Principal Units of Geological Time

Principal units of geological time are ways scientists organize Earth's history into manageable segments. The largest units are eons, like the Hadean or Phanerozoic, spanning hundreds of millions to billions of years. Eons are divided into eras (e.g., the Mesozoic or "Age of Reptiles"), which further split into periods (such as the Jurassic). Periods can contain epochs, representing more specific intervals. This hierarchy helps scientists study Earth's development, major events, and the evolution of life across vast timescales, providing a framework to understand the planet’s dynamic history in an organized and meaningful way.