Orthogneiss

Gneisses are rocks those can be identified by a typical banding. Orthogneiss, compared to the paragneiss, is richer in feldspar and muscovite dominates over biotite. This compositional difference results from the orthogneiss being a product of metamorphosed granite. Occasionally, relics of the original granitic texture can be seen in orthogneiss. This includes large feldspars crystal those are shattered and stretched by pressure.

The movement of Earth´s crust causes the formation of various cracks, faults, and deformation of the rocks. Individual blocks are pulled over each other or are uplifted or plugged to depths with increased temperature and pressure, where metamorphosis may occur. If the rock undergoing metamorphosis has an original granitic composition, it results in the formation of an orthogneiss. On the other hand, paragneiss is formed from metamorphosis of sedimentary rocks. The degree of metamorphosis suggests that the displayed red orthogneisses were formed under pressures of approximately 750 MPa and temperature around 600 °C.

The orthogneisses occur in Karlovy Vary Region in the areas of Horní Slavkov, Aš or around Stráž nad Ohří. A spectacular example of orthogneiss cliffs occur just beyond the Region border, is the Natural Reserve Sfinx near Měděnec.

Sample donated by town Stráž nad Ohří.