Roderick Impey Murchison, 1st Baronet KCB DCL FRS FRSE FLS PRGS PBA MRIA was a Scottish geologist who first described and investigated the Silurian system. Murchison was born at Muir of Ord, Ross-shire, the son of Kenneth Murchison, his wealthy father died in 1796, when Roderick was four years old, he was sent to Durham School three years and the Royal Military College, Great Marlow to be trained for the army. In 1808 he landed with Wellesley in Galicia, was present at the actions of Roliça and Vimeiro. Subsequently under Sir John Moore, he took part in the retreat to Corunna and the final battle there. After eight years of service Murchison left the army, married Charlotte Hugonin, the only daughter of General Hugonin, of Nursted House, Hampshire. Murchison and his wife spent two years in mainland Europe in Italy, they settled in Barnard Castle, County Durham, England in 1818 where Murchison made the acquaintance of Sir Humphry Davy. Davy urged Murchison to turn his energy to science, after hearing that he wasted his time riding to hounds and shooting.
Murchison became fascinated by the young science of geology and joined the Geological Society of London, soon becoming one of its most active members. His colleagues there included Adam Sedgwick, William Conybeare, William Buckland, William Fitton, Charles Lyell and Charles Darwin. Exploring with his wife, Murchison studied the geology of the south of England, devoting special attention to the rocks of the north-west of Sussex and the adjoining parts of Hampshire and Surrey, on which, aided by Fitton, he wrote his first scientific paper, read to the Geological Society of London in 1825. Turning his attention to Continental geology, he and Lyell explored the volcanic region of Auvergne, parts of southern France, northern Italy and Switzerland. A little with Sedgwick as his companion, Murchison attacked the difficult problem of the geological structure of the Alps, their joint paper giving the results of their study is a classic in the literature of Alpine geology. Murchison was an opponent of Charles Darwin's theory of evolution.
He supported successive creation. In 1831 he went to the England–Wales border, to attempt to discover whether the greywacke rocks underlying the Old Red Sandstone could be grouped into a definite order of succession; the result was the establishment of the Silurian system under which were grouped, for the first time, a remarkable series of formations, each replete with distinctive organic remains other than and different from those of the other rocks of England. These researches, together with descriptions of the coalfields and overlying formations in South Wales and the English border counties, were embodied in The Silurian System; the establishment of the Silurian system was followed by that of the Devonian system, an investigation in which Murchison assisted, both in the south-west of England and in the Rhineland. Soon afterwards Murchison projected an important geological campaign in Russia with the view of extending to that part of the Continent the classification he had succeeded in elaborating for the older rocks of western Europe.
He was accompanied by Édouard de Verneuil and Count Alexander von Keyserling, in conjunction with whom he produced a work on Russia and the Ural Mountains. The publication of this monograph in 1845 completes the first and most active half of Murchison's scientific career, he was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1840. In 1846 he was knighted, in the same year he presided over the meeting of the British Association at Southampton. During the years of his life a large part of his time was devoted to the affairs of the Royal Geographical Society, of which he was in 1830 one of the founders, he was president 1843–1845, 1851–1853, 1856–1859 and 1862–1871, he served on the Royal Commission on the British Museum. The chief geological investigation of the last decade of his life was devoted to the Highlands of Scotland, where he wrongly believed he had succeeded in showing that the vast masses of crystalline schists supposed to be part of what used to be termed the Primitive formations, were not older than the Silurian period, for that underneath them lay beds of limestone and quartzite containing Lower Silurian fossils.
James Nicol recognised the fallacy in the Murchison's extant theory and propounded his own ideas, in the 1880s these were superseded by the correct theory of Charles Lapworth, corroborated by Benjamin Peach and John Horne. Their subsequent research showed that the infraposition of the fossiliferous rocks is not their original place, but had been brought about by a gigantic system of dislocations, whereby successive masses of the oldest gneisses, have been exhumed from below and thrust over the younger formations. In 1855 Murchison was appointed director-general of the British Geological Survey and director of the Royal School of Mines and the Museum of Practical Geology in Jermyn Street, London, in succession to Sir Henry De la Beche, the first to hold these offices. Official routine now occupied much of his time, but he found opportunity for the Highland researches just alluded to, for preparing successive editions of his work Siluria, meant to present the main features of the original Silurian System together with a digest of subsequent discoveries of those that showed the extension of the Silurian classification into other countries.
