process of radioactive dating

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Process of radioactive dating

For an element to be useful for geochronology measuring geological time , the isotope must be reasonably abundant and produce daughter isotopes at a good rate. Either a whole rock or a single mineral grain can be dated. Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer such as in the argon-argon scheme.

Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes. The isotopes are then measured within the same machine by an attached mass spectrometer an example of this is SIMS analysis. Get our monthly emails for amazing animals, research insights and museum events. This is a common dating method mainly used by archaeologists, as it can only date geologically recent organic materials, usually charcoal, but also bone and antlers.

All living organisms take up carbon from their environment including a small proportion of the radioactive isotope 14C formed from nitrogen as a result of cosmic ray bombardment. The amount of carbon isotopes within living organisms reaches an equilibrium value, on death no more is taken up, and the 14C present starts to decay at a known rate.

The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism. This method faces problems because the cosmic ray flux has changed over time, but a calibration factor is applied to take this into account. Radiocarbon dating is normally suitable for organic materials less than 50 years old because beyond that time the amount of 14C becomes too small to be accurately measured.

This scheme was developed in but became more useful when mass spectrometers were improved in the late s and early s. However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. This technique is less used now. The dual decay of potassium K to 40Ar argon and 40Ca calcium was worked out between and This technique has become more widely used since the late s. Its great advantage is that most rocks contain potassium, usually locked up in feldspars, clays and amphiboles.

However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks. Argon-Argon dating 39ArAr. This technique developed in the late s but came into vogue in the early s, through step-wise release of the isotopes. This technique uses the same minerals and rocks as for K-Ar dating but restricts measurements to the argon isotopic system which is not so affected by metamorphic and alteration events.

It is used for very old to very young rocks. The decay of Sm to Nd for dating rocks began in the mids and was widespread by the early s. It is useful for dating very old igneous and metamorphic rocks and also meteorites and other cosmic fragments. However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes.

This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe. The Re-Os isotopic system was first developed in the early s, but recently has been improved for accurate age determinations.

The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes. This technique is good for iron meteorites and the mineral molybdenite. This system is highly favoured for accurate dating of igneous and metamorphic rocks, through many different techniques. It was used by the beginning of the s, but took until the early s to produce accurate ages of rocks.

The great advantage is that almost all igneous and metamorphic rocks contain sufficient U and Pb for this dating. It can be used on powdered whole rocks, mineral concentrates isotope dilution technique or single grains SHRIMP technique. It has revolutionised age dating using the U-Pb isotopic system.

Using the SHRIMP, selected areas of growth on single grains of zircon, baddeleyite, sphene, rutile and monazite can be accurately dated to less than years in some cases. It can even date nonradioactive minerals when they contain inclusions of zircons and monazite, as in sapphire grains. It can help fix the maximum age of sedimentary rocks when they contain enough accessory zircon grains usually need about grains.

Because of advancements in geochronology for over 50 years, accurate formation ages are now known for many rock sequences on Earth and even in space. The oldest accurately dated rocks on Earth are metamorphosed felsic volcanic rocks from north-west Western Australia. These were dated at about 4. Several minerals incorporate tiny amounts of uranium into their structure when they crystallise. The radioactive decay from the uranium releases energy and particles this strips away electrons leading to disorder in the mineral structure.

The travel of these particles through the mineral leaves scars of damage about one thousandth of a millimetre in length. These 'fission tracks' are formed by the spontaneous fission of U and are only preserved within insulating materials where the free movement of electrons is restricted. Because the radioactive decay occurs at a known rate, the density of fission tracks for the amount of uranium within a mineral grain can be used to determine its age.

To see the fission tracks, the mineral surface is polished, etched with acids, and examined with an electron microscope. An effective way to measure the uranium concentration is to irradiate the sample in a nuclear reactor and produce comparative artificial tracks by the induced fission of U. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time it takes for the parent atom to decay into the daughter atom s.

A g sample of Cs is allowed to decay. Calculate the mass of Cs that will be left after 90 years. The half-life of Cs is 30 years. Third half-life 90 years total : The remaining 25 grams of Cs decay and Boundless vets and curates high-quality, openly licensed content from around the Internet.

