Geoheritage of Grand Canyon National Park’s Rock Record

Telling Time at Grand Canyon

This article is part of a series adapted from the Natural Resources Report "Telling Time at Grand Canyon: 2020 Update". All of the articles, including numeric ages for rocks exposed at Grand Canyon, are available on the [article series] index page.

Boating down the Colorado River below Havasu Creek in Grand Canyon National Park.

NPS photo by Mark Lellouch.

Introduction

With one of the clearest exposures of the rock record and a long, diverse geologic history, Grand Canyon NP is an ideal place to gain a sense of geologic (or deep) time, especially given the great antiquity of those rocks. Rocks exposed in Grand Canyon’s walls record approximately one third of the planet’s history, from the Precambrian (Proterozoic Eon) to the Permian Period of the Paleozoic Era, and contain important information about the evolution and history of life (Santucci and Tweet 2020). These strata, along with younger deposits within the canyon, illustrate much of the tectonics, evolution, and geologic history of the western United States.

^{Figure 2. Earth history revealed: Vishnu Basement Rocks at the canyon’s bottom are nearly 2 billion years old. The youngest of the Grand Canyon strata on the South Rim skyline was deposited about 270 million years ago. The canyon landscape has been carved in only the past 5–6 million years
Chappell Aerial Photo courtesy of NPS.}

Highlights of Grand Canyon’s Geologic Ages

• The oldest basement rocks exposed in Grand Canyon (Figure 2) are ancient; 1,840 million years old.

• The Kaibab Formation, the youngest of Grand Canyon’s strata, holds up both the North and South rims. The Kaibab is 270 million years old, and was deposited prior to the age of the dinosaurs.

Figure 3. Lava flowed into the western Grand Canyon during the past 600,000 years. This cascade is called Devils Slide. The age of these younger surficial deposits help researchers understand modern landscape evolution.

Chappell aerial photo courtesy of NPS.

• Today’s canyon is geologically quite young, having been carved in the past 5–6 million years.

• Younger deposits within Grand Canyon, including Ice Age fossils in caves, 1,000-year-old lava flows that cascaded into the western canyon (Figure 3), recently-deposited debris flows, and river sediments that record oscillating climates and human influences, bring Grand Canyon’s geologic record to the present.

The geology of Grand Canyon and the long time frames encoded by its rocks can be hard to comprehend.

The first geologists who explored and studied Grand Canyon included John Strong Newberry, who was part of the Lieutenant Joseph Christmas Ives expedition of 1857–1858, and John Wesley Powell, who led the famous 1869 expedition down the Colorado River. They classified and correlated the canyon’s rock units based on fossils and the geologic knowledge that was available at the time. Early studies of Grand Canyon geology could only describe the age of Grand Canyon rocks in the broadest of parameters. With the later development and refinement of techniques that determine the numeric ages of rocks, geologists developed the ability to know the ages of rocks exposed in Grand Canyon with greater accuracy and precision. Advancements in geologic dating techniques are part of a renaissance of geologic research in the canyon that accelerated in the late 1990s.

Park Significance

Grand Canyon NP was established in 1919 and has been enlarged to encompass most of the physiographic Grand Canyon in northern Arizona (Figure 6). It is one of the most famous and highly visited parks in the National Park System, attracting visitors for many reasons. It has diverse animals and plants in ecosystems that span desert to mountain life zones. Its human history traces cultures back to more than 10,000 years ago. Its beauty has inspired artists and poets. Its societal importance involves all of these aspects and more. Behind it all, geology is its most prominent and important resource.

Figure 6. Grand Canyon National Park and associated federal lands in northern Arizona now include nearly the entire Grand Canyon from Lake Powell to Lake Mead.

NPS image.

