If you look about you there are stones that have holes in them. These are the Alveoli (small tafoni)
- What stage do you see the majority of the alveoli in? Explain.
- How are these formed based on the environment around you?
- Looking inside the holes, are there alveoli forming inside others?
- Do all of them appear on the same sides of rock or different sides?
- Add a photo of you from the location or of an identifiable item (paper with trackable name). Posting a photo pulled from the internet, or photoshopped will result in instant deletion.
Welcome to one of the mysteries of geology. I started looking into this and trying to figure out the causes and came across a vast amount of differing answers as to why these appear So here I go.
Alveoli
Similar to Tafoni. Though it seems that the difference is the size. It is a hole in the rock. Alveoli are smaller while tafoni are larger. Though some use the term tafoni for both. They are indentions or holes that form and develop in rocks.
How they form
Wind
The most obvious is erosion that is caused by the wind. The wind for whatever reason a tiny pocket starts forming. A tiny fragment of material can get inside the tiny depression and the wind can cause it to spin. Slowly eroding the hole deeper and deeper. Similar to potholes in streams. The water spinning a stone drilling it into the rock. Sand may blow out over time and eventually may be replaced with other grit in the hole, or other material may be deposit in the hole.
Salt
This is common in salt environments and the coast. The holes may or may not contain visible amounts of salt. The salt damages the rock in the process of drying and the salt forming. In much the same way ice cracks and damages rocks.
Freezing/Thawing
Similar to the salt methods. In environments where there is water the water can get into the pores of the rock, freeze, then break off tiny materials. The repeated thawing and freezing growing to cause enough damage to finally form the holes in the rock.
Combination
Any combination of the above ways is possible, and most likely true.
Speed of formation
In looking at the rate that these tend to form I get various information, and it varies by location. Similar stone in a similar environment seem to wear at a rate of .0001" per year. In other words a 1" deep hole would tak 10,000 years to form. Wet enviroments are 2-4 times faster. It is believed that having a wet period followed by a slow drying appears to increase the wear rate. This may allow the crystals in ice or salt form to break the particles loose.
Stages of formation
- No Tafoni - The rock surface is weathered or smooth
- Pitting - Small pits form in zones of weakness, low morosity or another fault in the stone. Less than 3/4" in diameter or deep
- Tafoni - Rounded cavities forming. 3/4" and greater
- Coalescence - Walls breaking down. Tafoni are starting to break into each other. Not well rounded.
- Remnants - A larger cavern may exist, all of the tafoni breaking into one. Base features are smooth and rounded with meandering tendrils, knobs etc. (the cycle can then begin again)
Boxerman, J. Z., 2005, The Evolutionary Cycle of the Tafone Weathering Pattern on Sandstone at Bean Hollow Beach, Northern California, Geological Society of America Sectional Meeting Abstract
—, 2006, The Evolution of Tafoni on Coastal Sandstones in Northern California, Unpublished Master's Thesis, Department of Geoscience, San Francisco State University, May, 2006.