Hoodoos are tall thin spires of rock that “grow” from the bottom of dry basins and badlands. They are composed of soft sedimentary rock, and are topped by a piece of harder, less easily eroded stone that protects the column from the elements. In general, the difference between hoodoos and pinnacles or spires is that hoodoos have a variable thickness, often described as having a “totem pole” shaped body.
A spire, on the other hand, has a more smooth profile or uniform thickness that tapers from the ground upward. Hoodoos are most commonly found in the High Plateaus region of the Colorado Plateau and in the Badlands region of the Northern Great Plains.
While hoodoos are scattered throughout these areas, the most massive concentration is found in the northern section of Bryce Canyon National Park. At Bryce Canyon, hoodoos range in size from that of an average human to heights exceeding a 10-story building. Formed in sedimentary rock, hoodoo shapes are affected by the different erosion of alternating hard and softer rock layers – siltstone, mudstone, and predominantly limestone. Mineral deposits within different rock types cause hoodoos to have different colors throughout their height.
In Bryce Canyon, hoodoos are formed by two weathering processes that continuously work together in eroding the edges of the Paunsaugunt Plateau. The primary force is frost wedging, where there are over 200 freeze/thaw cycles each year. In the winter, melting snow, in the form of water, seeps into the cracks and freezes at night. When water freezes it expands by almost 10%, gradually prying open cracks, making them ever wider.
In addition to frost wedging, what little rain falls in the area also sculpts the hoodoos. Although the Bryce Canyon region is far from major sources of atmospheric pollution, rainfall there is nevertheless slightly acidic. This weak carbon acid can slowly dissolve limestone grain by grain. It is this process that rounds the edges of hoodoos and gives them their lumpy and bulging profiles. Where internal mudstone and siltstone layers interrupt the limestone, the rock will be more resistant to the chemical weathering because of the comparative lack of limestone. Many of the more durable hoodoos are capped with a type of magnesium-rich limestone called dolomite. Dolomite, being fortified by the mineral magnesium, dissolves at a much slower rate, and consequently protects the weaker limestone underneath.
