The SHRIMP (Sensitive High Resolution Ion Microprobe) enables the accurate determination of the uranium-lead age of the mineral zircon, and this has revolutionized the understanding of the isotopic age of formation of zircon-bearing igneous granitic rocks.

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suggested that lead is one of the disintegration products of uranium, in which case the older a uranium-bearing mineral the greater should be its proportional part of lead.

Analyzing specimens whose relative geologic ages were known, Boltwood found that the ratio of lead to uranium did indeed increase with age.

These minerals are common to many igneous and metamorphic rocks.

Such techniques have had an enormous impact on scientific knowledge of Earth history because precise dates can now be obtained on rocks in all orogenic (mountain) belts ranging in age from the early Archean (about 4 billion years old) to the early Neogene (roughly 20 million years old).

Scientists determined the Earth's age using a technique called radiometric dating.

Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie.

Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth is difficult to determine.

It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.

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