This particular resource used the following sources:. Skip to main content. Nuclear Chemistry. Search for:. Dating Using Radioactive Decay. Learning Objective Calculate the age of a radioactive sample based on the half-life of a radioactive constituent. Key Points The best-known techniques for radioactive dating are radiocarbon dating, potassium-argon dating and uranium-lead dating. In any material containing a radioactive nuclide, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.

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It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and it can be used to date a wide range of natural and man-made materials. The best-known radiometric dating techniques include radiocarbon dating, potassium-argon dating, and uranium-lead dating. By establishing geological timescales, radiometric dating provides a significant source of information about the ages of fossils and rates of evolutionary change, and it is also used to date archaeological materials, including ancient artifacts.

The different methods of radiometric dating are accurate over different timescales, and they are useful for different materials. In many cases, the daughter nuclide is radioactive, resulting in a decay chain. This chain eventually ends with the formation of a stable, nonradioactive daughter nuclide. Each step in such a chain is characterized by a distinct half-life.

In these cases, the half-life of interest in radiometric dating is usually the longest one in the chain. This half-life will be the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter s. Systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e. However, in general, the half-life of a nuclide depends solely on its nuclear properties and is essentially a constant.

Therefore, in any material containing a radioactive nuclide, the proportion of the original 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 it takes for the parent atom to decay into the daughter atom s. A g sample of Cs is allowed to decay.

Calculate the mass of Cs that will be left after 90 years. The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism. This method faces problems because the cosmic ray flux has changed over time, but a calibration factor is applied to take this into account. Radiocarbon dating is normally suitable for organic materials less than 50 years old because beyond that time the amount of 14C becomes too small to be accurately measured.

This scheme was developed in but became more useful when mass spectrometers were improved in the late s and early s. However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. This technique is less used now. The dual decay of potassium K to 40Ar argon and 40Ca calcium was worked out between and This technique has become more widely used since the late s.

Its great advantage is that most rocks contain potassium, usually locked up in feldspars, clays and amphiboles. However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks.

Argon-Argon dating 39ArAr. This technique developed in the late s but came into vogue in the early s, through step-wise release of the isotopes. This technique uses the same minerals and rocks as for K-Ar dating but restricts measurements to the argon isotopic system which is not so affected by metamorphic and alteration events. It is used for very old to very young rocks. The decay of Sm to Nd for dating rocks began in the mids and was widespread by the early s. It is useful for dating very old igneous and metamorphic rocks and also meteorites and other cosmic fragments.

However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes. This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe. The Re-Os isotopic system was first developed in the early s, but recently has been improved for accurate age determinations.

The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes. This technique is good for iron meteorites and the mineral molybdenite. This system is highly favoured for accurate dating of igneous and metamorphic rocks, through many different techniques. It was used by the beginning of the s, but took until the early s to produce accurate ages of rocks.

The great advantage is that almost all igneous and metamorphic rocks contain sufficient U and Pb for this dating. It can be used on powdered whole rocks, mineral concentrates isotope dilution technique or single grains SHRIMP technique. It has revolutionised age dating using the U-Pb isotopic system.

Using the SHRIMP, selected areas of growth on single grains of zircon, baddeleyite, sphene, rutile and monazite can be accurately dated to less than years in some cases. It can even date nonradioactive minerals when they contain inclusions of zircons and monazite, as in sapphire grains. It can help fix the maximum age of sedimentary rocks when they contain enough accessory zircon grains usually need about grains.

Because of advancements in geochronology for over 50 years, accurate formation ages are now known for many rock sequences on Earth and even in space. The oldest accurately dated rocks on Earth are metamorphosed felsic volcanic rocks from north-west Western Australia. These were dated at about 4. Several minerals incorporate tiny amounts of uranium into their structure when they crystallise. The radioactive decay from the uranium releases energy and particles this strips away electrons leading to disorder in the mineral structure.

The travel of these particles through the mineral leaves scars of damage about one thousandth of a millimetre in length. These 'fission tracks' are formed by the spontaneous fission of U and are only preserved within insulating materials where the free movement of electrons is restricted. Because the radioactive decay occurs at a known rate, the density of fission tracks for the amount of uranium within a mineral grain can be used to determine its age.

