Garnet Gneiss of Western Australia

Like marble, this rock was formed by regional metamorphism, where deeper rocks under ground have collided when the Earth's plates have crashed into each other. It has been heated as well as put under pressure, and may have been so thoroughly changed it is impossible to tell what the original rock was, or even whether it was igneous, like a granite, or sedimentary, like an impure sandstone. Many gneisses were clay-based rocks like shales.

garnet gneiss

Photographed from the Geological Museum Perth

This gneiss rock in the picture has similar minerals to those of granite. It also has large pink crystals of Garnet.

The term "gneiss" is used for course grained, banded rocks formed at high temperature and usually under pressure during burial deep in the earth. The minerals making up the rock depend on two things: the original composition of the rock before it was metamorphosed, and 2nd, the temperatures and pressures it has reached during its history.

Quite often gneisses have had more than one period of metamorphism. The layers produced by one episode can be squeezed and folded by another, and new minerals might grow which cut across old structures. Unlike igneous rocks, metamorphic rocks do not become molten, though in some extreme cases parts of the rock may become mobilised and squeezed out as veins. No real changes to the composition of the rock are made, except that water and carbon dioxide are usually involved in the chemical reactions needed to make the new minerals and may be added or subtracted during metamorphism.

We would like to thank the "Geological Museum of the University of Western Australia" for this information supplied. You can visit this Geological Museum free of charge.