Geologic Time Scale Geologic time scale From Wikipedia, the free encyclopedia: See notes at the ends of "Major events" about carbon dioxide concentrations. The geologic time scale is a chronologic schema (or idealized Model ) relating stratigraphy to time that is used by geologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of Earth . The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy , and uses the standard color codes of the United States Geological Survey . Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Graphical timelines The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. Millions of Years The Holocene (the latest epoch ) is too small to be shown clearly on this timeline. Terminology Chron Generic term for any identifiable time period; not necessarily part of a hierarchy The largest defined unit of time is the supereon, composed of eons.

Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem , erathem , system , series , and stage are used to refer to the layers of rock that correspond to these periods of geologic time. Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper Jurassic ", and "Middle Cambrian ". Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone ," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition " or " fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is referred to as the Waucoban series that is then subdivided into zones based on trilobites .

The same timespan is split into Tommotian , Atdabanian and Botomian stages in East Asia and Siberia . A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.[ citations needed ] History of the time scale See the main articles: history of geology and history of paleontology . Earth history mapped to 24 hours The first geologic time scale was proposed in 1913 by the British geologist Arthur Holmes . [2] He greatly furthered the newly created discipline of geochronology and published the world renowned book The Age of the Earth in 1913 in which he estimated the Earth 's age to be at least 1.6 billion years. [3] Aristotle realized that fossil seashells from rocks were similar to those found on the beach, indicating the fossils were once living animals. He deduced that the positions of land and sea had changed and these changes occurred over long periods of time. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.

[4] One of the principles underlying geologic time scales was the principle of superposition of strata, first proposed in the 11th century by the Persian geologist , Avicenna (Ibn Sina). However, he rejected the explanation of fossils as organic remains. [5] While discussing the origins of mountains in The Book of Healing in 1027, he outlined the principle as follows: [6] [7] "It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on.

Over each layer there spread a substance of different material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimentary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains." His contemporary, Abū Rayhān Bīrūnī (973-1048), discovered the existence of shells and fossils in regions that once housed seas and later evolved into dry land, such as the Indian subcontinent . Based on this evidence, he realized that the Earth is constantly evolving and proposed that the Earth had an age, but that its origin was too distant to measure. [8] Later in the 11th century, the Chinese naturalist , Shen Kuo (1031-1095), also recognized the concept of ' deep time '.

[9] The principles underlying geologic (geological) time scales were later laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the law of superposition , which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th century geologists realized that: Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition ; Strata laid down at the same time in different areas could have entirely different appearances; The strata of any given area represented only part of the Earth's long history. A comparative geological timescale The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner , among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary.

Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 20th century.

In opposition to the then-popular Neptunist theories expounded by Werner (that all rocks had precipitated out of a single enormous flood), a major shift in thinking came with the reading by James Hutton of his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe before the Royal Society of Edinburgh in March and April 1785, events which "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology" [10] What Hutton proposed was that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood-oriented theory. The identification of strata by the fossils they contained, pioneered by William Smith , Georges Cuvier , Jean d'Omalius d'Halloy and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely.

It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today. The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for Wales ) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales. [11] The "Devonian" was named for the English county of Devon , and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm , Russia , because it was defined using strata in that region by a Scottish geologist Roderick Murchison . However, some periods were defined by geologists from other countries.

The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers ( Latin trias meaning triad) ï¿½ red beds , capped by chalk , followed by black shales ï¿½ that are found throughout Germany and Northwest Europe , called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains . The "Cretaceous" (from Latin creta meaning ' chalk ') as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin [12] and named for the extensive beds of chalk ( calcium carbonate deposited by the shells of marine invertebrates ). British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable.

While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible , early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils , and estimated the time scales based on studying rates of various kinds of weathering , erosion , sedimentation , and lithification . Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes ) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate. In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy ) started an effort to define global references ( Global Boundary Stratotype Sections and Points ) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al. [13] .

