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SCIENTIFIC AGE OF THE EARTH Before analyzing the arguments advanced by creation “scientists” for a very young Earth, I here summarize briefly the evidence that has convinced scientists that the Earth is 4.5 to 4.6 billion years old. There can be no doubt about the Earth’s antiquity; the evidence is abundant, conclusive, and readily available to all who care to examine it. The best evidence is contained in the Earth’s incomplete and complex but accurate stratigraphic record — a record that has been the subject of nearly two centuries of study. Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1. This was done by observing the relative age sequence of rock units in a given area and determining, from stratigraphic relations, which rock units are younger, which are older, and what assemblages of fossils are contained in each unit. Using fossils to correlate from area to area, geologists have been able to work out a relative worldwide order of rock formations and to divide the rock record and geologic time into the eras, periods, and epochs shown in Figure 1. The last modification to the geologic time scale of Figure 1 was in the 1930s, before radiometric dating was fully developed, when the Oligocene Epoch was inserted between the Eocene and the Miocene. Although early stratigraphers could determine the relative order of rock units and fossils, they could only estimate the lengths of time involved by observing the rates of present geologic processes and comparing the rocks produced by those processes with those preserved in the stratigraphic record. With the development of modern radiometric dating methods in the late 1940s and 1950s, it was possible for the first time not only to measure the lengths of the eras, periods, and epochs but also to check the relative order of these geologic time units. Radiometric dating verified that the relative time scale determined by stratigraphers and paleontologists (Figure 1) is absolutely correct, a result that could only have been obtained if both the relative time scale and radiometric dating methods were correct. The abundance and variety of fossils in Phanerozoic rocks have allowed geologists to decipher in considerable detail the past 600 million years or so of the Earth’s history. In Precambrian rocks, however, fossils are rare; thus, the geologic record of this important part of the Earth’s history has been especially difficult to decipher. Nonetheless, stratigraphy and radiometric dating of Precambrian rocks have clearly demonstrated that the history of the Earth extends billions of years into the past. Radiometric dating has not been applied to just a few selected rocks from the geologic record. Literally many tens of thousands of radiometric age measurements are documented in the scientific literature. Since beginning operation in the early 1960s, the Geochronology laboratories of the U. S. Geological Survey in Menlo Park, California, have alone produced more than 20,000 K-Ar, Rb-Sr, and 14C ages. Add to this number the age measurements made by from 50 to 100 other laboratories worldwide, and it is easy to see that the number of radiometric ages produced over the past two to three decades and published in the scientific literature must easily exceed 100,000. Taken as a whole, these data clearly prove that the Earth’s history extends backward from the present to at least 3.8 billion years into the past. A particularly fascinating question about the history of the Earth is “When did the Earth begin?” The answer to this question was provided by radiometric dating and is now known to within a few percent. Three basic approaches are used to determine the age of the Earth. The first is to search for and date the oldest rocks exposed on the surface of the Earth. These oldest rocks are metamorphic rocks with earlier but now erased histories, so the ages obtained in this way are minimum ages for the Earth. Because the Earth formed as part of the Solar System, a second approach is to date extraterrestrial objects, i.e., meteorites and samples from the Moon. Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets. The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites. This method is thought to represent the time when lead isotopes were last homogeneously distributed throughout the Solar System and, thus, the time that the planetary bodies were segregated into discrete chemical systems. The results from these methods indicate that the Earth, meteorites, the Moon, and, by inference, the entire Solar System are 4.5 to 4.6 billion years old. Before reviewing briefly the evidence for the age of the Earth, I emphasize that the formation of the Solar System and the Earth was not an instantaneous event but occurred over a finite period as a result of processes set in motion when the universe formed. It is, therefore, more correct to talk about formational intervals rather than discrete ages for the Solar System and the Earth. Present evidence indicates, however, that these intervals were rather short (100-200 million years) in comparison with the length of time that has elapsed since the Solar System formed some 4 to 5 billion years ago. Thus, the ages of the Earth, the Moon, and meteorites as measured by different methods represent slightly different events, although the differences in these ages are generally slight, and so, for the purposes of this chapter they are here treated as a single event. THE EARTH’S OLDEST ROCKS All the major continents contain a core of very old rocks fringed by younger rocks. These cores, called Precambrian shields, are all that remain of the Earth’s oldest crust. The rocks in these shields are mostly metamorphic, meaning they have been changed from other rocks into their present form by great heat and pressure beneath the surface; most have been through more than one metamorphism and have had very complex histories. A metamorphic event may change the apparent radiometric age of a rock. Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age. Not all metamorphisms completely erase the radiometric record of a rock’s age, although many do. Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock. Moreover, many of the oldest dated rocks intrude still older but undatable rocks. In all cases, the measured ages provide only a minimum age for the Earth. So far, rocks older than 3.0 billion years have been found in North America, India, Russia, Greenland, Australia, and Africa. The oldest rocks in North America, found in Minnesota, give a U-Pb discordia age of 3.56 billion years (Figure 5). The oldest rocks yet found on the Earth are in Greenland, South Africa, and India. The Greenland samples have been especially well studied. The Amitsoq Gneisses in western Greenland, for example, have been dated by five different methods (Table 6); within the analytical uncertainties, the ages are the same and indicate that these rocks are about 3.7 billion years old. Table 6: Radiometric Ages on the Amitsoq Gneisses, Western Greenland. Data from Baadsgaard (10), Moorbath et al. (89), Pettingill and Patchett (106) weighted mean age 3.67 ± 0.06 Method Age (billion years) Rb - Sr isochron 3.70 ± 0.14 Lu - Hf isochron 3.55 ± 0.22 Pb - Pb isochron 3.80 ± 0.12 U - Pb discordia 3.65 ± 0.05 Th - Pb discordia 3.65 ± 0.08 Whole-rock samples from the Sand River Gneisses in the Limpopo Valley, South Africa, have been dated by the Rb-Sr isochron method at 3.79 ± 0.06 billion years (15). These samples are from rocks that contain inclusions of still older but as yet undatable rocks. Recently, Basu and others (16) reported a nine-sample Sm-Nd isochron age of 3.78 ± 0.11 billion years for rocks in eastern India. Studies of the oldest rocks from the Precambrian shields show that the Earth is older than 3.8 billion years. The geology of these oldest rocks also indicates that there was a substantial period of history of the Earth before 3.8 billion years ago for which no datable geological record now exists. There are several possible reasons for the apparent absence of this earliest record. One reason is that during that period of Earth’s history not only was the first continental crust forming, but it was also being vigorously recycled and regenerated. A second reason is that the Moon and, by inference, the Earth, were subjected to intense bombardment by large meteorites from the time of their initial formation to about 3.8 billion years ago; this bombardment occurred because the Earth was still sweeping up material in its orbital path. A third reason may be that the record of the Earth’s early history exists somewhere but simply has not yet been found. The correct reason for the absence of data may well be some combination of the above. Whatever the reasons, if we are to learn more about the Earth’s history before 3.8 billion years ago, we must examine the evidence obtained from other, older sources, particularly meteorites and the Moon. AGES OF METEORITES There are two basic types of meteorites, stone and iron; other types are intermediate in composition between these two. Stone meteorites are composed primarily of the silicate minerals olivine and pyroxene, whereas iron meteorites consist primarily of nickel-iron alloy. Stone meteorites commonly contain small amounts of nickel-iron, and many iron meteorites include small amounts of silicate minerals. Once thought to be the remains of a shattered planet, meteorites probably originated from some 20 to 70 different parent bodies the size of large asteroids. Some meteorites are samples of the parent bodies that apparently were large enough to undergo partial melting and differentiation to produce different rock types. Others, primarily the stone meteorites called chondrites, seem to represent rocks essentially unchanged since condensation from the solar nebula. The orbits of meteorites indicate that they are parts of the Solar System, probably samples of the asteroids, and thus that their age is relevant to the age of the Earth. Like most things in nature, meteorites are not simple objects. This is especially true of those that have undergone differentiation, heating, and collisions with other bodies in space. To determine the age of the Solar System and the Earth, we must search for the oldest, least disturbed meteorites. K-Ar ages on stone meteorites range from about 400 million years to nearly 5 billion years, with a large concentration at 4.4 to 4.6 billion years. The younger ages reflect heating