Grand Canyon is one of the planet’s most iconic geologic landscapes. The purpose of Grand Canyon National Park is to preserve and protect its natural and cultural resources, and the ecological and physical processes of Grand Canyon along with its scenic, aesthetic, and scientific values for the benefit and enjoyment of the public (NPS 2017). Geology has always been recognized as central to the canyon’s significance, from its description as “the greatest eroded canyon in the United States” (1908 proclamation of Grand Canyon National Monument by Theodore Roosevelt) to its designation as a UNESCO World Heritage site for being “among the Earth’s greatest ongoing geological spectacles.”

Grand Canyon is probably the single location on the planet that provides the best opportunities for both researchers and students to learn about geology. The canyon remains an important field laboratory for active researchers. It also provides great opportunities for informal and formal science education via promotion of national and global geoscience literacy, both on site and remote learning.

All of the park’s natural and cultural resources are intertwined with its geology and geologic history. Therefore, telling geologic time and relating human and geologic timescales are important parts of the stories of Grand Canyon National Park.
Geologic time also provides the framework for understanding much more than bedrock geology, such as water supply for the park’s 6.5 million annual visitors, the waxing and waning of flow in the Colorado River, the history and future of mining in the Grand Canyon region, analysis of geologic hazards, and the nature and interaction of Grand Canyon’s ecosystems under changing climate regimes.

Understanding Geologic Time

A full understanding of geologic time encompasses diverse geologic topics including plate tectonics, stratigraphy, historical geology, paleontology, and geomorphology. Advances in understanding the history of our planet often begin in a well-exposed and well-known location like Grand Canyon, but quickly extend to include other areas. Grand Canyon is connected to other national parks on the Colorado Plateau, such as Arches, Bryce Canyon, and Zion that share an overall geologic history, and has a common erosional history with other parks located along the Colorado River and its tributaries, such as Black Canyon of the Gunnison National Park, Colorado National Monument, and Lake Mead National Recreation Area.

On an even broader scale, Grand Canyon’s rock record provides important information about the tectonic history of North America as it contains data about the formation of new continental crust early in its history and has been influenced by current tectonic environments. Grand Canyon is one of many park areas that has had an outsized role in the development of the science of geology in North America and an important locale for increasing geoscience literacy in the public. Many such park areas have been formed by dramatic events in planet’s history

Significance of Grand Canyon’s Rock Record

• Grand Canyon NP’s rock record has global significance and provides important information about the geologic history of the southwestern portion of the North American continent.

• The Vishnu Basement Rocks provide one of the best views into the early history of North America in the Colorado Plateau region where outcrops of basement rocks are few. The history of the Vishnu Basement Rocks can be compared to rocks of similar age exposed in Colorado National Monument, Black Canyon of the Gunnison National Park, Rocky Mountain National Park, and elsewhere in and around the Colorado Plateau. Together, these exposures provide a synthesis of the early history of North America that can then be compared to similar rocks on other continents) to reconstruct global plate configurations.

• The Grand Canyon Supergroup provides one of the best records in North America of the Proterozoic Eon from 1.25 to 0.7 billion years. Similar rocks of this age exist only in a few places like at Death Valley NP, central Arizona, and the Uinta Mountains in northern Utah. Hence, the Grand Canyon record anchors the scientific understanding of the geologic history of this time period).

• Units in the Layered Paleozoic Rocks are also proving to be of global importance, especially for understanding the Cambrian Period. Together with the mostly younger rocks exposed in the rest of the Colorado Plateau, Grand Canyon provides one of the world’s best sedimentary rock records for studying the evolution of life.

Learn More

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. https://doi.org/10.36967/nrr-2285173. [IRMA Portal]

Meet the Authors

Authors

• Dr. Karl Karlstrom is a Distinguished Professor of Geology at the University of New Mexico with a specialty in tectonics; he has researched Grand Canyon rocks of all ages over the past 35 years.
• Dr.Laurie Crossey is a Professor of Geology and Geochemistry at the University of New Mexico who has worked on Grand Canyon rocks and water issues over the past 20 years.
• Allyson Mathis is a Research Associate with the Northern Rockies Conservation Cooperative, and a geologist by training. Allyson worked for the National Park Service at Grand Canyon National Park from 1999 to 2013.
• Carl Bowman is a retired air quality specialist, formerly of Grand Canyon National Park, where he worked in various positions between 1980 and 2013.