A UML model for how the timescale is structured, relating it to the GSSP, is also available [14] . Table of geologic time The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. (See lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon.) The height of each table entry does not correspond to the duration of each subdivision of time. v ï¿½ d ï¿½ e Geologic time scale [hide] Ireland and Plate Tectonics Ireland and Plate Tectonics Enlarge image Plate tectonics is the name given to the titanic conflicts and rearrangements over hundreds of millions of years between the slow-moving large units or plates, which make up and determine the Earth's 'skin' or crust. At various periods in the past, the Earth's geography has been very different to what it is today. Continents have united and moved apart, and then regrouped in other ways. Oceans have appeared and disappeared.

Because of the movements, places that are now part of the present-day continents may once have been at very different latitudes, and may even have been linked to very different areas. These changes are driven by developments inside the Earth, most notably the uprising of molten magma from deep inside the Earth to or near to, the Earth's surface, and, at other parts of the surface, the sucking down or 'subduction' of old rock back inside the Earth. Magma appears mainly through a series of openings along the ridges that are to be found near the middle of the Earth's oceans. Here new rock is created and pushed outwards from the ridge by sea-floor spreading. In contrast subduction occurs at the great trenches that mark the deepest parts of the oceans. Studies of plate tectonics tell us that Ireland once had a very different setting within world geography. In fact some 450 million years ago north-west Ireland belonged to a landmass (Laurentia) that is now mainly represented in North America, whereas the rest of Ireland lay beyond an ocean several thousand miles to the south and belonged to another plate.

As the two plates moved toward each other, a mountain-building phase developed that is sometimes known as the Caledonian or Caledonide. The two parts of Ireland finally merged about 440-400 million years ago when the separating ocean closed and the two plates collided. The impact of that prolonged collision (or series of collisions) is still evident in Ireland today, being expressed in the way mountains over much of the island, particularly the north, display long axes that trend north-east to south-west - a direction that reflects the line along which the plates clashed. Enlarge image In later times the area that is now Ireland moved north, reaching close to the equator by approximately 300-350 million years ago. During a long period, now identified as the Carboniferous (which lasted over 50 million years), the sea extended across Ireland from the south. Initially sandstones, shales and mudstones were formed in shallow seas; then the constituents of what became thick deposits of limestones accumulated in quiet, warm seas similar to the present-day Persian Gulf. Later, after the formation of marine shales and other rocks, these areas became land.

In swampy tropical conditions, highly-organic materials which became coal measures accumulated. During another great mountain-building period, sometimes called the Variscan (but also widely known as the Hercynian or Armorican), pressure came from the south, producing strong folding in the southern part of Ireland. The legacy of this period is now evident in the east-west trending mountains of south Munster, with axes very different to the Caledonide-influenced mountains further north. Enlarge image Still later, the Atlantic re-opened and widened. About sixty million years ago Ireland experienced the impact of sea-flooring spreading. The north-east, particularly, was caught up in a prolonged period of magma-spreading and volcanic activity that also affected an area from western Scotland to Iceland and Greenland. This period, which may have lasted ten million years, was when lavas spread across much of what is now Antrim-Derry, producing distinctive black basalt rocks. It was during this time that the famous 'columnar' basalts exposed at the Giant's Causeway , on the north coast of Co. Antrim, were created.

About this time, too, the Alps were being created far to the south, prompting geologists to refer to this general period as the Alpine mountain-building phase. These great long-time shifts in plate relations now provide the framework within which the geology of Ireland is studied. As well as mountain-building, the impact of plate tectonics is expressed in earthquakes and in the faulting or fracturing of the earth's crust. These tectonic processes can produce the uplift and lowering of sections of a landmass. There is increasing evidence that such processes may have been significant in Ireland during the Tertiary period. For our study of the physical landscape, plate tectonics also contributes to two other relevant topics: the rock cycle , and the geological time scale . � P Q | Fforest Fawr Fforest Fawr P Palaeogene / Palaeogen Name given to the first (older) of two new periods which replace the Tertiary Period. It lasted 43 million years from 66 to 23 million years ago and precedes the Neogene.