and collision events, to which the K-Ar method is particularly susceptible, whereas the older ages record events near or equal to the time of meteorite formation. Many meteorites have now been dated by the 40Ar/39Ar age-spectrum method, which reveals that many meteorites were heated after their formation. The metallic phases in iron meteorites cannot be dated reliably by the K-Ar method because of their nearly negligible potassium content and cosmic-ray effects. However, silicate inclusions in several iron meteorites have been dated by the K-Ar method at 4.5 ± 0.2 billion years (19). Some of the most precise ages on meteorites have been obtained by the Rb-Sr isochron method. Table 7 lists some of these ages, from Faure’s (49) summary. Figure 3 plots the isochron for the meteorite Juvinas. Some iron meteorites containing small silicate inclusions have also been dated by the Rb-Sr isochron method; the results indicate that the least disturbed iron meteorites are of the same age (4.6 billion years) as the least disturbed stone meteorites. Table 7: Summary of Some Rb-Sr Isochron Ages of Meteorites From the Compilation of Faure (49) Note: All ages are based on a value of 1.39 × 10-11 y-1 for the decay constant of 87Rb. The currently accepted value of 1.42 × 10-11 yr-1 has the effect of lowering these ages slightly. Material Method Age (bil- lion years) Juvinas (achrondrite) Mineral isochron 4.60 ± 0.07 Allende (carbonaceous chrondrite) Mixed isochron 4.5-4.7 Colomera (silicate inclusion, iron meteorite) Mineral isochron 4.61 ± 0.04 Enstatite chondrites Whole-rock isochron 4.54 ± 0.13 Enstatite chondrites Mineral isochron 4.56 ± 0.15 Carbonaceous chon- drites Whole-rock isochron 4.69 ± 0.14 Amphoterite chon- drites Whole-rock isochron 4.56 ± 0.15 Bronzite chondrites Whole-rock isochron 4.69 ± 0.14 Hypersthene chon- drites Whole-rock isochron 4.48 ± 0.1 Krahenberg (amphoter- ite) Mineral isochron 4.70 ± 0.01 Norton County (achon- drite) Mineral isochron 4.7 ± 0.1 Meteorites have also been dated by the Sm-Nd isochron method. Jacobsen and Wasserburg (69), for example, showed that 10 chondrites and the achondrite Juvinas all fall on an isochron of 4.60 billion years. The results of radiometric dating on meteorites clearly indicate that these objects formed about 4.6 billion years ago. Because astrophysical considerations require that the formation of the planets and meteorites by condensation from the solar nebula was essentially simultaneous, we can infer with considerable certainty that the age of the most primitive meteorites also is the age of formation of the Earth. Even if we wished to deny this inference, we would still be forced to conclude that meteorites, which must at least post date the formation of the Solar System and the universe, are no less than 4.6 billion years old. AGES OF LUNAR ROCKS The Apollo missions, for the first time, gave scientists the exciting opportunity to study samples from another planet. Although all the samples provide important information about the history of the Moon, for data on the age of formation of the Moon we must again look at the oldest rocks. The surface of the Moon can be divided into the lunar highlands and the lunar maria. The highlands are mountainous upland areas that still preserve some aspects of the original impact morphology of the earliest Moon. The maria, or “seas,” are younger, lowland areas that were flooded by lava after impact by asteroid-size bodies. The Apollo missions returned samples from both the highlands and maria. Because of the severe impact history of the early Moon and the consequent heating and metamorphism of lunar samples, the conventional K-Ar method is not particularly useful in the study of lunar rock formation because it tends to date the latest heating and impact events rather than original rock ages. The ages of lunar rocks are known primarily from 40Ar/39Ar age-spectrum and Rb-Sr isochron dating; Table 8 lists some of these ages. As can be seen from this table, the rocks from each landing site give similar ages by both methods; this agreement cannot be a mere coincidence but must reflect the true ages of the rocks within the analytical uncertainties. Table 8, however, lists only data obtained before 1974; since that time, older rocks, from the lunar highlands, have been analyzed. Numerous 40Ar/39Ar age-spectrum ages of highland rocks fall between about 4.0 and 4.5 billion years. The oldest ages, however, have been measured by the Rb/Sr isochron method on samples from the Apollo 17 site. These include mineral isochron ages of 4.55 ± 0.1, 4.60 ± 0.1, and 4.43 ± 0.05 billion years for three different rock types. In addition, 40Ar/39Ar age-spectrum analyses from the Apollo 16 site have now shown two rocks with ages of 4.47 and 4.42 billion years (see summary in 75), and Sm-Nd isochron ages of 4.23 ± 0.05 and 4.34 ± 0.05 billion years have been determined for two Apollo 17 samples (23). Table 8: Summary of Some Radiometric Ages of Lunar Basalts. From the Compilation by Head (62) Location Age (billion years) Rock type Sample Method Apollo 14 – highlands 3.96 Al basalt 14053 Rb-Sr 3.95 Al basalt 14053 40Ar-39Ar 3.95 Al basalt 14321 Rb-Sr Apollo 17 – highlands 3.83 High-Ti basalt 75055 Rb-Sr 3.82 High-Ti