Geology of Bryce Canyon National Park
Learn about the geology of Bryce Canyon National Park.
Introduction
Bryce Canyon is known for distinctive hoodoos, spires and towers that appear as forests of rock. This national park is part of the Colorado Plateau, a region of the present-day Southwest United States that was once periodically flooded by freshwater and marine landscapes in which massive amounts of fine-grained sediment was deposited over millions of years, lithified into sedimentary rock, and were later uplifted by tectonic activity. Over time, the rock was subjected to the slow, powerful forces of weathering and erosion that molded the landscape into the layered columns seen today. Bryce Canyon is a picturesque fountain of information, informing geologists of the land’s history through the rock layers and unique structures.
Geology in Context: Colorado Plateau
The Colorado Plateau is a region in the Southwest US, encompassing parts of the Four Corners region (Utah, Colorado, Arizona, and New Mexico). This area includes parks such as the Grand Canyon, Arches National Park, Canyonlands, Mesa Verde, and Bryce Canyon. The Colorado Plateau is at a higher elevation than its surroundings, ranging from about 2000 -12,000 feet above sea level at its highest peaks. While some of the lowest (oldest) rocks in the region are metamorphic and igneous, the more visible and characteristic rocks are layered, sedimentary rocks with vibrant hues of rust-colored reds and orange.
There are three main rock types (sedimentary, metamorphic, and igneous), but Bryce Canyon is almost entirely composed of sedimentary rocks. This means it was formed as a result of the deposition of sediments (small pieces of rocks, biological material, or particles precipitated out of water) that are cemented together and form rock over time. Sedimentary rocks also record the history of the environment in a location. The size and type of sediment reveals the type of place the rock was deposited.
The rocks of Bryce Canyon tell a story, revealing the past environments of the area. At various times in the past, this region was a floodplain, part of a sea, and a desert. The Colorado Plateau, beginning about 50 million years ago, was a mostly flat part of a lake and floodplain system. This area was surrounded by higher elevations, carrying sediment downwards where it was deposited. The sediments were cemented over time to form various sedimentary rocks, including sandstones, dolostones, limestones, and mudstones. At this point, the region we now call the Colorado Plateau was near sea level.
The next major event was a collision of the North American Plate and the Farallon Plate, causing the Farallon plate to subduct and generate heat that drove the Colorado Plateau upward to its current elevation. This helps explain how we can have marine sediments at both a high elevation and in the middle of a continent. This uplift exposed the rocks to the elements, allowing weathering to create new formations and making the layering visible.
Hoodoos
The most distinctive feature of Bryce Canyon, hoodoos, are natural geologic features that create an otherworldly landscape. They are pillars of sandstone and other fine-grained sedimentary rocks created by the processes of uneven weathering (chemical and physical processes that break up rocks) and erosion (removal of sediment and rock due to weathering). The uplift of the Colorado Plateau caused the area that is now Bryce Canyon to move to a higher elevation. For ~200 days of the year, the region experiences both above and below freezing temperatures, allowing ice and rain to create the hoodoos. Water seeps into spaces between and within rock. When the temperature lowers, the water within the rock freezes and expands. ice wedging, because as water freezes it expands. This expansion, known as ice wedging, starts to break apart rocks, first into walls, then windows, then a fully formed hoodoo as water continues to melt and then refreeze and reenter the cracks. Slightly different formations and patterns of weathering can arise based on the exact rock and rock layering present in a particular place.
Closer look at geologic processes and history
Faults are fractures in the Earth’s crust along which movement occurs. Earthquakes happen when energy builds up along a fault plane and is released suddenly. While commonly associated with the boundaries of plates, faults can occur anywhere where there is a buildup of stress that causes rock to break and move from its original location. You can see evidence of faults by looking at the layering of rocks. In Bryce Canyon, there are horizontal layers of sediment. At a fault, part of the rock is displaced, so the horizontal layers are no longer continuous. Some examples at Bryce Canyon include the Bryce Point fault, the Peekaboo fault, and the Fairyland fault.
Folds happen when there is a buildup of stress, but the rock bends instead of breaking. This can happen if the rock is particularly ductile or warm or if the area experiences stress over a long period of time. One type of fold is a monocline, in which one side of a fold axis is bent downward. Folds in Bryce Canyon are not particularly visible, but often occur near fault zones.
