The best estimate for Earth's age is based on radiometric dating of fragments from the Canyon Diablo iron meteorite. From the fragments, scientists calculated the relative abundances of elements that formed as radioactive uranium decayed over billions of years. For the record, the universe is now thought to have debuted, at least in its latest incarnation, about Follow Life's Little Mysteries on Twitter llmysteries. Previously she was an assistant editor at Scholastic's Science World magazine.
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Jeanna has an English degree from Salisbury University, a Master's degree in biogeochemistry and environmental sciences from the University of Maryland, and a science journalism degree from New York University. 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. 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.
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. Robert Strutt tinkered with Rutherford's helium method until 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 that the oldest a sample from Ceylon was about 1. For example, he assumed that the samples had contained only uranium and no lead when they were formed. More important research was published in It showed that elements generally exist in multiple variants with different masses, or " isotopes ".
In the s, 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. His work was generally ignored until the s, though in 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' persistence finally began to pay off in , 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 in which he presented a range of 1. 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.
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. Thus, Arthur Holmes' 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.
Age of the Earth
Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine-tuned on an ongoing basis since the s. Forty or so different dating techniques have been utilized to date, working on a wide variety of materials.
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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.
An age of 4. 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. 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.
The Idea of Time
These have returned age dates of 4. Statistics for several meteorites that have undergone isochron dating are as follows: The Canyon Diablo meteorite was used because it is both large and representative of a particularly rare type of meteorite that contains sulfide minerals particularly troilite , FeS , metallic nickel - iron alloys, plus silicate minerals. 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. There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate. As one example, the first minerals to crystallize condense from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to plus or minus 2 million years!! That is pretty accurate!!! Other events on earth can be dated equally well given the right minerals.
For example, a problem I have worked on involving the eruption of a volcano at what is now Naples, Italy, occurred years ago with a plus or minus of years. Yes, radiometric dating is a very accurate way to date the Earth. We know it is accurate because radiometric dating is based on the radioactive decay of unstable isotopes. For example, the element Uranium exists as one of several isotopes, some of which are unstable. When an unstable Uranium U isotope decays, it turns into an isotope of the element Lead Pb. We call the original, unstable isotope Uranium the "parent", and the product of decay Lead the "daughter".
From careful physics and chemistry experiments, we know that parents turn into daughters at a very consistent, predictable rate.
- How is Earth's Age Calculated?;
- Age of the Earth - Wikipedia.
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A geologist can pick up a rock from a mountainside somewhere, and bring it back to the lab, and separate out the individual minerals that compose the rock. They can then look at a single mineral, and using an instrument called a mass spectrometer, they can measure the amount of parent and the amount of daughter in that mineral.
The ratio of the parent to daughter then can be used to back-calculate the age of that rock. The reason we know that radiometric dating works so well is because we can use several different isotope systems for example, Uranium-Lead, Lutetium-Halfnium, Potassium-Argon on the same rock, and they all come up with the same age. This gives geologists great confidence that the method correctly determines when that rock formed.
Hope that helps, and please ask if you'd like more details! I think that I will start by answering the second part of your question, just because I think that will make the answer to the first question clearer. Radiometric dating is the use of radioactive and radiogenic those formed from the decay of radioactive parents isotopes isotopes are atoms of the same element that have different numbers of neutrons in their nuclei to determine the age of something. It is commonly used in earth science to determine the age of rock formations or features or to figure out how fast geologic processes take place for example, how fast marine terraces on Santa Cruz island are being uplifted.
Radiometric dating relies on the principle of radioactive decay.
How is Earth's Age Calculated?
All radioactive isotopes have a characteristic half-life the amount of time that it takes for one half of the original number of atoms of that isotope to decay. By measuring the parent isotope radioactive and the daughter isotope radiogenic in a system for example, a rock , we can tell how long the system has been closed in our example, when the rock formed.
The process of radiogenic dating is usually done using some sort of mass spectrometer. A mass spectrometer is an instrument that separates atoms based on their mass.