Hoodoo (geology)

Occurrence

Hoodoos are found mainly in the desert in dry, hot areas. While visually similar to pinnacles (or spires), hoodoos are unique in their variable thickness, often described as having a totem pole-shaped body. A spire, conversely, has a smoother profile or uniform thickness that tapers from the ground upward.

Hoodoo formations are commonly found on the Colorado Plateau and in the Badland regions of the northern Great Plains of North America. While hoodoos are scattered throughout these areas, nowhere in the world are they so abundant as in the northern section of Bryce Canyon National Park in Utah. Hoodoos are also very prominent a few hundred miles away at Goblin Valley State Park on the eastern side of the San Rafael Swell, at Kasha-Katuwe Tent Rocks National Monument in north central New Mexico, and in the Chiricahua National Monument of Southeast Arizona. Some hoodoos are found in Sombrerete, Mexico at the Sierra de Organos National Park.

Hoodoos are also found in the Cappadocia region of Turkey, where houses have been carved into the formations. These hoodoos were depicted on the reverse of the Turkish 50 new lira banknote of 2005–2009.

In Armenia, hoodoos are found in Goris, Khndzoresk, Hin Khot and several other places in Syunik Province, where many were once carved into and inhabited or used.

In French, the formations are called demoiselles coiffées (ladies with hairdos) or cheminées de fées (fairy chimneys) and several are found in the Alpes-de-Haute-Provence; one of the best-known examples is the formation called Demoiselles Coiffées de Pontis.

The hoodoo stones of Yehliu on the northern coast of Taiwan are unusual for their coastal setting. The stones formed as the seabed rose rapidly out of the ocean during the Miocene epoch. Efforts have been made to slow the erosion in the case of iconic specimens in Wanli.

The Awa Sand Pillars found in Tokushima Prefecture, Japan, are hoodoos made from layers of compacted gravel and sandstone.

Đavolja Varoš (Devil's Town) hoodoos in Serbia feature about 200 formations described as earth pyramids or towers by local inhabitants. Since 1959, Đavolja Varoš has been protected by the state. The site was also a nominee in the New Seven Wonders of Nature campaign.

The hoodoos found in Drumheller, Alberta, are composed of clay and sand deposited between 70 and 75 million years ago during the Cretaceous Period. These hoodoos can maintain a unique mushroom-like appearance as the underlying base erodes at a faster rate compared to the capstones at a rate of nearly one centimeter per year, faster than most geologic structures.

In north-west Greece, distinctive chimney-like rock pillars, called bouharia, are found in the Grevena-Kozani Geopark, particularly near the village of Mikrovalto. Formed over thousands of years through erosion and weathering, they consist of loosely consolidated sediments topped by protective slate capstones, reaching heights of up to six meters. In the same geological park, sharp ridge formations, called noktaria, rise over ten meters high.

Formation

Hoodoos typically form in areas where a thick layer of a relatively soft rock, such as mudstone, poorly cemented sandstone, or tuff (consolidated volcanic ash), is covered by a thin layer of hard rock, such as well-cemented sandstone, limestone, or basalt. In glaciated mountainous valleys, the soft eroded material may be glacial till with the protective capstones being large boulders in the till. Over time, cracks in the resistant layer allow the much softer rock beneath to be eroded and washed away. Hoodoos form where a small cap of the resistant layer remains, and protects a cone of the underlying softer layer from erosion. The heavy cap pressing downward gives the pedestal of the hoodoo its strength to resist erosion. With time, erosion of the soft layer causes the cap to be undercut, eventually falling off, and the remaining cone is then quickly eroded.

Typically, hoodoos form from weathering processes that continuously work together in eroding the edges of a rock formation known as a fin. For example, the primary weathering force at Bryce Canyon is frost wedging. The hoodoos at Bryce Canyon experience more than 200 freeze-thaw cycles each year. In winter, melting snow in the form of water seeps into cracks and then freezes at night. When water freezes, it expands by almost ten percent, prying open cracks and gradually widening them, similarly to the way a pothole forms in a paved road.

Rain is another weathering process causing this erosion. In most places today, rainwater is slightly acidic, which lets the weak carbonic acid 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 is 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. Rain is also the chief source of erosion that washes away debris. For example, monsoon-type rainstorms travel through the Bryce Canyon region in summer, bringing short-duration high-intensity rain.