When next you drive the spectacular mountain road from San Pedro de Alcántara to Ronda, check out the rocks on the right hand side of the road. Best to have a designated driver for this investigation so as to avoid the precipitous drop on the other side. In the “old days” this route was fraught with dangers not least of which was the real possibility of ending up down the precipitous drop. Other dangers included bandits, fog and the lack of English speakers. The bandits have all been killed and the English are spreading up from the Coast. The fog remains.
A recent sharp-eyed guest of Los Castaños and avid geologist told me something about the rocks that I had never noticed in all the years of driving past them. They are unique and they are GREEN! And, something extraordinary happened in the Serrania de Ronda to produce them.
Loff told us that we are living in the world’s largest outcrop of a greenish rock called PERIDOTITE. I drove immediately to examine the rockface and sure enough it’s GREEN!
I have never been a great fan of rocks and indeed, when my elder daughter began a geology degree, I thought she was nuts. However, when you have the biggest, the best, the greenest rocks in the world in your backyard, it makes even dry old rocks interesting.
What follows is a brief account of the geology of the Serrania de Ronda as told by Loff. You may say, as I Peridotitedid, that you are not interested but I assure you, it is worth reading. And then come and see for yourself!
“The geology of this area holds a very special fascination for anyone seriously interested in rocks as it contains the world’s largest outcrop of a rock called PERIDOTITE.
This rock normally forms part of the Earth’s upper mantle which lies beneath the Earth’s crust and is around 400km in thickness. For this type of rock to appear at the Earth’s surface is very rare as it has such a long way to travel. In this area it had about 70km of crust to penetrate, and achieved this because it was molten and therefore less dense than the surrounding crust. It was thus able to ‘float’ to the surface as a ‘plume’, a huge mass of rising molten rock.
In order for it to have appeared in this area at all and in such a large volume, huge forces were necessary. These forces were supplied by the collision of Africa with Europe which began around 100 million years ago during a period of time called the Cretaceous. Africa is still gradually moving north today, creating the Alps and closing the Mediterranean Sea as it goes!
It is estimated that the molten plume began rising around 25 million years ago and finally cooled in its current position by around 4 million years ago. It would initially have been at 1100-1200 degrees centigrade and would have risen at more than one metre a year but slowing as it cooled. As this molten plume penetrated the crust pre-existing rock was metamorphosed due to the intense heat. This caused the limestone at or near the contact to become the marble which is now quarried in the area. As the plume rose, it thinned the crust above it causing it to rift. This is how the Alboran Sea was formed.
The peridotite in the Serrania de Ronda has become partly metamorphosed into a rock called SERPENTINITE* due to very hot fluids percolating through it which were hot enough to partially alter its mineral composition.”
Thank you, Loff, very much and, thank you, Karen, for transcribing Loff’s handwritten notes.