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Figure 1. Computer model of EarthClick to Enlarge ImageThe mathematical physicist Lord Kelvin produced one of the 19th century's most famous estimates of the age of the Earth: some 100 million years. The Earth is, in actuality, about 4.5 billion years old. Kelvin's result was based on the idea that the near-surface geothermal gradient reflects the conductive cooling of the solid Earth from an initial state of uniformly high temperature. Today many scientists, including professional geologists, believe that Kelvin's age was off because he was unaware that heat is generated within the Earth by radioactive decay (radioactivity not yet having been discovered). In fact, Kelvin's error, as his one-time assistant John Perry pointed out in 1895, was to regard the rocky mantle of the Earth as being rigid, whereas geologists now know that it is a viscous fluid that transfers heat by convection.

This article is adapted with permission of the Geological Society of America from a paper by England, Molnar and Richter, "John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics," which appeared in the January 2007 issue of GSA Today.

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