Why Grand Canyon's Rock Record Matters: Geologic Time in the Canyon Walls

Part of a National Park Service series adapted from the report Telling Time at Grand Canyon National Park: 2020 Update.

Boating the Colorado River below Havasu Creek. NPS photo by Mark Lellouch.

A third of Earth's history

Few places show the rock record as clearly as Grand Canyon National Park, which makes it an ideal place to grasp geologic, or "deep," time. Its walls record roughly a third of the planet's history, from the Proterozoic to the Permian, with much to say about the evolution of life, and together with younger deposits in the canyon they tell much of the tectonic and geologic story of the western United States.

Earth history on show: the Vishnu Basement Rocks at the bottom are nearly 2 billion years old; the youngest layer on the South Rim skyline was laid down about 270 million years ago; the canyon itself was carved in only the last 5–6 million years. Chappell aerial photo, courtesy of the NPS.

The ages at a glance:

• The oldest basement rocks are about 1,840 million years old.
• The Kaibab Formation, the youngest of the canyon's layers, caps both rims; it is about 270 million years old, older than the dinosaurs.
• The canyon itself is young — carved in the last 5–6 million years.
• Younger still are Ice Age fossils in caves, lava that poured into the western canyon, debris flows, and river sediments recording shifting climates and human influence, which carry the record to the present.

Devils Slide, where lava cascaded into the western canyon; lava has flowed in over the past 600,000 years, and dating these young deposits helps explain how today's landscape formed. Chappell aerial photo, courtesy of the NPS.

How the ages became known

The canyon's first geologists — John Strong Newberry, with Lieutenant Joseph Christmas Ives's expedition of 1857–1858, and John Wesley Powell, who led the famous 1869 river expedition — classified and correlated its rocks by their fossils and the knowledge of the time, which could place them only broadly. Later dating techniques let geologists give the rocks numeric ages with growing precision, part of a renaissance in canyon research that picked up in the late 1990s.

Why the park matters

Established in 1919 and enlarged since, the park now covers nearly the entire Grand Canyon, from Lake Powell to Lake Mead, and draws about 6.5 million visitors a year. Its ecosystems run from desert to mountain; its human history reaches back more than 10,000 years; it has inspired artists and poets. But geology is its most important resource. Theodore Roosevelt's 1908 proclamation of the national monument called it "the greatest eroded canyon in the United States," and UNESCO lists it as a World Heritage site among "the Earth's greatest ongoing geological spectacles."

Grand Canyon National Park and associated federal lands in northern Arizona. NPS.

It may be the best place on the planet to learn geology — a working field laboratory for researchers and a classroom, on site and remote, for students and the public. And geologic time frames far more than bedrock: the park's water supply, the Colorado River's rise and fall, the region's mining past and future, geologic hazards, and how its ecosystems respond to a changing climate.

A record of global importance

Understanding geologic time draws on plate tectonics, stratigraphy, historical geology, paleontology and geomorphology, and what is learned at Grand Canyon reaches outward — to the Colorado Plateau parks it shares a history with, such as Arches, Bryce Canyon and Zion, and to parks along the Colorado River system such as Black Canyon of the Gunnison, Colorado National Monument and Lake Mead. On a larger scale, its rocks record how new continental crust formed early in North America's history.

• Vishnu Basement Rocks — one of the best windows into North America's early history on the Colorado Plateau, where such outcrops are rare. Compared with rocks of similar age at Colorado National Monument, Black Canyon of the Gunnison and Rocky Mountain National Park, and with other continents, they help reconstruct ancient plate configurations.
• Grand Canyon Supergroup — one of North America's best records of the Proterozoic from 1.25 to 0.7 billion years ago; similar rocks survive only in a few places such as Death Valley, central Arizona and Utah's Uinta Mountains, so the canyon anchors understanding of that time.
• Layered Paleozoic Rocks — increasingly important worldwide, especially for the Cambrian; with the younger rocks of the rest of the plateau, they form one of the world's best sedimentary records of the evolution of life.

Sources

Based on "Geoheritage of Grand Canyon National Park's Rock Record," National Park Service, adapted from Karl Karlstrom, Laurie Crossey, Allyson Mathis and Carl Bowman, Telling Time at Grand Canyon National Park: 2020 Update, Natural Resource Report NPS/GRCA/NRR—2021/2246; published by the National Park Service. Two of its authors are University of New Mexico professors, so its words are paraphrased; rewritten in hubnx's own words. It cites Santucci and Tweet (2020) and the park's 2017 Foundation Statement.