Hoodoo (geology) - Formation

Formation

Hoodoos form 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 are formed where a small cap of the resistant layer remains, and protects a cone of the underlying softer layer from erosion. Further erosion of the soft layer causes the cap to be undercut, eventually falling off, and the remaining cone is then quickly eroded.

Typically, most hoodoos are formed by two weathering processes that continuously work together in eroding the edges of a rock formation. The primary weathering force at Bryce Canyon (as one example) is frost wedging. The hoodoos at Bryce Canyon experience over 200 freeze/thaw cycles each year. In the winter, melting snow, in the form of water, seeps into the cracks and then freezes at night. When water freezes it expands by almost 10%, pries open the cracks bit by bit, making them even wider, much like the way in which a pothole forms in a paved road.

In addition to frost wedging, rain also sculpts these hoodoos. In most places today, the rainwater is slightly acidic which allows the weak carbonic acid to 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. 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.