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If you want to know how old someone or something is, you can generally rely on some combination of simply asking questions or Googling to arrive at an accurate answer. This applies to everything from the age of a classmate to the number of years the United States has existed as a sovereign nation and counting as of But what about the ages of objects of antiquity, from a newly discovered fossil to the very age of the Earth itself? Sure, you can scour the Internet and learn rather quickly that the scientific consensus pins the age of of the planet at about 4. But Google didn't invent this number; instead, human ingenuity and applied physics have provided it. Specifically, a process called radiometric dating allows scientists to determine the ages of objects, including the ages of rocks, ranging from thousands of years old to billions of years old to a marvelous degree of accuracy. This relies on a proven combination of basic mathematics and knowledge of the physical properties of different chemical elements.
Early methods relied on uranium and thorium minerals, but potassium—argon, rubidium—strontium, samarium—neodymium, and carbon—carbon are now of considerable importance. Uranium decays to lead with a half-life of 4. It is important that the radioactive isotope be contained within the sample being dated. Carbon is contained within plant material, but potassium, argon, and uranium are contained satisfactorily only within crystals. Igneous rocks are the most suitable for dating. Fossils occur mostly in sedimentary rocks, however, so absolute dates can be calculated for them less commonly than might be supposed. The only exceptions are fossils occurring in glauconite, a clay mineral containing potassium and argon which forms authigenically on the bottom of shelf seas.
Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements. By "age" we mean the elapsed time from when the mineral specimen was formed. Radioactive elements "decay" that is, change into other elements by "half lives. The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life in other words raised to a power equal to the number of half-lives. If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula. To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.
Radiometric Dating: Methods, Uses & the Significance of Half-Life
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