19 May 2014

Tafoni – Nature's Rock Art

Tafoni – you may not have heard the term but the chance are, if you have visited an ocean shore, (or less likely a desert!) that you have seen them with your own eyes. Essentially they are caves (mostly in miniature) and their formation can often result in beautiful patterns. Yet why are they formed in the first place?

Found in all types of climates, tafoni are most common in tidal areas and, conversely, in deserts.  There is no exact scientific explanation to why they are formed – the jury is still out.  However, the number one explanation is that their formation is controlled by a combination of a weathering by salt and the length of time between when they are wet and dry.

Another explanation, where salt is not necessarily involved is that of cementation. As they are mostly found in granular rock (but also in granite, it should be added) cementation occurs when ions carried in water precipitate and form crystalline material.  This happens within sediment pores and how it becomes rock.  As the structure of any cemented material is structurally variable, parts of it are more susceptible to erosion than others and so these mini caves are formed over the years.

The cementation process can often take millions of years to complete.  However, when exposed to the elements it can take substantially less time to reduce the rock to granular form.  The word years was used in the previous paragraph rather than centuries or millennia as tafoni can substantially change within the lifespan of an average human.  They can cause coastal landscape retreat on a rapid scale although in deserts they weather much more slowly.  By the oceans, however, it is thought that tafoni cause up to ten percent of coastal retreat.

It is thought that some tafoni can be initiated by the action of mollusks – and probably other marine organisms like sea urchins and clams.  They create tiny pits in rock on which to cling and to digest minerals from it.  This bore hole would be enlarged over the life of the organism and once it dies it is exposed to the elements.  Then, the tafoni can be fully formed by the actions of forces such as the tide or the wind and rain.

Although tafoni are generally small they can come in many sizes and shapes.  In these pictures you can see the tiny pits which seem to congregate almost magnetically together.  Yet they can be much larger – sometimes even creating caves in which you can easily swing an animal of the feline variety should you chose to do so.

The word itself is something more of a mystery than the reasons why tafoni are formed.  There are several options here – Greek readers will immediately recognise the word taphos which means tomb.  In Sicilian and Corsican the word means windows, while in the latter language, tafonare means to perforate.

If tafoni look a little out of this world, then it should be no surprise that the Pathfinder mission to Mars took pictures of tafoni like structures on the surface of the red planet.  This might possibly indicate that there was once (at least) moisture on Mars but they may also have been caused by a different form of erosion.

Tafoni also come in three different forms – honeycomb, cellular and nested.  They were first defined scientifically in the nineteenth century and as a geomorphic topic have attracted many geologists as well as the more casual onlooker.  What unites the scientist and the layman is the sense of awe that they can inspire.