References Cited in Full Report

References

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• Breed, W.J., and Ford, T.D., 1973, Chapter Two-and-a-half of Grand Canyon history, or the Sixty Mile Formation: Plateau, v. 46, no. 1, p. 12-18.

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• Gehrels, G. E., R. Blakey, K. E. Karlstrom, J. M. Timmons, S. Kelley, B. Dickinson, and M. Pecha. 2011. Detrital zircon U-Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona. Lithosphere 3: 183–200. doi: 10.1130/L121.1.

• Hawkins, D. P., S. A. Bowring, B. R. Ilg, K. E. Karlstrom, and M. L. Williams. 1996. U-Pb geochronologic constraints on Proterozoic crustal evolution. Geological Society of America Bulletin 108: 1167–1181.

• Hintze, L. F. and B. J. Kowallis. 2009. Geologic History of Utah. Brigham Young University Geology Studies Special Publication 9, Provo, Utah.

• Holland, M. E., K. E. Karlstrom, G. Gehrels, O. P. Shufeldt, G. Begg, W. L. Griffin, and E. Belousova. 2018. The Paleoproterozoic Vishnu basin in southwestern Laurentia: implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave province: Precambrian Research 308: 1–17.

• Ilg, B. R., K. E. Karlstrom, D. Hawkins, and M. L. Williams. 1996. Tectonic evolution of Paleoproterozoic rocks in Grand Canyon, Insights into middle crustal processes. Geological Society of America Bulletin 108: 1149–1166.

• Karlstrom, K. E., S. A. Bowring, C. M. Dehler, A. H. Knoll, S. Porter, Z. Sharp, D. Des Marais, A. Weil, J. W. Geissman, M. Elrick, M. J. Timmons, K. Keefe, and L. Crossey. 2000. Chuar Group of the Grand Canyon: Record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma. Geology 28: 619–622.

• Karlstrom, K. E., and L. J. Crossey. 2019. The Grand Canyon Trail of Time companion: Geology essentials for your canyon adventure. Trail of Time Press, Albuquerque, New Mexico.

• Karlstrom, K. E., J. Hagadorn, G. G. Gehrels, W. Mathews, M. Schmitz, L. Madronich, J. Mulder, M. Pecha, D. Geisler, and L. J. Crossey. 2018. U-Pb dating of Sixtymile and Tonto Group in Grand Canyon defines 505–500 Ma Sauk transgression. Nature Geoscience 11: 438–443.

• Karlstrom, K. E., S. S. Harlan, M. L. Williams, J. McLelland, J. W. Geissman, and K. I. Ahall. 1999. Refining Rodinia: Geologic evidence for the Australia–Western U.S. connection for the Proterozoic. GSA Today 9(10): 1–7.

• Karlstrom, K. E., B. R. Ilg, M. L. Williams, D. P. Hawkins, S. A. Bowring, and S. J. Seaman. 2003. Paleoproterozoic rocks of the Granite Gorges. Pages 9–38 in S. S. Beus and M. Morales, editors. Grand Canyon geology, second edition. Oxford University Press, Oxford, United Kingdom.

• Karlstrom, K. E., M. T. Mohr, M. Schmitz, F. A. Sundberg, S. Rowland, J. Hagadorn, J. R. Foster, L. J. Crossey, C. Dehler, and R. Blakey. 2020. Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian timescale. Geology 48: 425–430. doi: 10.1130/G46755.1.

• Karlstrom, K., S. Semken, L. Crossey, D. Perry, E. D. Gyllenhaal, J. Dodick, M. Williams, J. Hellmich-Bryan, R. Crow, N. Bueno Watts, and C. Ault. 2008. Informal geoscience education on a grand scale: The Trail of Time exhibition at Grand Canyon. Journal of Geoscience Education 56: 354–361.

