Welcome to the Marina District of beautiful San Francisco!
Today’s lesson will take to a small jetty out by the Golden Gate Yacht Club. At the coordinates, you will find some large rocks that have been pitted all over due to years of weathering. This form of weathering as a whole is called tafoni. We will look at the many forms of tafoni, and you will determine which kind you see here!
Tafoni (singular: tafone) are small, circular caverns in rock faces often in areas protected from surface run-off. This means they are commonly seen in arid places; so in this case, standalone rocks that don’t receive a flow of runoff from higher-up places. Most tafoni form in softer stones, such as sandstone, which are much more soluble in water than their other rocky relatives. For instance, here you are standing at a riprap shoreline of the jetty. Riprap shorelines made of igneous or metamorphic rock often do not have rocks displaying this weathering phenomenon, even when experiencing the same weather/climate conditions.
In our case here, salt water and salty air have a profound effect on these rocks. In inland examples, tafoni will appear on rocks that already have a salty makeup (sandstone is made up of calcite [calcium carbonate, CaCO3]). With a porous rock like what we see here, salt water will splash up and enter very fine cracks and holes in the rock. When the salt water inevitably evaporates, it leaves behind a salty brine. The salt crystals from the water accumulate in the gaps and will eventually pry the rock apart in weaker areas. The ridges you see that encompass each small indent are harder veins within the rock that have not succumb to the relentless onslaught of salt crystals.
There are a few different types of tafoni. The first difference is small vs. large tafoni.
Small tafoni is simply measured on a centimeter- to decimeter-scale. Large tafoni is measured on a decimeter- to meter-scale.
Next, there is alveoli, or honeycomb weathering.
Alveoli is a cluster of small tafoni that have well-defined walls but are all grouped together like a collection of cells.
Lastly, there is nested tafoni.
Nested tafoni occurs when tafoni appear inside a former, larger tafone. This process takes even longer. Nested tafoni are very rare, as it is not often that wildly different scales of tafoni appear in the same geographic setting. However, it has been documented. Small tafoni can sometimes nest inside larger cavities that succumbed to the salt-weathering process long ago. To some, this is viewed as simply additional weathering occurring inside an older tafone, but to others, it as viewed as an indicator of other factors playing into the weathering processes of the rock.
To log this earthcache, please send me a message with the following:
- Name of this earthcache and the number of people in your group.
- What kind of tafoni is this? Small, large, alveoli (honeycomb), nested?
- Pick a tafone of your choice on the rock, and measure it (metric scale preferred). Does this support your claim of tafoni type? (If you don’t have a ruler, to-scale ones should be easily accessible on a smartphone device).
- Do you see any rocks in the vicinity that do not have any tafoni? What makes them different?
References:
-The Encyclopedia of Geomorphology (Goudie, 2003)
-https://oaklandgeology.com/2014/05/11/salt-weathering-tafoni/
-http://tafoni.com/Types.html#:~:text=Clusters%20of%20well%2Ddeveloped%20small,thin%20walls%20and%20appear%20delicate
-https://www.nps.gov/articles/tafoni.htm#:~:text=Known%20as%20honeycomb%20weathering%20or,arid%20or%20semi%2Darid%20deserts.