Geologic Time Scale Overview

Geologic Time Scale

• Represents Earth's history organized chronologically
• Based on geologists' study of rocks and fossils
• Arranged from the oldest (bottom) to most recent (top)

Divisions of Geologic Time

• Eons: Broadest divisions of time, characterized by four eons (Hadean, Archean, Proterozoic, and Phanerozoic)
• Eras: Longer spans of time within an eon, subdivided into periods (e.g., Paleozoic, Mesozoic, Cenozoic)
• Periods: Basic units of geologic time, lasting tens of millions of years, each period marking the time it takes to form a type of rock system
• Epochs: Subdivisions of periods, lasting several million years, marking the time a rock series is deposited, often further divided by early, middle, and late
• Ages: Referenced as "stages", lasting only a few million years, and an example of the present age is Meghalayan

Key Events and Life Forms

• Each segment is defined by major geologic events & appearance (and disappearance) of life-forms, starting with the formation of Earth's crust
• Precambrian eon encompasses Hadean, Archean, and Proterozoic eras and includes events like formation of the Earth's crust, oldest rocks, origin of life, early bacteria and algae, and the rise of complex multicelled organisms
• Paleozoic era (570-245 mya) is characterized by the evolution of invertebrates, fishes, and early terrestrial life forms, including extinction events and mountain building.
• Mesozoic era (245-65.5 mya) is known as the "Age of Reptiles" with dinosaurs as dominant life; includes extinction events and continental drift
• Cenozoic era (65.5 mya to the present) is the "Age of Mammals" with the rise of mammals and humans; encompasses North American events and extinction of large mammals