The Geologic Time Scale (GTS) is a standardized system used by Earth scientists to describe the chronology of Earth’s history. This scale organizes time based on significant geological and biological events recorded in rock layers, rather than fixed-length units. The total length of the GTS spans approximately 4.54 billion years, beginning with the formation of Earth. This vast duration provides the framework for organizing the major transformations the planet and its life forms have undergone.
The Hierarchical Structure of Geologic Time
The Geologic Time Scale is structured as a nested hierarchy to make Earth’s history manageable for study. The largest divisions are the Eons, which cover spans of hundreds of millions to billions of years. Earth’s history is divided into four Eons: the Hadean, Archean, Proterozoic, and the current Phanerozoic Eon.
The first three Eons—Hadean, Archean, and Proterozoic—are collectively and informally referred to as the Precambrian, which represents about 88% of all geologic time. The Phanerozoic, meaning “visible life,” encompasses the time of abundant and complex life forms. Eons are subdivided into the next-largest unit, the Eras, such as the Paleozoic, Mesozoic, and Cenozoic Eras within the Phanerozoic Eon.
Eras are further broken down into Periods, such as the Jurassic and Cretaceous. Periods are subdivided into Epochs, and then down to the smallest formal unit, the Ages. The duration of these units is not uniform; older Eons are immensely long, while more recent Epochs, where the rock record is more complete, are comparatively short.
Defining the Major Geologic Boundaries
The boundaries between these nested time units are not arbitrary dates, but are defined by evidence of profound global change recorded in the rock layers. The transitions between Eras and Periods often mark significant biological turnovers, such as mass extinction events. For example, the boundary between the Cretaceous Period and the Paleogene Period is defined by the global extinction that wiped out the non-avian dinosaurs about 66 million years ago.
To standardize these boundaries, geologists establish a Global Stratotype Section and Point (GSSP). A GSSP is a specific, physical location in a rock outcrop that precisely marks the base of a time unit. This marker is based on a primary event, such as the first appearance of a globally distributed fossil species or a distinct geochemical signal.
For instance, the start of the Phanerozoic Eon is marked by the appearance of complex, hard-shelled organisms during the Cambrian explosion. The GSSP system ensures that geologists worldwide can precisely correlate rock layers and events across different continents.
Establishing Numerical Ages
While the boundaries are defined by physical markers and events, a separate scientific process is required to assign numerical ages to them. This absolute dating is accomplished through radiometric dating techniques. Radiometric dating relies on the predictable, constant rate of decay of unstable radioactive isotopes (parent isotopes) into stable daughter isotopes.
The principle of half-life, the time required for half of the parent isotope to decay, provides the clock for deep time. For dating the oldest rocks and boundaries on the GTS, geologists utilize isotopes with very long half-lives, such as Uranium-238 decaying to Lead-206. Potassium-40 decaying into Argon-40 is also widely used, especially for volcanic rocks.
Radiometric dating cannot typically be applied directly to sedimentary rocks, which contain the fossil record and the GSSPs. Instead, scientists date layers of igneous rock, such as volcanic ash beds or lava flows, that are found immediately above and below the sedimentary layer of interest. By dating these surrounding igneous layers, geologists can bracket the age of the sedimentary rock and thus assign a precise numeric age to the boundaries of the Geologic Time Scale.