Palaeozoic / Palaeosoig Literally ‘ancient life’ – the era of geological time which comprises the Cambrian, Ordovician, Silurian, Devonian and Carboniferous periods. Succeeded by the Mesozoic Era. Pangaea or Pangea / Pangaea A super-continent which came into being during the Carboniferous Period before gradually breaking up to give the modern assemblage of continents. It was surrounded by Panthalassa – the global sea. Pennant Measures / Cystradau Pennant Subdivision of the Carboniferous Period. See Carboniferous timechart . Pennant Sandstone / Tywodfaen Pennant Collective name for various thick sandstones laid down during the Carboniferous Period and forming the plateaux into which the Welsh Coalfield valleys to the south of Fforest Fawr have been deeply cut. Pentre Bach Syncline / Synclin Pentre Bach A major NE-SW trending downfold of rocks of Silurian age seen to the SE of Llandovery. period / cyfnod A major division of geological time. The rocks formed within a period are termed a ‘system’. See geological timescale .

Permian / Permaidd Geological period which lasted 47 million years from 299 to 252 million years ago. See geological timescale . Plateau Beds / Haenau’r Llwyfandir A succession of hard-wearing sandstones laid down during the upper Devonian Period which form the summits of many of the higher hills of Fforest Fawr Geopark. Pontar-Llechau Formation / Ffurfiant Pontarllechau Thin succession of sandstones near the base of the Old Red Sandstone and of late Silurian age. The (officially mis-spelt) name comes from Pont ar Llechau over the Afon Sawdde. Precambrian / Cyn-Gambriaidd All of geological time before the start of the Cambrian Period 541 million years ago. Since the Earth is believed to have come into being nearly 4540 million years ago, it represents nearly 90% of all geological history. Pridoli / Pridoli Latest epoch of the Silurian Period during which the rocks of the Raglan and Temeside Mudstone Formations were laid down.

The Pridoli is also the earliest (ie lowermost) part of the Old Red Sandstone. See Silurian timechart . protalus (or pronival) rampart / cefnen eirdreulio Accumulation of rocky debris at the foot of a semi-permanent snow-patch. Once the snow has melted, it remains as a stony ridge. Psammosteus Limestone / Calchfaen Psammosteus Distinctive thin limestone between the Raglan Mudstone Formation and the St Maughan’s Formation. Q Quaternary / Cwaternaidd Current geological period which (by different definitions) has so far lasted 1.8, 2.4 or 2.6 million years. See Quaternary timechart . © Copyright Brecon Beacons 2011 | Design and technology by One Bright Space PALAEONTOLOGY[online] | Article: Fossil Focus > Fossil Focus: Coal swamps by Ben Slater *1 Introduction: Coal swamps are the classical terrestrial (land-based) ecosystems of the Carboniferous and Permian periods.

They are forests that grew during the Palaeozoic Era (encompassing the Carboniferous and Permian) in which the volume of plant biomass dying and being deposited in the ground was greater than the volume of clastic (grains of pre-existing rock) material, resulting in a build-up of peat. This was subsequently buried, and eventually turned into coal over geological time. These swamps gave rise to most of the major, industrial-grade coal reserves that are mined today. The palaeontology of these coal-forming ecosystems is well known from the Carboniferous rocks of Euramerica (modern day Europe and North America), owing to the history of coal exploitation in these regions. However, extensive swamp areas that produced thick coal reserves have also formed at other times in the Earth’s history, most notably in the Permian. During the Early Permian, the coal swamps of Euramerica continued to flourish in Cathaysia (the tectonic blocks that formed modern day China), and throughout the Permian, coal swamps dominated by seed plants called glossopterids were found on the Southern Hemisphere supercontinent Gondwana (formed from modern day India, Australia, Antarctica, Africa, Madagascar and South America).