• Karlstrom, K. E., and J. M. Timmons. 2012. Many unconformities make one “Great Unconformity” in J. M. Timmons and K. E. Karlstrom, editors. Grand Canyon geology: 2 billion years of Earth history. Geological Society of America Special Paper 489, Boulder, Colorado.

• Lillie, Robert J. 2005. Parks and plates: The geology of our national parks, monuments, and seashores. W. W. Norton, New York, New York.Mathis, A. 2006. Grand Canyon yardstick of geologic time: A guide to the canyon’s geologic history and origin. Grand Canyon Association, Grand Canyon, Arizona, USA.

• Mathis, A. and C. Bowman. 2005a. What’s in a number?: Numeric ages for rocks exposed within Grand Canyon. Nature Notes XXI(1). Grand Canyon National Park, Arizona.

• Mathis, A. and C. Bowman. 2005b. What’s in a number?: Numeric ages for rocks exposed within Grand Canyon. Part 2. Nature Notes, v. XXI(2). Grand Canyon National Park, Arizona.

• Mathis, A. and C. Bowman 2006a. The grand age of rocks Part 1— Numeric ages for rocks exposed within Grand Canyon. Boatman’s Quarterly Review 19(1):27–29. Grand Canyon River Guides, Flagstaff, Arizona. Available at https://www.gcrg.org/bqr/pdfs/19-1.pdf (accessed 01 Jun 2020).

• Mathis, A. and C. Bowman 2006b. The grand age of rocks Part 2—Grand Canyon’s three sets of rocks. Boatman’s Quarterly Review: Grand Canyon River Guides. 19(2):19–23. Grand Canyon River Guides, Flagstaff, Arizona. Available at https://www.gcrg.org/bqr/pdfs/19-2.pdf (accessed 01 Jun 2020).

• Mathis, A. and C. Bowman 2006c. The grand age of rocks Part 3—Geologic dating techniques. Boatman’s Quarterly Review 19(4):19–23. Grand Canyon River Guides, Flagstaff, Arizona. Available at https://www.gcrg.org/bqr/pdfs/19-4.pdf (accessed 01 Jun 2020).

• Mathis, A. and Bowman, C. 2007. Telling Time at Grand Canyon National Park. Park Science, 24(2): 78–83. Available at https://irma.nps.gov/Datastore/DownloadFile/615879 (accessed 07 May 2020).

• McKee, Edwin D. 1931. Ancient landscapes of the Grand Canyon region. Northland Press, Flagstaff, Arizona.

• McKee, Edwin D. 1969. Stratified rocks of the Grand Canyon. In The Colorado River Region and John Wesley Powell. USGS Professional Paper 669-B. Washington DC. Available at https://pubs.usgs.gov/pp/0669/report.pdf (accessed 11 May 2020).

• McKee, E. D. 1975. The Supai Group — Subdivision and nomenclature. U.S. Geological Survey Bulletin 1395-J, Washington, D.C , Washington, D.C.McKee, E. D., and C. E. Resser. 1945. Cambrian history of the Grand Canyon region. Carnegie Institution of Washington Publication 563.

• Mohr, M.T., M.D. Schmitz, N.L. Swanson-Hysell, K.E. Karlstrom; F.A. Macdonald, M.E. Holland, Y. Zhang, N. Anderson. In review. High-Precision U-Pb geochronology links magmatism in the SW Laurentia Large Igneous Province and Midcontinent Rift.

• Mulder, J. A., K. E. Karlstrom, K. Fletcher, M. T. Heizler, J. M. Timmons, L. J. Crossey, G. E. Gehrels, and M. Pecha. 2017. The syn-orogenic sedimentary record of the Grenville Orogeny in southwest Laurentia. Precambrian Research 294: 33–52.

• National Park Service. 2017. Foundation Statement. Grand Canyon National Park. Grand Canyon, Arizona.