basalt 70035 Rb-Sr 3.76 High-Ti basalt 75055 40Ar-39Ar 3.74 High-Ti basalt 75083 40Ar-39Ar Apollo 11 – mare 3.82 Low-K basalt 10062 40Ar-39Ar 3.71 Low-K basalt 10044 Rb-Sr 3.63 Low-K basalt 10058 Rb-Sr 3.68 High-K basalt 10071 Rb-Sr 3.63 High-K basalt 10057 Rb-Sr 3.61 High-K basalt 10024 Rb-Sr 3.59 High-K basalt 10017 Rb-Sr 3.56 High-K basalt 10022 40Ar-39Ar Luna 16 – highlands 3.45 Al basalt B-1 40Ar-39Ar 3.42 Al basalt B-1 Rb-Sr Apollo 15 – highlands 3.44 Quartz basalt 15682 Rb-Sr 3.40 Quartz basalt 15085 Rb-Sr 3.35 Quartz basalt 15117 Rb-Sr 3.33 Quartz basalt 15076 Rb-Sr 3.32 Olivine basalt 15555 Rb-Sr 3.31 Olivine basalt 15555 40Ar-39Ar 3.26 Quartz basalt 15065 Rb-Sr Apollo 12 – mare 3.36 Olivine basalt 12002 Rb-Sr 3.30 Olivine basalt 12063 Rb-Sr 3.30 Olivine basalt 12040 Rb-Sr 3.27 Quartz basalt 12051 40Ar-39Ar 3.26 Quartz basalt 12051 Rb-Sr 3.24 Olivine basalt 12002 40Ar-39Ar 3.24 Quartz basalt 12065 40Ar-39Ar 3.18 Quartz basalt 12064 Rb-Sr 3.16 Quartz basalt 12065 Rb-Sr The hundreds of radiometric ages on lunar rocks show clearly that the initial formation of the Moon was 4.5 to 4.6 billion years ago. There are, to be sure, some uncertainties about the exact chronology and events that led to the Moon we now see, but there is little doubt about when the Moon formed or about the date of the major volcanic events that produced the igneous rocks at the various Apollo sites. MODEL LEAD AGE OF METEORITES AND THE EARTH The generally accepted age of the Earth is based on a simple but elegant model for the evolution of lead isotopes. This model was developed independently by Houtermans (65) and Holmes (63), and first applied to meteorites and the Earth by Clair Patterson, now at the California Institute of Technology, in 1953. In his classic paper, Patterson (104) reasoned that if the Pb-isotopic composition were uniform in the solar nebula and, thus, uniform in the planetary bodies and meteorites at the time of their formation, and if these bodies contained differing amounts of uranium, then the Pb-isotopic composition of these bodies should fall on a straight-line isochron when the 207Pb/204Pb ratio is plotted against the 206Pb/204Pb ratio (Figure 8). The lower end of the isochron in Figure 8 represents the Pb-isotopic composition in a phase of iron meteorites (troilite, or iron sulfide) that contains no uranium; this point represents the initial Pb-isotopic composition of the Solar System. Figure 8: Meteoric lead-isotope isochron showing that the age of meteorites and the Earth is about 4.55 billion years. After Murthy and Patterson (98) and York and Farquhart (136). The Pb-isotopic compositions of iron and stone meteorites fall on an isochron age of 4.55 billion years (Figure 8). Note that this method, like the other isochron methods, is self-checking. Modern Earth leads, as represented by the Pb-isotopic compositions of some very young non-uranium-bearing minerals, also fall close to the meteoritic isochron,9 a result that we would expect if the Earth and meteorites formed contemporaneously. The ratios in lunar rocks have much larger values than those in terrestrial rocks and meteorites; they fall out of the field of Figure 8, but they do lie very close to the extension of the meteoritic isochron and, therefore, indicate a similar age. If the Earth, the Moon, and meteorites were not genetically related and of the same age, there would be no reason for their Pb-isotopic compositions to lie along the same isochron. This is convincing evidence that the planetary bodies, including the Earth, all formed about 4.55 billion years ago. Note that Patterson’s (104) original estimate of the age of the Earth has changed very little over the past three decades. In a recent reevaluation, Tera (125) concludes that the age of the Earth is about 4.54 billion years. Tera also summarizes several other lead models for the Earth’s age; they all give results within the range 4.43 to 4.59 billion years. Thus, although there is still some debate about the exact age of the Earth and the Solar System, scientists are quibbling only about the first one- or two-tenths of a billion years. The age of the Earth is known to within about one part in 45, i.e., about two percent. Age of the Earth From Wikipedia, the free encyclopedia Changes must be reviewed before being displayed on this page.show/hide details This article is about the scientific age of the Earth. For religious beliefs, see dating creation. Page semi-protected Key topics on Geology Grand Canyon NP-Arizona-USA.jpg Grand Canyon Overview[show] Dating methods[show] Сomposition and structure[show] Historical geology[show] Motion[show] Hydrogeology[show] Geophysics[show] History of geologic science[show] Earth Sciences Portal Category • Related topics v t e The age of the Earth is 4.54 ± 0.05 billion years (4.54 × 109 years ± 1%).[1][2][3] This age is based on evidence from radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples. Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.[4] The oldest such minerals analyzed to date – small crystals of zircon from the Jack Hills of Western Australia – are at least 4.404 billion years old.[5][6][7] Comparing the mass and luminosity of the Sun to those of other stars, it appears that the solar system cannot be much older than those rocks. Calcium-aluminium-rich inclusions – the oldest known solid constituents within meteorites that are formed within the solar system – are 4.567 billion years old,[8][9] giving an age for the solar system and an upper limit for the age of Earth. It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites. Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few millions up to about 100 million years, the exact age of Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. Contents [hide] 1 Development of modern geologic concepts 2 Early calculations 3 Radiometric dating 3.1 Overview 3.2 Convective mantle and radioactivity 3.3 Invention of radiometric dating 3.4 Arthur Holmes establishes radiometric dating 3.5 Modern radiometric dating 3.5.1 Why meteorites were used 3.5.2 Canyon Diablo meteorite 3.6 Helioseismic verification 4 See also 5 References 6 Bibliography 7 Further reading 8 External links Development of modern geologic concepts Main article: History of geology Further information: Relative dating Earth as seen from Apollo 17 Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence. These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.[10][11] Nicolas Steno in the 17th century was one of the first naturalists to appreciate the connection between fossil remains and strata.[11] His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality").[12] In the 1790s, William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.[13] William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old.[14] The naturalist Mikhail Lomonosov suggested in the mid-18th century that Earth had been created separately from the rest of the universe, several hundred thousand years before. Lomonosov's ideas were mostly speculative. In 1779 the Comte du Buffon tried to obtain a value for the age of Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling. This led him to estimate that Earth was about 75,000 years old. Other naturalists used these hypotheses to construct a history of Earth, though their timelines were inexact as they did not know how long it took to lay down stratigraphic layers. In 1830, geologist Charles Lyell, developing ideas found in James Hutton's works, popularized the concept that the features of Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant. This was a challenge to the traditional view, which saw the history of Earth as static, with changes brought about by intermittent catastrophes. Many naturalists were influenced by Lyell to become "uniformitarians" who believed that changes were constant and uniform. Early calculations William Thomson (Lord Kelvin) Further information: William Thomson, 1st Baron Kelvin § Age of the Earth: geology and theology In 1862, the physicist William Thomson published calculations that fixed the age of Earth at between 20 million and 400 million years.[15][16] He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface to cool to its present temperature. His calculations did not account for heat produced via radioactive decay (a process then unknown to science) or convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface.[15] Geologists such as Charles Lyell had trouble accepting such a short age for Earth. For biologists, even 100 million years seemed much too short to be plausible. In Darwin's theory of evolution, the process of random heritable variation with cumulative selection requires great durations of time. (Modern geneticists have measured the rate of genetic divergence of species, using the molecular clock, to date the last universal ancestor of all living organisms no later than 3.5 to 3.8 billion years ago). In a lecture in 1869, Darwin's great advocate, Thomas H. Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions. The physicist Hermann von Helmholtz (in 1856) and astronomer Simon Newcomb (in 1892) contributed their own calculations of 22 and 18 million years respectively to the debate: they independently calculated the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.[17] Their values were consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. The process of solar nuclear fusion was not yet known to science. Other scientists backed up Thomson's figures as well. Charles Darwin's son, the astronomer George H. Darwin, proposed that Earth and Moon had broken apart in their early days when they were both molten. He calculated the amount of time it would have taken for tidal friction to give Earth its current 24-hour day. His value of 56 million years added additional evidence that Thomson was on the right track.[17] The last estimate Thomson gave, in 1897, was: "that it was more than 20 and less than 40 million year old, and probably much nearer 20 than 40".[18] In 1899 and 1900, John Joly calculated the rate at which the oceans should have accumulated salt from erosion processes, and determined that the oceans were about 80 to 100 million years old.[17] Radiometric dating Main article: Radiometric dating Overview By their chemical nature, rock minerals contain certain elements and not others; but in rocks containing radioactive isotopes, the process of radioactive decay generates exotic elements over time. By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated.[19] Typical radioactive end products are argon from decay of potassium-40, and lead from decay of uranium and thorium.[19] If the rock becomes molten, as happens in Earth's mantle, such nonradioactive end products typically escape or are redistributed.[19] Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself. Convective mantle and radioactivity In 1892, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments. Kelvin calculated the age of Earth by using thermal gradients, and arrived at an estimate of 100 million years old.[20] He did not realize that Earth has a highly viscous fluid mantle, and this invalidated his estimate. In 1895, John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust.[20] Kelvin stuck by his estimate of 100 million years, and later reduced it to about 20 million years. The discovery of radioactivity introduced another factor in the calculation. After Henri Becquerel's initial discovery in 1896, Marie and Pierre Curie discovered the radioactive elements polonium and radium in 1898; and in 1903, Pierre Curie and Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour. Geologists quickly realized that this upset the assumptions underlying most calculations of the age of Earth. These had assumed that the original heat of the Earth and Sun had dissipated steadily into space, but radioactive decay meant that this heat had been continually replenished. George Darwin and John Joly were the first to point this out, in 1903.[21] Invention of radiometric dating This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2012) Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of radiometric dating. Ernest Rutherford in 1908. Ernest Rutherford and Frederick Soddy jointly had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or gamma radiation in the process. They also determined that a particular isotope of a radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "half-life", or the amount of time it takes half of a mass of that radioactive material to break down into its "decay product". Some radioactive materials have short half-lives; some have long half-lives. Uranium and thorium have long half-lives, and so persist in Earth's crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of Earth by determining the relative proportions of radioactive materials in geological samples. In reality, radioactive elements do not always decay into nonradioactive ("stable") elements directly, instead, decaying into other radioactive elements that have their own half-lives and so on, until they reach a stable element. Such "decay series", such as the uranium-radium and thorium series, were known within a few years of the discovery of radioactivity, and provided a basis for constructing techniques of radiometric dating. The pioneers of radioactivity were chemist Bertram B. Boltwood and the energetic Rutherford. Boltwood had conducted studies of radioactive materials as a consultant, and when Rutherford lectured at Yale in 1904,[22] Boltwood was inspired to describe the relationships between elements in various decay series. Late in 1904, Rutherford took the first step toward radiometric dating by suggesting that the alpha particles released by radioactive decay could be trapped in a rocky material as helium atoms. At the time, Rutherford was only guessing at the relationship between alpha particles and helium atoms, but he would prove the connection four years later. Soddy and Sir William Ramsay had just determined the rate at which radium produces alpha particles, and Rutherford proposed that he could determine the age of a rock sample by measuring its concentration of helium. He dated a rock in his possession to an age of 40 million years by this technique. Rutherford wrote, I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in trouble at the last part of my speech dealing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye, and cock a baleful glance at me! Then a sudden inspiration came, and I said, 'Lord Kelvin had limited the age of the earth, provided no new source was discovered. That prophetic utterance refers to what we are now considering tonight, radium!' Behold! the old boy beamed upon me.[23] Rutherford assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate, and that helium did not escape from the sample over time. Rutherford's scheme was inaccurate, but it was a useful first step. Boltwood focused on the end products of decay series. In 1905, he suggested that lead was the final stable product of the decay of radium. It was already known that radium was an intermediate product of the decay of uranium. Rutherford joined in, outlining a decay process in which radium emitted five alpha particles through various intermediate products to end up with lead, and speculated that the radium-lead decay chain could be used to date rock samples. Boltwood did the legwork, and by the end of 1905 had provided dates for 26 separate rock samples, ranging from 92 to 570 million years. He did not publish these results, which was fortunate because they were flawed by measurement errors and poor estimates of the half-life of radium. Boltwood refined his work and finally published the results in 1907.[4] Boltwood's paper pointed out that samples taken from comparable layers of strata had similar lead-to-uranium ratios, and that samples from older layers had a higher proportion of lead, except where there was evidence that lead had leached out of the sample. His studies were flawed by the fact that the decay series of thorium was not understood, which led to incorrect results for samples that contained both uranium and thorium. However, his calculations were far more accurate than any that had been performed to that time. Refinements in the technique would later give ages for Boltwood's 26 samples of 250 million to 1.3 billion years. Arthur Holmes establishes radiometric dating Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity. Boltwood gave up work on radiometric dating and went on to investigate other decay series. Rutherford remained mildly curious about the issue of the age of Earth but did little work on it. Robert Strutt tinkered with Rutherford's helium method until 1910 and then ceased. However, Strutt's student Arthur Holmes became interested in radiometric dating and continued to work on it after everyone else had given up. Holmes focused on lead dating, because he regarded the helium method as unpromising. He performed measurements on rock samples and concluded in 1911 that the oldest (a sample from Ceylon) was about 1.6 billion years old.[24] These calculations were not particularly trustworthy. For example, he assumed that the samples had contained only uranium and no lead when they were formed. More important research was published in 1913. It showed that elements generally exist in multiple variants with different masses, or "isotopes". In the 1930s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as "neutrons". In that same year, other research was published establishing the rules for radioactive decay, allowing more precise identification of decay series. Many geologists felt these new discoveries made radiometric dating so complicated as to be worthless. Holmes felt that they gave him tools to improve his techniques, and he plodded ahead with his research, publishing before and after the First World War. His work was generally ignored until the 1920s, though in 1917 Joseph Barrell, a professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes's findings in radiometric dating. Barrell's research determined that the layers of strata had not all been laid down at the same rate, and so current rates of geological change could not be used to provide accurate timelines of the history of Earth. Holmes's persistence finally began to pay off in 1921, when the speakers at the yearly meeting of the British Association for the Advancement of Science came to a rough consensus that Earth was a few billion years old, and that radiometric dating was credible. Holmes published The Age of the Earth, an Introduction to Geological Ideas in 1927 in which he presented a range of 1.6 to 3.0 billion years. No great push to embrace radiometric dating followed, however, and the die-hards in the geological community stubbornly resisted. They had never cared for attempts by physicists to intrude in their domain, and had successfully ignored them so far. The growing weight of evidence finally tilted the balance in 1931, when the National Research Council of the US National Academy of Sciences decided to resolve the question of the age of Earth by appointing a committee to investigate. Holmes, being one of the few people on Earth who was trained in radiometric dating techniques, was a committee member, and in fact wrote most of the final report.[25] The report concluded that radioactive dating was the only reliable means of pinning down geological time scales. Questions of bias were deflected by the great and exacting detail of the report. It described the methods used, the care with which measurements were made, and their error bars and limitations. Modern radiometric dating Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine-tuned for the past 50+ years. Forty or so different dating techniques have been utilized to date, working on a wide variety of materials. Dates for the same sample using these different techniques are in very close agreement on the age of the material. Possible contamination problems do exist, but they have been studied and dealt with by careful investigation, leading to sample preparation procedures being minimized to limit the chance of contamination. Why meteorites were used An age of 4.55 ± 0.07 billion years, very close to today's accepted age, was determined by C.C. Patterson using uranium-lead isotope dating (specifically lead-lead dating) on several meteorites including the Canyon Diablo meteorite and published in 1956.[26] Lead isotope isochron diagram showing data used by Patterson to determine the age of the Earth in 1956. The quoted age of Earth is derived, in part, from the Canyon Diablo meteorite for several important reasons and is built upon a modern understanding of cosmochemistry built up over decades of research. Most geological samples from Earth are unable to give a direct date of the formation of Earth from the solar nebula because Earth has undergone differentiation into the core, mantle, and crust, and this has then undergone a long history of mixing and unmixing of these sample reservoirs by plate tectonics, weathering and hydrothermal circulation. All of these processes may adversely affect isotopic dating mechanisms because the sample cannot always be assumed to have remained as a closed system, by which it is meant that either the parent or daughter nuclide (a species of atom characterised by the number of neutrons and protons an atom contains) or an intermediate daughter nuclide may have been partially removed from the sample, which will skew the resulting isotopic date. To mitigate this effect it is usual to date several minerals in the same sample, to provide an isochron. Alternatively, more than one dating system may be used on a sample to check the date. Some meteorites are furthermore considered to represent the primitive material from which the accreting solar disk was formed.[27] Some have behaved as closed systems (for some isotopic systems) soon after the solar disk and the planets formed. To date, these assumptions are supported by much scientific observation and repeated isotopic dates, and it is certainly a more robust hypothesis than that which assumes a terrestrial rock has retained its original composition. Nevertheless, ancient Archaean lead ores of galena have been used to date the formation of Earth as these represent the earliest formed lead-only minerals on the planet and record the earliest homogeneous lead-lead isotope systems on the planet. These have returned age dates of 4.54 billion years with a precision of as little as 1% margin for error.[28] Statistics for several meteorites that have undergone isochron dating are as follows:[29] 1. St. Severin (ordinary chondrite) 1. Pb-Pb isochron 4.543 ± 0.019 GY 2. Sm-Nd isochron 4.55  ± 0.33 GY 3. Rb-Sr isochron 4.51  ± 0.15 GY 4. Re-Os isochron 4.68  ± 0.15 GY 2. Juvinas (basaltic achondrite) 1. Pb-Pb isochron 4.556 ± 0.012 GY 2. Pb-Pb isochron 4.540 ± 0.001 GY 3. Sm-Nd isochron 4.56  ± 0.08 GY 4. Rb-Sr isochron 4.50  ± 0.07 GY 3. Allende (carbonaceous chondrite) 1. Pb-Pb isochron 4.553 ± 0.004 GY 2. Ar-Ar age spectrum 4.52  ± 0.02 GY 3. Ar-Ar age spectrum 4.55  ± 0.03 GY 4. Ar-Ar age spectrum 4.56  ± 0.05 GY Canyon Diablo meteorite Further information: Canyon Diablo (meteorite) Fragment of the Canyon Diablo iron meteorite. The Canyon Diablo meteorite was used because it is a very large representative of a particularly rare type of meteorite that contains sulfide minerals (particularly troilite, FeS), metallic nickel-iron alloys, plus silicate minerals. Barringer Crater, Arizona where the Canyon Diablo meteorite was found. This is important because the presence of the three mineral phases allows investigation of isotopic dates using samples that provide a great separation in concentrations between parent and daughter nuclides. This is particularly true of uranium and lead. Lead is strongly chalcophilic and is found in the sulfide at a much greater concentration than in the silicate, versus uranium. Because of this segregation in the parent and daughter nuclides during the formation of the meteorite, this allowed a much more precise date of the formation of the solar disk and hence the planets than ever before. The age determined from the Canyon Diablo meteorite has been confirmed by hundreds of other age determinations, from both terrestrial samples and other meteorites.[30] The meteorite samples, however, show a spread from 4.53 to 4.58 billion years ago. This is interpreted as the duration of formation of the solar nebula and its collapse into the solar disk to form the Sun and the planets. This 50 million year time span allows for accretion of the planets from the original solar dust and meteorites. The moon, as another extraterrestrial body that has not undergone plate tectonics and that has no atmosphere, provides quite precise age dates from the samples returned from the Apollo missions. Rocks returned from the moon have been dated at a maximum of around 4.4 and 4.5 billion years old. Martian meteorites that have landed upon Earth have also been dated to around 4.5 billion years old by lead-lead dating. Lunar samples, since they have not been disturbed by weathering, plate tectonics or material moved by organisms, can also provide dating by direct electron microscope examination of cosmic ray tracks. The accumulation of dislocations generated by high energy cosmic ray particle impacts provides another confirmation of the isotopic dates. Cosmic ray dating is only useful on material that has not been melted, since melting erases the crystalline structure of the material, and wipes away the tracks left by the particles. Altogether, the concordance of age dates of both the earliest terrestrial lead reservoirs and all other reservoirs within the solar system found to date are used to support the hypothesis that Earth and the rest of the solar system formed at around 4.53 to 4.58 billion years ago. Helioseismic verification The radiometric date of meteorites can be verified with studies of the Sun. The Sun can be dated using helioseismic methods that strongly agree with the radiometric dates found for the oldest meteorites.[31] picture pictures photo picture baby baby images baby images love words in love message love كلمات حب احلي صور اطفال car