The coal swamps of the Carboniferous (Fig. 1) and Early Permian formed primarily in tropical regions, whereas the Gondwanan coal swamps of the later Permian formed in higher-latitude temperate regions. Coal forests developed primarily in lowland areas such as river deltas, but there is a bias in the plant fossil record because fossilization is most likely to occur in these waterlogged habitats, meaning that fossils of drier, upland plant communities are much less common, so little is known of the plants that grew there. Figure 1 - Reconstruction of a Carboniferous coal swamp with stands of Lepidodendron and Calamites on a lake margin (John Watson © The Open University). Formation of coal: Coal is currently the foremost source of electricity in the world, and one of the largest sources of anthropogenic carbon dioxide. It is formed largely through the accumulation of dead plant matter, which builds up in layers of peat. If the peat accumulates in anoxic conditions (that is, in places in which it is not exposed to oxygen) such as at the base of a lake or swamp, or if a forest is flooded by rising seas, the carbon-rich plant material does not biodegrade.

This peat is then buried by sediments deposited on top of it, and is subject to increasing pressure and temperature. Eventually, the peats are lithified, or compacted into solid rock, and form coal. Coal balls: Much of the information that we have on coal-swamp plants comes from the detailed investigation of coal balls: plant matter that has been turned into fossils through permineralization, a process in which minerals, in this case calcium carbonate, seep into organic matter and form an internal cast of it. Coal balls often form in acidic peats, or when seawater permeates the compressed plant matter. The carbonate forms a hardened ball that resists compression throughout burial, thereby preserving the plant remains in exceptional detail; even cellular details can be retained. Such structures can be studied using a range of techniques. One of the most successful is the production of acetate peels, which involves cutting the coal ball using a high-powered saw, then dipping the cut surface in a bath of hydrofluoric acid to dissolve the silica and carbonate surrounding the fossil, leaving the organic remains standing just proud of the cut surface. Acetone is poured onto this surface and a sheet of acetate is laid on it, then peeled away.

This reveals a cross-section through the coal ball that shows the exceptional detail within (Fig. 2), which can be observed under a microscope. Figure 2 - Microscope image of plant material seen in an acetate peel. Part of a seed wall is visible. Plants of the coal swamps: Lepidodendron The name Lepidodendron was originally assigned to scaly trunk fossils found commonly in Carboniferous coal measures (Fig. 3b), but it now refers to the whole plant, which has been reconstructed as an enormous tree-sized organism. Lepidodendron dominated the Carboniferous coal swamps, and is thought to have reached heights of 40 metres. It is not closely related to the trees of today; instead, Lepidodendron is a lycopsid, more closely related to modern club mosses and quillworts. The thick trunk formed a pole, which had no branches, apart from the crown at the very top of the mature plant. The scale-like pattern on the trunk was produced by leaf scars (cushions where leaflets fell away). It was probably green in life because, unlike in modern trees, the trunk was composed of photosynthesizing tissues.

The branches at the crown of the mature Lepidodendron terminated in reproductive structures that look similar to cones (Fig. 3a). Reproduction was via spores as in modern lycopsids, rather than seeds as in most modern plants. It has been deduced from comparisons with modern lycopsids that many species of Lepidodendron reproduced only once, at the end of their lives. It has also been estimated that the plant may have grown to its full height in just 10–15 years. Lepidodendron grew in dense stands, as we know from assemblages of fossilized stumps, but the canopies of these forests would have been much more open than those of modern rainforests. Because Lepidodendron only branched to form a crown when mature, many of the tree-like plants in a forest would have been juvenile poles blocking out only a small amount of light. Figure 3 - a, Parts of Lepidodendron crown. b, Leaf cushions on stem of Lepidodendron. c, d, Fern-like fronds of Carboniferous age (images taken by Andrew Storey).

