1.
Geology
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Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also refer generally to the study of the features of any terrestrial planet. Geology gives insight into the history of the Earth by providing the evidence for plate tectonics, the evolutionary history of life. Geology also plays a role in engineering and is a major academic discipline. The majority of data comes from research on solid Earth materials. These typically fall into one of two categories, rock and unconsolidated 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 types of rock, igneous, sedimentary, and metamorphic. The rock cycle is an important concept in geology which illustrates the relationships between three types of rock, and magma. When a rock crystallizes from melt, it is an igneous rock, the sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure and is then weathered, eroded, deposited, and lithified, ultimately becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism, all three types of rocks may be re-melted, when this happens, a new magma is formed, from which an igneous rock may once again crystallize. Geologists also study unlithified material which typically comes from more recent deposits and these materials are superficial deposits which lie above the bedrock. Because of this, the study of material is often known as Quaternary geology. This includes the study of sediment and soils, including studies in geomorphology, sedimentology and this theory is supported by several types of observations, including seafloor spreading, and the global distribution of mountain terrain and seismicity. This coupling between rigid plates moving on the surface of the Earth and the mantle is called plate tectonics. The development of plate tectonics provided a basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries, mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, were explained as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes were explained as convergent boundaries, where one plate subducts under another, transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes. Plate tectonics also provided a mechanism for Alfred Wegeners theory of continental drift and they also provided a driving force for crustal deformation, and a new setting for the observations of structural geology
2.
Supercontinent
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In geology, a supercontinent is the assembly of most or all of Earths continental blocks or cratons to form a single large landmass. However, the definition of a supercontinent can be ambiguous, Many earth scientists, such as P. F. Hoffman, use the term supercontinent to mean a clustering of all continents. This definition leaves room for interpretation when labeling a continental body and is easier to apply to Precambrian times, the landmass of Pangaea is the collective name describing all of these continental masses when they were most recently in proximity to one another. This would classify Pangaea as a supercontinent, according to the definition by Rogers and Santosh, a supercontinent does not exist today. Supercontinents have assembled and dispersed multiple times in the geologic past, the positions of continents have been accurately determined back to the early Jurassic. However, beyond 200 Ma, continental positions are less certain. The following table displays historical supercontinents, using a general definition, there are two contrasting models for supercontinent evolution through geological time. The first model theorizes that at least two separate supercontinents existed comprising Vaalbara and Kenorland, the Neoarchean supercontinent consisted of Superia and Sclavia. These parts of Neoarchean age broke off at ~2300 and 2090 Ma, Nuna continued to develop during the Mesoproterozoic, primarily by lateral accretion of juvenile arcs, and in ~1000 Ma Nuna collided with other land masses, forming Rodinia. Between ~800 and 700 Ma Rodinia broke apart, however, before completely breaking up, some fragments of Rodinia had already come together to form Gondwana by ~530 Ma. Pangaea formed by ~300 Ma through the collision of Gondwana, Laurentia, Baltica, during the intervening periods, the poles conform to a unified apparent polar wander path. The explanation for the duration of the Protopangea-Paleopangea supercontinent appears to be that Lid Tectonics prevailed during Precambrian times. Plate Tectonics as seen on the contemporary Earth became dominant only during the part of geological times. The Phanerozoic supercontinent Pangaea began to break up 180 Ma and is doing so today. Because Pangaea is the most recent of Earths supercontinents, it is the most well known, contributing to Pangaeas popularity in the classroom is the fact that its reconstruction is almost as simple as fitting the present continents bordering the Atlantic-type oceans like puzzle pieces. A supercontinent cycle is the break-up of one supercontinent and the development of another, supercontinent cycles are not the same as the Wilson cycle, which is the opening and closing of an individual oceanic basin. The Wilson cycle rarely synchronizes with the timing of a supercontinent cycle, however, supercontinent cycles and Wilson cycles were both involved in the creation of Pangaea and Rodinia
3.