To see the fission tracks, the mineral surface is polished, etched with acids, and examined with an electron microscope. An effective way to measure the uranium concentration is to irradiate the sample in a nuclear reactor and produce comparative artificial tracks by the induced fission of U. Fission track dating is commonly used on apatite, zircon and monazite.

It helps to determine the rates of uplift for geomorphology studies , subsidence rates for petroleum exploration and sedimentary basin studies , and the age of volcanic eruptions this is because fission tracks reset after the eruption.

However, care is needed as some samples have fission tracks reset during bushfires, giving far too young ages. Fission track dating is mostly used on Cretaceous and Cenozoic rocks. The Australian Museum respects and acknowledges the Gadigal people of the Eora Nation as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands.

Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Skip to main content Skip to acknowledgement of country Skip to footer On this page

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The carbon ends up as a trace component in atmospheric carbon dioxide CO 2. A carbon-based life form acquires carbon during its lifetime. Plants acquire it through photosynthesis , and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years.

The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon an ideal dating method to date the age of bones or the remains of an organism. The carbon dating limit lies around 58, to 62, years. The rate of creation of carbon appears to be roughly constant, as cross-checks of carbon dating with other dating methods show it gives consistent results.

However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground nuclear bomb tests that were conducted into the early s.

Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon created in the atmosphere. This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium impurities. The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons.

This causes induced fission of U, as opposed to the spontaneous fission of U. The fission tracks produced by this process are recorded in the plastic film. The uranium content of the material can then be calculated from the number of tracks and the neutron flux. This scheme has application over a wide range of geologic dates. For dates up to a few million years micas , tektites glass fragments from volcanic eruptions , and meteorites are best used.

Older materials can be dated using zircon , apatite , titanite , epidote and garnet which have a variable amount of uranium content. The technique has potential applications for detailing the thermal history of a deposit. The residence time of 36 Cl in the atmosphere is about 1 week. Thus, as an event marker of s water in soil and ground water, 36 Cl is also useful for dating waters less than 50 years before the present. Luminescence dating methods are not radiometric dating methods in that they do not rely on abundances of isotopes to calculate age.

Instead, they are a consequence of background radiation on certain minerals. Over time, ionizing radiation is absorbed by mineral grains in sediments and archaeological materials such as quartz and potassium feldspar. The radiation causes charge to remain within the grains in structurally unstable "electron traps".

Exposure to sunlight or heat releases these charges, effectively "bleaching" the sample and resetting the clock to zero. The trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried.

Stimulating these mineral grains using either light optically stimulated luminescence or infrared stimulated luminescence dating or heat thermoluminescence dating causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial and specific properties of the mineral.

These methods can be used to date the age of a sediment layer, as layers deposited on top would prevent the grains from being "bleached" and reset by sunlight. Pottery shards can be dated to the last time they experienced significant heat, generally when they were fired in a kiln. Absolute radiometric dating requires a measurable fraction of parent nucleus to remain in the sample rock. For rocks dating back to the beginning of the solar system, this requires extremely long-lived parent isotopes, making measurement of such rocks' exact ages imprecise.

To be able to distinguish the relative ages of rocks from such old material, and to get a better time resolution than that available from long-lived isotopes, short-lived isotopes that are no longer present in the rock can be used. At the beginning of the solar system, there were several relatively short-lived radionuclides like 26 Al, 60 Fe, 53 Mn, and I present within the solar nebula.

These radionuclides—possibly produced by the explosion of a supernova—are extinct today, but their decay products can be detected in very old material, such as that which constitutes meteorites. By measuring the decay products of extinct radionuclides with a mass spectrometer and using isochronplots, it is possible to determine relative ages of different events in the early history of the solar system.

Dating methods based on extinct radionuclides can also be calibrated with the U-Pb method to give absolute ages. Thus both the approximate age and a high time resolution can be obtained. Generally a shorter half-life leads to a higher time resolution at the expense of timescale.

The iodine-xenon chronometer [35] is an isochron technique. Samples are exposed to neutrons in a nuclear reactor. This converts the only stable isotope of iodine I into Xe via neutron capture followed by beta decay of I. After irradiation, samples are heated in a series of steps and the xenon isotopic signature of the gas evolved in each step is analysed.

