The Great Desert


We saw a small city of tent rocks at the Valles Caldera overlook. Tent rocks are moderately common near caldera complex volcanos in arid regions. They can form beneath cliffs in soft material (like pumice) overlain by solid, fractured rock (like welded ash-flow tuff). Pieces of the solid rock protect the softer material below them from the erosion of rain and snow, while neighboring softer rock removed. In our Mesa Erosion lab exercise, some mesas showed the beginnings of tent rocks. Here, the soft material is air-fall tuff and pumice. At the nearby Kasa-Katuwe Tent Rocks National Monument, the soft material is a 'sandstone' made of pumice and volcanic ash.

Rocky top
Before the Storm

The Secret of how tent rocks form. Here is a tent column with its rock still on top. In time, the rock will fall off the column, leaving a 'bare' tent.
The softer rocks will erode more or less according to their strength. Here, the white "liberty cap" is probably soft ash-flow tuff, while weaker buff-colored rock beneath the cap is air-fall tuff or pumice.

Sometimes, uncapped tents erode to hollow columns or cylindrical spires. This happens because rhyolite volcanic ash can react chemically with rainwater. The glass in the ash dissolves in the water, and new minerals (zeolites and clays) precipitate from the water and cement the remaining ash fragments together. This process, called "case hardening," made the tuff cliffs of Bandelier National Monument ideal for cliff dwellings -- hard on the outside and easy to dig on the inside.
Weird and wonderful tent city.

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Copyright Allan Treiman, LPI.
Updated 09/06/03.
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