Stromatolite
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Fossilized stromatolites provide ancient records of life on Earth by these remains, some of which may date from 3.7 billion years ago. Stromatolites exhibit a variety of forms and structures, or morphologies, including conical, stratiform, branching, domal, Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. Very few ancient stromatolites contain fossilized microbes, while features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic precipitation. Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an area of research in geology. Time lapse photography of modern microbial mat formation in a laboratory setting gives some revealing clues to the behavior of cyanobacteria in stromatolites. In a novel experiment, the scientists projected a school logo onto a dish containing the organisms. The authors speculate that such motility allows the cyanobacteria to seek light sources to support the colony, in both light and dark conditions, the cyanobacteria form clumps that then expand outwards, with individual members remaining connected to the colony via long tendrils. This may be a mechanism that affords evolutionary benefit to the colony in harsh environments where mechanical forces act to tear apart the microbial mats. Thus these sometimes elaborate structures, constructed by microscopic organisms working somewhat in unison, are a means of providing shelter, some Archean rock formations show macroscopic similarity to modern microbial structures, leading to the inference that these structures represent evidence of ancient life, namely stromatolites. However, others regard these patterns as being due to natural material deposition or some other abiogenic mechanism, Stromatolites are a major constituent of the fossil record of the first forms of life on earth. The earliest fossils date to 3.7 billion years ago and they peaked about 1.25 billion years ago and subsequently declined in abundance and diversity, so that by the start of the Cambrian they had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victim to grazing creatures, Proterozoic stromatolite microfossils include cyanobacteria and possibly some forms of the eukaryote chlorophytes. One genus of very common in the geologic record is Collenia. Fluctuations in metazoan population and diversity may not have been the factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes, while prokaryotic cyanobacteria reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be responsible for increasing the amount of oxygen in the primeval earths atmosphere through their continuing photosynthesis. Cyanobacteria use water, carbon dioxide, and sunlight to create their food, a layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the habitat can become trapped within the mucus
4.
Sediment
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For sediment in beverages, see dregs. For example, sand and silt can be carried in suspension in water and on reaching the sea be deposited by sedimentation. Sediments are most often transported by water, but also wind, beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of transport and deposition. Glacial moraine deposits and till are ice-transported sediments, sediment can be classified based on its grain size and/or its composition. Sediment size is measured on a log base 2 scale, called the Phi scale, composition of sediment can be measured in terms of, parent rock lithology mineral composition chemical make-up. This leads to an ambiguity in which clay can be used as both a size-range and a composition, sediment is transported based on the strength of the flow that carries it and its own size, volume, density, and shape. Stronger flows will increase the lift and drag on the particle, causing it to rise, rivers and streams carry sediment in their flows. This sediment can be in a variety of locations within the flow and these relationships are shown in the following table for the Rouse number, which is a ratio of sediment fall velocity to upwards velocity. If the upwards velocity is less than the settling velocity, but still high enough for the sediment to move, it will move along the bed as bed load by rolling, sliding. If the upwards velocity is higher than the velocity, the sediment will be transported high in the flow as wash load. As there are generally a range of different particle sizes in the flow, sediment motion can create self-organized structures such as ripples, dunes, antidunes on the river or stream bed. These bedforms are often preserved in rocks and can be used to estimate the direction. Overland flow can erode soil particles and transport them downslope, the erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions. If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion, if overland flow is directly responsible for sediment entrainment but does not form gullies, it is called sheet erosion. If the flow and the substrate permit channelization, gullies may form, glaciers carry a wide range of sediment sizes, and deposit it in moraines. The overall balance between sediment in transport and sediment being deposited on the bed is given by the Exner equation and this expression states that the rate of increase in bed elevation due to deposition is proportional to the amount of sediment that falls out of the flow. This can be localized, and simply due to obstacles, examples are scour holes behind boulders, where flow accelerates
5.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
6.