Samples of a meteorite called Shallowater are usually included in the irradiation to monitor the conversion efficiency from I to Xe. This in turn corresponds to a difference in age of closure in the early solar system. Another example of short-lived extinct radionuclide dating is the 26 Al — 26 Mg chronometer, which can be used to estimate the relative ages of chondrules.

The 26 Al — 26 Mg chronometer gives an estimate of the time period for formation of primitive meteorites of only a few million years 1. From Wikipedia, the free encyclopedia. Technique used to date materials such as rocks or carbon. See also: Radioactive decay law. Main article: Closure temperature.

Main article: Uranium—lead dating. Main article: Samarium—neodymium dating. Main article: Potassium—argon dating. Main article: Rubidium—strontium dating. Main article: Uranium—thorium dating. Main article: Radiocarbon dating. Main article: fission track dating.

Main article: Luminescence dating. Earth sciences portal Geophysics portal Physics portal. Online corrected version: — " radioactive dating ". Part II. The disintegration products of uranium". American Journal of Science. Bibcode : AmJS S2CID In Roth, Etienne; Poty, Bernard eds. Nuclear Methods of Dating. Springer Netherlands. ISBN Applied Radiation and Isotopes. ISSN PMID Annual Review of Nuclear Science.

Bibcode : Natur. January Geochimica et Cosmochimica Acta. Bibcode : GeCoA.. Earth and Planetary Science Letters. Brent The age of the earth. Stanford, Calif. Radiogenic isotope geology 2nd ed. Cambridge: Cambridge Univ. Principles and applications of geochemistry: a comprehensive textbook for geology students 2nd ed.

OCLC Using geochemical data: evaluation, presentation, interpretation. Harlow : Longman. Cornell University. United States Geological Survey. Kramers June Hanson; M. Martin; S. Bowring; H. Jelsma; P. Dirks Journal of African Earth Sciences. Bibcode : JAfES.. Precambrian Research. Bibcode : PreR.. Vetter; Donald W. Davis Chemical Geology. Bibcode : ChGeo. South African Journal of Geology. Wilson; R. Carlson December In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults.

Sci Rep 10, The Swedish National Heritage Board. Archived from the original on 31 March Retrieved 9 March Dergachev As strontium forms, its ratio to strontium will increase. Strontium is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process. The amount of strontium in a given mineral sample will not change. It turns out to be a straight line with a slope of The corresponding half lives for each plotted point are marked on the line and identified.

It can be readily seen from the plots that when this procedure is followed with different amounts of Rb87 in different minerals , if the plotted half life points are connected, a straight line going through the origin is produced. These lines are called "isochrons".

The steeper the slope of the isochron, the more half lives it represents. When the fraction of rubidium is plotted against the fraction of strontium for a number of different minerals from the same magma an isochron is obtained. If the points lie on a straight line, this indicates that the data is consistent and probably accurate. An example of this can be found in Strahler, Fig However, if strontium 87 was present in the mineral when it was first formed from molten magma, that amount will be shown by an intercept of the isochron lines on the y-axis, as shown in Fig Thus it is possible to correct for strontium initially present.

Comparing figures The age of the sample can be obtained by choosing the origin at the y intercept. In Fig Note that the amounts of rubidium 87 and strontium 87 are given as ratios to an inert isotope, strontium However, in calculating the ratio of Rb87 to Sr87, we can use a simple analytical geometry solution to the plotted data. Again referring to Fig. Since the half-life of Rb87 is Therefore: log. When properly carried out, radioactive dating test procedures have shown consistent and close agreement among the various methods.

If the same result is obtained sample after sample, using different test procedures based on different decay sequences, and carried out by different laboratories, that is a pretty good indication that the age determinations are accurate. Of course, test procedures, like anything else, can be screwed up.

Mistakes can be made at the time a procedure is first being developed. Creationists seize upon any isolated reports of improperly run tests and try to categorize them as representing general shortcomings of the test procedure. This like saying if my watch isn't running, then all watches are useless for keeping time. Creationists also attack radioactive dating with the argument that half-lives were different in the past than they are at present. There is no more reason to believe that than to believe that at some time in the past iron did not rust and wood did not burn.

