Events that define the divisions within the Geologic Time Scale are primarily based on?

The divisions within the Geologic Time Scale are primarily defined by significant changes in life forms. This approach was established because the fossil record provides crucial evidence of the evolution and extinction of various organisms throughout Earth's history. Major events, such as mass extinctions or the emergence of new species, often serve as markers for the beginning or end of different geological periods and eras.

For example, the transition from the Paleozoic to the Mesozoic era is marked by the largest mass extinction event in Earth's history, which drastically reshaped the biodiversity of the planet. Similarly, the division between the Mesozoic and Cenozoic eras corresponds to the extinction of the dinosaurs and the rise of mammals and birds.

While climate changes, weather patterns, and rock formations can influence and reflect geological processes, it is the evolution and extinction of life forms that provide clear, observable criteria for defining the divisions in the Geologic Time Scale.