Petrology Tulane University Prof. Stephen A. Nelson Radiometric Dating Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Then, in , radioactivity was discovered. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer. It provided a means by which the age of the Earth could be determined independently. Principles of Radiometric Dating.
AGE OF THE EARTH
All rights reserved. Professor Willard Libby, a chemist at the University of Chicago, first proposed the idea of radiocarbon dating in Three years later, Libby proved his hypothesis correct when he accurately dated a series of objects with already-known ages.
Planet Earth doesn’t have a birth certificate to record its formation, which means scientists spent hundreds of years struggling to determine the age of the planet. So, just how old is Earth? By dating the rocks in Earth’s ever-changing crust, as well as the rocks in Earth’s neighbors, such as the moon and visiting meteorites, scientists have calculated that Earth is 4. Related: How Big is Earth?
Scientists have made several attempts to date the planet over the past years. They’ve attempted to predict the age based on changing sea levels, the time it took for Earth or the sun to cool to present temperatures, and the salinity of the ocean. As the dating technology progressed, these methods proved unreliable; for instance, the rise and fall of the ocean was shown to be an ever-changing process rather than a gradually declining one.
And in another effort to calculate the age of the planet, scientists turned to the rocks that cover its surface. Scientists also must battle an issue called the Great Unconformity, which is where sedimentary layers of rock appear to be missing at the Grand Canyon, for example, there’s 1.
Slideshows Videos Audio. Here of some of the well-tested methods of dating used in the study of early humans: Potassium-argon dating , Argon-argon dating , Carbon or Radiocarbon , and Uranium series. All of these methods measure the amount of radioactive decay of chemical elements; the decay occurs in a consistent manner, like a clock, over long periods of time.
Thermo-luminescence , Optically stimulated luminescence , and Electron spin resonance.
This new method for measuring Earth’s age not only confirms outcomes obtained with radiological methods, but it poses new problems and avenues of research.
An Essay on Radiometric Dating. Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth. All these methods point to Earth being very, very old — several billions of years old. Young-Earth creationists — that is, creationists who believe that Earth is no more than 10, years old — are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.
When I first became interested in the creation-evolution debate, in late , I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it. I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a non-geologist to understand them. Thus this essay, which is my attempt at producing such a source.
Video: How do we know the age of the Earth?
Encyclopedia of Scientific Dating Methods (Encyclopedia of Earth Sciences Series): Medicine & Health Science Books @
Geologist Ralph Harvey and historian Mott Greene explain the principles of radiometric dating and its application in determining the age of Earth. As the uranium in rocks decays, it emits subatomic particles and turns into lead at a constant rate. Measuring the uranium-to-lead ratios in the oldest rocks on Earth gave scientists an estimated age of the planet of 4. Segment from A Science Odyssey: “Origins. View in: QuickTime RealPlayer. Radiometric Dating: Geologists have calculated the age of Earth at 4.
But for humans whose life span rarely reaches more than years, how can we be so sure of that ancient date? It turns out the answers are in Earth’s rocks.
Dating the Earth, the Sun, and the Stars
This page has been archived and is no longer updated. Despite seeming like a relatively stable place, the Earth’s surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.
These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth’s surface is moving and changing. As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use.
Geochronology is the science of determining the age of rocks , fossils , and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes , whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios. By combining multiple geochronological and biostratigraphic indicators the precision of the recovered age can be improved.
Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloging and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, but merely places it within an interval of time at which that fossil assemblage is known to have coexisted.
Both disciplines work together hand in hand, however, to the point where they share the same system of naming strata rock layers and the time spans utilized to classify sublayers within a stratum. The science of geochronology is the prime tool used in the discipline of chronostratigraphy , which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies.
2 ways of dating fossils
October 23, The Earth is 4. How do scientists know that? Since there’s no “established in” plaque stuck in a cliff somewhere, geologists deduced the age of the Earth thanks to a handful of radioactive elements. With radiometric dating, scientists can put an age on really old rocks—and even good old Mother Earth.
The isotopic dating method can also be based on certain cosmogenic nuclides (e.g., 10Be, 21Ne, 26Al, 36Cl) that form in situ in the upper few meters of the Earth’s.
David H. Bailey does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment. In one respect, science and religion have been largely reconciled since the 19th century, when geologists such as Charles Lyell recognised the evidence for a very old Earth.
Within a few decades, most mainstream religious denominations accepted this view as well. But, much to the consternation of scientists, young-Earth creationism , which holds Earth is only about 6, years old, continues to be promoted in some quarters, and remains very popular with the public, especially in the United States. By contrast, and more representative of OECD countries, only about half as many Canadians espouse such beliefs. Such notions, of course, differ vastly to the findings of modern science, which pegs the age of the earth at 4.
While there are numerous experimental methods used to determine geologic ages, the most frequently employed technique is radiometric dating , based on measurements of various radioactive isotopes in rocks. The phenomenon of radioactivity is rooted in the fundamental laws of physics and follows simple mathematical formulae, taught to all calculus students. Dating schemes based on rates of radioactivity have been refined and scrutinised over several decades, and the latest high-tech equipment permits reliable results to be obtained even with microscopic rock samples.
Radiometric dating is self-checking, because the data after certain preliminary calculations are made are fitted to a straight line called an isochron by means of standard linear regression methods of statistics. The slope of the line determines the age of the rock, and the closeness of fit is a measure of the statistical reliability of this conclusion.
The graphic below gives the general idea, and more technical detail can be found here.
Aristotle thought the earth had existed eternally. Roman poet Lucretius, intellectual heir to the Greek atomists, believed its formation must have been relatively recent, given that there were no records going back beyond the Trojan War. The Talmudic rabbis, Martin Luther and others used the biblical account to extrapolate back from known history and came up with rather similar estimates for when the earth came into being.
An artistic depiction of the major events in the history of Earth. Geochronology is the science of determining the age of rocks, fossils, and sediments using These techniques date metamorphic, igneous and volcanic rocks. They are also used.
Dating , in geology , determining a chronology or calendar of events in the history of Earth , using to a large degree the evidence of organic evolution in the sedimentary rocks accumulated through geologic time in marine and continental environments. To date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques. These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.
Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present. The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere. Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled. This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the rocks or soil.
For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built. Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence. Although with clever detective work many complex time sequences or relative ages can be deduced, the ability to show that objects at two separated sites were formed at the same time requires additional information.
A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered. It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself.
Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated.
19.4 Isotopic Dating Methods
Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. This is different to relative dating, which only puts geological events in time order. Most absolute dates for rocks are obtained with radiometric methods.
Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth. All these methods point to Earth being very, very old
Originally, fossils only provided us with relative ages because, although early paleontologists understood biological succession, they did not know the absolute ages of the different organisms. It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils.
In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks in which they are found, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers. Isotopic dating of rocks, or the minerals within them, is based upon the fact that we know the decay rates of certain unstable isotopes of elements, and that these decay rates have been constant throughout geological time.
It is also based on the premise that when the atoms of an element decay within a mineral or a rock, they remain trapped in the mineral or rock, and do not escape. It has a half-life of 1. In order to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral. One good example is granite, which contains the mineral potassium feldspar Figure Potassium feldspar does not contain any argon when it forms.
Over time, the 40 K in the feldspar decays to 40 Ar. The atoms of 40 Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms. The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between 40 K and the much more abundant 39 K. The minerals biotite and hornblende are also commonly used for K-Ar dating.