Geologic Time Scales

Eras of the Earth
Law of Superposition
Radiometric Dating


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Geologic Time Scales


How old is the Earth? When did the first living organism evolve? How long ago did dinosaurs live? When did mammals and birds evolve?

Questions about the age of the Earth, and when major events occured surround our interest in Evolution, and the development of life on Earth. Geologist and Paleontologist have developed methods for placing more and more precise numbers on our timeframe. Still, the basic listing and arrangement of the eras of Earth's history was developed long before we could say that this event took place 200 million years ago (mya), or 70 mya. The following pages explain the Eras of Earth's history, and how scientist learned how to give a precise answer to the question, "when did dinosaurs walk the Earth?"