The rest of the uranium will have decayed to , atoms of other elements, ultimately to stable i. Wait another 4. Radiometric dating relies on the clock-like characteristics of radioactive decay. In one half-life, approximately half of a collection of radioactive atoms will decay. Source: NCSE. The best-known radiometric dating method involves the isotope carbon, with a half life of 5, years.
Every living organism takes in carbon during its lifetime. At this moment, your body is taking the carbon in your food and converting it to tissue, and the same is true of all other animals. Plants are taking in carbon dioxide from the air and turning it into roots, stems, and leaves. But a certain small percentage of the carbon in your body and every other living thing—no more than one carbon atom in every trillion—is in the form of radioactive carbon As long as an organism is alive, the carbon in its tissues is constantly renewed in the same small, part-per-trillion proportion that is found in the general environment.
All of the isotopes of carbon behave the same way chemically, so the proportions of carbon isotopes in the living tissue will be nearly the same everywhere, for all living things. When an organism dies, however, it stops taking in carbon of any form.
From the time of death, therefore, the carbon in the tissues is no longer replenished. Like a ticking clock, carbon atoms transmute by radioactive decay to nitrogen, atom-by-atom, to form an ever-smaller percentage of the total carbon. Scientists can thus determine the approximate age of a piece of wood, hair, bone, or other object by carefully measuring the fraction of carbon that remains and comparing it to the amount of carbon that we assume was in that material when it was alive.
If the material happens to be a piece of wood taken out of an Egyptian tomb, for example, we have a pretty good estimate of how old the artifact is and, by inference, when the tomb was built. The result: the two independent techniques yield exactly the same dates for ancient fossil wood. Carbon dating often appears in the news in reports of ancient human artifacts. In a highly publicized discovery in , an ancient hunter was found frozen in the ice pack of the Italian Alps Fig.
The technique provided similar age determinations for the tissues of the iceman, his clothing, and his implements Fowler Carbon dating revealed that he died about 5, years ago. Photo courtesy South Tyrol Museum of Archaeology, www. Carbon dating has been instrumental in mapping human history over the last several tens of thousands of years.
When an object is more than about 50, years old, however, the amount of carbon left in it is so small that this dating method cannot be used.
To date rocks and minerals that are millions of years old, scientists must rely on similar techniques that use radioactive isotopes of much greater half-life Table 1.
Among the most widely used radiometric clocks in geology are those based on the decay of potassium half-life of 1.
In these cases, geologists measure the total number of atoms of the radioactive parent and stable daughter elements to determine how many radioactive nuclei were present at the beginning. Thus, for example, if a rock originally formed a long time ago with a small amount of uranium atoms but no lead atoms, then the ratio of uranium-to-lead atoms today can provide an accurate geologic stop watch.
When you see geologic age estimates reported in scientific publications or in the news, chances are those values are derived from radiometric dating techniques. In the case of the early settlement of North America, for example, carbon-rich campfire remains and associated artifacts point to a human presence by about 13, years ago.
Much older events in the history of life, some stretching back billions of years, are often based on potassium dating.
This technique works well because fossils are almost always preserved in layers of sediments, which also record periodic volcanic ash falls as thin horizons. Volcanic ash is rich in potassium-bearing minerals, so each ash fall provides a unique time marker in a sedimentary sequence. The rise of humans about 2. Paleontologists rely on radiometric dating to determine the ages of fossils, such as this million-year-old trilobite, Ameura major , from near Kansas City, Kansas.
Photo courtesy Hazen Collection, Smithsonian Institution. The oldest known rocks, including basalt and other igneous formations, solidified from incandescent red-hot melts. These durable samples from the moon and meteorites are typically poor in potassium, but fortunately, they incorporate small amounts of uranium and other radioactive isotopes.
As soon as these molten rocks cool and harden, their radioactive elements are locked into place and begin to decay. The most ancient of these samples are several types of meteorites, in which slightly more than half of the original uranium has decayed to lead. These primordial space rocks, the leftovers from the formation of Earth and other planets, yield an age of about 4.
The oldest known moon rocks, at about 4. Only a few uranium-rich, sand-sized grains of the hardy mineral zircon, some as old as 4. Nevertheless, uranium-bearing rocks, on every continent provide a detailed chronology of the early Earth Hazen et al. The oldest Earth rocks, at about four billion years, point to the early origins of continents.
Rocks from almost 3. Distinctive uranium-rich sedimentary formations and layered deposits of iron oxides from about 2. Indeed, every stage of Earth history has been dated with exquisite accuracy and precision thanks to radiometric techniques. Stromatolites, such as this 2.
Radiometric methods provide an accurate approach to dating such ancient sediments. Photo courtesy of Dominic Papineau.
Overwhelming observational evidence confirms that Earth history is the story of the co-evolving geospheres and biospheres: Life has changed continuously over the course of Earth history.
As the work of Eugenie Scott has so forcefully defended, Earth must be billions of years old Scott However, such a conclusion is at odds with the doctrine of many Christian fundamentalists, who believe in the literal Biblical chronology of a universe no more than about 10, years old.