Ecology
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Ecology is the scientific analysis and study of interactions among organisms and their environment. It is a field that includes biology, geography. Ecology includes the study of interactions that organisms have with other, other organisms. Ecosystems are composed of dynamically interacting parts including organisms, the communities make up. Ecosystem processes, such as production, pedogenesis, nutrient cycling. These processes are sustained by organisms with specific life history traits, biodiversity, which refers to the varieties of species, genes, and ecosystems, enhances certain ecosystem services. Ecology is not synonymous with environment, environmentalism, natural history and it is closely related to evolutionary biology, genetics, and ethology. An important focus for ecologists is to improve the understanding of how biodiversity affects ecological function, Ecology is a human science as well. For example, the Circles of Sustainability approach treats ecology as more than the environment out there and it is not treated as separate from humans. Organisms and resources compose ecosystems which, in turn, maintain biophysical feedback mechanisms that moderate processes acting on living and non-living components of the planet, the word ecology was coined in 1866 by the German scientist Ernst Haeckel. Ecological thought is derivative of established currents in philosophy, particularly from ethics and politics, ancient Greek philosophers such as Hippocrates and Aristotle laid the foundations of ecology in their studies on natural history. Modern ecology became a more rigorous science in the late 19th century. Evolutionary concepts relating to adaptation and natural selection became the cornerstones of modern ecological theory, the scope of ecology contains a wide array of interacting levels of organization spanning micro-level to a planetary scale phenomena. Ecosystems, for example, contain abiotic resources and interacting life forms, an ecosystems area can vary greatly, from tiny to vast. A single tree is of consequence to the classification of a forest ecosystem. Several generations of a population can exist over the lifespan of a single leaf. Each of those aphids, in turn, support diverse bacterial communities, biodiversity describes the diversity of life from genes to ecosystems and spans every level of biological organization. The term has several interpretations, and there are ways to index, measure, characterize
7.
Induan
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The Induan is, in the geologic timescale, the first age of the Early Triassic epoch or the lowest stage of the Lower Triassic series. It spans the time between 252.17 Ma and 251.2 Ma and it is preceded by the Changhsingian and is followed by the Olenekian. The Induan is roughly coeval with the regional Feixianguanian stage of China, the Induan stage is named for the Indus region of Pakistan/India. The Russian subdivision of the Lower Triassic then slowly replaced the one used in the West, the global reference profile of the base of the Induan is situated in Changxing County, China. The top of the Induan stage is at the first appearance of ammonite species Meekoceras gracilitatis, the Induan age followed the mass extinction event at the end of the Permian period. Both global biodiversity and community-level diversity remained low through much of this stage of the Triassic, much of the world remained almost lifeless, deserted, hot, and dry. The lystrosaurids and the proterosuchids were the groups of land animals to dominate during the Induan stage. Other animals, such as the ammonites, fishes, insects, both the seas and much of the freshwater during the Induan were anoxic. Gradstein, F. M. Ogg, J. G. & Smith, A. G.2004, A Geologic Time Scale 2004, Cambridge University Press. Kiparisova, Lubov Dmitrievna & Popov, Yurij Nikolaivitch,1956, Расчленение нижнего отдела триасовой системы на ярусы, Doklady Akademii Nauk SSSR109, pp 842–845
8.
