from the accumulation of siliceous skeletons of microscopic organisms such as radiolaria and diatoms.

Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals, such as halite (rock salt), sylvite, baryte and gypsum.

Other sedimentary rocks
This fourth miscellaneous category includes volcanic tuff and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events.

Classification based on composition

Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy:
 * Siliciclastic sedimentary rocks, are dominantly composed of silicate minerals. The sediment that makes up these rocks was transported as bed load, suspended load, or by sediment gravity flows. Siliciclastic sedimentary rocks are subdivided into conglomerates and breccias, sandstone, and mudrocks.
 * Carbonate sedimentary rocks are composed of calcite (rhombohedral ), aragonite (orthorhombic ), dolomite, and other carbonate minerals based on the ion. Common examples include limestone and the rock dolomite.
 * Evaporite sedimentary rocks are composed of minerals formed from the evaporation of water. The most common evaporite minerals are carbonates (calcite and others based on ), chlorides (halite and others built on ), and sulfates (gypsum and others built on ). Evaporite rocks commonly include abundant halite (rock salt), gypsum, and anhydrite.
 * Organic-rich sedimentary rocks have significant amounts of organic material, generally in excess of 3% total organic carbon. Common examples include coal, oil shale as well as source rocks for oil and natural gas.
 * Siliceous sedimentary rocks are almost entirely composed of silica, typically as chert, opal, chalcedony or other microcrystalline forms.
 * Iron-rich sedimentary rocks are composed of >15% iron; the most common forms are banded iron formations and ironstones.
 * Phosphatic sedimentary rocks are composed of phosphate minerals and contain more than 6.5% phosphorus; examples include deposits of phosphate nodules, bone beds, and phosphatic mudrocks.

Sediment transport and deposition

Sedimentary rocks are formed when sediment is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. This sediment is often formed when weathering and erosion break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the hinterland (the source area of the sediment). However, some sedimentary rocks, such as evaporites, are composed of material that form at the place of deposition. The nature of a sedimentary rock, therefore, not only depends on the sediment supply, but also on the sedimentary depositional environment in which it formed.

Transformation (Diagenesis)

As sediments accumulate in a depositional environment, older sediments are buried by younger sediments, and they undergo diagenesis. Diagenesis includes all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by a sediment after its initial deposition. This includes compaction and lithification of the sediments. Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and is characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Some biochemical processes, like the activity of bacteria, can affect minerals in a rock and are therefore seen as part of diagenesis.

Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasing overburden (lithostatic) pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals (such as mica) are deformed, and pore space is reduced. Sediments are typically saturated with groundwater or seawater when originally deposited, and as pore space is reduced, much of these connate fluids are expelled. In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. The increased pressure and temperature stimulate further chemical reactions, such as the reactions by which organic material becomes lignite or coal.

Lithification follows closely on compaction, as increased temperatures at depth hasten the precipitation of cement that binds the grains together. Pressure solution contributes to this process of cementation, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. This further reduces porosity and makes the rock more compact and competent.

Unroofing of buried sedimentary rock is accompanied by telogenesis, the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sedimentary rock, such as leaching of some of the cement to produce secondary porosity.

At sufficiently high temperature and pressure, the realm of diagenesis makes way for metamorphism, the process that forms metamorphic rock.

Color
The color of a sedimentary rock is often mostly determined by iron, an element with two major oxides: iron(II) oxide and iron(III) oxide. Iron(II) oxide (FeO) only forms under low oxygen (anoxic) circumstances and gives the rock a grey or greenish colour. Iron(III) oxide (Fe2O3) in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. Thick sequences of red sedimentary rocks formed in arid climates are called red beds. However, a red colour does not necessarily mean the rock formed in a continental environment or arid climate.

The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at the bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales.

Texture

The size, form and orientation of clasts (the original pieces of rock) in a sediment is called its texture. The texture is a small-scale property of a rock, but determines many of its large-scale properties, such as the density, porosity or permeability.

The 3D orientation of the clasts is called the fabric of the rock. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water. The grain size of a rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes. The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of the same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'.

The form of the clasts can reflect the origin of the rock. For example, coquina, a rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters:
 * Surface texture describes the amount of small-scale relief of the surface of a grain that is too small to influence the general shape. For example, frosted grains, which are covered with small-scale fractures, are characteristic of eolian sandstones.
 * Rounding describes the general smoothness of the shape of a grain.
 * Sphericity describes the degree to which the grain approaches a sphere.
 * Grain form describes the three-dimensional shape of the grain.

Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. To describe such a texture, only the average size of the crystals and the fabric are necessary.

Mineralogy

Most sedimentary rocks contain either quartz (siliciclastic rocks) or calcite (carbonate rocks). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in a sedimentary rock may have been present in the original sediments or may formed by precipitation during diagenesis. In the second case, a mineral precipitate may have grown over an older generation of cement. A complex diagenetic history can be established by optical mineralogy, using a petrographic microscope.