In 1845, whilst visiting Carclew in Cornwall, he met several Cornish miners who were going to Australia. Believing that there might be gold there he asked them to s
Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region of Devonian age. It extends in the east across Great Britain and Norway, in the west along the northeastern seaboard of North America, it extends northwards into Greenland and Svalbard. In Britain it is a lithostratigraphic unit to which stratigraphers accord supergroup status and, of considerable importance to early paleontology. For convenience the short version of the term, ORS is used in literature on the subject; the term was coined to distinguish the sequence from the younger New Red Sandstone which occurs throughout Britain. The Old Red Sandstone describes a suite of sedimentary rocks deposited in a variety of environments during the Devonian but extending back into the late Silurian and on into the earliest part of the Carboniferous; the body of rock, or facies, is dominated by alluvial sediments and conglomerates at its base, progresses to a combination of dunes and river sediments. The familiar red colour of these rocks arises from the presence of iron oxide but not all the Old Red Sandstone is red or sandstone — the sequence includes conglomerates, mudstones and thin limestones and colours can range from grey and green through to red and purple.
These deposits are associated with the erosion of the Caledonian Mountain chain, thrown up by the collision of the former continents of Avalonia and Laurentia to form the Old Red Sandstone Continent- an event known as the Caledonian Orogeny. Many fossils are found within the rocks, including early fishes and plants; the rocks may appear paleontologically barren to amateur geologists but careful study with an accomplished fossil hunter, can uncover pockets of fossils. Rocks of this age were laid down in southwest England though these are of true marine origin and are not included within the Old Red Sandstone. Since the Old Red Sandstone consists predominantly of rocks of terrestrial origin, it does not contain marine fossils which would otherwise prove useful in correlating one occurrence of the rock with another, both between and within individual sedimentary basins. Accordingly, local stage names were devised and these remain in use to some extent today though there is an increasing use of international stage names.
Thus in the Anglo-Welsh Basin, there are frequent references to the Downtonian, Dittonian and Farlovian stages in the literature. The existence of a number of distinct sedimentary basins throughout Britain has been established; the Orcadian Basin extends over a wide area of the neighbouring seas. It encompasses the Moray Firth and adjoining land areas, Caithness and parts of Shetland. South of the Moray Firth, two distinct sub-basins are recognised at Rhynie; the sequence is more than 4 kilometres thick in parts of Shetland. The main basin is considered to be an intramontane basin resulting from crustal rifting associated with post-Caledonian extension accompanied by strike-slip faulting along the Great Glen Fault system. There are a scatter of exposures of the Old Red Sandstone around the Oban area on the West Highland coast where a conglomerate of andesite boulders rests unconformably on Dalradian schists of the Easdale Subgroup, they are interpreted as alluvial fans which filled a depositional basin from the northeast.
The deposits are obvious on Kerrera where they form the bedrock across half of the island. The Kerrera Sandstone Formation is up to 128m thick in its type area and consists of green and red sandstones and conglomerates accompanied by siltstones and limestones. Small outliers occur near either side of Loch Avich; the Midland Valley graben defined by the Highland Boundary Fault in the north and the Southern Uplands Fault in the south harbours not only a considerable amount of Old Red Sandstone sedimentary rocks but igneous rocks of this age associated with extensive volcanism. There is a continuous outcrop along the Highland Boundary Fault from Stonehaven on the North Sea coast to Helensburgh and beyond to Arran. A more disconnected series of outcrops occur along the line of the Southern Uplands Fault from Edinburgh to Girvan. Old Red Sandstone occurs in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve, Kincardineshire. A series of outcrops occur from East Lothian southwards through Berwickshire.