Plate tectonics describes the motion of the Earth’s lithosphere on top of the asthenosphere. The lithosphere is composed of the crust and upper mantle and acts as a brittle solid, while the asthenosphere is the rest of the mantle and acts as a liquid over long time scales. The lithosphere is broken into “plates,” or pieces that move due to convection in the mantle and the formation of new crust in mid oceanic ridges. These plates move on the surface, sometimes colliding at plate boundaries. Boundaries between plates are characterized as divergent (plates moving away from each other, usually through the generation of new crust), convergent (plates collide with one subducting under the other or compressing to build mountains), or transform (two plates slide against each other). Plate boundaries are not necessarily along continents, and the positions of continents are always changing (although too slow for us to feel on our relatively short time scales).
Rock Types
Rocks hold stories of a place’s past. In order to interpret these stories, it is important understand the underlying processes. There are three main types of rocks: sedimentary, metamorphic, and igneous. Sedimentary rocks form when sediments (gravel, sand, clay, silt), pile up and lithify (turn to rock through compaction and cementation over time). The material and fossils that compose the rock, as well as any patterns or inclusions in the rock, reveal where and when the rock was formed. Igneous rocks are formed when magma or lava cools. These are associated with volcanic activity and sea floor spreading. Metamorphic rocks are the result of subjecting another rock to extreme heat or pressure. Given that each of these types of rock form under particular conditions, knowing the types of rock common to an area help scientists understand what the region looked like in the past. For example, Bryce Canyon has many sandstones, which is a major indication that the area was once part of a marine environment.
Rock Strata in Bryce Canyon
Stratigraphy is the study of the order and relative position of rock layers, putting them in context of the geologic time scale. Strata are layers of rocks of the same composition and age, deposited at the same time, and spanning any size of area. Geologists construct stratigraphic columns to visualize a vertical slice a location and show the rock types and ages. This relies on several principles: the law of horizontal deposition (when rocks are deposited, the layer is flat until acted on by other forces), the law of superposition (younger rocks are deposited on top of older rocks), and the laws of inclusions and cross-cutting relationships (pieces of rock imbedded in another rock are older, and rocks or faults that cut through other rocks are younger).
The rocks in Bryce Canyon are separated by age into three main stratigraphic units: From youngest (highest elevation) to oldest (lowest elevation), they are Tertiary, Cretaceous, and Jurassic (which are only visible in certain areas). Additionally, there are younger Quaternary deposits in some places, mostly gravels and other sediments or rocks deposited as a result of erosion. These units are named based on the geologic time period in which they were deposited.
The Tertiary rocks are primarily the pink and white limestones characteristic of much of Bryce Canyon. The Pink Cliffs, Table Cliff Plateau, and Sevier River Formations are all part of the Claron Formation, which includes limestone strata as well as sandstone and shale. Also included in the Tertiary deposits is the Sevier River Formation, primarily composed of conglomerate sandstones and valley-fill deposits. Some of the clasts (particles included in sedimentary rocks) of this area are volcanic, indicating that volcanism had been active prior to the erosion and deposition of these rocks. The final well-known constituent of the Tertiary units is the Boat Mesa Conglomerates, the white or light-colored sandstone that form the caps on Bryce Point and Boat Mesa. The exact age of these rocks is uncertain, because there are no fossils or other age criteria that can be precisely dated, and there is an unconformity (gap in the geologic record as a result of erosion after one bed is deposited and before the next, indicating an indeterminate stretch of time for which there is no geologic information) between the Claron Formation rocks and the Boat Mesa Conglomerates.
The underlying Cretaceous rocks are primarily gray and white sandstones and shales, and are divided into five main units: the Kaiparowits formation, the Wahweap Formation, the Straight Cliffs Formation, the Tropic Shale, and the Dakota Sandstone (from youngest to oldest). Many of these rocks were deposited in flood plains or freshwater, though some of the older Straight Cliffs rocks and Tropic Shale rocks are of marine origin. The Dakota Sandstone rocks also contain coal, indicating that swamps were present. The contact between Dakota Sandstone and the Jurassic stratigraphic units is a clear change between the gray and brown Dakota rocks to the white and red Jurassic rocks, and a clear erosional surface separates the two.
The Jurassic rocks are generally not exposed in Bryce Canyon, but the sequence is exposed outside the park, allowing scientists to study the stratigraphy. These rocks are primarily sandstones.
The rocks of the Claron Formation comprise the spectacular structures that Bryce Canyon is known for. Limestones, which form much of the Claron Formation, are particularly susceptible to weathering and erosion through chemical reactions. Limestone is calcium carbonate (CaCO3), which can react with weak acids, such as the carbonic acid (H2CO3) that naturally occurs in rainwater from interactions between water vapor and carbon dioxide. This weak acid, over long time periods, can dissolve the limestone. Since not all the rocks are composed of limestone, there is uneven weathering, which is exacerbated by physical weathering (such as ice wedging) to create the spectacular structures.
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