Carbonate rocks predominantly consist of carbonate minerals such as calcite, aragonite or dolomite. Both the cement and the clasts (including fossils and ooids) of a carbonate sedimentary rock usually consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral.

The resistance of rock-forming minerals to weathering is expressed by the Goldich dissolution series. In this series, quartz is the most stable, followed by feldspar, micas, and finally other less stable minerals that are only present when little weathering has occurred. The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been weathered to clay minerals like kaolinite, illite or smectite.

Fossils

Among the three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification.

Dead organisms in nature are usually quickly removed by scavengers, bacteria, rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilisation. The chance of fossilisation is higher when the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity occurs) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare.

Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils, examples of which are burrows, footprints, etc.

As a part of a sedimentary rock, fossils undergo the same diagenetic processes as does the host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels, vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralization. The most common minerals involved in permineralization are various forms of amorphous silica (chalcedony, flint, chert), carbonates (especially calcite), and pyrite.

At high pressure and temperature, the organic material of a dead organism undergoes chemical reactions in which volatiles such as water and carbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form, graphite. This form of fossilisation is called carbonisation. It is particularly important for plant fossils. The same process is responsible for the formation of fossil fuels like lignite or coal.

Primary sedimentary structures

Structures in sedimentary rocks can be divided into primary structures (formed during deposition) and secondary structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field. Sedimentary structures can indicate something about the sedimentary environment or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers.

Sedimentary rocks are laid down in layers called beds or strata. A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks is called bedding. Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called lamination. Laminae are usually less than a few centimetres thick. Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at a (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called cross-bedding. Cross-bedding is characteristic of deposition by a flowing medium (wind or water).

The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. Differences in laminations are generally caused by cyclic changes in the sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar to tree rings) are called varves. Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term laminite. When sedimentary rocks have no lamination at all, their structural character is called massive bedding.

Graded bedding is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of turbidity currents.

The surface of a particular bed, called the bedform, can also be indicative of a particular sedimentary environment. Examples of bed forms include dunes and ripple marks. Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition.

Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where the current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current. Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats.

Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers.

Secondary sedimentary structures

Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within the sediment. They can be indicators of circumstances after deposition. Some can be used as way up criteria.

Organic materials in a sediment can leave more traces than just fossils. Preserved tracks and burrows are examples of trace fossils (also called ichnofossils). Such traces are relatively rare. Most trace fossils are burrows of molluscs or arthropods. This burrowing is called bioturbation by sedimentologists. It can be a valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult.

Secondary structures can also form by diagenesis or the formation of a soil (pedogenesis) when a sediment is exposed above the water level. An example of a diagenetic structure common in carbonate rocks is a stylolite. Stylolites are irregular planes where material was dissolved into the pore fluids in the rock. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions. Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be the result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots. In carbonate rocks such as limestone or chalk, chert or flint concretions are common, while terrestrial sandstones sometimes contain iron concretions. Calcite concretions in clay containing angular cavities or cracks are called septarian concretions.

After deposition, physical processes can deform the sediment, producing a third class of secondary structures. Density contrasts between different sedimentary layers, such as between sand and clay, can result in flame structures or load casts, formed by inverted diapirism. While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer. Sometimes, density contrasts occur or are enhanced when one of the lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes. The same process can form mud volcanoes on the surface where they broke through upper layers.

Sedimentary dykes can also be formed in a cold climate where the soil is permanently frozen during a large part of the year. Frost weathering can form cracks in the soil that fill with rubble from above. Such structures can be used as climate indicators as well as way up structures.

Density contrasts can also cause small-scale faulting, even while sedimentation progresses (synchronous-sedimentary faulting). Such faulting can also occur when large masses of non-lithified sediment are deposited on a slope, such as at the front side of a delta or the continental slope. Instabilities in such sediments can result in the deposited material to slump, producing fissures and folding. The resulting structures in the rock are syn-sedimentary folds and faults, which can be difficult to distinguish from folds and faults formed by tectonic forces acting on lithified rocks.

Depositional environments

The setting in which a sedimentary rock forms is called the depositional environment. Every environment has a characteristic combination of geologic processes, and circumstances. The type of sediment that is deposited is not only dependent on the sediment that is transported to a place (provenance), but also on the environment itself.