Hutton's famous unconformity at Siccar Point occurs within this basin - see History of study below. This large basin extends across much of South Wales from southern Pembrokeshire in the west through Carmarthenshire into Powys and Monmouthshire and through the southern Welsh Marches, notably into Herefordshire and Gloucestershire. Outliers in Somerset and north Devon complete the extent of this basin. With the exception of south Pembrokeshire, all parts of the basin are represented by a range of lithologies assigned to the Lower Devonian and to the Upper Devonian, the contact between the two being unconformable and representing the complete omission of any Middle Devonian sequence; the lowermost formations are of upper Silurian age, these being the Downton Castle Sandstone Formation and the overlying Raglan Mudstone Formation except in Pembrokeshire where a more complex series of formations is recognised. In the east of the basin, the top of the Raglan Mudstone is marked by a well-developed calcrete, the Bishop's Frome Limestone.
The lowermost Devonian formation is the St Maughans Formation, itself overlain by the Brownstones Formation though with an intervening Senni Formation over much of the area
Devon known as Devonshire, its common and official name, is a county of England, reaching from the Bristol Channel in the north to the English Channel in the south. It is part of South West England, bounded by Cornwall to the west, Somerset to the north east, Dorset to the east; the city of Exeter is the county town. The county includes the districts of East Devon, Mid Devon, North Devon, South Hams, Teignbridge and West Devon. Plymouth and Torbay are each geographically part of Devon, but are administered as unitary authorities. Combined as a ceremonial county, Devon's area is 6,707 km2 and its population is about 1.1 million. Devon derives its name from Dumnonia. During the British Iron Age, Roman Britain, the early Middle Ages, this was the homeland of the Dumnonii Brittonic Celts; the Anglo-Saxon settlement of Britain resulted in the partial assimilation of Dumnonia into the Kingdom of Wessex during the eighth and ninth centuries. The western boundary with Cornwall was set at the River Tamar by King Æthelstan in 936.
Devon was constituted as a shire of the Kingdom of England. The north and south coasts of Devon each have both cliffs and sandy shores, the county's bays contain seaside resorts, fishing towns, ports; the inland terrain is rural and hilly, has a lower population density than many other parts of England. Dartmoor is the largest open space in southern England, at 954 km2. To the north of Dartmoor are the Culm Measures and Exmoor. In the valleys and lowlands of south and east Devon the soil is more fertile, drained by rivers including the Exe, the Culm, the Teign, the Dart, the Otter; as well as agriculture, much of the economy of Devon is based on tourism. The comparatively mild climate and landscape make Devon a destination for recreation and leisure in England, with visitors attracted to the Dartmoor and Exmoor national parks; the name Devon derives from the name of the Britons who inhabited the southwestern peninsula of Britain at the time of the Roman conquest of Britain known as the Dumnonii, thought to mean "deep valley dwellers" from proto Celtic *dubnos'deep'.
In the Brittonic, Devon is known as Welsh: Dyfnaint, Breton: Devnent and Cornish: Dewnens, each meaning "deep valleys." Among the most common Devon placenames is -combe which derives from Brittonic cwm meaning'valley' prefixed by the name of the possessor. William Camden, in his 1607 edition of Britannia, described Devon as being one part of an older, wider country that once included Cornwall: THAT region which, according to the Geographers, is the first of all Britaine, growing straiter still and narrower, shooteth out farthest into the West, was in antient time inhabited by those Britans whom Solinus called Dumnonii, Ptolomee Damnonii For their habitation all over this Countrey is somewhat low and in valleys, which manner of dwelling is called in the British tongue Dan-munith, in which sense the Province next adjoyning in like respect is at this day named by the Britans Duffneit, to say, Low valleys, but the Country of this nation is at this day divided into two parts, knowen by names of Cornwall and Denshire, The term "Devon" is used for everyday purposes e.g. "Devon County Council" but "Devonshire" continues to be used in the names of the "Devonshire and Dorset Regiment" and "The Devonshire Association".
