Geologic Timescale, Geologic Dating Techniques, and Numeric Ages (U.S. National Park Service) (2024)

Historians identify major periods in human history and broad global eras with descriptive terms such as the Stone Age or the Renaissance that do not rely on specific dates but relate various periods of history that are more or less defined in the public’s mind. But historians have an advantage over geologists because people are more familiar with these terms and can infer the period of time from them.

Geologists have similar temporal subdivisions in the Geologic Timescale (Figure 7). The Paleoproterozoic, Mississippian, and Pliocene all define spans of time, but most laypeople are less familiar with these terms nor could they put them in chronological order.

The Geologic Timescale (Figure 7) was first constructed by geologists in the 1800s before the discovery of radioactive decay that provides the clock used in many numeric dating techniques. Hence, it was originally based entirely on relative dating and stratigraphic principles, such as the principles of superposition, lateral continuity, and the change in fossil assemblages through time (faunal succession). Geologists depend on index fossils and fossil biozones to help constrain the relative ages of sedimentary rocks containing fossils. Index fossils are fossils or assemblages of fossils that are diagnostic of a particular time in Earth history. Fossil biozones are stratigraphic units defined by the fossils that they contain. Most of the Paleozoic sedimentary rocks exposed in Grand Canyon contain a rich fossil record that provides important information about their age.

The Geologic Timescale is regularly updated and refined. This study utilizes the most recent version of the International Stratigraphic Chart (v 2020/01).

The primary subdivisions of geologic time (Figure 7) follow the major events in the evolution of life and history of the planet:

Numeric ages identify when in years specific events occurred. This term is used instead of absolute age because the ages are refined and updated as radiometric dating techniques improve

A variety of radioactive elements, each with its characteristic decay rate (and half life), have been used for numeric dating. Different elements can be used for different time spans, and/or to cross-check numeric dates obtained via other methods. Geologists identify these techniques by their radioactive parent and stable daughter elements and have used many of them, including uranium-lead (U-Pb), potassium-argon (K-Ar), rubidium-strontium (Rb-Sr), and rhenium-osmium (Re-Os), to determine numeric ages of Grand Canyon rocks.

^{Figure 8B. The age of sedimentary rocks can be bracketed by cross-cutting igneous rocks; in this case, the black dike cuts across, and hence, is younger than the reddish Hakatai Shale. This dike is dated as 1,104 ± 2 million years old.
Photo by Laurie Crossey.}

Dating detrital zircons (zircon crystals that eroded from igneous rocks and deposited in sedimentary rocks) (Figure 8C) is another technique that can help constrain the numeric age for sedimentary rocks by providing the maximum depositional age.

^{Figure 8C. The age of sedimentary rocks can also be bracketed by the age the youngest dated sedimentary grains (detrital zircon) within the sediment.}

Every analytical method used in scientific investigations, including radiometric age determinations, has a certain analytical error or precision that is expressed as a plus or minus from the measured age. Precision is different than accuracy, which is how close the measured date is to the real or actual age. Different dating techniques have different precisions due to limitations of the analytical equipment, while accuracy depends on a variety of geologic factors as well as the soundness of the relevant geological observations and interpretations. Geologists who use radiometric age determinations strive to both accurately and precisely measure geologic events by improving laboratory methods, controlling for geologic complexities, and applying multiple dating techniques when possible. Ages reported in the more recent geologic literature are commonly more precise and more accurate than ages published in the older literature due to advances in dating techniques.

Each new numeric age within a stratigraphic sequence brackets the age of the units above and below it. Thus each new date provides cross-checks, and leads to increasingly well-known ages and durations.