Geologic Timescale, Geologic Dating Techniques, and Numeric Ages

Simplified geologic timescale.
Figure 7. The Geologic Timescale is made up of Eons, Eras, Periods, and Epochs. Geologists generally made these subdivisions based on evolving life forms.

Time period boundaries are from the International Stratigraphic Chart v 2020/01 (Cohen et al. 2013, updated).


Geologists have many different ways to tell time. Some determine the order of events and others identify specific times in the past when geologic events took place. Geologists report the age of rock units and geologic events with descriptive terms and numeric ages. An understanding of how geologists tell time is important to understanding the age of rocks exposed in Grand Canyon National Park.

Geologic Timescale

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:

  • Hadean Eon (referring to the underworld)

  • Archean Eon (beginning life)

  • Proterozoic Eon (earlier life)

  • Paleozoic Era (old life)

  • Mesozoic Era (middle life)

  • Cenozoic Era (recent life).

Grand Canyon contains important rock records from the Proterozoic and the Paleozoic.

Geologic Dating Techniques

Scientists use two major categories of geologic dating techniques, to determine how old rocks are and identify major intervals in Earth’s history:

  • Relative dating
  • Numeric (or absolute) dating,

Relative Dating

Relative age dating determines the order in which a sequence of geologic events occurred (e.g., bottom-to-top sequential deposition of sedimentary strata, then carving of a geologically young canyon through those strata), but cannot determine how long ago events happened.

Numeric Ages

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.

Igneous and Metamorphic Rocks

For igneous rocks, radiometric age determinations directly measure when the minerals crystallized from a magma, essentially at the same time that the rock formed. For metamorphic rocks, most age determinations reflect the time when minerals formed during metamorphism or the time of cooling following metamorphism.

Sedimentary Rocks

Sedimentary rocks are harder to date because most grains within them were not formed when the sediment was deposited. But several circumstances are particularly helpful in determining the age of a sedimentary rock. Some sedimentary rocks include discrete layers of volcanic ash containing datable igneous minerals that were deposited with the sediment as ash fall deposits from a distant eruption (Figure 8A). Ash may also be reworked by rivers or oceans after its initial deposit. In these cases, the age of the ash bed indicates the maximum age for the enclosing sedimentary rock since the original ash fallout deposit must have occurred before it was reworked into the sedimentary rock. Another circumstance is when an igneous intrusion cuts across a sedimentary unit (Figure 8B). In this case, the age of the dike provided a minimum age for the sedimentary rock.

Photo of rock outcrop showing details of rock layers.

Figure 8A. Sedimentary rocks can be dated directly if they contain an igneous layer that was deposited within the sedimentary layers, like a volcanic ash. This 1-cm-thick ash bed in the uppermost Chuar Group provides a direct date of 729 ± 0.9 million years.
Photo by Laurie Crossey.

Photo of river side cliffs with a diagonal rock inrusion cross cutting the lower slopes.

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.

Photo of crystals as seen through a microscope.

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.

Learn More

Tiny image of the cover of a report titled Telling Time at Grand Canyon National Park.

To learn more about the age of Grand Canyon’s rocks, please see:

Karlstrom, K., L. Crossey, A. Mathis, and C. Bowman. 2021. Telling time at Grand Canyon National Park: 2020 update. Natural Resource Report NPS/GRCA/NRR—2021/2246. National Park Service, Fort Collins, Colorado. [IRMA Portal]

Photos and Illustrations

Part of a series of articles titled Telling Time at Grand Canyon National Park.

Grand Canyon National Park

Last updated: January 30, 2024