Final answer:
The geologic time scale divides Earth's history into five primary units: eons, eras, periods, epochs, and ages, each reflecting major geologic and biological changes. Eons are the largest units, within which are eras, then periods, epochs, and ages. Significant events like mass extinctions often define boundaries between these divisions.
Step-by-step explanation:
The geologic time scale is a system that divides Earth's 4.6 billion-year history into various divisions based on major events. At the broadest level, geologic time is split into eons, namely the Hadean, Archean, Proterozoic, and Phanerozoic. These first three eons are grouped together as a "super" eon called the Precambrian. Eons are further divided into eras, which are subdivided into periods, followed by epochs, and finally into the smallest units known as ages.
Each division reflects significant changes in Earth's geology, climate, and biological evolution. For example, the extinction of dinosaurs is a notable event that helped to define the boundary between the Mesozoic and Cenozoic eras. The geologic time scale uses colors, as shown in chart figures, to help distinguish these different periods, with golden spikes often indicating specific places used to define age boundaries.