Hoodoo (geology)
From Wikipedia, the free encyclopedia
- See Hoodoo for the folk practice of that name.
Hoodoos are tall thin spires of rock that protrude from the bottom of arid 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 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 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 regions of the Northern Great Plains (both in North America). While hoodoos are scattered throughout these areas, nowhere in the world are they as abundant as in the northern section of Bryce Canyon National Park. Walt Disney Imagineers notably based the design of the popular Big Thunder Mountain Railroad attraction around a series of hoodoos, albeit ones constructed out of steel and concrete. Geology purists note that only a tall formation should be called a hoodoo; any other shape is called a 'hoodoo rock'.
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 erosional patterns of alternating hard and softer rock layers. The name given to the rock layer that forms hoodoos at Bryce Canyon is the Claron Formation. This layer has several rock types including siltstones and mudstones but is predominantly limestone. 30 to 40 million years ago this rock was "born" in an ancient lake that covered much of western Utah. Minerals deposited within different rock types cause hoodoos to have different colors throughout their height.
Contents |
[edit] Hoodoo formation in Bryce Canyon
Hoodoos in the Bryce Canyon area 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 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 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, 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 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.
[edit] Destruction
Unfortunately, hoodoos do not last very long; the same processes that create hoodoos will also eventually destroy them. In the case of Bryce Canyon, the hoodoos' rate of erosion is calculated at 2–4 feet (0.6–1.3 m) every 100 years. So it is that Bryce Canyon, as it is known, will not always be here. As the canyon continues to erode to the west it will eventually capture (perhaps 3 million years from now) the watershed of the East Fork of the Sevier River. Once this river flows through the Bryce Amphitheater it will dominate the erosional pattern, replacing hoodoos with a "V" shaped canyon and steep cliff walls typical of the weathering and erosional patterns created by flowing water. Indeed a foreshadowing of this fate can be observed in Water Canyon while hiking the Mossy Cave Trail. For over 100 years a diversion canal has been taking a portion of the East Fork of the Sevier River through this section of the park and already it is easy to see the changes the flowing water has created.
[edit] Reference
- National Park Service: Bryce Canyon National Park: Nature and Geology - Hoodoos (adapted public domain text)
[edit] External links
[edit] Further reading
- DeCourten, Frank. 1994. Shadows of Time, the Geology of Bryce Canyon National Park. Bryce Canyon Natural History Association.
- Kiver, Eugene P., Harris, David V. 1999. Geology of U.S. Parklands 5th ed. John Wiley & Sons, Inc. 522-528.
- Sprinkel, Douglas A., Chidsey, Thomas C. Jr., Anderson, Paul B. 2000. Geology of Utah's Parks and Monuments. Publishers Press: 37-59