• Perry, D. L. 2012. What makes learning fun? Principles for the design of Intrinsically motivating museum exhibits. AltaMira Press, Lanham, Maryland.

• Powell, J. W. 1875. Exploration of the Colorado River of the West and its tributaries. Explored in 1869, 1870, 1871, and 1872, under the direction of the Secretary of the Smithsonian Institution. U.S. Government Printing Office, Washington, D.C., USA.

• Rooney, A. D., J. Austermann, E. F. Smith, Y. Li, D. Selby, C. M. Dehler, M. D. Schmitz, K. E. Karlstrom, and F. A. Macdonald. 2018. Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon. Geological Society of America Bulletin 130(7–8): 1085–1098.

• Santucci, V. L., and J. S. Tweet, editors. 2020. Grand Canyon National Park: Centennial paleontological resource inventory (non-sensitive version). Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado.

• Schuchert, Charles. 1918. The Cambrian of the Grand Canyon of Arizona. American Journal of Science (Series 4) 45: 368.

• Shufeldt, O. P., K. E. Karlstrom, G. E. Gehrels, and K. Howard. 2010. Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna reconstructions. Geology 38: 1099–1102.

• Spamer, Earle E. 1989. The development of geological studies in the Grand Canyon. Tryonia. Miscellaneous Publications of the Department of Malacology. The Academy of Sciences of Philadelphia No. 17. Philadelphia, Pennsylvania.

• Timmons, J. M., and K. E. Karlstrom, editors. 2012. Grand Canyon geology: 2 billion years of Earth history. Geological Society of America Special Paper 489. Boulder, Colorado.

• Timmons, J. M., K. E. Karlstrom, M. T. Heizler, S. A. Bowring, G. E. Gehrels, and L. J. Crossey. 2005. Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis. Geological Society of America Bulletin 117(11–12): 1573–1595.

• Whitmeyer, S., and K. E. Karlstrom. 2007. Tectonic model for the Proterozoic growth of North America.Geosphere 3(4): 220–259.

Glossary of Geologic Terms

Glossary

• Absolute age: a numeric age in years. Numeric age is the preferred term.

• Accuracy: measure of how close a numeric date is to the rock’s real age.

• Angular unconformity: a type of unconformity or a gap in the rock record where horizontal sedimentary layers (above) were deposited on tilted layers (below). At Grand Canyon, horizontal layers of the Layered Paleozoic Rocks lie on top of the tilted rocks of the Grand Canyon Supergroup.

• Basalt: a dark, fine-grained volcanic (extrusive igneous) rock with low silica (SiO2) content.

• Biochron: length of time represented by a fossil biozone.

• Carbonate: sedimentary rock such as limestone or dolostone largely composed of minerals containing carbonate (CO3^-2) ions.

• Contact: boundary between two bodies of rock or strata.

• Daughter isotope: the product of decay of a radioactive parent isotope.

• Detrital: pertaining to grains eroded from a rock that were transported and redeposited in another.

• Dike: a wall-like (planar) igneous intrusion that cuts across pre-existing layering.

• Diabase: a dark igneous rock similar in composition to basalt but with coarser (larger) grain size.

• Disconformity: a type of unconformity or gap in the rock record between two sedimentary layers caused by erosion or nondeposition where the layers are parallel to one another.

• Dolomite: the mineral calcium magnesium carbonate CaMg(CO3)2 that usually forms when magnesium-rich water alters calcium carbonate (CaCO3).

• Dolostone: a rock predominantly made of dolomite.

• Eon: longest subdivision of geologic time in the Geologic Timescale; for example, the Proterozoic Eon.

• Era: second-longest subdivision of geologic time below eon in the Geologic Timescale; for example, the Paleozoic Era.

• Epoch: fourth-longest subdivision of geologic time, shorter than a period and longer than a stage in the Geologic Timescale; for example, the Pleistocene Epoch.

• Faunal succession: the change in fossil assemblages through time which has a specific, reliable order.