Cyanobacteria
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The name cyanobacteria comes from the color of the bacteria. Sometimes, they are called blue-green algae, and incorrectly so, because cyanobacteria are prokaryotes, like other prokaryotes, cyanobacteria have no membrane-sheathed organelles. Photosynthesis is performed in distinctive folds in the membrane of the cell. Biologists commonly agree that chloroplasts found in eukaryotes have their ancestry in cyanobacteria, via a process called endosymbiosis, Cyanobacteria are a group of photosynthetic, nitrogen fixing bacteria that live in a wide variety of habitats such as moist soils and in water. They may be free-living or form relationships with plants or with lichen-forming fungi as in the lichen genus Peltigera. They range from unicellular to filamentous and include colonial species, colonies may form filaments, sheets, or even hollow balls. Cyanobacteria can fix nitrogen in anaerobic conditions by means of specialized cells called heterocysts. Heterocysts may also form under the environmental conditions when fixed nitrogen is scarce. Free-living cyanobacteria are present in the column in rice paddies, and cyanobacteria can be found growing as epiphytes on the surfaces of the green alga, Chara. Cyanobacteria such as, can provide rice plantations with biofertilizer, many cyanobacteria form motile filaments of cells, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than in the state. To break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, each individual cell of a cyanobacterium typically has a thick, gelatinous cell wall. They lack flagella, but hormogonia of some species can move about by gliding along surfaces, many of the multicellular filamentous forms of Oscillatoria are capable of a waving motion, the filament oscillates back and forth. In water columns, some cyanobacteria float by forming gas vesicles and these vesicles are not organelles as such. They are not bounded by membranes, but by a protein sheath. Cyanobacteria can be found in almost every terrestrial and aquatic habitat—oceans, fresh water, damp soil, temporarily moistened rocks in deserts, bare rock and soil and they can occur as planktonic cells or form phototrophic biofilms. They are found in almost every endolithic ecosystem, a few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form of camouflage, aquatic cyanobacteria are known for their extensive and highly visible blooms that can form in both freshwater and marine environments
9.
Miocene
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The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago. The Miocene was named by Sir Charles Lyell and its name comes from the Greek words μείων and καινός and means less recent because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene Epoch and is followed by the Pliocene Epoch, the earth went from the Oligocene through the Miocene and into the Pliocene, with the climate slowly cooling towards a series of ice ages. The Miocene boundaries are not marked by a single distinct global event, the apes arose and diversified during the Miocene, becoming widespread in the Old World. By the end of this epoch, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path, as in the Oligocene before it, grasslands continued to expand and forests to dwindle in extent. In the Miocene seas, kelp forests made their first appearance, the plants and animals of the Miocene were fairly modern. The Miocene faunal stages from youngest to oldest are typically named according to the International Commission on Stratigraphy, Two subdivisions each form the lower, middle, continents continued to drift toward their present positions. Mountain building took place in western North America, Europe, both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the North American Great Plains, India continued to collide with Asia, creating dramatic new mountain ranges. The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish–Arabian region between 19 and 12 Ma. The subsequent uplift of mountains in the western Mediterranean region and a fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea near the end of the Miocene. The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture, climates remained moderately warm, although the slow global cooling that eventually led to the Pleistocene glaciations continued. Although a long-term cooling trend was well underway, there is evidence of a period during the Miocene when the global climate rivalled that of the Oligocene. The Miocene warming began 21 million years ago and continued until 14 million years ago, by 8 million years ago, temperatures dropped sharply once again, and the Antarctic ice sheet was already approaching its present-day size and thickness. Greenland may have begun to have large glaciers as early as 7 to 8 million years ago, life during the Miocene Epoch was mostly supported by the two newly formed biomes, kelp forests and grasslands. This allows for more grazers, such as horses, rhinoceroses, ninety five percent of modern plants existed by the end of this epoch. The higher organic content and water retention of the deeper and richer grassland soils, with long term burial of carbon in sediments, produced a carbon and this, combined with higher surface albedo and lower evapotranspiration of grassland, contributed to a cooler, drier climate. The expansion of grasslands and radiations among terrestrial herbivores correlates to fluctuations in CO2
10.