A marine environment means that the rock was formed in a sea or ocean. Often, a distinction is made between deep and shallow marine environments. Deep marine usually refers to environments more than 200 m below the water surface (including the abyssal plain). Shallow marine environments exist adjacent to coastlines and can extend to the boundaries of the continental shelf. The water movements in such environments have a generally higher energy than that in deep environments, as wave activity diminishes with depth. This means that coarser sediment particles can be transported and the deposited sediment can be coarser than in deeper environments. When the sediment is transported from the continent, an alternation of sand, clay and silt is deposited. When the continent is far away, the amount of such sediment deposited may be small, and biochemical processes dominate the type of rock that forms. Especially in warm climates, shallow marine environments far offshore mainly see deposition of carbonate rocks. The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons. When these organisms die, their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warm shallow marine environments also are ideal environments for coral reefs, where the sediment consists mainly of the calcareous skeletons of larger organisms.

In deep marine environments, the water current working the sea bottom is small. Only fine particles can be transported to such places. Typically sediments depositing on the ocean floor are fine clay or small skeletons of micro-organisms. At 4 km depth, the solubility of carbonates increases dramatically (the depth zone where this happens is called the lysocline). Calcareous sediment that sinks below the lysocline dissolves; as a result, no limestone can be formed below this depth. Skeletons of micro-organisms formed of silica (such as radiolarians) are not as soluble and are still deposited. An example of a rock formed of silica skeletons is radiolarite. When the bottom of the sea has a small inclination, for example, at the continental slopes, the sedimentary cover can become unstable, causing turbidity currents. Turbidity currents are sudden disturbances of the normally quiet deep marine environment and can cause the near-instantaneous deposition of large amounts of sediment, such as sand and silt. The rock sequence formed by a turbidity current is called a turbidite.

The coast is an environment dominated by wave action. At a beach, dominantly denser sediment such as sand or gravel, often mingled with shell fragments, is deposited, while the silt and clay sized material is kept in mechanical suspension. Tidal flats and shoals are places that sometimes dry because of the tide. They are often cross-cut by gullies, where the current is strong and the grain size of the deposited sediment is larger. Where rivers enter the body of water, either on a sea or lake coast, deltas can form. These are large accumulations of sediment transported from the continent to places in front of the mouth of the river. Deltas are dominantly composed of clastic (rather than chemical) sediment.

A continental sedimentary environment is an environment in the interior of a continent. Examples of continental environments are lagoons, lakes, swamps, floodplains and alluvial fans. In the quiet water of swamps, lakes and lagoons, fine sediment is deposited, mingled with organic material from dead plants and animals. In rivers, the energy of the water is much greater and can transport heavier clastic material. Besides transport by water, sediment can be transported by wind or glaciers. Sediment transported by wind is called aeolian and is almost always very well sorted, while sediment transported by a glacier is called glacial till and is characterized by very poor sorting.

Aeolian deposits can be quite striking. The depositional environment of the Touchet Formation, located in the Northwestern United States, had intervening periods of aridity which resulted in a series of rhythmite layers. Erosional cracks were later infilled with layers of soil material, especially from aeolian processes. The infilled sections formed vertical inclusions in the horizontally deposited layers, and thus provided evidence of the sequence of events during deposition of the forty-one layers of the formation.

Sedimentary facies
The kind of rock formed in a particular depositional environment is called its sedimentary facies. Sedimentary environments usually exist alongside each other in certain natural successions. A beach, where sand and gravel is deposited, is usually bounded by a deeper marine environment a little offshore, where finer sediments are deposited at the same time. Behind the beach, there can be dunes (where the dominant deposition is well sorted sand) or a lagoon (where fine clay and organic material is deposited). Every sedimentary environment has its own characteristic deposits. When sedimentary strata accumulate through time, the environment can shift, forming a change in facies in the subsurface at one location. On the other hand, when a rock layer with a certain age is followed laterally, the lithology (the type of rock) and facies eventually change.

Facies can be distinguished in a number of ways: the most common are by the lithology (for example: limestone, siltstone or sandstone) or by fossil content. Coral, for example, only lives in warm and shallow marine environments and fossils of coral are thus typical for shallow marine facies. Facies determined by lithology are called lithofacies; facies determined by fossils are biofacies.

Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when the sea level drops (regression), when the surface rises (transgression) due to tectonic forces in the Earth's crust or when a river forms a large delta. In the subsurface, such geographic shifts of sedimentary environments of the past are recorded in shifts in sedimentary facies. This means that sedimentary facies can change either parallel or perpendicular to an imaginary layer of rock with a fixed age, a phenomenon described by Walther's Law.

The situation in which coastlines move in the direction of the continent is called transgression. In the case of transgression, deeper marine facies are deposited over shallower facies, a succession called onlap. Regression is the situation in which a coastline moves in the direction of the sea. With regression, shallower facies are deposited on top of deeper facies, a situation called offlap.

The facies of all rocks of a certain age can be plotted on a map to give an overview of the palaeogeography. A sequence of maps for different ages can give an insight in the development of the regional geography.