The roots of Lepidodendron are common fossils in their own right, and are given the name Stigmaria. Calamites Calamites are commonly found stem fossils of the coal measures (Fig. 4a,b). These stems are ridged with divided segments, some reaching 60 centimetres across, and wide enough to suggest that in life the plants might have reached up to 20 m in height. The plants that formed these stems are close relatives of modern horsetails. The leaves were arranged in circular whorls and the plants grew in the wettest areas of the coal swamp, around lakes and river margins. Figure 4 - a, b, Examples of Calamites stem cast fossils. c, Annularia leaf whorl. d, Cross section through chambered Vertebraria root of Permian age (images taken by Andrew Storey). Sphenopsids The sphenopsids are similar in appearance to Calamites, to which they are closely related, but are thought to have been much smaller plants with a range of heights. Some were vine-like scramblers. The leaves grew on the stems in whorls, termed Annularia (Fig. 4c).

Ferns Ferns were a common component of coal-forming ecosystems from the Carboniferous and Permian periods, just as in many environments today. They vary from small shrub-sized plants to large tree-ferns. Tree-sized ferns of the order Marattiales are common fossils of the British Carboniferous Coal Measures, occurring in coal balls and as flattened adpression fossils (formed of both a compression and an impression). Pteridosperms This diverse group of plants is known informally as the ‘seed-ferns’ because their leaf fronds superficially resemble those of true ferns, but unlike true ferns the pteridosperms reproduced via large seeds at the base of their leaves. The group is now thought to be paraphyletic, meaning that it includes several groups of plants that are only distantly related to each other. Plants termed pteridosperms were common in both Carboniferous and Permian wetland ecosystems. Glossopterids The glossopterids dominated the higher-latitude coal swamps of the Southern Hemisphere during the Middle and Late Permian. The name of the group comes from the common leaf fossil Glossopteris, a name that is now used for the entire reconstructed plant.

Glossopteris was tree-sized and bore large tongue-shaped leaves, which were possibly shed during the autumn at higher latitudes. The roots of Glossopteris are known as Vertebraria (Fig. 4d), named because they look like backbones when viewed lengthways. The Vertebraria roots were filled with air chambers, which may have been an adaptation to the boggy, waterlogged soils in which they grew. Large quantities of Glossopteris fossils were found among the remains of Captain Robert Falcon Scott and his four companions after the British Terra Nova Expedition to Antarctica (1910–13), which ended in disaster, with the death of all members of the expedition. The distribution of Glossopteris across the now dispersed southern continents (Fig. 5) was cited as early evidence in support of the theory of continental drift proposed by Alfred Wegener (1880–1930). Figure 5 - Distribution of Glossopteris fossils across the southern continents. Animals of the coal swamps: The rich habitats provided by the coal swamps were home to a diverse array of animal life, chiefly invertebrate arthropods.

Arachnids such as scorpions and trigonotarbids (Fig. 6a) were among the dominant predators of the Carboniferous forests. Trigonotarbids are extinct arachnids similar to modern spiders, but lacking the ability to spin silk webs. Myriapods (centipedes, millipedes and two smaller groups) were also present in these coal-forming terrestrial ecosystems. Millipedes (Fig. 6b) were among the first animals to have colonized the terrestrial environment, and were abundant as detritivores (feeding on decomposing organic matter) in Carboniferous forests. Some Carboniferous and Early Permian millipedes grew very large; one genus, Arthropleura, reached lengths of up to 3 m. Figure 6 - a, Trigonotarbid arthropod from a Carboniferous coal ball. b, Millipede from a Carboniferous coal ball. Other giants of the Palaeozoic coal swamps included the relatives of modern dragonflies, from the genus Meganeura (Fig. 7) in the Carboniferous and Meganeuropsis in the Permian, which attained wingspans of up to 75 cm.

