Meanwhile, Arthur Holmes (1890-1964) was finishing up a geology degree at the Imperial College of Science in London where he developed the technique of dating rocks using the uranium-lead method.By applying the technique to his oldest rock, Holmes proposed that the Earth was at least 1.6 billion years old.In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.

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Our planet was pegged at a youthful few thousand years old by Bible readers (by counting all the "begats" since Adam) as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of 100 million years.

Kelvin defended this calculation throughout his life, even disputing Darwin's explanations of evolution as impossible in that time period.

and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.

Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale.

The discovery gave scientists a tool for dating rocks that contain radioactive elements.

Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.

Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.

These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.

material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.