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Radioactive dating uses the decay rates of radioactive substances to measure absolute ages of rocks, minerals and carbon-based substances, according to How Stuff Works. Scientists know how quickly radioactive isotopes decay into other elements over thousands, millions and even billions of years. Scientists calculate ages by measuring how much of the isotope remains in the substance. The key to an age of a substance is the decay-product ratio. The ratio of the original isotope and its decay product determines how many half-lives have occurred since the sample formed.
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 red 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 isotope. 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 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 products, as 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 . Radioactive dating is only useful in dating igneous and metamorphic rock. Organic material that has not fossilized and does not exceed 70, years of age can . Mar 23, Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence.
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.
For biological objects older than 50, years, scientists use radioactive dating to determine the age of rocks surrounding where the material was found. By dating rocks, scientists can approximate ages of very old fossils, bones and teeth. Radiocarbon dating was invented in the s by Willard F. Libby. Dating the particles which make up the rock wouldn't give you the age of the rock itself. In addition, the redeposition process upsets the conditions necessary to achieve accurate results through radiometric dating. Scientists believe they can indirectly date sedimentary rocks using radiometric dating if they find igneous or metamorphic rock. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms.".
A popular way to determine the ages of biological substances no more than 50, years old is to measure the decay of carbon into nitrogen This process begins as soon as a living thing dies and is unable to produce more carbon Plants produce carbon through photosynthesis, while animals and people ingest carbon by eating plants.
Carbon has a half-life of 5, years.
Scientists determine the ages of once-living things by measuring the amount of carbon in the material. For biological objects older than 50, years, scientists use radioactive dating to determine the age of rocks surrounding where the material was found.
This technique is generally used to date igneous and metamorphic rock, which are rocks that were once melted due to extreme heat and pressure. Radiometric dating determines how long ago the liquid rock solidified into solid rock. Sedimentary rock on the other hand consists of sedimentary particles which were removed and deposited somewhere else by some sort of fluid generally wind and water.
The radiometric dating of suitable rocks (i.e. igneous or metamorphic) in proximity to their sedimentary counterparts, therefore allows the sedimentary rock to be dated as well. Sedimentary rock can't be dated using radioactive dating because it is composed of sediment, which is actually small pieces of different rocks. To date it, you have to use . Radiometric dating is a means of determining the age of very old objects, including the Earth itself. Radiometric dating depends on the decay of isotopes, which are different forms of the same element that include the same number of protons but different numbers of neutrons in their atoms.
The sedimentary particles predate the rock which they form. In addition, the redeposition process upsets the conditions necessary to achieve accurate results through radiometric dating.