These would have been top predators, certainly in the air during maturity and probably in the water during their nymph stage. Other arthropods found commonly in the coal-forest ecosystems are early relatives of cockroaches and mites. Vertebrates that lived in the Carboniferous forests included early relatives of the amphibians and the first reptiles. Fossil remains of some of the oldest true amniotic (egg-laying) reptiles, such as Hylonomus lyelli, are found inside the hollowed stumps of large plants of the Pennsylvanian (Upper Carboniferous) coal deposits of Joggins in Nova Scotia, Canada. It has been suggested that the reptiles either lived inside these broken stumps or sheltered there from forest fires, given that some of the remains are rich in charcoal. Alongside the terrestrial creatures, many aquatic animals lived in the lakes, pools and waterways of the coal swamps. These included crustaceans, bivalves, horseshoe crabs adapted to live in fresh or brackish waters, and fish including freshwater sharks. Figure 7 - Meganeura, Carboniferous relative of modern dragonflies.

Fate of the coal swamps: The coal swamps of tropical Euramerica gradually shrank towards the end of the Carboniferous, owing to a change in climate and because the lowlands that they occupied were being destroyed by mountain uplift. However, in wetter regions such as Cathaysia, the Carboniferous rainforests continued to flourish well into the Permian. Throughout the Permian the climate became increasingly warmer, leading to the reduction of the Southern Hemisphere ice cap. This favoured hardier, seed-bearing plants such as the glossopterids. The end of the Permian Period, 251 million years ago, saw the largest mass extinction known in the history of life, with an estimated 95% of all species on Earth dying out. The glossopterid-dominated coal swamps of Gondwana were among the casualties of this mass extinction. No coal deposits are known from the rocks of the Early Triassic . Thin coals of limited extent only return in Middle Triassic rocks, some ten million years later, after what is known to geologists and palaeontologists as the coal gap.

It is thought that this reflects the extinction of coal-forming ecosystems, and that it took many millions of years before new groups of plants adapted to create wetland peat-forming habitats. Suggestions for further reading: Cleal, C. J. & Thomas, B. A. 1994. Plant Fossils of the British Coal Measures. Dorchester: The Palaeontological Association. ISBN 0901702536 Cleal, C. J. & Thomas, B. A. 2009. An Introduction to Plant Fossils. Cambridge: Cambridge University Press. ISBN 978052188715.1 1 School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Share this: Geologic time scale - New World Encyclopedia Geologic time scale Next (Geological history of Earth) This clock representation shows some of the major units of geological time and definitive events of Earth history. The geologic time scale is used by geologists and other scientists to map the timing and relationships between events that have occurred during the history of the Earth .

Based on radiometric dating techniques, the Earth is estimated to be about 4,570 million years (4570 "Ma") old. The geological time scale is a means of mapping the history of the earth. It combines estimates of the age of geological formations as provided by radiometric dating techniques with the direct evidence of sequences and events in the rock record as assembled by geologists. In this way the geologic or deep time of Earth's past can be organized into various units according to events that took place in each period. Different spans of time on the time scale are usually delimited by major geologic or paleontologic events, such as mass extinctions . For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species . Contents The earth history mapped on the geologic time scale contrasts with that mapped by young-earth creationists , which see the earth as only thousands of years old. Terminology In the geological time scale, the largest defined unit of time is the eon, which is further divided successively into eras, periods, epochs, and stages.

Overlaid on this general pattern developed by geologists is a complementary mapping by paleontologists who have defined a system of faunal stages of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geologic nomenclature, though in general there are far more recognized faunal stages than defined geologic time units. Geologists tend to talk in terms of Upper/Late, Lower/Early, and Middle parts of periods and other units—for example, "Upper Jurassic ", "Middle Cambrian ". Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Early Cambrian is referred to as the Waucoban series, which is then subdivided into zones based on trilobites. The same time span is split into Tommotian, Atdabanian, and Botomian stages in East Asia and Siberia. It is a key aspect of the work of the International Commission on stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world.

History of the time scale Nicholas Steno laid down the principles underlying geologic time scales in the late seventeenth century. Steno argued that rock layers (strata) are laid down in succession, and that each represents a “slice” of time. He also formulated the principle of superposition, which states that any given stratum is probably older than those above it and younger than those below it. Steno's principles were simple, but applying them to real rocks proved complex. During the eighteenth century, geologists came to realize that: 1) Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition; 2) strata laid down at the same time in different areas could have entirely different appearances; and 3) the strata of any given area represented only part of the Earth's long history. The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late eighteenth century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: primary, secondary, tertiary, and quaternary.

Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" and "Quaternary" remained in use as names of geological periods well into the twentieth century. The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier , and Alexandre Brogniart in the early nineteenth century, enabled geologists to divide Earth history more finely and precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies of the strata and fossils of Europe produced between 1820 and 1850 formed the sequence of geological periods still used today. British geologists dominated the process, and the names of the periods reflect that dominance. The " Cambrian ," " Ordovician ," and " Silurian " periods were named for ancient British tribes (and defined using stratigraphic sequences from Wales).

The " Devonian " was named for the British county of Devon, and the name " Carboniferous " was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The " Permian ," though defined using strata in Russia , was delineated and named by British geologist Roderick Murchison. British geologists were also responsible for the grouping of periods into eras and the subdivision of the Tertiary and Quaternary periods into epochs. When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, there was no way to determine what time scale they represented. Young earth creationists proposed dates of only a few thousand years, while others suggested large (and even infinite) ages. For over one hundred years, the age of the Earth and of the rock strata was the subject of considerable debate until advances in the latter part of the twentieth century allowed radioactive dating to provide relatively firm dates to geologic horizons. In the intervening century and a half, geologists and paleontologists constructed time scales based solely on the relative positions of different strata and fossils.

In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references (Global Boundary Stratotype Section and Points) for geologic periods and faunal stages. Their most recent work is described in the 2004 geologic time scale of Gradstein, Ogg, and Smith (2005), and used as the foundation of the table on this page. The tables of geologic periods presented here are in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey. Graphical timelines An overview of the geologic time periods. The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. Millions of Years 4400 Ma—Oldest known mineral; 4570 Ma—Formation of Earth c.4570 Paleontologists often refer to faunal stages rather than geologic periods. The stage nomenclature is quite complex. Dates are slightly uncertain with differences of a few percent between various sources being common.

This is largely due to uncertainties in radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where they would be most useful. The dates and errors quoted above are according to the International Commission on Stratigraphy 2004 time scale. Dates labeled with a * indicate boundaries where a Global Boundary Stratotype Section and Point has been internationally agreed upon. Historically, the Cenozoic has been divided up into the Quaternary and Tertiary sub-eras, as well as the Neogene and Paleogene periods. However, the International Commission on Stratigraphy has recently decided to stop endorsing the terms Quaternary and Tertiary as part of the formal nomenclature. The start time for the Holocene epoch is here given as 11,430 years ago ± 130 years. For further discussion of the dating of this epoch, see Holocene . In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian Periods. The Proterozoic , Archean and Hadean are often collectively referred to as Precambrian Time , and sometimes also as the Cryptozoic.

Defined by absolute age (Global Standard Stratigraphic Age). Though commonly used, the Hadean is not a formal eon and no lower bound for the Eoarchean has been agreed upon. The Hadean has also sometimes been called the Priscoan. These four era names were taken from Lunar geologic timescale. Their use for Earth geology is unofficial. References Amthor, J. E., J. P. Grotzinger, S. Schroder, S. A. Bowring, J. Ramezani, M. W. Martin, and A. Matter. 2003. Extinction of Cloudina and Namacalathus at the Precambrian boundary in Oman. Geology 31(5):431-434. Bowring, S. A., D. H. Erwin, Y. G. Jin, M. W. Martin, K. Davidek, and W. Wang. 1998. U/Pb zircon geochronology and tempo of the end-Permian mass extinction. Science 280 (5366):1039-1045.

Gradstein, F. M., J. G. Ogg, and A. G. Smith. 2005. A Geologic Time Scale 2004. Cambridge: Cambridge University Press. Credits New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards . This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

Question: What is the name given to the geological time period of 363-290 million years ago during which coal measures were formed?
Answer: Carboniferous