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Earth Science Concepts

Geologic Time

Photo of Rebecca Port at Rocky Mountain National Park Bryce Canyon National Park by Heather Walborn Interpretive take at Shenandoah Rocky Mountain National Park Astronomy talk Bryce Canyon
National parks are a unique setting for youth educational programs that help promote appreciation for America's greatest classroom. (NPS Photos)

While a human life spans decades, the Earth's history spans 4.6 billion years! Geologists have created a time scale to organize Earth's history into eons, eras, periods, and epochs.

The geologic time scale began to take shape in the 1700s. Geologists used fundamental concepts to understand the chronological order of rocks around the world. It wasn't until the advent of radiometric dating techniques in the middle 1900s that reliable dates could be assigned to the previously named geologic time divisions.

The National Park System contains a magnificent record of geologic time because rocks from each period of the geologic time scale are preserved in park landscapes.

Fundamental Geologic Concepts

Geologists use these concepts to place sequences of rock in chronological order:

  • Uniformitarianism: Geologic processes operating on the Earth's crust have acted in the same manner and relative intensity throughout geologic time.
  • Superpostion: In an undisturbed sequence of sediments or rocks, the older layers occur at the bottom with successively younger layers on top.
  • Original Horizontality: Layers of sediment are originally deposited horiztonal to the Earth's surface.
  • Faunal Succession: Fossils often exhibit identifiable patterns or characteristics which progressively change over time.

Digital Geologic Map

Geologic Time Views Module

Views of the National Parks (Views) multimedia modules are part of the natural resource Science in Action series. The GeologicTime module focuses on the fundamental concepts needed to build knowledge of geologic time. Students will develop a better understanding of how geologic time is measured, learn about the events of the past, and explore interactive geologic time case studies at some of America's greatest National Parks.
Geologic Time Views Module

Earth from space

Video: Big Idea in Geoscience

From the American Geosciences Institute comes Big Idea 2: Earth is 4.6 Billion Years Old. Watch Earth form, and learn about Earth's history and the events of deep time. See what processes shaped the Earth we know today.
Learn more...

Digital Geologic Map

Geologic Time Scale

The divisions of the geologic time scale are organized stratigraphically, with the oldest at the bottom and youngest at the top. The green bar indicates the ages of geologic units that are mapped within Great Basin National Park. GRI map abbreviations for each geologic time division are in parentheses. Boundary ages are in millions of years ago (mya). Major North American life history and tectonic events are included. Compass directions in parentheses indicate the regional locations of events. Bold horizontal lines indicate major boundaries between eras. Graphic design by Trista Thornberry-Ehrlich (Colorado State University) and Rebecca Port (NPS Geologic Resources Division), adapted from geologic time scales published by the U.S. Geological Survey and the International Commission on Stratigraphy.
Geologic Time Scale

Educational Resources

Learning Activity: It's About Time

Space photo
Photo courtesy of NASA

Have you ever wondered how geologic time works? This interactive classroom learning activity helps build the basic understanding of geologic time for grades 4-9. It's About Time

Geology, Relatives, and Time

Ranger led program

Using a simple three or four generation family tree, students will construct a relatives time tree that mimics the major divisons of the geologic time scale (Precambrian, Paleozoic, Mesozoic, and Cenozoic). For Grades 9-12. Learn more...

Geologic Time Classroom Poster

GeoTime Poster

Every park contains a slice of geologic time. In this classroom resource we highlight a few parks associated with each geologic time period. Classroom Poster

The Age of the Earth

Ranger led program

Read about the age of the Universe and Earth while learning how scientists determine geologic age through Radiometic Dating. Learn more...

Fossils Through Geologic Time

GeoTime Poster

Geologic Time The Geologic Time Scale is a way of organizing Earth's 4.5 billion-year history. The time scale is divided into four large periods of time - the Precambrian, Paleozoic Era, Mesozoic Era, and Cenozoic Era. National parks preserve fossils from each of these time blocks. Learn more...

Oldest Rocks

Ranger led program

Learn about the oldest rocks found in the parks that range in age from 3 billion to 600 million years old. Learn more...

Related Links

Explore Further

| Plate Tectonics | Rocks & Minerals | Education & Outreach |

Last Updated: January 04, 2017