Gneisses

Hallandia gneissi

Hallandia gneiss

Gneisses and migmatites are metamorphic rock types that have been formed as a result of an igneous or sedimentary rock falling to a position close to the basal parts of the earth’s crust, where the severe pressure and high temperature cause alterations to take place.

Gneisses arise as a consequence of dark and light-coloured minerals separating out into different layers, giving them a streaky appearance. The light-coloured minerals in gneisses are typically quartz, plagioclase and/or potassium feldspar, while the dark minerals are micas (biotite and muscovite) or hornblende.