• Foliation: tectonic layering in metamorphic rocks caused by parallel alignment of minerals due to compression.

• Formation: the fundamental unit in stratigraphy and geologic mapping that consists of a set of strata with distinctive rock characteristics. Formations may consist of a single rock type (e.g., Tapeats Sandstone or Redwall Limestone), or a mixture of rock types (e.g. Hermit Formation, which includes sandstone, mudstone, and shale).

• Fossil: evidence of life in a geologic context usually consisting of the remains or traces of ancient life.

• Fossil biozone: stratigraphic unit defined by a distinctive assemblage of fossils.

• Ga: giga annum: billion years; in this paper, our usage implies billion years before present (or ago) when used for numeric ages.

• Gneiss: a high-grade metamorphic rock with strong foliation and light and dark bands of minerals.

• Granite: a high silica (SiO2) pink to white intrusive igneous rock composed mainly of feldspar and quartz.

• Granodiorite: a gray intrusive igneous rock composed of feldspar, quartz, biotite, and hornblende with less silica (SiO2) than granite.

• Group: a sequence of two or more related formations, with a stratigraphic rank higher than formation; for example, the Chuar Group is made up of the Nankoweap, Galeros, and Kwagunt formations.

• Igneous rock: a rock that solidified from molten material (magma or lava), either within the Earth (as an intrusive or plutonic rock) or after eruption onto the Earth’s surface (as an extrusive or volcanic rock).

• Inclusion: a fragment of an older rock within a younger rock.

• Index fossil: a fossil or assemblage of fossils that is diagnostic of a particular time in Earth history.

• Intrusion: an igneous rock body that crystallized underground. Intrusions may have any size or shape; large ones are known as plutons, thin ones parallel to layering are known as sills, and thin ones that cut across layering are called dikes.

• Isotope: one of the forms of a chemical element (with the same atomic number) that contains a different number of neutrons.

• Lateral continuity: a geologic principle that sedimentary rocks extend laterally, and that if they are now separated due to erosion, they were once laterally continuous; for example, the Kaibab Formation on the South Rim is laterally continuous with the Kaibab Formation on the North Rim.

• Lava: molten rock erupted onto the Earth’s surface.

• Ma: mega annum: million years; in this paper, our usage implies million years before present (or ago) when used for numeric ages.

• Magma: molten or partially molten rock material formed within the Earth.

• Member: a subdivision of a formation, usually on the basis of a different rock type or fossil content; for example, the Hotauta Conglomerate is a member of the Bass Formation.

• Metamorphic rock: a rock formed by recrystallization under intense heat and/or pressure, generally in the deep crust.

• Monadnock: a bedrock island that sticks above the general erosion level.

• Nonconformity: an unconformity or gap in the rock record where sedimentary layers directly overlie older and eroded igneous or metamorphic rocks.

• Numeric age: age of a rock in years (sometimes called absolute age).

• Numeric age determination: measurement of the age of a rock in years, often through the use of radiometricdating techniques.

• Orogeny: mountain building event, usually in a collisional tectonic environment.

• Parent isotope: the radioactive isotope that decays to a daughter isotope.

• Pegmatite: a type of intrusive igneous rock usually of granitic composition with large crystal size.

• Period: third-longest subdivision of geologic time shorter than an era and longer than an epoch in the Geologic Timescale; for example, the Permian Period.

• Plate tectonics: theory that describes the Earth’s outer shell as being composed of rigid plates that move relative to each other causing earthquakes, volcanism, and mountain building at their boundaries.

• Pluton: large intrusion of magma that solidified beneath the Earth’s surface.

• Precambrian: the period of time before the Cambrian Period that includes the Proterozoic, Archean, and Hadean eons and represents approximately 88% of geologic time.

• Precision: measure of the analytical uncertainty or reproducibility of an age determination.

• Proterozoic: geologic eon dominated by single-celled life extending from 2,500 to 541 million years ago; divided into the Paleoproterozoic (1,600–2,500 Ma), Mesoproterozoic (1,000–1,600 Ma), and Neoproterozoic (541–1,000 Ma) eras.

