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Until the early 1900’s it was impossible to fix an absolute age to any fossils. Before then, only relative ages could be determined by a method called superposition. Fossils found in lower rock layers were presumed older than fossils found in higher layers.
In 1905 a Yale chemist, Bertram Boltwood, found that there was a relationship between the amount of lead and uranium in mineral samples and their relative ages. Boltwood felt that the uranium (parent element), a radioactive element, gradually decayed into lead (daughter element). He was able to calculate a mathematical formula for the rate at which this decay occurs.
The rate of decay of radioactive elements is measured in “half-lifes.” A half-life is the length of time needed for one-half of the original amount of parent element to decay. The half-life of uranium was found to be 4,500,000,000 years. To determine the age of a mineral sample the ratio of parent/daughter element is calculated and from the decay curve a date is established.