Furthermore, astronomical data show that radioactive half-lives in elements in stars billions of light years away is the same as presently measured. On pages and of The Genesis Flood, creationist authors Whitcomb and Morris present an argument to try to convince the reader that ages of mineral specimens determined by radioactivity measurements are much greater than the "true" i.

Biblical ages. The mathematical procedures employed are totally inconsistent with reality. Henry Morris has a PhD in Hydraulic Engineering, so it would seem that he would know better than to author such nonsense. Apparently, he did know better, because he qualifies the exposition in a footnote stating:. This discussion is not meant to be an exact exposition of radiogenic age computation; the relation is mathematically more complicated than the direct proportion assumed for the illustration.

Nevertheless, the principles described are substantially applicable to the actual relationship. Morris states that the production rate of an element formed by radioactive decay is constant with time. This is not true, although for a short period of time compared to the length of the half life the change in production rate may be very small. Radioactive elements decay by half-lives. At the end of the first half life, only half of the radioactive element remains, and therefore the production rate of the element formed by radioactive decay will be only half of what it was at the beginning.

The authors state on p. If these elements existed also as the result of direct creation, it is reasonable to assume that they existed in these same proportions. Say, then, that their initial amounts are represented by quantities of A and cA respectively. This is not correct; radioactive elements decay by half lives, as explained in the first paragraphs of this post.

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Many different radioactive isotopes and techniques are used for dating. All rely on the fact that certain elements particularly uranium and potassium contain a number of different isotopes whose half-life is exactly known and therefore the relative concentrations of these isotopes within a rock or mineral can measure the age. For an element to be useful for geochronology measuring geological time , the isotope must be reasonably abundant and produce daughter isotopes at a good rate. Either a whole rock or a single mineral grain can be dated.

Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer such as in the argon-argon scheme. Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes. The isotopes are then measured within the same machine by an attached mass spectrometer an example of this is SIMS analysis.

Get our monthly emails for amazing animals, research insights and museum events. This is a common dating method mainly used by archaeologists, as it can only date geologically recent organic materials, usually charcoal, but also bone and antlers. All living organisms take up carbon from their environment including a small proportion of the radioactive isotope 14C formed from nitrogen as a result of cosmic ray bombardment.

The amount of carbon isotopes within living organisms reaches an equilibrium value, on death no more is taken up, and the 14C present starts to decay at a known rate. The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism.

This method faces problems because the cosmic ray flux has changed over time, but a calibration factor is applied to take this into account. Radiocarbon dating is normally suitable for organic materials less than 50 years old because beyond that time the amount of 14C becomes too small to be accurately measured. This scheme was developed in but became more useful when mass spectrometers were improved in the late s and early s.

However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. This technique is less used now. The dual decay of potassium K to 40Ar argon and 40Ca calcium was worked out between and This technique has become more widely used since the late s. Its great advantage is that most rocks contain potassium, usually locked up in feldspars, clays and amphiboles. However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks.

Argon-Argon dating 39ArAr. This technique developed in the late s but came into vogue in the early s, through step-wise release of the isotopes. This technique uses the same minerals and rocks as for K-Ar dating but restricts measurements to the argon isotopic system which is not so affected by metamorphic and alteration events. It is used for very old to very young rocks. The decay of Sm to Nd for dating rocks began in the mids and was widespread by the early s. It is useful for dating very old igneous and metamorphic rocks and also meteorites and other cosmic fragments.

However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes. This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe. The Re-Os isotopic system was first developed in the early s, but recently has been improved for accurate age determinations.

The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes. This technique is good for iron meteorites and the mineral molybdenite. This system is highly favoured for accurate dating of igneous and metamorphic rocks, through many different techniques. It was used by the beginning of the s, but took until the early s to produce accurate ages of rocks.

The great advantage is that almost all igneous and metamorphic rocks contain sufficient U and Pb for this dating. It can be used on powdered whole rocks, mineral concentrates isotope dilution technique or single grains SHRIMP technique. It has revolutionised age dating using the U-Pb isotopic system. Using the SHRIMP, selected areas of growth on single grains of zircon, baddeleyite, sphene, rutile and monazite can be accurately dated to less than years in some cases.

