Welcome to Grand Canyon National Park!
The Grand Canyon tells one of the world’s greatest geologic stories. Its distinctive features allow researchers to piece together the history of this unique location, one of America’s treasures and a UNESCO World Heritage Site. Thinking of the geologic record as a book is helpful to understand each page of Earth’s history. The beginning of the story starts at the bottom of the canyon and moves forward in time as you get closer to the rim.
Introduction to Grand Canyon Geologic Principles
Stratigraphy is the study of the rock layering, and reveals a wealth of information about what Earth was like when each layer formed. In the Grand Canyon, there are clear horizontal layers of different rocks that provide information about where, when, and how they were deposited, long before the canyon was even carved. The Law of Superposition states that sediment is deposited in layers in a sequence, the oldest rocks are on the bottom and the youngest rocks are on the top, similar to the way that sand piles up in an hour glass. This principle is a key part of determining the relative age of a rock layer. The three main rock layer sets in the Grand Canyon are grouped based on position and common composition and 1) Metamorphic basement rocks, 2) The Precambrian Grand Canyon Supergroup, and 3) Paleozoic strata. These three main sets of rocks were first described by the explorer and scientist John Wesley Powell during his expeditions of the Grand Canyon in the late 1860s and early 1870s. To learn more about the Powell expeditions, visit https://www.usgs.gov/Powell150. A USGS geologic field photograph map of the Grand Canyon can be viewed or downloaded here.
Unconformities are gaps in the geologic record that occur when rocks or sediments are eroded away and time elapses before new deposition occurs. New sediment eventually forms new rock layers on top of the eroded surface, but there is a period of geologic time that is not represented. You can think of unconformities as missing “pages” in the book of the geologic record. Missing layers may seem like a problem, but the very fact that there is this gap in the record provides information to geologists, indicating changing ocean levels or changes in the Earth’s crust. In the Grand Canyon, unconformities are common in the Grand Canyon Supergroup and the Paleozoic Strata.
The three main types of rock are igneous, sedimentary and metamorphic. Igneous rocks are cooled magma (melted rock found underground) or lava (molten rock found above ground). Granite (cooled from magma, known as an intrusive igneous rock) and basalt (cooled from lava, known as an extrusive igneous rock) are two types of igneous rocks. Sedimentary rocks are formed by smaller pieces of sand and mud stick together in layers. Examples include: sandstone, mudstone ,shale, siltsone, limestone, and more. Sedimentary rocks often contain fossils that can be used to help identify the age of the rock. Certain fossils, called index fossils, are particularly useful because they are abundant in a relatively narrow time range. Over time, pressure increases as sediment increases, and minerals help form these rock layers. Metamorphic rocks are formed when sedimentary or igneous rocks change due to exposure to heat and/or pressure. All three rock types can be found in the Grand Canyon, and each layer adds an important understanding to the geologic history of the region.
Metamorphic Basement Rocks
The oldest rocks in the Grand Canyon, found at the bottom of the canyon, are primarily metamorphic, with igneous intrusions (the name given to when magma or lava enters or cools on top of previously formed rock). The intrusive igneous rocks here are called Zoroaster granite. The name given to this rock set (the combination of metamorphic and igneous rock of a certain age found at this location) is Vishnu Basement Rocks. Primarily schist (metamorphic) with granite (igneous), these rocks have visible crystals and are about 1.7 billion years old, from an era early in Earth history known as the Proterozoic. On Powell’s expedition to explore and map the Grand Canyon, he named this part of the exposed rock “The Granite Gorge.” This rock set tells the story of the creation of North America, when volcanic islands collided with the continental landmass, forming metamorphic rocks through the intense heat and pressure. Volcanism continued after the collision and igneous intrusions continued after metamorphosis. 