One erroneous theory is that the "shire" suffix is due to a mistake in the making of the original letters patent for the Duke of Devonshire, resident in Derbyshire. However, there are references to "Defenascire" in Anglo-Saxon texts from before 1000 AD, which translates to modern English as "Devonshire"; the term Devonshire may have originated around the 8th century, when it changed from Dumnonia to Defenascir. Kents Cavern in Torquay had produced. Dartmoor is thought to have been occupied by Mesolithic hunter-gatherer peoples from about 6000 BC; the Romans held the area under military occupation for around 350 years. The area began to experience Saxon incursions from the east around 600 AD, firstly as small bands of settlers along the coasts of Lyme Bay and southern estuaries and as more organised bands pushing in from the east. Devon became a frontier between Brittonic and Anglo-Saxon Wessex, it was absorbed into Wessex by the mid 9th century. A genetic study carried out by the University of Oxford & University College London discovered separate genetic groups in Cornwall and Devon, not only were there differences on either side of the Tamar, with a division exactly along the modern county boundary dating back to the 6th Century but between Devon and the rest of Southern England, similarities with the modern northern France, including Brittany.
This suggests the Anglo-Saxon migration into Devon was limited rather than a mass movement of people. The border with Cornwall was set by King Æthelstan on the east bank of the River Tamar in 936 AD. Danish raids occurred sporadically along many coastal parts of Devon between around 800AD and just before the time of the Norman conquest, including the silver mint at Hlidaforda Lydford in 997 and Taintona in 1001. Devon has featured in most of th
England is a country, part of the United Kingdom. It shares land borders with Wales to Scotland to the north-northwest; the Irish Sea lies west of England and the Celtic Sea lies to the southwest. England is separated from continental Europe by the North Sea to the east and the English Channel to the south; the country covers five-eighths of the island of Great Britain, which lies in the North Atlantic, includes over 100 smaller islands, such as the Isles of Scilly and the Isle of Wight. The area now called England was first inhabited by modern humans during the Upper Palaeolithic period, but takes its name from the Angles, a Germanic tribe deriving its name from the Anglia peninsula, who settled during the 5th and 6th centuries. England became a unified state in the 10th century, since the Age of Discovery, which began during the 15th century, has had a significant cultural and legal impact on the wider world; the English language, the Anglican Church, English law – the basis for the common law legal systems of many other countries around the world – developed in England, the country's parliamentary system of government has been adopted by other nations.
The Industrial Revolution began in 18th-century England, transforming its society into the world's first industrialised nation. England's terrain is chiefly low hills and plains in central and southern England. However, there is upland and mountainous terrain in the west; the capital is London, which has the largest metropolitan area in both the United Kingdom and the European Union. England's population of over 55 million comprises 84% of the population of the United Kingdom concentrated around London, the South East, conurbations in the Midlands, the North West, the North East, Yorkshire, which each developed as major industrial regions during the 19th century; the Kingdom of England – which after 1535 included Wales – ceased being a separate sovereign state on 1 May 1707, when the Acts of Union put into effect the terms agreed in the Treaty of Union the previous year, resulting in a political union with the Kingdom of Scotland to create the Kingdom of Great Britain. In 1801, Great Britain was united with the Kingdom of Ireland to become the United Kingdom of Great Britain and Ireland.
In 1922 the Irish Free State seceded from the United Kingdom, leading to the latter being renamed the United Kingdom of Great Britain and Northern Ireland. The name "England" is derived from the Old English name Englaland, which means "land of the Angles"; the Angles were one of the Germanic tribes that settled in Great Britain during the Early Middle Ages. The Angles came from the Anglia peninsula in the Bay of Kiel area of the Baltic Sea; the earliest recorded use of the term, as "Engla londe", is in the late-ninth-century translation into Old English of Bede's Ecclesiastical History of the English People. The term was used in a different sense to the modern one, meaning "the land inhabited by the English", it included English people in what is now south-east Scotland but was part of the English kingdom of Northumbria; the Anglo-Saxon Chronicle recorded that the Domesday Book of 1086 covered the whole of England, meaning the English kingdom, but a few years the Chronicle stated that King Malcolm III went "out of Scotlande into Lothian in Englaland", thus using it in the more ancient sense.
