Granite under pressure


These are images of a pebble that was once granite.


The granite was subjected to high pressure and temperatures, and so its large crystals have undergone two types of transformation. 


They became distorted and changed their composition. The granite transformed into a rock called orthogneiss.


Orthogneiss rocks are also made up of large crystals.


Indeed, look at this series of photos: they often display broad areas of uniform colour, each of which is a crystal. Looking more closely, however, we can see that many of these areas contain shapes that are all elongated in the same way. These are mineral crystals that grew during the distortion of the rock at high pressure, and their arrangement depends on the way in which the pressure behaved. Other areas of the images have colours that form gradual shades. These colour variations are due to the plastic deformation of the quartz crystals.

Name: Orthogneiss
Type of rock: Metamorphic rock

Quartz, plagioclase, potassium feldspar, muscovite, garnet, sillimanite (rare). Calcite, epidote as alteration minerals.

Fossils: Absent
Location: Lazise (N 45° 30′ 39.3″ E 010° 46′ 25.6″))

Formation (deposit):

Formatione (pebble):

pebbles of the glacial deposits of the sarca (morainic amphitheater of the Garda)


Age (deposit):

Age (pebble):

Pleistocene (less than a million years)


How the pebble was formed:

How the rock was formed:

on the front of a lowland glacier



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