The geologic column by itself shows only the relative

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The geologic column by itself shows only the relative ages of the major periodsin Earth’s history. It tells us nothing about the length of time represented by a pe-riod.With the discovery of the radioactive decay of uranium and other elements,new tools for measuring geologic time were invented.They greatly enhanced ourunderstanding of time and of the history of Earth and provided numerical bench-marks for the standard geologic column.
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Geologic Time201RADIOMETRIC MEASUREMENTS OF TIMERadiometric dating provides a method for directly measuring geologic timein terms of a specific number of years (numeric age). It has been usedextensively during the last 50 years to provide a numerical time scale forthe events in Earth’s history.Unlike relative time, which specifies only a chronologic sequence of events,numer-ical ageis measured in hours, days, and years. In other words, numeric dating speci-fies quantitative (or absolute) lengths of time.Numeric time can be measured usingany regularly recurring event, such as the swing of a pendulum or the rotation ofEarth.You are very familiar with the concept of numeric dating in that it is the wayin which you record your own age. It is one matter to reveal that you are older thansomeone else (relative age) and something quite different to say that you are a spe-cific number of years old (numeric age). Using appropriate numerical dating tech-niques, geologists can estimate the time span between two geologic events.Before Hutton and Lyell, few people even thought about the age of Earth.AfterHutton presented his arguments for uniformitarianism,and Lyell further developedthe concept, much interest was generated in the magnitude of geologic time, andscientists explored several ways to estimate Earth’s age. Early attempts includedestimates based on how long it would take for the ocean to become salty (about100 million years) or how long it would take to accumulate the known thickness-es of fossil-bearing sedimentary strata (about 500 million years). By the end of the1800s, many geologists had come to accept an age for the Earth of about 100 mil-lion years.This was based on Lord Kelvin’s estimate of how long it would take theplanet to cool from an initially molten state.In short, before the 1900s there was no reliable method for measuring long pe-riods of geologic time, and there seemed to be little hope of finding the secret ofEarth’s age.Each method showed that Earth was far older than many had supposedbased on a reading of the Bible (6000 y), but the true dimensions of time remainedelusive.Then, a major breakthrough occurred when Henri Becquerel (1852–1908),a French physicist, discovered naturalradioactivityin 1896 and opened new vistasin many fields of science.Among the first to experiment with radioactive substanceswas the distinguished British physicist Lord Rutherford (1871–1937).After defin-ing the structure of the atom, Rutherford was the first to suggest that radioactivedecay could be used to calculate a numerical age for geologic events (Figure 8.8).

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Term
Fall
Professor
Swanson
Tags
Radioactive Decay, The Bible, Radiometric dating, geologist
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