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 million up to about 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. 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.
Nicolas Steno in the 17th century was one of the first naturalists to appreciate the connection between fossil remains and strata. In the midth century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe.
Lomonosov's ideas were mostly speculative.
In 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, 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.
This was a challenge to the traditional view, which saw the history of Earth as static, [ citation needed ] with changes brought about by intermittent catastrophes.
Many naturalists were influenced by Lyell to become "uniformitarians" who believed that changes were constant and uniform. In , the physicist William Thomson, 1st Baron Kelvin published calculations that fixed the age of Earth at between 20 million and million years. His calculations did not account for heat produced via radioactive decay a process then unknown to science or, more significantly, convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface.
Geologists such as Charles Lyell had trouble accepting such a short age for Earth. For biologists, even 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. According to modern biology, the total evolutionary history from the beginning of life to today has taken place since 3. In a lecture in , 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 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.
Age of the Earth - Wikipedia
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.
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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.
Dating on earth summary
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.
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.
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