geologic time, the extensive interval of time occupied by the geologic history of Earth. Formal geologic time begins at the start of the Archean Eon (4.0 billion to 2.5 billion years ago) and continues to the present day.

When did scientists make the geologic time scale?

Scientists noticed in the 1700s and 1800s that similar layers of sedimentary rocks all over the world contain similar fossils. They used relative dating to order the rock layers from oldest to youngest.

How was the geologic time scale established?

Scientists first developed the geologic time scale by studying rock layers and index fossils worldwide. With this information, scientists placed Earth's rock layers in order by relative age. Later, radioactive dating helped determine the absolute age of the divisions in the geologic time scale.