Mount Vadà

Mount Vadà is composed of very hard, erosion-resistant rock: the Occhiadi orthogneiss. It lies at a higher altitude than that reached by the glacier, and its slopes have been shaped by the action of freezing and thawing. Mount Vadà stands out from the ridgeline of Mount Spalavera and Mount Bavarione as it is largely composed of orthogneiss. It is evident here too, as with Mount Morissolo, that there is a close link between lithology (rocks) and morphology (landforms), with the terrain becoming more pronounced and rugged where the Occhiadi orthogneisses—rocks highly resistant to erosion—are present. Orthogneisses are metamorphic rocks derived from intrusive rocks of granitic composition, which belonged to the Ordovician magmatic cycle (approximately 470 Ma) and were subsequently metamorphosed during the Hercynian phase (around 340 Ma).

The orthogneiss outcropping here is characterised by large crystals of potassium feldspar (known as porphyroclasts), which were already present in the original granite. This mineral is rigid and does not deform during metamorphism, but rotates to align itself parallel to the foliation plane, which envelops it to form the characteristic ‘eyes’ (hence the term ‘occhiadino’, or ‘augengneiss’ in German).

Metamorphism has deformed the rock, but has not significantly altered its mineralogical composition, which consists of plagioclase (milky white), quartz (white-grey), biotite (black) and large grains of potassium feldspar (shiny white), as well as a small amount of muscovite with silvery sheen.

The presence of plagioclase and biotite indicates that metamorphism took place at temperatures in excess of approximately 500°: indeed, at lower temperatures, greenish minerals would have formed in their place – namely epidote (+ albite) from the plagioclase and chlorite from the biotite.