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In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
This estimate was a blow to geologists and supporters of Charles Darwin's theory of evolution, which required an older Earth to provide time for evolution to take place.
Thomson's calculations, however, were soon shown to be flawed when radioactivity was discovered in 1896.
Radioactivity is the tendency of certain atoms to decay into lighter atoms, emitting energy in the process.
For example, geologists measured how fast streams deposited sediment, in order to try to calculate how long the stream had been in existence.
Not surprisingly, these methods resulted in wildly different estimates, from a few million years to "quadrillions of years".