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In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed in terms of years. Most absolute age determinations in geology rely on radiometric methods.
The earth is billions of years old. The most useful methods for measuring the ages of geologic materials are the radiometric methods-the ones that make use of radioactive parent isotopes and their stable daughter productsas preserved in rocks, minerals, or other geologic materials.
The main condition for the method is that the production rate of isotopes stays the same through ages, i. The production of isotopes from chemical elements is known as decay rate and it is considered a constant. List of radioactive dating methods it is driven by sun activity it was always questioned. Recent article S. Is decay constant? One of the highlights is: No evidence of variable decay constants due to solar neutrinos.
An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the of neutrons it has in its nucleus. By definition, all atoms of a given element have the same of protons. However, they do not all have the same of neutrons.
The different s of neutrons possible in the atoms of a given element correspond to the different possible isotopes of that element. For example, all carbon atoms have 6 protons. Carbon is the isotope of carbon that has 6 neutrons. Carbon is the isotope of carbon that has 7 neutrons. Carbon has 8 neutrons in its List of radioactive dating methods, along with its 6 protons, which is not a stable combination.
That is why carbon is a radioactive isotope-it contains a combination of protons and neutrons in its nucleus that is not stable enough to hold together indefinitely. Eventually, it will undergo a spontaneous nuclear reaction and turn into a stable daughter product — a different isotope, which is not radioactive. Each type of radioactive isotope has a half-lifea length of time that it will take for half of the atoms in a sample of that isotope to decay into the stable daughter product.
Physicists have measured the half-lives of most radioactive isotopes to a high level of precision.
The properties of radioactive isotopes and the way they turn into their stable daughter products are not affected by variations in temperature, pressure, or chemistry. Therefore the half-lives and other properties of isotopes are unaffected by the changing conditions that a rock is subjected to as it moves through the rock cycle. If a granite crystallizes with minerals containing radioactive isotopes, it is as though the rock crystallizes with a built-in batch of stopwatches that begin ticking away as soon as the granite has cooled.
Development of C14 method by Dr. Measurement of the age of the Earth by Dr. Radiometric age determinations are expensive and time-consuming. A geologist has to be sure that an age of a rock will help answer an important research question before he or she devotes time and money to making a radiometric age measurement. Before determining the age of the granite, it must be analyzed under a powerful microscope, and with an electron microprobe, to make sure that its original minerals have not been cracked and altered by metamorphism since the rock first formed.
Separating the minerals from the granite is the next step in determining its age. High-precision laboratory analyses are then used to measure the amounts of radioactive parent isotope and stable daughter product in the minerals. Once these quantities have been measured, the half-life of the radioactive isotope is used to calculate absolute age of the granite. The dots in the cartoon below represent atoms of a parent isotope decaying to its stable daughter product through two half-lives. At time zero in the diagram, which could represent the crystallization of minerals in a rock, there are 32 red dots.
After one half-life has passed, there are 16 List of radioactive dating methods dots and 16 green dots. After two half-lives have passed, there are 8 red dots and 24 green dots. The following graph illustrates radioactive decay of a fixed amount of an List of radioactive dating methods. You can see how the proportions of the isotopes from the cartoon above are graphed as percentages at half-lives 0, 1, and 2 below. The following table lists a selection of isotope pairs that are used in making radiometric age determination.
Note that carbon has a relatively short half-life, which makes it useful only for young, carbon-rich geologic materials, less than about 70, years old. Igneous rocks and high-grade metamorphic rocks are the most likely to be entirely formed of minerals that crystallized when the rocks formed. As most fossils are found in clastic sedimentary rocks, which are made of weathered and eroded minerals and bits of rock of various ages, it is unlikely to be able to make an radiometric age determination of a rock in which a fossil is found. The age of a rock containing fossils can usually be narrowed down by measuring the ages of metamorphic or igneous rocks in stratigraphic relation to it, such as a lava flow on top of a layer of sedimentary rock.
List of radioactive dating methods Time. Search for:. Radiometric dating methods In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed in terms of years. Classic examples of key radiometric measurments are: Development of C14 method by Dr. Parent Isotopes, Daughter Isotopes, and Half-Lives The dots in the cartoon below represent atoms of a parent isotope decaying to its stable daughter product through two half-lives. Minerals include muscovite, biotite, K-feldspar.
Igneous or metamorphic rocks. Used for young organic materials, or surface-water samples: Wood, charcoal, peat, bone, tissue, carbonate minerals from surficial environments, water containing dissolved carbon.List of radioactive dating methods
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