How can science respond to such adamant claims? The testimony of the rocks is unambiguous: an enormous body of observational evidence points to the reality of deep time. Annual ice and rock layerings reveal a million years of Earth history. Geologic rates of mountain building, erosion and plate tectonics demand hundreds of millions of years.
Radiometric dating pushes the history back billions of years. And when these techniques overlap, their independent estimates of the timing of ancient events are internally consistent. And then, remarkably, he proceeds to describe how God created everything 10, years ago to look much older! But, no, according to the doctrine of created antiquity, the universe was created with light from those stars and galaxies already on its way to Earth.
We observe rocks with characteristic ancient ratios of radioactive and daughter isotopes. Presumably, the rocks are ancient. But no, those rocks were created with just the right mixtures of uranium, lead, potassium, and carbon to make them appear much older than they really are. Here, scientists are stymied. It is difficult to imagine any experiment or observation that could disprove the doctrine of created antiquity. I refuse to accept the idea that any God would bestow such precious gifts as our senses and reason, seemingly to understand His creation, and then try to fool us.
Every scientific idea must be testable by observations or experiments that can be independently confirmed. In principle, it must be possible to imagine outcomes that would prove the proposition wrong.
Without such independent confirmation, a hypothesis cannot be considered scientific. Created antiquity is not falsifiable. Consequently, the teaching of young-Earth creationism, as well as any other doctrine based on a miraculous creation of life, has been repeatedly prohibited in public schools not because the doctrine was proved wrong, but because it simply is not science. Aguillard , creationism is a religious belief that is inherently untestable by the techniques of science Working Group on Teaching Evolution Many lines of evidence point to the unfathomable antiquity of Earth.
In the s Nicolas Steno formulated our modern concepts of deposition of horizontal strata. He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil. This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law which nearly all of us would accept and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.
That is the background to the intellectual drama being played out in this series of papers. It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains.
We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds. Even less should we let that knowledge influence our judgment of the players, acting as they did in their own time, constrained by the concepts and data then available.
One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists.
Most notable is William Thomson, ennobled to become Lord Kelvin in , whose theories make up an entire section of this collection.
He was one of the dominant physicists of his time, the Age of Steam. His achievements ran from helping formulate the laws of thermodynamics to advising on the first transatlantic telegraph cable. Harlow Shapley, who wrote an article in on the subject, was an astronomer, responsible for the detection of the redshift in distant nebulae and hence, indirectly, for our present concept of an expanding universe. Russell, author of the article on radioactive dating, was familiar to me for his part in developing the Hetzsprung-Russell diagram for stars, but I was surprised to discover that he was also the Russell of Russell-Saunders coupling, important in atomic structure theory.
Sollas , assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift. The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics. The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity.
Lord Kelvin and his allies used three kinds of argument. The first of these referred to the rate of heat loss from the earth and the length of time it would have taken to form its solid crust. The second referred to such topics as the detailed shape of the earth bulging slightly at the equator and the dynamics of the earth-moon system.
The third referred to the heat of the sun, particularly the rate at which such heat is being lost, compared with the total amount of energy initially available. The first argument was completely undermined after taking into account the amount of heat generated by radioactive decay. This method of determining absolute age is called radiometric dating , and it involves the decay, or breakdown, of radioactive elements.
Using radiometric dating, scientists can determine the actual age of a rock. Radiometric dating requires an understanding of isotopes. Isotopes are different forms of the same element, which have a different number of neutrons. Neutrons are tiny particles inside the nucleus , or core, of an atom. The isotopes of unstable radioactive elements are called parent isotopes. They decay, or break down, into other, more stable elements called daughter isotopes. They do this in a predictable way in a certain amount of time called a half-life.
The half-life of an element is the amount of time required for exactly half of a quantity of that element to decay. Scientists can measure the number of parent isotopes that are left in a sample. They compare this to the number of daughter isotopes that are in the sample. This comparison is called a ratio. Using the half-life, they can calculate how long it would take for that number of daughter isotopes to form.
Using the ratio and the half-life, they can determine the age of a rock sample. Radiometric Dating Zeroes in on Earth's Age One problem with this approach to dating rocks and minerals on Earth is the presence of the rock cycle. During the rock cycle, rocks are constantly changing forms. Old rocks are destroyed as they slide back into the planet, and new rocks form when lava cools and solidifies.
The first rocks that formed on Earth are no longer here, and this makes finding an exact age for the planet difficult. The oldest rocks that have been found are about 3. To get around the difficulty presented by the rock cycle, scientists have looked elsewhere in the solar system for even older rock samples.
They have examined rocks from the moon and from meteorites, neither of which have been changed by the rock cycle. Radiometric dating has also been used on those rocks. All of the data from this planet and beyond has led scientists to estimate Earth's age at 4. The age of rocks is determined by radiometric dating, which looks at the proportion of two different isotopes in a sample.
Radioactive isotopes break down in a predictable amount of time, enabling geologists to determine the age of a sample using equipment like this thermal ionization mass spectrometer. Also called radioactivity. Also called radioactive dating.
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