Oligocene
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The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start. The name Oligocene comes from the Ancient Greek ὀλίγος and καινός, the Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period, the Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included an expansion of grasslands. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event, Oligocene faunal stages from youngest to oldest are, The Paleogene Period general temperature decline is interrupted by an Oligocene 7-million-year stepwise climate change. A deeper 8.2 °C,400, 000-year temperature depression leads the 2 °C, the stepwise climate change began 32.5 Ma and lasted through to 25.5 Ma, as depicted in the PaleoTemps chart. The Oligocene climate change was a increase in ice volume. The 7-million-year depression abruptly terminated within 1–2 million years of the La Garita Caldera eruption at 28–26 Ma, a deep 400, 000-year glaciated Oligocene Miocene boundary event is recorded at McMurdo Sound and King George Island. During this epoch, the continued to drift toward their present positions. Antarctica became more isolated and finally developed an ice cap, a brief marine incursion marks the early Oligocene in Europe. Marine fossils from the Oligocene are rare in North America, there appears to have been a land bridge in the early Oligocene between North America and Europe, since the faunas of the two regions are very similar. Sometime during the Oligocene, South America was finally detached from Antarctica and it also allowed the Antarctic Circumpolar Current to flow, rapidly cooling the Antarctic continent. Angiosperms continued their expansion throughout the world as tropical and sub-tropical forests were replaced by deciduous forests. Open plains and deserts became more common and grasses expanded from their habitat in the Eocene moving out into open tracts. However, even at the end of the period, grass was not quite enough for modern savannas. In North America, subtropical species dominated with cashews and lychee trees present, and temperate trees such as roses, beeches, the legumes spread, while sedges, bulrushes, and ferns continued their ascent. Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleocene epoch 30 million years earlier, marine faunas became fairly modern, as did terrestrial vertebrate fauna on the northern continents
11.
Eocene
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The Eocene Epoch, lasting from 56 to 33.9 million years ago, is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. As with other periods, the strata that define the start and end of the epoch are well identified. The name Eocene comes from the Ancient Greek ἠώς and καινός, the Eocene epoch is conventionally divided into early, middle, and late subdivisions. The corresponding rocks are referred to as lower, middle, the Ypresian stage constitutes the lower, the Priabonian stage the upper, and the Lutetian and Bartonian stages are united as the middle Eocene. The Eocene Epoch contained a variety of different climate conditions that includes the warmest climate in the Cenozoic Era. During this period of time, little to no ice was present on Earth with a difference in temperature from the equator to the poles. Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene-Oligocene transition at 34 million years ago. During this decrease ice began to reappear at the poles, greenhouse gases, in particular carbon dioxide and methane, played a significant role during the Eocene in controlling the surface temperature. For the early Eocene there is discussion on how much carbon dioxide was in the atmosphere. This is due to numerous proxies representing different atmospheric carbon dioxide content, for contrast, today the carbon dioxide levels are at 400 ppm or 0. 04%. At about the beginning of the Eocene Epoch the amount of oxygen in the atmosphere more or less doubled. During the early Eocene, methane was another gas that had a drastic effect on the climate. In comparison to carbon dioxide, methane has much effect on temperature as methane is ~34 times more effective per molecule than carbon dioxide on a 100-year scale. Most of the methane released to the atmosphere during this period of time would have been from wetlands, swamps, the atmospheric methane concentration today is 0. 000179% or 1.79 ppmv. Due to the climate and sea level rise associated with the early Eocene, more wetlands, more forests. Comparing the early Eocene production of methane to current levels of atmospheric methane, biogenic production of methane produces carbon dioxide and water vapor along with the methane, as well as yielding infrared radiation
12.
Radiometric dating
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Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale. Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating, by allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is used to date archaeological materials, including ancient artifacts. Different methods of radiometric dating vary in the timescale over which they are accurate, all ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus, a particular isotope of a particular element is called a nuclide. That is, at point in time, an atom of such a nuclide will undergo radioactive decay. This transformation may be accomplished in a number of different ways, including alpha decay, another possibility is spontaneous fission into two or more nuclides. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a daughter nuclide or decay product, Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years to over 100 billion years. For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant and it is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field. The only exceptions are nuclides that decay by the process of capture, such as beryllium-7, strontium-85. For all other nuclides, the proportion of the nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. The basic equation of radiometric dating requires that neither the parent nuclide nor the product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered and it is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body and this can reduce the problem of contamination. In uranium-lead dating, the diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample, the procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate
