Tafoni in the Forest of Koumares
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Here are the questions:
1. Based on the description, determine whether the feature described is a tafoni. Explain your reasoning.
2. Estimate how many holes you can find on the rock surface in an area of approximately 1 square meter.
3. Describe the holes in terms of depht, shape and size.
4. Take a photo of yourself standing next to the rock.. You can post it on the log or send it to us along with your answers.
Here you will learn about a special form of weathering. Rocks form over thousands of years and have different appearances, properties, and shapes. Naturally, a distinct process of weathering (or decay) can also be observed. Depending on the type of rock and weathering process, different forms can result. These can be easily recognized and observed in some rocks.
What is Tafoni (honeycomb weathering)?
In medium- to coarse-grained rocks, very specific weathering patterns can form. A kind of porous stone is created, which, depending on the number of holes, sometimes resembles a honeycomb. The special thing about tafoni is that the weathering process occurs chemically from the inside out. Unlike, for example, water eroding the rock from the outside. Spherical or oval holes/cavities are formed. The size can range from a few centimeters to several meters. With very extensive weathering, so-called weathering crusts made of minerals can also form. These consist of, for example, iron oxide, limonite, gypsum, or carbonate and form thin crusts.
Occurrence/Rock types
The most common rocks in which tafoni can form are: sandstone, greywacke, granite, granodiorite, and gneiss. They are less frequently found in: limestone, lava, and tuff. Crete offers a wide variety of different types of rock. But limestone can also be seen. Unfortunately, not all of these rock types are visible at the surface.
Formation
The exact chemical process of tafoni formation is still debated among geologists, but it is assumed that it is related to differences in moisture within the rock. Surface water seeps into the rock, evaporates, and releases dissolved minerals at the surface. Geochemical processes then form substances that decompose the stone from the inside. In the case of sandstone, this substance is, for example, salt (alum). When water evaporates from the pores of the sandstone, gypsum forms, which then deposits in nodules. Alum can form when oxygen and carbonic acid dissolve sulfur from the rock. This, in turn, can lead to the formation of sulfuric acid. When the water evaporates, alum crystallizes. Alum requires considerable space. During this crystallization process, the rock is "blown apart" and depressions form. In addition to this process, silica contributes to the consolidation of the sandstone. This results in the characteristic "honeycomb" structure. In contrast, in mechanical weathering, the rock is eroded from the outside in. This results in various surface patterns, but not circular or spherical weathering patterns. This is a key difference from tafoni, which is a form of "internal" weathering caused by chemical processes.
Sources: Wikipedia; personal observations on-site