Origin of granite
The geology of the promontory of Capo D’Orso, as well as of the whole Gallura territory, is related to the late events of the Hercynian orogeny and the formation of the Sardinia-Corsica batholith that occured around 300 million years ago. This orogeny derives from the collision between two plates forming two ancient continents, the Gondwana and Armorica.
The impact between these two plates caused the outcrop of the typical intrusive rock which is the fundamental element of the Sardinia-Corsica batholith: the granite.
Granite is an igneous rock, with a highly silica content formed from a solified fluid state, the magma. Intrusive or plutonic rocks originate when magma slowly cools inside the Earth’s crust; while vulcanic or effusive rocks originate when molten lava cools on the surface, losing some volatile components. Granite belongs to the intrusive rocks, characterized by the the presence of various minerals that, according to their quantity, determine the different classes of granite, also distinguished by the granule sizes and the specific relationships between them.
Granite classification is based on the quantity of silica minerals: quartz, plagioclase, potassic feldspar, micas. The colour of granite is identified according to the abundance or lack of these minerals, this characteristic being the most evident at a first analysis.
According to this classification the promontory consists, at the basis, of monzogranites which have a pink colour due to a high percentage of k-feldspar, while the upper part, mainly composed of leucogranites, has a whitish colour due to the absence of k-feldspars.
The Bear Rock
The erosive action of weather phenomena acts in a selective way. In fact, this phenomena not only act differently on different types of rocks, but also on various parts of the same rock, according to its major or minor resistance to weather conditions, thus giving the scenery a great variety of different, fascinating details and having determined the formation of this natural monument: the Bear Rock.
During different geological eras, granite has been attacked by weather phenomena, mainly marine spray and wind, but also by ice of quaternary glaciation, thus being shaped into two forms of primarly evidence: tafoni and rounded rocks.
The Bear Rock belongs to the first category, the tafoni, which are formed in the open air by the dynamic action of the wind combined to that of the chemical corrosion.
Warm wind bores into rock pores, thus accelerating the evaporation of salty water and causing the deposit of salt chrystals which expand exerting a pressure on the rock and causing its decay. These phenomenon, called honeycomb or alveolar weathering, is visible everywhere, particularly under the Bear’s head, in the upper part of it.
With time the hollows grow upward thus causing the exfoliation of the vault. Tafoni can appear in the form of small holes or wide cavities also called exfoliation niches