It can even date nonradioactive minerals when they contain inclusions of zircons and monazite, as in sapphire grains. It can help fix the maximum age of sedimentary rocks when they contain enough accessory zircon grains usually need about grains. Because of advancements in geochronology for over 50 years, accurate formation ages are now known for many rock sequences on Earth and even in space.

The oldest accurately dated rocks on Earth are metamorphosed felsic volcanic rocks from north-west Western Australia. These were dated at about 4. Several minerals incorporate tiny amounts of uranium into their structure when they crystallise. The radioactive decay from the uranium releases energy and particles this strips away electrons leading to disorder in the mineral structure. The travel of these particles through the mineral leaves scars of damage about one thousandth of a millimetre in length.

These 'fission tracks' are formed by the spontaneous fission of U and are only preserved within insulating materials where the free movement of electrons is restricted. Because the radioactive decay occurs at a known rate, the density of fission tracks for the amount of uranium within a mineral grain can be used to determine its age.

Although the span of time preceding the Cambrian period—the Precambrian —is nearly devoid of characteristic fossil remains and coincides with some of the primary rocks of certain early workers, it must, nevertheless, be evaluated in its temporal context. Historically, the subdivision of Precambrian rock sequences and, therefore, Precambrian time had been accomplished on the basis of structural or lithologic grounds.

With only minor indications of fossil occurrence mainly in the form of algal stromatolites , no effective method of quantifying this loosely constructed chronology existed until the discovery of radioactivity enabled dating procedures to be applied directly to the rocks in question. The quantification of geologic time remained an elusive matter for most human enquiry into the age of the Earth and its complex physical and biological history.

Although Hindu teachings accept a very ancient origin for the Earth, medieval Western concepts of Earth history were based for the most part on a literal interpretation of Old Testament references. Biblical scholars of Renaissance Europe and later considered paternity as a viable method by which the age of the Earth since its creation could be determined.

One such attempt was made by Archbishop James Ussher of Ireland, who in determined that the Creation had occurred during the evening of Oct. By his analysis of biblical genealogies, the Earth was not even 6, years old! As previously noted, fundamental to the principle was the premise that various Earth processes of the past operated in much the same way as those processes operate today.

The corollary to this was that the rates of the various ancient processes could be considered the same as those of the present day. Therefore, it should be possible to calculate the age of the Earth on the basis of the accumulated record of some process that has occurred at this determinable rate since the Creation. Many independent estimates of the age of the Earth have been proposed, each made using a different method of analysis. These chemical and physical arguments or a combination of both were all flawed to varying degrees because of an incomplete understanding of the processes involved.

The notion that all of the salts dissolved in the oceans were the products of leaching from the land was first proposed by the English astronomer and mathematician Edmond Halley in and restated by the Irish geologist John Joly in It was assumed that the ocean was a closed system and that the salinity of the oceans was an ever-changing and ever-increasing condition.

Based on these calculations, Joly proposed that the Earth had consolidated and that the oceans had been created between 80 and 90 million years ago. The subsequent recognition that the ocean is not closed and that a continual loss of salts occurs due to sedimentation in certain environments severely limited this novel approach. Equally novel but similarly flawed was the assumption that, if a cumulative measure of all rock successions were compiled and known rates of sediment accumulation were considered, the amount of time elapsed could be calculated.

While representing a reasonable approach to the problem, this procedure did not or could not take into account different accumulation rates associated with different environments or the fact that there are many breaks in the stratigraphic record. Even observations made on faunal succession proved that gaps in the record do occur. How long were these gaps? Do they represent periods of nondeposition or periods of deposition followed by periods of erosion?

Nevertheless, many attempts using this approach were made. William Thomson later Lord Kelvin applied his thermodynamic principles to the problems of heat flow , and this had implications for predicting the age of a cooling Sun and of a cooling Earth. From an initial estimate of million years for the development of a solid crust around a molten core proposed in , Thomson subsequently revised his estimate of the age of the Earth downward. Using the same criteria , he concluded in that the Earth was between 20 and 40 million years old.

His estimate came into question after the discovery of naturally occurring radioactivity by the French physicist Henri Becquerel in and the subsequent recognition by his colleagues, Marie and Pierre Curie , that compounds of radium which occur in uranium minerals produce heat. The Earth was, in effect, not cooling.