• Radioactive decay: the process by which the nuclei of an unstable (radioactive) isotope lose energy (or decay) by spontaneous changes in their composition which occurs at a known rate for each isotope (expressed as a half life); for example, the parent uranium (238U) isotope decays to the daughter lead (206Pb) isotope with a half life of 4.5 billion years.

• Radiometric dating: age determination method that uses the decay rate of radioactive isotopes and compares the ratio of parent and daughter isotopes within a mineral or rock to calculate when the rock or mineral formed.

• Regression: geologic process that occurs when the sea level drops relative to the land level; for example, by sea level fall and/or uplift of the land, causing the withdrawal of a seaway from a land area.

• Relative time: the chronological ordering of a series of events.

• Rift basin: a basin formed by stretching (extension) of the Earth’s crust. Rift basins are linear, fault-bounded basins that can become filled with sediments and/or volcanic rocks.

• Rodinia: a Neoproterozoic supercontinent that was assembled about 1.0 Ga (during Unkar Group time) and rifted about 750 Ma (during Chuar Group time).

• Sedimentary rock: a rock composed of sediments such as fragments of pre-existing rock (such as sand grains), fossils, and/or chemical precipitates such as calcium carbonate (CaCO3).

• Schist: a metamorphic rock with platy minerals such as micas that have a strong layering known as foliation or schistosity.

• Silica: silicon dioxide (SiO2), a common chemical “building block” of most major rock-forming minerals, either alone (i.e., as quartz) or in combination with other elements (in clays, feldspars, micas, etc.).

• Sill: a sheet-like igneous intrusion that is parallel to pre-existing layering.

• Snowball Earth: a hypothesis that the Earth’s surface became completely or mostly frozen between 717 and 635 million years ago.

• Stage: a short subdivision of geologic time in the Geologic Timescale often corresponding to the duration of a fossil assemblage.

• Stratigraphic age: the era, period, epoch, or stage a rock is assigned to based on its fossil biozones or numeric age.

• Stratigraphy: the study of layered rocks (strata), which usually consist of sedimentary rock layers, but may also include lava flows and other layered deposits.

• Stromatolite: a fossil form constructed of alternating layers (mats) of microbes (algal or bacterial) and finegrained sediment.

• Subduction zone: a plate boundary where two plates converge and one sinks (subducts) beneath the other.

• Supergroup: a sequence of related groups, with a higher stratigraphic rank than group; for example, the Grand Canyon Supergroup consists of the Unkar and Chuar groups.

• Superposition: principle of geology that the oldest layer in a stratigraphic sequence is at the bottom, and the layers get progressively younger upwards.

• Tectonics: large-scale processes of rock deformation that determine the structure of Earth’s crust and mantle.

• Trace fossil: a sign or evidence of past life, commonly consisting of fossil trackways or burrows.

• Transgression: a movement of the seaway across a land area, flooding that land area because of a relative sea level rise and/or land subsidence.

• Travertine: calcium carbonate (CaCO3) precipitated by a spring; most travertine deposits also contain some silica.

• Unconformity: a rock contact across which there is a time gap in the rock record formed by periods of erosion and/or nondeposition.

• Volcanic ash: small particles of rock, minerals, and volcanic glass expelled from a volcano during explosive eruptions. Volcanic ash may be deposited great distances (even hundreds of miles or kilometers) from the volcano in especially large eruptions.

• Yavapai orogeny: mountain building period that occurred approximately 1,700 million years ago when the Yavapai volcanic island arc collided with proto-North America.

• Zircon: a silicate mineral (ZrSiO4) that often forms in granite and other igneous rocks and incorporates uranium atoms, making it useful for radiometric dating.

Photos and Illustrations

Related Links

• Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA npshistory.org]

• NPS Geology—Geologic Time Scale

• NPS—Geologic Resources Division