The Geologic Time Scale and Gravimetric Analysis

Layers of rock exposed in the Grand Canyon provide a window on Earth s history. [Left Adapted from F. Press, 

Siever,). Gratzinger, and T. H. Jordan. Understanding Earth, 4th ed. (New York W. H. Freeman and Company, 200A). 

Right Carol Polich/Lonely Planet Images.) 

In the 1800s, geologists understood that new layers (strata) of rock are deposited on top of older layers. Characteristic fossils in each layer helped geologists to identify strata from the same geologic era all around the world. However, the age of each layer was unknown. 

Work by E. Rutherford, E Soddy, and B. Boltwood suggested that a uranium atom decays to one atom of Pb and eight atoms of He with a half-life of 4 J billion years. Rutherford realized that measuring the fraction of U converted to decay products gives the age of a rock. Boltwood and R. Strutt began such measurements. 

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