The Geological Time Scale (GTS) is the standardized framework for organizing Earth’s 4.54-billion-year history. This chronology is built upon evidence preserved in rock layers and the fossil record. The largest division of time is the Eon, which represents a significant phase in Earth’s development, spanning hundreds of millions to billions of years. Eons are broken down into smaller units that reflect major shifts in geology and life forms.
The Era: Primary Subdivision of the Eon
The primary subdivision of an Eon is the Era, which typically extends for tens to hundreds of millions of years. Eras are defined by major, global-scale changes in Earth’s biota, often involving widespread evolutionary diversification or mass extinction events.
The current Phanerozoic Eon, meaning “visible life,” began approximately 541 million years ago and is divided into three Eras. The Paleozoic Era (“ancient life”) started with the Cambrian Explosion and ended with the largest mass extinction event in history. The Mesozoic Era (“middle life”), or the Age of Dinosaurs, followed, lasting from about 252 million to 66 million years ago.
The Cenozoic Era (“recent life”) began 66 million years ago with the extinction of the non-avian dinosaurs and continues to the present day. This Era is characterized by the diversification and dominance of mammals and flowering plants.
The Period: Defining Shorter Timeframes
Eras are further subdivided into Periods, a unit of time that typically spans millions to tens of millions of years. Periods are defined by significant changes in rock strata and fossil assemblages, often marked by shifts in fossil types or evolutionary milestones.
Geologists often name Periods after the geographical location where the defining rock layers were first studied. For example, the Permian Period is named after the Perm region in Russia, and the Devonian Period is named after Devon in England. The Carboniferous Period (359 to 299 million years ago) is named for the immense coal deposits derived from vast swamp forests.
During the Carboniferous, land vertebrates diversified, with amphibians becoming dominant and the first reptiles evolving the amniotic egg. The Cretaceous Period (145 to 66 million years ago) saw the proliferation of flowering plants alongside the largest dinosaurs. This Period concluded with a major extinction event, linked to an asteroid impact, separating the Mesozoic and Cenozoic Eras.
Epochs and Ages: The Smallest Time Segments
Epochs represent the next level of division, subdividing a Period into smaller segments lasting hundreds of thousands to millions of years. These units provide a higher-resolution view of Earth’s history, detailing changes less significant than those defining a Period. For instance, the Quaternary Period of the Cenozoic Era includes the Pleistocene and Holocene Epochs.
Epochs are most detailed in the Cenozoic Era because the rock and fossil records for this recent time are more accessible. The Pleistocene Epoch is known for major ice ages that shaped the planet’s surface. The Holocene Epoch, which began approximately 11,700 years ago, marks the time since the last major glacial retreat and encompasses human civilization.
Ages are the smallest formal subdivisions on the GTS, representing the highest resolution of time, spanning thousands to a few million years. Ages are defined by highly specific, often short-lived, changes in the fossil record or climate. The current interval, the Meghalayan Age, began about 4,200 years ago, defined by a global mega-drought event.