Final answer:
Before the ability to use radioactive decay for absolute dating, geologists gauged Earth to be billions of years old. Methods included dating of the oldest rocks, analysis of the decay of uranium-238, and considering the age of primitive meteorites.
Step-by-step explanation:
Prior to the availability of absolute dating methods through radioactive decay, geologists inferred the Earth to be billions of years old. Their estimation was based on a variety of scientific observations and methods. One method was studying the oldest rocks and minerals on Earth's crust, with the assertion that Earth must be older than these. Uranium-lead dating revealed the oldest known rocks, the Jack Hills zircons from Australia, to be nearly 4.4 billion years old.
Other premises included the study of uranium-238 decay, understanding that the first rocks could have only solidified once Earth's surface had cooled sufficiently. This gave an idea of Earth's initial temperature, leading to an estimation of the Earth’s age of approximately 4.5×10⁹ years.
Also, the age of primitive meteorites, found to be close to 4.5 billion years, were taken into account. These meteorites, considered the fundamental building blocks, imply that planets formed and their crusts cooled within several millions of years from the inception of the solar system.
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