“Let this great wonder of nature remain as it now is. You cannot improve on it. But what you can do is keep it for your children, your children’s children, and all who come after you, as the one great sight which every American should see.”
–President Theodore Roosevelt on the dedication of Grand Canyon National Monument in 1908 (designated a National Park in 1919).
At 277 miles in length, up to 18 miles in width, and 6000 feet in maximum depth, it’s impossible to capture the vast grandeur of Grand Canyon National Park with a few photos. From any given point along the rim only part of the canyon is visible, but certain locations like the Desert View Watchtower offer slightly better views.
Amid its immense scale, the Grand Canyon exposes one of the most extensive geologic columns on earth. The nearly-horizontal sedimentary bedding records hundreds of millions of years of earth history, while the igneous and metamorphic basement rocks date back to around two billion years in age.
Beginning at least two billion years ago marine sediments were gradually deposited here near ancient shorelines. Around the 1.8 billion year mark, plate tectonics began to push a volcanic island arc into an early North American continent. This collision created immense heat and pressure that metamorphosed the existing sediments into the Vishnu Schist. Even at this age, lenses of marble suggest colonies of ancient algae lived in these primitive seas.
Later in this collisional regime subduction of the oceanic plate below the continental plate lead to crustal melting and upwelling of magma. Rising through the continent, these molten rocks intruded into the older crust and solidified to form the Zoroaster Granite. Some of that was then later metamorphosed into gneiss.
At around 1.6 billion years ago, uplift during the Mazatzal Orogeny created a mountain range that was subject to erosion rather than deposition. For this reason there were no sediments deposited here for the next few hundred million years.
After the mountains eroded, marine and volcanic deposition again occurred here between about 1.2 billion and 800 million years ago. Layers of sandstone, limestone, shale and lava now represent the Grand Canyon Supergroup. Starting around 800 million years ago, faulting and rifting tilted these beds about 15 degrees. Interestingly, these rocks aren’t exposed along most of the canyon walls. Instead the 1.7 billion-year-old Vishnu Schist makes contact with the 545 million-year-old Tapeats Sandstone, creating a gap of over a billion years in the geologic record (known as “The Great Unconformity”).
Beginning with the Tapeats Sandstone at around 545 million years in age, the horizontal sedimentary bedding is much less chaotic. These neat layers make up most of the canyon walls and tell a more straightforward story. From the Cambrian to Permian Periods (545-252 million years ago), repeating sequences of ocean advance and retreat lead to alternating sandstones, limestones, shales, and other deposits. These rocks preserve sediments and fossils from a variety of environments including rivers, deltas, beaches, deserts, tidal flats, and reefs.
Periodic inundation by shallow seas continued through the Triassic, Jurassic, and Cretaceous Periods of the Mesozoic Era. These “age of the dinosaurs” sediments were deposited between 252 and 65 million yeas ago, but uplift and erosion have since removed them locally.
Starting around 70 million years ago, the Laramide Orogeny represented a vast mountain-building event in the west. This period of uplift helped create the Rocky Mountains, but also helped elevate the Colorado Plateau. Later uplift brought the plateau even higher. This large elevation change created a steeper gradient and faster flow for the Colorado River, enhancing its ability to cut down into the rocks. Over the last few million years the river has downcut over a mile, and mass wasting erosion has helped widen the canyon to its current form. The resistant sandstone and limestone beds form steep cliffs along the canyon, while the recessive shales form slopes.
While the rocks of the Grand Canyon preserve a great deal of geologic and ecologic history, today many different plants and animals call the canyon home. Occupying a variety of ecosystems, these organisms carry on the legacy of life on earth. A person could get lost for weeks here observing the scientific and scenic wonder of this enormous park.