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The discovery of radioactivity not only invalidated Kelvin’s estimate of the Earth’s age, it also led to the development of isotopic dating.
Since the 1950s, geologists have scoured the planet to identify its oldest rocks. Rock samples from several localities (Wyoming, Canada, Greenland, and China) have yielded dates as old as 4.03 Ga.
(Recall that “Ga” means “billion years ago.”) Individual clastic grains of the mineral zircon have yielded dates of up to 4.4 Ga, indicating that rock as old as 4.4 Ga did once exist.
Geologists have come up with several ideas to explain the lack of extremely old rocks.
This dated column is commonly called the geologic time scale.
During the 18th and 19th centuries, before the discovery of isotopic dating, scientists came up with a great variety of clever solutions to the question, “How old is the Earth? Lord William Kelvin, a 19th century physicist renowned for his discoveries in thermodynamics, made the most inﬂuential scientiﬁc estimate of the Earth’s age of his time.
They argued that physical processes that shape the Earth and form its rocks, as well as the process of natural selection that yields the diversity of species, all take a very long time.
Geologists and physicists continued to debate the age issue for many years.
Geologists consider 4.57-Ga meteorites to be fragments of planetesimals like those from which the Earth ﬁrst formed.