Hoodoos are tall skinny spires of rock that protrude from the bottom of arid basins and “broken” lands. Hoodoos are most commonly found in the High Plateaus region of the Colorado Plateau and in the Badlands regions of the Northern Great Plains. Hoodoos, which may range from 1.5 to 45 metres (4.9 to 147.6 ft), typically consist of relatively soft rock topped by harder, less easily eroded stone that protects each column from the elements. They generally form within sedimentary rock and volcanic rock formations.
Hoodoos are found mainly in the desert in dry, hot areas. In common usage, 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 smoother profile or uniform thickness that tapers from the ground upward. An example of a single spire, as an earth pyramid, is found at Aultderg Burn, near Fochabers, Scotland.
Hoodoos range in size from the height of an average human to heights exceeding a 10-story building. Hoodoo shapes are affected by the erosional patterns of alternating hard and softer rock layers. Minerals deposited within different rock types cause hoodoos to have different colors throughout their height.
Hoodoos are formed by two weathering processes that continuously work together in eroding the edges of the Paunsaugunt Plateau. The primary weathering force at Bryce Canyon is frost wedging. Here we experience 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%, bit by bit prying open cracks, making them ever wider in the same way a pothole forms in a paved road.
In addition to frost wedging, what little rain we get here also sculpts the hoodoos. Even the crystal clear air of Bryce Canyon creates slightly acidic rainwater. This weak carbonic 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, you can expect the rock to be more resistant to the chemical weathering because of the comparative lack of limestone. Many of the more durable hoodoos are capped with a special kind 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 it in the same way a construction worker is protected by his/her hardhat.
Rain is also the chief source of erosion (the actual removal of the debris). In the summer, monsoon type rainstorms travel through the Bryce Canyon region bringing short duration high intensity rain.
Unfortunately hoodoos don’t last very long. The same processes that create hoodoos are equally aggressive and intent on their destruction. The average rate of erosion is calculated at 2-4 feet (0.6-1.3 m) every 100 years.