If you look at the first image you may ask yourself, “what is the deal with all of these holes and why are they only in one part of the rock wall”? That is a completely valid question as these features are a unique geological feature. These holes or cavities in our rock wall are called tafoni (also called honeycomb rock or alveoli – depending on their size). Throughout this post we will explore the tantalizing wonders of tafoni.
What is Tafoni?
Tafoni are defined as cavernously-weathered forms of a variety of sizes and shapes depending on the amount of weathering and erosion of the rocks they form in1. This unique erosion pattern was first noted in a 3500-year-old Minoan fresco painting2. The specific term “tafoni” was first used in 1882 by Hans Henrik Reusch while studying tafoni cells in Corsica3. The first description of tafoni from a geologic perspective was from the Mediterranean region, but Tafoni has been found on every continent; most specifically in mainly arid and semi-arid regions with a few exceptions (Finland, South Korea, Hong Kong)1.
How Does Tafoni Form?
Tafoni weathering patterns and the exact process in which these unique patterns form is still a bit of a geologic mystery, but a generalized process has been described. Tafoni forms typically in the following types of rocks: granite, sandstone, limestone, and tuff1. Formation of tafoni arises from a combination of wind action, moisture content in the rocks, and how warm the environment is. In arid or semi-arid regions, salt crystals typically form when water stored in holes or fractures in the rock evaporates, resulting in the loosening of sand grains or creating small fractures. Once these fractures develop, wind and rain will cause further breakdown of the rock face4. Over time the tafoni will continue to develop and as geologists observe it, is in different stages of development.
Terrific Tafoni in Tuff!
In the first image we saw our tafoni in some fabulous tuff. Since I am here in New Mexico writing this I wanted to share with you some information on the tuff. First and foremost, what is tuff? Tuff is a type of igneous rock that is a mixture of materials erupted from explosive volcanic eruptions5. As the volcano erupts it ejects rocks, ash, and other materials that get deposited either closer to the vent (typically larger fragments) or potentially very far away (ash carried in the atmosphere). Over time these different materials become cemented together creating the tuff. The tuff in which our tafoni formed is called the Bandelier tuff. The Bandelier tuff formed during a very large, explosive eruption associated with the Valles Caldera formation 1.25 million years ago6! The eruption ejected approximately 475km3 of material. The figure below shows the extent of ash deposits associated with this eruption.
1French, H. and Gugliemin, M., 2021, Tafoni weathering is an azonal process: Examples from Antarctica, Sardinia, and Australia: Geomorphology, v. 375, p. 1-7.
2Boxerman JZ (2005) The evolutionary cycle of the tafone weathering pattern on sandstone at Bean Hollow Beach, northern California. Geological Society of America Sectional Meeting. Abstract. Salt Lake City, UT.
3Groom, K., Allen, C.D., Mol, L., and Paradise, T.R., 2015, Defining tafoni: Re-examining terminological ambiguity for cavernous rock decay phenomena: Progress in Physical Geology, p. 1-19.
4Longstreth, D., 2015, Salt Point State Park: Geological Gems of California State Parks, v.16, p. 1-4.
5King, H.M., Tuff, https://geology.com/rocks/tuff.shtml. Accessed on 5/10/2021.
6Gardner, J.N., Goff, F., Kelly, S., and Jacobs, E., 2010, Rhyolites and associated deposits of the Valles-Toledo caldera complex: New Mexico Geology, v. 32, p. 1-16.