Sa Rocca Pinta is a spectacular rocky outcrop of ignimbriti, visible along the slopes that you cross through the Alghero-Bosa coastal road . It is considered a natural monument of Sardinia and is located in the municipality of Villanova Monteleone, in the province of Sassari. This stretch of the west coast of Sardinia (UNESCO heritage) is made up of explosive volcanic rocks with a rio-dacitic chemistry (i.e. with a high silicate component), dating back to the Oligo-Miocene age.
Sa Rocca Pinta (the painted rock) is so called due to the particular shapes naturally engraved on the vertical wall of the outcrop. These forms, made up of raised arched crests and cusps, are reminiscent of some kind of ancient writing; for this reason this rock seems to have been painted by man.
Genesis
The ignimbrites that make up Sa Rocca Pinta are pyroclastic rocks produced by the volcanism that affected the area from 32 to 15 million years ago. In this period, during the early stages of the formation of the Alps, Sardinia was still welded to the European plate and therefore was not yet an island. The volcanic activity that formed these large deposits of ashes and lapilli, was very strong and mostly explosive and can be traced back to subduction of the Adria micro plate under the European plate. The deposition and formation of these rocks is due to a vertical column of ashes and lapilli, the so-called burning cloud. The flow of a fiery cloud, made up of ash and supported by volcanic gases, is highly mobile, can be hundreds of meters thick and can extend up to tens of km from the source. The degree of welding of the ignimbrites varies according to the chemism, the granulometry and the temperature at which these ashes settle on the ground; in general, the hotter they are, the greater the degree of welding will be.
Interpretation of natural engravings, letters from nature
The ashes that make up Sa Rocca Pinta are poorly welded even with respect to the ignimbrite banks that rest above it. The soft, brittle and not very permeable nature of this rock allows the gravitational flow of rainwater to deeply affect the vertical wall. Furthermore, the presence of glassy inclusions (lapilli in the middle of the ashes) shields some parts of the wall from erosion and therefore, due to differential erosion, some parts of the rock remain in relief. Exposure to winds from the western quadrants accentuates the incision and to all this is added the chemical erosion, which occurs in the parts where marine humidity, loaded with CO2 and salts, stagnates (i.e. in the concavities facing downwards), breaking up the rock further and forming this beautiful natural art form.