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The Geologic Time Scale

The geologic time scale is a chronological framework that divides Earth’s history into distinct intervals based on significant geological and biological events. It was developed to provide a structured way to understand and discuss the vast span of time that Earth has existed. The timeline has been refined as scientists gained knowledge about the planet’s history through geological and paleontological studies.

Human history had long been divided into different time periods, but the idea of doing so for Earth’s history can be traced back to the 18th century, with notable contributions from scientists like Georges Cuvier and James Hutton. James Hutton in the 18th century was among the first to truly understand the lengths of time involved in “geologic time.”

In the 19th century, the timeline gained real structure and recognition. Geologists began to recognize rock layers and fossils as indicators of past environmental conditions and life forms, leading to the establishment of key boundaries and divisions.

The Structure of the Scale

The time scale is divided into eons, eras, periods, epochs, and ages, with each interval representing distinct geological and biological characteristics. These include major geological events (like the formation of mountain ranges) and the appearance or disappearance of significant life forms (such as dinosaurs or certain types of plants).

The timeline’s development and refinement have been aided by advances in paleontology, radiometric dating, and stratigraphy. Radiometric dating techniques, such as radiocarbon dating and potassium-argon dating, have allowed scientists to assign absolute ages to specific rock layers and fossils, helping to anchor the timeline in actual time spans. Before this, only an understanding of superposition was available to offer relative dating (e.g. this happened before that).

The timeline has been used for many purposes, such as understanding the history of life on Earth, studying past climate changes, and predicting future geological events. It provides a common language for geologists and paleontologists to communicate findings and compare research from different parts of the world. As our understanding of Earth’s history continues to evolve, the geologic time scale is continually refined and adjusted to accommodate new discoveries and insights, with an updated version published almost every year.

The Divisions

The geologic time scale is generally divided into comparable blocks. They are not comparable by length of time, but rather by the scale of changes noted as starting and ending points.

The divisions are:

  1. Eon: The largest division of time on the geological scale, representing a major span of Earth’s history. The current eon is the Phanerozoic Eon, which began around 541 million years ago and continues to the present day.
  2. Era: A subdivision of an eon, characterized by significant geological and biological events. The Phanerozoic Eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
  3. Period: A division within an era, marked by distinct changes in Earth’s geological and biological history. For example, the Mesozoic Era is divided into the Triassic, Jurassic, and Cretaceous Periods.
  4. Epoch: A smaller subdivision within a period, often characterized by specific evolutionary developments or changes in Earth’s climate. The Cenozoic Era is divided into several epochs, including the Paleogene and Neogene Epochs.
  5. Age: The smallest unit of geological time, representing a relatively short interval in Earth’s history. Ages are often defined by the appearance or disappearance of specific fossil groups and other geological events.

The Geologic Time Scale

Pleistocene Epoch

Holocene Epoch

Anthropocene Epoch (not fully accepted)

Geologic Timescale for Pinal County
Geologic Timescale for Pinal County