Sedimentary basins

Places where large-scale sedimentation takes place are called sedimentary basins. The amount of sediment that can be deposited in a basin depends on the depth of the basin, the so-called accommodation space. The depth, shape and size of a basin depend on tectonics, movements within the Earth's lithosphere. Where the lithosphere moves upward (tectonic uplift), land eventually rises above sea level and the area becomes a source for new sediment as erosion removes material. Where the lithosphere moves downward (tectonic subsidence), a basin forms and sediments are deposited.

A type of basin formed by the moving apart of two pieces of a continent is called a rift basin. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is stretched and thinned, so that the hot asthenosphere rises and heats the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting of volcanic deposits. When the basin grows due to continued stretching of the lithosphere, the rift grows and the sea can enter, forming marine deposits.

When a piece of lithosphere that was heated and stretched cools again, its density rises, causing isostatic subsidence. If this subsidence continues long enough, the basin is called a sag basin. Examples of sag basins are the regions along passive continental margins, but sag basins can also be found in the interior of continents. In sag basins, the extra weight of the newly deposited sediments is enough to keep the subsidence going in a vicious circle. The total thickness of the sedimentary infill in a sag basin can thus exceed 10 km.

A third type of basin exists along convergent plate boundaries – places where one tectonic plate moves under another into the asthenosphere. The subducting plate bends and forms a fore-arc basin in front of the overriding plate – an elongated, deep asymmetric basin. Fore-arc basins are filled with deep marine deposits and thick sequences of turbidites. Such infill is called flysch. When the convergent movement of the two plates results in continental collision, the basin becomes shallower and develops into a foreland basin. At the same time, tectonic uplift forms a mountain belt in the overriding plate, from which large amounts of material are eroded and transported to the basin. Such erosional material of a growing mountain chain is called molasse and has either a shallow marine or a continental facies.

At the same time, the growing weight of the mountain belt can cause isostatic subsidence in the area of the overriding plate on the other side to the mountain belt. The basin type resulting from this subsidence is called a back-arc basin and is usually filled by shallow marine deposits and molasse.

Influence of astronomical cycles
In many cases facies changes and other lithological features in sequences of sedimentary rock have a cyclic nature. This cyclic nature was caused by cyclic changes in sediment supply and the sedimentary environment. Most of these cyclic changes are caused by astronomic cycles. Short astronomic cycles can be the difference between the tides or the spring tide every two weeks. On a larger time-scale, cyclic changes in climate and sea level are caused by Milankovitch cycles: cyclic changes in the orientation and/or position of the Earth's rotational axis and orbit around the Sun. There are a number of Milankovitch cycles known, lasting between 10,000 and 200,000 years.

Relatively small changes in the orientation of the Earth's axis or length of the seasons can be a major influence on the Earth's climate. An example are the ice ages of the past 2.6 million years (the Quaternary period), which are assumed to have been caused by astronomic cycles. Climate change can influence the global sea level (and thus the amount of accommodation space in sedimentary basins) and sediment supply from a certain region. Eventually, small changes in astronomic parameters can cause large changes in sedimentary environment and sedimentation.

Sedimentation rates
The rate at which sediment is deposited differs depending on the location. A channel in a tidal flat can see the deposition of a few metres of sediment in one day, while on the deep ocean floor each year only a few millimetres of sediment accumulate. A distinction can be made between normal sedimentation and sedimentation caused by catastrophic processes. The latter category includes all kinds of sudden exceptional processes like mass movements, rock slides or flooding. Catastrophic processes can see the sudden deposition of a large amount of sediment at once. In some sedimentary environments, most of the total column of sedimentary rock was formed by catastrophic processes, even though the environment is usually a quiet place. Other sedimentary environments are dominated by normal, ongoing sedimentation.

In many cases, sedimentation occurs slowly. In a desert, for example, the wind deposits siliciclastic material (sand or silt) in some spots, or catastrophic flooding of a wadi may cause sudden deposits of large quantities of detrital material, but in most places eolian erosion dominates. The amount of sedimentary rock that forms is not only dependent on the amount of supplied material, but also on how well the material consolidates. Erosion removes most deposited sediment shortly after deposition.

Stratigraphy
That new rock layers are above older rock layers is stated in the principle of superposition. There are usually some gaps in the sequence called unconformities. These represent periods where no new sediments were laid down, or when earlier sedimentary layers were raised above sea level and eroded away.

Sedimentary rocks contain important information about the history of the Earth. They contain fossils, the preserved remains of ancient plants and animals. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains.

Provenance

Provenance is the reconstruction of the origin of sediments. All rock exposed at Earth's surface is subjected to physical or chemical weathering and broken down into finer grained sediment. All three types of rocks (igneous, sedimentary and metamorphic rocks) can be the source of sedimentary detritus. The purpose of sedimentary provenance studies is to reconstruct and interpret the history of sediment from the initial parent rocks at a source area to final detritus at a burial place.

title: Sedimentary rock