The Grand Canyon Supergroup
The middle rock set, the Grand Canyon Supergroup, is primarily sandstone and mudstone, both sedimentary rocks, with some areas of igneous rock. They are from the late Proterozoic, only slightly younger than the metamorphic basement rocks. These rocks do not contain many fossils, because they formed before complex life on Earth was common. The few fossils that are present include stromatolites, columns of sediment formed by cyanobacteria. The composition (sandstone) and presence of stromatolites indicate that this area was previously a very shallow sea. The rock layers in the Grand Canyon Supergroup have been tilted, whereas the other rocks above this set are horizontal. This is known as an angular unconformity. The top of these sediment layers was then eroded away, forming the Great Unconformity.
These layers are sedimentary, and primarily sandstone. This set makes up most of the typical reddish layers that you often see in images, and which come to mind when thinking of the Grand Canyon. Following the Great Unconformity, this set is much younger than the other rock layers, and fossils are prevalent. The most common fossils are small sea creatures, such as brachiopods, bryozoans, coral, and crinoids. Combined with the sandstone, this tells us that the region was a warm, shallow sea when these sediments were deposited.
Old Rocks, Young Canyon
A key feature of the Grand Canyon is, unsurprisingly, the canyon itself. The width and depth make it truly remarkable and expose the rock layers that were discussed above. After all the rocks were deposited, there was a period of uplift (where plate tectonics literally force a section of the Earth upward), setting the stage for canyon formation. It provided a high enough elevation that water could flow downward, cutting through the rock as it went. This incredible formation was carved over millions of years by the Colorado River. The canyon itself has formed much more recently than the deposition of rock layers, only about five million years ago (as opposed to the rocks, the youngest of which are a little less than 300 million years old). The canyon has since been forming at varying rates, with periods of intense erosion carving the canyon. The river must have had periods of quick movement, carving deep, not only wide. To view or download a 2018 report about the Colorado River downstream of Grand Canyon, click here.
The river continues to be an agent of change, reshaping the canyon over time. The canyon isn’t fully formed as long as there is water flowing. There is ongoing research about river flow, sediments, and geomorphology. The Glen Canyon Dam controls the Colorado River now, providing electricity to six states and changing the natural flow patterns. Since the construction of the dam in 1963, researchers have been studying how changes in river flow affect the erosion and deposition of sediment along the Colorado River and the changes to riparian vegetation and food webs.
More Information about Fossils
The Paleozoic Strata contain many fossils that help scientists learn about the geologic history of North America. Most of the fossils are ocean-dwelling creatures, telling us that the area now in the middle of Arizona was once a sea. Some of the most common fossils found in the Grand Canyon are listed below.
Trilobites were invertebrates that lived in shallow marine environments and varied widely in size. They are index fossils for the Paleozoic, and were particularly prominent during the Ordovician.
Tracks and burrows are known as trace fossils, because they are not preservations of the actual organism, but instead show where the organism moved and lived. They are commonly tunnels dug by trilobites and worms in muddy ocean sediment.
Brachiopods left shells behind that are quite common in Paleozoic rocks.
Geologic Map of the Grand Canyon: https://pubs.usgs.gov/imap/i-2688
The Grand Canyon Monitoring and Research Center: https://www.gcmrc.gov/about/about_default.aspx
Geologic Maps of the Grand Canyon: https://geomaps.wr.usgs.gov/arizona/
Information about John Wesley Powell and his expeditions: https://pubs.er.usgs.gov/publication/pp669
150th Anniversary of the 1869 Powell Expedition: https://pubs.er.usgs.gov/publication/cir1475
Grand Canyon Geology Training Manual: https://www.nps.gov/grca/learn/nature/geology_manual.htm
NPS Introduction to Grand Canyon Geology: https://www.nps.gov/grca/learn/nature/grca-geology.htm
Department of the Interior blog about Grand Canyon National Park: https://www.doi.gov/blog/13-things-you-didnt-know-about-grand-canyon-national-park
NPS Geologic Tours: https://www.nps.gov/subjects/geology/geologic-tour.htm
Fossils in the Grand Canyon: https://www.nps.gov/grca/learn/nature/fossils.htm
Geologic Field Photograph Map of the Grand Canyon Region: https://pubs.usgs.gov/gip/0189/gip189.pdf
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