According to the Oxford English Dictionary, its modern spelling was first used in 1538. The earliest attested reference to the Angles occurs in the 1st-century work by Tacitus, Germania, in which the Latin word Anglii is used; the etymology of the tribal name itself is disputed by scholars. How and why a term derived from the name of a tribe, less significant than others, such as the Saxons, came to be used for the entire country and its people is not known, but it seems this is related to the custom of calling the Germanic people in Britain Angli Saxones or English Saxons to distinguish them from continental Saxons of Old Saxony between the Weser and Eider rivers in Northern Germany. In Scottish Gaelic, another language which developed on the island of Great Britain, the Saxon tribe gave their name to the word for England. An alternative name for England is Albion; the name Albion referred to the entire island of Great Britain. The nominally earliest record of the name appears in the Aristotelian Corpus the 4th-century BC De Mundo: "Beyond the Pillars of Hercules is the ocean that flows round the earth.
In it are two large islands called Britannia. But modern scholarly consensus ascribes De Mundo not to Aristotle but to Pseudo-Aristotle, i.e. it was written in the Graeco-Roman period or afterwards. The word Albion or insula Albionum has two possible origins, it either derives from a cognate of the Latin albus meaning white, a reference to the white cliffs of Dover or from the phrase the "island of the Albiones" in the now lost Massaliote Periplus, attested through Avienus' Ora Maritima to which the former served as a source. Albion is now applied to England in a more poetic capacity. Another romantic name for England is Loegria, related to the Welsh word for England and made popular by its use in Arthurian legend; the earliest known evidence of human presence in the area now known as England was that of Homo antecessor, dating to approximate
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era; as with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes and coastlines, terrestrial arthropods are first found on land during the Silurian. However, terrestrial life would not diversify and affect the landscape until the Devonian; the Silurian system was first identified by British geologist Roderick Murchison, examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.
He named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales. This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, the germ of the modern geological time scale; as it was first identified, the "Silurian" series when traced farther afield came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland; the French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge.
He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, the stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils; the Llandovery Epoch lasted from 443.8 ± 1.5 to 433.4 ± 2.8 mya, is subdivided into three stages: the Rhuddanian, lasting until 440.8 million years ago, the Aeronian, lasting to 438.5 million years ago, the Telychian. The epoch is named for the town of Llandovery in Wales; the Wenlock, which lasted from 433.4 ± 1.5 to 427.4 ± 2.8 mya, is subdivided into the Sheinwoodian and Homerian ages. It is named after Wenlock Edge in England. During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia, appear; the complexity of later Gondwana plants like Baragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or Ordovician.
The first terrestrial animals appear in the Wenlock, represented by air-breathing millipedes from Scotland. The Ludlow, lasting from 427.4 ± 1.5 to 423 ± 2.8 mya, comprises the Gorstian stage, lasting until 425.6 million years ago, the Ludfordian stage. It is named for the town of Ludlow in England; the Přídolí, lasting from 423 ± 1.5 to 419.2 ± 2.8 mya, is the final and shortest epoch of the Silurian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area. In North America a different suite of regional stages is sometimes used: Cayugan Lockportian Tonawandan Ontarian Alexandrian In Estonia the following suite of regional stages is used: Ohessaare stage Kaugatuma stage Kuressaare stage Paadla stage Rootsiküla stage Jaagarahu stage Jaani stage Adavere stage Raikküla stage Juuru stage With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.
The high sea levels of the Silurian and the flat land resulted in a number of island chains, thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation; the southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity; the continents of Avalonia and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. When the proto-Europe coll
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine