Huxley , attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions. The physicist Hermann von Helmholtz in and astronomer Simon Newcomb in contributed their own calculations of 22 and 18 million years respectively to the debate: 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. In John Perry challenged Kelvin's figure on the basis of his assumptions on conductivity, and Oliver Heaviside entered the dialogue, considering it "a vehicle to display the ability of his operator method to solve problems of astonishing complexity.
Other scientists backed up Thomson's figures. 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 hour day. His value of 56 million years added additional evidence that Thomson was on the right track. The last estimate Thomson gave, in , was: 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. In , Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments.
Kelvin calculated the age of the Earth by using thermal gradients , and he arrived at an estimate of about million years. In , John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust.
The discovery of radioactivity introduced another factor in the calculation. After Henri Becquerel 's initial discovery in , Marie and Pierre Curie discovered the radioactive elements polonium and radium in ; and in , 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 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 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. These " decay chains ", 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 ,  Boltwood was inspired to describe the relationships between elements in various decay series.
Age of the Earth - Wikipedia
Late in , 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. 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!
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 , 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 had provided dates for 26 separate rock samples, ranging from 92 to 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 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.
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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 million to 2.
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Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity. Rutherford remained mildly curious about the issue of the age of Earth but did little work on it. Thus the transmutation medieval alchemist desired was in fact being done by nature continuously. Third, radioactive decay is proportional to the number of atoms present. Finally, helium might be a product of radioactive decay Dalrymple, , pp In Rutherford suggested that radioactive decay could be used as a method to calculate absolute time.
They were done without knowledge of isotopes or the decay rates and intermediate products of Uranium. Furthermore, the decay of Thorium to lead was unknown. Even so, early work done by Bertram Borden Boltwood , an American chemist and physicist demonstrated several important consistencies. Finally, as Arthur Holmes , a British geologist pointed out, ages obtained using these new methods were often in agreement with what many geologists had speculated Dalrymple, , pp.
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The fact that this new method of dating utilized empirical evidence gathered in the field and analyzed in the lab with methods based upon experimentally determined physical principles made it promising and powerful. It soon became clear that the Earth was much older than many of the earlier methods indicated. With time, radiometric dating became more sophisticated and accurate, by radiometric dating had proven to be the credible method for determining the age of igneous rocks.
For a good discussion on modern radiometric methods see The Age of the Earth by Dalrymple.
Radiometric Vocabulary Terms 1. Radiometric Dating-Measuring the passage of time by the regular rate of decay of radioactive isotopes. Isotopes-Same element, but different number of neutrons. There are different isotopes. Some isotopes are stable and others are radioactive. Because lead is usually found as a solid, this method was more promising. Like Rutherford's, Boltwood's attempt to apply the principle to the dating of rocks was technically flawed but a step forward.
Beginning in , Arthur Holmes began a long career of applying the concept of radiometric dating to rocks, and is given credit for ironing out the technical issues that hampered earlier attempts. After a century of applying the method we now know that thet oldest known Earth rocks are aprox 4.
The oldest in the Solar System are 4. Some commonly used radiometric systems: Note that the effective range of these dating systems is limited by the degree of error in measurement. Which rocks are useful for radiometric dating?
When you radiometrically date a mineral grain you are determining when it crystallized. Thus, you would like to use rocks whose crystals are roughly the same age. The easiest are igneous rocks in which all crystals are roughly the same age, having solidified at about the same time. The age of new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating. The problem is that metamorphism - the pressure-cooking of rocks - can occur over long intervals. Thus, different crystal grains can yield different ages.
With sedimentary rocks, one would end up dating the individual grains of sediment comprising the rock, not the rock as a whole. These grains could have radically different ages. So, geologists prefer to work with igneous rocks. Useful to archaeologists, maybe, but system is not typically used on rocks at all. Thus, sedimentary and metamorphic rocks can't be radiometrically dated.
Age of the Earth
Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded. Magnetostratigraphy The Earth generates a magnetic field that encompasses the entire planet. In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field.
It is, in essence a form of relative dating. Some magnetic minerals, such as magnetite occur naturally in igneous rocks. When their grains form, they align themselves with the Earth's magnetic field.
The Earth's magnetic field changes quickly i. Nevertheless, because of the orientation of their magnetic minerals, their intrinsic magnetic field records the orientation of the Earth's field as it existed when they formed. Such ancient magnetic fields are called remnant or paleomagnetism. The Earth's magnetic field has a north and south pole. For unknown reasons, at intervals of very roughly , years, the north and south poles trade places.
The result is that the paleomagnetic polarity of igneous rocks is either: